Author name code: shibata
ADS astronomy entries on 2022-09-14
=author:"Shibata, K." OR =author:"Shibata, Kazunari"
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Title: Extreme solar events
Authors: Cliver, Edward W.; Schrijver, Carolus J.; Shibata, Kazunari;
Usoskin, Ilya G.
Bibcode: 2022LRSP...19....2C
Altcode: 2022arXiv220509265C
We trace the evolution of research on extreme solar and
solar-terrestrial events from the 1859 Carrington event to the rapid
development of the last twenty years. Our focus is on the largest
observed/inferred/theoretical cases of sunspot groups, flares on the
Sun and Sun-like stars, coronal mass ejections, solar proton events,
and geomagnetic storms. The reviewed studies are based on modern
observations, historical or long-term data including the auroral and
cosmogenic radionuclide record, and Kepler observations of Sun-like
stars. We compile a table of 100- and 1000-year events based on
occurrence frequency distributions for the space weather phenomena
listed above. Questions considered include the Sun-like nature of
superflare stars and the existence of impactful but unpredictable solar
"black swans" and extreme "dragon king" solar phenomena that can involve
different physics from that operating in events which are merely large.
Title: Simultaneous TESS and ground-based spectroscopic observations
of stellar superflares: Exploring the connection between stellar
mass ejections and superflares
Authors: Maehara, Hiroyuki; Shibata, Kazunari; Honda, Satoshi; Nogami,
Daisaku; Ikuta, Kai; Notsu, Yuta; Namizaki, Keiichi; Inoue, Shun;
Namekata, Kosuke
Bibcode: 2022cosp...44.2462M
Altcode:
Solar flares are energetic explosions in the solar atmosphere
(corona). They are thought to be the rapid release of magnetic
energy through magnetic reconnection. The typical energy released
by a solar flare ranges from 10$ ^{29}$ erg to 10$ ^{32}$ erg. Much
larger flares called 'superflares' with the energy of 10$ ^{33}$-10$
^{38}$ erg have been observed on a wide variety of stars including
ordinary solar-type stars. Our knowledge on stellar superflares based
on photometric observations has been significantly improved in the past
decade thanks to the space-based high-precision photometry (e.g. Kepler
and TESS). However spectroscopic observations of superflares are very
limited due to the low occurrence rate of superflares. We performed
intensive spectroscopic observations of active G, K, and M-dwarfs in
the framework of OISTER (Optical and Infrared Synergetic Telescopes
for Education and Research) collaboration simultaneously with TESS. We
detected more than 20 stellar flares including 5 superflares with
the bolometric energy ranges from 10$ ^{33}$ - 10$ ^{35}$ erg. Among
them, a 10$ ^{33}$ erg superflare on the G-type star EK Dra (Namekata
et al. 2021) and a non-white light flare on the M-type star YZ CMi
(Maehara et al. 2021) exhibited signatures of stellar mass eruptions
in the H$\alpha$ line. Our results suggest that the estimated masses of
erupted material associated with stellar flares are roughly comparable
to those expected from the relation between CME mass and flare energy on
the Sun. Moreover, we found that the kinetic energies for these stellar
events are smaller than those extrapolated from the solar CME scaling
law. This discrepancy could be understood by taking into account the
difference between the velocity of filament eruptions and that of CMEs.
Title: Hunting for stellar mass ejections associated with stellar
superflares through time-resolved optical spectroscopy of superflare
stars
Authors: Maehara, Hiroyuki; Shibata, Kazunari; Honda, Satoshi; Nogami,
Daisaku; Ikuta, Kai; Notsu, Yuta; Namizaki, Keiichi; Inoue, Shun;
Namekata, Kosuke
Bibcode: 2022cosp...44.1378M
Altcode:
Solar flares are energetic explosions in the solar atmosphere
(corona). They are thought to be the rapid release of magnetic energy
through magnetic reconnection. Solar flares are sometimes accompanied
by mass ejections such as prominence/filament eruptions and coronal
mass ejections (CMEs). In the case of other stars, including ordinary
solar-type stars, much larger flares called 'superflares' with the
energy of 10$ ^{33}$-10$ ^{38}$ erg have been observed. Although our
knowledge on stellar superflares based on photometric observations has
been significantly improved in the past decade thanks to space-based
photometry missions like Kepler and TESS, it is still not well
understood whether stellar superflares are accompanied by super-CMEs. By
analogy with prominence/filament eruptions associated with solar flares,
mass ejections due to stellar flares are thought to be observed as the
blue-shifted emission/absorption components in chromospheric lines
(e.g. Odert et al. 2020). For example, blue-shifted enhancements in
the H$\alpha$ line with the velocity of a few hundred km/s observed on
M-type flare stars are thought to be caused by stellar mass ejections
(e.g. Vida et al. 2016). We preformed time-resolved optical spectroscopy
of active G, K, and M-dwarfs simultaneously with TESS and detected more
than 20 stellar flares including several superfalres with the bolometric
energy of 10$^{33}$-10$^{35}$ erg. Among these flares, a 10$^33$ erg
superflare on the G-type star EK Dra exhibited a blue-shifted absorption
component with the velocity of ~500 km/s in H-alpha line (Namekata
et al. 2021). The time evolution of the H$\alpha$ line during this
superflare is similar to that of the Sun-as-a-star spectrum during the
solar flare accompanying a filament eruption. This similarity suggests
that the observed blue-shifted absorption component is caused by the
filament eruption associated with a superflare on EK Dra. In addition,
we detected blue-shifted emission component in H-alpha line with the
velocity of ~100 km/s during a flare on the M-type star YZ CMi (Maehara
et al. 2021). This value is comparable to that of solar prominence
eruptions. The estimated masses of erupted material for these stellar
events based on optical spectroscopy are roughly comparable to those
expected from the empirical relation between CME mass and flare energy
on the Sun. However, the estimated kinetic energies for these stellar
events are smaller than those extrapolated from the relation between
the kinetic energy of CMEs and flare energy. This discrepancy could be
understood by the difference between the velocity of prominence/filament
eruptions and that of CMEs. Can these stellar prominence/filament
eruptions lead stellar CMEs? In the case of solar events, the CME
velocity is 4-8 times larger than that of prominence/filament eruptions
(e.g. Gopalswamy et al. 2003). This suggests that the observed stellar
prominence/filament eruptions could lead stellar CMEs. Further X-ray,
EUV and radio observations of stellar superflares simultaneously
with optical photometry/spectroscopy are necessary to understand the
connection between stellar flares and stellar CMEs.
Title: Present and Future of Connections between Solar and Stellar
CMEs
Authors: Shibata, Kazunari
Bibcode: 2022cosp...44.1402S
Altcode:
Recent developments of space observations of exoplanets such as by
Kepler and TESS mission have greatly advanced the study of superflares
on solar type stars (G-dwarfs) as well as M-dwarfs. These superflare
studies are important not only for the study of habitability of
exoplanets but also for understanding of the basic properties of
superflares on the past and future Sun. That is, the superflares on the
Sun-like stars can be a prototype of the future superflares on the Sun,
and so the observations of coronal mass ejections (CMEs) associated
with these superflares would be very important for understanding of
the effects of extreme space weather in the present and future Sun. In
this talk, we review recent developments of the observations of mass
ejections or candidates of CMEs on solar type stars and M dwarfs.
Title: Filament eruptions and CMEs from superflares on solar-type
stars
Authors: Namekata, Kosuke; Shibata, Kazunari; Honda, Satoshi; Maehara,
Hiroyuki; Nogami, Daisaku; Notsu, Yuta
Bibcode: 2022cosp...44.2446N
Altcode:
Solar flares are often accompanied by filament/prominence eruptions,
sometimes leading to coronal mass ejections (CMEs) that directly affect
the Earth's environment. Very large flares called `superflares' are
found on some active solar-type (G-type main-sequence) stars (Maehara
et al. 2012, Nature), but the association of filament eruptions/CMEs
is unknown. Here we show that our optical spectroscopic observation of
the young solar-type star EK Draconis reveals the first evidence for
a stellar filament eruption associated with a superflare on solar-type
stars (Namekata et al. 2021, Nature Astronomy, in press). We monitored
EK Draconis for about 30 nights with the TESS's optical photometry
and ground-based spectroscopy onboard the Seimei Telescope (Japan)
and Nayuta Telescope (Japan), and finally detected a superflare with
the TESS white-light and H-alpha. After the superflare with radiated
energy of 2.0×10$^{33}$ erg, a blue-shifted hydrogen absorption
component with a large velocity of -510 km s$^{-1}$ appeared. The
temporal changes in the spectra greatly resemble the Sun-as-a-star
spectra of solar filament eruptions observed by SMART telescope at Hida
Observatory, indicating a common picture of solar and stellar filament
eruptions. The comparison of this stellar filament eruption with solar
filament eruptions in terms of the length scale and velocity strongly
suggests that this would eventually become a stellar CME. The mass of
erupted filament of 1.1×10$^{18}$ g is surprisingly 10 times larger
than those of the largest solar CMEs, while the mass is consistent
with those extrapolated from the solar scaling law of filament/CME
mass-flare energy relation. The huge filament eruption and an associated
CME on the young solar-type star provide the opportunity to evaluate
how they affect the environment of young exoplanets/young Earth and
stellar mass/angular-momentum evolution.
Title: Universal Correlation between the Ejected Mass and Total
Flare Energy for Solar and Stellar Cold Plasma Ejection
Authors: Kotani, Yuji; Otsuji, Kenichi; Shibata, Kazunari; Asai,
Ayumi; Ichimoto, Kiyoshi; Ishii, Takako; Yamasaki, Daiki
Bibcode: 2022cosp...44.2450K
Altcode:
We often find spectral signatures of chromospheric plasma ejections
accompanied by flares in various spatial scales in the solar and stellar
atmospheres. Similar spectral signatures are found regardless of their
wide range of scale. However, no physical quantities such as mass and
energy have been estimated for flare energies covering over 10 orders
of magnitude until now. In this study, we analyzed the spectra of
cold plasma ejections associated with flares by performing H$\alpha$
imaging spectroscopy of the solar full-disk with SMART/SDDI. We
determined the ejected mass by cloud model fitting to the H$\alpha$
spectrum. We estimated flare energy by DEM analysis using SDO/AIA
for small-scale flares and by estimating the bolometric energy for
large-scale flares. In addition, we constructed a scaling law for
the total flare energy and the ejected mass and compared it with our
observation. The results are in good agreement with the scaling law for
small mass ejections with small flares in the quiet region for a coronal
field strength of 5 G and filament eruptions with flares for that of 5 -
50 G. We also compared it with the observations interpreted as stellar
filament eruptions, and found that they were roughly consistent with
the scaling law. These results suggest that cold plasma ejections with
flares taking place on the sun and stars in a wide range of the energy
scale are caused by a common mechanism.
Title: Nonlinear Alfvén Waves in the Chromosphere and Their Effects
on Spicules, Coronal Heating, and Wind Acceleration in the Sun
and M-dwarfs
Authors: Shibata, Kazunari; Sakaue, Takahito
Bibcode: 2022cosp...44.1333S
Altcode:
Alfvén waves are responsible for the transfer of magnetic energy in
magnetized plasma. They are involved in heating the solar atmosphere
and driving solar wind through various nonlinear processes. In this
study, we carried out one-dimensional magnetohydrodynamic simulations
of nonlinear propagation of Alfven waves from the photosphere to the
solar wind to reveal these nonlinear dynamics. The results show that
nonlinear Alfven waves and their associated dynamics naturally lead to
the formation of spicules, coronal heating and solar wind acceleration
in a self-consistent way. The application to the atmosphere and wind
in M dwarfs has also been studied, and it is found that nonlinear
Alfven waves remarkably well explain the formation of the chromosphere,
corona, and wind in M-dwarfs and the Sun (G-dwarfs) in a unified manner.
Title: Blue asymmetries in Balmer lines and possible mass ejections
during mid M dwarf flares
Authors: Notsu, Yuta; Shibata, Kazunari; Enoto, Teruaki; Hamaguchi,
Kenji; Honda, Satoshi; Maehara, Hiroyuki; Nogami, Daisaku; Ikuta,
Kai; Kowalski, Adam; Hawley, Suzanne; Davenport, James; Tristan,
Isaiah; Namekata, Kosuke
Bibcode: 2022cosp...44.1382N
Altcode:
Flares are releases of magnetic energy in the solar/stellar atmosphere,
and they have strong emissions from radio to X-rays. During some M
dwarf flares, chromospheric line profiles show blue asymmetries (Honda
et al. 2018), although red asymmetries are more commonly observed in
solar flares. Similar enhancements of the blue wings of Balmer lines
may provide clues for investigating the early phases of stellar coronal
mass ejections (CMEs) during flares (cf. Vida et al. 2016&2019), but
this is still controversial. Thus, we need simultaneous spectroscopic
and photometric observations of flares with high time resolution
to understand the relationship between mass ejections and flaring
events. We have conducted simultaneous spectroscopic and photometric
observations of mid M dwarf flare stars (YZCMi, EVLac, ADLeo) using
APO 3.5m/ARCES, SMARTS 1.5m/CHIRON, Nayuta 2m/MALLS (high-dispersion
spectroscopy), TESS (space high-precision single-color photometry),
ground-based 0.4-1m telescopes (ground-based photometry), and NICER
(X-ray spectroscopy). During 34 nights of observations, we detected more
than 46 flares in Balmer lines (e.g. H$\alpha$). Among them, at least
8 flare event showed clear blue asymmetries with velocities 100-200
km s$ ^{-1}$(Maehara et al. 2021 PASJ, Notsu et al. in prep). We
found various correspondences in the durations of blue asymmetries
and intensities of white light emissions. In most cases, while the
blue asymmetries were not significant in the higher-order Balmer lines
and other chromospheric lines such as the Ca II K/8542A line. As for
one event, we succeeded in the simultaneous observation of the blue
asymmetry and X-ray flare for the first time. These results might
be a clue to investigate the formation processes of the blue-shifted
components, and discuss how blue asymmteries are helpful for dicussing
CME properties. By assuming that the blue asymmetries were caused by
prominence eruptions, we estimate the mass and kinetic energy of the
upward-moving material to be 10$ ^{15}$ — 10$ ^{18}$ g and 10$ ^{29}$
—10$ ^{32}$ erg, respectively. The estimated masses are comparable
to expectations from the empirical relation between the flare X-ray
energy and mass of upward-moving material for stellar flares and
solar CMEs. In contrast, the estimated kinetic energies for these
non-white-light flares are roughly 2-3 orders of magnitude smaller than
that expected from the relation between flare X-ray energy and kinetic
energy for solar CMEs. This could be understood by the difference in
the velocity between CMEs and prominence eruptions.
Title: Sun-as-a-star Analysis of Hα Spectra of a Solar Flare Observed
by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening
Authors: Namekata, Kosuke; Ichimoto, Kiyoshi; Ishii, Takako T.;
Shibata, Kazunari
Bibcode: 2022ApJ...933..209N
Altcode: 2022arXiv220601395N
Stellar flares sometimes show red/blue asymmetries of the Hα
line, which can indicate chromospheric dynamics and prominence
activations. However, the origin of asymmetries is not completely
understood. For a deeper understanding of stellar data, we performed
a Sun-as-a-star analysis of Hα line profiles of an M4.2-class solar
flare showing dominant emissions from flare ribbons by using the
data of the Solar Dynamics Doppler Imager on board the Solar Magnetic
Activity Research Telescope at the Hida Observatory. Sun-as-a-star Hα
spectra of the flare show red asymmetry of up to ~95 km s-1
and line broadening of up to ~7.5 Å. The Sun-as-a-star Hα profiles
are consistent with spectra from flare regions with weak intensity, but
they take smaller redshift velocities and line widths by a factor of ~2
than those with strong intensity. The redshift velocities, as well as
line widths, peak out and decay more rapidly than the Hα equivalent
widths, which is consistent with the chromospheric condensation
model and spatially resolved flare spectra. This suggests that as
a result of superposition, the nature of chromospheric condensation
is observable even from stellar flare spectra. The time evolution of
redshift velocities is found to be similar to that of luminosities
of near-ultraviolet rays (1600 Å), while the time evolution of
line broadening is similar to that of optical white lights. These Hα
spectral behaviors in Sun-as-a-star view could be helpful to distinguish
whether the origin of Hα red asymmetry of stellar flares is a flare
ribbon or other phenomena.
Title: VizieR Online Data Catalog: 2344 superflares on 266 solar-type
stars (Okamoto+, 2021)
Authors: Okamoto, S.; Notsu, Y.; Maehara, H.; Namekata, K.; Honda,
S.; Ikuta, K.; Nogami, D.; Shibata, K.
Bibcode: 2022yCat..19060072O
Altcode:
We searched for superflares using the Kepler 30 minute (long) time
cadence data that were taken from 2009 May to 2013 May (quarters
0-17). We retrieved the data of the Kepler Date Release 25 (DR25) from
the Multimission Archive at the Space Telescope (MAST) We detected
2344 superflares on 266 solar-type stars from Kepler 30 minute (long)
time cadence data of ~4yr. (1 data file).
Title: Discovery of a Long-duration Superflare on a Young Solar-type
Star EK Draconis with Nearly Similar Time Evolution for Hα and
White-light Emissions
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Honda, Satoshi; Notsu,
Yuta; Okamoto, Soshi; Takahashi, Jun; Takayama, Masaki; Ohshima,
Tomohito; Saito, Tomoki; Katoh, Noriyuki; Tozuka, Miyako; Murata,
Katsuhiro L.; Ogawa, Futa; Niwano, Masafumi; Adachi, Ryo; Oeda, Motoki;
Shiraishi, Kazuki; Isogai, Keisuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2022ApJ...926L...5N
Altcode: 2022arXiv220109416N
Young solar-type stars are known to show frequent "superflares,"
which may severely influence the habitable worlds on young planets
via intense radiation and coronal mass ejections. Here we report an
optical spectroscopic and photometric observation of a long-duration
superflare on the young solar-type star EK Draconis (50-120 Myr age)
with the Seimei telescope and Transiting Exoplanet Survey Satellite. The
flare energy 2.6 × 1034 erg and white-light flare duration
2.2 hr are much larger than those of the largest solar flares, and
this is the largest superflare on a solar-type star ever detected by
optical spectroscopy. The Hα emission profile shows no significant
line asymmetry, meaning no signature of a filament eruption, unlike
the only previous detection of a superflare on this star. Also, it did
not show significant line broadening, indicating that the nonthermal
heating at the flare footpoints is not essential or that the footpoints
are behind the limb. The time evolution and duration of the Hα flare
are surprisingly almost the same as those of the white-light flare,
which is different from general M-dwarf (super-)flares and solar
flares. This unexpected time evolution may suggest that different
radiation mechanisms than general solar flares are predominant, such
as: (1) radiation from (off-limb) flare loops and (2) re-radiation
via radiative back-warming, in both of which the cooling timescales
of flare loops could determine the timescales of Hα and white light.
Title: Detection of Filament Eruption from a Superflare on a
Solar-Type Star
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Honda, Satoshi; Notsu,
Yuta; Okamoto, Soshi; Takahashi, Jun; Takayama, Masaki; Ohshima,
Tomohito; Saito, Tomoki; Katoh, Noriyuki; Tozuka, Miyako; Murata,
Katsuhiro; Ogawa, Futa; Niwano, Masafumi; Adachi, Ryo; Oeda, Motoki;
Shiraishi, Kazuki; Isogai, Keisuke; Seki, Daikichi; Ishii, Takako;
Ichimoto, Kiyoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021AGUFM.U43B..03N
Altcode:
Solar flares are often accompanied by filament/prominence eruptions,
sometimes leading to coronal mass ejections (CMEs) that directly affect
the Earth's environment. Very large flares called `superflares' are
found on some active solar-type (G-type main-sequence) stars (Maehara
et al. 2012, Nature), but the association of filament eruptions/CMEs
is unknown. Here we show that our optical spectroscopic observation of
the young solar-type star EK Draconis reveals the first evidence for
a stellar filament eruption associated with a superflare on solar-type
stars (Namekata et al. submitted). We monitored EK Draconis for about
30 nights with the TESS photometry and ground-based spectroscopy
onboard the Seimei Telescope (Japan) and Nayuta Telescope (Japan),
and finally detected a superflare with the TESS white-light and H-alpha
(Figure 1A). After the superflare with radiated energy of 2.0×1033 erg,
a blue-shifted hydrogen absorption component with a large velocity of
-510 km s-1 appeared (Figure 1B). The temporal changes in the spectra
greatly resemble those of the-Sun-as-a-star solar filament eruptions
observed by SMART telescope at Hida Observatory, indicating a common
process of solar and stellar filament eruptions. The comparison of this
eruption with solar filament eruptions in terms of the length scale and
velocity strongly suggests that this would eventually become a stellar
CME. The erupted filament mass of 1.1×1018 g is 10 times larger than
those of the largest solar CMEs. The huge filament eruption and an
associated CME on the young solar-type star provide the opportunity
to evaluate how they affect the environment of young exoplanets/young
Earth and stellar mass/angular-momentum evolution.
Title: Recent observations of stellar flares on G-, K-, and M-dwarf
stars and possible mass ejections
Authors: Notsu, Yuta; Kowalski, Adam; Maehara, Hiroyuki; Namekata,
Kosuke; Tristan, Isaiah; Okamoto, Soshi; Ikuta, Kai; Hawley, Suzanne;
Davenport, James; Enoto, Teruaki; Hamaguchi, Kenji; Nogami, Daisaku;
Shibata, Kazunari
Bibcode: 2021AGUFM.U43B..01N
Altcode:
Flares are frequent energetic explosions in the stellar atmosphere,
and are thought to occur by impulsive releases of magnetic energy
stored around starspots. Large flares (so called superflares) generate
strong high energy emissions and coronal mass ejections (CMEs), which
can greatly affect the planetary environment and habitability. Recent
Kepler/TESS photometric data have revealed the statistical properties
of superflares on G, K, M-type stars. Superflare stars are well
characterized by the existence of large starspots on the surface,
and their magnetic fluxes can explain well superflare energies. Flare
frequency/energy depends on stellar rotation period and stellar
temperature. Young rapidly-rotating stars and cooler stars tend to
have frequent flares, which can be more hazardous for the habitable
planets. However, we still do not know the emission mechanisms of
superflares, and how large CMEs are associated with superflares on these
active stars. Then recently, these active superflare stars have been
investigated in more detail thorugh recent multi-wavelength surveys. For
example, Hydrogen chromospheric lines during flares show blue-shifted
profiles, which can give us some hints on dynamics or mass ejections
during superflares. In the early part of this invited overview talk,
I briefly overview the recent statistical results of superflares from
Kepler/TESS data. Then in the latter part, I also briefly discuss the
results of recent multi-wavelength campaign observations of superflares,
and discuss possible detections of mass ejections (stellar CMEs).
Title: Probable detection of an eruptive filament from a superflare
on a solar-type star
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Honda, Satoshi; Notsu,
Yuta; Okamoto, Soshi; Takahashi, Jun; Takayama, Masaki; Ohshima,
Tomohito; Saito, Tomoki; Katoh, Noriyuki; Tozuka, Miyako; Murata,
Katsuhiro L.; Ogawa, Futa; Niwano, Masafumi; Adachi, Ryo; Oeda,
Motoki; Shiraishi, Kazuki; Isogai, Keisuke; Seki, Daikichi; Ishii,
Takako T.; Ichimoto, Kiyoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2022NatAs...6..241N
Altcode: 2021NatAs...6..241N; 2021arXiv211204808N; 2021NatAs.tmp..246N
Solar flares are often accompanied by filament/prominence eruptions
(~104 K and ~1010−11 cm−3),
sometimes leading to coronal mass ejections that directly affect
the Earth's environment1,2. `Superflares' are found on
some active solar-type (G-type main-sequence) stars3-5,
but the filament eruption-coronal mass ejection association has
not been established. Here we show that our optical spectroscopic
observation of the young solar-type star EK Draconis reveals evidence
for a stellar filament eruption associated with a superflare. This
superflare emitted a radiated energy of 2.0 × 1033 erg,
and a blueshifted hydrogen absorption component with a high velocity of
−510 km s−1 was observed shortly afterwards. The temporal
changes in the spectra strongly resemble those of solar filament
eruptions. Comparing this eruption with solar filament eruptions in
terms of the length scale and velocity strongly suggests that a stellar
coronal mass ejection occurred. The erupted filament mass of 1.1 ×
1018 g is ten times larger than those of the largest solar
coronal mass ejections. The massive filament eruption and an associated
coronal mass ejection provide the opportunity to evaluate how they
affect the environment of young exoplanets/the young Earth6
and stellar mass/angular momentum evolution7.
Title: Small-scale Turbulent Motion of the Plasma in a Solar Filament
as the Precursor of Eruption
Authors: Seki, Daikichi; Otsuji, Kenichi; Isobe, Hiroaki; Del Zanna,
Giulio; Ishii, Takako T.; Sakaue, Takahito; Ichimoto, Kiyoshi;
Shibata, Kazunari
Bibcode: 2021ApJ...918...38S
Altcode: 2021arXiv210611875S
A filament, a dense cool plasma supported by the magnetic fields in
the solar corona, often becomes unstable and erupts. It is empirically
known that the filament often demonstrates some activations such
as a turbulent motion prior to eruption. In our previous study, we
analyzed the Doppler velocity of an Hα filament and found that the
standard deviation of the line-of-sight velocity distribution in a
filament, which indicates the increasing amplitude of the small-scale
motions, increased prior to the onset of the eruption. Here, we
present a further analysis on this filament eruption, which initiated
approximately at 03:40 UT on 2016 November 5 in the vicinity of NOAA
Active Region 12605. It includes a coronal line observation and the
extrapolation of the surrounding magnetic fields. We found that both
the spatially averaged microturbulence inside the filament and the
nearby coronal line emission increased 6 and 10 hr prior to eruption,
respectively. In this event, we did not find any significant changes
in the global potential field configuration preceding the eruption for
the past 2 days, which indicates that there is a case in which it is
difficult to predict the eruption only by tracking the extrapolated
global magnetic fields. In terms of space weather prediction, our
result on the turbulent motions in a filament could be used as the
useful precursor of a filament eruption.
Title: An M Dwarf's Chromosphere, Corona, and Wind Connection via
Nonlinear Alfvén Waves
Authors: Sakaue, Takahito; Shibata, Kazunari
Bibcode: 2021ApJ...919...29S
Altcode: 2021arXiv210612752S
An M dwarf's atmosphere is expected to be highly magnetized. The
magnetic energy can be responsible for heating the stellar chromosphere
and corona and driving the stellar wind. The nonlinear propagation of
Alfvén waves is a promising mechanism for both heating the stellar
atmosphere and driving the stellar wind. Based on this Alfvén
wave scenario, we carried out a 1D compressive magnetohydrodynamic
simulation to reproduce the stellar atmospheres and winds of TRAPPIST-1,
Proxima Centauri, YZ CMi, AD Leo, AX Mic, and the Sun. The nonlinear
propagation of Alfvén waves from the stellar photosphere to the
chromosphere, corona, and interplanetary space is directly resolved
in our study. The simulation result particularly shows that the slow
shock generated through the nonlinear mode coupling of Alfvén waves
is crucially involved in both the dynamics of the stellar chromosphere
(stellar spicule) and stellar wind acceleration. Our parameter survey
further revealed the following general trends of the physical quantities
of the stellar atmosphere and wind. (1) The M dwarf coronae tend to
be cooler and denser than the solar corona. (2) The M dwarf stellar
winds can be characterized by a relatively faster velocity and much
smaller mass-loss rate compared to those of the solar wind. The
physical mechanisms behind these tendencies are clarified in this
paper, where the stronger stratification of the M dwarf's atmosphere
and relatively smaller Alfvén wave energy input from the M dwarf's
photosphere are remarkable.
Title: Time-resolved spectroscopy and photometry of an M dwarf flare
star YZ Canis Minoris with OISTER and TESS: Blue asymmetry in H-alpha
line during the non-white light flare
Authors: Maehara, Hiroyuki; Notsu, Yuta; Namekata, Kousuke; Honda,
Satoshi; Kowalski, Adam F.; Katoh, Noriyuki; Ohshima, Tomohito; Iida,
Kota; Oeda, Motoki; Murata, Katsuhiro L.; Yamanaka, Masayuki; Takagi,
Kengo; Sasada, Mahito; Akitaya, Hiroshi; Ikuta, Kai; Okamoto, Soshi;
Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021tsc2.confE..16M
Altcode:
We report the results from spectroscopic and photometric observations
of the M-type flare star YZ CMi in the framework of the Optical and
Infrared Synergetic Telescopes for Education and Research (OISTER)
collaborations during the Transiting Exoplanet Survey Satellite (TESS)
observation period. We detected 4 H-alpha flares and one of them did
not show clear brightening in the continuum; during this flare, the
H-alpha line exhibited blue-asymmetry which has lasted for (\sim 60)
min. The line of sight velocity of the blue-shifted component is (\sim
-80) km/s. Under the assumption of that observed blue-asymmetry in
H-alpha line was caused by a prominence eruption, the mass and kinetic
energy of the upward-moving material are estimated to be (10^{16}) -
(10^{18}) g and (10^{29.5}) - (10^{31.5}) erg, respectively. Although
the estimated mass is comparable to expectations from the empirical
relation between the X-ray flare energy and mass of solar coronal mass
ejections (CMEs), the estimated kinetic energy is roughly 2 orders
of magnitude smaller than that expected from the relation for solar
CMEs. This discrepancy could be understood by the difference in the
velocity between CMEs and prominence eruptions (Maehara et al. 2021
PASJ, 73, 44).
Title: Blue asymmetries in Balmer lines during mid M dwarf flares
Authors: Notsu, Yuta; Kowalski, Adam F.; Maehara, Hiroyuki; Namekata,
Kosuke; Honda, Satoshi; Enoto, Teruaki; Hamaguchi, Kenji; Tristan,
Isaiah; Hawley, Suzanne L.; Davenport, James R. A.; Okamoto, Soshi;
Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021tsc2.confE.118N
Altcode:
Flares are releases of magnetic energy in the stellar atmosphere, and
they have strong emissions from radio to X-rays. During some M dwarf
flares, chromospheric line profiles show blue asymmetries, although
red asymmetries are more commonly observed in solar flares. Similar
enhancements of the blue wings of Balmer lines may provide clues for
investigating the early phases of stellar coronal mass ejections (CMEs),
but this is still controversial. Thus, we need more observations to
understand the relationship between mass ejections and flares. We have
conducted simultaneous spectroscopic and photometric observations of
mid M dwarf flare stars using APO 3.5m/ARCES, SMARTS1.5m/CHIRON, TESS,
and etc. During 34 night observations, we detected 48 flares in Balmer
lines (e.g. Hα). At least 7 flares show clear blue asymmetries. Blue
asymmetry durations are different among the 7 events (20min ~
2hr). These results suggest upward flows of chromospheric plasma during
flare events. By assuming that the blue asymmetries were caused by
prominence eruptions, we estimated the mass and kinetic energy. The
estimated masses are comparable to expectations from the empirical
relation between the flare X-ray energy and mass of solar CMEs.
Title: Starspot mapping with parallel tempering for M-dwarf flare
stars
Authors: Ikuta, Kai; Maehara, Hiroyuki; Notsu, Yuta; Namekata, Kosuke;
Kato, Taichi; Okamoto, Soshi; Notsu, Shota; Honda, Satoshi; Nogami,
Daisaku; Shibata, Kazunari
Bibcode: 2021tsc2.confE.106I
Altcode: 2021tsc2.confE.106K
We implemented a code to decipher stellar surface information from
Kepler/TESS light curves specified by many stellar/spot parameters with
a parallel tempering (Ikuta et al. 2020) because it has been suggested
that there are many spots on the surface. The code enables to deduce
parameters, such as the spot location, size, emergence/decay rates,
and the stellar differential rotation, and to calculate the model
evidence in the Bayesian framework. We show degeneracies between all
the parameters and the comparison of the number of spots by revisiting
synthetic light curves emulating Kepler/TESS data. Then, we applied
the code to TESS light curves of M-dwarf flare stars, AU Mic, EV Lac,
and YZ CMi (Ikuta et al. 2021, to be submitted). As results, the spot
location and size are uniquely deduced and almost consistent with those
in other studies; the spot location is suggested to be uncorrelated
with flares in the light curve; the variation of the light curve
structure for AU Mic and YZ CMi in two TESS Cycles can be explained
by the stellar differential rotation or spot emergence/decay. We also
verify the validity of starspot mapping and propose other approaches
to resolve the conundrum.
Title: Erratum: Optical and X-ray observations of stellar flares
on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER,
and OISTER
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Sasaki, Ryo; Kawai,
Hiroki; Notsu, Yuta; Kowalski, Adam F.; Allred, Joel C.; Iwakiri,
Wataru; Tsuboi, Yoko; Murata, Katsuhiro L.; Niwano, Masafumi;
Shiraishi, Kazuki; Adachi, Ryo; Iida, Kota; Oeda, Motoki; Honda,
Satoshi; Tozuka, Miyako; Katoh, Noriyuki; Onozato, Hiroki; Okamoto,
Soshi; Isogai, Keisuke; Kimura, Mariko; Kojiguchi, Naoto; Wakamatsu,
Yasuyuki; Tampo, Yusuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021PASJ...73..485N
Altcode: 2021PASJ..tmp...16N
No abstract at ADS
Title: A three-dimensional velocity of an erupting prominence prior
to a coronal mass ejection
Authors: Gutierrez, Maria V.; Otsuji, Kenichi; Asai, Ayumi; Terrazas,
Raul; Ishitsuka, Mutsumi; Ishitsuka, Jose; Nakamura, Naoki; Yoshinaga,
Yusuke; Morita, Satoshi; Ishii, Takako T.; Ueno, Satoru; Kitai,
Reizaburo; Shibata, Kazunari
Bibcode: 2021PASJ...73..394G
Altcode: 2021arXiv210108575G; 2021PASJ..tmp...23G
We present a detailed three-dimensional (3D) view of a prominence
eruption, coronal loop expansion, and coronal mass ejections (CMEs)
associated with an M4.4 flare that occurred on 2011 March 8 in the
active region NOAA 11165. Full-disk Hα images of the flare and filament
ejection were successfully obtained by the Flare Monitoring Telescope
(FMT) following its relocation to Ica University, Peru. Multiwavelength
observation around the Hα line enabled us to derive the 3D velocity
field of the Hα prominence eruption. Features in extreme ultraviolet
were also obtained by the Atmospheric Imager Assembly onboard the Solar
Dynamic Observatory and the Extreme Ultraviolet Imager on board the
Solar Terrestrial Relations Observatory - Ahead satellite. We found
that, following collision of the erupted filament with the coronal
magnetic field, some coronal loops began to expand, leading to the
growth of a clear CME. We also discuss the succeeding activities of
CME driven by multiple interactions between the expanding loops and
the surrounding coronal magnetic field.
Title: Statistical Properties of Superflares on Solar-type Stars:
Results Using All of the Kepler Primary Mission Data
Authors: Okamoto, Soshi; Notsu, Yuta; Maehara, Hiroyuki; Namekata,
Kosuke; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021csss.confE.112O
Altcode:
Solar flares are energetic explosions in the solar atmosphere, and
superflares are the flares having the energy 10 - 106 times
larger than that of the largest solar flare. Recently, many superflares
on solar-type (G-type main-sequence; effective temperature is 5100 -
6000 K) stars were found in the initial 500 days data obtained by the
Kepler spacecraft (Maehara et al. 2012; Shibayama et al. 2013). Notsu
et al. (2019) conducted precise measurements and binarity check on
the basis of spectroscopic observations and the Gaia-DR2 data. As a
result, the number of Sun-like (effective temperature is 5600 - 6000
K and rotation period is over 20 days) superflare stars significantly
decreased.We report the latest statistical analyses of superflares
on solar-type stars using all of the Kepler primary mission data
and Gaia-DR2 catalog. We updated the flare detection method by using
highpass filter to remove rotational variations caused by starspots. We
also examined the sample biases on the frequency of superflares, taking
into account gyrochronology and flare detection completeness. The
sample size of solar-type stars and Sun-like stars are ~4 and ~12
times, respectively, compared with Notsu et al. (2019). As a result,
we found 2341 superflares on 265 solar-type stars, and 26 superflares
on 15 Sun-like stars. This enabled us to have a more well-established
view on the statistical properties of superflares. The observed upper
limit of the flare energy decreases as the rotation period increases
in solar-type stars. The frequency of superflares decreases as the
stellar rotation period increases. The maximum energy we found on
Sun-like stars is 4×1034 erg. Our analysis of Sun-like
stars suggest that the Sun can cause superflares with energies of
7×1033 erg (~X700-class flares) and ~1×1034
erg (~X1000-class flares) once every ~3,000 years and ~6,000 years,
respectively (Okamoto et al. 2021).
Title: Detection of mass ejection from a superflare on a solar-type
star
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Honda, Satoshi; Notsu,
Yuta; Okamoto, Soshi; Takahashi, Jun; Takayama, Masaki; Ohshima,
Tomohito; Saito, Tomoki; Katoh, Noriyuki; Tozuka, Miyako; Murata,
Katsuhiro; Ogawa, Futa; Niwano, Masafumi; Adachi, Ryo; Oeda, Motoki;
Shiraishi, Kazuki; Isogai, Keisuke; Ishii, Takako; Ichimoto, Kiyoshi;
Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021csss.confE.152N
Altcode:
Solar and stellar flares are caused by the sudden release of magnetic
energy on the surfaces. In the case of the Sun, mass ejections often
accompany solar flares and affect the Earth's environment. Active
solar-type stars (G-type main-sequence stars) sometimes show larger
`superflares' (Maehara et al. 2012) that may cause more huge mass
ejections than those of solar flares. The stellar mass ejections can
greatly affect the exoplanet habitability and the stellar mass evolution
(e.g. Airapetian et al. 2020). However, no observational indication
of mass ejection has been reported especially for solar-type stars.We
conducted spectroscopic monitoring observations of the active young
solar analog EK Dra (a famous zero-age main-sequence G-dwarf) by our
new 3.8-m Seimei telescope, simultaneously with TESS photometry. Our
time-resolved optical spectroscopic observation shows clear evidence for
a stellar mass ejection associated with a superflare on the solar-type
star (Namekata et al. submitted). After the superflare brightening
with the radiated energy of 2.0×1033 erg observed by TESS,
a blue-shifted H-alpha absorption component with a velocity of -510 km
s-1 appeared. The velocity gradually decayed in 2 hours and
the deceleration 0.34 km s-2 was consistent with the surface
gravity on EK Dra (0.30 ± 0.05 km s-2). The temporal changes
in the spectra greatly resemble that of a solar mass ejection observed
by the SMART telescope at Hida observatory. Moreover, the ejected
mass of 1.1×1018 g roughly corresponds to those predicted
from solar flare-energy/ejected-mass relation. These discoveries imply
that a huge stellar mass ejection occurs possibly in the same way as
solar ones. Our high-quality dataset can be helpful for future studies
to estimate its impacts on the young-planet atmosphere around young
solar-type stars as well as stellar mass/angular momentum evolution.
Title: Blue asymmetries in Balmer lines during mid M dwarf flares
Authors: Notsu, Yuta; Kowalski, Adam F.; Maehara, Hiroyuki; Namekata,
Kosuke; Honda, Satoshi; Enoto, Teruaki; Hamaguchi, Kenji; Tristan,
Isaiah; Hawley, Suzanne L.; Davenport, James R. A.; Okamoto, Soshi;
Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021csss.confE.103N
Altcode:
Flares are releases of magnetic energy in the solar/stellar atmosphere,
and they have strong emissions from radio to X-rays. During some
M dwarf flares, chromospheric line profiles show blue asymmetries
(Eason et al. 1992; Honda et al. 2018), although red asymmetries are
more commonly observed in solar flares. Similar enhancements of the
blue wings of Balmer lines may provide clues for investigating the
early phases of stellar coronal mass ejections (CMEs) during flares
(cf. Vida et al. 2016&2019), but this is still controversial. Thus,
we need more flare spectroscopic observations with high time resolution
to understand the relationship between mass ejections and flaring
events. The latter is helpful for estimating the impact on planets
from flares.We have conducted several simultaneous spectroscopic
and photometric observations of mid M dwarf flare stars using
APO 3.5m/ARCES, SMARTS 1.5m/CHIRON, TESS, and ground- based 0.4-1m
photometric telescopes. During 34 nights of observations, we detected
48 flares in Balmer lines (e.g. H-alpha). Among them, at least 7
flare events show clear blue asymmetries. Blue asymmetry durations
are different among the 7 events (20min ~ 2hr).These results suggest
upward flows of chromospheric plasma during flare events. By assuming
that the blue asymmetries were caused by prominence eruptions, we
estimate the mass and kinetic energy of the upward-moving material
to be 1015 - 1018 g and 1029 -
1032 erg, respectively. The estimated masses are comparable
to expectations from the empirical relation between the flare X-ray
energy and mass of upward-moving material for stellar flares and
solar CMEs. In contrast, the estimated kinetic energies for these
non-white-light flares are roughly 2-3 orders of magnitude smaller than
that expected from the relation between flare X-ray energy and kinetic
energy for solar CMEs. This could be understood by the difference in
the velocity between CMEs and prominence eruptions.
Title: Time-resolved spectroscopy and photometry of an M dwarf flare
star YZ Canis Minoris with OISTER and TESS: Blue asymmetry in H\alpha
line during the non-white light flare
Authors: Maehara, Hiroyuki; Notsu, Yuta; Namekata, Kousuke; Honda,
Satoshi; Kowalski, Adam F.; Katoh, Noriyuki; Ohshima, Tomohito; Iida,
Kota; Oeda, Motoki; Murata, Katsuhiro L.; Yamanaka, Masayuki; Takagi,
Kengo; Sasada, Mahito; Akitaya, Hiroshi; Ikuta, Kai; Okamoto, Soshi;
Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021csss.confE.142M
Altcode:
Flares are thought to be the rapid releases of magnetic energy through
magnetic reconnection in the corona. Blue asymmetries (enhancement of
the blue wing) in chromospheric lines have been widely observed during
flares on M dwarfs. They are thought to be caused by the upward motions
of cool plasma (e.g., chromospheric evaporations, filament/prominence
eruptions). As observed on the Sun, stellar filament/prominence
eruptions can evolve into stellar CMEs (coronal mass ejections).Here
we report the results from spectroscopic and photometric observations
of the M-type flare star YZ CMi in the framework of the Optical and
Infrared Synergetic Telescopes for Education and Research (OISTER)
collaborations during the Transiting Exoplanet Survey Satellite
(TESS) observation period.We detected 145 white-light flares from the
TESS light curve and 4 H(\alpha) flares from the OISTER observations
performed between 2019-01-16 and 2019-01-18. Among them, 3 H(\alpha)
flares were associated with white-light flares. However, one of them
did not show clear brightening in continuum; during this flare, the
H(\alpha) line exhibited blue-asymmetry which has lasted for (\sim
60) min. The line of sight velocity of the blue-shifted component is
(-80) - (-100) km s-1. By assuming that the blue-asymmetry
in H(\alpha) line was caused by a prominence eruption on YZ CMi, we
estimated the mass and kinetic energy of the upward-moving material
to be (10^{16}) - (10^{18}) g and (10^{29.5}) - (10^{31.5}) erg,
respectively.Although, the estimated mass is comparable to expectations
from the empirical relation between the flare X-ray energy and mass of
upward-moving material for solar CMEs, the estimated kinetic energy for
the non-white-light flare on YZ CMi is roughly 2 orders of magnitude
smaller than that expected from the relation between flare X-ray energy
and kinetic energy for solar CMEs. This could be understood by the
difference in the velocity between CMEs and prominence eruptions.
Title: Time-resolved spectroscopy and photometry of M dwarf flare
star YZ Canis Minoris with OISTER and TESS: Blue asymmetry in the
Hα line during the non-white light flare
Authors: Maehara, Hiroyuki; Notsu, Yuta; Namekata, Kousuke; Honda,
Satoshi; Kowalski, Adam F.; Katoh, Noriyuki; Ohshima, Tomohito; Iida,
Kota; Oeda, Motoki; Murata, Katsuhiro L.; Yamanaka, Masayuki; Takagi,
Kengo; Sasada, Mahito; Akitaya, Hiroshi; Ikuta, Kai; Okamoto, Soshi;
Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021PASJ...73...44M
Altcode: 2020PASJ..tmp..253M; 2020arXiv200914412M
In this paper, we present the results from spectroscopic and photometric
observations of the M-type flare star YZ CMi in the framework of the
Optical and Infrared Synergetic Telescopes for Education and Research
(OISTER) collaborations during the Transiting Exoplanet Survey
Satellite (TESS) observation period. We detected 145 white-light
flares from the TESS light-curve and four Hα flares from the OISTER
observations performed between 2019 January 16 and 18. Among them,
three Hα flares were associated with white-light flares. However,
one of them did not show clear brightening in the continuum; during
this flare, the Hα line exhibited blue asymmetry which lasted for
∼60 min. The line-of-sight velocity of the blueshifted component is
in the range from -80 to -100 km s-1. This suggests that
there can be upward flows of chromospheric cool plasma even without
detectable red/near-infrared (NIR) continuum brightening. By assuming
that the blue asymmetry in the Hα line was caused by a prominence
eruption on YZ CMi, we estimated the mass and kinetic energy of the
upward-moving material to be 1016-1018 g and
1029.5-1031.5 erg, respectively. The estimated
mass is comparable to expectations from the empirical relation between
the flare X-ray energy and mass of upward-moving material for stellar
flares and solar coronal mass ejections (CMEs). In contrast, the
estimated kinetic energy for the non-white-light flare on YZ CMi is
roughly two orders of magnitude smaller than that expected from the
relation between flare X-ray energy and kinetic energy for solar
CMEs. This could be understood by the difference in the velocity
between CMEs and prominence eruptions.
Title: Blue asymmetries in Balmer lines during mid M dwarf flares
Authors: Notsu, Y.; Kowalski, A.; Maehara, H.; Namekata, K.; Honda,
S.; Enoto, T.; Hamaguchi, K.; Tristan, I.; Hawley, S.; Davenport,
J.; Okamoto, S.; Ikuta, K.; Nogami, D.; Shibata, K.
Bibcode: 2021AAS...23751501N
Altcode:
Flares are releases of magnetic energy in the solar/stellar atmosphere,
and they have strong emissions from radio to X-rays. During some
M dwarf flares, chromospheric line profiles show blue asymmetries
(Honda et al. 2018), although red asymmetries are more commonly
observed in solar flares. Similar enhancements of the blue wings of
Balmer lines may provide clues for investigating the early phases
of stellar coronal mass ejections (CMEs) during flares (cf. Vida et
al. 2016&2019), but this is still controversial. Thus, we need
more flare spectroscopic observations with high time resolution
to understand the relationship between mass ejections and flaring
events. The latter is helpful for estimating the impact on planets
from flares. We have conducted several simultaneous spectroscopic
and photometric observations of mid M dwarf flare stars using APO
3.5m/ARCES, SMARTS 1.5m/CHIRON, Nayuta 2m/MALLS (high-dispersion
spectroscopy), TESS (space high-precision single-color photometry),
and ground-based 0.4-1m telescopes (ground-based photometry). During
~20 nights of observations, we detected more than 30 flares in Balmer
lines (e.g. Hα). Among them, at least 6 flare events (including one
already reported in Maehara et al. 2020) show clear blue asymmetries,
but none show brightening in the continuum. Blue asymmetry durations are
different among the 6 events (20min ~ 2hr). These results suggest upward
flows of chromospheric plasma during "non-white light" flare events. By
assuming that the blue asymmetries were caused by prominence eruptions,
we estimate the mass and kinetic energy of the upward-moving material to
be 1015-1018 g and 1029-1032
erg, respectively. The estimated masses are comparable to expectations
from the empirical relation between the flare X-ray energy and mass of
upward-moving material for stellar flares and solar CMEs. In contrast,
the estimated kinetic energies for these non-white-light flares are
roughly 2 orders of magnitude smaller than that expected from the
relation between flare X-ray energy and kinetic energy for solar
CMEs. This could be understood by the difference in the velocity
between CMEs and prominence eruptions.
Title: Superflares investigated with Kepler&TESS photometric
data and recent multi-wavelength campaign observations
Authors: Notsu, Yuta; Shibata, Kazunari; Enoto, Teruaki; Hamaguchi,
Kenji; Honda, Satoshi; Maehara, Hiroyuki; Nogami, Daisaku; Namekata,
Kosuke; Ikuta, Kai; Kowalski, Adam; Hawley, Suzanne; Davenport, James;
Okamoto, Soshi; Notsu, Shota
Bibcode: 2021cosp...43E1750N
Altcode:
Solar flares are frequent energetic explosions in the solar atmosphere,
and are thought to occur by impulsive releases of magnetic energy
stored around sunspots. Large solar flares sometimes can have large
impacts on our Earth and society (e.g., magnetic storms). Stars other
than the Sun also show flares. Many young stars, active M-dwarfs known
as flare stars, and close binary stars have ``superflares'', which are
flares that have a total energy 10--10$^{6}$ times larger than that of
the largest solar flares on the Sun ($\sim$10$^{32}$ erg). These stars
generally rotate very fast (Period $\sim$ 2--3 days). In contrast, the
Sun slowly rotates (Period $\sim$25 days). Then it had been thought that
superflares cannot occur on slowly-rotating G-type main-sequence stars
like the Sun. Recently, more than 1000 superflares on solar-type stars
(G-type main sequence stars) have been found using the photometric
data of Kepler spece telescope (and also TESS satellite). Using these
large number of data, it is now possible to do statistical studies
of superflares on solar-type stars. First, superflare stars are well
characterized by the existence of large starspots on the surface,
and their magnetic fluxes can explain well superflare energies. Then,
maximum superflare energy continuously decreases as the rotation period
increases. Superflares with their energy $\le \sim$ 5$\times$10$^{34}$
erg (a few hundred times larger than the largest solar flares) would
occur on old slowly-rotating Sun-like stars (Rotation Period $\sim$25
days) once every 2000-3000 years, while young rapidly-rotating
stars with Rotation Period $\sim$ a few days have superflares up
to 10$^{36}$ erg. These results presented in this work support that
even slowly-rotating stars similar to the Sun can have superflares,
considering long-term activity level changes. In addition to these
photometric observation results of solar-type superflare stars, cool
M-dwarf superflare stars have been investigated in more detail by
recent multi-wavelength surveys. For example, Hydrogen chromospheric
lines during flares show a lot of blue-shifted profiles, which can give
us some hints on dynamics or mass ejections during superflares. In
the main part of this review talk, I review the recent statistical
results of superflares from Kepler\&TESS data. Then in the latter
part, I also briefly discuss the results of recent multi-wavelength
campaign observations of M-dwarf superflares, and finally show the
future prospects of superflare studies, which are closely related with
solar physics and studies of effects on planets.
Title: First detection of stellar mass ejection from a superflare
on a solar-type star
Authors: Namekata, Kosuke; Ishii, Takako; Shibata, Kazunari; Ichimoto,
Kiyoshi; Honda, Satoshi; Maehara, Hiroyuki; Nogami, Daisaku; Notsu,
Yuta; Okamoto, Soshi
Bibcode: 2021cosp...43E1733N
Altcode:
Active solar-type stars sometimes show large `superflares' that may
cause huge mass ejections, greatly affecting the planetary environment
and the stellar mass evolution. However, no observational indication of
mass ejection has been reported especially for solar-type stars, and
furthermore, the signatures of mass ejections are very controversial
even for other kinds of magnetically active stars (e.g. cooler stars
and giant stars). We conducted a spectroscopic monitoring observations
of the active young solar-type star EK Draconis (EK Dra) by our new
3.8-m Seimei telescope, simultaneously with TESS satellite. Our
time-resolved optical spectroscopic observation shows the first,
conclusive evidence for a stellar mass ejection associated with a
superflare on the solar-type star. After the superflare brighntenings
with the radiated energy of $2.0\times10^{33}$ erg observed by TESS,
a blue-shifted H$\alpha$ absorption component with a velocity of
$-510$ km s$^{-1}$ appeared. The velocity gradually decayed in 1-2
hour and the deceleration 0.34 $km s$^{-2}$ was consistent with the
surface gravity on EK Dra (0.30 \pm 0.05 $km s$^{-2}$). The temporal
changes in the spectra greatly resemble that of solar mass ejections
observed by SMART telescope at Hida observatory. Moreover, the ejected
mass of $6.2\times10^{17}$ g corresponds to those predicted from solar
flare-energy/ejected-mass relation. These discoveries imply that a huge
stellar mass ejection did occur in the same way as solar ones, which
enables us to estimate its impacts on planets as well as mass/angular
momentum evolution.
Title: Nonlinear Alfvén Wave Model of Stellar Coronae and Winds
from the Sun to M Dwarfs
Authors: Sakaue, Takahito; Shibata, Kazunari
Bibcode: 2021ApJ...906L..13S
Altcode: 2020arXiv201210868S
An M dwarf's atmosphere and wind are expected to be highly
magnetized. The nonlinear propagation of Alfvén waves could play a
key role in both heating the stellar atmosphere and driving the stellar
wind. Using this Alfvén wave scenario, we carried out a one-dimensional
compressive magnetohydrodynamic simulation to examine the nonlinear
propagation of Alfvén waves from the M dwarf's photosphere,
chromosphere to the corona, and interplanetary space. Based on the
simulation results, we developed a semi-empirical method describing
the solar and M dwarf's coronal temperature, stellar wind velocity,
and wind's mass-loss rate. We find that M dwarfs' coronae tend to be
cooler than the solar corona, and that M dwarfs' stellar winds can be
characterized as having a faster velocity and much smaller mass-loss
rate compared to those of the solar wind.
Title: Statistical Properties of Superflares on Solar-type Stars:
Results Using All of the Kepler Primary Mission Data
Authors: Okamoto, Soshi; Notsu, Yuta; Maehara, Hiroyuki; Namekata,
Kosuke; Honda, Satoshi; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2021ApJ...906...72O
Altcode: 2020arXiv201102117O
We report the latest statistical analyses of superflares on solar-type
(G-type main-sequence; effective temperature is 5100-6000 K) stars
using all of the Kepler primary mission data and Gaia Data Release 2
catalog. We updated the flare detection method from our previous studies
by using a high-pass filter to remove rotational variations caused
by starspots. We also examined the sample biases on the frequency of
superflares, taking into account gyrochronology and flare detection
completeness. The sample sizes of solar-type and Sun-like stars
(effective temperature is 5600-6000 K and rotation period is over 20
days in solar-type stars) are ∼4 and ∼12 times, respectively,
compared with Notsu et al. As a result, we found 2341 superflares
on 265 solar-type stars and 26 superflares on 15 Sun-like stars;
the former increased from 527 to 2341 and the latter from three
to 26 events compared with our previous study. This enabled us to
have a more well-established view on the statistical properties of
superflares. The observed upper limit of the flare energy decreases as
the rotation period increases in solar-type stars. The frequency of
superflares decreases as the stellar rotation period increases. The
maximum energy we found on Sun-like stars is 4 × 1034
erg. Our analysis of Sun-like stars suggests that the Sun can cause
superflares with energies of ∼7 × 1033 erg (∼X700-class
flares) and ∼1 × 1034 erg (∼X1000-class flares) once
every ∼3000 and ∼6000 yr, respectively.
Title: Evaluation system of Stellar Superflares impacts on Planetary
Habitability
Authors: Yamashiki, Y. A.; Maehara, H.; Airapetian, V.; Notsu, Y.;
Sato, T.; Notsu, S.; Shimizu, R.; Kimura, N.; Sasaki, T.; Shibata, K.
Bibcode: 2020AGUFMP007.0004Y
Altcode:
We present the comprehensive quantitative impact evaluation system
of stellar flares on the habitability factors with an emphasis on
the impact of stellar proton events applicable for all 45 potential
habitable planets classified as CHZ. We derive the maximum flare energy
from stellar star spot sizes and examine the impacts of flare-associated
ionizing radiation on CO2, H2, and N2+
O2-rich atmospheres of a number of well-characterized
terrestrial type exoplanets. Our simulations based on the Particle
and Heavy Ion Transport code System suggest that the estimated
ground-level dose for each planet in the case of terrestrial-level
atmospheric pressure (1 bar) for most of exoplanets do not exceed the
critical dose for complex (multicellular) life to persist, even for
the planetary surface of Proxima Centauri b, Ross-128 b, and TRAPPIST-1
e. Large fluxes of coronal XUV emission from active stars induce high
atmospheric escape rates from close-in exoplanets, suggesting that the
atmospheric depth can be substantially smaller than that on Earth. In
this study we also consider that the atmospheric escape may be induced
both XUV emission and frequency of flares. By considering both impacts
some so-called habitable planets reaches near fatal levels with annual
frequency of flare occurrence from their host stars. The system can
be extended for evaluating historical transition of habitability of
terrestrial planets in our solar system, especially for the Marsian
surface radiation condition.
Title: IRIS Mg II Observations and Non-LTE Modeling of Off-limb
Spicules
Authors: Tei, A.; Gunár, S.; Heinzel, P.; Okamoto, T. J.; Štěpán,
J.; Jejčič, S.; Shibata, K.
Bibcode: 2020AGUFMSH0010008T
Altcode:
We investigated the off-limb spicules observed in the Mg II h and k
spectral lines by Interface Region Imaging Spectrograph (IRIS) in a
solar polar coronal hole. We analyzed the large data set of obtained
spectra to extract quantitative information about the line intensities,
line shifts, and line widths. The observed Mg II line profiles are
broad and double peaked at lower altitudes, broad but flat topped
at middle altitudes, and narrow and single peaked with the largest
Doppler shifts at higher altitudes. We used one-dimensional non-LTE
vertical slab models (i.e., models that consider departures from local
thermodynamic equilibrium) in single-slab and multi-slab configurations
to interpret the observations and to investigate how a superposition
of spicules along a line of sight (LOS) affects the synthetic Mg II
line profiles. The employed multi-slab models are either static,
i.e., without any LOS velocities, or assume randomly assigned LOS
velocities of individual slabs, representing the spicule dynamics. We
performed such single-slab and multi-slab modeling for a broad set of
model input parameters and examined the dependence of the Mg II line
profiles on these parameters. In this presentation, we demonstrate that
the observed line widths of the Mg h and k line profiles are strongly
affected by the presence of multiple spicules along the LOS. We also
show that the profiles obtained at higher altitudes can be reproduced
by single-slab models representing individual spicules. We found that
the multi-slab model with a random distribution of the LOS velocities
ranging from −25 to 25 km/s can well reproduce the width and the
shape of the Mg II profiles observed at middle altitudes.
Title: Numerical Simulation of Solar Photospheric Jet-Like Structures
Induced by Magnetic Reconnection
Authors: Kotani, Y.; Shibata, K.
Bibcode: 2020AGUFMSH0240005K
Altcode:
Jet phenomena with a bright loop in their footpoint, called anemone
jets, have been observed in the solar corona and chromosphere. These
jets are formed as a consequence of magnetic reconnection, and from the
scale universality of magnetohydrodynamics (MHD), it can be expected
that anemone jets exist even in the solar photosphere. However, it is
not necessarily apparent that jets can be generated as a result of
magnetic reconnection in the photosphere, where the magnetic energy
is not dominant. Furthermore, MHD waves generated from photospheric
jets could contribute to chromospheric heating and spicule formation;
however, this hypothesis has not yet been thoroughly investigated. In
this study, we perform three-dimensional MHD simulation including
gravity with the solar photospheric parameter to investigate anemone
jets in the solar photosphere. In the simulation, jet-like structures
were induced by magnetic reconnection in the solar photosphere. We
determined that these jet-like structures were caused by slow shocks
formed by the reconnection and were propagated approximately in the
direction of the background magnetic field. We also suggested that MHD
waves from the jet-like structures could influence local atmospheric
heating and spicule formation.
Title: Numerical simulation of solar photospheric jet-like phenomena
caused by magnetic reconnection
Authors: Kotani, Yuji; Shibata, Kazunari
Bibcode: 2020PASJ...72...75K
Altcode: 2020PASJ..tmp..221K; 2020arXiv200612511K
Jet phenomena with a bright loop in their footpoint, called anemone
jets, have been observed in the solar corona and chromosphere. These
jets are formed as a consequence of magnetic reconnection, and from the
scale universality of magnetohydrodynamics (MHD), it can be expected
that anemone jets exist even in the solar photosphere. However, it is
not necessarily apparent that jets can be generated as a result of
magnetic reconnection in the photosphere, where the magnetic energy
is not dominant. Furthermore, MHD waves generated from photospheric
jets could contribute to chromospheric heating and spicule formation;
however, this hypothesis has not yet been thoroughly investigated. In
this study, we perform three-dimensional MHD simulation including
gravity with the solar photospheric parameter to investigate anemone
jets in the solar photosphere. In the simulation, jet-like structures
were induced by magnetic reconnection in the solar photosphere. We
determined that these jet-like structures were caused by slow shocks
formed by the reconnection and were propagated approximately in the
direction of the background magnetic field. We also suggested that MHD
waves from the jet-like structures could influence local atmospheric
heating and spicule formation.
Title: Starspot Mapping with Adaptive Parallel
Tempering. I. Implementation of Computational Code
Authors: Ikuta, Kai; Maehara, Hiroyuki; Notsu, Yuta; Namekata, Kosuke;
Kato, Taichi; Notsu, Shota; Okamoto, Soshi; Honda, Satoshi; Nogami,
Daisaku; Shibata, Kazunari
Bibcode: 2020ApJ...902...73I
Altcode: 2020arXiv200805478I
Starspots are thought to be regions of locally strong magnetic
fields, similar to sunspots, and they can generate photometric
brightness modulations. To deduce stellar and spot properties, such
as spot emergence and decay rates, we implement a computational code
for starspot modeling. It is implemented with an adaptive parallel
tempering algorithm and an importance sampling algorithm for parameter
estimation and model selection in the Bayesian framework. For evaluating
the performance of the code, we apply it to synthetic light curves
produced with three spots. The light curves are specified in the spot
parameters, such as the radii, intensities, latitudes, longitudes,
and emergence/decay durations. The spots are circular with specified
radii and intensities relative to the photosphere, and the stellar
differential rotation coefficient is also included in the light
curves. As a result, stellar and spot parameters are uniquely deduced,
and the number of spots is correctly determined: the three-spot model
is preferable because the model evidence is much greater than that of
the two-spot model by orders of magnitude and more than that of the
four-spot model by a more modest factor, whereas the light curves are
produced to have two or one local minimum during one equatorial rotation
period by adjusting the values of longitude. The spot emergence and
decay rates can be estimated with error less than an order of magnitude,
considering the difference of the number of spots.
Title: Energy Transfer by Nonlinear Alfvén Waves in the Solar
Chromosphere and Its Effect on Spicule Dynamics, Coronal Heating,
and Solar Wind Acceleration
Authors: Sakaue, Takahito; Shibata, Kazunari
Bibcode: 2020ApJ...900..120S
Altcode: 2020arXiv200800643S
Alfvén waves are responsible for the transfer of magnetic energy in
magnetized plasma. They are involved in heating the solar atmosphere
and driving solar wind through various nonlinear processes. Because
the magnetic field configurations directly affect the nonlinearity of
Alfvén waves, it is important to investigate how they relate to the
solar atmosphere and wind structure through the nonlinear propagation
of Alfvén waves. In this study, we carried out one-dimensional
magnetohydrodynamic simulations to realize the above relation. The
results show that when the nonlinearity of Alfvén waves in the
chromosphere exceeds a critical value, the dynamics of the solar
chromosphere (e.g., spicule) and the mass-loss rate of solar wind
tend to be independent of the energy input from the photosphere. In
a situation where the Alfvén waves are highly nonlinear, the strong
shear torsional flow generated in the chromosphere "fractures" the
magnetic flux tube. This corresponds to the formation of chromospheric
intermediate shocks, which limit the transmission of the Poynting flux
into the corona by Alfvén waves and also inhibits the propagation of
chromospheric slow shock.
Title: Do superflares occur on the Sun? — Latest results using
all the Kepler 4-year data and Gaia-DR2 data
Authors: Notsu, Y.; Okamoto, S.; Maehara, H.; Namekata, K.; Nogami,
D.; Shibata, K.
Bibcode: 2020SPD....5121111N
Altcode:
Solar flares are energetic explosions in the solar atmosphere,
and superflares are the flares having the energy 10-106
times larger than that of the largest solar flare. It had been
thought that superflares cannot occur on slowly-rotating solar-type
(G-type main-sequence) stars like the Sun. Recently, many superflares
on solar-type stars were found in the initial 500 days data obtained by
the Kepler spacecraft (Maehara et al. 2012 Nature; Shibayama et al. 2013
ApJS). Notsu et al. (2019 ApJ) conducted precise measurements of the
stellar parameters and binarity check on the basis of spectroscopic
observations and the Gaia-DR2 data. As a result, the number of
Sun-like (slowly-rotating solar-type) superflare stars significantly
decreased. Then in our latest study (Okamoto, Notsu, Maehara et al. in
prep), we searched for superflares using all the Kepler 4-year primary
mission data covering ∼1500 days, adding the targets newly identified
as solar-type stars. We updated the flare detection method by using
high-pass filter to remove rotational variations caused by starspots. We
also took into account the effect of sample biases on the frequency
of superflares, by considering gyrochronology and flare detection
completeness. As a result, the number of superflares on Sun-like stars
in this study greatly increased by ∼12 times compared with Notsu et
al. (2019), which enabled us to discuss more well-established view on
statistical properties of superflares on Sun-like stars. As a result,
the upper limit of the flare energy decreases as the rotation period
(stellar age) increases in solar-type stars, while flare energy can
be explained by the magnetic energy stored around starspots. These
can be consistent with the result that the starspot coverage decrease
as the rotation period increases. Frequency of superflares decreases
as the stellar age increases, and flare frequency as a function of
flare energy shows power-law distributions (dN/dE ~ Eα
with α ≳ -2). As a result, Sun-like stars can cause superflares with
energies up to about 5×1034 erg once every ∼5000 years,
and this strongly supports the possiblity of superflares on the Sun.
Title: Optical and X-ray observations of stellar flares on an active
M dwarf AD Leonis with the Seimei Telescope, SCAT, NICER, and OISTER
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Sasaki, Ryo; Kawai,
Hiroki; Notsu, Yuta; Kowalski, Adam F.; Allred, Joel C.; Iwakiri,
Wataru; Tsuboi, Yohko; Murata, Katsuhiro L.; Niwano, Masafumi;
Shiraishi, Kazuki; Adachi, Ryo; Iida, Kota; Oeda, Motoki; Honda,
Satoshi; Tozuka, Miyako; Katoh, Noriyuki; Onozato, Hiroki; Okamoto,
Soshi; Isogai, Keisuke; Kimura, Mariko; Kojiguchi, Naoto; Wakamatsu,
Yasuyuki; Tampo, Yusuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2020PASJ...72...68N
Altcode: 2020arXiv200504336N; 2020PASJ..tmp..218N
We report on multi-wavelength monitoring observations of an M-dwarf
flare star AD Leonis with the Seimei Telescope (6150-7930 Å), SCAT
(Spectroscopic Chuo-university Astronomical Telescope; 3700-7500 Å),
and NICER (Neutron Star Interior Composition Explorer; 0.2-12.0
keV), with the collaboration of the OISTER (Optical and Infrared
Synergetic Telescopes for Education and Research) program. Twelve
flares are detected in total, including ten Hα, four X-ray, and
four optical-continuum flares; one of them is a superflare with a
total energy of ∼2.0 × 1033 erg. We found that: (1)
during the superflare, the Hα emission line full width at 1/8 maximum
dramatically increases to 14 Å from 8 Å in the low-resolution spectra
(R ∼ 2000) accompanied by large white-light flares, (2) some weak
Hα/X-ray flares are not accompanied by white-light emissions, and (3)
the non-flaring emissions show clear rotational modulations in X-ray
and Hα intensity in the same phase. To understand these observational
features, one-dimensional hydrodynamic flare simulations are performed
using the RADYN code. We find the simulated Hα line profiles with
hard and high-energy non-thermal electron beams to be consistent with
the initial phase line profiles of the superflares, while those with
a softer and/or weak-energy beam are consistent with those in decay
phases, indicating the changes in the energy fluxes injected to the
lower atmosphere. Also, we find that the relation between the optical
continuum and Hα intensity is nonlinear, which can be one cause of the
non-white-light flares. The flare energy budget exhibits diversity in
the observations and models, and more observations of stellar flares
are necessary for constraining the occurrence of various emission line
phenomena in stellar flares.
Title: From Jets to Superflares: Extraordinary Activity of Magnetized
Plasmas in the Universe
Authors: Shibata, K.
Bibcode: 2020SPD....5110001S
Altcode:
The development of astronomical observations of various objects in 20th
century has revealed that the universe is full of explosions (flares or
bursts) and plasma outflows such as high-speed jets. Why is our universe
filled with such extraordinary activity? When I started astrophysical
research in 1977, I was fascinated with a puzzle why the nuclei of
distant galaxies produce relativistic jets, collimated supersonic
outflows. Soon after I learned observations of astrophysical jets,
I learned solar observations, which show the importance of magnetic
field in the production of flares and jets, though detailed physics is
still not understood well at that time. I hypothesized that the jets
may be accelerated by magnetic force both on the Sun and galaxies: in
the case of the galaxies, magnetic field may be twisted by the rotation
of accretion disk plasma, whereas on the Sun magnetic field can be
twisted in the solar convection zone. During the untwisting process
of a twisted magnetic flux tube, the jet may be accelerated. Then
I started magnetohydrodynamic (MHD) numerical simulations of both
solar and astrophysical jets. Fortunately, I succeeded in reproducing
astrophysical jets from magnetized accretion disks using time dependent
MHD simulations for the first time (1985, 1986). I was also lucky since
I became a member of space solar observation missions Yohkoh (1991-2000)
and Hinode (2007-present) and discovered X-ray jets in the corona
(1992), as well as chromospheric anemone jets (2007). Both phenomena
were successfully explained by the magnetic reconnection model. From
observations of flares and jets on the Sun, I realized the importance
of plasmoid ejections in magnetic reconnection (1995) and proposed the
unified model of flares and jets on the basis of the plasmoid-induced
reconnection and fractal reconnection (2001). More recently, as an
extension of solar flare studies, I was fortunate enough to discover
superflares on solar type stars with young colleague (2012), which may
be important for the existence or survivability of human beings and
life on the Earth and exoplanets. In conclusion, through these studies,
I learned the reason why our universe is filled with extraordinary
activity is that magnetized plasmas are so active and dynamic.
Title: From Jets to Superflares: Extraordinary Activity of Magnetized
Plasmas in the Universe
Authors: Shibata, K.
Bibcode: 2020AAS...23630001S
Altcode:
The development of astronomical observations of various objects in 20th
century has revealed that the universe is full of explosions (flares or
bursts) and plasma outflows such as high-speed jets. Why is our universe
filled with such extraordinary activity? When I started astrophysical
research in 1977, I was fascinated with a puzzle why the nuclei of
distant galaxies produce relativistic jets, collimated supersonic
outflows. Soon after I learned observations of astrophysical jets,
I learned solar observations, which show the importance of magnetic
field in the production of flares and jets, though detailed physics is
still not understood well at that time. I hypothesized that the jets
may be accelerated by magnetic force both on the Sun and galaxies: in
the case of the galaxies, magnetic field may be twisted by the rotation
of accretion disk plasma, whereas on the Sun magnetic field can be
twisted in the solar convection zone. During the untwisting process
of a twisted magnetic flux tube, the jet may be accelerated. Then
I started magnetohydrodynamic (MHD) numerical simulations of both
solar and astrophysical jets. Fortunately, I succeeded in reproducing
astrophysical jets from magnetized accretion disks using time dependent
MHD simulations for the first time (1985, 1986). I was also lucky since
I became a member of space solar observation missions Yohkoh (1991-2000)
and Hinode (2007-present), and discovered X-ray jets in the corona
(1992), as well as chromospheric anemone jets (2007). Both phenomena
were successfully explained by the magnetic reconnection model. From
observations of flares and jets on the Sun, I realized the importance
of plasmoid ejections in magnetic reconnection (1995), and proposed the
unified model of flares and jets on the basis of the plasmoid-induced
reconnection and fractal reconnection (2001). More recently, as an
extension of solar flare studies, I was fortunate enough to discover
superflares on solar type stars with young colleague (2012), which may
be important for the existence or survivability of human beings and
life on the Earth and exoplanets. In conclusion, through these studies,
I learned the reason why our universe is filled with extraordinary
activity is that magnetized plasmas are so active and dynamic.
Title: The Seimei telescope project and technical developments
Authors: Kurita, Mikio; Kino, Masaru; Iwamuro, Fumihide; Ohta, Kouji;
Nogami, Daisaku; Izumiura, Hideyuki; Yoshida, Michitoshi; Matsubayashi,
Kazuya; Kuroda, Daisuke; Nakatani, Yoshikazu; Yamamoto, Kodai; Tsutsui,
Hironori; Iribe, Masatsugu; Jikuya, Ichiro; Ohtani, Hiroshi; Shibata,
Kazunari; Takahashi, Keisuke; Tokoro, Hitoshi; Maihara, Toshinori;
Nagata, Tetsuya
Bibcode: 2020PASJ...72...48K
Altcode: 2020PASJ..tmp..182K
An overview of the Seimei telescope, a 3.8 m optical infrared telescope
located on Mt. Chikurinji in the Okayama prefecture of Japan, is
presented. Seimei is a segmented-mirror telescope whose primary mirror
consists of 18 petal-shaped segments. The telescope tube supporting
the thin segmented mirrors is structurally incorporated within large
arc-rails providing the elevation axis. The tube has a light-weight
homologous structure designed with a genetic algorithm. The total
weight of the telescope tube, including 1.4-ton optics, is only 8
tons. By virtue of its light weight, the telescope is able to point
at an object anywhere in the observable sky within one minute. The
telescope is operated by Kyoto University in collaboration with
the National Astronomical Observatory of Japan (NAOJ). Half of the
telescope time is used by Kyoto University. The remaining time is
open to the Japanese astronomical community. NAOJ is responsible
for the management of the open-use time, including handling of the
observation proposals. The telescope is now regularly performing
scientific observations on the basis of a variety of proposals.
Title: Impact of space weather on climate and habitability of
terrestrial-type exoplanets
Authors: Airapetian, V. S.; Barnes, R.; Cohen, O.; Collinson,
G. A.; Danchi, W. C.; Dong, C. F.; Del Genio, A. D.; France, K.;
Garcia-Sage, K.; Glocer, A.; Gopalswamy, N.; Grenfell, J. L.; Gronoff,
G.; Güdel, M.; Herbst, K.; Henning, W. G.; Jackman, C. H.; Jin, M.;
Johnstone, C. P.; Kaltenegger, L.; Kay, C. D.; Kobayashi, K.; Kuang,
W.; Li, G.; Lynch, B. J.; Lüftinger, T.; Luhmann, J. G.; Maehara, H.;
Mlynczak, M. G.; Notsu, Y.; Osten, R. A.; Ramirez, R. M.; Rugheimer,
S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.; Sousa-Silva, C.;
Stamenković, V.; Strangeway, R. J.; Usmanov, A. V.; Vergados, P.;
Verkhoglyadova, O. P.; Vidotto, A. A.; Voytek, M.; Way, M. J.; Zank,
G. P.; Yamashiki, Y.
Bibcode: 2020IJAsB..19..136A
Altcode: 2019arXiv190505093A
The current progress in the detection of terrestrial type exoplanets has
opened a new avenue in the characterization of exoplanetary atmospheres
and in the search for biosignatures of life with the upcoming
ground-based and space missions. To specify the conditions favorable for
the origin, development and sustainment of life as we know it in other
worlds, we need to understand the nature of astrospheric, atmospheric
and surface environments of exoplanets in habitable zones around
G-K-M dwarfs including our young Sun. Global environment is formed by
propagated disturbances from the planet-hosting stars in the form of
stellar flares, coronal mass ejections, energetic particles, and winds
collectively known as astrospheric space weather. Its characterization
will help in understanding how an exoplanetary ecosystem interacts
with its host star, as well as in the specification of the physical,
chemical and biochemical conditions that can create favorable and/or
detrimental conditions for planetary climate and habitability along with
evolution of planetary internal dynamics over geological timescales. A
key linkage of (astro) physical, chemical, and geological processes can
only be understood in the framework of interdisciplinary studies with
the incorporation of progress in heliophysics, astrophysics, planetary
and Earth sciences. The assessment of the impacts of host stars on the
climate and habitability of terrestrial (exo)planets will significantly
expand the current definition of the habitable zone to the biogenic zone
and provide new observational strategies for searching for signatures
of life. The major goal of this paper is to describe and discuss the
current status and recent progress in this interdisciplinary field and
to provide a new roadmap for the future development of the emerging
field of exoplanetary science and astrobiology.
Title: Cost estimation for alternative aviation plans against
potential radiation exposure associated with solar proton events
for the airline industry
Authors: Yamashiki, Yosuke A.; Fujita, Moe; Sato, Tatsuhiko; Maehara,
Hiroyuki; Notsu, Yuta; Shibata, Kazunari
Bibcode: 2020arXiv200410869Y
Altcode:
We present a systematic approach to effectively evaluate potential
risk cost caused by exposure to solar proton events (SPEs) from solar
flares for the airline industry. We also evaluate associated health
risks from radiation, to provide relevant alternative ways to minimize
economic loss and opportunity. The estimated radiation dose induced by
each SPE for the passengers of each flight is calculated using ExoKyoto
and PHITS. We determine a few scenarios for the estimated dose limit
at 1 and 20mSv, corresponding to the effective dose limit for the
general public and occupational exposure, respectively, as well as
a higher dose induced an extreme superflare. We set a hypothetical
airline shutdown scenario at 1mSv for a single flight per passenger,
due to legal restrictions under the potential radiation dose. In such
a scenario, we calculate the potential loss in direct and opportunity
cost under the cancelation of the flight. At the same time, we
considered that, even under such a scenario, if the airplane flies
at a slightly lower altitude (from 12 to 9.5km: atmospheric depth
from 234 to 365g/cm$^{2}$), the total loss becomes much smaller than
flight cancelation, and the estimated total dose goes down from 1.2
to 0.45mSv, which is below the effective dose limit for the general
public. In case of flying at an even lower altitude (7km: atmospheric
depth 484g/cm$^{2}$), the estimated total dose becomes much smaller,
0.12 mSv. If we assume the increase of fuel cost is proportional to the
increase in atmospheric depth, the increase in cost becomes 1.56 and
2.07 for the case of flying at 9.5 km and at 7 km, respectively. Lower
altitude flights provide more safety for the potential risk of radiation
doses induced by severe SPEs. At the same time, since there is total
loss caused by flight cancelation, we propose that considering lower
flight altitude is the best protection against solar flares.
Title: VizieR Online Data Catalog: Spot parameters on KIC solar-type
stars (Namekata+, 2019)
Authors: Namekata, K.; Maehara, H.; Notsu, Y.; Toriumi, S.; Hayakawa,
H.; Ikuta, K.; Notsu, S.; Honda, S.; Nogami, D.; Shibata, K.
Bibcode: 2020yCat..18710187N
Altcode:
In order to assess the diversity and similarity of the star spots by
comparing them with the sunspots, we selected solar-type stars as target
stars from the Kepler data set on the basis of the stellar effective
temperature (Teff) and surface gravity (logg) listed in the Kepler
Input Catalog (see DR25; Mathur+, 2017, J/ApJS/229/30). In this study,
we defined solar-type stars with a criterion of 5000K<Teff<6000K
and logg>4.0. For each star, we used all of the available Kepler
pre-search data conditioning long-cadence (30min) data in which
instrumental effects are removed. (1 data file).
Title: Temporal Evolution of Spatially Resolved Individual Star
Spots on a Planet-hosting Solar-type Star: Kepler-17
Authors: Namekata, Kosuke; Davenport, James R. A.; Morris, Brett M.;
Hawley, Suzanne L.; Maehara, Hiroyuki; Notsu, Yuta; Toriumi, Shin;
Ikuta, Kai; Notsu, Shota; Honda, Satoshi; Nogami, Daisaku; Shibata,
Kazunari
Bibcode: 2020ApJ...891..103N
Altcode: 2020arXiv200201086N
Star spot evolution is visible evidence of the emergence/decay of the
magnetic field on a stellar surface, and it is therefore important for
the understanding of the underlying stellar dynamo and consequential
stellar flares. In this paper, we report the temporal evolution
of individual star spot areas on the hot-Jupiter-hosting, active
solar-type star Kepler-17, whose transits occur every 1.5 days. The
spot longitude and area evolution are estimated (1) from the stellar
rotational modulations of Kepler data and (2) from the brightness
enhancements during the exoplanet transits caused by existence of
large star spots. As a result of the comparison, the number of spots,
spot locations, and the temporal evolution derived from the rotational
modulations are largely different from those of in-transit spots. We
confirm that, although only two light-curve minima appear per rotation,
there are clearly many spots present on the star. We find that the
observed differential intensity changes are sometimes consistent with
the spot pattern detected by transits, but at other times they do not
match with each other. Although the temporal evolution derived from
the rotational modulation differs from those of in-transit spots to
a certain degree, the emergence/decay rates of in-transit spots are
within an order of magnitude of those derived for sunspots as well
as our previous research based only on rotational modulations. This
supports the hypothesis that the emergence/decay of sunspots and
extremely large star spots on solar-type stars occur through the same
underlying processes.
Title: SMART/SDDI Filament Disappearance Catalogue
Authors: Seki, Daikichi; Otsuji, Kenichi; Ishii, Takako T.; Hirose,
Kumi; Iju, Tomoya; UeNo, Satoru; Cabezas, Denis P.; Asai, Ayumi;
Isobe, Hiroaki; Ichimoto, Kiyoshi; Shibata, Kazunari
Bibcode: 2020arXiv200303454S
Altcode:
This paper describes a new SMART/SDDI Filament Disappearance Catalogue,
in which we listed almost all the filament disappearance events
that the Solar Dynamics Doppler Imager (SDDI) has observed since
its installation on the Solar Magnetic Activity Research Telescope
(SMART) in May 2016. Our aim is to build a database that can help
predict the occurrence and severity of coronal mass ejections
(CMEs). The catalogue contains miscellaneous information associated
with filament disappearance such as flare, CME, active region,
three-dimensional trajectory of erupting filaments, detection in
Interplanetary Scintillation (IPS), occurrence of interplanetary CME
(ICME) and Dst index. We also provide statistical information on the
catalogue data. The catalogue is available from the following website:
https://www.kwasan.kyoto-u.ac.jp/observation/event/sddi-catalogue/.
Title: Lifetimes and emergence/decay rates of star spots on solar-type
stars estimated by Kepler data in comparison with those of sunspots
Authors: Namekata, K.; Shibata, K.; Maehara, H.; Notsu, Y.; Nogami,
D.; Toriumi, S.; Davenport, J.; Hawley, S.; Morris, B.
Bibcode: 2020AAS...23514805N
Altcode:
Active solar-type stars show large quasi-periodic brightness variations
caused by stellar rotations with large star spots, and the amplitude
changes as the spots emerge and decay. Temporal evolution of star spots
has been hardly measured because of its difficulty in measurement,
especially on solar-type stars. The Kepler's long-term data are suitable
for investigations on the emergence and decay processes of star spots,
which are important to understand underlying stellar dynamo. In this
talk, we report the measurements of temporal evolution of individual
star-spot area on solar-type stars by using Kepler data. We estimated
it (i) by tracing local minima of the Kepler light curves (Namekata et
al. 2019) and (ii) by modeling the small brightness variation during
exoplanet transit (c.f. Morris et al. 2017, Namekata et al. submitted
to ApJ). We successfully obtained temporal evolution of individual star
spots showing clear emergence and decay, and derived the statistical
values of the lifetimes and emergence/decay rates of star spots. As
a result, we found that lifetimes (T) of star spots are ranging
from 10 to 350 days when spot areas (A) are 0.1-2.3% of a solar
hemisphere (SH). The lifetimes of star spots are much shorter than
those extrapolated from an empirical relation of sunspots, while being
consistent with other researches on star spot lifetimes. The emerging
and decay rates of star spots are typically 5×1020 Mx/h
(8 MSH/h) with the area of 0.1-2.3% of SH and are mostly consistent
with those expected from sunspots observations (Petrovay et al. 1997,
Norton et al. 2017). This strongly supports a possibility that the
emergence/decay mechanism of extremely large star spots (0.1-2.3%
of SH) is same as that of smaller sunspots (< 0.5% of SH), which
can constrain the stellar dynamo theory.
Title: Estimating the Temperature and Density of a Spicule from 100
GHz Data Obtained with ALMA
Authors: Shimojo, Masumi; Kawate, Tomoko; Okamoto, Takenori J.;
Yokoyama, Takaaki; Narukage, Noriyuki; Sakao, Taro; Iwai, Kazumasa;
Fleishman, Gregory D.; Shibata, Kazunari
Bibcode: 2020ApJ...888L..28S
Altcode: 2019arXiv191205714S
We succeeded in observing two large spicules simultaneously with the
Atacama Large Millimeter/submillimeter Array (ALMA), the Interface
Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory. One is a spicule seen
in the IRIS Mg II slit-jaw images and AIA 304 Å images (Mg II/304 Å
spicule). The other one is a spicule seen in the 100 GHz images obtained
with ALMA (100 GHz spicule). Although the 100 GHz spicule overlapped
with the Mg II/304 Å spicule in the early phase, it did not show any
corresponding structures in the IRIS Mg II and AIA 304 Å images after
the early phase. It suggests that the spicules are individual events and
do not have a physical relationship. To obtain the physical parameters
of the 100 GHz spicule, we estimate the optical depths as a function
of temperature and density using two different methods. One is using
the observed brightness temperature by assuming a filling factor,
and the other is using an emission model for the optical depth. As a
result of comparing them, the kinetic temperature of the plasma and
the number density of ionized hydrogen in the 100 GHz spicule are
∼6800 K and 2.2 × 1010 cm-3. The estimated
values can explain the absorbing structure in the 193 Å image, which
appear as a counterpart of the 100 GHz spicule. These results suggest
that the 100 GHz spicule presented in this Letter is classified to a
macrospicule without a hot sheath in former terminology.
Title: Blue asymmetries of Balmer lines during M-dwarf flares
investigated with multi-wavelength observations
Authors: Notsu, Y.; Kowalski, A.; Maehara, H.; Namekata, K.; Hawley,
S.; Davenport, J.; Enoto, T.; Hamaguchi, K.; Honda, S.; Notsu, S.;
Ikuta, K.; Nogami, D.; Shibata, K.
Bibcode: 2020AAS...23528805N
Altcode:
Flares are magnetic energy release in the solar/stellar atmosphere,
and they have strong emissions from radio to X-rays. During some
M-dwarf superflares, chromospheric line profiles show blue asymmetries
(Honda et al. 2018), though red asymmetries have been seen during many
ordinary solar flares. it is also thought that similar enhancements of
the blue wing of Balmer lines can provide clues for investigating mass
ejections from flares (stellar CMEs) (cf. Vida et al. 2016&2019),
but this is still very controversial. Thus, we need more flare
spectroscopic observations with high time resolution for understanding
how superflares occur and how large mass ejections occur during
superflares occur. The latter is helpful for estimating the impacts
on planets from superflares. We have conducted several simultaneous
spectroscopic and photometric observations of M-dwarf flare stars. In
2019 January, we observed a M-dwarf flare star YZCMi using APO3.5m/ARCES
(high-dispersion spectroscopy), APO/ARCSAT0.5m (multi-color photometry),
TESS (space high-precision single-color photometry), and NICER (soft
X-ray telescope on ISS). During the observation, we detected large
enhancements of chromospheric lines lasting for longer than 3 hours
(e.g., H- alpha and H-beta). H-alpha line profiles during this event
show some blue asymmetries. In this event, we also detected soft
X-ray intensity increases, but a bit strangely and a bit different
from previous expectations, the photometric data (optical continuum
white light data) show no clear flare-like brightness increases. This
might suggest that these intensity increases of chromospheric lines
(with possible blue asymmetries) and soft X-rays occurred as a
"non white-light" flare events, which are often seen in the case of
solar flares (e.g., Watanabe et al. 2017). We also observed another
M-dwarf flare star AU Mic using CTIO/SMART1.5m/CHIRON (high-dispersion
spectroscopy), LCO (U&V-band photometry), and XMM-Newton (soft
X-ray), and detected several flares in Oct 2018. In contrast to the
above "non-white light" events, these flares show enhancements in
Balmer lines (e.g., H-alpha), optical continuum white light, and soft
X-ray. Then this event is a so-called "white-light" flare. Moreover,
this "white-light" event does not show clear blue asymmetries, which are
different from the above YZCMi "non-white light" event. In this poster,
we introduce ongoing results on the analyses of these two events.
Title: IRIS Mg II Observations and Non-LTE Modeling of Off-limb
Spicules in a Solar Polar Coronal Hole
Authors: Tei, Akiko; Gunár, Stanislav; Heinzel, Petr; Okamoto,
Takenori J.; Štěpán, Jiří; Jejčič, Sonja; Shibata, Kazunari
Bibcode: 2020ApJ...888...42T
Altcode: 2019arXiv191112243T
We investigated the off-limb spicules observed in the Mg II h and k
lines by IRIS in a solar polar coronal hole. We analyzed the large data
set of obtained spectra to extract quantitative information about the
line intensities, shifts, and widths. The observed Mg II line profiles
are broad and double peaked at lower altitudes, broad but flat topped
at middle altitudes, and narrow and single peaked with the largest
Doppler shifts at higher altitudes. We use one-dimensional non-LTE
vertical slab models (I.e., models that consider departures from local
thermodynamic equilibrium) in single-slab and multi-slab configurations
to interpret the observations and to investigate how a superposition
of spicules along the line of sight (LOS) affects the synthetic Mg
II line profiles. The used multi-slab models either are static, I.e.,
without any LOS velocities, or assume randomly assigned LOS velocities
of individual slabs, representing the spicule dynamics. We conducted
such single-slab and multi-slab modeling for a broad set of model
input parameters and showed the dependence of the Mg II line profiles
on these parameters. We demonstrated that the observed line widths
of the h and k line profiles are strongly affected by the presence
of multiple spicules along the LOS. We later showed that the profiles
obtained at higher altitudes can be reproduced by single-slab models
representing individual spicules. We found that the multi-slab model
with a random distribution of the LOS velocities ranging from -25 to
25 km s-1 can well reproduce the width and the shape of Mg
II profiles observed at middle altitudes.
Title: Ionizing Radiation in the Martian Atmosphere during Young Sun.
Authors: Yamashiki, Y. A.; Maehara, H.; Notsu, Y.; Sato, T.;
Airapetian, V.; Notsu, S.; Shibata, K.
Bibcode: 2019AGUFM.P23B3480Y
Altcode:
The enhanced magnetic activity of the young sun was an important in
the atmospheric evolution and dynamics of the early solar system. Lower
solar luminosity associated provided planets a cooler atmospheric and
surface environment on the early Earth and Martian surfaces and required
production of potent greenhouse gasses in their atmospheres. Within
these environments, the dose of ionizing radiation from solar energetic
particles (SEPs) from the young Sun could have played an important
role in the early phases of planetary habitability. Here, we present
a theoretical model of the exposure of the early Mars to high fluence
and hard spectra SEPs from the young sun by evaluating the possible
magnitude and frequency of solar flares by using possible star spot
area, derived from their rotational period. We evaluated annual and
maximum flare energy in different cases of sunspot areas for 3 %, 5 %
and 10 % of their surfaces. The maximum dose at the Martian top of
the atmosphere reaches 5.86 ×103 Gy (2.65 ×102
Sv) at the time when the sunspot area is ~10 % of the solar surface.
Title: SMART/SDDI Filament Disappearance Catalogue
Authors: Seki, D.; Otsuji, K.; Ishii, T.; Hirose, K.; Iju, T.; UeNo,
S.; Cabezas, D.; Asai, A.; Isobe, H.; Ichimoto, K.; Shibata, K.
Bibcode: 2019SunGe..14...95S
Altcode:
This paper describes a new "SMART/SDDI Filament Disappearance
Catalogue," in which we listed almost all the filament disappearance
events that the Solar Dynamics Doppler Imager (SDDI) has observed since
its installation on the Solar Magnetic Activity Research Telescope
(SMART) in May 2016. Our aim is to build a database that can help
predict the occurrence and severity of coronal mass ejections
(CMEs). The catalogue contains miscellaneous information associated
with filament disappearance such as flare, CME, active region,
three-dimensional trajectory of erupting filaments, detection in
Interplanetary Scintillation (IPS), occurrence of interplanetary CME
(ICME) and Dst index. We also provide statistical information on the
catalogue data. The catalogue is available from the following website:
https://www.kwasan.kyoto-u.ac.jp/observation/event/sddi-catalogue/.
Title: Effect of Ionization and Recombination on the Evolution of
the Harris-type Current Sheet in Partially Ionized Plasmas
Authors: Singh, K. A. P.; Sakaue, Takahito; Nakamura, Naoki; Kawamura,
Akito D.; Isobe, Hiroaki; Shibata, Kazunari
Bibcode: 2019ApJ...884..161S
Altcode:
Two-dimensional magnetohydrodynamics (MHD) simulations, treating
plasma and neutral populations (hereafter, neutrals) as two
separate components of the magneto-fluid, are performed in order
to investigate the effect of ionization and recombination (or I/R)
on the time evolution of the Harris-type current sheet in partially
ionized plasmas. Our MHD simulations, including the effect of ambipolar
diffusion (arising due to ion-neutral interactions) along with the I/R,
show that the current sheet thinning occurs due to the diffusion of
neutral particles from the current sheet. In addition to ambipolar
diffusion, frictional heating also appears and affects the evolution
of the current sheet. In a current sheet that is formed in a partially
ionized plasma, the neutral population tries to spread outward and the
plasma population tries to converge toward the center of the current
sheet, and the overall process is influenced by the I/R. One of the
important feature that is captured in our 2D simulations is that the
escape of neutrals from the current sheet is sometimes suppressed due
to the increase in ionization rate at the center of the current sheet,
for the case of collisional I/R. As long as the ionization degree
is kept low inside the current sheet, the current sheet thinning and
elongation takes place and the current sheet becomes unstable due to
the tearing-mode and plasmoid formation. The ion-neutral interactions
coupled with I/R and the dynamics of the magnetic reconnection play
an important role in plasmoid-mediated reconnection, therefore, the
present study on the current sheet thinning and plasmoid formation
could serve as a key for understanding bursty and intermittent plasma
ejections observed in the solar chromosphere.
Title: Dynamic Processes of the Moreton Wave on 2014 March 29
Authors: Cabezas, Denis P.; Asai, Ayumi; Ichimoto, Kiyoshi; Sakaue,
Takahito; UeNo, Satoru; Ishitsuka, José K.; Shibata, Kazunari
Bibcode: 2019ApJ...883...32C
Altcode: 2019arXiv190803534C
On 2014 March 29, an intense solar flare classified as X1.0 occurred
in active region 12017. Several associated phenomena accompanied this
event, among them a fast-filament eruption, large-scale propagating
disturbances in the corona and the chromosphere including a Moreton
wave, and a coronal mass ejection. This flare was successfully detected
in multiwavelength imaging in the Hα line by the Flare Monitoring
Telescope (FMT) at Ica University, Peru. We present a detailed study
of the Moreton wave associated with the flare in question. Special
attention is paid to the Doppler characteristics inferred from the
FMT wing (Hα ± 0.8 Å) observations, which are used to examine
the downward/upward motion of the plasma in the chromosphere. Our
findings reveal that the downward motion of the chromospheric material
at the front of the Moreton wave attains a maximum velocity of 4 km
s-1, whereas the propagation speed ranges between 640 and
859 km s-1. Furthermore, using the weak-shock approximation
in conjunction with the velocity amplitude of the chromospheric motion
induced by the Moreton wave, we derive the Mach number of the incident
shock in the corona. We also performed the temperature-emission measure
analysis of the coronal wave based on the Atmospheric Imaging Assembly
observations, which allowed us to derive the compression ratio, and to
estimate Alfvén and fast-mode Mach numbers on the order of 1.06-1.28
and 1.05-1.27. Considering these results and the magnetohydrodynamics
linear theory, we discuss the characteristics of the shock front and
the interaction with the chromospheric plasma.
Title: Dynamic Evolution of Current Sheets, Ideal Tearing, Plasmoid
Formation and Generalized Fractal Reconnection Scaling Relations
Authors: Singh, K. A. P.; Pucci, Fulvia; Tenerani, Anna; Shibata,
Kazunari; Hillier, Andrew; Velli, Marco
Bibcode: 2019ApJ...881...52S
Altcode: 2019arXiv190400755S
Magnetic reconnection may be the fundamental process allowing energy
stored in magnetic fields to be released abruptly, with solar flares and
coronal mass ejection being archetypal natural plasma examples. Magnetic
reconnection is much too slow of a process to be efficient on the
large scales, but accelerates once small enough scales are formed in
the system. For this reason, the fractal reconnection scenario was
introduced to explain explosive events in the solar atmosphere; it was
based on the recursive triggering and collapse via tearing instability
of a current sheet originally thinned during the rise of a filament in
the solar corona. Here we compare the different fractal reconnection
scenarios that have been proposed, and derive generalized scaling
relations for the recursive triggering of fast, “ideal” —i.e.,
Lundquist number independent—tearing in collapsing current sheet
configurations with arbitrary current profile shapes. An important
result is that the Sweet-Parker scaling with Lundquist number, if
interpreted as the aspect ratio of the singular layer in an ideally
unstable sheet, is universal and does not depend on the details of
the current profile in the sheet. Such a scaling, however, must not
be interpreted in terms of stationary reconnection, rather it defines
a step in the accelerating sequence of events of the ideal tearing
mediated fractal cascade. We calculate scalings for the expected number
of plasmoids for such generic profiles and realistic Lundquist numbers,
showing that in ideal tearing scenarios a smaller number of plasmoids,
by orders of magnitude, is generated compared to the original fractal
model.
Title: Impact of Stellar Superflares on Planetary Habitability
Authors: Yamashiki, Yosuke A.; Maehara, Hiroyuki; Airapetian, Vladimir;
Notsu, Yuta; Sato, Tatsuhiko; Notsu, Shota; Kuroki, Ryusuke; Murashima,
Keiya; Sato, Hiroaki; Namekata, Kosuke; Sasaki, Takanori; Scott,
Thomas B.; Bando, Hina; Nashimoto, Subaru; Takagi, Fuka; Ling,
Cassandra; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2019ApJ...881..114Y
Altcode: 2019arXiv190606797Y
High-energy radiation caused by exoplanetary space weather events from
planet-hosting stars can play a crucial role in conditions promoting
or destroying habitability in addition to the conventional factors. In
this paper, we present the first quantitative impact evaluation system
of stellar flares on the habitability factors with an emphasis on the
impact of stellar proton events. We derive the maximum flare energy
from stellar star spot sizes and examine the impacts of flare-associated
ionizing radiation on CO2, H2, and N2+
O2-rich atmospheres of a number of well-characterized
terrestrial type exoplanets. Our simulations based on the Particle and
Heavy Ion Transport code System suggest that the estimated ground-level
dose for each planet in the case of terrestrial-level atmospheric
pressure (1 bar) for each exoplanet does not exceed the critical dose
for complex (multicellular) life to persist, even for the planetary
surface of Proxima Centauri b, Ross-128 b, and TRAPPIST-1 e. However,
when we take into account the effects of the possible maximum flares
from those host stars, the estimated dose reaches fatal levels at
the terrestrial lowest atmospheric depth on TRAPPIST-1 e and Ross-128
b. Large fluxes of coronal X-ray and ultraviolet radiation from active
stars induce high atmospheric escape rates from close-in exoplanets,
suggesting that the atmospheric depth can be substantially smaller
than that on Earth. In a scenario with the atmospheric thickness
of one-tenth of Earth’s, the radiation dose from close-in planets
including Proxima Centauri b and TRAPPIST-1 e reaches near fatal levels
with annual frequency of flare occurrence from their host stars.
Title: Non-relativistic and relativistic magnetic reconnection with
the effects of optically thin synchrotron cooling
Authors: Takeshige, Satoshi; Takahashi, Hiroyuki R.; Shibata, Kazunari
Bibcode: 2019PASJ...71...63T
Altcode: 2019PASJ..tmp...56T
We performed special relativistic resistive magnetohydrodynamic
simulations of Petscheck-type magnetic reconnection including an
optically thin synchrotron cooling. The magnetization parameter,
σ0, which is the ratio of Poynting flux to mass flux in the
upstream plasma, is taken to be 0.01 and 3. For the non-relativistic
plasma (σ0 = 0.01), the radiative cooling subtracts
thermal energy mainly in the upstream plasma and a plasma is strongly
compressed at the slow shock. The cooling in the post-shock region
and plasmoid also reduces the thermal energy and it forms a narrower
outflow. The reconnection rate slightly increases as a result of
the radiative cooling, since the plasma beta in the inflow region
becomes small. The effect of decreasing thermal energy in the outflow
region is more prominent for the relativistic plasma (σ0
= 3). In this case, the outflow temperature increases and the plasma
internal energy becomes comparable to the plasma rest mass energy. The
subtraction of this thermal energy by radiative cooling leads to a
decrease in plasma inertia and the outflows are more accelerated than
without radiative cooling. The reconnection rate is also enhanced by its
Lorentz contraction effect. For both non-relativistic and relativistic
simulations, it is concluded that the reconnection rate is determined
by the plasma beta in the inflow region.
Title: Do Kepler superflare stars really include slowly-rotating
Sun-like stars ? - Results using APO 3.5m telescope spectroscopic
observations and Gaia-DR2 data -
Authors: Notsu, Yuta; Maehara, Hiroyuki; Honda, Satoshi; Hawley,
Suzanne L.; Davenport, James R. A.; Namekata, Kosuke; Notsu, Shota;
Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2019AAS...23412202N
Altcode:
Solar and stellar flares are the energetic explosions in the solar and
stellar atmosphere, and superflares are very large flares that release
total energy 10∼104 times greater than that of the biggest
solar flares (∼1032 erg). Recent Kepler-space-telescope
observations found more than 1000 superflares on a few hundred
solar-type stars. We report the latest view of Kepler solar-type
(G-type main-sequence) superflare stars, including recent updates
with Apache Point Observatory (APO) 3.5m telescope spectroscopic
observations and Gaia-DR2 data. First, more than half (43 stars) are
confirmed to be "single" stars, among 64 superflare stars in total
that have been spectroscopically investigated so far in this APO3.5m
and our previous Subaru/HDS observations. The measurements of v sin i
(projected rotational velocity) and chromospheric lines (Ca II H&K
and Ca II 8542 A) support the brightness variation of superflare stars
is caused by the rotation of a star with large starspots. Then, we
investigated again the statistical properties of Kepler solar- type
superflare stars by incorporating Gaia-DR2 stellar radius values. As
a result, the maximum superflare energy continuously decreases as the
rotation period Prot increases. Superflares with their energy <
5×1034 erg would occur on old slowly-rotating Sun-like
stars (Prot ∼25 days) once every 2000-3000 years, while
young rapidly- rotating stars with Prot ∼ a few days
have superflares up to 1036 erg. The maximum starspot area
does not depend on the rotation period when the star is young rapidly-
rotating, but as the rotation slows down, it starts to steeply decrease
at Prot 12 days for Sun-like stars. These two decreasing
trends can be consistent since the magnetic energy stored around
starspots explains the flare energy, but other factors like spot
magnetic structure should be also considered. These results presented
in this work support that even slowly-rotating stars similar to the
Sun can have large starspots necessary for superflares, considering
long-term (1,000-10,000 years) activity level changes.
Title: Small-scale motions in solar filaments as the precursors
of eruptions
Authors: Seki, Daikichi; Otsuji, Kenichi; Isobe, Hiroaki; Ishii,
Takako T.; Ichimoto, Kiyoshi; Shibata, Kazunari
Bibcode: 2019PASJ...71...56S
Altcode: 2019arXiv190208718S; 2019PASJ..tmp...48S
Filaments, the dense cooler plasma floating in the solar corona
supported by magnetic fields, generally exhibit certain activations
before they erupt. In our previous study (Seki et al. 2017, ApJ, 843,
L24), we observed that the standard deviation of the line-of-sight (LOS)
velocities of the small-scale motions in a filament increased prior
to its eruption. However, because that study only analyzed one event,
it is unclear whether such an increase in the standard deviation of
LOS velocities is common in filament eruptions. In this study, 12
filaments that vanished in Hα line center images were analyzed in
a manner similar to the one in our previous work; these included two
quiescent filaments, four active region filaments, and six intermediate
filaments. We verified that in all the 12 events, the standard deviation
of the LOS velocities increased before the filaments vanished. Moreover,
we observed that the quiescent filaments had approximately 10 times
longer duration of an increase in the standard deviation than the
other types of filaments. We concluded that the standard deviation
of the LOS velocities of the small-scale motions in a filament can
potentially be used as a precursor of a filament eruption.
Title: Lifetimes and emergence/decay rates of star spots on solar-type
stars estimated by Kepler data in comparison with those of sunspots
Authors: Namekata, Kosuke; Maehara, H.; Davenport, J.; Morris, B.;
Hawley, S.; Notsu, Y.; Toriumi, S.; Hayakawa, H.; Honda, S.; Notsu,
S.; Ikuta, K.; Nogami, D.; Shibata, K.
Bibcode: 2019shin.confE..42N
Altcode:
Active solar-type stars show large quasi-periodic brightness variations
caused by stellar rotations with large star spots, and the amplitude
change as the spots emerge and decay. Temporal evolution of star spots
has been hardly measured because of its difficulty in measurement,
especially on solar-type stars. The Kepler’s long-term data is
suitable for investigations on the emergence and decay processes
of star spots, which are important to understand underlying stellar
dynamo. In this talk, we report the measurements of temporal evolution
of individual star-spot area on solar-type stars by using Kepler
data. We estimated it (i) by tracing local minima of the Kepler
light curves (Namekata et al. 2019) and (ii) by modeling the small
brightness variation during exoplanet transit (c.f. Morris et al. 2017,
Namekata et al. in prep). We successfully obtained temporal evolution
of individual star spots showing clear emergence and decay, and derived
the statistical values of the lifetimes and emergence/decay rates of
star spots. As a result, we found that lifetimes (T) of star spots
are ranging from 10 to 350 days when spot areas (A) are 0.1-2.3% of
a solar hemisphere (SH). The lifetimes of star spots are much shorter
than those extrapolated from an empirical relation of sunspots, while
being consistent with other researches on star spot lifetimes. The
emerging and decay rates of star spots are typically 5×10^20 Mx/h
(8 MSH/h) with the area of 0.1-2.3% of SH and are mostly consistent
with those expected from sunspots observations (Petrovay et al. 1997,
Norton et al. 2017). This strongly supports a possibility that the
emergence/decay mechanism of extremely large star spots (0.1-2.3%
of SH) is same as that of smaller sunspots (<0.5% of SH), which
can constrain the stellar dynamo theory.
Title: Do Kepler Superflare Stars Really Include Slowly Rotating
Sun-like Stars?—Results Using APO 3.5 m Telescope Spectroscopic
Observations and Gaia-DR2 Data
Authors: Notsu, Yuta; Maehara, Hiroyuki; Honda, Satoshi; Hawley,
Suzanne L.; Davenport, James R. A.; Namekata, Kosuke; Notsu, Shota;
Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2019ApJ...876...58N
Altcode: 2019arXiv190400142N
We report the latest view of Kepler solar-type (G-type main-sequence)
superflare stars, including recent updates with Apache Point
Observatory (APO) 3.5 m telescope spectroscopic observations and
Gaia-DR2 data. First, we newly conducted APO 3.5 m spectroscopic
observations of 18 superflare stars found from Kepler 1-minute
time-cadence data. More than half (43 stars) are confirmed to be
“single” stars, among 64 superflare stars in total that have
been spectroscopically investigated so far in this APO 3.5 m and
our previous Subaru/HDS observations. The measurements of v sin i
(projected rotational velocity) and chromospheric lines (Ca II H and K
and Ca II λ8542) support that the brightness variation of superflare
stars is caused by the rotation of a star with large starspots. We then
investigated the statistical properties of Kepler solar-type superflare
stars by incorporating Gaia-DR2 stellar radius estimates. As a result,
the maximum superflare energy continuously decreases as the rotation
period P rot increases. Superflares with energies ≲5
× 1034 erg occur on old, slowly rotating Sun-like stars
(P rot ∼ 25 days) approximately once every 2000-3000 yr,
while young, rapidly rotating stars with P rot ∼ a few
days have superflares up to 1036 erg. The maximum starspot
area does not depend on the rotation period when the star is young,
but as the rotation slows down, it starts to steeply decrease at P
rot ≳ 12 days for Sun-like stars. These two decreasing
trends are consistent since the magnetic energy stored around starspots
explains the flare energy, but other factors like spot magnetic
structure should also be considered.
Title: Do superflares really occur on slowly-rotating Sun-like stars
in the long-term activity changes? -- Latest statistical results
using Kepler and Gaia-DR2 data -
Authors: Notsu, Yuta; Maehara, Hiroyuki; Honda, Satoshi; Hawley,
Suzanne L.; Davenport, James R. A.; Namekata, Kosuke; Notsu, Shota;
Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2019shin.confE..17N
Altcode:
Superflares are very large flares that release total energy 10-10^4
times greater than that of the biggest solar flares (?10^32 erg). Recent
Kepler space telescope observations found more than 1000 superflares
on a few hundred solar-type stars. Our previous statistical studies
of these data suggested that superflares with energy up to 10^35 erg
could occur on Sun-like slowly-rotating stars (Rotation period Prot
? 25 days) once in a few thousand years. On the other hand, the
recent Gaia-DR2 stellar radius data have suggested the possibility of
severe contaminations of subgiant stars in the classification of Kepler
solar-type (G-type main-sequence) stars used for the above previous
studies. Then in this new study (Notsu+2019, ApJ, 876, 58), we
investigated again the statistical properties of Kepler solar-type
superflare stars by incorporating Gaia-DR2 stellar radius values. More
than 40% of the original solar-type superflare stars in our previous
studies are now classified as subgiant stars and are rejected from the
following statistical results. As a result, the maximum superflare
energy continuously decreases as the Prot increases. Superflares
with their energy 5 × 10^34 erg would occur on old slowly-rotating
Sun-like stars (Prot ?25 days) once every 2000-3000 years. In contrast,
superflares up to ?10^36 erg can occur on young rapidly-rotating
stars (Prot ? a few days), and the flare frequency of such young
rapidly-rotating stars is at least 100 times higher compared with the
old slowly-rotating Sun-like stars. The maximum starspot area does not
depend on the rotation period when the star is young rapidly-rotating,
but as the rotation slows down, it starts to steeply decrease at Prot
12 days for Sun-like stars. These two decrease trends can be consistent
since the magnetic energy stored around the starspots explains the
flare energy, but other factors like spot magnetic structure should
be also considered.
Title: Reconstructing Extreme Space Weather From Planet Hosting Stars
Authors: Airapetian, Vladimir; Adibekyan, V.; Ansdell, M.; Alexander,
D.; Barklay, T.; Bastian, T.; Boro Saikia, S.; Cohen, O.; Cuntz,
M.; Danchi, W.; Davenport, J.; DeNolfo, G.; DeVore, R.; Dong, C. F.;
Drake, J. J.; France, K.; Fraschetti, F.; Herbst, K.; Garcia-Sage,
K.; Gillon, M.; Glocer, A.; Grenfell, J. L.; Gronoff, G.; Gopalswamy,
N.; Guedel, M.; Hartnett, H.; Harutyunyan, H.; Hinkel, N. R.; Jensen,
A. G.; Jin, M.; Johnstone, C.; Kahler, S.; Kalas, P.; Kane, S. R.;
Kay, C.; Kitiashvili, I. N.; Kochukhov, O.; Kondrashov, D.; Lazio, J.;
Leake, J.; Li, G.; Linsky, J.; Lueftinger, T.; Lynch, B.; Lyra, W.;
Mandell, A. M.; Mandt, K. E.; Maehara, H.; Miesch, M. S.; Mickaelian,
A. M.; Mouschou, S.; Notsu, Y.; Ofman, L.; Oman, L. D.; Osten, R. A.;
Oran, R.; Petre, R.; Ramirez, R. M.; Rau, G.; Redfield, S.; Réville,
V.; Rugheimer, S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.;
Schnittman, J. D.; Soderblom, David; Strugarek, A.; Turner, J. D.;
Usmanov, A.; Van Der Holst, B.; Vidotto, A.; Vourlidas, A.; Way, M. J.;
Wolk, Scott J.; Zank, G. P.; Zarka, P.; Kopparapu, R.; Babakhanova,
S.; Pevtsov, A. A.; Lee, Y.; Henning, W.; Colón, K. D.; Wolf, E. T.
Bibcode: 2019BAAS...51c.564A
Altcode: 2019astro2020T.564A; 2019arXiv190306853A
The goal of this white paper is to identify and describe promising key
research goals to aid the theoretical characterization and observational
detection of ionizing radiation from quiescent and flaring upper
atmospheres of planet hosts as well as properties of stellar coronal
mass ejections (CMEs) and stellar energetic particle (SEP) events.
Title: The extreme space weather event in September 1909
Authors: Hayakawa, Hisashi; Ebihara, Yusuke; Cliver, Edward W.;
Hattori, Kentaro; Toriumi, Shin; Love, Jeffrey J.; Umemura, Norio;
Namekata, Kosuke; Sakaue, Takahito; Takahashi, Takuya; Shibata,
Kazunari
Bibcode: 2019MNRAS.484.4083H
Altcode: 2018MNRAS.tmp.3046H
We evaluate worldwide low-latitude auroral activity associated with the
great magnetic storm of September 1909 for which a minimum Dst value
of -595 nT has recently been determined. From auroral observations,
we calculate that the equatorward boundary of the auroral oval in
the 1909 event was in the range from 31°-35° invariant latitude
(assuming auroral height of 400 km) to 37°-38° (800 km). These
locations compare with satellite-based observations of precipitating
auroral electrons down to 40° magnetic latitude for the March 1989
storm with its comparable minimum Dst value of -589 nT. According
to Japanese auroral records, bluish colour started to appear first,
followed by reddish colour. The colour change can be attributed to
the transition from sunlit aurora to the usual low-latitude reddish
aurora. Telegraph communications were disrupted at mid/low latitudes,
coincidently with the storm main phase and the early recovery
phase. The telegraphic disturbances were caused by geomagnetically
induced currents associated with the storm-time ring current and
substorm current wedge. From the calculated CME energy - based on the
24.75 hr separation between the flare-associated magnetic crochet
and the geomagnetic storm sudden commencement and interplanetary
conditions inferred from geomagnetic data - and consideration of the
∼-40 nT crochet amplitude, we estimated that the soft X-ray class
of the 24 September 1909 flare was ≥X10. As is the case for other
extreme storms, strong/sharp excursions in the horizontal component
of the magnetic field observed at low-latitude magnetic stations were
coincident with the observation of low-latitude aurora.
Title: First Unambiguous Imaging of Large-scale Quasi-periodic
Extreme-ultraviolet Wave or Shock
Authors: Shen, Yuandeng; Chen, P. F.; Liu, Ying D.; Shibata, Kazunari;
Tang, Zehao; Liu, Yu
Bibcode: 2019ApJ...873...22S
Altcode: 2019arXiv190108199S
We report the first unambiguous quasi-periodic large-scale
extreme-ultraviolet (EUV) wave or shock that was detected by
the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. During the whip-like unwinding eruption of a small
filament on 2012 April 24, multiple consecutive large-scale wavefronts
emanating from AR11467 were observed simultaneously along the solar
surface and a closed transequatorial loop system. In the meantime,
an upward propagating dome-shaped wavefront was also observed, whose
initial speed and deceleration are about 1392 km s-1
and 1.78 km s-2, respectively. Along the solar surface,
the quasi-peridoic wavefronts had a period of about 163 ± 21 s and
propagated at a nearly constant speed of 747 ± 26 km s-1
they interacted with active region AR11469 and launched a sympathetic
upward propagating secondary EUV wave. The wavefronts along the loop
system propagated at a speed of 897 km s-1, and they were
reflected back at the southern end of the loop system at a similar
speed. In addition to the propagating waves, a standing kink wave was
also present in the loop system simultaneously. Periodicity analysis
reveals that the period of the wavefronts was consistent with that of
the unwinding helical structures of the erupting filament. Based on
these observational facts, we propose that the observed quasi-periodic
EUV wavefronts were most likely excited by the periodic unwinding
motion of the filament helical structures. In addition, two different
seismological methods are applied to derive the magnetic field strength
of the loop system, and for the first time the reliability of these
inversion techniques are tested with the same magnetic structure.
Title: Lifetimes and Emergence/Decay Rates of Star Spots on Solar-type
Stars Estimated by Kepler Data in Comparison with Those of Sunspots
Authors: Namekata, Kosuke; Maehara, Hiroyuki; Notsu, Yuta; Toriumi,
Shin; Hayakawa, Hisashi; Ikuta, Kai; Notsu, Shota; Honda, Satoshi;
Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2019ApJ...871..187N
Altcode: 2018arXiv181110782N
Active solar-type stars show large quasi-periodic brightness variations
caused by stellar rotation with star spots, and the amplitude changes
as the spots emerge and decay. The Kepler data are suitable for
investigations of the emergence and decay processes of star spots,
which are important to understand the underlying stellar dynamo and
stellar flares. In this study, we measured the temporal evolution of
the star-spot area with Kepler data by tracing the local minima of the
light curves. In this analysis, we extracted the temporal evolution of
star spots showing clear emergence and decay without being disturbed
by stellar differential rotation. We applied this method to 5356
active solar-type stars observed by Kepler and obtained temporal
evolution of 56 individual star spots. We calculated the lifetimes
and emergence/decay rates of the star spots from the obtained temporal
evolution of the spot area. As a result, we found that the lifetimes
(T) of star spots range from 10 to 350 days when the spot areas (A)
are 0.1%-2.3% of the solar hemisphere. We also compared them with
sunspot lifetimes and found that the lifetimes of star spots are much
shorter than those extrapolated from an empirical relation of sunspots
(T ∝ A), while being consistent with other research on star-spot
lifetimes. The emergence and decay rates of star spots are typically 5
× 1020 Mx hr-1 (8 MSH hr-1) with an
area of 0.1%-2.3% of the solar hemisphere and mostly consistent with
those expected from sunspots, which may indicate the same underlying
processes.
Title: Recent progress in Asia-Pacific solar physics and astrophysics
Authors: Chen, P. F.; Shibata, K.; Matsumoto, R.
Bibcode: 2018RvMPP...2....5C
Altcode:
More than 40 participants from the solar/astrophysical community
attended the First Asia-Pacific Conference on Plasma Physics. Among
them, four colleagues presented invited talks in the plenary session. In
the Solar/Astron session, there were 23 invited talks and 14 contributed
talks, with another two posters. These talks cover recent progress
obtained in a wide spectrum of topics, including solar and galactic
dynamo, solar and stellar flares, solar and galactic filaments, solar
and astrophysical jets, solar and accretion disk winds, plasma waves and
coronal heating, solar coronal mass ejections, magnetic reconnection in
non-relativistic and relativistic regimes, star and planetary formation,
shock-medium interactions, and even gravitational waves. Laboratory
laser experiments and some new rocket and space missions were also
introduced.
Title: Observational study on the fine structure and dynamics of a
solar jet. II. Energy release process revealed by spectral analysis
Authors: Sakaue, Takahito; Tei, Akiko; Asai, Ayumi; Ueno, Satoru;
Ichimoto, Kiyoshi; Shibata, Kazunari
Bibcode: 2018PASJ...70...99S
Altcode: 2017arXiv171008441S; 2018PASJ..tmp....6S
We report on a solar jet phenomenon associated with the C5.4 class
flare on 2014 November 11. The data of the jet was provided by the Solar
Dynamics Observatory, the X-Ray Telescope (XRT) aboard Hinode, and the
Interface Region Imaging Spectrograph and Domeless Solar Telescope (DST)
at Hida Observatory, Kyoto University. These plentiful data enabled
us to present this series of papers to discuss all the processes of
the observed phenomena, including energy storage, event trigger,
and energy release. In this paper, we focus on the energy release
process of the observed jet, and mainly describe our spectral analysis
on the Hα data of DST to investigate the internal structure of the
Hα jet and its temporal evolution. This analysis reveals that in the
physical quantity distributions of the Hα jet, such as line-of-sight
velocity and optical thickness, there is a significant gradient in
the direction crossing the jet. We interpret this internal structure
as the consequence of the migration of the energy release site, based
on the idea of ubiquitous reconnection. Moreover, by measuring the
horizontal flow of the fine structures in the jet, we succeeded in
deriving the three-dimensional velocity field and the line-of-sight
acceleration field of the Hα jet. The analysis result indicates that
part of the ejecta in the Hα jet experienced additional acceleration
after it had been ejected from the lower atmosphere. This secondary
acceleration was found to occur in the vicinity of the intersection
between the trajectories of the Hα jet and the X-ray jet observed by
Hinode/XRT. We propose that a fundamental cause of this phenomenon is
magnetic reconnection involving the plasmoid in the observed jet.
Title: Space Weather Prediction from the Ground: Case of CHAIN
Authors: Seki, Daikichi; Ueno, Satoru; Isobe, Hiroaki; Otsuji, Kenichi;
Cabezas, Denis P.; Ichimoto, Kiyoshi; Shibata, Kazunari; Chain Team
Bibcode: 2018SunGe..13..157S
Altcode:
In this article, we insist on the importance and the challenges of the
prediction of solar eruptive phenomena including flares, coronal mass
ejections (CME), and filament eruptions fully based on the ground-based
telescopes. It is true that satellites' data are indispensable for the
space weather prediction, but they are vulnerable to the space weather
effects. Therefore, the ground-based telescopes can be complementary
to them from the viewpoint of space weather prediction. From this
view point, one possible new flare prediction method that makes use of
H-alpha, red wings, and blue wings images obtained by the SDDI/SMART,
the ground-based telescope at Hida Observatory, is presented. And
in order to show the possibility for the actual operation based on
that method, the recent progress of CHAIN project, the international
observation network, is mentioned in terms of their outcomes and
capacity buildings.
Title: Blue-wing enhancement of the chromospheric Mg II h and k
lines in a solar flare
Authors: Tei, Akiko; Sakaue, Takahito; Okamoto, Takenori J.; Kawate,
Tomoko; Heinzel, Petr; UeNo, Satoru; Asai, Ayumi; Ichimoto, Kiyoshi;
Shibata, Kazunari
Bibcode: 2018PASJ...70..100T
Altcode: 2018PASJ..tmp...61T; 2018arXiv180305237T
We performed coordinated observations of AR 12205, which showed a
C-class flare on 2014 November 11, with the Interface Region Imaging
Spectrograph (IRIS) and the Domeless Solar Telescope (DST) at Hida
Observatory. Using spectral data in the Si IV 1403 Å, C II 1335 Å,
and Mg II h and k lines from IRIS and the Ca II K, Ca II 8542 Å, and
Hα lines from DST, we investigated a moving flare kernel during the
flare. In the Mg II h line, the leading edge of the flare kernel showed
an intensity enhancement in the blue wing and a smaller intensity of the
blue-side peak (h2v) than that of the red-side one (h2r). The blueshift
lasted for 9-48 s with a typical speed of 10.1 ± 2.6 km s-1,
which was followed by a high intensity and a large redshift with a speed
of up to 51 km s-1 detected in the Mg II h line. The large
redshift was a common property for all six lines, but the blueshift
prior to it was found only in the Mg II lines. Cloud modeling of the
Mg II h line suggests that the blue-wing enhancement with such a peak
difference could have been caused by a chromospheric-temperature (cool)
upflow. We discuss a scenario in which an upflow of cool plasma is
lifted up by expanding hot plasma owing to the deep penetration of
non-thermal electrons into the chromosphere. Furthermore, we found
that the blueshift persisted without any subsequent redshift in the
leading edge of the flare kernel during its decaying phase. The cause
of such a long-lasting blueshift is also discussed.
Title: Numerical simulation on the stellar wind from TRAPPIST-1.
Authors: Sakaue, T.; Shibata, K.
Bibcode: 2018AGUFM.P43H3843S
Altcode:
We report the results of one-dimensional magnetohydrodynamic (MHD)
simulation of the stellar wind from TRAPPIST-1. TRAPPIST-1 is a M8-type
main sequence star with cooler temperature (2559K) and lower mass
(0.08Msun). The star is confirmed to have seven planets
(Gillon et al. 2017) and thus greatly interested in not only by stellar
astrophysics but also by astrobiology. The stellar wind or radiation
from TRAPPIST-1 is the most important research subject to comprehend the
interplanetary environment of TRAPPIST-1. Several solar wind theories,
which have been examined observationally, are applied to the case of
TRAPPIST-1. Among them, it is the plausible idea that the dissipation
of the incompressible MHD wave leads to the heating and driving the
stellar wind. For instance, Garraffo et al. 2017 succeeded in the
three-dimensional MHD simulation of the stellar wind from TRAPPIST-1,
by extending their solar wind model in which the incompressible MHD
wave is exhausted by the turbulent dissipation. The incompressible MHD
wave, on the other hand, is also affected not only by the turbulent
dissipation but also by the nonlinear mode coupling, which leads
to the formation of the shock wave and consequently contribute to
heating the wind. This nonlinear process has been well investigated
to account for the dynamics of the lower solar atmosphere (Kudoh &
Shibata 1999) and driving the solar wind (Suzuki & Inutsuka 2005),
but not discussed for the stellar wind from TRAPPIST-1. In this study,
therefore, we performed the one-dimensional MHD simulation to consider
the nonlinear process of the incompressible MHD wave leading to the
shock formation, and succeeded in reproducing the stellar wind from
TRAPPIST-1.
Title: Space Weather Prediction from the Ground: Case of CHAIN
Authors: Seki, Daikichi; Ueno, Satoru; Isobe, Hiroaki; Otsuji, Kenichi;
Cabezas, Denis P.; Ichimoto, Kiyoshi; Shibata, Kazunari; CHAIN team
Bibcode: 2018arXiv180806295S
Altcode:
In this article, we insist on the importance and the challenges of the
prediction of solar eruptive phenomena including flares, coronal mass
ejections (CME), and filament eruptions fully based on the ground-based
telescopes. It is true that satellites' data are indispensable for the
space weather prediction, but they are vulnerable to the space weather
effects. Therefore, the ground-based telescopes can be complementary
to them from the viewpoint of space weather prediction. From this
view point, one possible new flare prediction method that makes use of
H-alpha, red wings, and blue wings images obtained by the SDDI/SMART,
the ground-based telescope at Hida Observatory, is presented. And
in order to show the possibility for the actual operation based on
that method, the recent progress of CHAIN project, the international
observation network, is mentioned in terms of their outcomes and
capacity buildings.
Title: Sunspot drawings by Japanese official astronomers in 1749-1750
Authors: Hayakawa, Hisashi; Iwahashi, Kiyomi; Fujiyama, Masashi;
Kawai, Toshiki; Toriumi, Shin; Hotta, Hideyuki; Iijima, Haruhisa;
Imada, Shinsuke; Tamazawa, Harufumi; Shibata, Kazunari
Bibcode: 2018PASJ...70...63H
Altcode: 2018arXiv180408614H; 2018PASJ..tmp...87H
Sunspot observations with telescopes in the 18th century
were carried out in Japan as well as elsewhere. One of these sunspot
observations is recorded in an account called Sansaizusetsu narabini
Kansei irai Jissoku Zusetsu (Charts of Three Worlds and Diagrams of
Actual Observations since Kansei Era). We have analyzed manuscripts
of this account to show a total of 15 sunspot drawings during
1749-1750. These observations are considered to be carried out by
contemporary official astronomers in Japan, with telescopes covered
by zongurasus (< zonglas in Dutch, corresponding to "sunglass"
in English). We counted their group number of sunspots to locate
them in long-term solar activity and show that their observations
were situated near the solar maximum in 1750. We also computed their
locations and areas, while we have to admit differences of the variant
manuscripts with one another. These observational records show the
spread of sunspot observations not only in Europe, but also in Japan,
and hence may contribute to crosscheck, or possibly to improve the
known sunspot indices.
Title: Time-resolved spectroscopic observations of an M-dwarf flare
star EV Lacertae during a flare
Authors: Honda, Satoshi; Notsu, Yuta; Namekata, Kosuke; Notsu, Shota;
Maehara, Hiroyuki; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2018PASJ...70...62H
Altcode: 2018arXiv180403771H; 2018PASJ..tmp...70H
We have performed five night spectroscopic observations of the Hα line
of EV Lac with a medium wavelength resolution (R ∼ 10000) using the
2 m Nayuta telescope at the Nishi-Harima Astronomical Observatory. EV
Lac always possesses the Hα emission line; however, its intensity was
stronger on 2015 August 15 than during the other four night periods. On
this night, we observed a rapid rise (∼20 min) and a subsequent slow
decrease (∼1.5 hr) of the emission-line intensity of Hα, which was
probably caused by a flare. We also found an asymmetrical change in
the Hα line on the same night. The enhancement has been observed in
the blue wing of the Hα line during each phase of this flare (from the
flare start to the flare end), and absorption components were present in
its red wing during the early and later phases of the flare. Such blue
enhancement (blue asymmetry) of the Hα line is sometimes seen during
solar flares, but only during the early phases. Even for solar flares,
little is known about the origin of the blue asymmetry. Compared
with solar flare models, the presented results can lead to better
understanding of the dynamics of stellar flares.
Title: Spectroscopic observations of Kepler/TESS solar-type supeflare
stars
Authors: Notsu, Yuta; Maehara, Hiroyuki; Honda, Satoshi; Hawley,
Suzanne; Davenport, James; Notsu, Shota; Namekata, Kosuke; Ikuta,
Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2018csss.confE..14N
Altcode:
Recent Kepler-space-telescope observations found more than 1000
superflares on 300 solar-type stars (e.g., Maeahra+2012 Nature;
Shibayama+2013 ApJS). Many of the superflare stars show quasi-periodic
brightness variations with the typical period of 1-30 days and the
typical amplitude of 0.1-10 percent. We conducted spectroscopic
observations of these superflare stars using Subaru/HDS and APO 3.5m
telescope (Notsu+2015a&b, 2018 in prep). The projected rotation
velocity (v sin i) values are consistent with brightness variation
period, and there is a good correlation between Kepler brightness
variation amplitude and the intensity of Ca II lines (Ca II H&K, Ca
II 8542Å). These results support that the above brightness variations
are caused by stellar rotation with large starspots, and existence of
large starspots should be a key to understand superflares.More detailed
spectroscopic studies (e.g., activity cycle) of superflare stars are
important, but Kepler target stars are faint and not appropriate for
such detailed studies. TESS satellite, launched in April 2018, brings us
a large sample of brighter (e.g., V < 12 mag) superflare stars. We
have started spectroscopic monitoring observations of nearby active
solar-type stars (superflare candidate stars) in the TESS field. These
results can have good collaborations with multi-wavelength project
observations (e.g., X-ray, UV, polarimetry) of young solar-type stars.
Title: The Great Space Weather Event during 1872 February Recorded
in East Asia
Authors: Hayakawa, Hisashi; Ebihara, Yusuke; Willis, David M.; Hattori,
Kentaro; Giunta, Alessandra S.; Wild, Matthew N.; Hayakawa, Satoshi;
Toriumi, Shin; Mitsuma, Yasuyuki; Macdonald, Lee T.; Shibata, Kazunari;
Silverman, Sam M.
Bibcode: 2018ApJ...862...15H
Altcode: 2018arXiv180705186H
The study of historical great geomagnetic storms is crucial for
assessing the possible risks to the technological infrastructure of
a modern society, caused by extreme space-weather events. The normal
benchmark has been the great geomagnetic storm of 1859 September, the
so-called “Carrington Event.” However, there are numerous records
of another great geomagnetic storm in 1872 February. This storm,
which occurred about 12 years after the Carrington Event, resulted in
comparable magnetic disturbances and auroral displays over large areas
of the Earth. We have revisited this great geomagnetic storm in terms
of the auroral and sunspot records in historical documents from East
Asia. In particular, we have surveyed the auroral records from East Asia
and estimated the equatorward boundary of the auroral oval to be near
24.°2 invariant latitude, on the basis that the aurora was seen near
the zenith at Shanghai (20° magnetic latitude, MLAT). These results
confirm that this geomagnetic storm of 1872 February was as extreme as
the Carrington Event, at least in terms of the equatorward motion of
the auroral oval. Indeed, our results support the interpretation of the
simultaneous auroral observations made at Bombay (10° MLAT). The East
Asian auroral records have indicated extreme brightness, suggesting
unusual precipitation of high-intensity, low-energy electrons during
this geomagnetic storm. We have compared the duration of the East
Asian auroral displays with magnetic observations in Bombay and found
that the auroral displays occurred in the initial phase, main phase,
and early recovery phase of the magnetic storm.
Title: Starspots on late-type stars and their correlation with
flare activity
Authors: Maehara, Hiroyuki; Notsu, Yuta; Notsu, Shota; Namekata,
Kousuke; Ikuta, Kai; Honda, Satoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2018csss.confE..69M
Altcode:
Recent space-based observations (e.g., Kepler mission) found thousands
of ``superflares’’ on G-, K-, and M-type stars. These superflare
stars show quasi-periodic brightness variations caused by the rotation
of the star with starspots. We analyzed the statistical properties of
starspots on G-, K-, and M-type stars and their correlation with the
flare activity. The analysis shows that the fraction of stars showing
large-amplitude rotational variations, which are thought to be the
signature of the large starspots, decreases as the rotation period
increases. We found that there is a good correlation between the
bolometric energy of the largest superflares and area of starspots
estimated from the amplitude of rotational light variations. The
bolometric energy released by the largest flare on the star is
consistent with the magnetic energy stored near the starspots. We
also found that the frequency of superflares correlates with the
starspot area. The average frequency of flares with a given bolometric
energy is roughly proportional to the area of starspots. Our results
suggest that flare activity level (e.g., energy of the largest flares,
occurrence frequency) of the late-type stars can be determined by the
area of starspots.
Title: VizieR Online Data Catalog: Statistical studies of solar
white-light flares (Namekata+, 2017)
Authors: Namekata, K.; Sakaue, T.; Watanabe, K.; Asai, A.; Maehara,
H.; Notsu, Y.; Notsu, S.; Honda, S.; Ishii, T. T.; Ikuta, K.; Nogami,
D.; Shibata, K.
Bibcode: 2018yCat..18510091N
Altcode:
Our white-light flare (WLF) catalog contains M- and X-class solar
flares that occurred from 2011 to 2015 and were observed by both Solar
Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) and
RHESSI. The 43 flares in our catalog that occurred from 2011 to 2014
were taken from Kuhar+ (2016ApJ...816....6K); we enlarged the sample
by adding 10 flares that occurred in 2015. To present whether the
cadence of SDO/HMI is short enough to resolve the evolution of WLFs,
we compared the obtained light curves with those observed by the Solar
Magnetic Activity Research Telescope (SMART) at Hida Observatory of
Kyoto University for one event and the Hinode/Solar Optical Telescope
(SOT) for four events. (3 data files).
Title: Blue wing enhancement of the chromospheric Mg II h and k
lines in a solar flare
Authors: Tei, Akiko; Shibata, Kazunari; Asai, Ayumi; Ichimoto,
Kiyoshi; Heinzel, Petr; Ueno, Satoru; Okamoto, Joten; Sakaue, Takahito;
Kawate, Tomoko
Bibcode: 2018cosp...42E3350T
Altcode:
We performed coordinated observations of NOAA AR 12205, which produced
a C-class flare on 2014 November 11, with the Interface Region Imaging
Spectrograph (IRIS) and the Domeless Solar Telescope (DST) at Hida
Observatory.Using spectral data in the Si IV 1403 Å, C II 1335 Å,
and Mg II h and k lines from IRIS and the Ca II K, Ca II 8542 Å,
and Hα lines from DST, we investigated the temporal and spatial
evolution around a moving flare kernel.In the Mg II h line, the leading
edge of the kernel showed intensity enhancement in the blue wing and
difference between the blue-side peak and red-side one (I_{h2v} <
I_{h2r}).Then, the drastic change of the intensity in the red wing
occurred.The blueshift lasted for 9-48 s with a speed of 10.1 ± 2.6
km s^{-1} and it was followed by the strong redshift with a speed of
up to 51 km s^{-1} detected in the Mg II h line.The strong redshift
was a common property for all six lines but the blueshift prior to it
was found only in the Mg II lines.A cloud modeling of the Mg II h line
suggests that the blue wing enhancement with such peak difference can
be caused by a chromospheric-temperature (cool) upflow.We discuss a
scenario in which an upflow of cool plasma is lifted up by expanding
(hot) plasma owing to the deep penetration of non-thermal electrons
into the chromosphere.In addition, at the leading edge of the final
flare footpoints, the blueshift persisted in the Mg II h line, which
was not followed by any large redshift and intensity enhancement.Such
long-lasting blueshift can be explained by cool upflow caused by small
energy flux into the lower atmosphere.
Title: Statistical Studies of Solar White-light Flares and Comparisons
with Superflares on Solar-type Stars
Authors: Namekata, Kosuke; Ishii, Takako; Watanabe, Kyoko; Shibata,
Kazunari; Asai, Ayumi; Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki;
Notsu, Shota; Nogami, Daisaku; Sakaue, Takahito; Ikuta, Kai
Bibcode: 2018cosp...42E2404N
Altcode:
Recently, many superflares on solar-type stars have been discovered
as white-light flares. Our statistical study found a correlation
between their energies (E) and durations (t): t∝ E^{0.39} (Maehara
et al. 2015, EP&S), similar to those of solar hard/soft X-ray
flares: t∝ E^{0.2-0.33}. This indicates a universal mechanism
of energy release on solar and stellar flares, i.e., magnetic
reconnection. We here carried out a statistical research on 50 solar
white-light flares with SDO/HMI and examined the correlation between
the energies and durations, aiming to universally explain solar and
stellar white-light flares. As a result, the t-E relation on solar
white-light flares (t∝ E^{0.38}) is similar to that on stellar
superflares (t∝ E^{0.39}). However, the durations of stellar
superflares are one order of magnitude shorter than those expected
from solar white-light flares. We proposed that the discrepancy can be
understood by applying a scaling law (t∝ E^{1/3}B^{-5/3}) which is
derived from the magnetic reconnection theory (Namekata et al. 2017,
ApJ, 851, 91). In this case, the observed superflares are expected to
have 2-4 times stronger magnetic field than solar flares. Although
there might be another effect like the cooling time of white-light
flares, the scaling law has a potential to estimate coronal magnetic
field strength of spatially unresolved stellar flares.
Title: Can Flare Loops Contribute to the White-light Emission of
Stellar Superflares?
Authors: Heinzel, P.; Shibata, K.
Bibcode: 2018ApJ...859..143H
Altcode: 2018arXiv180409656H
Since the discovery of stellar superflares by the Kepler satellite,
these extremely energetic events have been studied in analogy to
solar flares. Their white-light (WL) continuum emission has been
interpreted as being produced by heated ribbons. In this paper,
we compute the WL emission from overlying flare loops depending
on their density and temperature and show that, under conditions
expected during superflares, the continuum brightening due to
extended loop arcades can significantly contribute to stellar flux
detected by Kepler. This requires electron densities in the loops of
1012-1013 cm-3 or higher. We show
that such densities, exceeding those typically present in solar-flare
loops, can be reached on M-dwarf and solar-type superflare stars with
large starspots and much stronger magnetic fields. Quite importantly,
the WL radiation of loops is not very sensitive to their temperature
and thus both cool as well as hot loops may contribute. We show that
the WL intensity emergent from optically thin loops is lower than the
blackbody radiation from flare ribbons, but the contribution of loops
to total stellar flux can be quite important due to their significant
emitting areas. This new scenario for interpreting superflare emission
suggests that the observed WL flux is due to a mixture of the ribbon
and loop radiation and can be even loop-dominated during the gradual
phase of superflares.
Title: Exploring Extreme Space Weather Factors of Exoplanetary
Habitability
Authors: Airapetian, V. S.; Adibekyan, V.; Ansdell, M.; Cohen, O.;
Cuntz, M.; Danchi, W.; Dong, C. F.; Drake, J. J.; Fahrenbach, A.;
France, K.; Garcia-Sage, K.; Glocer, A.; Grenfell, J. L.; Gronoff,
G.; Hartnett, H.; Henning, W.; Hinkel, N. R.; Jensen, A. G.; Jin,
M.; Kalas, P.; Kane, S. R.; Kobayashi, K.; Kopparapu, R.; Leake, J.;
López-Puertas, M.; Lueftinger, T.; Lynch, B.; Lyra, W.; Mandell,
A. M.; Mandt, K. E.; Moore, W. B.; Nna-Mvondo, D.; Notsu, Y.; Maehara,
H.; Yamashiki, Y.; Shibata, K.; Oman, L. D.; Osten, R. A.; Pavlov,
A.; Ramirez, R. M.; Rugheimer, S.; Schlieder, J. E.; Schnittman,
J. D.; Shock, E. L.; Sousa-Silva, C.; Way, M. J.; Yang, Y.; Young,
P. A.; Zank, G. P.
Bibcode: 2018arXiv180303751A
Altcode:
It is currently unknown how common life is on exoplanets, or how long
planets can remain viable for life. To date, we have a superficial
notion of habitability, a necessary first step, but so far lacking an
understanding of the detailed interaction between stars and planets
over geological timescales, dynamical evolution of planetary systems,
and atmospheric evolution on planets in other systems. A planet mass,
net insolation, and atmospheric composition alone are insufficient
to determine the probability that life on a planet could arise
or be detected. The latter set of planetary considerations, among
others, underpin the concept of the habitable zone (HZ), defined
as the circumstellar region where standing bodies of liquid water
could be supported on the surface of a rocky planet. However, stars
within the same spectral class are often treated in the same way in HZ
studies, without any regard for variations in activity among individual
stars. Such formulations ignore differences in how nonthermal emission
and magnetic energy of transient events in different stars affect the
ability of an exoplanet to retain its atmosphere.In the last few years
there has been a growing appreciation that the atmospheric chemistry,
and even retention of an atmosphere in many cases, depends critically
on the high-energy radiation and particle environments around these
stars. Indeed, recent studies have shown stellar activity and the
extreme space weather, such as that created by the frequent flares and
coronal mass ejections (CMEs) from the active stars and young Sun, may
have profoundly affected the chemistry and climate and thus habitability
of the early Earth and terrestrial type exoplanets. The goal of this
white paper is to identify and describe promising key research goals
to aid the field of the exoplanetary habitability for the next 20 years.
Title: Statistical Study of Solar White-light Flares and Comparison
with Superflares on Solar-type Stars
Authors: Namekata, Kosuke; Sakaue, Takahito; Watanabe, Kyoko; Asai,
Ayumi; Maehara, Hiroyuki; Notsu, Yuta; Notsu, Shota; Honda, Satoshi;
Ishii, Takako T.; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2018IAUS..340..221N
Altcode: 2018arXiv180407122N
Recently, many superflares on solar-type stars were discovered as
white-light flares (WLFs). A correlation between the energies (E)
and durations (t) of superflares is derived as t~E0.39,
and this can be theoretically explained by magnetic reconnection
(t~E1/3). In this study, we carried out a statistical
research on 50 solar WLFs with SDO/HMI to examine the t-E relation. As
a result, the t-E relation on solar WLFs (t~E0.38) is quite
similar stellar superflares, but the durations of stellar superflares
are much shorter than those extrapolated from solar WLFs. We present
the following two interpretations; (1) in solar flares, the cooling
timescale of WL emission may be longer than the reconnection one,
and the decay time can be determined by the cooling timescale;
(2) the distribution can be understood by applying a scaling law
t~E1/3B-5/3 derived from the magnetic reconnection
theory.
Title: Iwahashi Zenbei's Sunspot Drawings in 1793 in Japan
Authors: Hayakawa, Hisashi; Iwahashi, Kiyomi; Tamazawa, Harufumi;
Toriumi, Shin; Shibata, Kazunari
Bibcode: 2018SoPh..293....8H
Altcode: 2017arXiv171108143H
Three Japanese sunspot drawings associated with Iwahashi Zenbei (1756
- 1811) are shown here from contemporary manuscripts and woodprint
documents with the relevant texts. We reveal the observational date of
one of the drawings to be 26 August 1793, and the overall observations
lasted for over a year. Moreover, we identify the observational site
for the dated drawing as Fushimi in Japan. We then compare Zenbei's
observations with the group sunspot number and the raw group count
from the Sunspot Index and Long-term Solar Observations (SILSO) to
reveal the context of the data, and we conclude that these drawings
fill gaps in our understanding that are due to the fragmental sunspot
observations around 1793. These drawings are important as a clue to
evaluate astronomical knowledge of contemporary Japan in the late
eighteenth century and are valuable as a non-European observation,
considering that most sunspot observations up to the middle of the
nineteenth century are from Europe.
Title: Long-lasting Extreme Magnetic Storm Activities in 1770 Found
in Historical Documents
Authors: Hayakawa, Hisashi; Iwahashi, Kiyomi; Ebihara, Yusuke;
Tamazawa, Harufumi; Shibata, Kazunari; Knipp, Delores J.; Kawamura,
Akito D.; Hattori, Kentaro; Mase, Kumiko; Nakanishi, Ichiro; Isobe,
Hiroaki
Bibcode: 2017ApJ...850L..31H
Altcode: 2017arXiv171100690H
Dim red aurora at low magnetic latitudes is a visual and recognized
manifestation of magnetic storms. The great low-latitude auroral
displays seen throughout East Asia on 1770 September 16-18 are
considered to manifest one of the greatest storms. Recently found,
111 historical documents in East Asia attest that these low-latitude
auroral displays appeared in succession for almost nine nights during
1770 September 10-19 in low magnetic latitude areas (<30°). This
suggests that the duration of the great magnetic storm is much longer
than usual. Sunspot drawings from 1770 reveal that the sunspot areas
were twice as large as those observed in another great storm of 1859,
which substantiates these unusual storm activities in 1770. These spots
likely ejected several huge, sequential magnetic structures in short
duration into interplanetary space, resulting in spectacular worldwide
aurorae in mid-September of 1770. These findings provide new insight
into the history, duration, and effects of extreme magnetic storms that
may be valuable for those who need to mitigate against extreme events.
Title: Statistical Studies of Solar White-light Flares and Comparisons
with Superflares on Solar-type Stars
Authors: Namekata, Kosuke; Sakaue, Takahito; Watanabe, Kyoko; Asai,
Ayumi; Maehara, Hiroyuki; Notsu, Yuta; Notsu, Shota; Honda, Satoshi;
Ishii, Takako T.; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2017ApJ...851...91N
Altcode: 2017arXiv171011325N
Recently, many superflares on solar-type stars have been discovered
as white-light flares (WLFs). The statistical study found a
correlation between their energies (E) and durations (τ): τ \propto
{E}0.39, similar to those of solar hard/soft X-ray flares,
τ \propto {E}0.2{--0.33}. This indicates a universal
mechanism of energy release on solar and stellar flares, i.e., magnetic
reconnection. We here carried out statistical research on 50 solar
WLFs observed with Solar Dynamics Observatory/HMI and examined the
correlation between the energies and durations. As a result, the E-τ
relation on solar WLFs (τ \propto {E}0.38) is quite similar
to that on stellar superflares (τ \propto {E}0.39). However,
the durations of stellar superflares are one order of magnitude
shorter than those expected from solar WLFs. We present the following
two interpretations for the discrepancy: (1) in solar flares, the
cooling timescale of WLFs may be longer than the reconnection one, and
the decay time of solar WLFs can be elongated by the cooling effect;
(2) the distribution can be understood by applying a scaling law (τ
\propto {E}1/3{B}-5/3) derived from the magnetic
reconnection theory. In the latter case, the observed superflares
are expected to have 2-4 times stronger magnetic field strength than
solar flares.
Title: Records of auroral candidates and sunspots in Rikkokushi,
chronicles of ancient Japan from early 7th century to 887
Authors: Hayakawa, Hisashi; Iwahashi, Kiyomi; Tamazawa, Harufumi;
Ebihara, Yusuke; Kawamura, Akito Davis; Isobe, Hiroaki; Namiki,
Katsuko; Shibata, Kazunari
Bibcode: 2017PASJ...69...86H
Altcode: 2017arXiv170801045H
We present the results of the surveys on sunspots and auroral candidates
in Rikkokushi, Japanese official histories from the early 7th century
to 887, to review the solar and auroral activities. In total, we found
one sunspot record and 13 auroral candidates in Rikkokushi. We then
examine the records of the sunspots and auroral candidates, compare the
auroral candidates with the lunar phase to estimate their reliability,
and compare the records of the sunspots and auroral candidates with the
contemporary total solar irradiance reconstructed from radioisotope
data. We also identify the locations of the observational sites to
review possible equatorward expansion of the auroral oval. These
discussions suggest a major gap in auroral candidates from the late
7th to early 9th centuries, which includes the candidate of the grand
minimum reconstructed from the radioisotope data, a similar tendency
as the distributions of sunspot records in contemporary China, and a
relatively high magnetic latitude of observational sites with a higher
potential for observing aurorae more frequently than at present.
Title: Observational study on the fine structure and dynamics of a
solar jet. I. Energy build-up process around a satellite spot
Authors: Sakaue, Takahito; Tei, Akiko; Asai, Ayumi; Ueno, Satoru;
Ichimoto, Kiyoshi; Shibata, Kazunari
Bibcode: 2017PASJ...69...80S
Altcode: 2017arXiv170701262S
We report on a solar jet phenomenon associated with successive flares
on 2014 November 10. These explosive events were involved with the
satellite spots' emergence around a δ-type sunspot in the decaying
active region NOAA 12205. The data for this jet were provided by the
Solar Dynamics Observatory, the X-Ray Telescope aboard Hinode, and the
Interface Region Imaging Spectrograph and Domeless Solar Telescope
at Hida Observatory, Kyoto University. These abundant data enabled
us to present this series of papers to discuss the entire process of
the observed phenomena, including the energy storage, event trigger,
and energy release. In this paper, we focus on the energy build-up and
trigger phases, by analyzing the photospheric horizontal flow field
around the active region by an optical flow method. The analysis shows
the following results: (1) The observed explosive phenomena involved
three satellite spots, the magnetic fluxes of which successively
reconnected with their pre-existing ambient fields; (2) All of these
satellite spots emerged in the moat region of a pivotal δ-type
sunspot, especially near its convergent boundary with the neighboring
supergranules or moat regions of adjacent sunspots; (3) Around the
jet ejection site, the positive polarities of the satellite spot and
adjacent emerging flux encountered the global magnetic field with a
negative polarity in the moat region of the pivotal δ-type sunspot,
and thus the polarity inversion line was formed along the convergent
boundary of the photospheric horizontal flow channels.
Title: Quasi-periodic Oscillations in Flares and Coronal Mass
Ejections Associated with Magnetic Reconnection
Authors: Takahashi, Takuya; Qiu, Jiong; Shibata, Kazunari
Bibcode: 2017ApJ...848..102T
Altcode: 2017arXiv170905234T
We propose a mechanism for quasi-periodic oscillations of both
coronal mass ejections (CMEs) and flare loops as related to magnetic
reconnection in eruptive solar flares. We perform two-dimensional
numerical MHD simulations of magnetic flux rope eruption, with three
different values of the global Lundquist number. In the low Lundquist
number run, no oscillatory behavior is found. In the moderate Lundquist
number run, on the other hand, quasi-periodic oscillations are excited
both at the bottom of the flux rope and at the flare loop top. In the
high Lundquist number run, quasi-periodic oscillations are also excited;
in the meanwhile, the dynamics become turbulent owing to the formation
of multiple plasmoids in the reconnection current sheet. In high and
moderate Lundquist number runs, thin reconnection jets collide with
the flux rope bottom or flare loop top and dig them deeply. Steep
oblique shocks are formed as termination shocks where reconnection
jets are bent (rather than decelerated) in the horizontal direction,
resulting in supersonic backflows. The structure becomes unstable, and
quasi-periodic oscillations of supersonic backflows appear at locally
confined high-beta regions at both the flux rope bottom and flare loop
top. We compare the observational characteristics of quasi-periodic
oscillations in erupting flux ropes, post-CME current sheets, flare
ribbons, and light curves with corresponding dynamical structures
found in our simulation.
Title: A Theoretical Model of X-Ray Jets from Young Stellar Objects
Authors: Takasao, Shinsuke; Suzuki, Takeru K.; Shibata, Kazunari
Bibcode: 2017ApJ...847...46T
Altcode: 2017arXiv170805388T
There is a subclass of X-ray jets from young stellar objects that are
heated very close to the footpoint of the jets, particularly DG Tau
jets. Previous models have attributed the strong heating to shocks
in the jets. However, the mechanism that localizes the heating at the
footpoint remains puzzling. We presented a different model of such X-ray
jets, in which the disk atmosphere is magnetically heated. Our disk
corona model is based on the so-called nanoflare model for the solar
corona. We show that the magnetic heating near the disks can result in
the formation of a hot corona with a temperature of ≳106
K, even if the average field strength in the disk is moderately weak,
≳1 G. We determine the density and the temperature at the jet base
by considering the energy balance between the heating and cooling. We
derive the scaling relations of the mass-loss rate and terminal
velocity of jets. Our model is applied to the DG Tau jets. The
observed temperature and estimated mass-loss rate are consistent
with the prediction of our model in the case of a disk magnetic field
strength of ∼20 G and a heating region of <0.1 au. The derived
scaling relation of the temperature of X-ray jets could be a useful
tool for estimating the magnetic field strength. We also find that the
jet X-ray can have a significant impact on the ionization degree near
the disk surface and the dead zone size.
Title: Starspot activity and superflares on solar-type stars
Authors: Maehara, Hiroyuki; Notsu, Yuta; Notsu, Shota; Namekata,
Kosuke; Honda, Satoshi; Ishii, Takako T.; Nogami, Daisaku; Shibata,
Kazunari
Bibcode: 2017PASJ...69...41M
Altcode: 2017arXiv170207141M
We analyze the correlation between starspots and superflares on
solar-type stars using observations from the Kepler mission. The
analysis shows that the observed fraction of stars with superflares
decreases as the rotation period increases and as the amplitude of
photometric variability associated with rotation decreases. We found
that the fraction of stars with superflares among the stars showing
large-amplitude rotational variations, which are thought to be the
signature of the large starspots, also decreases as the rotation period
increases. The small fraction of superflare stars among the stars
with large starspots in the longer-period regime suggests that some
of the stars with large starspots show a much lower flare activity
than the superflare stars with the same spot area. Assuming simple
relations between spot area and lifetime and between spot temperature
and photospheric temperature, we compared the size distribution of
large starspot groups on slowly rotating solar-type stars with that of
sunspot groups. The size distribution of starspots shows the power-law
distribution and the size distribution of larger sunspots lies on this
power-law line. We also found that frequency-energy distributions
for flares originating from spots with different sizes are the same
for solar-type stars with superflares and the Sun. These results
suggest that the magnetic activity we observe on solar-type stars with
superflares and on the Sun is caused by the same physical processes.
Title: Statistical studies of superflares on G-, K-, M- type stars
from Kepler data
Authors: Notsu, Yuta; Maehara, Hiroyuki; Honda, Satoshi; Notsu, Shota;
Namekata, Kosuke; Ikuta, Kai; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2017reph.conf40003N
Altcode:
Flares are thought to be sudden releases of magnetic energy stored
around starspots. Recent space high-precision photometry shows
“superflares”, 10-104 times more energetic than
the largest solar flares, occur on many G, K, M-type stars (e.g.,
Maehara+2012 Nature). Harmful UV/X-ray radiation and high-energy
particles such as protons are caused by such superflares. This
may suggest that exoplanet host stars have severe effects on
the physical and chemical evolution of exoplanetary atmospheres
(cf. Segura+2010 Astrobiology, Takahashi+2016 ApJL).We here present
statistical properties of superflares on G, K, M-type stars on the
basis of our analyses of Kepler photometric data (Maehara+2012 Nature,
Shibayama+2013 ApJS, Notsu+2013 ApJ, Canderaresi+2014 ApJ, Maehara+2015
EPS, Maehara+2017 PASJ). We found more than 5000 superflares on 800 G,
K, M-type main-sequence stars, and the occurrence frequency (dN/dE)
of superflares as a function of flare energy (E) shows the power-law
distribution with the index of -1.8 -1.9. This power-law distribution
is consistent with that of solar flares.Flare frequency increases
as stellar temperature decreases. As for M-type stars, energy of
the largest flares is smaller compared with G,K-type stars, but more
frequent “hazardous” flares for the habitable planets since the
habitable zone around M-type stars is much smaller compared with G,
K-type stars.Rotation period and starspot coverage can be estimated from
the quasi-periodic brightness variation of the superflare stars. The
intensity of Ca II 8542 line of superflare stars, which is measured from
spectroscopic observations with Subaru Telescope, has a well correlation
with the brightness variation amplitude (Notsu+2015a&b PASJ).Flare
frequency has a correlation with rotation period, and this suggests
young rapidly-rotating stars (like “young Sun”) have more severe
impacts of flares on the planetary atmosphere (cf. Airapetian+2016
ApJL). Flare energy and frequency also depends on starspot coverage,
and this suggests existence of large starspots is important factor
of superflares.These statistical properties of superflares discussed
here can be one of the basic information for considering the impacts
of flares on planet-host stars.
Title: Sheath-accumulating Propagation of Interplanetary Coronal
Mass Ejection
Authors: Takahashi, Takuya; Shibata, Kazunari
Bibcode: 2017ApJ...837L..17T
Altcode: 2017arXiv170206607T
Fast interplanetary coronal mass ejections (ICMEs) are the drivers of
strong space weather storms such as solar energetic particle events and
geomagnetic storms. The connection between the space-weather-impacting
solar wind disturbances associated with fast ICMEs at Earth and the
characteristics of causative energetic CMEs observed near the Sun is
a key question in the study of space weather storms, as well as in the
development of practical space weather prediction. Such shock-driving
fast ICMEs usually expand at supersonic speeds during the propagation,
resulting in the continuous accumulation of shocked sheath plasma
ahead. In this paper, we propose a “sheath-accumulating propagation”
(SAP) model that describes the coevolution of the interplanetary sheath
and decelerating ICME ejecta by taking into account the process of
upstream solar wind plasma accumulation within the sheath region. Based
on the SAP model, we discuss (1) ICME deceleration characteristics; (2)
the fundamental condition for fast ICMEs at Earth; (3) the thickness
of interplanetary sheaths; (4) arrival time prediction; and (5) the
super-intense geomagnetic storms associated with huge solar flares. We
quantitatively show that not only the speed but also the mass of the
CME are crucial for discussing the above five points. The similarities
and differences between the SAP model, the drag-based model, and
the“snow-plow” model proposed by Tappin are also discussed.
Title: Validation of a scaling law for the coronal magnetic field
strength and loop length of solar and stellar flares
Authors: Namekata, Kosuke; Sakaue, Takahito; Watanabe, Kyoko; Asai,
Ayumi; Shibata, Kazunari
Bibcode: 2017PASJ...69....7N
Altcode: 2016arXiv161009811N
Shibata and Yokoyama (1999, ApJ, 526, L49; 2002, ApJ, 577, 422)
proposed a method of estimating the coronal magnetic field strength
(B) and magnetic loop length (L) of solar and stellar flares, on the
basis of magnetohydrodynamic simulations of the magnetic reconnection
model. Using the scaling law provided by Shibata and Yokoyama (1999,
ApJ, 526, L49; 2002, ApJ, 577, 422), we obtain B and L as functions of
the emission measure (EM = n2L3) and temperature
(T) at the flare peak. Here, n is the coronal electron density of
the flares. This scaling law enables the estimation of B and L for
unresolved stellar flares from the observable physical quantities EM
and T, which is helpful for studying stellar surface activities. To
apply this scaling law to stellar flares, we discuss its validity for
spatially resolved solar flares. Quantities EM and T are calculated from
GOES (Geostationary Operational Environmental Satellite) soft X-ray flux
data, and B and L are theoretically estimated using the scaling law. For
the same flare events, B and L were also observationally estimated
with images taken by the Solar Dynamics Observatory (SDO)/Helioseismic
and Magnetic Imager (HMI) Magnetogram and Atmospheric Imaging Assembly
(AIA) 94 Å pass band. As expected, a positive correlation was found
between the theoretically and observationally estimated values. We
interpret this result as indirect evidence that flares are caused
by magnetic reconnection. Moreover, this analysis makes us confident
about the validity of applying this scaling law to stellar flares as
well as solar flares.
Title: “Dandelion” Filament Eruption and Coronal Waves Associated
with a Solar Flare on 2011 February 16
Authors: Cabezas, Denis P.; Martínez, Lurdes M.; Buleje, Yovanny J.;
Ishitsuka, Mutsumi; Ishitsuka, José K.; Morita, Satoshi; Asai, Ayumi;
UeNo, Satoru; Ishii, Takako T.; Kitai, Reizaburo; Takasao, Shinsuke;
Yoshinaga, Yusuke; Otsuji, Kenichi; Shibata, Kazunari
Bibcode: 2017ApJ...836...33C
Altcode: 2017arXiv170100308C
Coronal disturbances associated with solar flares, such as Hα Moreton
waves, X-ray waves, and extreme ultraviolet (EUV) coronal waves,
are discussed herein in relation to magnetohydrodynamic fast-mode
waves or shocks in the corona. To understand the mechanism of
coronal disturbances, full-disk solar observations with high spatial
and temporal resolution over multiple wavelengths are of crucial
importance. We observed a filament eruption, whose shape is like a
“dandelion,” associated with the M1.6 flare that occurred on 2011
February 16 in Hα images taken by the Flare Monitoring Telescope at
Ica University, Peru. We derive the three-dimensional velocity field
of the erupting filament. We also identify winking filaments that are
located far from the flare site in the Hα images, whereas no Moreton
wave is observed. By comparing the temporal evolution of the winking
filaments with those of the coronal wave seen in the EUV images data
taken by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory and by the Extreme Ultraviolet Imager on board the Solar
Terrestrial Relations Observatory-Ahead, we confirm that the winking
filaments were activated by the EUV coronal wave.
Title: Lithium Abundance of the Solar-Type Superflare Stars
Authors: Honda, Satoshi; Notsu, Yuta; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2017nuco.confb0303H
Altcode:
We performed the high dispersion spectroscopy of solar-type superflare
stars with Subaru/HDS, and estimated the stellar parameters and lithium
abundance of the stars to compare them with those of the Sun [1]. Our
spectroscopic analysis of superflare stars shows more than half of
targets have no evidence of binary system and the stellar parameters
are in the range of solar-type stars [2, 3]. We also investigate the
correlations of lithium abundance with stellar atmospheric parameters,
rotational velocity, and superflare activities to understand the
nature of superflare stars and the possibility of the nucleosynthesis
of lithium by superflares. The derived lithium abundance in superflare
stars does not show the correlation with stellar parameters. As compared
with the lithium abundance in Hyades cluster which is younger than the
sun, it is suggested that half of the observed stars are young. However,
some objects have the low lithium abundances and slowly rotate on the
basis of the estimated v sin i and period of brightness variation. These
results indicate that the superflare stars are not only young stars
but also old stars like our Sun. In our observations, we could not
find the any evidence of lithium productions by superflare.
Title: Spectroscopic observations of active solar-analog stars with
high X-ray luminosity, as a proxy of superflare stars
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Namekata, Kosuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2017PASJ...69...12N
Altcode: 2016arXiv161103659N
Recent studies of solar-type superflare stars have suggested that even
old slowly rotating stars similar to the Sun can have large starspots
and superflares. We conducted high-dispersion spectroscopy of 49 nearby
solar-analog stars (G-type main-sequence stars with Teff
≈ 5600-6000 K) identified as ROSAT soft X-ray sources, which are
not binary stars from previous studies. We expected that these stars
could be used as a proxy of bright solar-analog superflare stars, since
superflare stars are expected to show strong X-ray luminosity. More than
half (37) of the 49 target stars show no evidence of binarity, and their
atmospheric parameters (temperature, surface gravity, and metallicity)
are within the range of ordinary solar-analog stars. We measured the
intensity of Ca ii 8542 and Hα lines, which are good indicators of the
stellar chromospheric activity. The intensity of these lines indicates
that all the target stars have large starspots. We also measured v
sin i (projected rotational velocity) and lithium abundance for the
target stars. Li abundance is a key to understanding the evolution
of the stellar convection zone, which reflects the stellar age, mass
and rotational history. We confirmed that many of the target stars
rapidly rotate and have high Li abundance, compared with the Sun,
as suggested by many previous studies. There are, however, also some
target stars that rotate slowly (v sin i = 2-3 km s-1)
and have low Li abundance like the Sun. These results support that old
and slowly rotating stars similar to the Sun could have high activity
levels and large starspots. This is consistent with the results of
our previous studies of solar-type superflare stars. In the future,
it is important to conduct long-term monitoring observations of these
active solar-analog stars in order to investigate detailed properties
of large starspots from the viewpoint of stellar dynamo theory.
Title: Superflares on Sun-like stars
Authors: Shibata, Kazunari
Bibcode: 2017symm.conf..195S
Altcode:
No abstract at ADS
Title: Solar flares in GOES X-ray flux forecast based on SDO/HMI
and SDO/AIA images.
Authors: Hada-Muranushi, Y.; Muranushi, T.; Asai, A.; Nemoto, S.;
Shibata, K.
Bibcode: 2016AGUFMSH11C2239H
Altcode:
We have been studying and operationg methods for automated solar
flare forecasts. The automated forecast of solar flares and other
space-weather related events have two crucial goals. One is to enable
real-time forecast and thus provide truely predictive test for the
space weather theories. The other is to enable numerous variation of
tailor-made space weather forecasts for various space weather users. We
have been building space weather prediction system UFCORIN (Universal
Forecast Constructor by Optimized Regression of INputs), a software
framework that can provide forecast based on generic time-series
data. Recently, we have been updating UFCORIN so that it can handle
image time-series data in addition to scalar-values timeseries,
with the help of convolutional neural network. We have been operating
space weather forecast since August, 2015 that provides 24-hour-ahead
forecast of solar flares, every 12 minutes, based on the time-series
data of GOES X-ray flux and wavelet features of line-of-sight magnetic
field images in SDO/HMI. However, the TSS (True Skill Statistics)
for M and C class flares achieved so far has been approximately 0.3,
much less than those values of 0.7-0.9 reported by simulated forecast
studies. Especially, it is difficult to predict rim flares and those
flares that occur on the East side of the Sun, where active regions
have small, noisy features in the magnetic field images. In order to
better predict rim flares, we are now studying the effect of adding
extreme-ultraviolet images in SDO/AIA to the input set, which includes
solar rim information. In this presentation, we report the methods
and prediction results of the system. In addition, we will report the
results of adding SDO/AIA images to the input data.
Title: Scaling Relations in Coronal Mass Ejections and Energetic
Proton Events Associated with Solar Superflares
Authors: Takahashi, Takuya; Mizuno, Yoshiyuki; Shibata, Kazunari
Bibcode: 2016ApJ...833L...8T
Altcode: 2016arXiv161106015T
In order to discuss the potential impact of solar “superflares” on
space weather, we investigated statistical relations among energetic
proton peak flux with energy higher than 10 MeV (F p ),
CME speed near the Sun (V CME) obtained by Solar and
Heliospheric Observatory/LASCO coronagraph, and flare soft X-ray peak
flux in the 1-8 Å band (F SXR) during 110 major solar
proton events recorded from 1996 to 2014. The linear regression
fit results in the scaling relations {V}{CME}\propto
{F}{SXR}α , {F}p\propto
{F}{SXR}β , and {F}p\propto
{V}{CME}γ with α = 0.30 ± 0.04, β = 1.19
± 0.08, and γ = 4.35 ± 0.50, respectively. On the basis of simple
physical assumptions, on the other hand, we derive scaling relations
expressing CME mass (M CME), CME speed, and energetic
proton flux in terms of total flare energy (E flare)
as {M}{CME}\propto {E}{flare}2/3,
{V}{CME}\propto {E}{flare}1/6, and
{F}p\propto {E}{flare}5/6\propto
{V}{CME}5, respectively. We then combine the
derived scaling relations with observation and estimated the upper
limit of V CME and F p to be associated with
possible solar superflares.
Title: East Asian observations of low-latitude aurora during the
Carrington magnetic storm
Authors: Hayakawa, Hisashi; Iwahashi, Kiyomi; Tamazawa, Harufumi;
Isobe, Hiroaki; Kataoka, Ryuho; Ebihara, Yusuke; Miyahara, Hiroko;
Kawamura, Akito Davis; Shibata, Kazunari
Bibcode: 2016PASJ...68...99H
Altcode: 2016arXiv160807702H; 2016PASJ..tmp..100H
A magnetic storm around 1859 September 2, caused by a so-called
Carrington flare, was the most intense in the history of modern
scientific observations, and hence is considered to be a benchmark
event concerning space weather. The magnetic storm caused worldwide
observations of auroras, even at very low latitudes, such as Hawaii,
Panama, or Santiago. Available magnetic-field measurements at Bombay,
India, showed two peaks: the main was the Carrington event, which
occurred in day time in East Asia; a second storm after the Carrington
event occurred at night in East Asia. In this paper, we present
results from surveys of aurora records in East Asia, which provide new
information concerning the aurora activity of this important event. We
found some new East Asian records of low-latitude aurora observations
caused by a storm which occurred after the Carrington event. The size
of the aurora belt of the second peak of the Carrington magnetic storm
was even wider than that of usual low-latitude aurora events.
Title: Comparative Studies of Prediction Strategies for Solar X-ray
Time Series
Authors: Muranushi, T.; Hattori, T.; Jin, Q.; Hishinuma, T.; Tominaga,
M.; Nakagawa, K.; Fujiwara, Y.; Nakamura, T.; Sakaue, T.; Takahashi,
T.; Seki, D.; Namekata, K.; Tei, A.; Ban, M.; Kawamura, A. D.;
Hada-Muranushi, Y.; Asai, A.; Nemoto, S.; Shibata, K.
Bibcode: 2016AGUFMSH11C2240M
Altcode:
Crucial virtues for operational space weather forecast are
real-timeforecast ability, forecast precision and customizability
to userneeds. The recent development of deep-learning makes it
veryattractive to space weather, because (1) it learns gradually
incomingdata, (2) it exhibits superior accuracy over conventional
algorithmsin many fields, and (3) it makes the customization
of the forecasteasier because it accepts raw images.However, the
best deep-learning applications are only attainable bycareful human
designers that understands both the mechanism of deeplearning and the
application field. Therefore, we need to foster youngresearchers to
enter the field of machine-learning aided forecast. So,we have held
a seminar every Monday with undergraduate and graduatestudents from
May to August 2016.We will review the current status of space weather
science and theautomated real-time space weather forecast engine
UFCORIN. Then, weintroduce the deep-learning space weather forecast
environments wehave set up using Python and Chainer on students' laptop
computers.We have started from simple image classification neural
network, thenimplemented space-weather neural network that predicts
future X-rayflux of the Sun based on the past X-ray lightcurve and
magnetic fieldline-of-sight images.In order to perform each forecast
faster, we have focused on simplelightcurve-to-lightcurve forecast,
and performed comparative surveysby changing following parameters: The size and topology of the neural network Batchsize Neural network
hyperparameters such as learning rates to optimize the preduction
accuracy, and time for prediction.We have found how to design compact,
fast but accurate neural networkto perform forecast. Our forecasters can
perform predictionexperiment for four-year timespan in a few minutes,
and achieveslog-scale errors of the order of 1. Our studies is ongoing,
and inour talk we will review our progress till December.
Title: Observational Evidence of Particle Acceleration Associated
with Plasmoid Motions
Authors: Takasao, Shinsuke; Asai, Ayumi; Isobe, Hiroaki; Shibata,
Kazunari
Bibcode: 2016ApJ...828..103T
Altcode: 2016arXiv161100108T
We report a strong association between the particle acceleration and
plasma motions found in the 2010 August 18 solar flare. The plasma
motions are tracked in the extreme ultraviolet (EUV) images taken by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory and the Extreme UltraViolet Imager (EUVI) on the Solar
Terrestrial Relations Observatory spacecraft Ahead, and the signature of
particle acceleration was investigated by using Nobeyama Radioheliograph
data. In our previous paper, we reported that in EUV images many plasma
blobs appeared in the current sheet above the flare arcade. They were
ejected bidirectionally along the current sheet, and the blobs that
were ejected sunward collided with the flare arcade. Some of them
collided or merged with each other before they were ejected from
the current sheet. We discovered impulsive radio bursts associated
with such plasma motions (ejection, coalescence, and collision with
the post flare loops). The radio bursts are considered to be the
gyrosynchrotron radiation by nonthermal high energy electrons. In
addition, the stereoscopic observation by AIA and EUVI suggests
that plasma blobs had a three-dimensionally elongated structure. We
consider that the plasma blobs were three-dimensional plasmoids (I.e.,
flux ropes) moving in a current sheet. We believe that our observation
provides clear evidence of particle acceleration associated with the
plasmoid motions. We discuss possible acceleration mechanisms on the
basis of our results.
Title: Lithium Abundance Of The Solar-Type Superflare Stars
Authors: Honda, Satoshi; Notsu, Yuta; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2016csss.confE..85H
Altcode:
We performed the high dispersion spectroscopy of solar-type superflare
stars by Subaru/HDS, and estimate the stellar parameters and lithium
abundance of the stars to compare with the Sun. Our spectroscopic
analysis of superflare stars show more than half of targets have no
evidence of binary system and the stellar parameters are in the range
of solar-type stars (Notsu et al. 2015a&b). We also investigate the
correlations of Lithium abundance with stellar atmospheric parameters,
rotational velocity, and superflare activities to understand the
nature of superflare stars and the possibility of the nucleosynthesis
of lithium by superflares. The derived lithium abundance in superflare
stars do not show the correlation with stellar parameters. As compared
with the lithium abundance in Hyades cluster which is younger than the
sun, it is suggested that half of observed stars are young. However,
there are some objects which show the low lithium and slowly rotate
from the estimated v sin(i) and period of brightness variation. These
results indicate that the superflare stars are not only young stars
but also old stars like our sun. In our observations, we could not
find the any evidence of lithium productions by superflare.
Title: Statistical Properties Of Superflares On Solar-Type Stars
With Kepler Data
Authors: Notsu, Yuta; Maehara, Hiroyuki; Shibayama, Takuya; Honda,
Satoshi; Notsu, Shota; Namekata, Kosuke; Nogami, Daisaku; Shibata,
Kazunari
Bibcode: 2016csss.confE.119N
Altcode: 2016arXiv160800186N
Superflares are flares that release total energy 10¥sim10(4) times
greater than
that of the biggest solar flares with energy of
¥sim10(32) erg.
We searched superflares on solar-type stars
(G-type main sequence stars)
using the Kepler 30-min (long) and
1-min (short) cadence data.
We found more than 1500 superflares on
279 stars from 30-min cadence data (Q0-6)
and 187 superflares on
23 stars from 1-min cadence data (Q0-17).
The bolometric energy
of detected superflares ranges from the order of 10(32) erg to 10(36)
erg.
Using these data, we found that the occurrence frequency
(dN/dE) of superflares is
expressed as a power-law function of
flare energy (E) with the index of -1.5 for 10(33}<E<10({36))
erg. Most of the superflare stars show quasi-periodic light variations
with the amplitude of a few percent,
which can be explained by
the rotation of the star with large starspots.
¥¥
¥ ¥
¥ ¥ ¥
The bolometric energy released by flares is consistent
with the magnetic energy stored around such large starspots.
Furthermore, our analyses indicate that the occurrence frequency of
superflares depends on the rotation period,
and that the flare
frequency increases as the rotation period decreases.
However,
the energy of the largest flares observed in a given period bin does
not show any clear correlation with the rotation period.
We
also found that the duration of superflares increases with the flare
energy as E(0.39+/-0.03) .
This can be explained if we assume the
time-scale of flares is determined by the Alfv¥acute{¥rm{e}}n time.
Title: Time Resolved Spectroscopic Observations of an M-Dwarf Flare
Star EV Lac During a Flare.
Authors: Honda, Satoshi; Notsu, Yuta; Notsu, Shota; Maehara, Hiroyuki;
Namekata, Kosuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2016csss.confE..87H
Altcode:
Flares are explosions on the surface of the Sun and stars. We can find
stellar flares frequently on M-dwarf stars. EV Lac, one of the M-dwarf
flare stars, is a good target for investigation of stellar flares. We
have carried out continuous spectroscopic observations of the Halpha
line with medium wavelength resolution (R 10,000) by using 2m Nayuta
telescope at Nishi-Harima Astronomical Observatory in Japan from
August to December, 2015. EV Lac always has Halpha emission line and
it was stronger on 15 August than on the other nights. In this night,
we observed a rapid enhancement ( 20min) and following slow decrease
( 1.5h) of the emission-line intensity of Halpha, which was probably
caused by a flare. We also found an asymmetrical change in the blue
region of the Halpha line. In many cases, red asymmetry has been
observed in the Halpha line profile during solar flares, but blue
asymmetry has been sometimes observed during stellar flares.
Title: High Dispersion Spectroscopy of Solar-Type Superflare Stars
With Subaru/HDS
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Namekata, Kosuke; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2016csss.confE..98N
Altcode: 2015arXiv151008143N
Superflares are flares that release total energy 10-104times
greater than that of the biggest solar flares ( 1032
erg). Recent Kepler-space-telescope observations found more than
1000superflares on a few hundred solar-type stars (Maehara et
al. 2012, Nature; Shibayama et al. 2013, ApJS, Maehara et al. 2015
EPS).Suchsuperflare stars show quasi-periodicbrightnessvariations with
the typical period of from one to a few tens of days. Such variations
are thought to be caused by the rotation of the star with large
starspots (Notsu et al. 2013, ApJ). However,
spectroscopic
observations are needed in order to confirm whether the variation
is really due to the rotation and whether superflares can occur on
ordinary single stars similar to our Sun.Then we have carried out
spectroscopic observations for 50 solar-type superflare stars with
Subaru/HDS (Notsu et al. 2015a&b, PASJ). As a result, more than
half (34 stars) of the targetstars show no evidence of binarity,
and the atmospheric parameters of these stars are in the range of
solar-type stars.The detailed analyses for these 34 stars show that
(1) the projected rotational velocities (v sin i) are consistent with
the rotational velocities estimated from the brightness variations,
(2)there is a correlation between the brightness variation amplitude
and the intensity of Ca II IR triplet line. These results support that
the brightness variation discussed above is explained bythe rotation of
a star with large starspots.
(The contents of this poster were
already summarized in the proceeding of IAU Symposium S320 (Notsu et
al. 2016 IAUS in press, arXiv:1510.08143))
Title: Above-the-loop-top Oscillation and Quasi-periodic Coronal
Wave Generation in Solar Flares
Authors: Takasao, Shinsuke; Shibata, Kazunari
Bibcode: 2016ApJ...823..150T
Altcode: 2016arXiv160609354T
Observations revealed that various kinds of oscillations are excited
in solar flare regions. Quasi-periodic pulsations (QPPs) in flare
emissions are commonly observed in a wide range of wavelengths. Recent
observations have found that fast-mode magnetohydrodynamic (MHD)
waves are quasi-periodically emitted from some flaring sites
(quasi-periodic propagating fast-mode magnetoacoustic waves;
QPFs). Both QPPs and QPFs imply a cyclic disturbance originating
from the flaring sites. However, the physical mechanisms remain
puzzling. By performing a set of two-dimensional MHD simulations of
a solar flare, we discovered the local oscillation above the loops
filled with evaporated plasma (above-the-loop-top region) and the
generation of QPFs from such oscillating regions. Unlike all previous
models for QPFs, our model includes essential physics for solar flares
such as magnetic reconnection, heat conduction, and chromospheric
evaporation. We revealed that QPFs can be spontaneously excited by
the above-the-loop-top oscillation. We found that this oscillation is
controlled by the backflow of the reconnection outflow. The new model
revealed that flare loops and the above-the-loop-top region are full
of shocks and waves, which is different from the previous expectations
based on a standard flare model and previous simulations. In this paper,
we show the QPF generation process based on our new picture of flare
loops and will briefly discuss a possible relationship between QPFs
and QPPs. Our findings will change the current view of solar flares to
a new view in which they are a very dynamic phenomenon full of shocks
and waves.
Title: A Deep-Learning Approach for Operation of an Automated Realtime
Flare Forecast
Authors: Hada-Muranushi, Yuko; Muranushi, Takayuki; Asai, Ayumi;
Okanohara, Daisuke; Raymond, Rudy; Watanabe, Gentaro; Nemoto, Shigeru;
Shibata, Kazunari
Bibcode: 2016arXiv160601587H
Altcode:
Automated forecasts serve important role in space weather science,
by providing statistical insights to flare-trigger mechanisms, and
by enabling tailor-made forecasts and high-frequency forecasts. Only
by realtime forecast we can experimentally measure the performance of
flare-forecasting methods while confidently avoiding overlearning. We
have been operating unmanned flare forecast service since August,
2015 that provides 24-hour-ahead forecast of solar flares, every 12
minutes. We report the method and prediction results of the system.
Title: Roles of Ground-based Solar Observations of Hida Observatory
toward the Solar-C Era
Authors: Ueno, S.; Shibata, K.; Ichimoto, K.; Nagata, S.; Dorotovič,
I.; Shahamatnia, E.; Ribeiro, R. A.; Fonseca, J. M.
Bibcode: 2016ASPC..504..309U
Altcode:
For the realization of the Solar-C satellite, discussions about
scientific themes and preliminary observations are internationally
carried out now. At Hida Observatory of Kyoto University, we will play
the following roles toward the Solar-C era by utilizing the Domeless
Solar Telescope (DST) and the international solar chromospherirc
full-disk observation network (CHAIN project) that includes the
Solar Magnetic Activity Research Telescope (SMART) with international
collaborations, for example, such as the development of image-analysis
software by UNINOVA (Portugal) and so on.
Title: Statistical properties of superflares on solar-type stars
based on the Kepler 1-min cadence data
Authors: Maehara, Hiroyuki; Shibayama, Takuya; Notsu, Yuta; Notsu,
Shota; Honda, Satoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2016IAUS..320..144M
Altcode: 2016arXiv160101132M
We searched for superflares on solar-type stars using the Kepler
short-cadence (1-min sampling) data in order to detect superflares
with short duration. We found 187 superflares on 23 solar-type stars
whose bolometric energy ranges from the order of 1032 erg
to 1036 erg. Using these new data combined with the results
from the data with 30-min sampling, we found the occurrence frequency
(dN/dE) of superflares as a function of flare energy (E) shows the
power-law distribution (dN/dE ~ E -α) with α=1.5 for
1033 < E < 1036 erg. The upper limit of
energy released by superflares is basically comparable to a fraction of
the magnetic energy stored near starspots which is estimated from the
amplitude of brightness variations. We also found that the duration
of superflares (τ) increases with the flare energy (E) as τ ~
E 0.39+/-0.03. This can be explained if we assume the
time-scale of flares is determined by the Alfvén time.
Title: Fractal Reconnection in Solar and Stellar Environments
Authors: Shibata, K.; Takasao, S.
Bibcode: 2016ASSL..427..373S
Altcode: 2016arXiv160609401S
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from
small scale reconnection (observed as nanoflares) to large scale
one (observed as long duration flares or giant arcades). Often the
magnetic reconnection events are associated with mass ejections or jets,
which seem to be closely related to multiple plasmoid ejections from
fractal current sheet. The bursty radio and hard X-ray emissions from
flares also suggest the fractal reconnection and associated particle
acceleration. We shall discuss recent observations and theories related
to the plasmoid-induced-reconnection and the fractal reconnection in
solar flares, and their implication to reconnection physics and particle
acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model
of solar flares to much a wider parameter space suitable for stellar
flares are also discussed.
Title: {High dispersion spectroscopy of solar-type superflare stars
with Subaru/HDS†
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2016IAUS..320..138N
Altcode:
We carried out spectroscopic observations with Subaru/HDS of 50
solar-type superflare stars found from Kepler data. More than half
(34 stars) of the target stars show no evidence of the binary system,
and we confirmed atmospheric parameters of these stars are roughly in
the range of solar-type stars. We then conducted the detailed
analyses for these 34 stars. First, the value of the ``v sin i''
(projected rotational velocity) measured from spectroscopic results is
consistent with the rotational velocity estimated from the brightness
variation. Second, there is a correlation between the amplitude of the
brightness variation and the intensity of Ca II IR triplet line. All
the targets expected to have large starspots because of their large
amplitude of the brightness variation show high chromospheric activities
compared with the Sun. These results support that the brightness
variation of superflare stars is explained by the rotation of a star
with large starspots.
Title: Solar and stellar flares and their impact on planets
Authors: Shibata, Kazunari
Bibcode: 2016IAUS..320....3S
Altcode:
Recent observations of the Sun revealed that the solar atmosphere is
full of flares and flare-like phenomena, which affect terrestrial
environment and our civilization. It has been established that
flares are caused by the release of magnetic energy through magnetic
reconnection. Many stars show flares similar to solar flares, and
such stellar flares especially in stars with fast rotation are much
more energetic than solar flares. These are called superflares. The
total energy of a solar flare is 1029 - 1032
erg, while that of a superflare is 1033 - 1038
erg. Recently, it was found that superflares (with 1034 -
1035 erg) occur on Sun-like stars with slow rotation with
frequency once in 800 - 5000 years. This suggests the possibility
of superflares on the Sun. We review recent development of solar
and stellar flare research, and briefly discuss possible impacts of
superflares on the Earth and exoplanets.
Title: UFCORIN: A fully automated predictor of solar flares in GOES
X-ray flux
Authors: Muranushi, Takayuki; Shibayama, Takuya; Muranushi, Yuko Hada;
Isobe, Hiroaki; Nemoto, Shigeru; Komazaki, Kenji; Shibata, Kazunari
Bibcode: 2015SpWea..13..778M
Altcode: 2015arXiv150708011M
We have developed UFCORIN, a platform for studying and automating
space weather prediction. Using our system we have tested 6160
different combinations of Solar Dynamic Observatory/Helioseismic and
Magnetic Imager data as input data, and simulated the prediction of
GOES X-ray flux for 2 years (2011-2012) with 1 h cadence. We have
found that direct comparison of the true skill statistic (TSS) from
small cross-validation sets is ill posed and used the standard scores
(z) of the TSS to compare the performance of the various prediction
strategies. The z of a strategy is a stochastic variable of the
stochastically chosen cross-validation data set, and the z for the
three strategies best at predicting X-, ≥M-, and ≥C-class flares
are better than the average z of the 6160 strategies by 2.3σ, 2.1σ,
and 3.8σ confidence levels, respectively. The best three TSS values
were 0.75 ± 0.07, 0.48 ± 0.02, and 0.56 ± 0.04, respectively.
Title: Numerical Study on the Emergence of Kinked Flux Tube for
Understanding of Possible Origin of δ-spot Regions
Authors: Takasao, Shinsuke; Fan, Yuhong; Cheung, Mark C. M.; Shibata,
Kazunari
Bibcode: 2015ApJ...813..112T
Altcode: 2015arXiv151102863T
We carried out an magnetohydrodynamic simulation where a subsurface
twisted kink-unstable flux tube emerges from the solar interior to the
corona. Unlike the previous expectations based on the bodily emergence
of a knotted tube, we found that the kinked tube can spontaneously
form a complex quadrupole structure at the photosphere. Due to the
development of the kink instability before the emergence, the magnetic
twist at the kinked apex of the tube is greatly reduced, although the
other parts of the tube are still strongly twisted. This leads to the
formation of a complex quadrupole structure: a pair of the coherent,
strongly twisted spots and a narrow complex bipolar pair between it. The
quadrupole is formed by the submergence of a portion of emerged magnetic
fields. This result is relevant for understanding the origin of the
complex multipolar δ-spot regions that have a strong magnetic shear
and emerge with polarity orientations not following Hale-Nicholson
and Joy Laws.
Title: Nonlinear instability and intermittent nature of magnetic
reconnection in solar chromosphere
Authors: Singh, K. A. P.; Hillier, Andrew; Isobe, Hiroaki; Shibata,
Kazunari
Bibcode: 2015PASJ...67...96S
Altcode: 2016arXiv160201999S; 2015PASJ..tmp..234S
The recent observations of Singh et al. (2012, ApJ, 759, 33) have shown
multiple plasma ejections and the intermittent nature of magnetic
reconnection in the solar chromosphere, highlighting the need for
fast reconnection to occur in highly collisional plasma. However,
the physical process through which fast magnetic reconnection occurs
in partially ionized plasma, like the solar chromosphere, is still
poorly understood. It has been shown that for sufficiently high
magnetic Reynolds numbers, Sweet-Parker current sheets can become
unstable leading to tearing mode instability and plasmoid formation,
but when dealing with a partially ionized plasma the strength of
coupling between the ions and neutrals plays a fundamental role
in determining the dynamics of the system. We propose that as the
reconnecting current sheet thins and the tearing instability develops,
plasmoid formation passes through strongly, intermediately, and weakly
coupled (or decoupled) regimes, with the time scale for the tearing
mode instability depending on the frictional coupling between ions
and neutrals. We present calculations for the relevant time scales for
fractal tearing in all three regimes. We show that as a result of the
tearing mode instability and the subsequent non-linear instability due
to the plasmoid-dominated reconnection, the Sweet-Parker current sheet
tends to have a fractal-like structure, and when the chromospheric
magnetic field is sufficiently strong the tearing instability can
reach down to kinetic scales, which are hypothesized to be necessary
for fast reconnection.
Title: High dispersion spectroscopy of solar-type superflare
stars. III. Lithium abundances†
Authors: Honda, Satoshi; Notsu, Yuta; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015PASJ...67...85H
Altcode: 2015PASJ..tmp..215H; 2015arXiv150506050H
We report on the abundance analysis of Li in solar-type (G-type main
sequence) superflare stars which were found by the analysis of Kepler
photometric data. Li is a key element to understand the evolution of the
stellar convection zone, which reflects the age of solar-type stars. We
performed the high-dispersion spectroscopy of solar-type superflare
stars with Subaru/High Dispersion Spectrograph and confirmed that
34 stars show no evidence of binarity in our previous study. In this
study, we derived the Li abundances of these 34 objects. We investigate
correlations of Li abundance with stellar atmospheric parameters,
rotational velocity, and superflare activities to understand the nature
of superflare stars and the possibility of the nucleosynthesis of Li by
superflares. We confirm the large dispersion in the Li abundance, and
the correlation with stellar parameters is not seen. When compared with
the Li abundance in the Hyades cluster, which is younger than the Sun,
it is suggested that half of the observed stars are younger than Hyades
cluster. The measured value of v sin i (projected rotational velocity)
supports that those objects are younger than the Sun. However, there
are some objects which show the low Li abundance and slowly rotate
on the basis of the estimated v sin i and P (period of brightness
variation). This result indicates that superflare stars are not only
young stars but also old stars like our Sun. In our observations, we
could not find any evidence of Li production by superflares. Further
research on Li isotope abundances of superflare stars would clarify
the issue of Li production by stellar flares.
Title: Division II: Commission 49: Interplanetary Plasma and the
Heliosphere
Authors: Gopalswamy, Natchimuthuk; Mann, Ingrid; Bougeret, Jean-Louis;
Briand, Carine; Lallement, Rosine; Lario, David; Manoharan, P. K.;
Shibata, Kazunari; Webb, David F.
Bibcode: 2015IAUTB..28..112G
Altcode:
The President of IAU Commission 49 (C49; Interplanetary Plasma and the
Heliosphere), Nat Gopalswamy, chaired the business meeting of C10,
which took place on August 23, 2012 in the venue of the IAU General
Assembly in Beijing (2:00 - 3:30 PM, Room 405).
Title: Statistical properties of superflares on solar-type stars
based on the Kepler 1-min cadence data
Authors: Maehara, Hiroyuki; Shibayama, Takuya; Notsu, Yuta; Notsu,
Shota; Honda, Satoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015IAUGA..2251867M
Altcode:
We searched for superflares on solar-type stars using the Kepler
short-cadence (1-min sampling) data in order to detect superflares
with short duration. We found 187 superflares on 23 solar-type stars
whose bolometric energy ranges from the order of 1032
erg to 1036 erg. Some superflares show multiple peaks
with the peak separation of the order of 100-1000 seconds which is
comparable to the periods of quasi-periodic pulsations in solar and
stellar flares. Using these new data combined with the results from
the data with 30 min sampling, we found the occurrence frequency
(dN/dE) of superflares as a function of flare energy (E) shows the
power-law distribution (dN/dE ∝ E-α) with α=1.5 for
1033<E<1036 erg. The average occurrence rate
of superflares with the energy of 1033 erg on Sun-like stars
(early G-dwarfs with rotation periods comparable to that of the Sun)
which is equivalent to X100 solar flares is estimated to be about once
in 500-600 years. The upper limit of energy released by superflares is
basically comparable to a fraction of the magnetic energy stored near
starspots which is estimated from the photometry. We also found that
the duration of superflares (τ) increases with the flare energy (E)
as τ ∝ E0.39±0.03. This can be explained if we assume
the time-scale of flares is determined by the Alfvén time.
Title: Solar and Stellar Flares and Their Effects on Planets
Authors: Shibata, Kazunari
Bibcode: 2015IAUGA..2255465S
Altcode:
Recent space observations of the Sun revealed that the solar atmosphere
is full of explosions, such as flares and flare-like phenomena. These
flares generate not only strong electromagnetic emissions but also
nonthermal particles and bulk plasma ejections, which sometimes lead
to geomagnetic storms and affect terrestrial environment and our
civilization, damaging satellite, power-grids, radio communication
etc. Solar flares are prototype of various explosions in our universe,
and hence are important not only for geophysics and environmental
science but also for astrophysics. The energy source of solar flares
is now established to be magnetic energy stored near sunspots. There
is now increasing observational evidence that solar flares are caused
by magnetic reconnection, merging of anti-parallel magnetic field
lines and associated magneto-plasma dynamics (Shibata and Magara
2011, Living Review). It has also been known that many stars show
flares similar to solar flares, and often such stellar flares are
much more energetic than solar flares. The total energy of a solar
flare is typically 10^29 - 10^32 erg. On the other hand, there are
much more energetic flares (10^33 - 10^38 erg) in stars, especially
in young stars. These are called superflares. We argue that these
superflares on stars can also be understood in a unified way based on
the reconnection mechanism. Finally we show evidence of occurrence of
superflares on Sun-like stars according to recent stellar observations
(Maehara et al. 2012, Nature, Shibayama et al. 2013), which revealed
that superflares with energy of 10^34 - 10^35 erg (100 - 1000 times of
the largest solar flares) occur with frequency of once in 800 - 5000
years on Sun-like stars which are very similar to our Sun. Against the
previous belief, these new observations as well as theory (Shibata et
al. 2013) suggest that we cannot deny the possibility of superflares
on the present Sun. Finally, we shall discuss possible impacts of
these superflares on the Earth as well as exoplanets around these
superflare stars.
Title: A Real Source of a Stealth CME - Energetics of a Filament
Eruption and Giant Arcade Formation
Authors: Asai, Ayumi; Ishii, Takako T.; Otsuji, Kenichi; Ichimoto,
Kiyoshi; Shibata, Kazunari
Bibcode: 2015IAUGA..2255125A
Altcode:
Various active phenomena occurring on the solar surface are sources
of disturbances in the solar-terrestrial environment. It is, however,
sometimes said that solar flares, the most energetic explosions in the
active phenome on the sun, are not crucially important for space weather
researches, but coronal mass ejections (CMEs) are more significant. This
is because not all flares are associated with CMEs, and therefore, not
geo-effective, and because geo-effective CMEs sometimes occur without
any notable active phenomena (such as flares) on the sun. The latter
is sometimes called as a “stealth CME” event. However, for even
such cases, we often see filament eruptions in H-alpha observations
and formations of giant arcade in X-ray and/or extreme ultraviolet
(EUV) observations.The geomagnetic storm with the Dst index of -105
nT that occurred on October 8, 2012 was such a stealth event. We,
on the other hand, recognize formation of an X-ray giant arcade and
activation of an H-alpha filament on October 5, 2012. We examined the
velocity field of the filament by using the H-alpha wing data obtained
with SMART telescope at Hida Observatory, Kyoto University. We also
derived the temperature and the volume emission measure by using the
X-ray and EUV data obtained by Hinode/XRT and SDO/AIA. We discuss the
energetics of this event on the solar surface
Title: High dispersion spectroscopy of solar-type superflare stars
with Subaru/HDS
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015IAUGA..2213744N
Altcode:
Superflares are very large flares that release total energy
10-104 times greater than that of the biggest solar flares
with energy of ~1032 erg. Recent Kepler-space-telescope
observations found more than 1000 superflares on a few hundred
solar-type stars (Maehara et al. 2012, Nature, 485, 478; Shibayama et
al. 2013, ApJS, 209, 5). Such superflare stars show quasi-periodic
brightness variations with the typical period of from one to a
few tens of days . Such variations are thought to be caused by the
rotation of the star with large starspots (Notsu et al. 2013, ApJ,
771, 127). However, spectroscopic observations are needed in order
toconfirm whether the variation is really due to the rotation and
whether superflares can occur on ordinary single stars similar to our
Sun.We have carried out spectroscopic observations for 50 solar-type
superflare stars with Subaru/HDS. As a result, more than half (34
stars) of the target stars show no evidence of the binary system, and
we confirmed stellar atmospheric parameters of these stars are roughly
in the range of solar-type stars on the basis of our spectroscopic
data.We then conducted the detailed analyses for these 34 stars. First,
the value of the "v sin i" (projected rotational velocity) measured
from spectroscopic results is consistent with the rotational velocity
estimated from the brightness variation. Second, there is a correlation
between the amplitude of the brightness variation and the intensity of
Ca II IR triplet line. All the targets expected to have large starspots
because of their large amplitude of the brightness variation show high
chromospheric activities compared to the Sun. These support that the
brightness variation discussed above is explained by the rotation of
a star with large starspots.Reference:Notsu et al. 2015a & 2015b,
PASJ in press (arXiv:1412.8243, 1412.8245)Nogami et al. 2014, PASJ,
66, L4Notsu et al. 2013, PASJ, 65, 112
Title: A Theoretical Model of a Thinning Current Sheet in the
Low-β Plasmas
Authors: Takeshige, Satoshi; Takasao, Shinsuke; Shibata, Kazunari
Bibcode: 2015ApJ...807..159T
Altcode: 2015arXiv150405677T
Magnetic reconnection is an important physical process in various
explosive phenomena in the universe. In previous studies, it was
found that fast reconnection takes place when the thickness of a
current sheet becomes on the order of a microscopic length such as
the ion Larmor radius or the ion inertial length. In this study, we
investigated the pinching process of a current sheet by the Lorentz
force in a low-β plasma using one-dimensional magnetohydrodynamics
(MHD) simulations. It is known that there is an exact self-similar
solution for this problem that neglects gas pressure. We compared the
non-linear MHD dynamics with the analytic self-similar solution. From
the MHD simulations, we found that with the gas pressure included the
implosion process deviates from the analytic self-similar solution as
t\to {t}0, where t0 is the explosion time when
the thickness of a current sheet of the analytic solution becomes
0. We also found that a pair of MHD fast-mode shocks is generated
and propagates after the formation of the pinched current sheet as
t\to {t}0. On the basis of the Rankine-Hugoniot relations,
we derived the scaling law of the physical quantities with respect to
the initial plasma beta in the pinched current sheet. Our study could
help us estimate the physical quantities in the pinched current sheet
formed in a low-β plasma.
Title: Magnetohydrodynamic Shocks in and above Post-flare Loops:
Two-dimensional Simulation and a Simplified Model
Authors: Takasao, Shinsuke; Matsumoto, Takuma; Nakamura, Naoki;
Shibata, Kazunari
Bibcode: 2015ApJ...805..135T
Altcode: 2015arXiv150405700T
Solar flares are an explosive phenomenon where super-sonic flows and
shocks are expected in and above the post-flare loops. To understand
the dynamics of post-flare loops, a two-dimensional magnetohydrodynamic
(2D MHD) simulation of a solar flare has been carried out. We found
new shock structures in and above the post-flare loops, which were not
resolved in the previous work by Yokoyama & Shibata. To study the
dynamics of flows along the reconnected magnetic field, the kinematics
and energetics of the plasma are investigated along selected field
lines. It is found that shocks are crucial to determine the thermal
and flow structures in the post-flare loops. On the basis of the 2D MHD
simulation, we developed a new post-flare loop model, which we defined
as the pseudo-2D MHD model. The model is based on the one-dimensional
(1D) MHD equations, where all variables depend on one space dimension,
and all the three components of the magnetic and velocity fields
are considered. Our pseudo-2D model includes many features of the
multi-dimensional MHD processes related to magnetic reconnection
(particularly MHD shocks), which the previous 1D hydrodynamic models are
not able to include. We compared the shock formation and energetics of
a specific field line in the 2D calculation with those in our pseudo-2D
MHD model, and found that they give similar results. This model will
allow us to study the evolution of the post-flare loops in a wide
parameter space without expensive computational cost or neglecting
important physics associated with magnetic reconnection.
Title: High dispersion spectroscopy of solar-type superflare
stars. I. Temperature, surface gravity, metallicity, and vsin i
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015PASJ...67...32N
Altcode: 2014arXiv1412.8243N; 2015PASJ..tmp..162N
We conducted high-dispersion spectroscopic observations of 50
superflare stars with Subaru High Dispersion Spectrograph (HDS),
and measured the stellar parameters of them. These 50 targets were
selected from the solar-type (G-type main sequence) superflare stars
that we had discovered from the Kepler photometric data. As a result
of these spectroscopic observations, we found that more than half
(34) of our 50 targets have no evidence of binary systems. We then
estimated the effective temperature (Teff), surface gravity
(log g), metallicity ([Fe/H]), and projected rotational velocity (vsin
i) of these 34 superflare stars on the basis of our spectroscopic
data. The accuracy of our estimations is higher than that of the
Kepler Input Catalog (KIC) values, and the differences between our
values and KIC values [(ΔTeff)rms ∼ 219 K,
(Δlog g)rms ∼ 0.37 dex, and (Δ[Fe/H])rms
∼ 0.46 dex] are comparable to the large uncertainties and systematic
differences of KIC values reported by the previous researchers. We
confirmed that the estimated Teff and log g values of the
34 superflare stars are roughly in the range of solar-type stars. In
particular, these parameters and the brightness variation period
(P0) of nine of the stars are in the range of "Sun-like"
stars (5600 ≤ Teff ≤ 6000 K, log g ≥ 4.0, and
P0 > 10 d). Five of the 34 target stars are fast rotators
(vsin i ≥ 10 km s-1), while 22 stars have relatively low
vsin i values (vsin i < 5 km s-1). These results suggest
that stars that have spectroscopic properties similar to the Sun can
have superflares, and this supports the hypothesis that the Sun might
cause a superflare.
Title: High dispersion spectroscopy of solar-type superflare
stars. II. Stellar rotation, starspots, and chromospheric activities
Authors: Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota;
Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015PASJ...67...33N
Altcode: 2015PASJ..tmp..177N; 2014arXiv1412.8245N
We conducted high dispersion spectroscopic observations of 50 superflare
stars with Subaru/HDS. These 50 stars were selected from the solar-type
superflare stars that we had discovered from the Kepler data. More than
half (34 stars) of these 50 target superflare stars show no evidence
of binarity, and we estimated stellar parameters of these 34 stars in
our previous study (Notsu et al. 2015, PASJ, 67, 32). According to our
previous studies using Kepler data, superflare stars show quasi-periodic
brightness variations whose amplitude (0.1%-10%) is much larger than
that of the solar brightness variations (0.01%-0.1%) caused by the
existence of sunspots on the rotating solar surface. In this study,
we investigated whether these quasi-periodic brightness variations of
superflare stars are explained by the rotation of a star with fairly
large starspots, by using stellar parameters derived in Paper I. First,
we confirmed that the value of the projected rotational velocity, v
sin i, is consistent with the rotational velocity estimated from the
period of the brightness variation. Next, we measured the intensity
of Ca II infrared triplet lines and Hα line, good indicators of the
stellar chromospheric activity, and compared them with other stellar
properties. The intensity of Ca II infrared triplet lines indicates
that the mean magnetic field strength (<fB>) of the target
superflare stars can be higher than that of the Sun. A correlation
between the amplitude of the brightness variation and the intensity of
Ca II triplet line was found. All the targets expected to have large
starspots because of their large amplitude of the brightness variation
show high chromospheric activities compared to the Sun. These results
support the idea that the brightness variation of superflare stars is
due to the rotation with large starspots.
Title: Statistical properties of superflares on solar-type stars
based on 1-min cadence data
Authors: Maehara, Hiroyuki; Shibayama, Takuya; Notsu, Yuta; Notsu,
Shota; Honda, Satoshi; Nogami, Daisaku; Shibata, Kazunari
Bibcode: 2015EP&S...67...59M
Altcode: 2015arXiv150400074M
We searched for superflares on solar-type stars using Kepler data with
1-min sampling in order to detect superflares with a short duration. We
found 187 superflares on 23 solar-type stars whose bolometric energy
ranges from the order of 1032 to 1036 erg. Some
superflares show multiple peaks with the peak separation of the order
of 100 to 1,000 s which is comparable to the periods of quasi-periodic
pulsations in solar and stellar flares. Using these new data combined
with the results from the data with 30-min sampling, we found that the
occurrence frequency (dN/dE) of superflares as a function of flare
energy ( E) shows the power-law distribution (dN/dE∝ E - α
) with α∼-1.5 for 1033< E<1036
erg which is consistent with the previous results. The average
occurrence rate of superflares with the energy of 1033
erg which is equivalent to X100 solar flares is about once in 500
to 600 years. The upper limit of energy released by superflares is
basically comparable to a fraction of the magnetic energy stored near
starspots which is estimated from the photometry. We also found that
the duration of superflares ( τ) increases with the flare energy (
E) as τ∝ E 0.39 ± 0.03. This can be explained if we
assume the time scale of flares is determined by the Alfvén time.
Title: Prominence Activation By Coronal Fast Mode Shock
Authors: Takahashi, Takuya; Asai, Ayumi; Shibata, Kazunari
Bibcode: 2015ApJ...801...37T
Altcode: 2015arXiv150101592T
An X5.4 class flare occurred in active region NOAA11429 on 2012 March
7. The flare was associated with a very fast coronal mass ejection
(CME) with a velocity of over 2500 km s-1. In the images
taken with the Solar Terrestrial Relations Observatory-B/COR1, a
dome-like disturbance was seen to detach from an expanding CME bubble
and propagated further. A Type-II radio burst was also observed at the
same time. On the other hand, in extreme ultraviolet images obtained
by the Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA),
the expanding dome-like structure and its footprint propagating to the
north were observed. The footprint propagated with an average speed
of about 670 km s-1 and hit a prominence located at the
north pole and activated it. During the activation, the prominence was
strongly brightened. On the basis of some observational evidence, we
concluded that the footprint in AIA images and the ones in COR1 images
are the same, that is, the MHD fast mode shock front. With the help
of a linear theory, the fast mode Mach number of the coronal shock is
estimated to be between 1.11 and 1.29 using the initial velocity of the
activated prominence. Also, the plasma compression ratio of the shock
is enhanced to be between 1.18 and 2.11 in the prominence material,
which we consider to be the reason for the strong brightening of the
activated prominence. The applicability of linear theory to the shock
problem is tested with a nonlinear MHD simulation.
Title: Superflare Occurrence and Energies on G-, K-, and M-type Dwarfs
Authors: Candelaresi, S.; Hillier, A.; Maehara, H.; Brandenburg, A.;
Shibata, K.
Bibcode: 2014ApJ...792...67C
Altcode: 2014arXiv1405.1453C
Kepler data from G-, K-, and M-type stars are used to study conditions
that lead to superflares with energies above 1034 erg. From
the 117,661 stars included, 380 show superflares with a total of 1690
such events. We study whether parameters, like effective temperature
or rotation rate, have any effect on the superflare occurrence
rate or energy. With increasing effective temperature we observe a
decrease in the superflare rate, which is analogous to the previous
findings of a decrease in dynamo activity with increasing effective
temperature. For slowly rotating stars, we find a quadratic increase
of the mean occurrence rate with the rotation rate up to a critical
point, after which the rate decreases linearly. Motivated by standard
dynamo theory, we study the behavior of the relative starspot coverage,
approximated as the relative brightness variation. For faster rotating
stars, an increased fraction of stars shows higher spot coverage,
which leads to higher superflare rates. A turbulent dynamo is used
to study the dependence of the Ohmic dissipation as a proxy of the
flare energy on the differential rotation or shear rate. The resulting
statistics of the dissipation energy as a function of dynamo number is
similar to the observed flare statistics as a function of the inverse
Rossby number and shows similarly strong fluctuations. This supports
the idea that superflares might well be possible for solar-type G stars.
Title: Progress and challenges in advanced ground-based
gravitational-wave detectors
Authors: Adier, M.; Aguilar, F.; Akutsu, T.; Arain, M. A.; Ando,
M.; Anghinolfi, L.; Antonini, P.; Aso, Y.; Barr, B. W.; Barsotti,
L.; Beker, M. G.; Bell, A. S.; Bellon, L.; Bertolini, A.; Blair, C.;
Blom, M. R.; Bogan, C.; Bond, C.; Bortoli, F. S.; Brown, D.; Buchler,
B. C.; Bulten, H. J.; Cagnoli, G.; Canepa, M.; Carbone, L.; Cesarini,
E.; Champagnon, B.; Chen, D.; Chincarini, A.; Chtanov, A.; Chua,
S. S. Y.; Ciani, G.; Coccia, E.; Conte, A.; Cortese, M.; Daloisio,
M.; Damjanic, M.; Day, R. A.; De Ligny, D.; Degallaix, J.; Doets,
M.; Dolique, V.; Dooley, K.; Dwyer, S.; Evans, M.; Factourovich,
M.; Fafone, V.; Farinon, S.; Feldbaum, D.; Flaminio, R.; Forest,
D.; Frajuca, C.; Frede, M.; Freise, A.; Fricke, T.; Friedrich, D.;
Fritschel, P.; Frolov, V. V.; Fulda, P.; Geitner, M.; Gemme, G.;
Gleason, J.; Goßler, S.; Gordon, N.; Gräf, C.; Granata, M.; Gras,
S.; Gross, M.; Grote, H.; Gustafson, R.; Hanke, M.; Heintze, M.;
Hennes, E.; Hild, S.; Huttner, S. H.; Ishidoshiro, K.; Izumi, K.;
Kawabe, K.; Kawamura, S.; Kawazoe, F.; Kasprzack, M.; Khalaidovski,
A.; Kimura, N.; Koike, S.; Kume, T.; Kumeta, A.; Kuroda, K.; Kwee, P.;
Lagrange, B.; Lam, P. K.; Landry, M.; Leavey, S.; Leonardi, M.; Li,
T.; Liu, Z.; Lorenzini, M.; Losurdo, G.; Lumaca, D.; Macarthur, J.;
Magalhaes, N. S.; Majorana, E.; Malvezzi, V.; Mangano, V.; Mansell,
G.; Marque, J.; Martin, R.; Martynov, D.; Mavalvala, N.; McClelland,
D. E.; Meadors, G. D.; Meier, T.; Mermet, A.; Michel, C.; Minenkov,
Y.; Mow-Lowry, C. M.; Mudadu, L.; Mueller, C. L.; Mueller, G.; Mul,
F.; Nanda Kumar, D.; Nardecchia, I.; Naticchioni, L.; Neri, M.; Niwa,
Y.; Ohashi, M.; Okada, K.; Oppermann, P.; Pinard, L.; Poeld, J.; Prato,
M.; Prodi, G. A.; Puncken, O.; Puppo, P.; Quetschke, V.; Reitze, D. H.;
Risson, P.; Rocchi, A.; Saito, N.; Saito, Y.; Sakakibara, Y.; Sassolas,
B.; Schimmel, A.; Schnabel, R.; Schofield, R. M. S.; Schreiber, E.;
Sequino, V.; Serra, E.; Shaddock, D. A.; Shoda, A.; Shoemaker, D. H.;
Shibata, K.; Sigg, D.; Smith-Lefebvre, N.; Somiya, K.; Sorazu, B.;
Stefszky, M. S.; Strain, K. A.; Straniero, N.; Suzuki, T.; Takahashi,
R.; Tanner, D. B.; Tellez, G.; Theeg, T.; Tokoku, C.; Tsubono, K.;
Uchiyama, T.; Ueda, S.; Vahlbruch, H.; Vajente, G.; Vorvick, C.;
van den Brand, J. F. J.; Wade, A.; Ward, R.; Wessels, P.; Williams,
L.; Willke, B.; Winkelmann, L.; Yamamoto, K.; Zendri, J. -P.
Bibcode: 2014GReGr..46.1749A
Altcode:
The Amaldi 10 Parallel Session C3 on Advanced Gravitational Wave
detectors gave an overview of the status and several specific challenges
and solutions relevant to the instruments planned for a mid-decade
start of observation. Invited overview talks for the Virgo, LIGO,
and KAGRA instruments were complemented by more detailed discussions
in presentations and posters of some instrument features and designs.
Title: A Chain of Winking (Oscillating) Filaments Triggered by an
Invisible Extreme-ultraviolet Wave
Authors: Shen, Yuandeng; Ichimoto, Kiyoshi; Ishii, Takako T.; Tian,
Zhanjun; Zhao, Ruijuan; Shibata, Kazunari
Bibcode: 2014ApJ...786..151S
Altcode: 2014arXiv1403.7705S
Winking (oscillating) filaments have been observed for many
years. However, observations of successive winking filaments in
one event have not yet been reported. In this paper, we present the
observations of a chain of winking filaments and a subsequent jet that
are observed right after the X2.1 flare in AR11283. The event also
produced an extreme-ultraviolet (EUV) wave that has two components: an
upward dome-like wave (850 km s-1) and a lateral surface
wave (554 km s-1) that was very weak (or invisible)
in imaging observations. By analyzing the temporal and spatial
relationships between the oscillating filaments and the EUV waves,
we propose that all the winking filaments and the jet were triggered
by the weak (or invisible) lateral surface EUV wave. The oscillation
of the filaments last for two or three cycles, and their periods,
Doppler velocity amplitudes, and damping times are 11-22 minutes,
6-14 km s-1, and 25-60 minutes, respectively. We further
estimate the radial component magnetic field and the maximum kinetic
energy of the filaments, and they are 5-10 G and ~1019 J,
respectively. The estimated maximum kinetic energy is comparable to the
minimum energy of ordinary EUV waves, suggesting that EUV waves can
efficiently launch filament oscillations on their path. Based on our
analysis results, we conclude that the EUV wave is a good agent for
triggering and connecting successive but separated solar activities
in the solar atmosphere, and it is also important for producing solar
sympathetic eruptions.
Title: Two sun-like superflare stars rotating as slow as the Sun*
Authors: Nogami, Daisaku; Notsu, Yuta; Honda, Satoshi; Maehara,
Hiroyuki; Notsu, Shota; Shibayama, Takuya; Shibata, Kazunari
Bibcode: 2014PASJ...66L...4N
Altcode: 2014arXiv1402.3772N
We report on the results of high dispersion spectroscopy
of two "superflare stars," KIC 9766237 and KIC 9944137 with
Subaru/HDS. Superflare stars are G-type main sequence stars, but show
gigantic flares compared to the Sun, which have recently been discovered
in the data obtained with the Kepler spacecraft. Though most of these
stars are thought to have a rotation period shorter than 10 d on the
basis of photometric variabilities, the two targets of the present
paper are estimated to have rotation periods of 21.8 d and 25.3 d. Our
spectroscopic results clarified that these stars have stellar parameters
similar to those of the Sun in terms of the effective temperature,
surface gravity, and metallicity. The projected rotational velocities
derived by us are consistent with the photometric rotation period,
indicating a fairly high inclination angle. The average strength of
the magnetic field on the surface of these stars are estimated to be
1-20 G, by using the absorption line of Ca II 8542. We could not detect
any hint of binarity in our spectra, although more data are needed to
firmly rule out the presence of an unseen low-mass companion. These
results claim that the spectroscopic properties of these superflare
stars are very close to those of the Sun, and support the hypothesis
that the Sun might cause a superflare.
Title: Superflares on Late-Type Stars
Authors: Maehara, Hiroyuki; Shibayama, Takuya; Notsu, Yuta; Notsu,
Shota; Nagao, Tanashi; Honda, Satoshi; Nogami, Daisaku; Shibata,
Kazunari
Bibcode: 2014IAUS..293..393M
Altcode:
We present the results of an extensive survey of superflares on
late-type stars (G, K, and M-type main sequence stars) using the Kepler
satellite data. Wefound about 6,800 superflares on late-type stars
from the data of about 120,000 stars observed over 500 days. The
total bolometric energy of superflares in oursample ranges from
1032 erg to 1036 erg. Our data suggest that the
occurrencefrequency of superflares depends on the surface temperature
and the rotationperiod of stars. Superflares on M-type stars occur
about 10-100 times morefrequently than those on G-type stars. Our
results suggest that the average frequency ofsuperflares releasing
1034-1035 erg of energy (100-1,000 times larger
than the largestsolar flares) on M-type stars and Sun-like stars is
once in 10 years and once in a few thousand years respectively.
Title: International Collaboration and Academic Exchange of the
CHAIN Project in this Three Years (Period)
Authors: Ueno, Satoru; Shibata, Kazunari; Morita, Satoshi; Kimura,
Goichi; Asai, Ayumi; Kitai, Reizaburo; Ichimoto, Kiyoshi; Nagata,
Shin'ichi; Ishii, Takako; Nakatani, Yoshikazu; Masashi, Yamaguchi;
et al.
Bibcode: 2014SunGe...9...97U
Altcode:
We will introduce contents of international collaboration and
academic exchange of the CHAIN project in recent three years (ISWI
period). After April of 2010, we have not obtained any enough budget
for new instruments. Therefore, we have not been able to install new
Flare Monitoring Telescopes (FMT) in new countries, such as Algeria. On
the other hand, however, we have continued international academic
exchange through scientific and educational collaboration with mainly
Peru, such as data-analysis training, holding scientific workshops
etc. Additionally, in this year, King Saudi University of Saudi Arabia
and CRAAG of Algeria have planned to build a new FMT in their university
by their own budget. Therefore, we have started some collaboration in
the field of technical advices of instruments and scientific themes
etc. Moreover, Pakistan Space and Upper Atmosphere Research Commission
(SUPARCO) also offered us participation in the CHAIN-project. We would
like to continue to consider the possibility of academic collaboration
with such new positive developing nations, too.
Title: Within the International Collaboration CHAIN: a Summary of
Events Observed with Flare Monitoring Telescope (FMT) in Peru
Authors: Ishitsuka, J.; Asai, A.; Morita, S.; Terrazas, R.; Cabezas,
D.; Gutierrez, V.; Martinez, L.; Buleje, Y.; Loayza, R.; Nakamura,
N.; Takasao, S.; Yoshinaga, Y.; Hillier, A.; Otsuji, K.; Shibata, K.;
Ishitsuka, M.; Ueno, S.; Kitai, R.; Ishii, T.; Ichimoto, K.; Nagata,
S.; Narukage, N.
Bibcode: 2014SunGe...9...85I
Altcode:
In 2008 we inaugurated the new Solar Observatory in collaboration with
Faculty of Sciences of San Luis Gonzaga de Ica National University,
300 km south of Lima. In March of 2010 a Flare Monitoring Telescope
of Hida Observatory of Kyoto University arrived to Ica, part of CHAIN
Project (Continuous H-alpha Imaging Network). In October of the same
year we hosted the First FMT Workshop in Ica, then in July of 2011 the
Second FMT Workshop was opened. Since that we are focused on two events
registered by FMT in Peru to publish results. FMT is a good tool to
introduce young people from universities into scientific knowledge;
it is good also for education in Solar Physics and outreach. Details
of this successful collaboration will be explained in this presentation.
Title: The Role of a Plasmoid Ejection in Magnetic Reconnection in
Solar Flares
Authors: Nishida, Keisuke; Nishizuka, Naoto; Shibata, Kazunari
Bibcode: 2014cosp...40E2289N
Altcode:
The plasmoid-induced reconnection model explains how a plasmoid ejection
(flux rope eruption) leads fast reconnection in various scales. In
this model, a plasmoid ejection induces reconnection inflow to the
reconnection site, and enhances reconnection current, finally leading to
fast reconnection. This model has been verified in solar observations,
laboratory experiments, and 2-dimensional (2D) magnetohydrodynamic
(MHD) simulations. To answer the question of whether this is valid even
for 3D configuration, we performed a 3D resistive MHD simulation of a
solar flare by simply extending We succeeded in reproducing a current
sheet and bi-directional reconnection outflows just below the flux
rope during the eruption in our 3D simulations. We found that there is
a positive feedback between the ejection speed of a flux rope and the
reconnection rate both in the 2D and 3D simulations, and we conclude
that the plasmoid-induced reconnection model can be applied to 3D. We
also found that small scale plasmoids are formed inside a current
sheet and make it turbulent. These small scale plasmoid ejections have
a role in locally increasing the reconnection rate intermittently as
observed in solar flares, coupled with a global eruption of a flux rope.
Title: The interplanetary causes of geomagnetic activity during the
7-17 March 2012 interval: a CAWSES II overview
Authors: Tsurutani, Bruce T.; Echer, Ezequiel; Shibata, Kazunari;
Verkhoglyadova, Olga P.; Mannucci, Anthony J.; Gonzalez, Walter D.;
Kozyra, Janet U.; Pätzold, Martin
Bibcode: 2014JSWSC...4A..02T
Altcode:
This overview paper presents/discusses the major solar, interplanetary,
magnetospheric, and ionospheric features of the CAWSES II interval of
study: 7-17 March 2012. Magnetic storms occurred on 7, 9, 12, and 15
March with peak SYM-H intensities of -98 nT, -148 nT, -75 nT (pressure
corrected), and -79 nT, respectively. These are called the S1, S2,
S3, and S4 events. Although three of the storm main phases (S1, S3,
and S4) were caused by IMF Bsouth sheath fields and the S2
event was associated with a magnetic cloud (MC), the detailed scenario
for all four storms were different. Two interplanetary features with
unusually high temperatures and intense and quiet magnetic fields were
identified located antisunward of the MCs (S2 and S3). These features
are signatures of either coronal loops or coronal sheaths. A high speed
stream (HSS) followed the S4 event where the presumably southward IMF Bz
components of the Alfvén waves extended the storm "recovery phase" by
several days. The ICME-associated shocks were particularly intense. The
fast forward shock for the S2 event had a magnetosonic Mach number of
~9.4, the largest in recorded history. All of the shocks associated
with the ICMEs created sudden impulses (SI+s) at Earth. The
shocks preceding the S2 and S3 magnetic storms caused unusually high
SI+ intensities of ~60 and 68 nT, respectively. Many further
studies on various facets of this active interval are suggested for
CAWSES II researchers and other interested parties.
Title: Multi-Wavelength Observations of Large Amplitude Prominence
Oscillations
Authors: Shen, Yuandeng; Shibata, Kazunari; Ichimoto, Kiyoshi; Liu, Yu
Bibcode: 2014cosp...40E3016S
Altcode:
Multi-Wavelength observations of large amplitude prominence oscillations
are important in diagnosing the physical property and eruption
mechanism of prominences, as well as their ambient coronal magnetic
fields. Such studies has led to a new discipline dubbed ‘’Prominence
Seismology’’. However, up to the present, high-resolution and
multi-wavelength observations of large amplitude oscillations are very
scarce. Using high-resolution spectroscopic Halpha observations taken
by the Solar Magnetic Activity Research Telescope (SMART) and the
Solar Dynamics Observatory, we studied a series of intriguing large
amplitude prominence (filament) oscillation events. We find that large
amplitude horizontal and vertical prominence oscillations are often
launched by large-scale shock waves associated with remote flares,
while large amplitude longitudinal prominence oscillations are often
associated with nearby micro jets or flare activities. Sometimes,
longitudinal oscillations can also be launched by large-scale
shocks. With the spectroscopic observations taken by the SMART, we can
measure the Doppler velocity and even the three-dimensional velocity
of the oscillations, with the so-called ``Clould Model’’. The
oscillation period, amplitude, and damping time are also determined
from the Halpha observations. These prominence parameters are used to
estimate the magnetic fields of the prominence and the surrounding
corona using the method of prominence seismology. Other property of
large amplitude oscillation prominences such as restoring forces and
damping mechanisms are also discussed in our study.
Title: Superflares on Solar type Stars and Their Impacts on
Habitability of Exoplanets
Authors: Shibata, Kazunari; Maehara, Hiroyuki; Shibayama, Takuya;
Notsu, Yuta; Notsu, Shota; Honda, Satoshi; Nogami, Daisaku; Isobe,
Hiroaki
Bibcode: 2014cosp...40E3035S
Altcode:
Using Kepler data, Maehara et al. (2012) have discovered 365 superflares
(10(34) -10(36) erg) on 148 solar type stars (G type dwarfs). They
revealed that the occurrence frequency of superflares of 10(34) erg is
once in 800 years, and that of 10(35) erg is once in 5000 years. It was
also found that these superflare stars show quasi-periodic brightness
variation, which can be interpreted as a result of rotation of stars
with large star spots (Notsu Y. et al. 2013). This interpretation
is consistent with theory of solar flares and dynamo (Shibata et
al. 2013). Furthermore, there were no evidence of hot Jupiters around
these superflare stars, suggesting the possibility that superflares may
occur on our Sun without hot Jupiters. Superflares tend to occur more in
cooler stars (K and M type dwarfs) than G type dwarfs. More recently,
Shibayama et al. (2013) extended Maehara et al.'s work to find 1547
superflares on 279 solar type stars from 500 days Kepler data. They
basically confirmed the results of Maehara et al., but found that in
some G-type dwarfs the occurrence rate of superflares was extremely
high, 57 superflares in 500 days (i.e., once in 10 days). Such an
extreme superflare activity would give a strong influence on the
environmental condition of exoplanets around these stars. We shall
discuss implication of these observations for habitability of exoplanets
around solar type stars. References Maehara et al. (2012) Nature 485,
478; Shibata et al. (2013) PASJ 65, 49; Shibayama et al. (2013) ApJS
209, 5; Notsu, Y. et al. (2013) ApJ 771,127; Notsu, S. et al. (2013)
PASJ 65, 112
Title: VizieR Online Data Catalog: Superflares of Kepler
stars. I. (Shibayama+, 2013)
Authors: Shibayama, T.; Maehara, H.; Notsu, S.; Notsu, Y.; Nagao,
T.; Honda, S.; Ishii, T. T.; Nogami, D.; Shibata, K.
Bibcode: 2013yCat..22090005S
Altcode:
Kepler carries an optical telescope with a 95cm aperture and
105deg2< field-of-view (about 12° diameter), which
is in Cygnus, Lyra, and Draco. This spacecraft is designed to obtain
high-precision and long-period light curves of many stars. The typical
precision is 0.1mmag for a star of 12mag and the number of observed
stars is more than 160000 (Koch et al. 2010ApJ...713L..79K). The
data we used were taken during the period from 2009 May to 2010
September. We retrieved the data from the Multimission Archive at
the Space Telescope Science Institute and analyzed the long-cadence
(the time resolution is about 30 minutes) corrected flux of 9511,
75598, 82811, 82586, 89188, 86248, and 82052 stars in quarters 0, 1,
2, 3, 4, 5, and 6, respectively (all public light curves of G-type
dwarfs observed by Kepler). Since the aim of this study is to detect
superflares on G-type dwarfs, we selected G-type dwarfs in all of
the observed stars using the Kepler Input Catalog (Brown et al. 2011,
Cat. J/AJ/142/112). The condition is 5100K<=Teff<6000K
and logg>4.0 and the number of selected stars is about 80000 in
160000. (2 data files).
Title: Superflares on Solar-type Stars Observed with
Kepler. I. Statistical Properties of Superflares
Authors: Shibayama, Takuya; Maehara, Hiroyuki; Notsu, Shota; Notsu,
Yuta; Nagao, Takashi; Honda, Satoshi; Ishii, Takako T.; Nogami,
Daisaku; Shibata, Kazunari
Bibcode: 2013ApJS..209....5S
Altcode: 2013arXiv1308.1480S
By extending our previous study by Maehara et al., we searched for
superflares on G-type dwarfs (solar-type stars) using Kepler data
for a longer period (500 days) than that (120 days) in our previous
study. As a result, we found 1547 superflares on 279 G-type dwarfs,
which is much more than the previous 365 superflares on 148 stars. Using
these new data, we studied the statistical properties of the occurrence
rate of superflares, and confirmed the previous results, i.e., the
occurrence rate (dN/dE) of superflares versus flare energy (E) shows
a power-law distribution with dN/dEvpropE -α, where α ~
2. It is interesting that this distribution is roughly similar to that
for solar flares. In the case of the Sun-like stars (with surface
temperature 5600-6000 K and slowly rotating with a period longer
than 10 days), the occurrence rate of superflares with an energy of
1034-1035 erg is once in 800-5000 yr. We also
studied long-term (500 days) stellar brightness variation of these
superflare stars and found that in some G-type dwarfs the occurrence
rate of superflares was extremely high, ~57 superflares in 500 days
(i.e., once in 10 days). In the case of Sun-like stars, the most active
stars show a frequency of one superflare (with 1034 erg) in
100 days. There is evidence that these superflare stars have extremely
large starspots with a size about 10 times larger than that of the
largest sunspot. We argue that the physical origin of the extremely
high occurrence rate of superflares in these stars may be attributed
to the existence of extremely large starspots.
Title: High-Dispersion Spectroscopy of the Superflare Star KIC 6934317
Authors: Notsu, Shota; Honda, Satoshi; Notsu, Yuta; Nagao, Takashi;
Shibayama, Takuya; Maehara, Hiroyuki; Nogami, Daisaku; Shibata,
Kazunari
Bibcode: 2013PASJ...65..112N
Altcode: 2013arXiv1307.4929N
We conducted high-resolution spectroscopic observation with Subaru/HDS
for a G-type star (KIC 6934317). We selected this star from the
data of the Kepler spacecraft. It produces many superflares, and the
total energy of the largest recorded superflare on this star was ∼
103-times larger (∼ 2.2 × 1035 erg) than
that of the most energetic flare on the Sun (∼ 1032
erg). The core depth and the emission flux of the Ca II infrared
triplet lines and the Hα line show high chromospheric activity in
this star, in spite of its low lithium abundance and small amplitude
of the rotational modulation. Using empirical relations between the
emission flux of chromospheric lines and the X-ray flux, this star
is considered to show much higher coronal activity than that of the
Sun. It probably has large starspots that can store a large amount
of magnetic energy, sufficient to give rise to superflares. We also
estimated the stellar parameters, such as the effective temperature,
surface gravity, metallicity, projected rotational velocity (v sin i),
and radial velocity. KIC 6934317 is then confirmed to be an early G-type
main-sequence star. The value of v sin i is estimated to be ∼ 1.91
km s-1. In contrast, the rotational velocity is calculated
to be ∼ 20 km s-1 by using the period of the brightness
variation as the rotation period. This difference can be explained by
its small inclination angle (nearly pole-on). The small inclination
angle is also supported by the contrast between the large superflare
amplitude and the small stellar brightness variation amplitude. The
lithium abundance and isochrones implies that the age of this star is
more than about a few Gyr, though a problem why this star with such
an age has such strong activity remains unsolved.
Title: The Role of a Flux Rope Ejection in a Three-dimensional
Magnetohydrodynamic Simulation of a Solar Flare
Authors: Nishida, Keisuke; Nishizuka, Naoto; Shibata, Kazunari
Bibcode: 2013ApJ...775L..39N
Altcode: 2013arXiv1308.0442N
We investigated the dynamic evolution of a three-dimensional (3D) flux
rope eruption and magnetic reconnection process in a solar flare by
simply extending the two-dimensional (2D) resistive magnetohydrodynamic
simulation model of solar flares with low β plasma to a 3D model. We
succeeded in reproducing a current sheet and bi-directional reconnection
outflows just below the flux rope during the eruption in our 3D
simulations. We calculated four cases of a strongly twisted flux rope
and a weakly twisted flux rope in 2D and 3D simulations. The time
evolution of a weakly twisted flux rope in the 3D simulation shows
behaviors similar to those of the 2D simulation, while a strongly
twisted flux rope in the 3D simulation clearly shows a different
time evolution from the 2D simulation except for the initial phase
evolution. The ejection speeds of both strongly and weakly twisted
flux ropes in 3D simulations are larger than in the 2D simulations,
and the reconnection rates in 3D cases are also larger than in the 2D
cases. This indicates positive feedback between the ejection speed of
a flux rope and the reconnection rate even in the 3D simulation, and we
conclude that the plasmoid-induced reconnection model can be applied to
3D. We also found that small-scale plasmoids are formed inside a current
sheet and make it turbulent. These small-scale plasmoid ejections have
a role in locally increasing the reconnection rate intermittently as
observed in solar flares, coupled with a global eruption of a flux rope.
Title: Diffraction, Refraction, and Reflection of an
Extreme-ultraviolet Wave Observed during Its Interactions with Remote
Active Regions
Authors: Shen, Yuandeng; Liu, Yu; Su, Jiangtao; Li, Hui; Zhao, Ruijuan;
Tian, Zhanjun; Ichimoto, Kiyoshi; Shibata, Kazunari
Bibcode: 2013ApJ...773L..33S
Altcode: 2013arXiv1307.6098S
We present observations of the diffraction, refraction, and reflection
of a global extreme-ultraviolet (EUV) wave propagating in the solar
corona. These intriguing phenomena are observed when the wave interacts
with two remote active regions, and together they exhibit properties
of an EUV wave. When the wave approached AR11465, it became weaker
and finally disappeared in the active region, but a few minutes
later a new wavefront appeared behind the active region, and it was
not concentric with the incoming wave. In addition, a reflected wave
was also simultaneously observed on the wave incoming side. When the
wave approached AR11459, it transmitted through the active region
directly and without reflection. The formation of the new wavefront
and the transmission could be explained with diffraction and refraction
effects, respectively. We propose that the different behaviors observed
during the interactions may be caused by different speed gradients at
the boundaries of the two active regions. We find that the EUV wave
formed ahead of a group of expanding loops a few minutes after the
start of the loops' expansion, which represents the initiation of
the associated coronal mass ejection (CME). Based on these results,
we conclude that the EUV wave should be a nonlinear magnetosonic wave
or shock driven by the associated CME, which propagated faster than
the ambient fast mode speed and gradually slowed down to an ordinary
linear wave. Our observations support the hybrid model that includes
both fast wave and slow non-wave components.
Title: Superflares on Solar-type Stars Observed with Kepler
II. Photometric Variability of Superflare-generating Stars: A
Signature of Stellar Rotation and Starspots
Authors: Notsu, Yuta; Shibayama, Takuya; Maehara, Hiroyuki; Notsu,
Shota; Nagao, Takashi; Honda, Satoshi; Ishii, Takako T.; Nogami,
Daisaku; Shibata, Kazunari
Bibcode: 2013ApJ...771..127N
Altcode: 2013arXiv1304.7361N
We performed simple spot-model calculations for quasi-periodic
brightness variations of solar-type stars showing superflares
using Kepler photometric data. Most of the superflare stars show
quasi-periodic brightness modulations with a typical period of one
to a few tens of days. Our results indicate that these brightness
variations can be explained by the rotation of a star with fairly large
starspots. Using the results of the period analysis, we investigated
the relation between the energy and frequency of superflares and the
rotation period. Stars with relatively slower rotation rates can still
produce flares that are as energetic as those of more rapidly rotating
stars although the average flare frequency is lower for more slowly
rotating stars. We found that the energy of superflares is related
to the total coverage of the starspot. The correlation between the
spot coverage and the flare energy in superflares is similar to that
in solar flares. These results suggest that the energy of superflares
can be explained by the magnetic energy stored around the starspots.
Title: Can Superflares Occur on Our Sun?
Authors: Shibata, Kazunari; Isobe, Hiroaki; Hillier, Andrew; Choudhuri,
Arnab Rai; Maehara, Hiroyuki; Ishii, Takako T.; Shibayama, Takuya;
Notsu, Shota; Notsu, Yuta; Nagao, Takashi; Honda, Satoshi; Nogami,
Daisaku
Bibcode: 2013PASJ...65...49S
Altcode: 2012arXiv1212.1361S
Recent observations of Sun-like stars, similar to our Sun in their
surface temperature (5600-6000 K) and slow rotation (rotational period
> 10 d), using the Kepler satellite by Maehara et al. (2012, Nature,
485, 478) have revealed the existence of superflares (with energy
of 1033-1035 erg). From statistical analyses
of these superflares, it was found that superflares with energy
of 1034 erg occur once in 800 yr, and superflares with
1035 erg occur once in 5000 yr. In this paper, we examine
whether superflares with energy of 1033-1035
erg could occur on the present Sun through the use of simple
order-of-magnitude estimates based on current ideas related to
the mechanisms of the solar dynamo. If magnetic flux is generated
by differential rotation at the base of the convection zone, as
assumed in typical dynamo models, it is possible that the present Sun
would generate a large sunspot with a total magnetic flux of ∼2 ×
1023 Mx (= G cm2) within one solar cycle period,
and lead to superflares with an energy of 1034 erg. To
store a total magnetic flux of ∼1024 Mx, necessary for
generating 1035 erg superflares, it would take ∼40 yr. Hot
Jupiters have often been argued to be a necessary ingredient for the
generation of superflares, but we found that they do not play any
essential role in the generation of magnetic flux in the star itself,
if we consider only the magnetic interaction between the star and the
hot Jupiter. This seems to be consistent with Maehara et al.'s finding
of 148 superflare-generating solar-type stars that do not have a hot
Jupiter-like companion. Altogether, our simple calculations, combined
with Maehara et al.'s analysis of superflares on Sun-like stars,
show that there is a possibility that superflares of 1034
erg would occur once in 800 yr on our present Sun.
Title: Numerical Simulations of Solar Chromospheric Jets Associated
with Emerging Flux
Authors: Takasao, Shinsuke; Isobe, Hiroaki; Shibata, Kazunari
Bibcode: 2013PASJ...65...62T
Altcode: 2013arXiv1301.7325T
We studied the acceleration mechanisms of chromospheric jets associated
with emerging flux using a two-dimensional magnetohydrodynamic (MHD)
simulation. We found that slow-mode shock waves generated by magnetic
reconnection in the chromosphere and the photosphere play key roles
in the acceleration mechanisms of chromospheric jets. An important
parameter is the height of magnetic reconnection. When magnetic
reconnection takes place near the photosphere, the reconnection outflow
collides with the region where the plasma beta is much larger than
unity. Then, the plasma moves along a magnetic field. This motion
generates a slow-mode wave. The slow-mode wave develops to a strong
slow shock as it propagates upward. When the slow shock crosses the
transition region, this region is lifted up. As a result, we obtain
a chromospheric jet as the lifted transition region. When magnetic
reconnection takes place in the upper chromosphere, the chromospheric
plasma is accelerated due to the combination of the Lorentz force
and the whip-like motion of the magnetic field. We found that the
chromospheric plasma is further accelerated through the interaction
between the transition region (steep density gradient) and a slow shock
emanating from the reconnection point. In the process, the magnetic
energy released by magnetic reconnection is efficiently converted into
the kinetic energy of jets. This is an MHD effect that has not been
discussed before.
Title: Simulations of the Dynamics of the Magnetic Rayleigh-Taylor
Instability in Solar Prominences
Authors: Hillier, A.; Berger, T.; Shibata, K.; Isobe, H.
Bibcode: 2013ASPC..474..147H
Altcode:
The magnetic Rayleigh-Taylor instability plays an important role in the
mass and magnetic flux transport in many astrophysical bodies. Solar
prominences also display this instability and recent observations using
the Solar Optical Telescope onboard the Hinode satellite have revealed
these dynamics in amazing detail. The observations show rising plumes,
approximately 1 Mm in width, that propagate through the dense prominence
material from low-density bubbles, i.e. the situation expected when the
magnetic Rayleigh-Taylor instability occurs. To study this phenomenon,
we performed 3D simulations of the magnetic Rayleigh-Taylor instability
in the Kippenhahn-Schlüter prominence model. The plumes formed in
these simulations are filamentary structures that are aligned with
the magnetic field created as 3D modes of the magnetic Rayleigh-Taylor
instability. The plumes rise, developing large structures from smaller
structures through an inverse cascade process driven by nonlinear
interaction. The results suggest that the plumes observed in the
prominence may be used to study the conditions inside the prominence.
Title: Fast Extreme-ultraviolet Dimming Associated with a Coronal
Jet Seen in Multi-wavelength and Stereoscopic Observations
Authors: Lee, K. -S.; Innes, D. E.; Moon, Y. -J.; Shibata, K.; Lee,
Jin-Yi; Park, Y. -D.
Bibcode: 2013ApJ...766....1L
Altcode:
We have investigated a coronal jet observed near the limb on 2010
June 27 by the Hinode/X-Ray Telescope (XRT), EUV Imaging Spectrograph
(EIS), and Solar Optical Telescope (SOT), and by the Solar Dynamics
Observatory (SDO)/Atmospheric Imaging Assembly (AIA), and on the disk
by STEREO-A/EUVI. From EUV (AIA and EIS) and soft X-ray (XRT) images we
have identified both cool and hot jets. There was a small loop eruption
seen in Ca II images of the SOT before the jet eruption. We found that
the hot jet preceded its associated cool jet by about 2 minutes. The
cool jet showed helical-like structures during the rising period which
was supported by the spectroscopic analysis of the jet's emission. The
STEREO observation, which enabled us to observe the jet projected
against the disk, showed dimming at 195 Å along a large loop connected
to the jet. We measured a propagation speed of ~800 km s-1
for the dimming front. This is comparable to the Alfvén speed in the
loop computed from a magnetic field extrapolation of the photospheric
field measured five days earlier by the SDO/Helioseismic and Magnetic
Imager, and the loop densities obtained from EIS Fe XIV λ264.79/274.20
line ratios. We interpret the dimming as indicating the presence of
Alfvénic waves initiated by reconnection in the upper chromosphere.
Title: Fermi Acceleration in Plasmoids Interacting with Fast Shocks
of Reconnection via Fractal Reconnection
Authors: Nishizuka, Naoto; Shibata, Kazunari
Bibcode: 2013PhRvL.110e1101N
Altcode: 2013arXiv1301.6242N
We propose the particle acceleration model coupled with multiple
plasmoid ejections in a solar flare. Unsteady reconnection produces
plasmoids in a current sheet and ejects them out to the fast shocks,
where particles in a plasmoid are reflected upstream the shock front
by magnetic mirror effect. As the plasmoid passes through the shock
front, the reflection distance becomes shorter and shorter driving
Fermi acceleration, until it becomes proton Larmor radius. The fractal
distribution of plasmoids may also have a role in naturally explaining
the power-law spectrum in nonthermal emissions.
Title: Numerical Simulation of Three-dimensional Asymmetric
Reconnection and Application to a Physical Mechanism of Penumbral
Microjets
Authors: Nakamura, Naoki; Shibata, Kazunari; Isobe, Hiroaki
Bibcode: 2012ApJ...761...87N
Altcode:
Three-dimensional (3D) component reconnection, where reconnecting
field lines are not perfectly anti-parallel, is studied with a
3D magnetohydrodynamic simulation. In particular, we consider the
asymmetry of the field strength of the reconnecting field lines. As
the asymmetry increases, the generated reconnection jet tends to be
parallel to stronger field lines. This is because weaker field lines
have higher gas pressure in the initial equilibrium, and hence the
gas pressure gradient along the reconnected field lines is generated,
which accelerates the field-aligned plasma flow. This mechanism may
explain penumbral microjets and other types of jets that are parallel
to magnetic field lines.
Title: The March 7-18 2012 CAWSES-SCOSTEP Interplanetary Events and
Their Magnetospheric and Ionospheric Effects
Authors: Tsurutani, B.; Shibata, K.; Echer, E.; Mannucci, A.;
Verkhoglyadova, O. P.; Mlynczak, M. G.
Bibcode: 2012AGUFMSH43C..06T
Altcode:
The recent March 7-18, 2012 complex solar event associated with AR
1429 has been selected by CAWSES-SCOSTEP as an interval for focused
studies. We will give an overview of the solar and interplanetary
main features. In addition, we will also examine some of the detailed
magnetospheric, ionospheric and atmospheric effects during these
magnetic storms.
Title: Multiple Plasma Ejections and Intermittent Nature of Magnetic
Reconnection in Solar Chromospheric Anemone Jets
Authors: Singh, K. A. P.; Isobe, H.; Nishizuka, N.; Nishida, K.;
Shibata, K.
Bibcode: 2012ApJ...759...33S
Altcode:
The recent discovery of chromospheric anemone jets with the Solar
Optical Telescope (SOT) on board Hinode has shown an indirect evidence
of magnetic reconnection in the solar chromosphere. However, the basic
nature of magnetic reconnection in chromosphere is still unclear. We
studied nine chromospheric anemone jets from SOT/Hinode using Ca II H
filtergrams, and we found multiple bright, plasma ejections along the
jets. In most cases, the major intensity enhancements (larger than
30% relative to the background intensity) of the loop correspond to
the timing of the plasma ejections. The typical lifetime and size of
the plasma ejecta are about 20-60 s and 0.3-1.5 Mm, respectively. The
height-time plot of jet shows many sub-structures (or individual jets)
and the typical lifetime of the individual jet is about one to five
minutes. Before the onset of the jet activity, a loop appears in Ca II
H and gradually increases in size, and after few minutes several jets
are launched from the loop. Once the jet activity starts and several
individual jets are launched, the loop starts shrinking with a speed
of ~4 km s-1. In some events, a downward moving blob with a
speed of ~35 km s-1 was observed, associated with the upward
moving plasma along one of the legs of the loop hosting the jets. The
upward moving plasma gradually developed into jets. Multiple plasma
ejections in chromospheric anemone jet show the strongly time-dependent
as well as intermittent nature of magnetic reconnection in the solar
chromosphere.
Title: Systematic Motion of Fine-scale Jets and Successive
Reconnection in Solar Chromospheric Anemone Jet Observed with the
Solar Optical Telescope/Hinode
Authors: Singh, K. A. P.; Isobe, H.; Nishida, K.; Shibata, K.
Bibcode: 2012ApJ...760...28S
Altcode:
The Solar Optical Telescope (SOT) on board Hinode allows observations
with high spatiotemporal resolution and stable image quality. A
λ-shaped chromospheric anemone jet was observed in high resolution with
SOT/Hinode. We found that several fine-scale jets were launched from one
end of the footpoint to the other. These fine-scale jets (~1.5-2.5 Mm)
gradually move from one end of the footpoint to the other and finally
merge into a single jet. This process occurs recurrently, and as time
progresses the jet activity becomes more and more violent. The time
evolution of the region below the jet in Ca II H filtergram images
taken with SOT shows that various parts (or knots) appear at different
positions. These bright knots gradually merge into each other during
the maximum phase. The systematic motion of the fine-scale jets is
observed when different knots merge into each other. Such morphology
would arise due to the emergence of a three-dimensional twisted flux
rope in which the axial component (or the guide field) appears in the
later stages of the flux rope emergence. The partial appearance of
the knots could be due to the azimuthal magnetic field that appears
during the early stage of the flux rope emergence. If the guide field
is strong and reconnection occurs between the emerging flux rope and
an ambient magnetic field, this could explain the typical feature of
systematic motion in chromospheric anemone jets.
Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence
Model. II. Reconnection-triggered Downflows
Authors: Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger,
Thomas
Bibcode: 2012ApJ...756..110H
Altcode: 2011arXiv1106.2613H
The launch of the Hinode satellite has allowed high-resolution
observations of supersonic bright downflows in quiescent prominences,
known as prominence knots. We present observations in the Ca
II H spectral line using the Solar Optical Telescope on board
the Hinode satellite of a descending plasma knot of size ~900
km. The knot initially undergoes ballistic motion before undergoing
impulsive accelerations at the same time as experiencing increases
in intensity. We also present a subset of our three-dimensional
magnetohydrodynamic simulations, performed to investigate the
nonlinear stability of the Kippenhahn-Shlüter prominence model
to the magnetic Rayleigh-Taylor instability in which interchange
reconnection occurs. The interchange reconnection in the model
breaks the force balance along the field lines which initiates the
downflows. The downflows propagate with a downward fluid velocity
of ~15 km s-1 and a characteristic size of ~700 km. We
conclude that the observed plasma blob and the simulated downflow are
driven by the breaking of the force balance along the magnetic field
as a result of a change in magnetic topology caused by reconnection
of the magnetic field.
Title: Observation of Reconnection Features in the Chromosphere
through a Chromospheric Jet Observed by SOT/Hinode
Authors: Singh, K. A. P.; Isobe, H.; Shibata, K.
Bibcode: 2012ASPC..454...99S
Altcode:
High-resolution observations from Solar Optical Telescope (SOT)
onboard Hinode has shown number of jet-like structures in the solar
chromosphere. One of the key features in the observations is the clear
presence of tiny, inverted Y-shaped jets called Chromospheric Anemone
Jets. These jets are supposed to be formed as a result of the magnetic
reconnection, however, whether and how fast magnetic reconnection
is realized in partially ionized, fully collisional chromosphere is
poorly understood. In this paper, we report the observation of a well
resolved jet phenomenon observed from SOT. The jets were found to recur
at the same location. We observed multiple blobs ejected along the
jet. The jets occur after the ejection of blobs. It is noticed that
the brightness enhancements at the footpoint of the jet are related
with the height of the jet. These features indicate an important role
of plasmoid dynamics and intermittent nature of the chromospheric
reconnection. The lifetime of the plasmoid is 30 s - 50 s. We noticed
the undulations in chromospheric anemone jets. The evolution of a
single jet is consistent with the Sweeping-Magnetic-Twist mechanism
proposed by Shibata and Uchida (1986).
Title: Implications for Coronal Heating from Coronal Rain
Authors: Antolin, P.; Shibata, K.; Carlsson, M.; Rouppe van der Voort,
L.; Vissers, G.; Hansteen, V.
Bibcode: 2012ASPC..454..171A
Altcode:
Coronal rain is a phenomenon above active regions in which cool plasma
condensations fall down from coronal heights. Numerical simulations of
loops have shown that such condensations can naturally form in the case
of footpoint concentrated heating through the “catastrophic cooling”
mechanism. In this work we analize high resolution limb observations in
Ca II H and Hα of coronal rain performed by Hinode/SOT and by Crisp of
SST and derive statistical properties. We further investigate the link
between coronal rain and the coronal heating mechanisms by performing
1.5-D MHD simulations of a loop subject to footpoint heating and to
Alfvén waves generated in the photosphere. It is found that if a loop
is heated predominantly from Alfvén waves coronal rain is inhibited
due to the characteristic uniform heating they produce. Hence coronal
rain can point both to the spatial distribution of the heating and to
the agent of the heating itself, thus acting as a marker for coronal
heating mechanisms.
Title: Spicule Dynamics over Plage Region
Authors: Anan, T.; Kitai, R.; Hillier, A.; Kawate, T.; Ichimoto, K.;
Shibata, K.
Bibcode: 2012ASPC..454...91A
Altcode:
We have studied spicular jets over a plage region and derived their
dynamic characteristics using Hinode Solar Optical Telescope (SOT)
high-resolution Ca II H images. We have identified 169 spicules over
the target plage. This sample size permits us to derive statistically
reliable results regarding spicular dynamics. The properties of plage
spicules can be summarized as follows: (1) In a plage area, we clearly
identify spicular jet features. (2) They are shorter in length than
the quiet-region limb spicules, and follow ballistic motion under
constant deceleration. (3) The majority (80%) of the plage spicules
show a full rise and retreat (which we call ‘parabolic’ spicules),
while 10% of them fade out without a complete retreat phase(which we
call ‘fade out’ spicules). (4) The deceleration of the spicule is
proportional to the velocity of ejection (i.e. the initial velocity).
Title: Chromospheric Anemone Jets Observed with Hinode/SOT and Hida
Ca II Spectroheliograph
Authors: Morita, S.; Shibata, K.; Ueno, S.; Ichimoto, K.; Kitai, R.;
Otsuji, K.
Bibcode: 2012ASPC..454...95M
Altcode:
We present the first simultaneous observations of chromospheric
“anemone” jets in active regions with the Ca II H broadband
filetergram on the Hinode/SOT and with the Ca II K spetroheliogram on
the Domeless Solar Telescope (DST) at the Hida Observatory. During
coordinated observation period, 9 chromospheric anemone jets were
simultaneously observed with the two instruments. These observations
revealed: (1) the jets are generated in the low chromosphere because
these cannot be seen in Ca II K3, (2) these jets are
associated with mixed polarity regions which are either small emerging
flux regions or moving magnetic features, (3) the Ca II K line often
show red or blue asymmetry in K2/K1 component;
the footpoint of the jets associated with emerging flux regions often
show red asymmetry (2-16 km s-1), while the one with moving
magnetic features show blue asymmetry (∼5 km s-1). The
magnetic cancellations were observed at the footpoint of the jets. The
canceling rates are of order of 1016 Mx s-1, and
the resulting magnetic energy release rate (1.1-10)×1024 erg
s-1, with the total energy release (1-13)×1026
erg for the duration of the magnetic cancellations, ∼130 s. These
are comparable to the estimated total energy, ∼1026 erg,
in a single chromospheric anemone jet.
Title: Propagating Slow Magnetoacoustic Waves along the Continuous
Outflows Observed with EIS/Hinode
Authors: Nishizuka, N.; Matsumoto, T.; Morita, S.; Hara, H.;
Shibata, K.
Bibcode: 2012ASPC..454..157N
Altcode:
The high temporal relation data obtained from EIS/Hinode has been
analyzed. In the ‘sit-and-stare’ mode observations, we focused
on continuous outflows at the edge of the active region NOAA 10942
on 2007 February 20. We found that the Doppler blueshift (∼20-50 km
s-1) components, in Fe XII 195 Å emission line, propagating
with the continuous plasma outflows from the edge of the active region,
with the transverse velocity ranging 140-160 km s-1. This
suggests slow magnetoacoustic wave propagation along the open field
line. We also found a jet around the active region, whose transverse
velocity ∼170 km s-1 and line-of-sight Doppler velocity
∼150 km s-1.
Title: Will Superflares Occur on Our Sun ?
Authors: Shibata, Kazunari
Bibcode: 2012cosp...39.1786S
Altcode: 2012cosp.meet.1786S
Supeflares are very big flares that release total energy much greater
than that of the biggest solar flares ever observed (3 x 10^{32}
erg). The famous Carrington flare in 1859 may correspond to the biggest
solar flare. If such superflares will occur on our Sun, we would
have extreme space weather events, which might lead to big hazards of
terrestrial environments and our civilization. Astronomical observations
revealed that young stars or fast rotating stars often show superflares
(10^{34} - 10^{38} erg). Hence it has been thought that our Sun would
have produced superflares when it was young and rotating faster (>
10 km/s). However it was not clear whether superflares would occur on
the present Sun or not, since the present Sun is not young and is now
slowly rotating (at 2 km/s). Recent observations of solar type stars
with Kepler satellite (Maehara et al. 2012) have revealed existence of
superflares (with energy of 10^{34}-10^{35} erg) on solar twins which
are quite similar to our Sun on surface temperature (5600 K - 6000 K)
and slow rotation (< 10 km/s). From the statistical analysis of these
superflare observations, it is suggested that superflares with energy
10^{34} erg occur once in 500 years and superflares with 10^{35} erg
occur once in 5000 years on solar twins and/or our present Sun. Finally,
we will also give theoretical arguments whether superflares will occur
on the present Sun or not on the basis of modern theories of flares
and dynamo.
Title: Reconnection Processes in the Chromosphere and Corona
Authors: Shibata, Kazunari
Bibcode: 2012cosp...39.1785S
Altcode: 2012cosp.meet.1785S
Magnetic reconnection is a fundamental key physical process in
magnetized plasmas. Recent space solar observations revealed that
magnetic reconnection is ubiquitous in the solar chromospheres and
corona. Especially recent Hinode observations has found various
types of tiny chromospheric jets, such as chromospheric anemone jets
(Shibata et al. 2007), penumbral microjets (Katsukawa et al. 2007),
light bridge jets from sunspot umbra (Shimizu et al. 2009), etc. It
was also found that the corona is full of tiny X-ray jets (Cirtain
et al. 2007). Often they are seen as helical spinning jets (Shimojo
et al. 2007, Patsourakos et al. 2008, Pariat et al. 2009, Filippov
et al. 2009, Kamio et al. 2010) with Alfvenic waves (Nishizuka et
al. 2008, Liu et al. 2009) and there are increasing evidence of
magnetic reconnection in these tiny jets. We can now say that as
spatial resolution of observations become better and better, smaller
and smaller flares and jets have been discovered, which implies that the
magnetized solar atmosphere consist of fractal structure and dynamics,
i.e., fractal reconnection. Bursty radio and hard X-ray emissions
from flares also suggest the fractal reconnection and associated
particle acceleration. Since magnetohydrodynamics (MHD) does not
contain any characteristic length and time scale, it is natural that
MHD structure, dynamics, and reconnection, tend to become fractal in
ideal MHD plasmas with large magnetic Reynolds number such as in the
solar atmosphere. We would discuss recent observations and theories
related to fractal reconnection in the chromospheres and corona,
and discuss possible implication to chromospheric and coronal heating.
Title: Isolated Coronal Mass Ejections and Associated Phenomena:
MHD Simulations and STEREO Observations
Authors: Lugaz, Noé; Roussev, Ilia; Sokolov, Igor; Shibata, Kazunari;
Schwadron, Nathan; Downs, Cooper
Bibcode: 2012cosp...39.1118L
Altcode: 2012cosp.meet.1118L
Coronal Mass Ejections (CMEs), the most energetic events occurring
in our solar system, are associated with a range of other phenomena
such as EUV waves, dimming regions and solar energetic particles
(SEPs). With the advancement of global numerical simulations and
the new fleet of spacecraft observing the Sun and the heliosphere,
it is possible to combine simulations with data analyses to gain
new insight into the complex Sun-Earth system. In this talk, I will
discuss magneto-hydrodynamic (MHD) simulations of CMEs and associated
phenomena. In particular, I will focus on the changes in magnetic
connectivity and the opening of previously closed field lines during
and after an eruption and discuss the consequences for the acceleration
and transport of energetic particles. I will also present combined
numerical simulations and observations by SDO and STEREO/SECCHI of
CMEs and EUV waves, which have led to new insights into CME deflection,
expansion and rotation.
Title: The role of a flux rope in three-dimensional
magnetohydrodynamic simulation of a solar flare
Authors: Nishida, Keisuke; Nishizuka, Naoto; Shibata, Kazunari
Bibcode: 2012cosp...39.1375N
Altcode: 2012cosp.meet.1375N
We investigated the dynamical evolution of a three-dimensional (3D)
flux rope eruption and associated magnetic reconnection in a solar
flare, extending two-dimensional (2D) magnetohydrodynamic simulation
model of solar flares (Chen and Shibata 2000, Nishida et al. 2009)
to 3D. We succeeded in reproducing a current sheet and bi-directional
reconnection outflows just below the flux rope during the eruption
in our 3D simulations. We calculated four cases of a strongly twisted
flux rope and a weakly twisted flux rope in 2D and 3D simulations. The
time evolution of a weakly twisted flux rope in 3D plane shows similar
behaviors to 2D simulation, while a strongly twisted flux rope shows
clearly different time evolution from 2D simulation except for the
initial phase evolution. The ejection speeds of both strongly and
weakly twisted flux ropes in 3D simulation are larger than those in 3D
simulation, and the reconnection rates in 2D cases are also larger than
those in 2D cases. Especially, in the case of a strongly twisted flux
rope, both ejection speed and reconnection rate are much larger than
those in 2D cases. This indicates that there exists a positive feedback
between the ejection speed of a flux rope and the reconnection rate
even in the 3D simulation, and that the plasmoid-induced reconnection
model (Shibata and Tanuma 2001) can be applied to 3D.
Title: Effect of Interacting Rarefaction Waves on Relativistically
Hot Jets
Authors: Matsumoto, Jin; Masada, Youhei; Shibata, Kazunari
Bibcode: 2012ApJ...751..140M
Altcode: 2012arXiv1204.5697M
The effect of rarefaction acceleration on the propagation dynamics and
structure of relativistically hot jets is studied through relativistic
hydrodynamic simulations. We emphasize the nonlinear interaction of
rarefaction waves excited at the interface between a cylindrical
jet and the surrounding medium. From simplified one-dimensional
(1D) models with radial jet structure, we find that a decrease in
the relativistic pressure due to the interacting rarefaction waves
in the central zone of the jet transiently yields a more powerful
boost of the bulk jet than that expected from single rarefaction
acceleration. This leads to a cyclic in situ energy conversion between
thermal and bulk kinetic energies, which induces radial oscillating
motion of the jet. The oscillation timescale is characterized by the
initial pressure ratio of the jet to the ambient medium and follows
a simple scaling relation, τoscillationvprop(P jet,
0/P amb, 0)1/2. Extended two-dimensional
simulations confirm that this radial oscillating motion in the 1D
system manifests as modulation of the structure of the jet in a more
realistic situation where a relativistically hot jet propagates through
an ambient medium. We find that when the ambient medium has a power-law
pressure distribution, the size of the reconfinement region along
the propagation direction of the jet in the modulation structure λ
evolves according to a self-similar relation λvpropt α/2,
where α is the power-law index of the pressure distribution.
Title: Fast EUV Dimming Associated with a Coronal Jet Seen in
Multi-Wavelength and Stereoscopic Observations
Authors: Lee, Kyoung-Sun; Innes, D. E.; Moon, Y.; Shibata, K.; Lee, J.
Bibcode: 2012AAS...22020120L
Altcode:
We have investigated a coronal jet observed near the limb on 2010 June
27 by the Hinode/X-Ray Telescope (XRT), EUV Imaging Spectrograph (EIS),
and Solar Optical Telescope (SOT), and the SDO/Atmospheric Imaging
Assembly (AIA), Helioseismic and Magnetic Imager (HMI), and on the
disk by STEREO-A/EUVI. From EUV (AIA and EIS) and soft X-ray (XRT)
images we have identified both cool and hot jets. There was a small
loop eruption in Ca II images of the SOT before the jet eruption. We
found that the hot jet preceded its associated cool jet by about
2 minutes. The cool jet showed helical-like structures during the
rising period. According to the spectroscopic analysis, the jet’s
emission changed from blue to red shift with time, implying helical
motions in the jet. The STEREO observation, which enabled us to observe
the jet projected against the disk, showed that there was a dim loop
associated with the jet. We measured a propagation speed of ∼ 800 km
s-1 for the dimming front. This is comparable to the Alfven
speed in the loop computed from a magnetic field extrapolation of the
HMI photospheric field measured 5 days earlier and the loop densities
obtained from EIS Fe XIV λ 264.79/274.20 line ratios. We interpret
the dimming as indicating the presence of Alfvenic waves initiated by
reconnection in the upper chromosphere.
Title: Revision of Solar Spicule Classification
Authors: Zhang, Y. Z.; Shibata, K.; Wang, J. X.; Mao, X. J.; Matsumoto,
T.; Liu, Y.; Su, J. T.
Bibcode: 2012ApJ...750...16Z
Altcode: 2012arXiv1202.4518Z
Solar spicules are the fundamental magnetic structures in the
chromosphere and may play a key role in channeling the chromosphere and
corona. Recently, it was suggested by De Pontieu et al. that there were
two types of spicules with very different dynamic properties, which were
detected by the space-time plot technique in the Ca II H line (3968 Å)
wavelength from Hinode/Solar Optical Telescope observations. A "Type
I" spicule with a 3-7-minute lifetime undergoes a cycle of upward and
downward motions; by contrast, a "Type II" spicule fades away within
dozens of seconds without a descending phase. We are motivated by the
fact that for a spicule with complicated three-dimensional motion the
space-time plot, which is made through a slit on a fixed position, could
not match the spicule behavior all the time and might lose its real life
story. By revisiting the same data sets, we identify and trace 105 and
102 spicules in the quiet Sun (QS) and coronal hole (CH), respectively,
and obtain their statistical dynamic properties. First, we have not
found a single convincing example of "Type II" spicules. Second, more
than 60% of the identified spicules in each region show a complete
cycle, i.e., the majority are "Type I" spicules. Third, the lifetimes
of the spicules in the QS and CH are 148 s and 112 s, respectively,
but there is no fundamental lifetime difference between the spicules
in the QS and CH reported earlier. Therefore, the suggestion of coronal
heating by "Type II" spicules should be taken with caution.
Title: Simulations of the Magnetic Rayleigh-Taylor Instability in
the Kippenhahn-Schlüter Prominence Model
Authors: Hillier, A.; Berger, T.; Shibata, K.; Isobe, H.
Bibcode: 2012ASPC..456..157H
Altcode:
The launch of the Hinode satellite, with the Solar Optical Telescope,
allowed for high resolution, high time cadence observations of
prominences to be performed in the seeing free environment of
space. The most striking discovery from these observations is of
plumes, approximately 1 Mm in width, that propagate through the
prominence material. The plumes initiate from underdense bubbles that
form beneath prominences, rise at constant speeds of approximately 20
km s-1 and are formed in the conditions required for the
magnetic Rayleigh-Taylor instability to occur. To study this phenomenon,
we performed 3D simulations of the magnetic Rayleigh-Taylor instability
in the Kippenhahn-Schlüter prominence model. The plumes formed in
these simulations are filamentary structures that are aligned with
the magnetic field created as 3D modes of the magnetic Rayleigh-Taylor
instability. The plumes rise, developing large structures from smaller
structures through an inverse cascade process driven by nonlinear
interaction. The results suggest that the plumes observed in the
prominence may be used to study the conditions inside the prominence.
Title: Dynamic Features of Current Sheet Associated with the 2010
August 18 Solar Flare
Authors: Takasao, S.; Asai, A.; Isobe, H.; Shibata, K.
Bibcode: 2012ASPC..456..221T
Altcode:
We report the observation of the magnetic reconnection site in the 2010
August 18 flare. We simultaneously found both reconnection inflow and
outflow. By using these velocities, we estimated the nondimensional
reconnection rate and found that it varied from 0.20 to 0.055. We
also observed dynamic plasma blobs in the sheet structure. The plasma
blobs collided with the hot loops and radio emissions were found at
this site, which may suggest particle acceleration. We hypothesize
that the sheet structure is the current sheet and that these plasma
blobs are plasmoids, which could be important for understanding the
dynamics of the reconnection region.
Title: Superflares on solar-type stars
Authors: Maehara, Hiroyuki; Shibayama, Takuya; Notsu, Shota; Notsu,
Yuta; Nagao, Takashi; Kusaba, Satoshi; Honda, Satoshi; Nogami, Daisaku;
Shibata, Kazunari
Bibcode: 2012Natur.485..478M
Altcode:
Solar flares are caused by the sudden release of magnetic energy stored
near sunspots. They release 1029 to 1032 ergs of
energy on a timescale of hours. Similar flares have been observed on
many stars, with larger `superflares' seen on a variety of stars, some
of which are rapidly rotating and some of which are of ordinary solar
type. The small number of superflares observed on solar-type stars has
hitherto precluded a detailed study of them. Here we report observations
of 365 superflares, including some from slowly rotating solar-type
stars, from about 83,000 stars observed over 120 days. Quasi-periodic
brightness modulations observed in the solar-type stars suggest that
they have much larger starspots than does the Sun. The maximum energy
of the flare is not correlated with the stellar rotation period,
but the data suggest that superflares occur more frequently on
rapidly rotating stars. It has been proposed that hot Jupiters may
be important in the generation of superflares on solar-type stars,
but none have been discovered around the stars that we have studied,
indicating that hot Jupiters associated with superflares are rare.
Title: Commission 49: Interplanetary Plasma and Heliosphere
Authors: Gopalswamy, Natchimuthuk; Mann, Ingrid; Bougeret, Jean-Louise;
Briand, Carine; Lallement, Rosine; Lario, David; Manoharan, P. K.;
Shibata, Kazunari; Webb, David F.
Bibcode: 2012IAUTA..28...95G
Altcode:
Commission 49 (Interplanetary Plasma and Heliosphere) is part of
IAU Division II (Sun and Heliosphere). The research topics include
large-scale solar disturbances such as coronal mass ejections (CMEs),
shocks, and corotating interaction regions (CIRs) propagating into
the heliosphere. The disturbances propagate through the solar wind,
which essentially defines the heliosphere. The solar disturbances
provide large-scale laboratory to study plasma processes over various
time and spatial scales, the highest spatial scale being the size
of the heliosphere itself (~100 AU). These solar disturbances are
related to solar activity in the form of active regions and coronal
holes. Solar eruptions are accompanied by particle acceleration and
the particles can be hazardous to life on earth in various ways from
modifying the ionosphere to damaging space technology and increasing
lifetime radiation dosage to astronauts and airplane crew. Particle
acceleration in solar eruptions poses fundamental physics questions
because the underlying mechanisms are not fully understood. One of
important processes is the particle acceleration by shocks, which occurs
throughout the heliosphere. The heliosphere has both neutral and ionized
material, with interesting interaction between the two components.
Title: Fractal Reconnection and Particle Acceleration in the Solar
Corona
Authors: Shibata, K.
Bibcode: 2012EGUGA..14.3226S
Altcode:
Recent space observations of the Sun such as with Hinode revealed that
magnetic reconnection is ubiquitous in the solar corona, ranging from
small scale reconnection (nanoflares) to large scale one (CME related
flares). These observations imply that the current sheet in these
reconnection events consists of self-similar dynamical structure, i.e.,
fractal structure, which is consistent with basic magnetohydrodynamics
(MHD) theory, since MHD does not contain any characteristic length
and time scale, and it is natural that MHD structure, dynamics, and
reconnection, tend to become fractal in ideal MHD plasmas with large
magnetic Reynolds number such as in the solar atmosphere. We would
discuss recent observations with Hinode and theories related to fractal
reconnection, and discuss possible implication to reconnection physics,
coronal heating, and particle acceleration.
Title: Observation of Dynamic Features of Current Sheet Associated
with 2010 August 18 Solar Flare
Authors: Takasao, S.; Asai, A.; Isobe, H.; Shibata, K.
Bibcode: 2012decs.confE..93T
Altcode:
We report the simultaneous extreme-ultraviolet observation of magnetic
reconnection inflow and outflow in a flare on 2010 August 18 observed
by SDO/AIA. We found that during the rise phase of the flare, some
plasma blobs appeared in a sheet structure above hot loops. The
plasma blobs were ejected bidirectionally along the sheet structure
(i.e. reconnection outflow). Simultaneously, bright threads visible
in the extreme-ultraviolet images moved toward the sheet structure
(i.e. reconnection inflow). Using the velocities of the inflow and
outflow, we estimated the non-dimensional reconnection rate and found
it varies during this period from 0.20 to 0.055. We also found that
the plasma blobs in the sheet structure collided and possibly merged
with each other before they were ejected from the sheet structure. From
these observational results, we hypothesize that the sheet structure
is the current sheet and that these plasma blobs are plasmoids or
magnetic islands. This observational report could be important for
understanding the dynamics of the reconnection region.
Title: First Simultaneous Observation of an Hα Moreton Wave, EUV
Wave, and Filament/Prominence Oscillations
Authors: Asai, Ayumi; Ishii, Takako T.; Isobe, Hiroaki; Kitai,
Reizaburo; Ichimoto, Kiyoshi; UeNo, Satoru; Nagata, Shin'ichi; Morita,
Satoshi; Nishida, Keisuke; Shiota, Daikou; Oi, Akihito; Akioka, Maki;
Shibata, Kazunari
Bibcode: 2012ApJ...745L..18A
Altcode: 2011arXiv1112.5915A
We report on the first simultaneous observation of an Hα Moreton wave,
the corresponding EUV fast coronal waves, and a slow and bright EUV
wave (typical EIT wave). We observed a Moreton wave, associated with
an X6.9 flare that occurred on 2011 August 9 at the active region
NOAA 11263, in the Hα images taken by the Solar Magnetic Activity
Research Telescope at Hida Observatory of Kyoto University. In the
EUV images obtained by the Atmospheric Imaging Assembly on board the
Solar Dynamic Observatory we found not only the corresponding EUV fast
"bright" coronal wave, but also the EUV fast "faint" wave that is not
associated with the Hα Moreton wave. We also found a slow EUV wave,
which corresponds to a typical EIT wave. Furthermore, we observed,
for the first time, the oscillations of a prominence and a filament,
simultaneously, both in the Hα and EUV images. To trigger the
oscillations by the flare-associated coronal disturbance, we expect
a coronal wave as fast as the fast-mode MHD wave with the velocity of
about 570-800 km s-1. These velocities are consistent with
those of the observed Moreton wave and the EUV fast coronal wave.
Title: A Study on Red Asymmetry of Hα Flare Ribbons Using a
Narrowband Filtergram in the 2001 April 10 Solar Flare
Authors: Asai, Ayumi; Ichimoto, Kiyoshi; Kita, Reizaburo; Kurokawa,
Hiroki; Shibata, Kazunari
Bibcode: 2012PASJ...64...20A
Altcode: 2011arXiv1112.5912A
We report on a detailed examination of the ``red asymmetry'' of the
Hα emission line seen during the 2001 April 10 solar flare by using
a narrowband filtergram. We investigated the temporal evolution and
the spatial distribution of the red asymmetry by using Hα data taken
with the 60-cm Domeless Solar Telescope at Hida Observatory, Kyoto
University. We confirmed that the red asymmetry clearly appeared all
over the flare ribbons, and the strong red asymmetry is located on the
outer narrow edges of the flare ribbons, with a width of about 1.5"-3.0"
(1000-2000 km), where strong energy releases occur. Moreover, we found
that the red asymmetry, which also gives a measure of the Doppler
shift of the Hα emission line, concentrates on a certain value,
not depending on the intensity of the Hα kernels. This implies not
only that the temporal evolutions of the red asymmetry and those of
the intensity are not synchronous in each flare kernel, but also that
the peak asymmetry (or velocity of the chromospheric condensation)
of individual kernel is not a strong function of their peak intensity.
Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence Model. I. Formation
of Upflows
Authors: Hillier, Andrew; Berger, Thomas; Isobe, Hiroaki; Shibata,
Kazunari
Bibcode: 2012ApJ...746..120H
Altcode:
The launch of the Hinode satellite led to the discovery of rising
plumes, dark in chromospheric lines, that propagate from large
(~10 Mm) bubbles that form at the base of quiescent prominences. The
plumes move through a height of approximately 10 Mm while developing
highly turbulent profiles. The magnetic Rayleigh-Taylor instability
was hypothesized to be the mechanism that drives these flows. In this
study, using three-dimensional (3D) MHD simulations, we investigate the
nonlinear stability of the Kippenhahn-Schlüter prominence model for the
interchange mode of the magnetic Rayleigh-Taylor instability. The model
simulates the rise of a buoyant tube inside the quiescent prominence
model, where the interchange of magnetic field lines becomes possible
at the boundary between the buoyant tube and the prominence. Hillier
et al. presented the initial results of this study, where upflows of
constant velocity (maximum found 6 km s-1) and a maximum
plume width ≈1.5 Mm which propagate through a height of approximately
6 Mm were found. Nonlinear interaction between plumes was found to be
important for determining the plume dynamics. In this paper, using
the results of ideal MHD simulations, we determine how the initial
parameters for the model and buoyant tube affect the evolution of
instability. We find that the 3D mode of the magnetic Rayleigh-Taylor
instability grows, creating upflows aligned with the magnetic field
of constant velocity (maximum found 7.3 km s-1). The width
of the upflows is dependent on the initial conditions, with a range
of 0.5-4 Mm which propagate through heights of 3-6 Mm. These results
are in general agreement with the observations of the rising plumes.
Title: Simultaneous Observation of Reconnection Inflow and Outflow
Associated with the 2010 August 18 Solar Flare
Authors: Takasao, Shinsuke; Asai, Ayumi; Isobe, Hiroaki; Shibata,
Kazunari
Bibcode: 2012ApJ...745L...6T
Altcode: 2011arXiv1112.1398T
We report the simultaneous extreme-ultraviolet observation of
magnetic reconnection inflow and outflow in a flare on 2010 August
18 observed by the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory. We found that during the rise phase of the
flare, some plasma blobs appeared in the sheet structure above the
hot loops. The plasma blobs were ejected bidirectionally along the
sheet structure (outflow), at the same time as the threads visible
in extreme-ultraviolet images moved toward the sheet structure
(inflow). The upward and downward ejection velocities are 220-460 km
s-1 and 250-280 km s-1, respectively. The inflow
speed changed from 90 km s-1 to 12 km s-1 in 5
minutes. By using these velocities, we estimated the nondimensional
reconnection rate, which we found to vary during this period from 0.20
to 0.055. We also found that the plasma blobs in the sheet structure
collided or merged with each other before they were ejected from
the sheet structure. We hypothesize that the sheet structure is the
current sheet and that these plasma blobs are plasmoids or magnetic
islands, which could be important for understanding the dynamics of
the reconnection region.
Title: Solar Flares: Magnetohydrodynamic Processes
Authors: Shibata, Kazunari; Magara, Tetsuya
Bibcode: 2011LRSP....8....6S
Altcode:
This paper outlines the current understanding of solar flares,
mainly focused on magnetohydrodynamic (MHD) processes responsible for
producing a flare. Observations show that flares are one of the most
explosive phenomena in the atmosphere of the Sun, releasing a huge
amount of energy up to about 1032 erg on the timescale
of hours. Flares involve the heating of plasma, mass ejection, and
particle acceleration that generates high-energy particles. The key
physical processes for producing a flare are: the emergence of magnetic
field from the solar interior to the solar atmosphere (flux emergence),
local enhancement of electric current in the corona (formation of a
current sheet), and rapid dissipation of electric current (magnetic
reconnection) that causes shock heating, mass ejection, and particle
acceleration. The evolution toward the onset of a flare is rather
quasi-static when free energy is accumulated in the form of coronal
electric current (field-aligned current, more precisely), while the
dissipation of coronal current proceeds rapidly, producing various
dynamic events that affect lower atmospheres such as the chromosphere
and photosphere. Flares manifest such rapid dissipation of coronal
current, and their theoretical modeling has been developed in accordance
with observations, in which numerical simulations proved to be a strong
tool reproducing the time-dependent, nonlinear evolution of a flare. We
review the models proposed to explain the physical mechanism of flares,
giving an comprehensive explanation of the key processes mentioned
above. We start with basic properties of flares, then go into the
details of energy build-up, release and transport in flares where
magnetic reconnection works as the central engine to produce a flare.
Title: Simulations of the magnetic Rayleigh-Taylor instability in
a quiescent prominence model to study the dark upflows observed
in prominences
Authors: Hillier, A. S.; Berger, T. E.; Shibata, K.; Isobe, H.
Bibcode: 2011AGUFMSH33A2033H
Altcode:
Observations of quiescent prominences by the Solar Optical Telescope
(SOT) on board the Hinode satellite show plumes of hot, underdense
material rising through the prominence. These plumes form at the
boundary between the prominence and low density bubbles, approximately
10 Mm in size, that appear beneath the prominence, and then rise
through the prominence material at speeds of approximately 20 km/s and
widths of approximately 1.5 Mm. The plume profile ranges from highly
turbulent to smooth, suggesting that the prominence conditions, as well
as those of the bubble, are important in determining the dynamics. To
investigate this phenomenon, we perform simulations of the magnetic
Rayleigh-Taylor instability in a local prominence model. The instability
creates rising plumes of hot, underdense material that propagate through
the prominence material at a velocity of approximately 6-7 km/s and
widths of approximately 1.5 Mm, in rough agreement with the Hinode
observations. Nonlinear effects, in which the interaction between
plumes drives an inverse cascade process creating large plumes from
smaller plumes, are found to be important. Increasing the magnetic
field strength creates smoother plume structures. The addition of a
strong guide field, which is suggested in some prominence models, does
not hinder plume formation but does change the dynamic scaling. The
Rayleigh-Taylor instability drives an upward flow of magnetic energy
and a downward flow of mass. The results from the simulations well
match the characteristics of the observed plumes, suggesting that
the magnetic Rayleigh-Taylor instability could be important in
determining prominence structure as well as changing the magnetic
energy distribution in overlying coronal cavities which ultimately
erupt as coronal mass ejections.
Title: Multi-wavelength and Stereoscopic observations of a coronal
jet supporting the emerging flux reconnection model
Authors: Lee, K.; Innes, D.; Moon, Y.; Shibata, K.
Bibcode: 2011AGUFMSH13B1940L
Altcode:
We have investigated a coronal jet near the limb on 2010 June 27 by
Hinode/X-Ray Telescope (XRT), EUV Imaging Spectrograph (EIS), Solar
Optical Telescope (SOT), SDO/Atmospheric Imaging Assembly (AIA), and
STEREO. From EUV (AIA and EIS) and soft X-ray (XRT) images we identify
the erupting jet feature in cool and hot temperatures. It is noted
that there was a small loop eruption at the low temperature from the
SOT observation before the jet eruption. Using the high temporal and
multi wavelength AIA images, we found that the hot jet preceded its
associated cool jet. The jet also shows the helical-like structures
during the rising period. According to the spectroscopic analysis, the
jet structure changes from blue shift to red one with time, implying
the helical structure of the jet. The STEREO observation, which enables
us to observe this jet on the disk, shows that there was a dim loop
associated with the jet. Comparing observations from the AIA and
STEREO, the dim loop corresponds to the jet structure which implies
the heated loop. Considering that the structure of its associated
active region seen in STEREO is similar to that in AIA observed 5 days
before, we compared the jet morphology on the limb with the magnetic
fields extrapolated from a HMI vector magnetogram observed on the
disk. Interestingly, the comparison also shows that the open field
corresponds to the jet which is seen as the dim loop in STEREO. Our
observations (XRT, SDO, SOT, and STEREO) are well consistent with the
numerical simulation of the emerging flux reconnection model predicted.
Title: Chromospheric anemone jets and magnetic reconnection in
partially ionized solar atmosphere
Authors: Singh, K. A. P.; Shibata, K.; Nishizuka, N.; Isobe, H.
Bibcode: 2011PhPl...18k1210S
Altcode:
The solar optical telescope onboard Hinode with temporal resolution
of less than 5 s and spatial resolution of 150 km has observed the
lower solar atmosphere with an unprecedented detail. This has led to
many important findings, one of them is the discovery of chromospheric
anemone jets in the solar chromosphere. The chromospheric anemone jets
are ubiquitous in solar chromosphere and statistical studies show
that the typical length, life time and energy of the chromospheric
anemone jets are much smaller than the coronal events (e.g.,
jets/flares/CMEs). Among various observational parameters, the apparent
length and maximum velocity shows good correlation. The velocity of
chromospheric anemone jets is comparable to the local Alfvén speed
in the lower solar chromosphere. Since the discovery of chromospheric
anemone jets by Hinode, several evidences of magnetic reconnection
in chromospheric anemone jets have been found and these observations
are summarized in this paper. These observations clearly suggest that
reconnection occurs quite rapidly as well as intermittently in the solar
chromosphere. In the solar corona (λi > δSP), anomalous resistivity
arises due to various collisionless processes. Previous MHD simulations
show that reconnection becomes fast as well as strongly time-dependent
due to anomalous resistivity. Such processes would not arise in the
solar chromosphere which is fully collisional and partially-ionized. So,
it is unclear how the rapid and strongly time-dependent reconnection
would occur in the solar chromosphere. It is quite likely that the
Hall and ambipolar diffusion are present in the solar chromosphere
and they could play an important role in driving such rapid, strongly
time-dependent reconnection in the solar chromosphere.
Title: Statistical Study on the Nature of Solar-Flux Emergence
Authors: Otsuji, Kenichi; Kitai, Reizaburo; Ichimoto, Kiyoshi;
Shibata, Kazunari
Bibcode: 2011PASJ...63.1047O
Altcode: 2011arXiv1106.1955O
We studied 101 flux emergence events ranging from small ephemeral
regions to large emerging flux regions that were observed with the
Hinode Solar Optical Telescope filtergram. We investigated how the
total magnetic flux of the emergence event controls the nature of
emergence. To determine the modes of emergences, horizontal velocity
fields of the global motion of the magnetic patches in the flux emerging
sites were measured by local correlation tracking. Between two main
polarities of the large emerging flux regions with more than around 2
× 1019 Mx, there were converging flows of anti-polarity
magnetic patches. On the other hand, small ephemeral regions showed
no converging flow, but a simple diverging pattern. When we looked
into the detailed features in the emerging sites, irrespective of
the total flux and the spatial size, all of the emergence events
were observed to consist of single or multiple elementary emergence
unit(s). The typical size of unitary emergence is 4 Mm, and consistent
with simulation results. From a statistical study of the flux emergence
events, the maximum spatial distance between two main polarities, the
magnetic flux growth rate and the mean separation speed were found
to follow the power-law functions of the total magnetic flux with
indices of 0.27, 0.57, and -0.16, respectively. From a discussion on
the observed power-law relations, we obtained a physical view of solar
flux emergence, in which the emerging magnetic fields float and evolve
while balancing to the surrounding turbulent atmosphere.
Title: Numerical Investigation of a Coronal Mass Ejection from an
Anemone Active Region: Reconnection and Deflection of the 2005 August
22 Eruption
Authors: Lugaz, N.; Downs, C.; Shibata, K.; Roussev, I. I.; Asai,
A.; Gombosi, T. I.
Bibcode: 2011ApJ...738..127L
Altcode: 2011arXiv1106.5284L
We present a numerical investigation of the coronal evolution of a
coronal mass ejection (CME) on 2005 August 22 using a three-dimensional
thermodynamic magnetohydrodynamic model, the space weather modeling
framework. The source region of the eruption was anemone active
region (AR) 10798, which emerged inside a coronal hole. We validate
our modeled corona by producing synthetic extreme-ultraviolet (EUV)
images, which we compare to EIT images. We initiate the CME with an
out-of-equilibrium flux rope with an orientation and chirality chosen
in agreement with observations of an Hα filament. During the eruption,
one footpoint of the flux rope reconnects with streamer magnetic field
lines and with open field lines from the adjacent coronal hole. It
yields an eruption which has a mix of closed and open twisted field
lines due to interchange reconnection and only one footpoint line-tied
to the source region. Even with the large-scale reconnection, we find
no evidence of strong rotation of the CME as it propagates. We study
the CME deflection and find that the effect of the Lorentz force is
a deflection of the CME by about 3° R -1 sun
toward the east during the first 30 minutes of the propagation. We
also produce coronagraphic and EUV images of the CME, which we compare
with real images, identifying a dimming region associated with the
reconnection process. We discuss the implication of our results for
the arrival at Earth of CMEs originating from the limb and for models
to explain the presence of open field lines in magnetic clouds.
Title: Numerical Simulations of the Magnetic Rayleigh-Taylor
Instability in the Kippenhahn-Schlüter Prominence Model
Authors: Hillier, Andrew; Isobe, Hiroaki; Shibata, Kazunari; Berger,
Thomas
Bibcode: 2011ApJ...736L...1H
Altcode: 2011arXiv1107.4882A
The launch of the Hinode satellite has allowed unprecedented
high-resolution, stable images of solar quiescent prominences to
be taken over extended periods of time. These new images led to the
discovery of dark upflows that propagated from the base of prominences,
developing highly turbulent profiles. As yet, how these flows are driven
is not fully understood. To study the physics behind these phenomena,
we use three-dimensional magnetohydrodynamic simulations to investigate
the nonlinear stability of the Kippenhahn-Shlüter prominence model
to the magnetic Rayleigh-Taylor instability. The model simulates the
rise of a buoyant tube inside a quiescent prominence, where the upper
boundary between the tube and prominence model is perturbed to excite
the interchange of magnetic field lines. We found upflows of constant
velocity (maximum found 6 km s-1) and a maximum plume
width ≈1500 km which propagate through a height of approximately 6
Mm in the no guide field case. The case with the strong guide field
(initially By = 2Bx ) results in a large plume
that rises through the prominence model at ~5 km s-1 with
width ~900 km (resulting in width of 2400 km when viewed along the
axis of the prominence), reaching a height of ~3.1 Mm. In both cases,
nonlinear processes were important for determining plume dynamics.
Title: MHD Simulation of a CME from an Anemone Active Region and
Comparison with EUV Images
Authors: Lugaz, Noe; Downs, C.; Roussev, I.; Shibata, K.; Asai, A.
Bibcode: 2011shin.confE..41L
Altcode:
We present a numerical investigation of the coronal evolution of
the coro- nal mass ejection (CME) on 2005 August 22 using a 3-D
thermodynamics MHD model, the SWMF. The source region of the eruption
was anemone AR 10798, which emerged inside a coronal hole. We validate
our modeled corona by producing synthetic EUV images, which we compare
to EIT images. The eruption yields a mix of open and closed field lines
due to interchange re- connection. We discuss the CME reconnection
process with the ambient magnetic field of the AR and the surrounding
coronal hole and show how it is related to a long-lasting dimming
region.
Title: Special relativistic magnetohydrodynamic simulation of
two-component outflow powered by magnetic explosion on compact stars
Authors: Matsumoto, Jin; Masada, Youhei; Asano, Eiji; Shibata, Kazunari
Bibcode: 2011IAUS..274..220M
Altcode:
The nonlinear dynamics of the outflow driven by magnetic explosion
on the surface of compact object is investigated through special
relativistic magnetohydrodynamic simulations. We adopt, as an
initial equilibrium state, a spherical stellar object embedded in
the hydrostatic plasma which has a density ρ(r) ~ r-α
and is threaded by a dipole magnetic field. The injection of magnetic
energy at the surface of compact star breaks the dynamical equilibrium
and triggers two-component outflow. At the early evolutionary stage,
the magnetic pressure increases rapidly in time around the stellar
surface, initiating a magnetically driven outflow. Then it excites a
strong forward shock, shock driven outflow. The expansion velocity
of the magnetically driven outflow is characterized by the Alfvén
velocity on the stellar surface, and follows a simple scaling
relation υmag ~ υA1/2. When the
initial density profile declines steeply with radius, the strong
shock is accelerated self-similarly to relativistic velocity ahead
of the magnetically driven component. We find that the evolution of
the strong forward shock can be described by a self-similar relation
Γsh ~ rsh, where Γsh is the Lorentz
factor of the plasma measured at the shock surface rsh. It
should be stressed that the pure hydrodynamic process is responsible
for the acceleration of the shock driven outflow. Our two-component
outflow model, which is the natural outcome of the magnetic explosion,
would deepen the understanding of the magnetic active phenomena on
various magnetized stellar objects.
Title: Propagation of Moreton Waves
Authors: Zhang, Yuzong; Kitai, Reizaburo; Narukage, Noriyuki;
Matsumoto, Takuma; Ueno, Satoru; Shibata, Kazunari; Wang, Jingxiu
Bibcode: 2011PASJ...63..685Z
Altcode:
With the Flare-Monitoring Telescope (FMT) and Solar Magnetic Activity
Research Telescope (SMART) at Hida observatory of Kyoto University,
13 events of Moreton waves were captured at Hα center, Hα ±0.5 Å,
and Hα ±0.8 Å wavebands since 1997. With such samples, we have
studied the statistical properties of the propagation of Moreton
waves. Moreton waves were all restricted in sectorial zones with a
mean value of 92°. However, their accompanying EIT waves, observed
simultaneously with SOHO/EIT at extreme-ultraviolet wavelength, were
very isotropic with a quite extended scope of 193°. The average
propagation speeds of the Moreton waves and the corresponding
EIT waves were 664 km s-1 and 205 km s-1,
respectively. Moreton waves propagated either under large-scale
close magnetic flux loops, or firstly in the sectorial region where
two sets of magnetic loops separated from each other and diverged,
and then stopped before the open magnetic flux region. The location
swept by Moreton waves had a relatively weak magnetic field as compared
to the magnetic fields at their sidewalls. The ratio of the magnetic
flux density between the sidewall and the path falls in the range of
1.4 to 3.7 at a height of 0.01 solar radii. Additionally, we roughly
estimated the distribution of the fast magnetosonic speed between the
propagating path and sidewalls in an event on 1997 November 3, and
found a relatively low-fast magnetosonic speed in the path. We also
found that the propagating direction of Moreton waves coincided with
the direction of filament eruption in a few well-observed events. This
favors an interpretation of the ``Piston'' model, although further
studies are necessary for any definitive conclusion.
Title: Detailed analysis of fan-shaped jets in three dimensional
numerical simulation
Authors: Jiang, Rong-Lin; Shibata, K.; Isobe, H.; Fang, Cheng
Bibcode: 2011RAA....11..701J
Altcode: 2011arXiv1101.4598J
We performed three dimensional resistive magnetohydrodynamic simulations
to study the magnetic reconnection using an initially shearing magnetic
field configuration (force free field with a current sheet in the middle
of the computational box). It is shown that there are two types of
reconnection jets: the ordinary reconnection jets and fan-shaped jets,
which are formed along the guide magnetic field. The fan-shaped jets
are significantly different from the ordinary reconnection jets which
are ejected by magnetic tension force. There are two driving forces
for accelerating the fan-shaped jets. One is the Lorentz force which
initially dominates the motion of fluid elements, and then the gas
pressure gradient force accelerates the fluid elements in the later
stage. The dependence on magnetic reconnection angle and resistivity
value has also been studied. The formation and evolution of these jets
provide a new understanding of dynamic magnetohydrodynamic jets.
Title: Special Relativistic Magnetohydrodynamic Simulation of a
Two-component Outflow Powered by Magnetic Explosion on Compact Stars
Authors: Matsumoto, Jin; Masada, Youhei; Asano, Eiji; Shibata, Kazunari
Bibcode: 2011ApJ...733...18M
Altcode: 2011arXiv1103.1939M
The nonlinear dynamics of outflows driven by magnetic explosion on the
surface of a compact star is investigated through special relativistic
magnetohydrodynamic simulations. We adopt, as the initial equilibrium
state, a spherical stellar object embedded in hydrostatic plasma
which has a density ρ(r) vprop r -α and is threaded
by a dipole magnetic field. The injection of magnetic energy at
the surface of a compact star breaks the equilibrium and triggers a
two-component outflow. At the early evolutionary stage, the magnetic
pressure increases rapidly around the stellar surface, initiating a
magnetically driven outflow. A strong forward shock driven outflow is
then excited. The expansion velocity of the magnetically driven outflow
is characterized by the Alfvén velocity on the stellar surface and
follows a simple scaling relation v mag vprop v A
1/2. When the initial density profile declines steeply with
radius, the strong shock is accelerated self-similarly to relativistic
velocity ahead of the magnetically driven component. We find that it
evolves according to a self-similar relation Γsh vprop r
sh, where Γsh is the Lorentz factor of the plasma
measured at the shock surface r sh. A purely hydrodynamic
process would be responsible for the acceleration mechanism of the shock
driven outflow. Our two-component outflow model, which is the natural
outcome of the magnetic explosion, can provide a better understanding
of the magnetic active phenomena on various magnetized compact stars.
Title: Multi-wavelength Observation Of A Coronal Jet Supporting The
Emerging Flux Reconnection Model
Authors: Lee, Kyoung-Sun; Innes, D.; Moon, Y.; Shibata, K.
Bibcode: 2011SPD....42.2109L
Altcode: 2011BAAS..43S.2109L
We have investigated a coronal jet near the limb on 2010 June 27
by Hinode/X-Ray Telescope (XRT), EUV Imaging Spectrograph (EIS),
SDO/Atmospheric Imaging Assembly (AIA), and STEREO. From EUV (AIA
and EIS) and soft X-ray (XRT) images we identify the erupting jet
feature in cool and hot temperatures. Using the high temporal and
multi wavelength AIA images, we found that the hot jet preceded its
associated cool jet and their structures are well consistent with the
numerical simulation of the emerging flux-reconnection model. From
the spectroscopic analysis, we found that the jet structure changes
from blue shift to red one with time, which may indicate the helical
structure of the jet. The STEREO observation, which enables us to
observe this jet on the disk, shows that there was a dim loop associated
with the jet. Considering that the structure of its associated active
region seen in STEREO is similar to that in AIA observed 5 days
before, we compared the jet morphology on the limb with the magnetic
fields extrapolated from a HMI vector magnetogram observed on the
disk. Interestingly, the comparison shows that the open and closed
magnetic fields correspond to the jet and the dim loop, respectively,
as the emerging flux reconnection model predicted.
Title: Statistical Study of Chromospheric Anemone Jets Observed
with Hinode/SOT
Authors: Nishizuka, N.; Nakamura, T.; Kawate, T.; Singh, K. A. P.;
Shibata, K.
Bibcode: 2011ApJ...731...43N
Altcode:
The Solar Optical Telescope on board Hinode has revealed numerous
tiny jets in all regions of the chromosphere outside of sunspots. A
typical chromospheric anemone jet has a cusp-shaped structure and
bright footpoint, similar to the shape of an X-ray anemone jet observed
previously with the Soft X-ray Telescope on board Yohkoh. The similarity
in the shapes of chromospheric and X-ray anemone jets suggests that
chromospheric anemone jets are produced as a result of the magnetic
reconnection between a small bipole (perhaps a tiny emerging flux)
and a pre-existing uniform magnetic field in the lower chromosphere. We
examine various chromospheric anemone jets in the solar active region
near the solar limb and study the typical features (e.g., length, width,
lifetime, and velocity) of the chromospheric anemone jets. Statistical
studies show that chromospheric anemone jets have: (1) a typical
length ~1.0-4.0 Mm, (2) a width ~100-400 km, (3) a lifetime ~100-500
s, and (4) a velocity ~5-20 km s-1. The velocity of the
chromospheric anemone jets is comparable to the local Alfvén speed in
the lower solar chromosphere (~10 km s-1). The histograms
of chromospheric anemone jets near the limb and near the disk center
show similar averages and shapes of distributions, suggesting that the
characteristic behavior of chromospheric anemone jets is independent
of whether they are observed on the disk or at the limb. The observed
relationship between the velocity and length of chromospheric anemone
jets shows that the jets do not follow ballistic motion but are more
likely accelerated by some other mechanism. This is consistent with
numerical simulations of chromospheric anemone jets.
Title: Dependence of the Magnetic Energy of Solar Active Regions on
the Twist Intensity of the Initial Flux Tubes
Authors: Toriumi, Shin; Miyagoshi, Takehiro; Yokoyama, Takaaki; Isobe,
Hiroaki; Shibata, Kazunari
Bibcode: 2011PASJ...63..407T
Altcode: 2011arXiv1101.0978T
We present a series of numerical experiments that model the evolution
of magnetic flux tubes with a different amount of initial twist. As a
result of calculations, tightly twisted tubes reveal a rapid two-step
emergence to the atmosphere with a slight slowdown at the surface,
while weakly twisted tubes show a slow two-step emergence waiting
longer the secondary instability to be triggered. This picture of the
two-step emergence is highly consistent with recent observations. These
tubes show multiple magnetic domes above the surface, indicating that
the secondary emergence is caused by an interchange mode of magnetic
buoyancy instability. In the case of the weakest twist, the tube
exhibits an elongated photospheric structure, and never rises into
the corona. The formation of the photospheric structure is due to an
inward magnetic tension force of the azimuthal field component of
the rising flux tube (i.e., tube's twist). When the twist is weak,
the azimuthal field cannot hold the tube's coherency, and the tube
extends laterally at the subadiabatic surface. In addition, we newly
found that the total magnetic energy measured above the surface depends
on the initial twist. Strong twist tubes follow the initial relation
between the twist and the magnetic energy, while weak twist tubes
deviate from this relation, because these tubes store their magnetic
energy in the photospheric structure.
Title: Magneto-thermal convection in solar prominences
Authors: Berger, Thomas; Testa, Paola; Hillier, Andrew; Boerner, Paul;
Low, Boon Chye; Shibata, Kazunari; Schrijver, Carolus; Tarbell, Ted;
Title, Alan
Bibcode: 2011Natur.472..197B
Altcode:
Coronal cavities are large low-density regions formed by
hemispheric-scale magnetic flux ropes suspended in the Sun's outer
atmosphere. They evolve over time, eventually erupting as the dark
cores of coronal mass ejections. Although coronal mass ejections are
common and can significantly affect planetary magnetospheres, the
mechanisms by which cavities evolve to an eruptive state remain poorly
understood. Recent optical observations of high-latitude `polar crown'
prominences within coronal cavities reveal dark, low-density `bubbles'
that undergo Rayleigh-Taylor instabilities to form dark plumes rising
into overlying coronal cavities. These observations offered a possible
mechanism for coronal cavity evolution, although the nature of the
bubbles, particularly their buoyancy, was hitherto unclear. Here we
report simultaneous optical and extreme-ultraviolet observations of
polar crown prominences that show that these bubbles contain plasma at
temperatures in the range (2.5-12)×105 kelvin, which is
25-120 times hotter than the overlying prominence. This identifies a
source of the buoyancy, and suggests that the coronal cavity-prominence
system supports a novel form of magneto-thermal convection in the solar
atmosphere, challenging current hydromagnetic concepts of prominences
and their relation to coronal cavities.
Title: Solar Activity in 1992-2003: Solar Cycle 23 Observed by Flare
Monitoring Telescope
Authors: Shibata, Kazunari; Kitai, Reizaburo; Ueno, Satoru; Nogami,
Daisaku; Ishii, Takako T.; Katoda, Miwako; Nasuji, Yoko; Kamobe, Mai;
Kaneda, Naoki; Komori, Hiroyuki; Yagi, Shozo; Asai, Ayumi
Bibcode: 2011fmt..book.....S
Altcode:
No abstract at ADS
Title: MHD simulations of quiescent prominence upflows in the
Kippenhahn-Schlüter prominence model
Authors: Hillier, A. S.; Isobe, H.; Shibata, K.; Berger, T. E.
Bibcode: 2011ASInC...2..331H
Altcode:
Images from the Hinode satellite have led to the discovery of dark
upflows that propagate from the base of prominences, developing highly
turbulent profiles. The magnetic Rayleigh-Taylor instability has been
hypothesized as the mechanism to create these plumes. To study the
physics behind this phenomenon we use 3D magnetohydrodynamic simulations
to investigate the nonlinear stability of the Kippenhahn-Shlüter
prominence model to the magnetic Rayleigh-Taylor instability. The model
simulates the rise of a buoyant tube inside a quiescent prominence,
where the upper boundary between the tube and prominence model is
perturbed to excite the interchange of magnetic field lines. We
find upflows of constant velocity (maximum found 6 km s^{-1}) and a
maximum plume width ≈ 1500 km which propagate through a height of
approximately 6 Mm, in general agreement with the Hinode observations.
Title: Fan-shaped Jets in Three-dimensional Reconnection Simulation
as a Model of Ubiquitous Solar Jets
Authors: Jiang, Rong Lin; Shibata, Kazunari; Isobe, Hiroaki; Fang,
Cheng
Bibcode: 2011ApJ...726L..16J
Altcode: 2010arXiv1012.2551J
Magnetic reconnection is a fundamental process in space and
astrophysical plasmas in which the oppositely directed magnetic
field changes its connectivity and eventually converts its energy
into kinetic and thermal energy of the plasma. Recently, ubiquitous
jets (for example, chromospheric anemone jets, penumbral microjets,
umbral light bridge jets) have been observed by the Solar Optical
Telescope on board the satellite Hinode. These tiny and frequently
occurring jets are considered to be a possible evidence of small-scale
ubiquitous reconnection in the solar atmosphere. However, the details
of three-dimensional (3D) magnetic configuration are still not very
clear. Here, we propose a new model based on 3D simulations of magnetic
reconnection using a typical current sheet magnetic configuration with
a strong guide field. The most interesting feature is that the jets
produced by the reconnection eventually move along the guide field
lines. This model provides a fresh understanding of newly discovered
ubiquitous jets and moreover a new observational basis for the theory
of astrophysical magnetic reconnection.
Title: Evolution of the Kippenhahn-Schlüter Prominence Model Magnetic
Field under Cowling Resistivity
Authors: Hillier, Andrew; Shibata, Kazunari; Isobe, Hiroaki
Bibcode: 2010PASJ...62.1231H
Altcode: 2010arXiv1007.1909H
We present the results from 1.5D diffusion simulations of the
Kippenhahn-Schlüter prominence model magnetic field evolution under the
influence of the ambipolar terms of Cowling resistivity. We show that
initially the evolution is determined by the ratio of the horizontal
and vertical magnetic fields, which gives current sheet thinning
(thickening) when this ratio is small (large) and a marginal case
where a new characteristic current sheet length scale is formed. After
a timespan greater than the Cowling resistivity time, the current
sheet thickens as a power law of t independent of the ratio of the
field strengths. These results imply that when Cowling resistivity is
included in the model, the tearing instability time scale is reduced
by more than one order of magnitude when the ratio of the horizontal
field to the vertical field is 20% or less. These results imply that,
over the course of its lifetime, the structure of the prominence can
be significantly altered by Cowling resistivity, and in some cases
will allow the tearing instability to occur.
Title: Magnetohydrodynamic Modeling for a Formation Process of
Coronal Mass Ejections: Interaction Between an Ejecting Flux Rope
and an Ambient Field
Authors: Shiota, Daikou; Kusano, Kanya; Miyoshi, Takahiro; Shibata,
Kazunari
Bibcode: 2010ApJ...718.1305S
Altcode: 2010arXiv1006.0307S
We performed a magnetohydrodynamic simulation of a formation
process of coronal mass ejections (CMEs), focusing on the interaction
(reconnection) between an ejecting flux rope and its ambient field. We
examined three cases with different ambient fields: one had no ambient
field, while the other two had dipole fields with opposite directions,
parallel and anti-parallel to that of the flux rope surface. We found
that while the flux rope disappears in the anti-parallel case, in the
other cases the flux ropes can evolve to CMEs and show different amounts
of flux rope rotation. The results imply that the interaction between
an ejecting flux rope and its ambient field is an important process for
determining CME formation and CME orientation, and also show that the
amount and direction of the magnetic flux within the flux rope and the
ambient field are key parameters for CME formation. The interaction
(reconnection) plays a significant role in the rotation of the flux
rope especially with a process similar to "tilting instability" in a
spheromak-type experiment of laboratory plasma.
Title: Observations of Chromospheric Anemone Jets with Hinode Ca II
Broadband Filtergraph and Hida Ca II Spectroheliograph
Authors: Morita, Satoshi; Shibata, Kazunari; UeNo, Satoru; Ichimoto,
Kiyoshi; Kitai, Reizaburo; Otsuji, Ken-ichi
Bibcode: 2010PASJ...62..901M
Altcode: 2010arXiv1002.2143M
We present the first simultaneous observations of chromospheric
"anemone" jets in solar active regions with Hinode SOT Ca II H
broadband filetergram and Ca II K spetroheliogram on the Domeless
Solar Telescope (DST) at Hida Observatory. During the coordinated
observation, 9 chromospheric anemone jets were simultaneously observed
with the two instruments. These observations revealed three important
features, i.e.: (1) the jets are generated in the lower chromosphere,
(2) the length and lifetime of the jets are 0.4-5 Mm and 40-320 sec,
(3) the apparent velocity of the jets with Hinode SOT are 3-24 km/s,
while Ca II K3 component at the jets show blueshifts (in 5 events) in
the range of 2- 6 km/s. The chromospheric anemone jets are associated
with mixed polarity regions which are either small emerging flux regions
or moving magnetic features. It is found that the Ca II K line often
show red or blue asymmetry in K2/K1 component: the footpoint of the
jets associated with emerging flux regions often show redshift (2-16
km/s), while the one with moving magnetic features show blueshift
(around 5 km/s). Detailed analysis of magnetic evolution of the jet
foaming regions revealed that the reconnection rate (or canceling
rate) of the total magnetic flux at the footpoint of the jets are of
order of 10^{16} Mx/s, and the resulting magnetic energy release rate
(1.1-10) x 10^{24} erg/s, with the total energy release (1-13) x 10^{26}
erg for the duration of the magnetic cancellations, 130s. These are
comparable to the estimated total energy, 10^{26} erg, in a single
chromospheric anemone jet. An observation-based physical model of the
jet is presented. The relation between chromospheric anemone jets and
Ellerman bombs is discussed.
Title: Internal Fine Structure of Ellerman Bombs
Authors: Hashimoto, Yuki; Kitai, Reizaburo; Ichimoto, Kiyoshi; Ueno,
Satoru; Nagata, Shin'ichi; Ishii, Takako T.; Hagino, Masaoki; Komori,
Hiroyuki; Nishida, Keisuke; Matsumoto, Takuma; Otsuji, Kenichi;
Nakamura, Tahei; Kawate, Tomoko; Watanabe, Hiroko; Shibata, Kazunari
Bibcode: 2010PASJ...62..879H
Altcode:
We conducted coordinated observations of Ellerman bombs (EBs) between
Hinode Satellite and Hida Observatory (HOP12). CaII H broad-band
filter images of NOAA 10966 on 2007 August 9 and 10 were obtained
with the Solar Optical Telescope (SOT) aboard the Hinode Satellite,
and many bright points were observed. We identified a total of 4
bright points as EBs, and studied the temporal variation of their
morphological fine structures and spectroscopic characteristics. With
high-resolution CaII H images of SOT, we found that the EBs, thus far
thought of as single bright features, are composed of a few of fine
subcomponents. Also, by using Stokes I/V filtergrams with Hinode/SOT,
and CaII H spectroheliograms with Hida/Domeless Solar Telescope (DST),
our observation showed: (1) The mean duration, the mean width, the
mean length, and the mean aspect ratio of the subcomponents were
390 s, 170 km, 450 km, and 2.7, respectively. (2) Subcomponents
started to appear on the magnetic neutral lines, and extended their
lengths from the original locations. (3) When the CaII H line of EBs
showed the characteristic blue asymmetry, they are associated with the
appearance or re-brightening of subcomponents. Summarizing our results,
we obtained an observational view that elementary magnetic reconnections
take place one by one successively and intermittently in EBs, and that
their manifestation is the fine subcomponents of the EB phenomena.
Title: Generation of Alfvén Waves by Magnetic Reconnection
Authors: Kigure, Hiromitsu; Takahashi, Kunio; Shibata, Kazunari;
Yokoyama, Takaaki; Nozawa, Satoshi
Bibcode: 2010PASJ...62..993K
Altcode: 2010arXiv1002.1360K
In this paper, the results of 2.5-dimensional magnetohydrodynamical
simulations are reported for the magnetic reconnection of non-perfectly
antiparallel magnetic fields. The magnetic field has a component
perpendicular to the computational plane, that is, a guide field. The
angle θ between the magnetic field lines in two half regions was a key
parameter in our simulations, whereas the initial distribution of the
plasma was assumed to be simple; the density and pressure were uniform,
except for the current sheet region. Alfvén waves were generated
at the reconnection point and propagated along the reconnected field
line. The energy fluxes of the Alfvén waves and the magneto-acoustic
waves (slow mode and fast mode) generated by magnetic reconnection
were measured. Each flux shows a similar time evolution independent of
θ. The percentages of the energies (time integral of energy fluxes)
carried by the Alfvén waves and magneto-acoustic waves to the released
magnetic energy were calculated. The Alfvén waves carry 38.9%, 36.0%,
and 29.5% of the released magnetic energy at the maximum (θ = 80°)
in the cases of β = 0.1, 1, and 20, respectively, where β is the
plasma β (the ratio of gas pressure to magnetic pressure). The
magneto-acoustic waves carry 16.2% (θ = 70°), 25.9% (θ = 60°),
and 75.0% (θ = 180°) of the energy at the maximum. Implications of
these results for solar coronal heating and acceleration of high-speed
solar wind are discussed.
Title: Solar-Type Magnetic Reconnection Model for Magnetar Giant
Flares
Authors: Masada, Youhei; Nagataki, Shigehiro; Shibata, Kazunari;
Terasawa, Toshio
Bibcode: 2010PASJ...62.1093M
Altcode:
We present a theoretical model describing magnetar giant flares on
the basis of solar flare/coronal mass ejection theory. In our model,
preflare activity plays a crucial role in driving evaporating flows
and supplying baryonic matter to the magnetosphere. Loaded baryonic
matter, which is called ``prominence'', is then gradually uplifted via
crustal cracking while maintaining a quasi-force-free equilibrium of
the magnetosphere. Finally, the prominence is erupted by a magnetic
pressure force due to the loss of equilibrium triggered by explosive
magnetic reconnection. A giant flare should be induced as a final
outcome of prominence eruption accompanied by large-scale field
reconfigurations. An essential difference between the solar flare
and the magnetar flare is the control process of their evolutionary
dynamics. The flaring activity on magnetars is mainly controlled
by the radiative process, unlike a solar flare governed by electron
conduction. It is full of suggestion that our model accounts for the
physical properties of an extraordinary giant flare observed on 2004
December 27 from SGR 1806-20, including the source of baryonic matter
loaded in the expanding ejecta observed after the giant burst.
Title: Spicule Dynamics over a Plage Region
Authors: Anan, Tetsu; Kitai, Reizaburo; Kawate, Tomoko; Matsumoto,
Takuma; Ichimoto, Kiyoshi; Shibata, Kazunari; Hillier, Andrew; Otsuji,
Kenichi; Watanabe, Hiroko; Ueno, Satoru; Nagata, Shin'ichi; Ishii,
Takako T.; Komori, Hiroyuki; Nishida, Keisuke; Nakamura, Tahei; Isobe,
Hiroaki; Hagino, Masaoki
Bibcode: 2010PASJ...62..871A
Altcode: 2010arXiv1002.2288A
We studied spicular jets over a plage area and derived their
dynamic characteristics using Hinode Solar Optical Telescope (SOT)
high-resolution images. A target plage region was near to the west limb
of the solar disk. This location permitted us to study the dynamics
of spicular jets without any overlapping effect of spicular structures
along the line of sight. In this work, to increase the ease with which
we could identify spicules on the disk, we applied the image processing
method `MadMax' developed by Koutchmy et al. (1989). It enhances fine,
slender structures (like jets), over a diffuse background. We identified
169 spicules over the target plage. This sample permited us to derive
statistically reliable results regarding spicular dynamics. The
properties of plage spicules can be summarized as follows: (1) In a
plage area, we clearly identified spicular jet features. (2) They were
shorter in length than the quiet region limb spicules, and followed a
ballistic motion under constant deceleration. (3) The majority (80%)
of the plage spicules showed a cycle of rise and retreat, while 10% of
them faded out without a complete retreat phase. (4) The deceleration
of the spicule was proportional to the velocity of ejection (i.e.,
the initial velocity).
Title: CaII K Spectral Study of an Emerging Flux Region using the
Domeless Solar Telescope in Hida Observatory
Authors: Otsuji, Kenichi; Kitai, Reizaburo; Matsumoto, Takuma;
Ichimoto, Kiyoshi; Ueno, Satoru; Nagata, Shin'ichi; Isobe, Hiroaki;
Shibata, Kazunari
Bibcode: 2010PASJ...62..893O
Altcode: 2010arXiv1005.2025O
A cooperative observation with Hida Observatory and the Hinode
satellite was performed on an emerging flux region. Successive CaII
K spectro-heliograms of the emerging flux region were taken by the
Domeless Solar Telescope of Hida Observatory. Hinode observed the
emerging flux region with CaII H and FeI Stokes IQUV filtergrams. In
this study, detailed dynamics and the temporal evolution of the
magnetic flux emergence was studied observationally. The event was first
detected in the photospheric magnetic field signals; 3 minutes later,
a horizontal expansion of the dark area was detected. Then, 7 minutes
later than the horizontal expansion, the emerging loops were detected
with a maximal rise speed of 2.1 km s-1 at chromospheric
heights. The observed dynamics of the emerging magnetic flux from the
photosphere to the upper chromosphere was very consistent with the
results of previous simulation studies. A gradual rising phase of flux
tubes with a weak magnetic strength was confirmed by our observation.
Title: Energetic Relations between the Disappearing Solar Filaments
and the Associated Flare Arcades
Authors: Morimoto, Taro; Kurokawa, Hiroki; Shibata, Kazunari; Ishii,
Takako T.
Bibcode: 2010PASJ...62..939M
Altcode:
We present the temporal and statistical relations between the mechanical
energies of disappearing solar filaments and the thermal energies
of the associated flare arcades in soft X-rays. Measuring the 3-D
velocity fields of 10 eruptive filaments, we calculated their mechanical
energy gain rate, ɛmc, per unit volume and compared it to
the thermal energy release rate per unit volume, ɛth,
derived with Yohkoh/SXT data. For the statistical relation, we
found a relation that can be approximated as ɛth ∝
ɛ1.9mc. This relation can be explained by
interpreting the energy input to an arcade via the Poynting flux in
the magnetic reconnection process and the acceleration of a filament
by the Lorentz force. This explanation is also supported by the strong
dependence of the observed increase rates of both the thermal and
mechanical energy densities on the mean magnetic field strength of
the source region. We also investigated their temporal variations,
and found that the start time of increase in the mechanical energy of
a filament preceded that of the thermal energy of the coronal arcade
in some cases. These relations imply that the basic mechanisms that
accelerate a filament and create a hot plasma are different, and
both energy increase rates are determined primary by the magnetic
field strengths.
Title: Continuous H-alpha Imaging Network Project (CHAIN) with Ground-
based Solar Telescopes for Space Weather Research
Authors: Ueno, S.; Shibata, K.; Ichimoto, K.; Kitai, R.; Nagata, S.;
Kimura, G.; Nakatani, Y.
Bibcode: 2010AfrSk..14...17U
Altcode:
No abstract at ADS
Title: Coronal Rain as a Marker for Coronal Heating Mechanisms
Authors: Antolin, P.; Shibata, K.; Vissers, G.
Bibcode: 2010ApJ...716..154A
Altcode: 2009arXiv0910.2383A
Reported observations in Hα, Ca II H, and K or other chromospheric
lines of coronal rain trace back to the days of the Skylab
mission. Corresponding to cool and dense plasma, coronal rain is often
observed falling down along coronal loops in active regions. A physical
explanation for this spectacular phenomenon has been put forward
thanks to numerical simulations of loops with footpoint-concentrated
heating, a heating scenario in which cool condensations naturally
form in the corona. This effect has been termed "catastrophic cooling"
and is the predominant explanation for coronal rain. In this work, we
further investigate the link between this phenomenon and the heating
mechanisms acting in the corona. We start by analyzing observations of
coronal rain at the limb in the Ca II H line performed by the Hinode
satellite, and derive interesting statistical properties concerning
the dynamics. We then compare the observations with 1.5-dimensional
MHD simulations of loops being heated by small-scale discrete events
concentrated toward the footpoints (that could come, for instance,
from magnetic reconnection events), and by Alfvén waves generated at
the photospheric level. Both our observation and simulation results
suggest that coronal rain is a far more common phenomenon than
previously thought. Also, we show that the structure and dynamics of
condensations are far more sensitive to the internal pressure changes
in loops than to gravity. Furthermore, it is found that if a loop is
predominantly heated from Alfvén waves, coronal rain is inhibited due
to the characteristic uniform heating they produce. Hence, coronal
rain may not only point to the spatial distribution of the heating
in coronal loops but also to the agent of the heating itself. We thus
propose coronal rain as a marker for coronal heating mechanisms.
Title: Same-beam VLBI observations of SELENE for improving lunar
gravity field model
Authors: Liu, Q.; Kikuchi, F.; Matsumoto, K.; Goossens, S.; Hanada,
H.; Harada, Y.; Shi, X.; Huang, Q.; Ishikawa, T.; Tsuruta, S.; Asari,
K.; Ishihara, Y.; Kawano, N.; Kamata, S.; Iwata, T.; Noda, H.; Namiki,
N.; Sasaki, S.; Ellingsen, S.; Sato, K.; Shibata, K.; Tamura, Y.;
Jike, T.; Iwadate, K.; Kameya, O.; Ping, J.; Xia, B.; An, T.; Fan,
Q.; Hong, X.; Yang, W.; Zhang, H.; Aili, Y.; Reid, B.; Hankey, W.;
McCallum, J.; Kronschnabl, G.; Schlüter, W.
Bibcode: 2010RaSc...45.2004L
Altcode:
The Japanese lunar mission, Selenological and Engineering Explorer
(Kaguya), which was successfully launched on 14 September 2007,
consists of a main satellite and two small satellites, Rstar and
Vstar. Same-beam very long baseline interferometry (VLBI) observations
of Rstar and Vstar were performed for 15.4 months from November 2007
to February 2009 using eight VLBI stations. In 2008, S band same-beam
VLBI observations totaling 476 h on 179 days were undertaken. The
differential phase delays were successfully estimated for most (about
85%) of the same-beam VLBI observation periods. The high success
rate was mainly due to the continuous data series measuring the
differential correlation phase between Rstar and Vstar. The intrinsic
measurement error in the differential phase delay was less than 1
mm RMS for small separation angles and increased to approximately
2.5 mm RMS for the largest separation angles (up to 0.56 deg). The
long-term atmospheric and ionospheric delays along the line of sight
were reduced to a low level (several tens of milimeters) using the
same-beam VLBI observations, and further improved through application of
GPS techniques. Combining the eight-station (four Japanese telescopes of
VLBI Exploration of Radio Astrometry and four international telescopes)
S band same-beam VLBI data with Doppler and range data, the accuracy of
the orbit determination was improved from a level of several tens of
meters when only using Doppler and range data to a level of 10 m. As
a preliminary test of the technique, the coefficient sigma degree
variance of the lunar gravity field was compared with and without 4
months of VLBI data included. A significant reduction below around 10
deg (especially for the second degree) was observed when the VLBI data
were included. These observations confirm that the VLBI data contribute
to improvements in the accuracy of the orbit determination and through
this to the lunar gravity field model.
Title: The Role Of Torsional Alfvén Waves in Coronal Heating
Authors: Antolin, P.; Shibata, K.
Bibcode: 2010ApJ...712..494A
Altcode: 2009arXiv0910.0962A
In the context of coronal heating, among the zoo of magnetohydrodynamic
(MHD) waves that exist in the solar atmosphere, Alfvén waves receive
special attention. Indeed, these waves constitute an attractive
heating agent due to their ability to carry over the many different
layers of the solar atmosphere sufficient energy to heat and maintain
a corona. However, due to their incompressible nature these waves
need a mechanism such as mode conversion (leading to shock heating),
phase mixing, resonant absorption, or turbulent cascade in order
to heat the plasma. Furthermore, their incompressibility makes their
detection in the solar atmosphere very difficult. New observations with
polarimetric, spectroscopic, and imaging instruments such as those on
board the Japanese satellite Hinode, or the Crisp spectropolarimeter of
the Swedish Solar Telescope or the Coronal Multi-channel Polarimeter,
are bringing strong evidence for the existence of energetic Alfvén
waves in the solar corona. In order to assess the role of Alfvén
waves in coronal heating, in this work we model a magnetic flux tube
being subject to Alfvén wave heating through the mode conversion
mechanism. Using a 1.5 dimensional MHD code, we carry out a parameter
survey varying the magnetic flux tube geometry (length and expansion),
the photospheric magnetic field, the photospheric velocity amplitudes,
and the nature of the waves (monochromatic or white-noise spectrum). The
regimes under which Alfvén wave heating produces hot and stable coronae
are found to be rather narrow. Independently of the photospheric wave
amplitude and magnetic field, a corona can be produced and maintained
only for long (>80 Mm) and thick (area ratio between the photosphere
and corona >500) loops. Above a critical value of the photospheric
velocity amplitude (generally a few km s-1) the corona can
no longer be maintained over extended periods of time and collapses due
to the large momentum of the waves. These results establish several
constraints on Alfvén wave heating as a coronal heating mechanism,
especially for active region loops.
Title: Multiple Plasmoid Ejections and Associated Hard X-ray Bursts
in the 2000 November 24 Flare
Authors: Nishizuka, N.; Takasaki, H.; Asai, A.; Shibata, K.
Bibcode: 2010ApJ...711.1062N
Altcode: 2013arXiv1301.6241N
The Soft X-ray Telescope (SXT) on board Yohkoh revealed that the
ejection of X-ray emitting plasmoid is sometimes observed in a solar
flare. It was found that the ejected plasmoid is strongly accelerated
during a peak in the hard X-ray (HXR) emission of the flare. In
this paper, we present an examination of the GOES X 2.3 class flare
that occurred at 14:51 UT on 2000 November 24. In the SXT images,
we found "multiple" plasmoid ejections with velocities in the range
of 250-1500 km s-1, which showed blob-like or loop-like
structures. Furthermore, we also found that each plasmoid ejection
is associated with an impulsive burst of HXR emission. Although some
correlation between plasmoid ejection and HXR emission has been
discussed previously, our observation shows similar behavior for
multiple plasmoid ejection such that each plasmoid ejection occurs
during the strong energy release of the solar flare. As a result
of temperature-emission measure analysis of such plasmoids, it was
revealed that the apparent velocities and kinetic energies of the
plasmoid ejections show a correlation with the peak intensities in
the HXR emissions.
Title: Nonlinear Propagation of Alfvén Waves Driven by Observed
Photospheric Motions: Application to the Coronal Heating and Spicule
Formation
Authors: Matsumoto, Takuma; Shibata, Kazunari
Bibcode: 2010ApJ...710.1857M
Altcode: 2010arXiv1001.4307M
We have performed MHD simulations of Alfvén wave propagation along
an open flux tube in the solar atmosphere. In our numerical model,
Alfvén waves are generated by the photospheric granular motion. As the
wave generator, we used a derived temporal spectrum of the photospheric
granular motion from G-band movies of Hinode/Solar Optical Telescope. It
is shown that the total energy flux at the corona becomes larger and
the transition region's height becomes higher in the case when we use
the observed spectrum rather than the white/pink noise spectrum as
the wave generator. This difference can be explained by the Alfvén
wave resonance between the photosphere and the transition region. After
performing Fourier analysis on our numerical results, we have found that
the region between the photosphere and the transition region becomes
an Alfvén wave resonant cavity. We have confirmed that there are at
least three resonant frequencies, 1, 3, and 5 mHz, in our numerical
model. Alfvén wave resonance is one of the most effective mechanisms
to explain the dynamics of the spicules and the sufficient energy flux
to heat the corona.
Title: Unified picture of large and small scale: micro-flares,
flares, particle acceleration
Authors: Shibata, Kazunari
Bibcode: 2010cosp...38.2837S
Altcode: 2010cosp.meet.2837S
Recent space observations of the Sun revealed that magnetic reconnection
is ubiquitous in the solar atmosphere, ranging from small scale
reconnection (observed as nanoflares) to large scale one (observed as
long duration flares or giant arcades). Often these reconnections are
associ-ated with mass ejections or jets. Coronal mass ejections (CMEs)
are among the largest one associated with magnetic reconnection. Recent
Hinode satellite has revealed even smaller re-connection events
and associated jets, i.e., tiny chromosphere jets observed with
Hinode/SOT. These chromospheric jets have a width of only a few
100 km. As spatial resolution of obser-vations become better and
better, smaller and smaller flares and jets have been discovered,
which implies that the magnetized solar atmosphere consist of fractal
structure and dynamics, i.e., fractal reconnection. Bursty radio and
hard X-ray emissions from flares also suggest the fractal reconnection
and associated particle acceleration. Since magnetohydrodynamics (MHD)
does not contain any characteristic length and time scale, it is natural
that MHD structure, dynamics, and reconnection, tend to become fractal
in ideal MHD plasmas with large magnetic Reynolds number such as in the
solar atmosphere. We would discuss recent observations and theories
related to fractal reconnection, and discuss possible implication to
coronal heating, reconnection physics, and particle acceleration.
Title: Signatures of Coronal Heating Mechanisms
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
Bibcode: 2010ASSP...19..277A
Altcode: 2010mcia.conf..277A; 2009arXiv0903.1766A
Alfvén waves created by sub-photospheric motions or by magnetic
reconnection in the low solar atmosphere seem good candidates for
coronal heating. However, the corona is also likely to be heated more
directly by magnetic reconnection, with dissipation taking place
in current sheets. Distinguishing observationally between these
two heating mechanisms is an extremely difficult task. We perform
1.5-dimensional MHD simulations of a coronal loop subject to each
type of heating and derive observational quantities that may allow
these to be differentiated. This work is presented in more detail in
Antolin et al. (2008).
Title: MHD simulations of upflows in the Kippenhahn-Schlueter
prominence model
Authors: Hillier, Andrew; Shibata, Kazunari; Isobe, Hiroaki; Berger,
Thomas
Bibcode: 2010cosp...38.2914H
Altcode: 2010cosp.meet.2914H
The launch of SOT on the Hinode satellite, with it's previously
unprecedented high resolution, high cadence images of solar prominences,
led to the discovery of small scale, highly dynamic flows in quiescent
prominences. Berger et al. (2008) reported dark upflows that propagated
from the base of the prominence through a height of approximately 10
Mm before ballooning into the familiar mushroom shape often associated
with the Rayleigh-Taylor instability. Whether such phenomena can be
driven by instabilities and, if so, how the instability evolve is yet
to be fully investigated. In this study, we use the Kippenhahn-Schlueter
(K-S) prominence model as the base for 3D numerical MHD simulations. The
K-S prominence model is linearly stable for ideal MHD perturbationss,
but can be made unstable through nonlinear perturbations, which we
impose through inserting a low density (high temperature) tube through
the centre of the prominence. Our simulations follow the linear and
nonlinear evolution of upflows propagating from the hot tube through the
K-S prominence model. We excited Rayleigh-Taylor like modes inside the
K-S model with a wave along the contact discontinuity created between
the hot tube and the K-S prominence, and solved the pertur-bations
of this system. For such a complex setting, the linear evolution of
the instability has 0.7 not been studied, and we found the growth
rate to be ∼ ( ρ+ -ρ- - 0.05)k 0.22 . The most ρ+ +ρ- unstable
wavelength was ∼ 100 km which, through the inverse cascade process,
created upflows of ∼ 300 km. The rising plumes obtained a constant
rise velocity in the nonlinear stage due to the creation of adverse
magnetic and gas pressure gradients at the top of the plume.
Title: MHD modeling for Formation Process of Coronal Mass Ejections:
Interaction between Ejecting Flux Rope and Ambient Field
Authors: Shiota, Daikou; Kusano, Kanya; Miyoshi, Takahiro; Shibata,
Kazunari
Bibcode: 2010cosp...38.1856S
Altcode: 2010cosp.meet.1856S
Coronal mass ejections (CMEs), in which large amount of magnetic
flux is launched into the interplanetary space, are most explosive
phenomena in the solar corona. Due to their large influences to the
space environment near the Earth, it is very important to make cleat
how CMEs are formed and how determine the field orientations within
CMEs. In order to examine the sufficient conditions, we performed three
dimensional magnetohydrodynamic simulation of formation processes of
CMEs, focusing on interaction (reconnection) between an ejecting flux
rope and its ambient field. We examined three cases with different
ambient fields: no ambient field, and cases with dipole field of
two opposite directions which are parallel and anti-parallel to
that of the flux rope surface. As the results, while the flux rope
disappears in the anti-parallel case, in other cases the flux ropes
can evolve to CMEs and however shows different amount of rotation
of the flux rope. The results mean that the interaction between an
ejecting flux rope and its ambient field is a significant process for
determining CME formation and CME orientation, and also show that the
amount and direction of magnetic flux within the flux rope and the
ambient field are key parameters for CME formation. Especially, the
interaction (reconnection) plays a significant role to the rotation
of the flux rope, with a process similar to "tilting instability"
in a spheromak-type experiment of laboratory plasma.
Title: Nonlinear Propagation of Alfven Waves Driven by Observed
Photospheric Motions: Application to the Coronal Heating and Spicule
Formation
Authors: Matsumoto, Takuma; Shibata, Kazunari
Bibcode: 2010cosp...38.2919M
Altcode: 2010cosp.meet.2919M
We have performed MHD simulations of Alfven wave propagation along an
open ux tube in the solar atmosphere. In our numerical model, Alfven
waves are generated by the photospheric granular motion. As the wave
generator, we used a derived temporal spectrum of the photo-spheric
granular motion from G-band movies of Hinode/SOT. It is shown that the
total energy ux at the corona becomes larger and the transition region
height becomes higher in the case when we use the observed spectrum
rather than white/pink noise spectrum as the wave gener-ator. This
difference can be explained by the Alfven wave resonance between
the photosphere and the transition region. After performing Fourier
analysis on our numerical results, we have found that the region
between the photosphere and the transition region becomes an Alfven
wave resonant cavity. We have conrmed that there are at least three
resonant frequencies, 1, 3 and 5 mHz, in our numerical model. Alfven
wave resonance is one of the most effective mechanisms to explain
the dynamics of the spicules and the sufficient energy ux to heat
the corona.
Title: Fractal Reconnection and Particle Acceleration in the Solar
Atmosphere
Authors: Nishizuka, Naoto; Shibata, Kazunari
Bibcode: 2010cosp...38.1959N
Altcode: 2010cosp.meet.1959N
Recent space observations of the Sun with Yohkoh, SOHO, TRACE, RHESSI
and Hinode revealed that magnetic reconnection is ubiquitous in the
solar atmosphere, ranging from small scale reconnection (nanoflares)
to large scale one (Coronal Mass Ejection related flares). These
observations imply that the solar atmosphere consists of self-similar
structure, i.e., fractal structure. It is also proposed that even
current sheet might have a fractal structure, which is favorable
for particle acceleration. Here we propose that nonthermal electrons
are efficiently accelerated by Fermi process at fast shocks, coupled
with the dynamics of multiple plasmoid ejections. Multiple plasmoids
collide with an oblique fast shock, which is naturally formed below the
reconnection site. The accelerated particles are trapped in a plasmoid
and reflected at the shock front due to magnetic mirror upstream of the
fast shock. As a plasmoid passing through the shock front, thetrapping
distance becomes shorter and shorter, driving the first-order Fermi
acceleration until it becomes electron larmor radius. We performed
2.5D resistive MHD simulation and test particle simulation, and showed
that particles can be accelerated more efficiently during the plasmoid
ejections. Furthermore, when we consider fractal plasmoid ejections,
we can naturally explainthe power-law distributions of hard X-ray
observations.
Title: Solar Plages: Observational Study of Their Chromospheric
Heating and Spicular Mass Ejections
Authors: Kitai, R.; Hashimoto, Y.; Anan, T.; Matsumoto, T.; Kawate,
T.; Watanabe, H.; Otsuji, K.; Nakamura, T.; Nishizuka, N.; Nishida,
K.; Ueno, S.; Nagata, S.; Shibata, K.
Bibcode: 2009ASPC..415...19K
Altcode:
We investigated the chromospheric dynamics of plage area. From our
spectroscopic analysis of the CaII K line, we have found that their
periodic variations are due to the propagation of acoustic waves from
the lower layers. Another observational work on Hinode CaII H images,
gave us a new result that there are numerous spicular jets in plage
area, thanks to the stable observing condition of Hinode. The present
paper is an extended abstract of our works which will be published
fully in our future papers.
Title: Magnetohydrodynamics Study of Three-Dimensional Fast Magnetic
Reconnection for Intermittent Snake-Like Downflows in Solar Flares
Authors: Shimizu, T.; Kondo, K.; Ugai, M.; Shibata, K.
Bibcode: 2009ApJ...707..420S
Altcode:
Three-dimensional instability of the spontaneous fast magnetic
reconnection is studied with magnetohydrodynamics (MHD)
simulation, where the two-dimensional model of the spontaneous
fast magnetic reconnection is destabilized in three dimensions. In
two-dimensional models, every plasma condition is assumed to be
uniform in the sheet current direction. In that case, it is well
known that the two-dimensional fast magnetic reconnection can be
caused by current-driven anomalous resistivity, when an initial
resistive disturbance is locally put in a one-dimensional current
sheet. In this paper, it is studied whether the two-dimensional fast
magnetic reconnection can be destabilized or not when the initial
resistive disturbance is three dimensional, i.e., that which has weak
fluctuations in the sheet current direction. According to our study,
the two-dimensional fast magnetic reconnection is developed to the
three-dimensional intermittent fast magnetic reconnection which is
strongly localized in the sheet current direction. The resulting fast
magnetic reconnection repeats to randomly eject three-dimensional
magnetic loops which are very similar to the intermittent downflows
observed in solar flares. In fact, in some observations of solar flares,
the current sheet seems to be approximately one dimensional, but the
fast magnetic reconnection is strongly localized in the sheet current
direction, i.e., fully three dimensional. In addition, the observed
plasma downflows as snake-like curves. It is shown that those observed
features are consistent with our numerical MHD study.
Title: Alfvén Wave and Nanoflare Reconnection Heating: How to
Distinguish Them Observationally?
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
Bibcode: 2009ASPC..415..247A
Altcode:
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is nanoflare reconnection heating, in which energy is released
through many small magnetic reconnection events. Distinguishing the
observational features of each mechanism is an extremely difficult
task. By setting up a 1.5D MHD model of a loop we test both heating
mechanisms and derive observational quantities. The obtained coronae
differ in many aspects; for instance, in the flow patterns along
the loop, flow velocities, and the simulated intensity profile that
Hinode/XRT would observe. The heating events in the loop exhibit
power-law distributions in frequency, whose indexes differ considerably
depending on the heating mechanism and its location along the loop. We
thus test the observational signatures of the power-law index as a
diagnostic tool for the above coronal heating mechanisms.
Title: The tandem Fabry Perot Full-Disk Solar Vector Magnetogram
system for the Solar Magnetic Activity Research Telescope (SMART)
Authors: Otsuji, K.; Nagata, S.; Ueno, S.; Kitai, R.; Kimura, G.;
Nakatani, Y.; Ishii, T.; Morita, S.; Shibata, K.
Bibcode: 2009AGUFMSH33B1498O
Altcode:
For solar and heliospheric physics, energy release mechanism of
solar flares and onset of CMEs are one of the most the most important
issues. Solar Magnetic Activity Research Telescope (SMART) in Hida
observatory Kyoto University aims to investigate the relation between
the solar flares and photospheric vector magnetic field. Full disk high
resolution H-alpha images as well as and photospheric vector magnetic
field using Fe I 6302 line have been taken regularly since 2005. In
order to improve the vector magnetic field measurement accuracy,
a tandem Fabry Perot Stokes polarimeter is newly constructed for
the SMART. The system consist of a tandem FP with FWHM of ~0.01nm,
polarization beam splitter and two CCD cameras taking orthogonally
polarized images simultaneously. In this poster we present the
system design and laboratory test results, and expected observational
capabilities. We also present the flux emergence activities observed
with SMART and Hinode satellite.
Title: Study of Chromospheric Jets Using Hinode Observations and MHD
Simulations: Evidence of Propagating Alfvén Waves and Reconnection,
and Its Implication to the Coronal Heating Problem
Authors: Nishizuka, N.; Shibata, K.
Bibcode: 2009ASPC..415..188N
Altcode:
We discuss the following subjects: 1) the discovery of chromospheric
jets by Hinode/SOT and comparison with the reconnection model, 2) the
discovery of propagating Alfvén waves associated with a chromospheric
jet and the generation of the wave via magnetic reconnection, 3) the
energy estimation of the propagating Alfvén wave and its implications
for coronal heating.
Title: Kyoto 3.8m segmented telescope project
Authors: Shibata, K.
Bibcode: 2009iac..talk..167S
Altcode: 2009iac..talk..110S
No abstract at ADS
Title: The tandem Fabry-Perot filter imaging spectro-polarimeter
for the Solar Magnetic Activity Research Telescope (SMART)
Authors: Nagata, Shin'ichi; Otsuji, Kenichi; Ishii, Takako T.;
Ichimoto, Kiyoshi; Ueno, Satoru; Kitai, Reizaburo; Kimura, Goichi;
Shibata, Kazunari; Nakatani, Yoshikazu; Morita, Satoshi
Bibcode: 2009SPIE.7438E..0VN
Altcode: 2009SPIE.7438E..22N
In order to perform precise and high time cadence magnetic field
measurement across the solar surface, the Tandem Fabry-Perot filter
imaging spectro-polarimeter for the Solar Magnetic Activity Research
Telescope (SMART) is revised. By using the CCD with moderate frame
rate of 30fps, full Stokes vectors on the field-of-view 320"x240" can
be obtained at 4 wavelengths around FeI6302 line within about 15s. The
optical performance of the Tandem Fabry-Perof filters is investigated
by using the spectrograph at the Domeless Solar Telescope at Hida
Observatory. The test results show the full-width-half-maximum (FWHM)
of the tandem filters is about 0.017nm over the 60mm clear aperture
is achieved. The system is developed to start the regular observations
from 2010.
Title: Reconnection in solar flares: Outstanding questions
Authors: Isobe, Hiroaki; Shibata, Kazunari
Bibcode: 2009JApA...30...79I
Altcode:
No abstract at ADS
Title: Magnetorotational Instability in Viscous Media: Application
to the Central Engine of Gamma-Ray Bursts
Authors: Masada, Y.; Shibata, K.
Bibcode: 2009RMxAC..36..163M
Altcode:
In ultra dense and hot regions realized in stellar core-collapse,
neutrinos take a major role in the energy and momentum transports. We
investigate the growth of the magnetorotational instability (MRI) in
neutrino viscous media by using linear and nonlinear calculations. It
is found from the local linear analysis that the neutrino viscosity
can suppress the MRI in the regime of weak magnetic field (
B 10^14G). This suggests that MHD turbulence sustained by the MRI
might not be driven efficiently in neutrino viscous media. Applying
this result to a collapsar disk, which is known as the central engine
of gamma-ray bursts (GRB), we find that the MRI can be suppressed
only in its inner region. Based on this finding, a new evolutionary
scenario of a collapsar disk, the ``Episodic Disk Accretion Model''
is proposed. Finally, we report our recent numerical study on the
nonlinear evolution of the MRI in neutrino viscous media.
Title: Coupled chemistry climate model simulations of stratospheric
temperatures and their trends for the recent past
Authors: Austin, J.; Wilson, R. J.; Akiyoshi, H.; Bekki, S.; Butchart,
N.; Claud, C.; Fomichev, V. I.; Forster, P.; Garcia, R. R.; Gillett,
N. P.; Keckhut, P.; Langematz, U.; Manzini, E.; Nagashima, T.; Randel,
W. J.; Rozanov, E.; Shibata, K.; Shine, K. P.; Struthers, H.; Thompson,
D. W. J.; Wu, F.; Yoden, S.
Bibcode: 2009GeoRL..3613809A
Altcode:
Temperature results from multi-decadal simulations of coupled chemistry
climate models for the recent past are analyzed using multi-linear
regression including a trend, solar cycle, lower stratospheric
tropical wind, and volcanic aerosol terms. The climatology of the
models for recent years is in good agreement with observations for the
troposphere but the model results diverge from each other and from
observations in the stratosphere. Overall, the models agree better
with observations than in previous assessments, primarily because of
corrections in the observed temperatures. The annually averaged global
and polar temperature trends simulated by the models are generally in
agreement with revised satellite observations and radiosonde data over
much of their altitude range. In the global average, the model trends
underpredict the radiosonde data slightly at the top of the observed
range. Over the Antarctic some models underpredict the temperature
trend in the lower stratosphere, while others overpredict the trends.
Title: New Observation of Failed Filament Eruptions: The Influence
of Asymmetric Coronal Background Fields on Solar Eruptions
Authors: Liu, Y.; Su, J.; Xu, Z.; Lin, H.; Shibata, K.; Kurokawa, H.
Bibcode: 2009ApJ...696L..70L
Altcode:
Failed filament eruptions not associated with a coronal mass ejection
(CME) have been observed and reported as evidence for solar coronal
field confinement on erupting flux ropes. In those events, each
filament eventually returns to its origin on the solar surface. In
this Letter, a new observation of two failed filament eruptions is
reported which indicates that the mass of a confined filament can be
ejected to places far from the original filament channel. The jetlike
mass motions in the two failed filament eruptions are thought to be
due to the asymmetry of the background coronal magnetic fields with
respect to the locations of the filament channels. The asymmetry of the
coronal fields is confirmed by an extrapolation based on a potential
field model. The obvious imbalance between the positive and negative
magnetic flux (with a ratio of 1:3) in the bipolar active region is
thought to be the direct cause of the formation of the asymmetric
coronal fields. We think that the asymmetry of the background fields
can not only influence the trajectories of ejecta, but also provide
a relatively stronger confinement for flux rope eruptions than the
symmetric background fields do.
Title: Preface
Authors: Tsurutani, Bruce T.; Shibata, Kazunari; Akasofu, Syun-Ichi;
Oka, Mitsuo
Bibcode: 2009EP&S...61..553T
Altcode:
No abstract at ADS
Title: A two-step scenario for both solar flares and magnetospheric
substorms: Short duration energy storage
Authors: Tsurutani, Bruce T.; Shibata, Kazunari; Akasofu, Syun-Ichi;
Oka, Mitsuo
Bibcode: 2009EP&S...61..555T
Altcode: 2009EP&S...61L.555T
The basic observations for magnetic storms and substorms at Earth and
for flares at the Sun are reviewed for background. We present a common
scenario of double magnetic reconnection for both substorms and flares
based on previous interplanetary observations and substorm-triggering
results. Central to the scenario is that the first magnetic reconnection
phase is the source of energy loading for possible substorms and
flares. The energy placed in the magnetotail or magnetosphere/at the
sun lasts for only a short duration of time however. The energy gets
dissipates away rapidly (in some less dramatic form). This scenario
predicts that if the initial reconnection process is sufficiently
intense and rapid, concomitant substorms and flares occur soon
thereafter. If the energy input is less rapid, there may be lengthy
delays for the onset of substorms and flares. If external influences
(shocks, etc.) occur during the latter energy buildup, the "trigger"
will cause a sudden release of this energy. The model also explains
reconnection without subsequent substorms and flares. The model
addresses the question why strong triggering events are sometimes
ineffective.
Title: Prominence Formation Associated with an Emerging Helical
Flux Rope
Authors: Okamoto, Takenori J.; Tsuneta, Saku; Lites, Bruce W.; Kubo,
Masahito; Yokoyama, Takaaki; Berger, Thomas E.; Ichimoto, Kiyoshi;
Katsukawa, Yukio; Nagata, Shin'ichi; Shibata, Kazunari; Shimizu,
Toshifumi; Shine, Richard A.; Suematsu, Yoshinori; Tarbell, Theodore
D.; Title, Alan M.
Bibcode: 2009ApJ...697..913O
Altcode: 2009arXiv0904.0007O
The formation and evolution process and magnetic configuration of
solar prominences remain unclear. In order to study the formation
process of prominences, we examine continuous observations of a
prominence in NOAA AR 10953 with the Solar Optical Telescope on
the Hinode satellite. As reported in our previous Letter, we find
a signature suggesting that a helical flux rope emerges from below
the photosphere under a pre-existing prominence. Here we investigate
more detailed properties and photospheric indications of the emerging
helical flux rope, and discuss their relationship to the formation of
the prominence. Our main conclusions are: (1) a dark region with absence
of strong vertical magnetic fields broadens and then narrows in Ca II
H-line filtergrams. This phenomenon is consistent with the emergence
of the helical flux rope as photospheric counterparts. The size of the
flux rope is roughly 30,000 km long and 10,000 km wide. The width is
larger than that of the prominence. (2) No shear motion or converging
flows are detected, but we find diverging flows such as mesogranules
along the polarity inversion line. The presence of mesogranules may
be related to the emergence of the helical flux rope. (3) The emerging
helical flux rope reconnects with magnetic fields of the pre-existing
prominence to stabilize the prominence for the next several days. We
thus conjecture that prominence coronal magnetic fields emerge in
the form of helical flux ropes that contribute to the formation and
maintenance of the prominence.
Title: Two-dimensional numerical study for relativistic outflow from
strongly magnetized neutron stars
Authors: Matsumoto, Jin; Masada, Youhei; Asano, Eiji; Shibata, Kazunari
Bibcode: 2009IAUS..259..139M
Altcode:
Using special relativistic magnetohydrodynamic simulation, the nonlinear
dynamics of the magnetized outflow triggered on the magnetar surface
is investigated. It is found that the strong shock propagates in the
circumstellar medium in association with the expanding outflow. The
shock velocity vsh depends on the strength of the
dipole field anchored to the stellar surface Bdipole
and is described by a simple scaling relation vsh ∝
Bdipole0.5. In addition, the outflow-driven
shock can be accelerated self-similarly to the relativistic velocity
when the density profile of the circumstellar medium is steeper than
the critical density profile, that is α ≡ d logρ(r)/d log r ≲
αcrit = -5.0, where the density is set as a power law
distribution with an index α and r is the cylindrical radius. Our
results suggest that the relativistic outflow would be driven by the
flaring activity in a circumstellar medium with a steep density profile.
Title: A Two-Step Scenario for Both Solar Flares and Magnetospheric
Substorms: Short Duration Energy Storage
Authors: Tsurutani, B. T.; Shibata, K.; Akasofu, S. -I.; Oka, M.
Bibcode: 2009EGUGA..11.2256T
Altcode:
We present a common scenario of double magnetic reconnection for both
substorms and flares based on previous interplanetary observations
and substorm-triggering results. Central to the scenario is that
the first magnetic reconnection process is the source of energy
loading for possible substorms and flares. The energy placed in the
magnetotail or magnetosphere/at the sun lasts for only a short duration
of time however. The energy gets dissipates away rapidly (in some less
dramatic form). This scenario predicts that if the initial reconnection
process is sufficiently rapid, substorms and flares occur in short
order. If the energy input is less rapid, there may be lengthy delays
for the onset of substorms and flares. If external influences (shocks,
etc.) occur during the latter energy buildup, the "trigger" will cause a
sudden release of this energy. The model explains reconnection without
subsequent substorms and flares. It also addresses the question why
some strong triggering events are ineffective.
Title: The Power-Law Distribution of Flare Kernels and Fractal
Current Sheets in a Solar Flare
Authors: Nishizuka, N.; Asai, A.; Takasaki, H.; Kurokawa, H.;
Shibata, K.
Bibcode: 2009ApJ...694L..74N
Altcode: 2013arXiv1301.6244N
We report a detailed examination of the fine structure inside flare
ribbons and the temporal evolution of this fine structure during
the X2.5 solar flare that occurred on 2004 November 10. We examine
elementary bursts of the C IV (~1550 Å) emission lines seen as local
transient brightenings inside the flare ribbons in the ultraviolet
(1600 Å) images taken with Transition Region and Coronal Explorer,
and we call them C IV kernels. This flare was also observed in Hα
with the Sartorius 18 cm Refractor telescope at Kwasan observatory,
Kyoto University, and in hard X-rays (HXR) with Reuven Ramaty High
Energy Solar Spectroscopic Imager. Many C IV kernels, whose sizes were
comparable to or less than 2'', were found to brighten successively
during the evolution of the flare ribbon. The majority of them were
well correlated with the Hα kernels in both space and time, while
some of them were associated with the HXR emission. These kernels were
thought to be caused by the precipitation of nonthermal particles at the
footpoints of the reconnecting flare loops. The time profiles of the C
IV kernels showed intermittent bursts, whose peak intensity, duration,
and time interval were well described by power-law distribution
functions. This result is interpreted as evidence for "self-organized
criticality" in avalanching behavior in a single flare event, or for
fractal current sheets in the impulsive reconnection region.
Title: Evolution of the anemone AR NOAA 10798 and the related
geo-effective flares and CMEs
Authors: Asai, Ayumi; Shibata, Kazunari; Ishii, Takako T.; Oka,
Mitsuo; Kataoka, Ryuho; Fujiki, Ken'ichi; Gopalswamy, Nat
Bibcode: 2009JGRA..114.0A21A
Altcode: 2009JGRA..11400A21A; 2008arXiv0812.2063A
We present a detailed examination of the features of the active region
(AR) NOAA 10798. This AR generated coronal mass ejections (CMEs) that
caused a large geomagnetic storm on 24 August 2005 with the minimum Dst
index of -216 nT. We examined the evolution of the AR and the features
on/near the solar surface and in the interplanetary space. The AR
emerged in the middle of a small coronal hole, and formed a sea anemone
like configuration. Hα filaments were formed in the AR, which have
southward axial field. Three M class flares were generated, and the
first two that occurred on 22 August 2005 were followed by Halo-type
CMEs. The speeds of the CMEs were fast, and recorded about 1200 and
2400 km s-1, respectively. The second CME was especially
fast, and caught up and interacted with the first (slower) CME during
their travelings toward Earth. These acted synergically to generate
an interplanetary disturbance with strong southward magnetic field of
about -50 nT, which was followed by the large geomagnetic storm.
Title: Numerical Examination of Plasmoid-Induced Reconnection
Model for Solar Flares: The Relation between Plasmoid Velocity and
Reconnection Rate
Authors: Nishida, Keisuke; Shimizu, Masaki; Shiota, Daikou; Takasaki,
Hiroyuki; Magara, Tetsuya; Shibata, Kazunari
Bibcode: 2009ApJ...690..748N
Altcode: 2008arXiv0809.0797N
The plasmoid-induced reconnection model explaining solar flares based on
bursty reconnection produced by an ejecting plasmoid suggests a possible
relation between the ejection velocity of a plasmoid and the rate of
magnetic reconnection. In this study, we focus on the quantitative
description of this relation. We performed magnetohydrodynamic
simulations of solar flares by changing the values of resistivity
and the plasmoid velocity. The plasmoid velocity has been changed by
applying an additional force to the plasmoid to see how the plasmoid
velocity affects the reconnection rate. An important result is that the
reconnection rate has a positive correlation with the plasmoid velocity,
which is consistent with the plasmoid-induced reconnection model for
solar flares. We also discuss an observational result supporting this
positive correlation.
Title: Climate and Weather of the Sun-Earth System (CAWSES)
Authors: Tsuda, T.; Fujii, R.; Shibata, K.; Geller, M. A.
Bibcode: 2009cwse.conf.....T
Altcode:
No abstract at ADS
Title: Commission 49: Interplanetary Plasma and Heliosphere
Authors: Bougeret, Jean-Louis; von Steiger, Rudolf; Webb, David
F.; Ananthakrishnan, Subramanian; Cane, Hilary V.; Gopalswamy,
Natchimuthuk; Kahler, Stephen W.; Lallement, Rosine; Sanahuja, Blai;
Shibata, Kazunari; Vandas, Marek; Verheest, Frank
Bibcode: 2009IAUTA..27..124B
Altcode:
Commission 49 covers research on the solar wind, shocks and particle
acceleration, both transient and steady-state, e.g., corotating,
structures within the heliosphere, and the termination shock and
boundary of the heliosphere.
Title: Three-Dimensional Magnetic Topology in the AR10930 based on
the Non-Linear Force- Free Modeling
Authors: Inoue, S.; Shiota, D.; Kusano, K.; Asano, E.; Matsumoto,
T.; Kataoka, R.; Miyoshi, T.; Magara, T.; Yamamoto, T.; Ogino, T.;
Shibata, K.
Bibcode: 2008AGUFMSH41A1606I
Altcode:
Three-dimensional (3D) magnetic field cannot be directly observed by
ground observatories and space satellites, because the magnetic field is
unfortunately obtained only solar surface. Therefore the reconstruction
of 3D coronal magnetic field using only 2D data on the photosphere is
the strong tool to help the understanding not only 3D structure but
also the solar activities. We have also developed a new Non- linear
Force-Free (NLFF) field extrapolation method, based on the extended
magnetofrictional model and applied this NLFF solver to the AR NOAA
10930 observed by Hinode/SOT. As a result, we succeeded to reproduce the
strong sheared structure on the neutral line before the flare and the
post flare loop structure after the flare. However we have some problems
such as narrow calculation domain caused by Hinode/SOT,imposed on the
artificial sides and top boundary conditions, 180 degree ambiguity
and so on. In this study, to overcome these problems, first, the
calculation domain is extended 2 times for Hinode/SOT region in the
East-West direction and 4 times in the North-South one by connecting
between Hinode/SOT data and SOHO/MDI data. Sides and top boundaries are
determined by global potential field model form SOHO/MDI (Shiota et
al. 2008). From this result, the strong sheared region was reproduce
as same as previous one, however, the field line connectivity is
different. This result suggests the difference of separatrix structure
between every each different boundary condition. Furthermore we will
report the results of the 3D field line topology in detail by not only
different boundary conditions but also 180 degree ambiguity. We will
check the validity of these structures compared with Hinode/XRT.
Title: Cooperative observation of solar atmospheric heating by Hida
observatory and Hinode
Authors: Kitai, R.; Hashimoto, Y.; Anan, T.; Watanabe, H.; Ishii,
T. T.; Kawate, T.; Matsumoto, T.; Otsuji, K.; Nakamura, T.; Morita,
S.; Nishizuka, N.; Nishida, K.; Ueno, S.; Nagata, S.; Ichimoto, K.;
Shibata, K.
Bibcode: 2008AGUFMSH41B1625K
Altcode:
At Hida observatory of Kyoto University, we continue to study solar
activities and fine structures with Domeless Solar Telescope (DST)
and Solar Magnetic Activity Research Telescope (SMART). In this work,
we will report some recent cooperative observational results with
Hinode on the following topics: (1) Plage heating and waves Analysis
of a long time series of CaII K spectrograms at a plage area showed
us a clear co-existence of 3- and 5-min oscillation in Doppler
velocity. We simulated the response of the VAL model atmosphere to
the input of 3-min/5-min acoustic disturbances, in 1-D geometry and
found that plage chromosphere is heated unsteadily by acoustic shock
waves as was proposed by Carlsson and Stein (1997). (2) Disk spicules
in and around plage regions We clearly identified numerous ejecting
features in a plage area. Their morphological shapes of thin tapered
cylinder and their dynamics strongly suggest that they are spicules
in plage area. Plage spicules were observed to move under constant
deceleration, which are driven by acoustic shock waves predicted by
Shibata and Suematsu (1980) and Hansteen et al. (2007). Our results
will be discussed from the view point of Type I, II classification
of limb spicules ( de Pontieu et al. 2007). (3) Umbral dots We have
confirmed that umbral dots are manifestation of magneto-convection in
strong magnetic filed from the analysis of Hinode/SOT/BFI&SP. We
will discuss the plausibility of monolithic umbral model from the
oscillatory brightening of umbral dots. (4) X-ray brightenings in the
supergranular network XRT showed us numerous bright points in solar
quiet regions. Possible relation between these XBPs and supergranular
network pattern in quiet chromosphere was studied. XBPs were found to be
located in the network not in the cell center. Many of network bright
XBPs were consisted of magnetically bipolar loops. (5) Ellerman bombs
By studying the fine structure of Ellerman bomb, we have found core-halo
structure and loop like fine-structures in the chromosphere. Discussions
on the origin of bombs will be given from the viewpoint of magnetic
reconnection theory.
Title: Data-driven Multiscale Simulation Study of Solar Eruption
Based on Hinode Vector Magnetogram
Authors: Kusano, K.; Shiota, D.; Inoue, S.; Kataoka, R.; Asano, E.;
Matsumoto, T.; Shibata, K.
Bibcode: 2008AGUFMSH52A..06K
Altcode:
Solar eruptions, which arise as flares and coronal mass ejection (CME),
are the most energetic phenomena in our solar system, and can often
influence even the geo-space environment. However, the initiation
mechanism and the physical condition for the onset of them are not
yet well understood. The objective of this study is to clarify how the
multiscale interaction between small scale magnetic reconnection and
the large scale evolution of the solar coronal magnetic field interact
with each other for the initiation of solar eruptions. In order to
obtain that, we have developed a realistic magnetohydrodynamics (MHD)
simulation, which is driven by a high resolution vector magnetogram
observed by Solar Optical Telescope (SOT) onboard Hinode. The simulation
is performed by incorporating the advanced magnetofrictional model
for the nonlinear force-free extrapolation and the three different
MHD models for active region, global corona, and the interplanetary
space, respectively. Using the model, we have successfully performed
the first-ever data- driven simulation of the eruptive event caused
by the X-class solar flare, which occurred in the active region NOAA
10930 on Dec. 13, 2006. The simulation result indicates that magnetic
reconnection triggered in a strongly sheared region causes a large scale
eruption, in which a plasmoid with helical magnetic flux is ejected at
super-Alfvenic speed. The direction and speed of plasmoid ejection are
well consistent with the observation by EUV imaging spectrometer (EIS)
onboard Hinode. The evolution of field line topology is also consistent
with the SOT observation of flare ribbons. The propagation of CME,
which is formed as a result of the plasmoid ejection, is calculated
by handing over the output data of the active region model to the
coronal model. The results indicate that the tether cutting scenario
is well consistent with the observation, and also suggest us that the
data-driven simulation might be usable for some kind of now-casting
simulation of solar eruption, which could assess what type of solar
eruption is able to arise from particular active regions.
Title: Launching Process of Coronal Mass Ejections
Authors: Shiota, D.; Kusano, K.; Miyoshi, T.; Shibata, K.
Bibcode: 2008AGUFMSH41B1621S
Altcode:
Coronal mass ejections (CMEs) are one the most spectacular explosive
phenomena, in which large amount of mass and magnetic flux are ejected
to the interplanetary space, as a result of a disruption of coronal
magnetic field. It is very important for space weather science to
understand the whole process of CMEs because of their close relation
with geoeffective events. However, the physics of how and when CMEs
are launched have not yet been understood. Although disruptions of
coronal field (eruptions) are often observed as flares, in many cases,
they are not accompanied by CMEs. The fact implies that occurrences of
eruptions are not sufficient condition for CMEs and they are affected
by some kinds of factor, for example, the interactions between magnetic
field structure in an eruption and the ambient global scale magnetic
field, such as confinement and reconnection. In order to examine the
condition whether the eruption of coronal field can be launched as
a CME, we performed a three-dimensional MHD simulation of a twisted
flux rope ejected from a small and strong magnetic field active region
surrounded by a global coronal magnetic field. We carried out the
simulations for various configurations aiming to systematically reveal
the condition for the capability of CME formation. As a result, we
found, for example, that a flux rope cannot be ejected as a CME due to
magnetic tension force of anchored field under weak surrounding field,
while it can be ejected under moderately strong surrounding field. In
the case with strong surrounding field, the significant amount of
the magnetic flux inside of the ejecting flux rope reconnects with
the ambient field and then the footpoint of the flux rope appears to
move outward into weak field region. As the results, inward magnetic
tension force of the large scale magnetic field become weak, while
outward one becomes strong due to relaxation of the complex structure
just after reconnection. The ejected flux rope shows tilting rotation
in the direction perpendicular to the ejection line in the case where
the CME is successfully formed. The tilting motion, which results
from a relaxation of complex field structure, is much important for
the determination of the field structure inside a propagating CME as
well as for forecasting the orientation of magnetic field at the orbit
of the Earth.
Title: Predicting Observational Signatures of Coronal Heating by
Alfvén Waves and Nanoflares
Authors: Antolin, P.; Shibata, K.; Kudoh, T.; Shiota, D.; Brooks, D.
Bibcode: 2008ApJ...688..669A
Altcode:
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is nanoflare reconnection heating, in which energy is released
through many small magnetic reconnection events. Distinguishing the
observational features of each mechanism is an extremely difficult
task. On the other hand, observations have shown that energy release
processes in the corona follow a power-law distribution in frequency
whose index may tell us whether small heating events contribute
substantially to the heating or not. In this work we show a link
between the power-law index and the operating heating mechanism in
a loop. We set up two coronal loop models: in the first model Alfvén
waves created by footpoint shuffling nonlinearly convert to longitudinal
modes which dissipate their energy through shocks; in the second model
numerous heating events with nanoflare-like energies are input randomly
along the loop, either distributed uniformly or concentrated at the
footpoints. Both models are based on a 1.5-dimensional MHD code. The
obtained coronae differ in many aspects; for instance, in the flow
patterns along the loop and the simulated intensity profile that
Hinode XRT would observe. The intensity histograms display power-law
distributions whose indexes differ considerably. This number is found
to be related to the distribution of the shocks along the loop. We
thus test the observational signatures of the power-law index as a
diagnostic tool for the above heating mechanisms and the influence of
the location of nanoflares.
Title: Three Successive and Interacting Shock Waves Generated by a
Solar Flare
Authors: Narukage, Noriyuki; Ishii, Takako T.; Nagata, Shin'ichi;
UeNo, Satoru; Kitai, Reizaburo; Kurokawa, Hiroki; Akioka, Maki;
Shibata, Kazunari
Bibcode: 2008ApJ...684L..45N
Altcode:
We discovered three successive Moreton waves generated by a single solar
flare on 2005 August 3. Although this flare was not special in magnitude
or configuration, Moreton waves (shock waves) successively occurred
three times. Multiple shock waves generated during a single flare have
not been reported before. Furthermore, the faster second-generated
Moreton wave caught up and merged with the slower first-generated
one. This is the first report of shock-shock interaction associated with
a solar flare. The shock-plasma interaction was also detected. When
the third-generated Moreton wave passed through an erupting filament,
the filament was accelerated by the Moreton wave. In this event,
filaments also erupted three times. On the basis of this observation,
we consider that filament eruption is indispensable to the generation
of Moreton waves.
Title: Spectropolarimetric Observation of an Emerging Flux Region:
Triggering Mechanisms of Ellerman Bombs
Authors: Watanabe, H.; Kitai, R.; Okamoto, K.; Nishida, K.; Kiyohara,
J.; Ueno, S.; Hagino, M.; Ishii, T. T.; Shibata, K.
Bibcode: 2008ApJ...684..736W
Altcode: 2008arXiv0805.4266W
A high spatial resolution observation of an emerging flux region
(EFR) was made using a vector magnetograph and a Hα Lyot filtergraph
with the Domeless Solar Telescope at Hida Observatory on 2006 October
22. In Hα wing images, we could see many Ellerman bombs (EBs) in the
EFR. Observations in two modes, slit scan and slit fixed, were performed
with the vector magnetograph, along with the Hα filtergraph. Using the
Hα wing images, we detected 12 EBs during the slit scan observation
period and 9 EBs during the slit fixed observation period. With the
slit scan observation, we found that all the EBs were distributed in
the area where the spatial gradient of vertical field intensity was
large, which indicates the possibility of rapid topological change in
the magnetic field in the area of EBs. With the slit fixed observation,
we found that EBs were distributed in the areas of undulatory magnetic
fields, in both the vertical and horizontal components. This paper is
the first to report the undulatory pattern in the horizontal components
of the magnetic field, which is also evidence for emerging magnetic flux
triggered by the Parker instability. These results allow us to confirm
the association between EBs and emerging flux tubes. Three triggering
mechanisms for EBs are discussed with respect to emerging flux tubes:
9 out of 21 EBs occurred at the footpoints of emerging flux tubes,
8 occurred at the top of emerging flux tubes, and 4 occurred in the
unipolar region. Each case can be explained by magnetic reconnection
in the low chromosphere.
Title: Resistive General Relativistic MHD Simulations of Jet Formation
around Kerr Black Hole
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro
Bibcode: 2008mgm..conf.1585K
Altcode:
Recent general relativistic magnetohydrodynamic (GRMHD) simulations
of jet formation show inevitable formation of anti-parallel magnetic
field in black hole magnetospheres. In such situation, magnetic
reconnections should take place and influence the jet acceleration
drastically. However, all of present GRMHD simulations assumed electric
resistivity to be zero, where the magnetic reconnection is forbidden. To
investigate the magnetic reconnection near the black hole, we develop
the numerical method of the resistive GRMHD.
Title: Long-Time Simulations of Astrophysical Jets: Energy Structure
and Quasi-Periodic Ejection
Authors: Ibrahim, Ahmed; Shibata, Kazunari
Bibcode: 2008PASJ...60..871I
Altcode: 2007arXiv0704.2918I
We have performed self-consistent 2.5-dimensional nonsteady MHD
numerical simulations of jet formation as long as possible, including
the dynamics of accretion disks. Previous simulations showed that,
in the case where the calculation time of the simulations is very
short as compared with the time scale of observed jets, there
is no significant difference between the characteristics of the
nonsteady and steady MHD simulations. Thus, we have investigated
long-time evolutions of the mass-accretion rate, mass-outflow
rate, jet velocity, and various energy fluxes. We found that the
ejection of a jet is quasi-periodic. The period of the ejection,
Tejection, is related to the time needed for the initial
magnetic field, B0, to be twisted to generate a toroidal
filed, Tejection ∝ VA-1 ∝
B0-1 ∝ Emg-1/2, where
VA is the Alfvén velocity and Emg the initial
magnetic energy. We compared our results with both the steady-state
theory and the previous 2.5-dimensional nonsteady MHD simulations. We
found that the time-averaged velocity of the jet, Vjet,avg,
is ∼ 0.1 VK and ∼ 0.1 Vjet,max, where
VK is the Keplerian velocity at (r, z) = (1. 0) and
Vjet,max the maximum velocity of the jet. Nevertheless,
the characteristics of our simulations are consistent with those of
the steady solution and previous short-time simulations. We found that
the dependences of the time-averaged velocity and the mass-outflow
rate, dot{M}w,avg, on the initial magnetic field are
approximately Vjet,avg ∝ B00.3 and
dot{M}w,avg ∝ B00.32, respectively.
Title: Giant Chromospheric Anemone Jet Observed with Hinode and
Comparison with Magnetohydrodynamic Simulations: Evidence of
Propagating Alfvén Waves and Magnetic Reconnection
Authors: Nishizuka, N.; Shimizu, M.; Nakamura, T.; Otsuji, K.; Okamoto,
T. J.; Katsukawa, Y.; Shibata, K.
Bibcode: 2008ApJ...683L..83N
Altcode: 2008arXiv0810.3384N
Hinode discovered a beautiful giant jet with both cool and hot
components at the solar limb on 2007 February 9. Simultaneous
observations by the Hinode SOT, XRT, and TRACE 195 Å satellites
revealed that hot (~5 × 106 K) and cool (~104
K) jets were located side by side and that the hot jet preceded the
associated cool jet (~1-2 minutes). A current-sheet-like structure
was seen in optical (Ca II H), EUV (195 Å), and soft X-ray emissions,
suggesting that magnetic reconnection is occurring in the transition
region or upper chromosphere. Alfvén waves were also observed with
Hinode SOT. These propagated along the jet at velocities of ~200
km s-1 with amplitudes (transverse velocity) of ~5-15 km
s-1 and a period of ~200 s. We performed two-dimensional MHD
simulation of the jets on the basis of the emerging flux-reconnection
model, by extending Yokoyama and Shibata's model. We extended the model
with a more realistic initial condition (~106 K corona) and
compared our model with multiwavelength observations. The improvement
of the coronal temperature and density in the simulation model allowed
for the first time the reproduction of the structure and evolution of
both the cool and hot jets quantitatively, supporting the magnetic
reconnection model. The generation and the propagation of Alfvén
waves are also reproduced self-consistently in the simulation model.
Title: The Correlation among the Rise Velocity of a Soft X-Ray Loop,
the Ejection Velocity of a Plasmoid, and the Height above the Loop Top
of the Hard X-Ray Source in Masuda-Type Flares, and Its Interpretation
Based on the Reconnection Model of Flares
Authors: Shimizu, M.; Nishida, K.; Takasaki, H.; Shiota, D.; Magara,
T.; Shibata, K.
Bibcode: 2008ApJ...683L.203S
Altcode:
One of the most important results on solar flares obtained by Yohkoh
is finding impulsive compact-loop flares associated with a hard X-ray
(HXR) loop-top source well above a soft X-ray (SXR) loop, which are
called Masuda-type flares. This finding supports the reconnection
model of flares in which magnetic reconnection occurs above the closed
loop observed in the soft X-ray during a flare. Although this model
qualitatively explains the observed feature of Masuda-type flares,
quantitative investigations into physical processes in these flares are
still insufficient, which is the main subject of this Letter. We used
15 Masuda-type flares (seven are newly found and eight are previously
reported) to examine the correlation among the rise velocity of an
SXR loop, the ejection velocity of a plasmoid, and the height of the
HXR source above the SXR loop (i.e., the difference in the apparent
heights of the HXR and SXR sources). The main conclusion is that there
is a positive correlation among these three quantities, and we explain
the physical origin of this correlation using a reconnection model
of flares.
Title: Cooperative Observation of Ellerman Bombs between the Solar
Optical Telescope aboard Hinode and Hida/Domeless Solar Telescope
Authors: Matsumoto, Takuma; Kitai, Reizaburo; Shibata, Kazunari;
Nagata, Shin'ichi; Otsuji, Kenichi; Nakamura, Tahei; Watanabe, Hiroko;
Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto, Kiyoshi; Shimizu,
Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.; Lites, Bruce W.;
Shine, Richard A.; Title, Alan M.
Bibcode: 2008PASJ...60..577M
Altcode:
High-resolution CaIIH broad-band filter images of NOAA10933 on 2007
January 5 were obtained by the Solar Optical Telescope aboard the Hinode
satellite. Many small-scale (∼1") bright points were observed outside
the sunspot and inside the emerging flux region. We identified some of
these bright points with Ellerman bombs (EBs) by using Hα images taken
by the Domeless Solar Telescope at Hida observatory. The sub-arcsec
structures of two EBs seen in CaIIH were studied in detail. Our
observation showed the following two aspects: (1) The CaIIH bright
points identified with EBs were associated with the bipolar magnetic
field structures, as reported by previous studies. (2)The structure
of the CaIIH bright points turned out to consist of the following two
parts: a central elongated bright core (0.7" × 0.5") located along
the magnetic neutral line and a diffuse halo (1.2"×1.8").
Title: Emergence of a helical flux rope and prominence formation
Authors: Okamoto, T. J.; Tsuneta, S.; Lites, B. W.; Kubo, M.; Yokoyama,
T.; Berger, T. E.; Ichimoto, K.; Katsukawa, Y.; Nagata, S.; Shibata,
K.; Shimizu, T.; Shine, R. A.; Suematsu, Y.; Tarbell, T. D.; Title,
A. M.
Bibcode: 2008AGUSMSP43B..06O
Altcode:
We report a discovery about emergence of a helical flux rope. The
episode may be related to the formation and evolution of an active
region prominence. Statistical studies by previous authors indicate that
numerous prominences have the inverse-polarity configuration suggesting
the helical magnetic configurations. There are two theoretical
models about formation of such a coronal helical magnetic field in
association with prominences: flux rope model and sheared-arcade
model. We have so far no clear observational evidence to support
either model. In order to find a clue about the formation of the
prominence, we had continuous observations of NOAA AR 10953 with the
SOT during 2007 April 28 to May 9. A prominence was located over the
polarity inversion line in the south-east of the main sunspot. These
observations provided us with a time series of vector magnetic fields
on the photosphere under the prominence. We found four new features:
(1) The abutting opposite-polarity regions on the two sides along
the polarity inversion line first grew laterally in size and then
narrowed. (2) These abutting regions contained vertically-weak,
but horizontally-strong magnetic fields. (3) The orientations of
the horizontal magnetic fields along the polarity inversion line on
the photosphere gradually changed with time from a normal- polarity
configuration to an inverse-polarity one. (4) The horizontal-magnetic
field region was blueshifted. These indicate that helical flux rope
emerges from below the photosphere into the corona along the polarity
inversion line under the prominence. We suggest that this supply of a
helical magnetic flux possibly into the corona is related to formation
and maintenance of active-region prominences.
Title: Predicting observational signatures of coronal heating by
Alfvén waves and nanoflares
Authors: Antolin, Patrick; Shibata, Kazunari; Kudoh, Takahiro; Shiota,
Daiko; Brooks, David
Bibcode: 2008IAUS..247..279A
Altcode: 2007IAUS..247..279A
Alfvén waves can dissipate their energy by means of nonlinear
mechanisms, and constitute good candidates to heat and maintain the
solar corona to the observed few million degrees. Another appealing
candidate is the nanoflare-reconnection heating, in which energy is
released through many small magnetic reconnection events. Distinguishing
the observational features of each mechanism is an extremely difficult
task. On the other hand, observations have shown that energy release
processes in the corona follow a power law distribution in frequency
whose index may tell us whether small heating events contribute
substantially to the heating or not. In this work we show a link
between the power law index and the operating heating mechanism in
a loop. We set up two coronal loop models: in the first model Alfvén
waves created by footpoint shuffling nonlinearly convert to longitudinal
modes which dissipate their energy through shocks; in the second model
numerous heating events with nanoflare-like energies are input randomly
along the loop, either distributed uniformly or concentrated at the
footpoints. Both models are based on a 1.5-D MHD code. The obtained
coronae differ in many aspects, for instance, in the simulated intensity
profile that Hinode/XRT would observe. The intensity histograms display
power law distributions whose indexes differ considerably. This number
is found to be related to the distribution of the shocks along the
loop. We thus test the observational signatures of the power law index
as a diagnostic tool for the above heating mechanisms and the influence
of the location of nanoflares.
Title: Coronal Jet Observed by Hinode as the Source of
a3He-rich Solar Energetic Particle Event
Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wiedenbeck, Mark E.;
Cohen, Christina M. S.; Krucker, Säm; Hannah, Iain G.; Shimojo,
Masumi; Shibata, Kazunari
Bibcode: 2008ApJ...675L.125N
Altcode:
We study the solar source of the 3He-rich solar
energetic particle (SEP) event observed on 2006 November 18. The
SEP event showed a clear velocity dispersion at energies below 1 MeV
nucleon-1, indicating its solar origin. We associate the SEP
event with a coronal jet in an active region at heliographic longitude
of W50°, as observed in soft X-rays. This jet was the only noticeable
activity in full-disk X-ray images around the estimated release time of
the ions. It was temporally correlated with a series of type III radio
bursts detected in metric and longer wavelength ranges and was followed
by a nonrelativistic electron event. The jet may be explained in terms
of the model of an expanding loop reconnecting with a large-scale
magnetic field, which is open to interplanetary space for the particles
to be observed at 1 AU. The open field lines appear to be anchored at
the boundary between the umbra and penumbra of the leading sunspot,
where a brightening is observed in both soft and hard X-rays during
the jet activity. Other flares in the same region possibly associated
with 3He-rich SEP events were not accompanied by a jet,
indicative of different origins of this type of SEP event.
Title: A quantitative MHD study of the relation among arcade shearing,
flux rope formation, and eruption due to the tearing instability
Authors: Shiota, D.; Kusano, K.; Miyoshi, T.; Nishikawa, N.;
Shibata, K.
Bibcode: 2008JGRA..113.3S05S
Altcode:
The quantitative relationship between the magnetohydrodynamic (MHD)
activity of solar coronal arcade and the magnetic helicity injection,
which is caused by shearing motion, has been investigated, using
azimuthally symmetric model of MHD simulation. We have calculated
several cases in which the width of the shearing region is varied
and examined the relationship between the magnetic arcade dynamics
and magnetic helicity evolution. As a result, it is found that as
the shearing motion is imposed on narrower regions along each side
of the magnetic inversion line, the magnetic arcade can be easily
destabilized by the resistive tearing mode. However, in this case,
even though reconnection driven by the tearing mode produces plasmoids,
the plasmoid elevation is almost in proportion to the total amount of
magnetic helicity contained in the arcade, and it is too slow to explain
the trigger process of coronal mass ejections (CMEs). On the other hand,
in the case where the shearing motion is imposed on the entire region,
much larger magnetic helicity injection is required to injected arcade
in order to destabilize the system, compared to practical helicity
injection measured in the solar corona. The results suggest that it may
be difficult to trigger a CME just by the axisymmetric shearing motion
and that some other mechanisms should be involved in the triggering
process of a CME. The results also imply that the relation between
the magnetic helicity and the overlying magnetic flux can be a key
parameter for the CME occurrence.
Title: Hot and Cool Plasmoid Ejections Associated with a Solar Flare
Authors: Ohyama, Masamitsu; Shibata, Kazunari
Bibcode: 2008PASJ...60...85O
Altcode:
A 1993 May 14 flare was associated with both X-ray plasma ejection
as hot plasmoid ejection and Hα filament eruption as cool plasma
ejection. The flare proceeded through two stages according to a
GOES soft X-ray observation. In the first stage, an X-ray plasma
ejection, an Hα filament eruption, and a chain of pointlike Hα
brightenings occurred. In the second stage, an Hα two-ribbon flare
and an X-ray arcade structure were seen in Hα and soft X-ray images,
respectively. The X-ray plasmoid and the eruptive Hα filament were
in the same current sheet. The X-ray plasmoid started to rise with
a speed of ∼270kms-1 temporally after the Hα filament
eruption. The top part of the X-ray plasmoid moved together with the
eruptive filament. They were then decelerated before the main peak of
the hard X-ray emission. The X-ray plasmoid was not a bloblike feature,
as the eruptive Hα filament, but a loop structure. Our results indicate
that the X-ray plasmoid was not the heated part of the Hα filament.
Title: Height Dependence of Gas Flows in an Ellerman Bomb
Authors: Matsumoto, Takuma; Kitai, Reizaburo; Shibata, Kazunari;
Otsuji, Kenichi; Naruse, Takuya; Shiota, Daikou; Takasaki, Hiroyuki
Bibcode: 2008PASJ...60...95M
Altcode:
We performed spectroscopic observations of Ellerman bombs (EBs) in
an active region of NOAA 10705 at Hida Observatory on 2004 November
24. The photospheric velocity fields of EBs have for the first time
been investigated spectroscopically. From the Doppler shifts of a
TiII absorption line (6559.576Å) and a broad Hα emission line,
we derived the photospheric velocity and the lower chromospheric
velocity, respectively. The photospheric velocity of EBs was ∼
0.2kms-1, indicating downward flow, on average. We found
that the photospheric velocity variation of EBs has a good temporal
correlation with the Hα wing emission variation. On the other hand, the
chromospheric velocity showed an upward flow of ∼1-3kms-1
on the average. From the characteristics of the flow field, we
conclude that the observed EB occurred at the upper photospheric
level. We suggest that it is important to know the motions of EBs in
the photosphere because a plausible triggering mechanism of EBs is
magnetic reconnection in the low-lying atmosphere.
Title: Emergence of a Helical Flux Rope under an Active Region
Prominence
Authors: Okamoto, Takenori J.; Tsuneta, Saku; Lites, Bruce W.; Kubo,
Masahito; Yokoyama, Takaaki; Berger, Thomas E.; Ichimoto, Kiyoshi;
Katsukawa, Yukio; Nagata, Shin'ichi; Shibata, Kazunari; Shimizu,
Toshifumi; Shine, Richard A.; Suematsu, Yoshinori; Tarbell, Theodore
D.; Title, Alan M.
Bibcode: 2008ApJ...673L.215O
Altcode: 2008arXiv0801.1956O
Continuous observations were obtained of NOAA AR 10953 with the Solar
Optical Telescope (SOT) on board the Hinode satellite from 2007 April
28 to May 9. A prominence was located over the polarity inversion
line (PIL) to the southeast of the main sunspot. These observations
provided us with a time series of vector magnetic fields on the
photosphere under the prominence. We found four features: (1) The
abutting opposite-polarity regions on the two sides along the PIL first
grew laterally in size and then narrowed. (2) These abutting regions
contained vertically weak but horizontally strong magnetic fields. (3)
The orientations of the horizontal magnetic fields along the PIL on
the photosphere gradually changed with time from a normal-polarity
configuration to an inverse-polarity one. (4) The horizontal magnetic
field region was blueshifted. These indicate that helical flux rope
was emerging from below the photosphere into the corona along the PIL
under the preexisting prominence. We suggest that this supply of a
helical magnetic flux to the corona is associated with evolution and
maintenance of active region prominences.
Title: Characteristics of Anemone Active Regions Appearing in Coronal
Holes Observed with the Yohkoh Soft X-Ray Telescope
Authors: Asai, Ayumi; Shibata, Kazunari; Hara, Hirohisa; Nitta,
Nariaki V.
Bibcode: 2008ApJ...673.1188A
Altcode: 2008arXiv0805.4474A
Coronal structure of active regions appearing in coronal holes is
studied, using data that were obtained with the Soft X-Ray Telescope
(SXT) aboard Yohkoh between 1991 November and 1993 March. The following
characteristics are found. Many of the active regions (ARs) appearing
in coronal holes show a structure that looks like a sea anemone. Such
active regions are called anemone ARs. About one-fourth of all active
regions that were observed with SXT from their births showed the
anemone structure. For almost all the anemone ARs, the order of the
magnetic polarities is consistent with the Hale-Nicholson polarity
law. These anemone ARs also showed, to a greater or lesser extent,
an east-west asymmetry in the X-ray intensity distribution, such that
the following (eastern) part of the AR was brighter than its preceding
(western) part. This, as well as the anemone shape itself, is consistent
with the magnetic polarity distribution around the anemone ARs. These
observations also suggest that an active region appearing in coronal
holes has a simpler (less sheared) and more preceding-spot-dominant
magnetic structure than those appearing in other regions.
Title: Plasma ejections and shock waves in the solar atmosphere
Authors: Magara, Tetsuya; Shibata, Kazunari
Bibcode: 2008JASTP..70..546M
Altcode:
Recent space observations have revealed that the solar atmosphere is
much more dynamic than had been thought and is full of plasma ejections
and shock waves. It is interesting to note that as observational
accuracy becomes better and better, more and more tiny jets, shocks,
and small flares (microflares and nanoflares) have been found. It
is also interesting to note that the structure and dynamics of these
tiny phenomena are often similar to those of larger phenomena. This
led us to develop a unified model that explains both large-scale
and small-scale eruptions observed in the solar atmosphere, in which
magnetic reconnection plays a key role. In this paper, we review plasma
ejections and shock waves observed in the solar atmosphere and discuss
how these dynamic phenomena have been studied using magnetohydrodynamic
(MHD) simulations. We also report recent solar observational projects
in Japan; one of them is a space mission called Hinode (previously
called Solar-B) of JAXA/ISAS with US/UK/ESA collaboration, and the
other is the solar magnetic activity research telescope (SMART) built
at Hida Observatory of Kyoto University.
Title: Multi-scale Interlocked Simulation of Solar Eruption
Authors: Kusano, Kanya; Sugiyama, Tooru; Inoue, Satoshi; Shiota, Daiko;
Asano, Eiji; Matsumoto, Takuma; Kataoka, Ryuho; Shibata, Kazunari
Bibcode: 2008cosp...37.1659K
Altcode: 2008cosp.meet.1659K
The onset process of solar eruption, which arises as solar flares and/or
coronal mass ejections (CME), is one of the most important subject
in space and astrophysical plasma physics, because it is the typical
phenomena of the explosive energy liberation in plasma as well as the
primary cause of space weather disturbances. However, not only the onset
mechanism but even the physical condition to trigger it are not yet well
clarified. In particular, the mutual relationship between large-scale
magnetic configuration and small-scale reconnection dynamics in the
CME initiation is hardly understood, although it is quite important
from the view point both of the multi-scale plasma physics and the
space weather forecast. The objective of this paper is to develop a
new type of simulation framework to shed a light to this long-standing
problem. Our simulation is performed by the incorporation of multiple
models, each of which can calculate the different dynamics at different
scales. They are constituted of the active region model, the global
corona model, the interplanetary space model, and the fluid-particle
interlocked model, which is able to handle the calculation of energetic
particle acceleration in macro-scale magnetic environment. We have
applied the new model to simulate the eruptive event caused by the
X-class flare occurred on December 13, 2006, using vector magnetic
field data observed by Hinode, Solar Optical Telescope. In this talk,
after a brief review of the theories proposed so far for the flare
and CME initiation, we show the basic algorithm of our model. Then,
we present the result of the first-ever datadriven simulation of the
solar eruption. The detail comparison between the simulation and the
observation is also reported. Finally, we are discussing about the
predictability of solar eruption, based on the numerical experiments
with the multi-scale interlocked model.
Title: Giant chromospheric jet observed with Hinode and magnetic
reconnection model
Authors: Nishizuka, Naoto; Shimizu, Masaki; Nakamura, Tahei; Otsuji,
Kenichi; Okamoto, Takenori; Shibata, Kazunari; Katsukawa, Yukio
Bibcode: 2008cosp...37.2239N
Altcode: 2008cosp.meet.2239N
Heating of the solar chromosphere and corona is one of the long-standing
puzzles in astronomy and also a key to understand the Sun-Earth
connection through solar wind and EUV/X-ray radiation. A solar space
telescope mission Hinode revealed that solar chromosphere is much more
dynamic than had been thought and is full of tiny jets, which may be
a key to resolve the puzzle of chromospheric and coronalheating. It
has long been observed that H-alpha jets called surges often occur in
the chromosphere. They have been believed to be produced by magnetic
reconnection, which is an energy conversion mechanism from magnetic
energy into thermal and kinetic energies of plasma when anti-parallel
magnetic fields encounter and reconnect with each other. Hinode's
new chromospheric observations (with Calcium II H broad band filter)
revealed that jets are ubiquitous in the chromosphere and some of the
jets show evidence of magnetic reconnection. However, there have not
been simultaneous observations of the chromospheric jets at X-ray,
EUV, and Optical (at Calcium II H line) wavelengths until now. Here
we report first multi-wavelength observations of a chromospheric
jet with Solar Optical Telescope (SOT) and X-Ray Telescope (XRT)
on board Hinode and TRACE 195A filter. With its unprecedented high
quality instruments at both optical and X-rays, Hinode discovered a
beautiful, giant jet with both cool (104 K) and hot (5x106 K) components
at the solar limb. TRACE satellite also observed the same jet with
EUV telescope and revealed the existence of both hot (106 K) and cool
(unknown temperature) components. These data set are probably the best
multi-wavelength observations of solar jets until now. We also performed
magnetohydrodynamic simulation of the jet based on the reconnection
model and found that it can explain various observational facts very
well. It has often been argued that some of solar jets are produced by
magnetic reconnection, but previous observations had a limitation on
spatial and temporal resolutions and temperature coverage. Using these
new data and twodimensional magnetohydrodynamic simulations of the jet,
we showed, for the first time, how hot and cool jets are heated and
accelerated during the reconnection, including associated generation of
Alfvén waves. This jet formation dynamics would show a proto-tyep of
reconnection e model of solar jets and can be applied for other small
jets discovered by Hinode, which might heat the chromosphere and corona.
Title: Ubiquitous and Fractal Reconnection in the Solar Atmosphere
Authors: Shibata, Kazunari
Bibcode: 2008cosp...37.2858S
Altcode: 2008cosp.meet.2858S
Recent space observations of the Sun with Yohkoh, SOHO, TRACE,
and RHESSI revealed that magnetic reconnection is ubiquitous in the
solar corona, ranging from small scale reconnection (nanoflares) to
large scale one (CME related flares). These reconnection events are
often associated with mass ejections with various sizes, from small
scale jets to large scale plasmoid ejections and CMEs. Recent Hinode
satellite has revealed also that magnetic reconnection is ubiquitous
in the solar chromosphere, and that even smaller reconnection events
occur in the solar chromopshere, which are associated with tiny
jets. These observations imply that the solar atmosphere consists of
self-similar structure, i.e., fractal structure, which is consistent
with basic magnetohydrodynamics (MHD) theory, since MHD does not
contain any characteristic length and time scale. It is natural that
MHD reconnection tend to become fractal in ideal MHD plasmas with
large magnetic Reynolds number such as in the solar atmosphere, and
it is proposed that even current sheet might have a fractal structure,
which is favorable for particle acceleration. We would discuss recent
observations and theories related to fractal reconnection, and discuss
possible implication to coronal heating, reconnection physics, and
particle acceleration.
Title: A solar observing station for education and research in Peru
Authors: Kaname, José Iba, Ishitsuka; Ishitsuka, Mutsumi; Trigoso
Avilés, Hugo; Takashi, Sakurai; Yohei, Nishino; Miyazaki, Hideaki;
Shibata, Kazunari; Ueno, Satoru; Yumoto, Kiyohumi; Maeda, George
Bibcode: 2007BASI...35..709K
Altcode:
Since 1937 Carnegie Institution of Washington made observations of
active regions of the Sun with a Hale type spectro-helioscope in
Huancayo observatory of the Instituto Geofísico del Perú (IGP). IGP
has contributed significantly to geophysical and solar sciences
in the last 69 years. Now IGP and the Faculty of Sciences of the
Universidad Nacional San Luis Gonzaga de Ica (UNICA) are planning to
refurbish the coelostat at the observatory with the support of National
Astronomical Observatory of Japan. It is also planned to install a
solar Flare Monitor Telescope (FMT) at UNICA, from Hida observatory
of Kyoto University. Along with the coelostat, the FMT will be useful
to improve scientific research and education.
Title: Interlocked MHD modeling of the Launching and Propagation of
Coronal Mass Ejections
Authors: Shiota, D.; Kusano, K.; Kataoka, R.; Asano, E.; Inoue, S.;
Ogino, T.; Shibata, K.
Bibcode: 2007AGUFMSM41A0312S
Altcode:
Coronal mass ejections (CMEs) are not only one of the most energetic
phenomena in the solar corona but also a key process in the space
weather study. The numerical modeling of the launching and propagation
of CMEs play a crucial role for space weather forecast. However, due
to the vast difference in the spatial scale among solar, solar wind,
and terrestrial plasmas, it is difficult and wasteful of numerical
resource to simulate the whole process associated with the initiation
and propagation of CMEs with a single simulation model. Therefore, we
have recently developed a new numerical model, which can continuously
calculate the whole process from the onset of CMEs to the geomagnetic
impact of that, using the interlocking of several numerical models. The
model consists of four numerical models; the solar active region
model, the global corona model, the interplanetary space model, and
the geospace model. In this paper, we explain the basic algorithm of
the interlocked modeling, and show some important results obtained
with it. First, we investigate the physical relationship between the
solar magnetic field structure and the launching of CMEs. CMEs are
ejections of a large amount of mass and magnetic flux, which result
from eruptions of coronal magnetic field. Although the eruptions may
cause solar flares, many observations show that all flares are not
necessarily associated with CMEs, implying that there are some kinds
of criteria. For example, it is likely that an eruption is confined by
the ambient global magnetic field if the eruption is not large enough
or if its direction of magnetic field is not appropriate. In order
to examine the condition whether the eruption of coronal field can be
launched as a CME, we performed a three-dimensional MHD simulation of
an eruption in various global coronal magnetic fields, and succeeded to
distinguish the condition capable of a CME. For example, an eruption of
a small and strong magnetic field active region cannot be ejected as
a CME due to magnetic tension force of anchored field. In this paper,
we summarize the relationship between the key parameters for the
ejection; the amount of magnetic flux and field directions. Second,
our interlocked model is applied onto the CME event caused by the
X-class flare occurring on the active region NOAA 10930 on December
13, 2006. The numerical experiment is initiated by the magnetic field
model, which is constructed based on the magnetogram data observed by
the Solar Optical Telescope (SOT) boarded on Hinode satellite, and the
simulated variation of magnetic field and plasma state at the Earth
orbit is compared with the in-situ observation by ACE. As a result,
it is found that a relatively good agreement can be obtained between
the simulation and observation if we parameterize the magnetic field
on the CME launching site.
Title: Chromospheric Anemone Jets as Evidence of Ubiquitous
Reconnection
Authors: Shibata, Kazunari; Nakamura, Tahei; Matsumoto, Takuma; Otsuji,
Kenichi; Okamoto, Takenori J.; Nishizuka, Naoto; Kawate, Tomoko;
Watanabe, Hiroko; Nagata, Shin'ichi; UeNo, Satoru; Kitai, Reizaburo;
Nozawa, Satoshi; Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto, Kiyoshi;
Shimizu, Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.; Berger,
Thomas E.; Lites, Bruce W.; Shine, Richard A.; Title, Alan M.
Bibcode: 2007Sci...318.1591S
Altcode: 2008arXiv0810.3974S
The heating of the solar chromosphere and corona is a long-standing
puzzle in solar physics. Hinode observations show the ubiquitous
presence of chromospheric anemone jets outside sunspots in active
regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers
long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their
velocity is 10 to 20 kilometers per second. These small jets have an
inverted Y-shape, similar to the shape of x-ray anemone jets in the
corona. These features imply that magnetic reconnection similar to that
in the corona is occurring at a much smaller spatial scale throughout
the chromosphere and suggest that the heating of the solar chromosphere
and corona may be related to small-scale ubiquitous reconnection.
Title: CHAIN-project and installation of the flare monitoring
telescopes in developing countries
Authors: Ueno, Satoru; Shibata, Kazunari; Kimura, Goichi; Nakatani,
Yoshikazu; Kitai, Reizaburo; Nagata, Shin'ichi
Bibcode: 2007BASI...35..697U
Altcode:
The Flare Monitoring Telescope (FMT) was constructed in 1992 at the
Hida Observatory in Japan to investigate the long-term variation of
solar activity and explosive events, as a project of the international
coordinated observations programme (STEP). The FMT consists of five
solar imaging telescopes and one guide telescope. The five telescopes
simultaneously observe the full-disk Sun at different wavelengths around
H-alpha absorption line or in different modes. Therefore, the FMT can
measure the three-dimensional velocity field of moving structures on
the full solar disk without the atmospheric seeing effect. The science
target of the FMT is to monitor solar flares and erupting filaments
continuously all over the solar disk and as many events as possible
and to investigate the relationship between such phenomena and space
weather. Now we are planning to start a new worldwide project called
as ``Continuous H-alpha Imaging Network (CHAIN)-project''. As part of
this project, we are examining the possibility of installing telescopes
similar to the FMT in developing countries with cooperative help by the
United Nations. We have selected Peru as the candidate country where
the first oversea FMT will be installed, and are beginning to study
the natural environment, the seeing conditions, the proper design of
the telescope for Peru and the training and education programme of
operating staff, etc.
Title: MHD Simulation of the Magnetic Storm on the Solar Flare Event
in December 2006
Authors: Ogino, T.; Kataoka, R.; Obara, T.; Omura, Y.; Kusano, K.;
Shibata, K.
Bibcode: 2007AGUFMSM41A0313O
Altcode:
It is an important subject on space weather study to make a physical
model and to simulate a series of phenomena from origins of disturbances
in the sun to the responses of magnetosphere and ionosphere in
earth. Under the Creative Scientific Research "The Basic Study of
Space Weather Prediction", we have tried such a series of modeling
on the solar flare event in December 13-16 2006 and the geomagnetic
storms. Following the space weather modeling (1) from the sun to
solar wind, we present the space weather modeling (2) from the solar
wind to the magnetosphere-ionosphere response in the earth. Large
interplanetary disturbances were generated in association with the
strong solar activity of X-class flare. Characteristic features of
the event are two X-class flares on 12/13 and 12/14 in the interval of
rather quiet solar activity, north-south fluctuation of IMF and a long
duration of southward IMF from 12/14, arrival of a high speed solar wind
during the time for southward IMF. Propagation of the disturbances from
the sun to the earth is simulated by using 3D global solar wind model
following evolution of solar disturbances. A 3D global MHD simulation
of interaction between the solar wind and earth's magnetosphere is
carried out by using the output of the 3D solar wind simulation. In
the simulation, the high speed solar wind compresses the magnetosphere
and magnetic reconnection occurs in the tail as long as at the dayside
magnetopause for rapid southward turning of IMF from northward IMF and
hot plasmas are injected around the geosynchronous orbit from plasma
sheet. Moreover, the magnetosphere and ionosphere coupling and the
ionosphere convection in the polar region are discussed in association
with geomagnetic storms generated by the solar flare event.
Title: Coronal Transverse Magnetohydrodynamic Waves in a Solar
Prominence
Authors: Okamoto, T. J.; Tsuneta, S.; Berger, T. E.; Ichimoto, K.;
Katsukawa, Y.; Lites, B. W.; Nagata, S.; Shibata, K.; Shimizu, T.;
Shine, R. A.; Suematsu, Y.; Tarbell, T. D.; Title, A. M.
Bibcode: 2007Sci...318.1577O
Altcode: 2008arXiv0801.1958O
Solar prominences are cool 104 kelvin plasma clouds
supported in the surrounding 106 kelvin coronal plasma by
as-yet-undetermined mechanisms. Observations from Hinode show fine-scale
threadlike structures oscillating in the plane of the sky with periods
of several minutes. We suggest that these represent Alfvén waves
propagating on coronal magnetic field lines and that these may play
a role in heating the corona.
Title: Small-Scale Magnetic-Flux Emergence Observed with Hinode
Solar Optical Telescope
Authors: Otsuji, Kenichi; Shibata, Kazunari; Kitai, Reizaburo; Ueno,
Satoru; Nagata, Shin'ichi; Matsumoto, Takuma; Nakamura, Tahei;
Watanabe, Hiroko; Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto,
Kiyoshi; Shimizu, Toshifumi; Katsukawa, Yukio; Tarbell, Theodore D.;
Lites, Bruce; Shine, Richard A.; Title Alan M.
Bibcode: 2007PASJ...59S.649O
Altcode: 2007arXiv0709.3207O
We observed small-scale magnetic-flux emergence in a sunspot moat region
by the Solar Optical Telescope (SOT) aboard the Hinode satellite. We
analyzed filtergram images observed at wavelengths of Fe 6302Å, G band,
and CaII H. In Stokes I images of Fe 6302Å, emerging magnetic flux was
recognized as dark lanes. In the G band, they showed to be their shapes
almost the same as in Stokes I images. These magnetic fluxes appeared
as dark filaments in CaII H images. Stokes V images of Fe 6302Å showed
pairs of opposite polarities at footpoints of each filament. These
magnetic concentrations were identified to correspond to bright points
in G band/CaII H images. From an analysis of time-sliced diagrams, we
derived the following properties of emerging flux, which are consistent
with those of previous studies: (1) Two footpoints separate each other
at a speed of 4.2kms-1 during the initial phase of evolution,
and decrease to about 1kms-1 10minutes later. (2) CaII H
filaments appear almost simultaneously with the formation of dark lanes
in Stokes I in an observational cadence of 2minutes. (3) The lifetime
of the dark lanes in the Stokes I and G band is 8minutes, while that
of Ca filament is 12minutes. An interesting phenomena was observed,
that an emerging flux tube expanded laterally in the photosphere with a
speed of 3.8kms-1. A discussion on the horizontal expansion
of the flux tube is given with refernce to previous simulation studies.
Title: Umbral Fine Structures in Sunspots Observed with Hinode Solar
Optical Telescope
Authors: Kitai, Reizaburo; Watanabe, Hiroko; Nakamura, Tahei; Otsuji,
Ken-ichi; Matsumoto, Takuma; UeNo, Satoru; Nagata, Shin'ichi; Shibata,
Kazunari; Muller, Richard; Ichimoto, Kiyoshi; Tsuneta, Saku; Suematsu,
Yoshinori; Katsukawa, Yukio; Shimizu, Toshifumi; Tarbell, Theodore D.;
Shine, Richard A.; Title, Alan M.; Lites, Bruce
Bibcode: 2007PASJ...59S.585K
Altcode: 2007arXiv0711.3266K
A high resolution imaging observation of a sunspot umbra was made with
the Hinode Solar Optical Telescope. Filtergrams at wavelengths of the
blue and green continua were taken during three consecutive days. The
umbra consisted of a dark core region, several diffuse components,
and numerous umbral dots. We derived basic properties of umbral dots
(UDs), especially their temperatures, lifetimes, proper motions,
spatial distribution, and morphological evolution. The brightness
of UDs is confirmed to depend on the brightness of their surrounding
background. Several UDs show fission and fusion. Thanks to the stable
condition of the space observation, we could for the first time follow
the temporal behavior of these events. The derived properties of the
internal structure of the umbra are discussed from the viewpoint of
magnetoconvection in a strong magnetic field.
Title: Velocity Structure of Jets in a Coronal Hole
Authors: Kamio, Suguru; Hara, Hirohisa; Watanabe, Tetsuya; Matsuzaki,
Keiichi; Shibata, Kazunari; Culhane, Len; Warren, Harry P.
Bibcode: 2007PASJ...59S.757K
Altcode: 2007arXiv0711.2848K
The velocity structures of jets in a coronal hole have been derived
for the first time. Hinode observations revealed the existence
of many bright points in coronal holes. They are loop-shaped and
sometimes associated with coronal jets. Spectra obtained with the
Extreme-ultraviolet Imaging Spectrometer aboard Hinode were analyzed
to infer the Doppler velocity of bright loops and jets in a coronal
hole of the north polar region. Elongated jets above bright loops are
found to be blue-shifted by 30kms-1 at maximum, while foot
points of bright loops are red-shifted. Blue-shifts detected in coronal
jets are interpreted as being upflows produced by magnetic reconnection
between emerging flux and the ambient field in the coronal hole.
Title: Magnetohydrodynamic Numerical Simulations of Emerging Flux
Tube for studying Effects of Twist Intensity
Authors: Miyagosh, T.; Isobe, H.; Yokoyama, T.; Shibata, K.
Bibcode: 2007ASPC..369..377M
Altcode:
We present results from numerical MHD simulations of the emergence
of twisted magnetic flux tubes from below the photosphere into the
corona. The aim is to study the influence of the field line twist on the
emergence process. In almost all previous studies of the emergence of
flux tubes, very strong twist (more than one rotation around the buoyant
region of the initial tube) was imposed. Observations of flux emergence,
however, suggest that flux tubes emerging into the solar atmosphere
carry lesser twist. By varying the amount of twist in the initial tube,
we have carried out a parameter study with particular emphasis on the
weakly twisted regime. We found the followings. (1) In weak twist
case, tube fragments once around the photosphere, and emerging motion is
rapidly suppressed. Then the tube expands to horizontal direction. As
time goes on, emergence starts again. (2) In weak twist case, at the
photosphere the magnetic tension force, which keeps tube coherent,
is weak so it expands more largely than strong twist case. So magnetic
fields strength in flux tube is weaker than that of strong twist case
because it expands more largely. As a result of this, buoyancy force
becomes weak and emerging motion is rapidly decelerated. Then expansion
to the horizontal direction drastically occurs.
Title: Evolution of Hα Kernels and Energy Release in an X-Class Flare
Authors: Asai, A.; Yokoyama, T.; Shimojo, M.; Masuda, S.; Shibata, K.
Bibcode: 2007ASPC..369..461A
Altcode:
The investigation on the evolution of Hα kernels allows us to
derive some key information on the energy release processes and the
particle acceleration mechanisms during a flare. We report a detailed
examination on the relationship between the evolution of the Hα flare
ribbons and the released magnetic energy during an X2.3 solar flare
which occurred on 2001 April 10. In the Hα images, several bright
kernels were observed in the flare ribbons. We identified the conjugated
footpoints, by analyzing the light curve at each Hα kernel, and showed
their connectivities during the flare. Then, based on the magnetic
reconnection model, we calculated quantitatively the released energy
by using the photospheric magnetic field strengths and the separation
speeds of the fronts of the Hα flare ribbons. We confirmed that the
estimated energy release rate corresponds to the nonthermal emission
light curves at the strong emission sources. Finally, we examined the
downward motions at the Hα kernels. The "red-asymmetry" features,
generated by the precipitation of the nonthermal particles and/or
thermal conduction into the chromospheric plasma, were observed for all
the flare ribbons. We also found that the stronger the red-asymmetry
tends to be associated with the brighter Hα kernel.
Title: Three-Dimensional MHD Simulations of Emerging Flux and
Associated Magnetic Reconnection
Authors: Isobe, H.; Miyagoshi, T.; Shibata, K.; Yokoyama, T.
Bibcode: 2007ASPC..369..355I
Altcode:
We present the results of high-resolution three-dimensional MHD
simulations of an emerging flux region. The high-resolution simulations
that have been enabled by the develpment of high-performance computers
have successfully simulated the evolution of fine structure in the
global three-dimension dynamics of the emerging flux and magnetic
reconnection. It has been found that (1) filamentary structure similar
to an arch filament system is spontaneously formed in the emerging
flux due to the magnetic Rayleigh-Taylor instability, (2) filamentary
current sheets are formed in the emerging flux as a result of the
nonlinear development of the magnetic Rayleigh-Taylor instability,
and (3) magnetic reconnection between the emerging flux and the
coronal field occurs in spatially intermittent way, because of the
interchanging of the current sheet due to the magnetic Rayleigh-Taylor
instability. Possible observational signatures expected from Solar-B
are discussed.
Title: A Simulation of Flares in YSOs with Non-thermal Heating and
Expected Hard X-ray Spectrum
Authors: Kawamichi, T.; Shibata, K.
Bibcode: 2007ASPC..369..505K
Altcode:
We performed a one-dimensional hydrodynamic simulation of YSO flare
loops, applying the flare mechanism for the sun. We assumed a flare
loop connecting the central star and accretion disk. We found that the
hard X-ray spectrum expected from our simulation has a very similar
shape to the solar one obserbed by RHESSI, except that the energy band
was 10 times higher than that of the solar flare.
Title: XRT and EIS Observations of Reconnection Associated Phenomena
Authors: Shiota, D.; Isobe, H.; Brooks, D. H.; Chen, P. -F.;
Shibata, K.
Bibcode: 2007ASPC..369..439S
Altcode:
Based on the results of numerical simulations, we sythesized EIS and
XRT images of reconnection flows associated with an eruptive flare. The
results suggest that reconnection inflows can be observed as pairs of
blue- and red-shifted components just above the X-ray arcade in the
low temperature lines. It is found that the Doppler velocities are
larger in the lower temperature line spectra. Reconnection outflows
can also be observed as pairs of blue- and red-shifted components with
an asymmetry in the high temperature line spectra.
Title: New Solar Physics with Solar-B Mission
Authors: Shibata, K.; Nagata, S.; Sakurai, T.
Bibcode: 2007ASPC..369.....S
Altcode:
No abstract at ADS
Title: Triggering Mechanism for the Filament Eruption on 2005
September 13 in NOAA Active Region 10808
Authors: Nagashima, Kaori; Isobe, Hiroaki; Yokoyama, Takaaki; Ishii,
Takako T.; Okamoto, Takenori J.; Shibata, Kazunari
Bibcode: 2007ApJ...668..533N
Altcode: 2007arXiv0706.3519N
On 2005 September 13 a filament eruption accompanied by a halo coronal
mass ejection (CME) occurred in the most flare-productive active region,
NOAA 10808, in solar cycle 23. Using multiwavelength observations
before the filament eruption on September 13, we investigate the
processes leading to the catastrophic eruption. We find that the
filament slowly ascended at a speed of 0.1 km s-1 over 2
days before the eruption. During slow ascension, many small flares were
observed close to the footpoints of the filament, where new magnetic
elements were emerging. On the basis of the observational facts, we
discuss the triggering mechanism leading to the filament eruption. We
suggest that the process toward the eruption is as follows. First,
a series of small flares played a role in changing the topology of
the loops overlying the filament. Second, the small flares gradually
changed the equilibrium state of the filament and caused the filament
to ascend slowly over 2 days. Finally, a C2.9 flare that occurred when
the filament was close to the critical point for loss of equilibrium
directly led to the catastrophic filament eruption right after it.
Title: Oblique Shocks in the Reconnection Jet in Solar Flares
Authors: Tanuma, S.; Shibata, K.
Bibcode: 2007ASPC..369..443T
Altcode:
The strong hard X-ray emission of energetic electrons is observed
in some solar flares. The origin of energetic electrons is, however,
not known fully. Then, we suggest that the internal shocks are created
in the reconnection jet in solar flares, and that energetic electrons
are accelerated by the shocks. In this paper, we examine 2D MHD
simulations of magnetic reconnection with a high spatial resolution. As the results, the magnetic reconnection occurs after the secondary
tearing instability at the current sheet. We find that, during the
non-steady Petschek reconnection, the oblique strong shocks are created
by the Kelvin-Helmholtz-like instability in the reconnection jet when
we assume an anomalous resistivity model. The oblique shocks can
be possible sites of the particle acceleration in the solar flares.
Title: Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar
Disks: A Possible Origin of Short-Term Variability in the Prompt
Emission of Gamma-Ray Bursts
Authors: Masada, Youhei; Kawanaka, Norita; Sano, Takayoshi; Shibata,
Kazunari
Bibcode: 2007ApJ...663..437M
Altcode: 2006astro.ph.12664M
The central engine of gamma-ray bursts (GRBs) is believed to be a
hot and dense disk with hyperaccretion onto a few solar-mass black
hole. We investigate where the magnetorotational instability (MRI)
actively operates in the hyperaccretion disk, which can cause angular
momentum transport in the disk. The inner region of hyperaccretion
disks can be neutrino opaque, and the energy and momentum transport by
neutrinos could affect the growth of the MRI significantly. Assuming
reasonable disk models and a weak magnetic field B<~1014
G, it is found that the MRI is strongly suppressed by the neutrino
viscosity in the inner region of hyperaccretion disks. On the other
hand, the MRI can drive active magnetohydrodynamic turbulence in the
outer neutrino-transparent region regardless of the field strength. This
suggests that the baryonic matter is accumulated into the inner dead
zone, where the MRI grows inactively and the angular momentum transport
is inefficient. When the dead zone gains a large amount of mass and
becomes gravitationally unstable, intense mass accretion onto the
central black hole would occur episodically through the gravitational
torque. This process can be a physical mechanism of the short-term
variability in the prompt emission of GRBs. Finally, the origin of
flaring activities in the X-ray afterglow is predicted in the context
of our episodic accretion scenario.
Title: A Simulation of Protostar Flares and the Expected X-Ray
Spectrum
Authors: Kawamichi, T.; Shibata, K.
Bibcode: 2007ASPC..362..304K
Altcode:
We performed one-dimensional hydrodynamic simulations of flare loops
on protostars by applying the reconnection model for a solar flare. A
magnetic loop from the central star that penetrates an accretion
disk was assumed. We found that the plasma in the loop was heated by
conduction, and that evaporation of the disk matter made the plasma
denser. This effect was seen only in the half-loop on the reconnection
side. The hard X-ray spectrum expected from our simulation has a
shape very similar to the solar one observed by RHESSI}, except that
the energy band (sim200 keV) is 10 times higher than that of the
solar flare.
Title: Discovery Of Cool Cloud-like Structures In The Corona With
Hinode Solar Optical Telescope
Authors: Okamoto, Takenori; Tsuneta, S.; Katsukawa, Y.; Ichimoto,
K.; Suematsu, Y.; Shimizu, T.; Nagata, S.; Shibata, K.; Tarbell, T.;
Shine, R.; Berger, T.; Lites, B.; Myers, D.
Bibcode: 2007AAS...210.9426O
Altcode: 2007BAAS...39..221O
A solar observation satellite Hinode (Japanese for sun rise) was
launched in September 2006.Hinode carried 3 advanced solar telescopes,
visible light telescope, EUV imaging spectrometer, and X-ray telescope
to simultaneously observe the photosphere, chromosphere, transition
region, and corona. In the performance verification phase of the Hinode
spacecraft with its telescopes, we observed an active region AR10921
near the west limb of the solar disk on November 9 2006. At this point,
we planned to observe spicules on the limb with a broadband filter
dedicated to Ca II H line (3968A). Ca II-H emission line (3968A) comes
from plasma with temperature of approx. 10(4) K, which is much lower
than the coronal temperature of 10(6-7) K. In addition to spectacular
spicules, we find a large cloud-like structure located 10,000-20,000
km above the limb. The cloud has a very complex fine structure with
dominant horizontal thread-like structure. Some features are moving
horizontally and also have clear vertical oscillatory motions. The
periods and amplitudes of these oscillations are 130-250 seconds and
200-850 km, respectively. The vertical oscillatory motion sometimes
has a coherence length as long as 16,000 km. We conclude that from
various observational features this vertical oscillation is a signature
of Alfven waves propagating along the horizontal magnetic fields. We
will discuss their origin and implications.
Title: Oblique Shocks in the Magnetic Reconnection Jet in Solar Flares
Authors: Tanuma, Syuniti; Shibata, Kazunari
Bibcode: 2007PASJ...59L...1T
Altcode:
Strong radio emission of energetic electrons is observed in some
solar flares. The origin of energetic electrons is, however, not fully
known. In this letter we suggest that oblique shocks are created in
reconnection jets in solar flares, and that energetic electrons are
accelerated by shocks. We examine 2D MHD simulations of magnetic
reconnection with high spacial resolution by assuming an anomalous
resistivity model. As a result, magnetic reconnection is found to
occur after a secondary tearing instability at the current sheet. We
find that, during nonsteady Petschek reconnection, oblique shocks
are created by an even mode of the Kelvin-Helmholtz-like instability
in the reconnection jet when we assume an anomalous resistivity
model. Furthermore, bursty, time-dependent reconnection ejects many
plasmoids from the diffusion region, and creates shocks. We suggest
that these shocks can be possible sites of particle acceleration in
solar flares.
Title: The Effect of Neutrino Radiation on Magnetorotational
Instability in Proto-Neutron Stars
Authors: Masada, Youhei; Sano, Takayoshi; Shibata, Kazunari
Bibcode: 2007ApJ...655..447M
Altcode: 2006astro.ph.10023M
Neutrino radiation takes a major role in the momentum, heat, and
lepton transports in proto-neutron stars (PNSs). These diffusive
processes affect the growth of the magnetorotational instability
(MRI) in PNSs. We perform a local linear analysis for the axisymmetric
and nonaxisymmetric MRI including the effects of neutrino transport
and ohmic dissipation. We find that the MRI can grow even in the
multidiffusive situations that are realized in neutrino-loaded
PNSs. When the toroidal magnetic component dominates over the poloidal
one, nonaxisymmetric MRI modes grow much faster than axisymmetric
modes. These results suggest the importance of the nonaxisymmetric MRI
in PNSs. Thus, understanding the three-dimensional nonlinear evolution
of the MRI is necessary for revealing the explosion mechanism of
core-collapse supernovae.
Title: Magnetohydrodynamic Numerical Simulations of Emerging Flux
and Associated Active Phenomena in the Solar Atmosphere
Authors: Miyagoshi, T.; Isobe, H.; Yokoyama, T.; Shibata, K.
Bibcode: 2006ASPC..359..164M
Altcode:
It is suggested that emerging flux build up magnetic energy in the
solar corona that could become the source of flares or jets. Emerging
flux also plays an important role in active region formation and
disappearance. To investigate these phenomena, it is necessary
to study the evolution of an emerging flux from the convection
zone to the corona. The solar atmosphere is highly stratified by
gravity. For example, the density ratio between photosphere and
corona is about ∼ 10^8. However, to study emerging flux process by
numerical simulations, it is needed to treat this process with that
highly stratified calculation domain. In this paper, we introduce
our numerical simulation results for (1) X-ray jets associated with
emerging flux and magnetic reconnection in the solar corona, (2)
twisted magnetic flux tube emergence into the solar corona, and (3)
surge caused by emerging flux tube.
Title: Investigating the state of the Sun-Earth system during extreme
events: First science results of a worldwide online conference series
Authors: Kozyra, J. U.; Shibata, K.; Fox, N. J.; Basu, S.; Coster,
A. J.; Davila, J. M.; Gopalswamy, N.; Liou, K.; Lu, G.; Mann, I. R.;
Pallamraju, D.; Paxton, L. J.; Peterson, W. K.; Talaat, E. R.;
Weatherwax, A. T.; Young, C. A.; Zanetti, L. J.
Bibcode: 2006AGUFMSA43A..01K
Altcode:
This presentation reports on new science results from an online
conference entitled "Return to the Auroral Oval for the Anniversary
of the IGY" designed to bring together researchers worldwide: (1)
to investigate newly reported features in the auroral oval during
substorms that occur in the main phase of superstorms and how these
features map throughout geospace, (2) to explore implications for the
state of the geospace system, (3) to identify signatures associated
with this geospace state from equatorial to polar latitudes,
(4) to investigate the unusual aspects of the solar sources, and
(5) to understand how propagation from Sun to Earth modified the
observed solar drivers. The main focus of the first conference is on
worldwide data exchange, the construction of global data products and
assimilative global views, and identifying coupled chains of events
from sun-to-Earth. The collaborative conference data products and
enhanced understanding of the observed features of the events will
form the basis for a follow-on conference in 2007 focused primarily
on theoretical studies and collaborative simulation efforts between
modeling groups, observers and data analysts. This conference is the
first in a series of sun-Earth connection online conferences, sponsored
by CAWSES, IHY, eGY, ICESTAR, NASA/LWS, and NSF Atmospheric Science
Programs, and designed to bring interdisciplinary researchers together
with the vast developing cyber-infrastructure of large international
data sets, high performance computing and advanced visualizations to
address grand challenge science issues in a way not previously possible.
Title: Flare Ribbon Expansion and Energy Release
Authors: Asai, Ayumi; Yokoyama, Takaaki; Shimojo, Masumi; Masuda,
Satoshi; Shibata, Kazunari
Bibcode: 2006JApA...27..167A
Altcode:
We report a detailed examination about the relationship between the
evolution of the Hα flare ribbons and the released magnetic energy
during the April 10 2001 flare. In the Hα images, several bright
kernels are observed in the flare ribbons.We identified the conjugated
footpoints, by analyzing the lightcurves at each Hα kernels, and showed
their connectivities during the flare. Then, based on the magnetic
reconnection model, we calculated quantitatively the released energy by
using the photospheric magnetic field strengths and separation speeds
of the Hα flare ribbons. Finally, we examined the downward motions
which are observed at the Hα kernels. We found that the stronger the
red-asymmetry tends to be associated with the brighter the Hα kernel.
Title: Anemone structure of Active Region NOAA 10798 and related
geo-effective flares/ CMEs
Authors: Asai, A.; Ishii, T. T.; Shibata, K.; Gopalswamy, N.
Bibcode: 2006IAUJD...3E..72A
Altcode:
Introduction: We report the evolution and the coronal features of an
active region NOAA 10798, and the related magnetic storms. Method:
We examined in detail the photospheric and coronal features of
the active region by using observational data in soft X-rays, in
extreme ultraviolet images, and in magnetogram obtained with GOES,
SOHO satellites. We also examined the interplanetary disturbances from
the ACE data. Results: This active region was located in the middle of
a small coronal hole, and generated 3 M-class flares. The flares are
associated with high speed CMEs up to 2000 km/s. The interplanetary
disturbances also show a structure with southward strong magnetic
field. These produced a magnetic storm on 2005 August 24. Conclusions:
The anemone structure may play a role for producing the high-speed
and geo-effective CMEs even the near limb locations.
Title: Magnetic Field Configuration and Evolution of a Highly
Flare-producitve Region NOAA 10808 (2005-Sep)
Authors: Ishii, T. T.; Nagashima, K.; Kurokawa, H.; Kitai, R.; Ueno,
S.; Nagata, S.; Shibata, K.
Bibcode: 2006IAUJD...3E..59I
Altcode:
Active regions on the Sun have different flare productivities with each
other. Some regions produce many large flares, while others produce no
flares. The key factor of a high flare productivity is the complexity
of magnetic field configuration of the region. In our previous studies,
we found that the twisted structure of emerging magnetic flux bundles
is the essential feature of flare-productive active regions. Vector
magnetic field data sets are necessary to examine the twisted
magnetic field structures (e.g., shear and helicity). Recently we
have constructed our new telescope, Solar Magnetic Activity Research
Telescope (SMART) at Hida Observatory, Kyoto University. We can obtain
high resolution full disk H-alpha images (with Lyot filter; H-alpha
center, blue and reg wing) and full disk vector magnetograms with
SMART. The active region NOAA 10808 showed the highest flare activity
during the current solar cycle (cycle 23) in September 2005. We
studied the formation process of delta-type magnetic configuration
using SOHO/MDI magnetograms and flares using TRACE data. We also
studied the evolution of magnetic shear and H-alpha filaments using
H-alpha full disk images and full disk vector magnetograms obtained
with SMART. In this paper, we summarize the characteristics of magnetic
field configuration of this region and discuss the relation between
the configuration and the high flare activity.
Title: SMART Observation of Magnetic Helicity in Solar Filaments
Authors: Hagino, M.; Kitai, R.; Shibata, K.
Bibcode: 2006IAUJD...3E..60H
Altcode:
We examined the magnetic helicity of solar filaments from their
structure in the chromosphere and corona. The H-alpha telescope of
the Solar Magnetic Activity Research Telescope (SMART) observed 239
intermediate filaments from 2005 July 1 to 2006 May 15. The intermediate
filament usually locates between two active regions. Using these images,
we identified the filament spine and its barbs, and determined the
chromospheric filament helicity from the mean angle between each barbs
and a spine. We found that 71% (78 of 110) of intermediate filaments
in the northern hemisphere are negative helicity and 67% (87 of 129) of
filaments in the southern hemisphere are positive, which agreed with the
well-known hemispheric tendency of the magnetic helicity. Additionally,
we studied the coronal helicity of intermediate filaments. The coronal
filament helicity is defined as the crossing angle of threads formed
a filament. The helicity pattern of coronal filaments obtained with
EIT/SOHO 171A also shows the helicity hemispheric tendency. Namely, 65%
(71 of 110) of coronal filaments in the northern hemisphere exhibit
negative helicity and the 65% (84 of 129) of filaments in the southern
hemisphere show negative helicity. These data were observed in the
same day with the SMART H-alpha data. Moreover, we found 12 filament
eruptions in our data. The 7 of 12 filaments show the clear opposite
sign of the hemispheric tendency of the magnetic helicity. The helicity
seems to be change during temporal evolution. This results suggest
that filament instability may be driven by the opposite sign helicity
injection from the foot point of the barb.
Title: Jet formation driven by the expansion of magnetic bridges
between the ergosphere and the disk around a rapidly rotating
black hole
Authors: Koide, Shinji; Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 2006PhRvD..74d4005K
Altcode:
We report two-dimensional numerical results of jet formation driven
by a magnetic field due to a current loop near a rapidly rotating
black hole. We initially set the current loop along the intersection
of the equatorial plane and the surface of the ergosphere around
the black hole. In such magnetic configurations, there are magnetic
flux tubes which bridge the region between the ergosphere and the
corotating disk. The magnetic flux tube, which we call a “magnetic
bridge,” is twisted rapidly by the plasma in the ergosphere due
to the frame-dragging effect. The magnetic pressure of the magnetic
flux tube increases and the strong magnetic pressure blows off the
plasma near the ergosphere to form outflow. The outflow is pinched
by the magnetic tension of the magnetic flux tube. Then, eventually,
the jet is formed. That is, the magnetic bridges cannot be stationary,
and they expand explosively to form a jet. The parameter survey of
the background pressure shows that the radius of the collimated jet
depends on the gas pressure of the corona. However, this does not mean
the gas pressure collimates the jet. The gas pressure decelerates the
jet and the pinch effect by the magnetic field becomes significant.
Title: A Sun-to-Earth Campaign Joining Observations from the Great
Observatory with Worldwide Satellite and Ground-Based Resources to
Investigate System Science Frontiers
Authors: Kozyra, J. U.; Shibata, K.; Barnes, R. J.; Basu, S.; Davila,
J. M.; Fox, N. J.; Gopalswamy, N.; Kuznetsova, M. M.; Pallamraju,
D.; Paxton, L. J.; Ridley, A.; Weiss, M.; Young, C. A.; Zanetti, L. J.
Bibcode: 2006AGUSMSM23A..03K
Altcode:
An Internet-based cross-disciplinary analysis campaign that will
make heavy use of Great Observatory missions as well as international
satellite and ground-based assets is being undertaken with joint support
from the CAWSES, IHY, LWS, and ICESTAR programs planned for late
April or early May 2006. An evolving list of open science questions
that serve as sun-to-Earth focus areas for the worldwide campaign
were identified during a small interdisciplinary CAWSES workshop
at Stanford University in December 2005 as well as during a joint
CAWSES/ICESTAR session at the CEDAR meeting in Boulder the preceding
summer. The analysis campaign will take place over the Internet in the
form of virtual poster sessions with message boards and monitors that
summarize the important science issues and new results daily. Poster
authors will be asked to closely monitor their message boards during
the day of their poster session as well as the following day. Outreach
to other disciplines and international students will take the form of
tutorial talks that place campaign science issues into the context of
the current state of knowledge in each discipline area. Global models
and data sets (TEC, magnetometer maps, ULF wave maps, assimilative
models, MHD model outputs, continuous solar images) will be available
to provide context for local and regional observations. The Community
Coordinated Data Center (CCMC) is developing a small number of new data
display formats that extract data from global models and place it in
the same format as the observations either for ground-based stations
or along satellite tracks. Other ideas being explored include real
time upload of additional posters in response to issues raised during
the poster session, library of related articles, reference archive of
observations, etc. A summary of which aspects and/or tools worked and
which were less useful will be presented.
Title: Three-Dimensional Simulation of Solar Emerging Flux Using
the Earth Simulator I. Magnetic Rayleigh-Taylor Instability at the
Top of the Emerging Flux as the Origin of Filamentary Structure
Authors: Isobe, Hiroaki; Miyagoshi, Takehiro; Shibata, Kazunari;
Yokoyama, Takaaki
Bibcode: 2006PASJ...58..423I
Altcode:
We present the results of three-dimensional magnetohydrodynamic
simulations of solar emerging flux and its interaction with preexisting
coronal field. In order to resolve the fine structures and the current
sheets, we used high-resolution grids with up to 800×400×620 points;
the calculation was carried out using the Earth Simulator. The model
set up is an extension of a previous two-dimensional simulation by
Yokoyama and Shibata (1995) to include the variation along the third
direction. Based on the same simulation result, we reported in our
previous paper (Isobe et al. 2005): (1) Dense filaments similar to Hα
arch filament system are spontaneously formed in the emerging flux
by the magnetic Rayleigh-Taylor type instability. (2) Filamentary
current sheets are created in the emerging flux due to a nonlinear
development of the magnetic Rayleigh-Taylor instability, which may
cause an intermittent, nonuiform heating of the corona. (3) A magnetic
reconnection between the emerging flux and preexisting coronal field
occurs in a spatially intermittent way. In this paper we describe
the simulation model and discuss the origin and the properties of the
magnetic Rayleigh-Taylor instability in detail. It is shown that the
top-heavy configuration that causes the instability is formed by the
intrinsic dynamics of the emerging flux.
Title: Statistical Analysis of Reconnection Inflows in Solar Flares
Observed with SOHO EIT
Authors: Narukage, Noriyuki; Shibata, Kazunari
Bibcode: 2006ApJ...637.1122N
Altcode:
We report observations of reconnection inflows in extreme ultraviolet
(EUV) Fe XII λ195 images with the Extreme Ultraviolet Imaging Telescope
(EIT) on board the Solar and Heliospheric Observatory (SOHO). Yokoyama
and colleagues reported the first example observed on 1999 March 18. We
survey the EIT data from 1996 to 2000 and find six new inflow events. We
measure the inflow velocity vinflow for each event and find
that vinflow is about 2.6-38 km s-1. Furthermore,
using the six EIT inflow events observed simultaneously with Yohkoh SXT
(including the Yokoyama event), we calculate the reconnection rate as
MA≡vinflow/vA=0.001-0.07. It is also
found that the plasmoid ejection and/or coronal mass ejection (CME)
are closely related to the inflow. The velocity of the CME exhibits
a correlation with the inflow velocity.
Title: Why are there stationary EIT wave fronts
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2006AdSpR..38..456C
Altcode:
EIT waves are often observed to be propagating EUV enhancements followed
by an expanding dimming region after the launch of CMEs. It was widely
assumed that they are the coronal counterparts of the chromospheric
Moreton waves, though the former are three or more times slower. The
existence of a stationary “EIT wave” front in some events,
however, posed a big challenge to the wave explanation. Simulations
are performed to reproduce the stationary “EIT wave” front, which
is exactly located near the footpoint of the magnetic separatrix,
consistent with observations. The formation of the stationary front
is explained in the framework of our model where “EIT waves” are
supposed to be generated by successive opening of the field lines
covering the erupting flux rope in CMEs.
Title: A simulation on protostar flare and the expected hard X-ray
spectrum
Authors: Kawamichi, T.; Shibata, K.
Bibcode: 2006cosp...36.3055K
Altcode: 2006cosp.meet.3055K
Many X-ray observations have revealed that protostars have strong flare
activities The released energy sometimes reaches 10 36-37 erg It is
widely accepted that these flares are driven by magnetic reconnection
like solar flares Applying the solar flare mechanism to protostars we
performed hydrodynamic simulations and predicted the hard X-ray spectra
We assumed a magnetic loop of 14R odot in the halfway point of which
an accretion disk exists A flare energy of 10 36 erg which consists
of thermal and non-thermal energies is released in the half loop We
found that the evaporation flows from the star and the disk occured so
that hot and dense flare loop was formed there The conduction front
was blocked by the disk and hence the other half loop remained to be
a pre-flare loop However non-thermal electrons having enough energy
could penetrate the disk and released their energy beyond the disk The
expected hard X-ray spectra had a spectral break at about 100 keV and
the shapes were very similar to thouse of solar flares We also found
that the spectral energy distribution was different at some regions
in the flare loop due to the collision by non-thermal electrons This
may provide a diagnostic tool to test the existence of the accretion
disk in the flare loop
Title: Self-consistent MHD modeling of a coronal mass ejection,
coronal dimming, and a giant cusp-shaped arcade formation
Authors: Shiota, D.; Isobe, H.; Chen, P. F.; Yamamoto, T. T.; Sakajiri,
T.; Shibata, K.
Bibcode: 2006cosp...36.1994S
Altcode: 2006cosp.meet.1994S
Coronal mass ejections CMEs are one of the most spectacular phenomena in
the solar corona Recent observations revealed that CMEs are associated
with either solar flares or giant arcade formations in which magnetic
reconnection is thought to play an essential role Considering these
observations Shibata 1996 1999 suggest that CMEs filament eruptions
flares giant arcades can be understood in a unified view mass ejection
and magnetic energy release via magnetic reconnection We performed
magnetohydrodynamic simulations of CMEs and associated giant arcade
formations The soft X-ray images synthesized from the numerical
results are compared with the soft X-ray images taken with the Soft
X-Ray Telescope aboard Yohkoh The comparison between synthesized and
observed soft X-ray images provides new interpretations of various
features associated with CMEs and giant arcades 1 It is likely that
the Y-shaped ejecting structure observed in the giant arcade on 1992
January 24 corresponds to slow and fast shocks associated with magnetic
reconnection 2 Soft X-ray twin dimming corresponds to the rarefaction
induced by reconnection 3 The inner boundary of the dimming region
corresponds to the slow shocks 4 The three-part structure of a CME
can be explained by our numerical results 5 The numerical results
also suggest that a backbone feature of a flare giant arcade may
correspond to the fast shock formed by the collision of the downward
reconnection outflow
Title: Close causal relation between emergence of twisted flux rope
and strong flares
Authors: Kurokawa, H.; Ishii, T.; Ueno, S.; Nagata, S.; Kitai, R. .;
Shibata, K.
Bibcode: 2006cosp...36.2230K
Altcode: 2006cosp.meet.2230K
Solar flares are considered to occur in the solar corona by magnetic
field reconnection Shibata 1995 The flare energy which is stored in the
twisted magnetic field is supplied to the corona through the photosphere
from the convection zone For this reason observations of evolutional
changes in the magnetic field configuration of flare-productive regions
are fundamentally important for the flare energy build-up study and
the forecast of strong flares Hagyard 1984 showed that flares occurred
along the neutral line of strong magnetic shear The magnetic shear is
however not a sufficient condition for strong flare occurrence and it
is important to study the active region evolutions and examine which
type of magnetic shear development produces strong flares Until now
several works made detailed studies of magnetic shear developments in
flare-productive sunspot regions and suggested that the emergence of a
twisted magnetic flux rope which is originally formed in the convection
zone must be the source of the strong magnetic shear development in a
sunspot region to produce a strong flare activity Kurokawa 1987 Tanaka
1991 Ishii et al 1998 Kurokawa 2002 In this paper we present our recent
studies of energy build-up processes of flare-productive active regions
We studied the evolutional changes of 11 super active regions which
produced more than three X-class flares observed from 1998 through 2005
during the 23 solar sunspot cycle in details We found all these regions
show some common magnetic evolutional features in the course of the
Title: CAWSES Related Projects in Japan : Grant-in-Aid for Creative
Scientific Research ügBasic Study of Space Weather Predictionüh
and CHAIN (Continuous H Alpha Imaging Network)
Authors: Shibata, K.; Kurokawa, H.
Bibcode: 2006cosp...36.3288S
Altcode: 2006cosp.meet.3288S
The Grant-in-Aid for Creative Scientific Research of the Ministry of
Education Science Sports Technology and Culture of Japan The Basic
Study of Space Weather Prediction PI K Shibata Kyoto Univ has started
in 2005 as 5 years projects with total budget 446Myen The purpose
of this project is to develop a physical model of solar-terrestrial
phenomena and space storms as a basis of space weather prediction by
resolving fundamental physics of key phenomena from solar flares and
coronal mass ejections to magnetospheric storms under international
cooperation program CAWSES Climate and Weather of the Sun-Earth System
Continuous H Alpha Imaging Network CHAIN Project led by H Kurokawa
is a key project in this space weather study enabling continuous H
alpha full Sun observations by connecting many solar telescopes in
many countries through internet which provides the basis of the study
of space weather prediction
Title: Anemone structure of AR NOAA 10798 and related geo-effective
flares and CMEs
Authors: Asai, A.; Ishii, T. T.; Shibata, K.; Gopalswamy, N.
Bibcode: 2006cosp...36.2406A
Altcode: 2006cosp.meet.2406A
We report coronal features of an active region NOAA 10798 This
active region was located in the middle of a small coronal hole and
generated 3 M-class flares The flares are associated with high speed
CMEs which produced a magnetic storm on 2005 August 24 We examined
the coronal features by using observational data in soft X-rays in
extreme ultraviolets and in microwaves obtained with GOES SOHO TRACE
satellites and Nobeyama Radioheliograph
Title: Three-dimensional Magnetohydrodynamic Simulations of Jets
from Accretion Disks
Authors: Kigure, Hiromitsu; Shibata, Kazunari
Bibcode: 2005ApJ...634..879K
Altcode: 2005astro.ph..8388K
We report the results of three-dimensional magnetohydrodynamic (MHD)
simulations of a jet formation by the interaction between an accretion
disk and a large-scale magnetic field. The disk is not treated as a
boundary condition but is solved self-consistently. To investigate
the stability of the MHD jet, the accretion disk is perturbed with a
nonaxisymmetric sinusoidal or random fluctuation of the rotational
velocity. The dependences of the jet velocity (vz),
mass outflow rate (M˙w), and mass accretion rate
(M˙a) on the initial magnetic field strength in both
nonaxisymmetric cases are similar to those in the axisymmetric
case. That is, vz~B1/30,
M˙w~B0, and
M˙a~B1.40, where
B0 is the initial magnetic field strength. The
former two relations are consistent with Michel's steady solution,
vz~(B20/M˙w)1/3,
although the jet and accretion do not reach the steady state. In both
perturbation cases, a nonaxisymmetric structure with m=2 appears in
the jet, where m is the azimuthal wavenumber. This structure cannot
be explained by Kelvin-Helmholtz instability and seems to originate
in the accretion disk. Nonaxisymmetric modes in the jet reach almost
constant levels after about 1.5 orbital periods of the accretion disk,
while all modes in the accretion disk grow with oscillation. As for the
angular momentum transport by Maxwell stress, the vertical component,
<BφBz/4π>, is comparable to the radial
component, <BφBr/4π>, in the wide range
of initial magnetic field strength.
Title: Magnetic reconnection in solar plasmas
Authors: Shibata, K.
Bibcode: 2005AGUFMSM13C..01S
Altcode:
Recent space observations of the solar atmosphere such as by Yohkoh,
SOHO, TRACE, have revealed that the solar atmosphere is much more
dynamic than had been thought, and is full of microflares, nanoflares,
jets, and mass ejections, which are more or less related to magnetic
reconnection. We can now say that magnetic reconnection is ubiquitous
in the solar atmosphere, and play major role in producing flares
and flare-like phenomena, and possibly play even an important role
in heating the corona. Solar B mission will be launched next summer,
and we expect that many fundamental puzzles related to reconnection
and coronal heating will be solved by Solar B. Here we will discuss
recent progress and future prospects (especially with Solar B) in the
study of magnetic reconnection in solar plasmas from both theoretical
and observational point of view.
Title: 3-D GRMHD Simulations of Disk-Jet Coupling and Emission
Authors: Nishikawa, K. -I.; Mizuno, Y.; Fuerst, S.; Wu, K.; Hardee,
P.; Richardson, G.; Koide, S.; Shibata, K.; Kudoh, T.; Fishman, G. J.
Bibcode: 2005AIPC..801..184N
Altcode: 2005astro.ph..9601N
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from a thin
accretion disk around a Schwarzschild black hole with a free-falling
corona. The initial simulation results show that a bipolar jet
(velocity nearly 0.3c) is created as shown by previous two-dimensional
axisymmetric simulations with mirror symmetry at the equator. The 3-D
simulation ran over one hundred light-crossing time units which is
considerably longer than the previous simulations. We show that the
jet is initially formed as predicted due in part to magnetic pressure
from the twisting the initially uniform magnetic field and from gas
pressure associated with shock formation in the region around r =
3r_S. At later times, the accretion disk becomes thick and the jet fades
resulting in a wind that is ejected from the surface of the thickened
(torus-like) disk. It should be noted that no streaming matter from a
donor is included at the outer boundary in the simulation (an isolated
black hole not binary black hole). The wind flows outwards with a wider
angle than the initial jet. The widening of the jet is consistent with
the outward moving torsional Alfven waves (TAWs). This evolution of
disk-jet coupling suggests that the jet fades with a thickened accretion
disk due to the lack of streaming material from an accompanying star.
Title: Self-Consistent Magnetohydrodynamic Modeling of a Coronal Mass
Ejection, Coronal Dimming, and a Giant Cusp-shaped Arcade Formation
Authors: Shiota, Daikou; Isobe, Hiroaki; Chen, P. F.; Yamamoto,
Tetsuya T.; Sakajiri, Takuma; Shibata, Kazunari
Bibcode: 2005ApJ...634..663S
Altcode: 2005astro.ph..8478S
We performed magnetohydrodynamic simulations of coronal mass ejections
(CMEs) and associated giant arcade formations, and the results suggest
new interpretations of observations of CMEs. We performed two cases of
the simulation: with and without heat conduction. Comparing between
the results of the two cases, we found that the reconnection rate
in the conductive case is a little higher than that in the adiabatic
case and that the temperature of the loop top is consistent with the
theoretical value predicted by the Yokoyama-Shibata scaling law. The
dynamical properties such as velocity and magnetic field are similar
in the two cases, whereas thermal properties such as temperature and
density are very different. In both cases, slow shocks associated
with magnetic reconnection propagate from the reconnection region
along the magnetic field lines around the flux rope, and the shock
fronts form spiral patterns. Just outside the slow shocks, the plasma
density decreases greatly. The soft X-ray images synthesized from
the numerical results are compared with the soft X-ray images of a
giant arcade observed with the Soft X-Ray Telescope aboard Yohkoh;
it is confirmed that the effect of heat conduction is significant
for the detailed comparison between simulation and observation. The
comparison between synthesized and observed soft X-ray images provides
new interpretations of various features associated with CMEs and
giant arcades. (1) It is likely that the Y-shaped ejecting structure,
observed in the giant arcade on 1992 January 24, corresponds to slow and
fast shocks associated with magnetic reconnection. (2) Soft X-ray twin
dimming corresponds to the rarefaction induced by reconnection. (3)
The inner boundary of the dimming region corresponds to the slow
shocks. (4) The ``three-part structure'' of a CME can be explained by
our numerical results. (5) The numerical results also suggest that a
backbone feature of a flare/giant arcade may correspond to the fast
shock formed by the collision of the downward reconnection outflow.
Title: Magnetic Fields and Intensity Changes in Coronal Dimming
Regions
Authors: Attrill, G. D. R.; Narukage, N.; Shibata, K.; Harra, L. K.
Bibcode: 2005ESASP.596E..11A
Altcode: 2005ccmf.confE..11A
No abstract at ADS
Title: Magnetic-Reconnection and Current-Sheet Model for the Radio
Arc and Threads in the Galactic Center
Authors: Sofue, Yoshiaki; Kigure, Hiromitsu; Shibata, Kazunari
Bibcode: 2005PASJ...57L..39S
Altcode: 2005astro.ph..7568S
We propose a new mechanism to explain the radio arc and threads in
the Galactic center by current sheets produced by local magnetic
shears due to the interaction of a moving cloud and a vertical field
based on three-dimensional magnetohydrodynamical simulations. Magnetic
reconnection and acceleration of cosmic-ray electrons in the current
sheet would result in a high contrast of the radio emissivity inside
and outside the arc and threads.
Title: Measurement of the Energy Release Rate and the Reconnection
Rate in Solar Flares
Authors: Isobe, Hiroaki; Takasaki, Hiroyuki; Shibata, Kazunari
Bibcode: 2005ApJ...632.1184I
Altcode: 2005astro.ph..7327I
By using the method presented by Isobe et al., the nondimensional
reconnection rate vin/vA has been determined for
the impulsive phase of three two-ribbon flares, where vin is
the velocity of the reconnection inflow and vA is the Alfvén
velocity. The nondimensional reconnection rate is important to make
a constraint on the theoretical models of magnetic reconnection. In
order to reduce the uncertainty of the reconnection rate, it is
important to determine the energy release rate of the flares from
observational data as accurately as possible. To this end, we have
carried out one-dimensional hydrodynamic simulations of a flare loop
and synthesized the count rate detected by the Soft X-Ray Telescope
(SXT) aboard the Yohkoh satellite. We found that the time derivative
of the thermal energy contents in a flare arcade derived from SXT data
is smaller than the real energy release rate by a factor of 0.3-0.8,
depending on the loop length and the energy release rate. The results
of the simulations are presented in the paper and used to calculate
the reconnection rate. We found that the reconnection rate is 0.047
for the X2.3 flare on 2000 November 24, 0.015 for the M3.7 flare on
2000 July 14, and 0.071 for the C8.9 flare on 2000 November 16. These
values are similar to that derived from the direct observation of the
reconnection inflow by Yokoyama et al. and consistent with the fast
reconnection models such as that of Petschek.
Title: Evolution of Barb Angle and Filament Eruption
Authors: Su, J. T.; Liu, Y.; Zhang, H. Q.; Kurokawa, H.; Yurchyshyn,
V.; Shibata, K.; Bao, X. M.; Wang, G. P.; Li, C.
Bibcode: 2005ApJ...630L.101S
Altcode:
Hα observations of a quiescent U-shaped filament were obtained at Big
Bear Solar Observatory and at Hida Observatory with the Flare Monitoring
Telescope. The filament was located in the southern hemisphere on
1998 November 4. We study the evolution of the angle of a barb with
respect to the axis of the filament and find the evolution can be
divided into two phases: a rise from the acute phase to the obtuse
phase and a fall. Thus, this indicates that the chirality of this barb
changes with time. Moreover, in the process of evolution, we find that
interconnection of the part of the filament bearing the barb with the
whole filament became either weakened or strengthened. We impute the
final eruption of the filament to the chirality evolution of the barb.
Title: MHD Flares and Jets in the Sun, Stars, and Accretion Disks
Authors: Shibata, K.
Bibcode: 2005AIPC..784..153S
Altcode:
Recent observations of the Sun, stars, and accretion disks (active
galactic nuclei, close binary systems, young stellar objects) show that
these objects are much more dynamic than it had been thought and are
full of flares and jets with many common properties. In this article,
we give unified view and model of these flares and jets, in the Sun,
stars, protostars, accretion disks, and gamma ray bursts, on the basis
of magnetohydrodynamic (MHD) interpretation.
Title: Production of Filaments by Surges
Authors: Liu, Yu; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2005ApJ...631L..93L
Altcode:
We have recently developed a new method for automatic detection of
solar surges and other dark dynamic features. For the first time we
find that some filaments can be quickly formed by trapping the cold
material supplied by surges originating from the chromosphere. Two clear
examples are presented in this Letter, showing the special process for
new filament formation by surge injection. The data used here were
taken from Hida Observatory and the Global Hα Network operated by
Big Bear Solar Observatory. Both the filaments existed on the solar
surface for not less than 20 hr, with an average length about 200",
and had obvious helical structures and barbs. The surge material
was injected from one terminal along the main axis of the filaments
or the filament channels. We conclude that there are two necessary
conditions for new filament formation by surges at one location:
(1) an ``empty'' filament channel, or magnetic trap, and (2) enough
mass supplied by surge activity. In most other surge-filament events,
apart from our two examples, the surges are observed moving toward
and acting on a preexisting nearby filament. The close relationship
between surge activity and filament formation and maintenance suggests
that there should be a direct link between the filament axial fields
and the large-scale background fields along which the surge material
can be driven into the filament channel. On the other hand, it also
supports the idea that the frequent injection of flow from below is an
important way to convey mass and energy into the corona through magnetic
reconnections driven by successive emerging flux and converging flow.
Title: Observations of an Emerging Flux Region Surge: Implications
for Coronal Mass Ejections Triggered by Emerging Flux
Authors: Liu, Y.; Su, J. T.; Morimoto, T.; Kurokawa, H.; Shibata, K.
Bibcode: 2005ApJ...628.1056L
Altcode:
It is well known that coronal mass ejections (CMEs) are often associated
with flares and filament eruptions. Previous studies of CMEs, however,
have not established any association between CMEs and surges. In this
paper, we present a detailed analysis of a large emerging flux region
(EFR) surge and a jetlike CME, both observed on 1998 April 16. Our
analysis shows a close temporal and spatial relationship between
the two. Using observations from the Large Angle and Spectrometric
Coronagraph (LASCO) and Hida Flare Monitoring Telescope (Hα, Hα+/-0.8
Å), we found that the CME's onset time and central position angle were
coincident with the surge features. Magnetograms and Hα filtergrams
showed that the surge resulted from the successive emergence of a
bipolar sunspot group, NOAA Active Region 8203, which was the only
active region in the northern hemisphere. The surge was impulsively
accelerated at around the peak time of the GOES SXR flux. The associated
CME appeared in the field of view of LASCO C2 16 minutes after the
surge disappeared. Importantly, observations from the EUV Imaging
Telescope at λ195 Å clearly demonstrate topological changes in the
coronal field due to its interaction with the EFR. An initially closed
EFR-loop system opened up during the surge. There was no filament
involved in this surge-CME event. We propose that the onset of the
CME resulted from the significant restructuring of the large-scale
coronal magnetic field as a result of flux emergence in the active
region. This surge-CME event strongly suggests that emerging flux
may not only trigger a surge but also simultaneously trigger a CME by
means of small-scale reconnection in the lower atmosphere.
Title: Internal Shocks in the Magnetic Reconnection Jet in Solar
Flares: Multiple Fast Shocks Created by the Secondary Tearing
Instability
Authors: Tanuma, S.; Shibata, K.
Bibcode: 2005ApJ...628L..77T
Altcode: 2005astro.ph..3005T
Space solar missions such as Yohkoh and RHESSI observe the hard X-
and gamma-ray emission from energetic electrons in impulsive solar
flares. Their energization mechanism, however, is unknown. In
this Letter, we suggest that the internal shocks are created in
the reconnection jet and that they are possible sites of particle
acceleration. We examine how magnetic reconnection creates the multiple
shocks by performing two-dimensional resistive magnetohydrodynamic
simulations. In this Letter, we use a very small grid to resolve
the diffusion region. As a result, we find that the current sheet
becomes thin due to the tearing instability, and it collapses to a
Sweet-Parker sheet. The thin sheet becomes unstable to the secondary
tearing instability. Fast reconnection starts by the onset of anomalous
resistivity immediately after the secondary tearing instability. During
the bursty, time-dependent magnetic reconnection, the secondary tearing
instability continues in the diffusion region where the anomalous
resistivity is enhanced. As a result, many weak shocks are created in
the reconnection jet. This situation produces turbulent reconnection. We
suggest that multiple fast shocks are created in the jet and that the
energetic electrons can be accelerated by these shocks.
Title: General relativistic magnetohydrodynamic simulations of
collapsars: Rotating black hole cases
Authors: Mizuno, Y.; Yamada, S.; Koide, S.; Shibata, K.
Bibcode: 2005NCimC..28..423M
Altcode:
We have performed 2.5-dimensional general relativistic
magnetohydrodynamic (MHD) simulations of collapsars including a rotating
black hole. Initially, we assume that the core collapse has failed in
this star. A rotating black hole of a few solar masses is inserted by
hand into the calculation. The simulation results show the formation
of a disklike structure and the generation of a jetlike outflow near
the central black hole. The jetlike outflow propagates and accelerated
mainly by the magnetic field. The total jet velocity is ∼ 0.3c. When
the rotation of the black hole is faster, the magnetic field is twisted
strongly owing to the frame-dragging effect. The magnetic energy stored
by the twisting magnetic field is directly converted to kinetic energy
of the jet rather than propagating as an Alfvén wave. Thus, as the
rotation of the black hole becomes faster, the poloidal velocity of
the jet becomes faster.
Title: A General Relativistic Magnetohydrodynamic Simulation of
Jet Formation
Authors: Nishikawa, K. -I.; Richardson, G.; Koide, S.; Shibata, K.;
Kudoh, T.; Hardee, P.; Fishman, G. J.
Bibcode: 2005ApJ...625...60N
Altcode: 2004astro.ph..3032N
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from
a thin accretion disk around a Schwarzschild black hole with a
free-falling corona. The initial simulation results show that a
bipolar jet (velocity ~0.3c) is created, as shown by previous
two-dimensional axisymmetric simulations with mirror symmetry
at the equator. The three-dimensional simulation ran over 100
light crossing time units (τS=rS/c, where
rS≡2GM/c2), which is considerably longer than
the previous simulations. We show that the jet is initially formed as
predicted owing in part to magnetic pressure from the twisting of the
initially uniform magnetic field and from gas pressure associated with
shock formation in the region around r=3rS. At later times,
the accretion disk becomes thick and the jet fades resulting in a
wind that is ejected from the surface of the thickened (torus-like)
disk. It should be noted that no streaming matter from a donor is
included at the outer boundary in the simulation (an isolated black
hole not binary black hole). The wind flows outward with a wider angle
than the initial jet. The widening of the jet is consistent with the
outward-moving torsional Alfvén waves. This evolution of disk-jet
coupling suggests that the jet fades with a thickened accretion disk
because of the lack of streaming material from an accompanying star.
Title: A Full View of EIT Waves
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2005ApJ...622.1202C
Altcode:
Early observations by the EUV Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory indicated that propagating diffuse wave fronts,
now conventionally referred to as ``EIT waves,'' can often be seen on
the solar disk with a propagation velocity several times smaller than
that of Hα Moreton waves. They are almost always associated with
coronal mass ejections. We have previously confirmed the existence
of such a wave phenomenon with numerical simulations, which indicate
that there does exist a slower moving ``wave'' much behind the coronal
counterpart of the Hα Moreton wave. Further observations have disclosed
many new features of the EIT waves: the waves stop near the separatrix
between active regions, sometimes they experience acceleration from
the active region to the quiet region, and so on. Here we report on
MHD simulations performed to demonstrate how the typical features of
EIT waves can all be accounted for within our theoretical model, in
which the EIT waves are thought to be formed by successive stretching
or opening of closed field lines driven by an erupting flux rope. The
relationship between EIT waves, Hα Moreton waves, and type II radio
bursts is discussed, with an emphasis on reconciling the discrepancies
among different views of the ``EIT wave'' phenomenon.
Title: Filamentary structure on the Sun from the magnetic
Rayleigh-Taylor instability
Authors: Isobe, Hiroaki; Miyagoshi, Takehiro; Shibata, Kazunari;
Yokoyama, Takaaki
Bibcode: 2005Natur.434..478I
Altcode:
Magnetic flux emerges from the solar surface as dark filaments
connecting small sunspots with opposite polarities. The regions around
the dark filaments are often bright in X-rays and are associated
with jets. This implies plasma heating and acceleration, which are
important for coronal heating. Previous two-dimensional simulations
of such regions showed that magnetic reconnection between the coronal
magnetic field and the emerging flux produced X-ray jets and flares,
but left unresolved the origin of filamentary structure and the
intermittent nature of the heating. Here we report three-dimensional
simulations of emerging flux showing that the filamentary structure
arises spontaneously from the magnetic Rayleigh-Taylor instability,
contrary to the previous view that the dark filaments are isolated
bundles of magnetic field that rise from the photosphere carrying the
dense gas. As a result of the magnetic Rayleigh-Taylor instability,
thin current sheets are formed in the emerging flux, and magnetic
reconnection occurs between emerging flux and the pre-existing coronal
field in a spatially intermittent way. This explains naturally the
intermittent nature of coronal heating and the patchy brightenings in
solar flares.
Title: The Acceleration Mechanism of Resistive Magnetohydrodynamic
Jets Launched from Accretion Disks
Authors: Kuwabara, Takuhito; Shibata, Kazunari; Kudoh, Takahiro;
Matsumoto, Ryoji
Bibcode: 2005ApJ...621..921K
Altcode: 2004astro.ph.11712K
We analyzed the results of nonlinear resistive magnetohydrodynamic
(MHD) simulations of jet formation to study the acceleration
mechanism of axisymmetric, resistive MHD jets. The initial state
is a constant angular momentum, polytropic torus threaded by weak
uniform vertical magnetic fields. The time evolution of the torus is
simulated by applying the CIP-MOCCT scheme extended for resistive MHD
equations. We carried out simulations up to 50 rotation periods at the
innermost radius of the disk created by accretion from the torus. The
acceleration forces and the characteristics of resistive jets were
studied by computing forces acting on Lagrangian test particles. Since
the angle between the rotation axis of the disk and magnetic field
lines is smaller in resistive models than in ideal MHD models,
magnetocentrifugal acceleration is smaller. The effective potential
along a magnetic field line has a maximum around z~0.5r0
in resistive models, where r0 is the radius at which the
density of the initial torus is maximum. Jets are launched after the
disk material is lifted to this height by pressure gradient force. Even
in this case, the main acceleration force around the slow magnetosonic
point is the magnetocentrifugal force. The power of the resistive MHD
jet is comparable to the mechanical energy liberated in the disk by mass
accretion. Joule heating is not essential for the formation of jets.
Title: Downflow motions associated with impulsive nonthermal emissions
Authors: Asai, Ayumi; Shimojo, Masumi; Yokoyama, Takaaki; Shibata,
Kazunari
Bibcode: 2005ARAOJ...7....8A
Altcode:
No abstract at ADS
Title: Theories of Eruptive Flares
Authors: Shibata, K.
Bibcode: 2005IAUS..226..241S
Altcode:
Recent progress of theories of eruptive flares (and CMEs) is reviewed
within a framework of reconnection model with emphasis on development
of basic idea and concept.
Title: Enhancement of Sunward Double-Probe Electric Field Observed
By Geotail During the Solar Flare
Authors: Takei, Y.; Terasawa, T.; Nakamura, M.; Mukai, T.; Hayakawa,
H.; Matsuoka, A.; Takasaki, H.; Shibata, K.
Bibcode: 2005fmpp.conf..301T
Altcode:
No abstract at ADS
Title: Flare ribbon expansion and energy release rate
Authors: Asai, Ayumi; Shimojo, Masumi; Yokoyama, Takaaki; Masuda,
Satoshi; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2005ARAOJ...7....7A
Altcode:
No abstract at ADS
Title: Observations of Flare-Associated Waves with SolarB
Authors: Narukage, N.; Shibata, K.
Bibcode: 2004ASPC..325..389N
Altcode:
In Hα, a flare-associated chromospheric wave (called a Moreton
wave) was discovered in 1960, and after that such waves are
sometimes observed. Uchida (1968, 1974) identified the Moreton
wave as the intersection of a coronal MHD fast-mode shock and the
chromosphere. Recently, the Soft X-ray Telescope (SXT) on board Yohkoh
observed coronal wave-like disturbances (X-ray waves). Narukage et
al. (2002, 2004) showed two X-ray waves are MHD fast-mode shock,
i.e. coronal counterparts of the Moreton waves. The SolarB has
Solar Optical Telescope (SOT), X-Ray Telescope (XRT) and EUV Imaging
Spectrometer (EIS) on board and will be launched in 2006. We expect
SOT, XRT and EIS will detect chromospheric Moreton waves, coronal X-ray
waves and line-of-sight velocity of waves, respectively. In preparation
for SolarB, we examine the detectable possibility of waves with these
telescopes and suggest methods for observation.
Title: Spectroscopic Detection of Magnetic Reconnection Evidence in
the Solar Atmosphere with SolarB/EIS
Authors: Brooks, D. H.; Isobe, H.; Shibata, K.; Chen, P. F.; Lanzafame,
A. C.
Bibcode: 2004ASPC..325..367B
Altcode:
2.5D MHD simulations of CMEs and flares are combined with improved
accuracy density sensitive line emission contribution functions from
the ADAS database to study profiles of spectral lines which will fall
within the wavelength range of the SolarB Extreme ultraviolet Imaging
Spectrometer (EIS). The objective is to study the signatures of magnetic
reconnection associated flow phenomena in the line profiles and provide
a set of recommended lines for EIS observations planning. Here, we
illustrate our methods by considering the profile of the well-known
Ion{Fe{XII}} 195 AA line and its ability to detect reconnection
inflows. We also discuss the effects of altering simulation parameters
such as electron temperature and the inclusion of the effect of heat
conduction. The table of recommended lines following these methods is
being prepared and will be presented in a separate paper.
Title: Simulated XRT and EIS Observations of Magnetic Reconnection
in Coronal Mass Ejection and X-Ray Arcade
Authors: Shiota, D.; Isobe, H.; Brooks, D. H.; Shibata, K.; Chen, P. F.
Bibcode: 2004ASPC..325..373S
Altcode:
We perform a numerical simulation of a coronal mass ejection and an
associated X-ray arcade. Based on the numerical results, we synthesize
theoretical images taken with XRT and EIS aboard SolarB, and discuss
how reconnection inflow and slow shocks would be observed with XRT
and EIS. The mechanism of dimming is also discussed.
Title: Downflow as a Reconnection Outflow
Authors: Asai, A.; Shibata, K.; Yokoyama, T.; Shimojo, M.
Bibcode: 2004ASPC..325..361A
Altcode:
We present a detailed examination about the evolution of TRACE downflow
motions (sunward motions) seen above post-flare loops. We found that
the times when the downflow motions are seen correspond to those of
the bursts of nonthermal emissions in hard X-rays and microwave. These
results mean that the downflows occurred when strong magnetic energy
was released, and that they are, or at least correlated with, the
reconnection outflows. We also propose an observation of downflows as
the reconnection outflows by SolarB.
Title: Nonlinear Alfvén Wave Model for Solar Coronal Heating and
Nanoflares
Authors: Moriyasu, S.; Shibata, K.
Bibcode: 2004ESASP.575...80M
Altcode: 2004soho...15...80M
No abstract at ADS
Title: The Red-Asymmetry Distribution at Hα Flare Kernels Observed
in the 2001 April 10 Solar Flare
Authors: Asai, A.; Ichimoto, K.; Shibata, K.; Kitai, R.; Kurokawa, H.
Bibcode: 2004AGUFMSH13A1134A
Altcode:
We report a detailed examination about the evolution of the Hα flare
kernels during an X2.3 solar flare which occurred on 2001 April 10. The
Hα red-asymmetry, that is, the red-shifted Hα emission, is observed
at almost all Hα flare kernels, during the impulsive phase of the
flare. At Hα kernels nonthermal particles and/or thermal conduction
precipitate into the chromospheric plasma, and this is thought to lead
the downward compression of the chromospheric plasma, which is observed
as the reddening of Hα emission (e.g. Ichimoto & Kurokawa 1984). We
examined the evolution of the flare kernels inside the flare ribbons
by using the Hα images obtained with the Domeless Solar Telescope
at Hida Observatory, Kyoto University. We also examined the spatial
distribution of the Hα kernels which show the red-asymmetry and their
relationship with the intensity of the Hα kernels. We found that the
stronger the red-asymmetry is, the brighter the Hα kernel is. Then, we
compared the strengthes of the Hα red-asymmetry at hard X-ray emitting
sources with those at the Hα kernels without the hard X-ray emissions.
Title: Three-Dimensional MHD Simulation of Convection and Emerging
Flux
Authors: Isobe, H.; Shibata, K.
Bibcode: 2004ASPC..325...63I
Altcode:
We present results of three dimensional magnetohydrodynamic simulation
of convection and emerging flux. Although simulations of emerging flux
tube have been presented in the literature, effect of convection on the
dynamics of emerging flux has not been studied in detail. We carried out
simulation of emergence of twisted flux tube from vigorously convecting
convection zone. It is found that the flux tube emerges to the upper
atmosphere and forms loop like structures, but finally it is broken
by the convecting flows and loses its coherence in several turnover
time of convection.
Title: Jet Phenomena in the Solar Atmosphere caused by Interaction
between Emerging Flux and Coronal Fields
Authors: Miyagoshi, T.; Yokoyama, T.; Isobe, H.; Shibata, K.
Bibcode: 2004ASPC..325...69M
Altcode:
We have studied jet phenomena in the solar atmosphere with MHD numerical
simulations. Emerging flux could interact coronal magnetic fields
through magnetic reconnection. Then, magnetic energy is released and
magnetic topology drastically changes. Our numerical simulation results
show that coronal jets are produced through this process.
Title: General Relativistic Magnetohydrodynamic Simulations of
Collapsars: Rotating Black Hole Cases
Authors: Mizuno, Yosuke; Yamada, Shoichi; Koide, Shinji; Shibata,
Kazunari
Bibcode: 2004ApJ...615..389M
Altcode: 2003astro.ph.10017M
We have performed 2.5-dimensional general relativistic
magnetohydrodynamic (MHD) simulations of collapsars including a
rotating black hole. This paper is an extension of our previous
paper. The current calculation focuses on the effect of black hole
rotation using general relativistic MHD with simplified microphysics;
i.e., we ignore neutrino cooling, physical equation of state, and
photodisintegration. Initially, we assume that the core collapse
has failed in this star. A rotating black hole of a few solar masses
is inserted by hand into the calculation. We consider two cases, a
corotating case and a counterrotating case with respect to the black
hole rotation. Although the counterrotating case may be unrealistic for
collapsars, we perform it as the maximally dragging case of a magnetic
field. The simulation results show the formation of a disklike structure
and the generation of a jetlike outflow near the central black hole. The
jetlike outflow propagates outwardly with the twisted magnetic field
and becomes collimated. We have found that the jets are generated and
accelerated mainly by the magnetic field. The total jet velocity in
the rotating black hole case is comparable to that of the nonrotating
black hole case, ~0.3c. When the rotation of the black hole is faster,
the magnetic field is twisted strongly owing to the frame-dragging
effect. The magnetic energy stored by the twisting magnetic field is
directly converted to kinetic energy of the jet rather than propagating
as an Alfvén wave. Thus, as the rotation of the black hole becomes
faster, the poloidal velocity of the jet becomes faster. In the rapidly
rotating black hole case the jetlike outflow can be produced by the
frame-dragging effect only through twisting of the magnetic field,
even if there is no stellar rotation.
Title: A Study of a Tiny Two-Ribbon Flare Driven by Emerging Flux
Authors: Sakajiri, Takuma; Brooks, David H.; Yamamoto, Tetsuya;
Shiota, Daikou; Isobe, Hiroaki; Akiyama, Sachiko; Ueno, Satoru; Kitai,
Reizaburo; Shibata, Kazunari
Bibcode: 2004ApJ...616..578S
Altcode:
We present observations of the eruption of a miniature filament that
occurred near NOAA Active Region 9537 on 2001 July 14. The eruption was
observed by the Hida Observatory Domeless Solar Telescope, in the Hα
line center and +/-0.4 Å wings, the Solar and Heliospheric Observatory
EUV Imaging Telescope (EIT) and Michelson Doppler Imager, and the Yohkoh
Soft X-Ray Telescope (SXT). The miniature filament began to form and
was clearly visible in Hα images by around 06:50 UT. It erupted about
25 minutes later, accompanied by a small two-ribbon subflare (with
an area of 61 arcsec2). The two ribbons were also found to
approach each other at a speed of 3.33 km s-1. We found that
this event was caused by the emergence of new magnetic flux in a quiet
region. The emerging flux appeared as a bright region in the EIT and
SXT images taken on the previous day. It moved southward into an area
of preexisting opposite-polarity flux, where a cancelling magnetic
flux region was formed. The miniature filament then appeared, and we
suggest that it played some role in inhibiting the release of energy
by delaying reconnection between the emerging and preexisting flux, as
evidenced by the disappearance of the bright region between opposite
polarities in the EUV and soft X-ray images. Consequently, magnetic
energy was stored as a result of the slow converging motion of the two
opposite-polarity flux regions (0.17 km s-1). Reconnection
below the filament provoked the filament eruption, and the two-ribbon
flare occurred. Miniature filaments are thought to be small-scale
analogs of large-scale filaments. Our observations also suggest some
common properties between small-scale and large-scale flares. These
results support the view that a unified magnetic reconnection model
may be able to explain all scales of flares.
Title: A Quantitative Study of the Homologous Flares on 2000
November 24
Authors: Takasaki, Hiroyuki; Asai, Ayumi; Kiyohara, Junko; Shimojo,
Masumi; Terasawa, Toshio; Takei, Yasuhiro; Shibata, Kazunari
Bibcode: 2004ApJ...613..592T
Altcode:
We present an examination of multiwavelength observations of three
X-class homologous flares that occurred on 2000 November 24. By
investigating the behavior of the two-ribbon flares in Hα and the
ultraviolet (1600 Å), we found that the temporal variation of the
distance between the two ribbons shows a good correlation with the
soft X-ray light curve. From this finding we can derive the relation
dISXR(t)/dt~Vrib(t), where ISXR(t)
is the soft X-ray intensity and Vrib(t) is the separation
velocity of the two ribbons. This relation is similar to the well-known
empirical law, the Neupert effect. We also measured the rise time,
velocity of the plasmoid/filament ejection, and separation velocity of
the two ribbons for each of these homologous flares. Since the magnetic
reconnection model predicts that each of these physical parameters has
a dependence on the coronal magnetic field strength Bc,
we derived the relative Bc between the three flares from
each of the parameters. We compared the relative Bc values,
which are derived from those parameters, and found that they are roughly
equal. Our results successfully support the magnetic reconnection
model. Moreover, the relative hard X-ray maximum intensity in each
flare is consistent with the relative Bc derived above,
if we assume that the hard X-ray intensity is proportional to the
energy release rate as implied by the Neupert effect.
Title: General Relativistic MHD Simulations of the Gravitational
Collapse of a Rotating Star with Magnetic Field as a Model of
Gamma-Ray Bursts
Authors: Mizuno, Y.; Yamada, S.; Koide, S.; Shibata, K.
Bibcode: 2004AIPC..727..392M
Altcode: 2003astro.ph.12060M
We have performed 2.5-dimensional general relativistic
magnetohydrodynamic (MHD) simulations of the gravitational collapse
of a magnetized rotating massive star as a model of gamma ray bursts
(GRBs). This simulation showed the formation of a disk-like structure
and the generation of a jet-like outflow inside the shock wave launched
at the core bounce. We have found the jet is accelerated by the magnetic
pressure and the centrifugal force and is collimated by the pinching
force of the toroidal magnetic field amplified by the rotation and
the effect of geometry of the poloidal magnetic field. The maximum
velocity of the jet is mildly relativistic (~ 0.3 c).
Title: Computational Relativistic Fluids and Jet Formation
Authors: Richardson, G.; Nishikawa, K. -I.; Koide, S.; Shibata, K.
Bibcode: 2004AIPC..727..286R
Altcode:
We present our methodology for numerically solving the fluid equations
in general relativistic environments with magnetic fields. Our
motivation for the development of such a method is to study the
environment around a rotating black hole, specifically the dynamics
of the accretion disk and the associated formation of relativistic
jets. We present our three-dimensional results confirming previous
two-dimensional simulations, which demonstrate the initial stages of
jet formation. Without injecting matter, the generated jet evolves
into a wind. Consequently, a state change of the jet and accretion
disk system is observed.
Title: H I Jet G40-15 from a Rotating Cloud in the 4-kpc Molecular
Ring: Magnetized Outflow and Formation of a Dense Star Cluster
Authors: Sofue, Yoshiaki; Kudoh, Takahiro; Kawamura, Akiko; Shibata,
Kazunari; Fujimoto, Mitsuaki
Bibcode: 2004PASJ...56..633S
Altcode:
A giant H I spur centered on (l, b)=(40°, -15°), (G40 - 15) emanates
from G30 - 00 toward negative high latitude. The spur is known as
the Smith cloud and as a high-velocity cloud HVC 40 - 15 + 100,
while we call it H I jet G40 - 15. The radial velocity ranges from
80 to 120 km s-1 with the center velocity at ∼ 100 km
s-1. Both the kinematical and positional coordinates are
coincident with those of the tangential point of the 4-kpc molecular
ring of the Galaxy. We interpret this spur as being a magnetized H I
jet emanating from the 4-kpc molecular ring of the galactic disk. The
kinematical distance is estimated to be 6.9kpc from the Sun, and the
jet's top (G50 - 26) is -3.4 kpc high from the galactic plane. The
whole extent is 4.4kpc in length and 300pc in width. The total H I
mass is ∼ 3 × 105 Msolar, and the energy to
lift the mass from the galactic plane is ∼ 1052 erg. We
propose an acceleration mechanism of such an extremely high-altitude
coherent H I structure by a twisting magnetic jet associated with a
collapsing molecular cloud in the galactic disk. We suggest a scenario
for the formation of a dense star cluster, such as a globular cluster,
from a collapsing cloud core with a twisting magnetic outflow.
Title: Numerical Examination of the Stability of an Exact
Two-dimensional Solution For Flux Pile-up Magnetic Reconnection
Authors: Hirose, Shigenobu; Litvinenko, Yuri E.; Tanuma, Syuniti;
Shibata, Kazunari; Takahashi, Masaaki; Tanigawa, Takayuki; Sasaqui,
Takahiro; Noro, Ayato; Uehara, Kazuhiro; Takahashi, Kunio; Taniguchi,
Takashi; Terekhova, Yuliya A.
Bibcode: 2004ApJ...610.1107H
Altcode: 2003astro.ph..9033H
The Kelvin-Helmholtz (KH) and tearing instabilities are likely to
be important for the process of fast magnetic reconnection that is
believed to explain the observed explosive energy release in solar
flares. Theoretical studies of the instabilities, however, typically
invoke simplified initial magnetic and velocity fields that are not
solutions of the governing magnetohydrodynamic (MHD) equations. In
the present study, the stability of a reconnecting current sheet
is examined using a class of exact global MHD solutions for steady
state incompressible magnetic reconnection, discovered by Craig &
Henton. Numerical simulation indicates that the outflow solutions where
the current sheet is formed by strong shearing flows are subject to
the KH instability. The inflow solutions where the current sheet is
formed by a fast and weakly sheared inflow are shown to be tearing
unstable. Although the observed instability of the solutions can be
interpreted qualitatively by applying standard linear results for
the KH and tearing instabilities, the magnetic field and plasma flow,
specified by the Craig-Henton solution, lead to the stabilization of
the current sheet in some cases. The sensitivity of the instability
growth rate to the global geometry of magnetic reconnection may help
in solving the trigger problem in solar flare research.
Title: Quasi-periodic Inward Shock Formations in the System of a
Black Hole and an Accretion Disk and Application to Quasi-periodic
Oscillations in Galactic Black Hole Candidates
Authors: Aoki, Seiichiro I.; Koide, Shinji; Kudoh, Takahiro; Nakayama,
Kunji; Shibata, Kazunari
Bibcode: 2004ApJ...610..897A
Altcode:
We performed 1.5-dimensional general relativistic hydrodynamic
simulations with a Kerr metric to construct a model for high-frequency
quasi-periodic oscillations (QPOs) in microquasars. The simulations were
performed assuming an initial accretion disk without viscosity rotating
around a Kerr black hole at sub-Keplerian velocity (sub-Keplerian case),
which induces various wave modes everywhere in the disk. We found
that quasi-periodic inward shock waves propagate from the accretion
disk toward the black hole. The frequency of the shock formation is
about the maximum epicyclic frequency in the disk (κmax),
which depends on the rotation of the black hole. In order to understand
the mechanism of the shock formation, we also performed a simulation
assuming an initial linear perturbation injected at one point in the
Keplerian disk (linear perturbation case) and found an oscillation with
frequency ~κ at the point where the perturbation injection occurred. To
explain the simulation result, we derived an analytic solution for the
time evolution of the linear perturbation of physical variables near the
point of the perturbation injection and found that the time evolution
of the oscillation can be described well. From comparison of the result
in the sub-Keplerian case with that of the linear perturbation case,
we found that the periodicity of the quasi-periodic shock formation in
the sub-Keplerian case is due to a filtering effect by the epicyclic
frequency distribution in the disk, which acts on the wave propagation
toward the black hole. The only necessary condition for quasi-periodic
shock formation is having a nonsteady character for the disks,
which can be a source of acoustic waves. The frequency of the shock
formation (~κmax) is on the order of the frequency of the
high-frequency QPOs in microquasars and depends on the rotation of the
black hole. Hence, we can estimate the spin parameter (a) of a black
hole candidate (BHC) in a microquasar by comparing the frequency of
the high-frequency QPO with κmax. The spin parameters of
the BHCs in microquasars are roughly estimated to be a=0.345+/-0.345
for GRS 1915+105 and a=0.895+/-0.105 for GRO J1655-40.
Title: Flare Ribbon Expansion and Energy Release Rate
Authors: Asai, Ayumi; Yokoyama, Takaaki; Shimojo, Masumi; Masuda,
Satoshi; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2004ApJ...611..557A
Altcode:
We have examined the relation between the evolution of the Hα
flare ribbons and the released magnetic energy in a solar flare that
occurred on 2001 April 10. Based on the magnetic reconnection model, the
released energy was quantitatively calculated by using the photospheric
magnetic field strengths and separation speeds of the fronts of the
Hα flare ribbons. We compared the variation of the released energy
with the temporal and spatial fluctuations in the nonthermal radiation
observed in hard X-rays and microwaves. These nonthermal radiation
sources indicate when and where large energy releases occur. We also
estimated the magnetic energy released during the flare. The estimated
energy release rates in the Hα kernels associated with the hard X-ray
sources are locally large enough to explain the difference between the
spatial distributions of the Hα kernels and the hard X-ray sources. We
also reconstructed the peaks in the nonthermal emission by using the
estimated energy release rates.
Title: Filament Oscillations and Moreton Waves Associated with
EIT Waves
Authors: Okamoto, Takenori J.; Nakai, Hidekazu; Keiyama, Atsushi;
Narukage, Noriyuki; UeNo, Satoru; Kitai, Reizaburo; Kurokawa, Hiroki;
Shibata, Kazunari
Bibcode: 2004ApJ...608.1124O
Altcode:
In this paper we compare EUV Imaging Telescope (EIT) waves with
simultaneous phenomena seen in Hα in order to address the question
of what an EIT wave is. We surveyed the events associated with solar
flares larger than GOES M-class in 1999-2002. The Hα data are taken
with the Flare-monitoring Telescope (FMT) at the Hida Observatory
of Kyoto University. Among 14 simultaneous observations of EIT
waves and Hα, 11 were found to have filament eruptions, three were
associated with Moreton waves, and one was found to have only filament
oscillations. This shows that we cannot see clear wave fronts in
Hα even if EIT waves exist, but that it is possible to recognize
invisible waves by means of filament oscillations. The nature of
filament oscillations and Moreton waves associated with EIT waves is
examined in detail, and it is found that the filament oscillations
were caused by EIT waves.
Title: General Relativistic Magnetohydrodynamic Simulations of
Collapsars
Authors: Mizuno, Yosuke; Yamada, Shoichi; Koide, Shinji; Shibata,
Kazunari
Bibcode: 2004ApJ...606..395M
Altcode: 2004astro.ph..4152M
We have performed 2.5-dimensional general relativistic
magnetohydrodynamic (MHD) simulations of the gravitational collapse
of a magnetized rotating massive star as a model of gamma-ray bursts
(GRBs). The current calculation focuses on general relativistic MHD with
simplified microphysics (we ignore neutrino cooling, physical equation
of state, and photodisintegration). Initially, we assume that the core
collapse has failed in this star. A few Msolar black hole
is inserted by hand into the calculation. The simulations presented in
the paper follow the accretion of gas into a black hole that is assumed
to have formed before the calculation begins. The simulation results
show the formation of a disklike structure and the generation of a
jetlike outflow inside the shock wave launched at the core bounce. We
have found that the jet is accelerated by the magnetic pressure and
the centrifugal force and is collimated by the pinching force of the
toroidal magnetic field amplified by the rotation and the effect of
geometry of the poloidal magnetic field. The maximum velocity of the jet
is mildly relativistic (~0.3c). The velocity of the jet becomes larger
as the initial rotational velocity of stellar matter gets faster. On
the other hand, the dependence on the initial magnetic field strength
is a bit more complicated: the velocity of the jet increases with the
initial field strength in the weak field regime, then is saturated at
some intermediate field strength, and decreases beyond the critical
field strength. These results are related to the stored magnetic energy
determined by the balance between the propagation time of the Alfvén
wave and the rotation time of the disk (or twisting time).
Title: X-Ray Expanding Features Associated with a Moreton Wave
Authors: Noriyuki, Narukage; Morimoto, Taro; Kadota, Miwako; Kitai,
Reizaburo; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2004PASJ...56L...5N
Altcode:
No abstract at ADS
Title: Magnetohydrodynamic Accretion Flows: Formation of Magnetic
Tower Jet and Subsequent Quasi-Steady State
Authors: Kato, Y.; Mineshige, S.; Shibata, K.
Bibcode: 2004ApJ...605..307K
Altcode: 2003astro.ph..7306K
We present three-dimensional magnetohydrodynamic (MHD) simulations of
radiatively inefficient accretion flow around black holes. General
relativistic effects are simulated by using the pseudo-Newtonian
potential. We start calculations with a rotating torus threaded by
localized poloidal magnetic fields with plasma-β, a ratio of the gas
pressure to the magnetic pressure, β=10 and 100. When the bulk of
torus material reaches the innermost region close to a central black
hole, a magnetically driven jet emerges. This magnetic jet is derived
by vertically inflating toroidal fields (``magnetic tower'') and has a
two-component structure: low-β (<~1) plasmas threaded with poloidal
(vertical) fields are surrounded by those with toroidal fields. The
collimation width of the jet depends on external pressure, the pressure
of ambient medium; the weaker the external pressure is, the wider
and the longer lasting becomes the jet. Unless the external pressure
is negligible, the bipolar jet phase ceases after several dynamical
timescales at the original torus position and a subsequent quasi-steady
state starts. The black hole is surrounded by a quasi-spherical
zone with highly inhomogeneous structure in which toroidal fields
are dominant except near the rotation axis. Mass accretion takes
place mainly along the equatorial plane. Comparisons with other MHD
simulation results and observational implications are discussed.
Title: Downflow Motions Associated with Impulsive Nonthermal Emissions
Observed in the 2002 July 23 Solar Flare
Authors: Asai, Ayumi; Yokoyama, Takaaki; Shimojo, Masumi; Shibata,
Kazunari
Bibcode: 2004ApJ...605L..77A
Altcode:
We present a detailed examination of downflow motions above flare
loops observed in the 2002 July 23 flare. The extreme-ultraviolet
images obtained with the Transition Region and Coronal Explorer show
dark downflow motions (sunward motions) above the postflare loops, not
only in the decay phase but also in the impulsive and main phases. We
also found that the times when the downflow motions start to be
seen correspond to the times when bursts of nonthermal emissions in
hard X-rays and microwaves are emitted. This result implies that the
downflow motions occurred when strong magnetic energy was released
and that they are, or are correlated with, reconnection outflows.
Title: A Reexamination of the Evidence for Reconnection Inflow
Authors: Chen, P. F.; Shibata, K.; Brooks, D. H.; Isobe, H.
Bibcode: 2004ApJ...602L..61C
Altcode:
In the flare event of 1999 March 18, a threadlike structure observed
in EUV Imaging Telescope images was found to move inward and collapse
to an X-shaped configuration below the ejecta, strongly suggestive
of the occurrence of magnetic reconnection. On the basis of the
numerical results of a coronal mass ejection (CME) flare model, a
similar threadlike structure in the Fe XII 195 Å image is reproduced
in this Letter. It is found that, as in the observations, the thread
experiences an outward motion in the preflare phase, which is followed
by an inward motion. Our simulation suggests that its formation and
outward motion in the preflare phase result from the CME expansion;
after the onset of the flare, the threadlike structure is always
located on the upstream side of the interface between the reconnection
inflow and outflow. Its apparent inward motion, which is several times
slower than the in situ reconnection inflow, is mainly attributed to
the rising motion of the reconnection X-point.
Title: Quasi-Periodic Inward Shock Formation from an Accretion Disk
to a Black Hole and Its Application to Quasi-Periodic Oscillations
in Microquasars
Authors: Aoki, S. I.; Koide, S.; Kudoh, T.; Nakayama, K.; Shibata, K.
Bibcode: 2004PThPS.155..307A
Altcode:
We performed 1.5D general relativistic simulations using a
Kerr metricwithout disk viscosity for a model of high frequency
quasi-periodic oscillations (QPOs) in microquasars. We found that
quasi-periodic inward shock waves propagate from the accretion disk
toward the black hole when we initially inject a disturbance into the
disk. The frequency of the shock formation is approximately equal
to the maximum epicyclic frequency in the disk (κmax),
which is on the order of the frequency of high frequency QPOs in
microquasars. κmax depends on the rotation of the black
hole, and therefore we can estimate the value of the spin parameter
of a black hole candidate (BHC) in a microquasar by comparing the high
frequency QPO with κmax. We found that such values can be
roughly estimated as 0.0 ≤ a < 0.4 for GRS 1915+105 and 0.85 <
a < 1.0 for GRO J1655-40.
Title: Why are there stationary EIT wave fronts?
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2004cosp...35..276C
Altcode: 2004cosp.meet..276C
EIT waves are nearly circularly propagating emission enhancements
followed by expanding dimming regions, which have been found to be
closely related with coronal mass ejections. They are generally believed
to correspond to some kind of wave phenomenon. We have explained the
EIT waves as the propagating structures associated with the gradual
opening of coronal mass ejections. However, occasionally a stationary
front can be observed for several hours, which prompted the doubt
about whether the so-called "EIT waves" are pseudo-phenomena. Through
numerical simulations, this paper illustrates how a propagating EIT
wave stops to form the stationary front when it meets another active
region or a coronal hole.
Title: 2D MHD Simulations of Internal Shocks and Turbulence in the
Reconnection Jet
Authors: Tanuma, S.; Shibata, K.
Bibcode: 2004IAUS..223..487T
Altcode: 2005IAUS..223..487T
To explain the origin of hard X-ray emission, we suggest the internal
shocks could be created in the reconnection jet and accelerate the
energetic particles in the solar flares. We examine its possibility by
performing 2D resistive MHD simulations. We use very small very small
grid to resolve the diffusion region and remove the effect of numerical
noise. As the results, the current sheet becomes thin by tearing
instability, and collapses to Sweet-Parker sheet. It becomes unstable
again so that the secondary teaing instability occurs. Immediately after
the plasmoid ejection, anomalous resistivity sets in and Petschek-like
reconnection starts. During the Petschek reconnection, many plasmoids
are created by the secondary tearing instability and ejected along the
current sheet. The multiple fast shocks are created in the current
sheet. We suggest that the internal shocks shown in this paper are
possible sites for the particle acceleration in the solar flares.
Title: Internal Shocks in the Magnetic Reconnection Jet in Solar
Flares
Authors: Tanuma, S.; Shibata, K.
Bibcode: 2004IAUS..219..771T
Altcode: 2003IAUS..219E..81T
The satellites such as Yohkoh and RHESSI observe the X- and gamma-ray
emissions from the high energy particles in solar flare: for example at
looptop and foot points of impulsive flare. In this paper we suggest
that the multiple fast shock are created in the reconnection jet and
they create the high energy particles by performing two-dimensional
numerical resistive magnetohydrodynamic simulations. As the results
we find that the current sheet becomes thin by tearing instability
and it collapses to Sweet-Parker current sheet. The thin current sheet
becomes unstable to the tearing instability again. The fast reconnection
starts immediately after the plasmoid-ejection which are created by
the secondary tearing instability. The internal shocks are created
inside the reconnection jet due to the nonsteady plasmoid-ejection
created by the secondary tearing instability. In the next phase the
reconnection jet starts to oscillate which is due to Kelvin-Helmholtz
instability or turbulence reconnection. The reconnection jet collides
with two standing slow shocks so that the fast shocks are created as
the oblique shocks. The fast shocks created by the magnetic reconnection
is possible sites for the particle acceleration.
Title: The Nonlinear Alfvén Wave Model for Solar Coronal Heating
and Nanoflares
Authors: Moriyasu, Satoshi; Kudoh, Takahiro; Yokoyama, Takaaki;
Shibata, Kazunari
Bibcode: 2004ApJ...601L.107M
Altcode:
The mechanism of solar coronal heating has been unknown since the
discovery that the coronal plasma temperature is a few million
degrees. There are two promising mechanisms, the Alfvén wave model
and the nanoflare-reconnection model. Recent observations favor the
nanoflare model since it readily explains the ubi-quitous small-scale
brightenings all over the Sun. We have performed magnetohydrodynamic
(MHD) simulations of the nonlinear Alfvén wave coronal heating model
that include both heat conduction and radiative cooling in an emerging
flux loop and found that the corona is episodically heated by fast- and
slow-mode MHD shocks generated by nonlinear Alfvén waves via nonlinear
mode-coupling. We also found that the time variation of the simulated
extreme-ultraviolet and X-ray intensities of these loops, on the basis
of the Alfvén wave model, is quite similar to the observed one, which
is usually attributed to nanoflare or picoflare heating. This suggests
that the observed nanoflares may not be a result of reconnection but
in fact may be due to nonlinear Alfvén waves, contrary to current
widespread opinion.
Title: Moreton waves observed at Hida Observatory
Authors: Narukage, Noriyuki; Eto, Shigeru; Kadota, Miwako; Kitai,
Reizaburo; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2004IAUS..223..367N
Altcode: 2005IAUS..223..367N
Moreton waves are flare-associated waves observed to propagate across
the solar disk in Halpha, especially in the wing of Halpha. The Flare
Monitoring Telescope at Hida Observatory of Kyoto University observed
12 events associated with flare waves (i.e., Moreton waves and/or
filament oscillations) in Halpha from 1997 to 2002. We review our
studies of Moreton waves based on these observations; relation between
EIT wave and Moreton wave (Eto et al. 2002),simultaneous observation
with X-ray wave (Narukage et al. 2002),three dimensional structure of
flare-associated wave (Narukage et al. 2004),relation between Moreton
waves and filament eruptions.
Title: The Log-normal Distributions of Physical Quantities of Flare
associated Coronal Mass Ejections (CMEs), and Flare/CME Model of
Gamma-ray Bursts
Authors: Aoki, Seiichiro I.; Yashiro, Seiji; Shibata, Kazunari
Bibcode: 2004astro.ph..1352A
Altcode:
We investigated the statistical distributions of physical quantities
of solar flares and associated coronal mass ejections (CMEs). We
found that the distributions of the X-ray peak fluxes of CME-related
flares, their time intervals, and speeds of associated CMEs are in good
agreement with log-normal distributions. One possible interpretation
of this is that only large energetic mass ejections can escape from
the solar corona, which become CMEs in the interplanetary space. This
``filtering effect'' may be the reason why statistical distributions of
some physical quantities are similar to log-normal distribution. It is
known that the distributions of several physical quantities of gamma-ray
bursts (GRBs) are also log-normal distributions, and that the time
variations of gamma-ray intensity in GRBs are very similar to those
of hard X-rays of solar flares. On the basis of these similarities
and physical consideration of magnetohydrodynamic properties of an
accretion disk around a black hole, which is supposed to be situated
in the central engine of a GRB, we propose a new model of the central
engine of GRBs, the {\it flare/CME model}, in which GRBs are formed
by nonsteady jets consisting of intermittent ejections of mass (like
solar CMEs) associated with reconnection events (like solar flares)
in the accretion disk corona. Such nonsteady MHD jets would produce
many shocks as well as local reconnection events far from the central
engine. In this model, only large energetic mass ejections can escape
from the accretion disk corona, so that statistical distributions of
some physical quantities are similar to log-normal distributions.
Title: General Relativistic MHD Simulations of the Gravitational
Collapse of a Rotating Star with Magnetic Field as a Model of
Gamma-Ray Bursts
Authors: Mizuno, Y.; Shibata, K.; Yamada, S.; Koide, S.
Bibcode: 2004PThPS.155..387M
Altcode:
We have performed 2.5-dimensional general relativistic
magnetohydrodynamic (MHD) simulations of collapsars as a model of
gamma ray bursts (GRBs). This simulation showed the formation of a
disk-like structure and the generation of a mildly relativistic jet
(∼ 0.3 c). We have found the jet is accelerated by the magnetic
pressure and the centrifugal force and is collimated by the pinching
force of the toroidal magnetic field amplified by the rotation.
Title: Long Term Simulations of Astrophysical Jets
Authors: Ibrahim, A.; Sato, K.; Shibata, K.
Bibcode: 2004PThPS.155..343I
Altcode:
We have performed self-consistent 2.5-dimensional nonsteady MHD
numerical simulations of jet formation, including the dynamics of
accretion disks. Although the previous nonsteady MHD simulations for
astrophysical jets revealed that the characteristics of nonsteady
jets are very similar to those of steady jets, the calculation time
of these simulations is very short compared with the time scale of
observed jets. Thus we have investigated long term evolutions of
the mass accretion rate (dot{M}a), mass outflow rate
(dot{M}w), and energy of the toroidal magnetic field
(Emgt). We found that the jet is ejected intermittently
with a period of around 2π which is similar to the growth time of
the magnetorotational instability (MRI).
Title: TRACE Downflows and Energy Release
Authors: Asai, A.; Yokoyama, T.; Shimojo, M.; Tandokoro, R.; Fujimoto,
M.; Shibata, K.
Bibcode: 2004ESASP.547..163A
Altcode: 2004soho...13..163A
We have examined in detail the evolution of a big two-ribbon flare which
occurred on 2002 July 23. The extreme ultraviolet images obtained with
TRACE show dark downflow motions (sunward motions) above the post-flare
loop, not only in the decay phase but also in the impulsive and main
phase. We found that the times when the downflow motions are seen
correspond to those of the bursts of nonthermal emissions in hard X-ray
and microwave. This result means that the downflow motions occurred when
strong magnetic energy was released, and that they are, or correlated
with, the reconnection outflows. We also found the ascending motions
of super hot plasma region seen in TRACE and RHESSI associating with
the light curves in hard X-rays and microwaves. This result supports
the Neupert effect.
Title: Statistical analysis of reconnection inflows in solar flares
Authors: Narukage, N.; Shibata, K.
Bibcode: 2004cosp...35.3696N
Altcode: 2004cosp.meet.3696N
Solar flare is an explosion on the solar surface and releases huge
energy in a short time. It is widely believed that the energy
is stored in magnetic field and the flare is caused by magnetic
reconnection. Yohkoh, a Japanese solar X-ray satellite launched in 1991,
discovered various evidence of the magnetic reconnection, e.g. cusp
shaped loops, plasmoid ejections, and etc. The reconnection model
has been established at least phenomenologically by Yohkoh. However,
the inflow, which is predicted by the reconnection theory, has not been
discovered, and it remains mystery. In 2000, Yokoyama et al. discovered
such reconnection inflow in a solar flare observed with SOHO/EIT,
but no other observation of reconnection inflow has been reported
until now. The generality of inflow remains doubtful. We surveyed
the reconnection inflow in the full Sun movies taken with SOHO/EIT
and discovered 13 inflows. We measured the line-of-sight velocity of
the inflows using SOHO/EIT images, the angle between magnetic neutral
line and line-of-sight direction in the inflows using SOHO/MDI images,
and the energy release rate of the flares associated with the inflows
using Yohkoh/SXT images. Based on this results, we estimated some
physical quantities of the inflows, e.g. the real inflow velocity and
reconnection rate. This result is conclusive evidence of reconnection
and the clue to the solution of flare mechanism.
Title: Flare Ribbon Expansion and Energy Release Rate
Authors: Asai, Ayumi; Yokoyama, Takaaki; Shimojo, Masumi; Masuda,
Satoshi; Shibata, Kazunari
Bibcode: 2004IAUS..223..443A
Altcode: 2005IAUS..223..443A
We report a detailed examination about the relationship between
the evolution of the Halpha flare ribbons and the released magnetic
energy during an X2.3 solar flare which occurred on 2001 April 10. We
successfully evaluated the released energy quantitatively, based on the
magnetic reconnection model. We measured the photospheric magnetic field
strengths and the separation speeds of the fronts of the Halpha flare
ribbon, and estimated the released magnetic energy at the flare by using
those values. Then, we compared the estimated energy release rates with
the nonthermal behaviors observed in hard X-rays and microwaves. We
also estimated the magnetic energy released during the flare. The
estimated energy release rates in the Halpha kernels associated
with the hard X-ray sources are locally large enough to explain the
difference between the spatial distribution of the Halpha kernels and
the hard X-ray sources. Furthermore, we reconstructed the peaks in
the nonthermal emission by using the estimated energy release rates.
Title: Magnetohydrodynamical Accretion Flows: Formation of Magnetic
Tower Jet and Subsequent Quasi-Steady State
Authors: Kato, Y.; Mineshige, S.; Shibata, K.
Bibcode: 2004PThPS.155..353K
Altcode:
We present three-dimensional (3-D) magnetohydrodynamical (MHD)
simulations of both radiatively inefficient accretion flow (RIAF)
into and jet from black holes (BHs). When the bulk of torus material
reaches the innermost region close to a central BH, a magnetically
driven jet emerges. This magnetic jet is derived by vertically inflating
toroidal fields (`magnetic tower') and has a two-component structure:
low-β (< 1) plasmas threaded with poloidal fields are surrounded by
that with toroidal fields. The collimation width of the jet depends on
external pressure, pressure of ambient medium; the weaker the external
pressure is, the wider and the longer-lasting becomes the jet. Unless
the external pressure is negligible, the bipolar jet phase ceases
after several dynamical timescales at the original torus position and
a subsequent quasi-steady state starts.
Title: Tropical Cumulus Convection and Upward-Propagating Waves in
Middle-Atmospheric GCMs.
Authors: Horinouchi, T.; Pawson, S.; Shibata, K.; Langematz, U.;
Manzini, E.; Giorgetta, M. A.; Sassi, F.; Wilson, R. J.; Hamilton,
K.; de Grandpré, J.; Scaife, A. A.
Bibcode: 2003JAtS...60.2765H
Altcode:
It is recognized that the resolved tropical wave spectrum can vary
considerably among general circulation models (GCMs) and that these
differences can have an important impact on the simulated climate. A
comprehensive comparison of low-latitude waves is presented for
the December January February period using high-frequency data
from nine GCMs participating in the GCM Reality Intercomparison
Project for Stratospheric Processes and Their Role in Climate (GRIPS;
SPARC). Quantitative measures of the wavenumber-frequency structure
of resolved waves and their impacts on the zonal mean circulation are
given. Space time spectral analysis reveals that the wave spectrum
throughout the middle atmosphere is linked to the variability of
convective precipitation, which is determined by the parameterized
convection. The variability of the precipitation spectrum differs
by more than an order of magnitude among the models, with additional
changes in the spectral distribution (especially the frequency). These
differences can be explained primarily by the choice of different
cumulus parameterizations: quasi-equilibrium mass-flux schemes tend
to produce small variability, while the moist-convective adjustment
scheme is the most active. Comparison with observational estimates
of precipitation variability suggests that the model values are
scattered around the observational estimates. Among the models, only
those that produce the largest precipitation variability can reproduce
the equatorial quasi-biennial oscillation (QBO). This implies that in
the real atmosphere, the forcing from the waves, which are resolvable
with the typical resolutions of present-day GCMs, is insufficient
to drive the QBO. Parameterized cumulus convection also impacts the
nonmigrating tides in the equatorial region. In most of the models,
momentum transport by diurnal nonmigrating tides in the mesosphere is
comparable to or larger than that by planetary-scale Kelvin waves,
being more significant than has been thought. It is shown that the
westerly accelerations in the equatorial semi-annual oscillation in
the models examined are driven mainly by gravity waves with periods
shorter than 3 days, with some contribution from parameterized gravity
waves, and that the contribution from the wavenumber-1 Kelvin waves
is negligible. These results provide a state-of-the-art assessment of
the links between convective parameterizations and middle-atmospheric
waves in present-day middle-atmosphere climate models.
Title: MHD simulations of jets from accretion disks
Authors: Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 2003Ap&SS.287...99K
Altcode:
We present the MHD simulation including accretion flows in disks,
acceleration of outflows from disks, and collimation of the outflows
self-consistently. Although it was considered that this kind of
simulations only shows the transient phenomena of jets, we found
that the outflow and accretion flow reached a quasi-steady state by
performing a long-term calculation in a large calculation region. Though
the final stage is not exactly the steady state, the acceleration and
collimation mechanisms of the outflow were the same as those of the
steady theory. The scale of the calculation is approaching to the
scale that was observed by the VLBI technique, which provides the
current highest resolution for YSO jets.
Title: Hydrodynamic Modeling of a Flare Loop Connecting the Accretion
Disk and Central Core of Young Stellar Objects
Authors: Isobe, Hiroaki; Shibata, Kazunari; Yokoyama, Takaaki;
Imanishi, Kensuke
Bibcode: 2003PASJ...55..967I
Altcode:
Many young stellar objects, such as protostars and T-Tauri stars,
show strong flare activity. In this paper we present a hydrodynamic
simulation of a flare loop that connects the central star and the
accretion disk, and discuss the evaporation of the chromosphere of the
central star and the disk. We assumed a long ( > 10 Rodot)
loop length, and that the flare energy is deposited near the half-way
point between the disk and the stellar surface. We found that in some
cases all of the plasma in the accretion disk is heated to the flare
temperature and spreads over the flare loop. The condition for this
``disk disappearance'' was examined. The X-ray spectrum expected when we
observe the simulation result was synthesized by taking into account the
instrumental response of ASCA/GIS. However, we could not find any clear
observational signature of the existence of the disk, because the bulk
properties of a flare loop are determined by the flare heating flux
and loop length, and not by the involvement of the disk. We found that
the synthesized spectrum is reasonably fitted with a two-temperature
model, and that the temperature of the hotter component is several
factors lower than the maximum temperature of the simulation result.
Title: Erratum: ``Analysis of the Temperature and Emission Measure of
Solar Coronal Arcades and Test of a Scaling Law of Flare/Arcade Loop
Length'' (ApJ, 579, L45 [2002])
Authors: Yamamoto, Tetsuya T.; Shiota, Daikou; Sakajiri, Takuma;
Akiyama, Sachiko; Isobe, Hiroaki; Shibata, Kazunari
Bibcode: 2003ApJ...592L.107Y
Altcode:
Recently, we found an error in our analysis program and have
reanalyzed data. As a result, Table 1 and the right panel of
Figure 3 were altered in the above Letter. The figure shows a
slight change, but our conclusion is not affected. However,
typical values changed to n0=2×108
cm-3, B=5.6 G, narc=4×108
cm-3, EMarc=3.2×1047
cm-3, and β~=0.3, and equation (6)
becameLtheor~=1010(EM3.2×1047
cm-3)3/5(n02×108
cm-3)-2/5(T3.7×106
K)-8/5cm.(6)
Title: General Relativistic MHD Simulations of the Gravitational
Collapse of a Rotating Star with Magnetic Field as a Model of
Gamma-Ray Bursts
Authors: Mizuno, Y.; Mizuno, Yosuke; Yamada, Shoichi; Shibata,
Kazunari; Koide, Shinji
Bibcode: 2003ICRC....5.2733M
Altcode: 2003ICRC...28.2733M
We have performed 2.5-dimensional general relativistic MHD simulations
of the gravitational collapse of a rotating massive star with magnetic
field as a model of gamma ray bursts (GRBs). This simulation showed
the generation of jet-like-outflow from the gravitational collapse. We
found the jet is accelerated by the gradient force of gas pressure
and the centrifugal force and is collimated by the pinching force of
toroidal magnetic field and the effect of geometry of magnetic field.
Title: The Log-Normal Distributions of Coronal Mass Ejection-Related
Solar Flares and the Flare/CME Model of Gamma-Ray Bursts
Authors: Aoki, Seiichiro; Yashiro, S.; Shibata, K.
Bibcode: 2003ICRC....5.2729A
Altcode: 2003ICRC...28.2729A
It is known that gamma-ray bursts (GRBs) are highly time-variable and
the statistical distribution of physical variables show log-normal. The
solar flares are also highly time-variable phenomena, but the physical
quantities of them show power-law distribution. GRBs are believed to be
the emission from the relativistic outflow from the central engine. On
the other hand, the outflow from the Sun is known as coronal mass
ejections (CMEs). We analyze the distributions of some quantities of
CMEs and the solar flares related to CMEs (CME-related solar flares)
to compare with the distributions of GRBs. We found the distributions
of X-ray peak fluxes of CME-related solar flares and the speed of CMEs
are very similar to log-normal distribution. Hence the distributions
of GRBs are similar to those of CME-related solar flares and CMEs,
and the ejection mechanisms of outflow from the central engine of
GRBs and the Sun might be similar, that is, magnetic reconnection. We
propose the new model of the central engine of GRBs by the analogy of
solar flares and CMEs.
Title: MHD Simulations of the Internal Shocks in Magnetic Reconnection
Jet in the Solar Flare: Possibility of the Particle Acceleration
Authors: Tanuma, Syuniti; Shibata, K.
Bibcode: 2003ICRC....6.3351T
Altcode: 2003ICRC...28.3351T
The solar atmosphere is filled with the high-energy particles. The
satellites such as Yohkoh and RHESSI observe the Xand gamma-ray
emissions from them. They are accelerated in solar flares. The their
origin is, however, not fully known yet. In this paper, we examine
how the magnetic reconnection creates the fast shocks, by performing
two-dimensional numerical resistive magnetohydro dynamic simulations. As
the results, we find that the current sheet becomes thin by tearing
instability, and it collapses to Sweet-Parker current sheet. The thin
current sheet becomes unstable to the tearing instability again. The
fast reconnection starts immediately after the plasmoid ejection,
which are created by the secondary tearing instability. The internal
shocks are created inside the reconnection jet due to the non-steady
plasmoid-ejection. In the next phase, the magnetic reconnection jet
oscillates in the current sheet, which is due to Kelvin-Helmholtzlike
instability, or turbulent reconnection. The reconnection jet collides
with two standing slow shocks, so that the fast shocks are created by
oblique shocks. The fast shocks created by the magnetic reconnection
are possible sites for the particle acceleration in the solar flare.
Title: Magnetohydrodynamic Numerical Simulations of Coronal Mass
Ejections and Associated Giant Arcades
Authors: Shiota, D.; Shibata, K.; Isobe, H.; Brooks, D. H.; Chen, P. F.
Bibcode: 2003ICRC....6.3379S
Altcode: 2003ICRC...28.3379S
By extending the Chen-Shibata [1] model of coronal mass ejections
(CMEs), we develop physical model of CMEs and associated giant
arcades just below CMEs in two and half dimension, incorporating heat
conduction. On the basis of the simulation results, the theoretical
soft X-ray images are calculated and compared with observations of
CMEs and giant arcades with Yohkoh/SXT (soft X-ray telescope). Detailed
comparison between simulated X-ray images and observations revealed that
(1) the Y-shaped ejection features, often seen at the bottom of some
CMEs, might corresp ond to slow and fast mode MHD shocks associated with
reconnection [3], (2) the dimming, often observed both sides of arcades,
can be produced at least partly by reconnection inflow, (3) the back
bone like bright soft X-ray features seen at the top of some arcades
might corresp ond to fast mode MHD shocks just below reconnection jet.
Title: MHD Simulations of Magnetic Reconnection in the Galaxy:
The Origin of Diffuse X-Ray Gas and High Energy Particles
Authors: Tanuma, Syuniti; Shibata, K.
Bibcode: 2003ICRC....4.2277T
Altcode: 2003ICRC...28.2277T
Many X-ray and non-thermal emissions are observed in the Galaxy. It
is, however, unknown what is the origin of hot(∼ 7 keV) diffuse
X-ray gas such as Galactic ridge X-ray emission, including non-thermal
component. Tanuma et al.(1999) suggested that the X-ray gas are created
by magnetic reconnection which occurs in the locally strong(∼ 30
µG) interstellar magnetic field in the Galaxy. In this paper, we
suggest that the non-thermal emission from high energy particles are
also created by the reconnection, and examine this model by performing
two-dimensional numerical resistive magnetohydro dynamic simulations. As
the results, we find that the fast reconnection starts immediately
after the plasmoid ejection, which are created by the secondary tearing
instability. The internal shocks are created in the reconnection
jet due to the non-steady plasmoid-ejection. In the next phase,
the reconnection jet oscillates due to KelvinHelmholtz instability,
so that the multiple fast (oblique) shocks are created. The magnetic
reconnection and fast shocks are possible mechanism to generate the
diffuse X-ray gas and high energy particles in the Galaxy.
Title: Evolution of Flare Ribbons and Energy Release
Authors: Asai, A.; Yokoyama, Takaaki; Shimojo, Masumi; Masuda, Satoshi;
Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2003ICRC....6.3367A
Altcode: 2003ICRC...28.3367A
We examined the relation between evolutions of flare ribb ons and
released magnetic energies at a solar flare which occurred on 2001
April 10 in the active region NOAA 9415. We successfully evaluated
the released energy quantitatively, based on the magnetic reconnection
model. We measured the photospheric magnetic field strengths and the
separation speeds of the fronts of the Hα flare ribb on, and estimated
the released magnetic energy at the flare by using those values. Then,
we compared the estimated energy release rates with the nonthermal
behaviors observed in hard X-rays and microwaves. We found that those
at the Hα kernels associated with the HXR sources are locally large
enough to explain the difference between the spatial distribution
the Hα kernels and the hard X-ray sources. Their temporal evolution
of the energy release rates also shows peaks corresponding to hard
X-ray bursts.
Title: Solar Gamma Ray Events Detected by the GEOTAIL Plasma
Instrument
Authors: Takei, Y.; Terasawa, Toshio; Yoshikawa, Ichiro; Saito,
Yoshifumi; Mukai, Toshifumi; Takasaki, Hiroyuki; Shibata, Kazunari
Bibcode: 2003ICRC....6.3223T
Altcode: 2003ICRC...28.3223T
We have found the MCP (micro channel plate) of the LEP (low energy
plasma) experiment aboard the GEOTAIL satellite was activated during
large solar gamma ray events (such as the 6 Nov 1997 event). We have
studied the statistical relationship between the MCP activations and
solar events,@and found that significant activations occurred for
X-class solar flares. Since the altitude of GEOTAIL is as high as 8-30
Re, our 'data coverage' for solar events could be nearly 100 percent,
if we can separate the gamma ray 'signals' from the nominal counts
by plasma particles. Therefore GEOTAIL may provide useful information
of solar gamma ray events when the data from other satellites such as
YOHKOH and RHESSI are not available.
Title: Slow and Fast MHD Shocks Associated with a Giant Cusp-Shaped
Arcade on 1992 January 24
Authors: Shiota, Daikou; Yamamoto, Tetsuya T.; Sakajiri, Takuma;
Isobe, Hiroaki; Chen, Peng-Fei; Shibata, Kazunari
Bibcode: 2003PASJ...55L..35S
Altcode:
We performed magnetohydrodynamic (MHD) simulations of a giant arcade
formation with a model of magnetic reconnection coupled with heat
conduction, to investigate the dynamical structure of slow and fast MHD
shocks associated with reconnection. Based on the numerical results,
theoretical soft X-ray images were calculated and compared with the
Yohkoh soft X-ray observations of a giant arcade on 1992 January 24. The
Y-shaped structure observed in the event was identified to correspond
to the slow and fast shocks associated with the magnetic reconnection.
Title: The Formation of relativistic jets from Kerr Black Holes
Authors: Nishikawa, K. -I.; Richardson, G.; Preece, R.; Hardee, P.;
Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.; Hughes, J. P.; Fishman, J.
Bibcode: 2003AAS...202.0602N
Altcode: 2003AAS...202..602N; 2003BAAS...35Q.706N
We have performed the first fully three-dimensional general relativistic
magnetohydrodynamics (GRMHD) simulation for Schwarzschild and Kerr
black holes with a free falling corona and thin accretion disk. The
initial simulation results with a Schwarzschild metric show that
a jet is created as in the previous axisymmetric simulations with
mirror symmetry at the equator. However, the time to form the jet
is slightly longer than in the 2-D axisymmetric simulation. We
expect that the dynamics of jet formation are modified due to the
additional freedom in the azimuthal dimension without axisymmetry
with respect to the z axis and reflection symmetry with respect to
the equatorial plane. This simulation ran for 128.9 light-crossing
time (τ S = r S/c) which is much larger than
the previous simulation time 52 τ S. The jet which is
initially formed due to the twisted magnetic fields and shocks becomes
a wind at the later time. The wind flows out with a much wider angle
than the initial jet. The twisted magnetic fields at the earlier time
were untwisted and less pinched. The accretion disk became thicker than
the initial condition. Further simulations with initial perturbations
will provide insights for accretion dynamics with instabilities such
as magnetorotational instability (MRI) and accretion-eject instability
(AEI). These instabilities may contribute to variablities observed in
microquasars and AGN jets.
Title: Progress on Numerical Simulations of Solar Flares and Coronal
Mass Ejections
Authors: Shibata, K.
Bibcode: 2003ASPC..289..381S
Altcode: 2003aprm.conf..381S
Recent progress of numerical simulations of solar flares and coronal
mass ejections is discussed with emphasis on MHD simulations of magnetic
reconnection and their application to recent space observations such
as those by Yohkoh.
Title: MHD Jets, Flares, and Gamma Ray Bursts
Authors: Shibata, Kazunari; Aoki, Seiichiro
Bibcode: 2003astro.ph..3253S
Altcode:
Recent numerical simulations of MHD jets from accretion disks are
briefly reviewed with emphasis on the scaling law for jet speed and
the role of magnetic reconnection in relation to time variability in
accretion disks, jets, and flares. On the basis of these studies,
possible interpretation is given on why statistical properties
of peak intensity, peak interval, and peak duration of gamma ray
bursts (log-normal distribution) are different from those in solar
flares and black hole accretion disks (power-law distribution). From
these considerations, a new model, ``magnetized plasmoid model'',
is proposed for a central engine of gamma ray bursts. (see
http://www.kwasan.kyoto-u.ac.jp/~shibata/grb_final/grb.pdf for pdf
files including both text and figures)
Title: Reconnection, Alfven Wave, and Coronal Heating
Authors: Shibata, K.; Moriyasu, S.
Bibcode: 2003ASPC..286..377S
Altcode: 2003ctmf.conf..377S
No abstract at ADS
Title: Magnetic Reconnection Triggered by the Parker Instability in
the Galaxy: Two-dimensional Numerical Magnetohydrodynamic Simulations
and Application to the Origin of X-Ray Gas in the Galactic Halo
Authors: Tanuma, Syuniti; Yokoyama, Takaaki; Kudoh, Takahiro; Shibata,
Kazunari
Bibcode: 2003ApJ...582..215T
Altcode: 2002astro.ph..9008T
We propose the Galactic flare model for the origin of the X-ray gas
in the Galactic halo. For this purpose, we examine the magnetic
reconnection triggered by Parker instability (magnetic buoyancy
instability), by performing the two-dimensional resistive numerical
magnetohydrodynamic simulations. As a result of numerical simulations,
the system evolves through the following phases. Parker instability
occurs in the Galactic disk. In the nonlinear phase of Parker
instability, the magnetic loop inflates from the Galactic disk into
the Galactic halo and collides with the antiparallel magnetic field, so
that the current sheets are created in the Galactic halo. The tearing
instability occurs and creates the plasmoids (magnetic islands). Just
after the plasmoid ejection, further current sheet thinning occurs in
the sheet, and the anomalous resistivity sets in. Petschek reconnection
starts and heats the gas quickly in the Galactic halo. It also creates
the slow and fast shock regions in the Galactic halo. The magnetic
field (B~3 μG), for example, can heat the gas (n~10-3
cm-3) to a temperature of ~106 K via the
reconnection in the Galactic halo. The gas is accelerated to Alfvén
velocity (~300 km s-1). Such high-velocity jets are the
evidence of the Galactic flare model we present in this paper, if the
Doppler shift of the bipolar jet is detected in the Galactic halo.
Title: 3-D GRMHD simulations of generating jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.
Bibcode: 2003nvm..conf..109N
Altcode: 2003nvm..conf..101N; 2002astro.ph..8092N
We have performed a first fully 3-D GRMHD simulation with Schwarzschild
black hole with a free falling corona. The initial simulation results
show that a jet is created as in the previous simulations using the
axisymmetric geometry with the mirror symmetry at the equator. However,
the time to generate the jet is longer than in the 2-D axisymmetric
simulations. We expect that due to the additional freedom in the
azimuthal dimension without axisymmetry with respect to the z axis and
reflection symmetry with respect to the equatorial plane, the dynamics
of jet formation can be modified. Further simulations are required
for the study of instabilities along the azimuthal direction such as
accretion-ejection instability
Title: 3-D GRMHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Richardson, G.; Preece, R.; Fishiman,
G. J.; Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.; Li, L. X.; Hughes,
J. P.; Hardee, P.; Blandford, R.
Bibcode: 2003ASPC..290..353N
Altcode: 2003agnc.conf..353N
We have performed a first fully 3-D GRMHD simulation with Schwarzschild
black hole with a free falling corona. The simulation results show
that a jet is created as in previous axisymmetric simulations. However,
the time to generate the jet is longer than in the 2-D simulations. We
expect that due to the additional azimuthal dimension the dynamics of
jet formation can be modified. We will report formation mechanisms of
jet from an accreting black hole.
Title: The Resistive Magnetohydrocynamics of Protoplanetary Jets
Authors: Kuwabara, Takuhito; Shibata, Kazunari; Matsumoto, Ryoji;
Ko, Chung-Ming
Bibcode: 2003IAUS..221P.160K
Altcode:
The mass outflow (jet) phenomenon like HH-object is observed in
star formation region. The existence of an accretion disk at the
foot point of jet is confirmed by the observations of Hubble Space
Telescope. From these results we can say that the accretion disk
plays an important role for mass outflow phenomenon. On the other
hand it is confirmed that the existence of knot like structure in the
jet. Furthermore We think that the magnetic field plays an important
role in jet formation region with accretion disk and studied the mass
outflow from protoplanetary disk. We studied the uniform magnetic
diffusivity case and the non-uniform magnetic diffusivity case given
by Sano et al.(2000). They derived this distribution of magnetic
diffusivity by taking into account the spatial distribution of many
kinds of charged particles in protoplanetary disk. From their
result the region where the magnetorotational instability is stabilized
exists between 1AU and 20AU from a central star. We discuss about how
the difference of diffusivity distribution between these two cases
affects the mass outflow from protoplanetary disks
Title: Jet Formation with 3-D General Relativistic MHD Simulations
Authors: Richardson, G. A.; Nishikawa, K. -I.; Preece, R.; Hardee, P.;
Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.; Hughes, J. P.; Fishman, J.
Bibcode: 2002AAS...201.1509R
Altcode: 2002BAAS...34R1123R
We have investigated the dynamics of an accretion disk around
Schwarzschild black holes initially threaded by a uniform poloidal
magnetic field in a non-rotating corona (in a steady-state infalling
state) around a non-rotating black hole using 3-D GRMHD with the
``axisymmetry'' along the z-direction. The magnetic field is tightly
twisted by the rotation of the accretion disk, and plasmas in the
shocked region of the disk are accelerated by the J x B force to form
bipolar relativistic jets. In order to investigate variabilities of
generated relativistic jets and the magnetic field structure inside
jets, we have performed calculations using the 3-D GRMHD code with a
full 3-dimensional system without the axisymmetry. We have investigated
how the third dimension affects the global disk dynamics and jet
generation. We will perform simulations with various incoming flows
from an accompanying star.
Title: MHD Simulation of a Solar Flare and Derived Scaling Law between
the Temperature and the Emission Measure of Stellar/Solar Flares
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 2002ASPC..277..615Y
Altcode: 2002sccx.conf..615Y
No abstract at ADS
Title: ``Dawn-dusk asymmetry'' in solar coronal arcade formations
Authors: Isobe, Hiroaki; Shibata, Kazunari; Machida, Shinobu
Bibcode: 2002GeoRL..29.2014I
Altcode: 2002GeoRL..29u..10I
Solar flares/arcades and magnetospheric substorms are believed
to have a common physical process: energy release via magnetic
reconnection. In substorms, it is known that the reconnection point
tends to occur more in the dusk side of the magnetotail than in the
dawn side. This asymmetry is called the dawn-dusk asymmetry. However,
no one has examined so far the existence of such asymmetry in the
solar corona. We investigated the directions of arcade formation and
-vin × B electric field for 32 events, and found that more
arcades successively formed in the direction of the electric field than
in the opposite direction. This may be a solar analogue of dawn-dusk
asymmetry. We also found that velocity of successive formation of
arcades increases with the aspect ratio of the arcade.
Title: Analysis of the Temperature and Emission Measure of Solar
Coronal Arcades and Test of a Scaling Law of Flare/Arcade Loop Length
Authors: Yamamoto, Tetsuya T.; Shiota, Daikou; Sakajiri, Takuma;
Akiyama, Sachiko; Isobe, Hiroaki; Shibata, Kazunari
Bibcode: 2002ApJ...579L..45Y
Altcode:
We analyze 17 arcades to study the relations between solar flares and
arcades. Soft X-ray images taken with Yohkoh's soft X-ray telescope are
used to derive T0, EM0, and temporal variation
of Tarc and EMarc, where Tarc and
T0 are the temperatures of an arcade and prearcade region
and EMarc and EM0 are the volume emission measures
of an arcade and prearcade region. It is found that T0~2 MK
and Tarc~4 MK. We also estimate prearcade coronal electron
density n0 and arcade electron density narc
to find that narc is comparable to n0
(narc~n0~108 cm-3). Using
these observed EM, T, and n0, we calculate the theoretical
loop length Ltheor based on the scaling law for solar and
stellar flares derived by Shibata & Yokoyama and compare it with
observed flare/arcade loop length Lobs. The result shows a
good correlation between them (Ltheor~Lobs)
and indicates the need of plasma β for the scaling law
(Ltheor~EM3/5T-8/5n-
2/50β-6/5). This supports the theory of
the scaling law and is indirect evidence that flares and arcades are
heated by the same magnetic reconnection mechanism.
Title: Difference between Spatial Distributions of the Hα Kernels
and Hard X-Ray Sources in a Solar Flare
Authors: Asai, Ayumi; Masuda, Satoshi; Yokoyama, Takaaki; Shimojo,
Masumi; Isobe, Hiroaki; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2002ApJ...578L..91A
Altcode: 2002astro.ph..9106A
We present the relation of the spatial distribution of Hα kernels
with the distribution of hard X-ray (HXR) sources seen during the 2001
April 10 solar flare. This flare was observed in Hα with the Sartorius
telescope at Kwasan Observatory, Kyoto University, and in HXRs with
the hard X-ray telescope (HXT) on board Yohkoh. We compared the spatial
distribution of the HXR sources with that of the Hα kernels. While many
Hα kernels are found to brighten successively during the evolution
of the flare ribbons, only a few radiation sources are seen in the
HXR images. We measured the photospheric magnetic field strengths
at each radiation source in the Hα images and found that the Hα
kernels accompanied by HXR radiation have magnetic strengths about 3
times larger than those without HXR radiation. We also estimated the
energy release rates based on the magnetic reconnection model. The
release rates at the Hα kernels with accompanying HXR sources are
16-27 times larger than those without HXR sources. These values are
sufficiently larger than the dynamic range of HXT, which is about 10,
so that the difference between the spatial distributions of the Hα
kernels and the HXR sources can be explained.
Title: A Hertzsprung-Russell-like Diagram for Solar/Stellar Flares
and Corona: Emission Measure versus Temperature Diagram
Authors: Shibata, Kazunari; Yokoyama, Takaaki
Bibcode: 2002ApJ...577..422S
Altcode: 2002astro.ph..6016S
In our previous paper, we presented a theory to explain the observed
universal correlation between the emission measure (EM=n2V)
and temperature (T) for solar/stellar flares on the basis of the
magnetic reconnection model with heat conduction and chromospheric
evaporation. Here n is the electron density and V is the volume. By
extending our theory to general situations, we examined the
EM-T diagram in detail and found the following properties: (1)
The universal correlation sequence (``main-sequence flares'') with
EM~T17/2 corresponds to the case of constant heating flux
or, equivalently, the case of constant magnetic field strength in the
reconnection model. (2) The EM-T diagram has a forbidden region, in
which gas pressure of flares exceeds magnetic pressure. (3) There is a
coronal branch with EM~T15/2 for T<107 K and
EM~T13/2 for T>107 K. This branch is situated
on the left side of the main-sequence flares in the EM-T diagram. (4)
There is another forbidden region determined by the length of flare
loop; the lower limit of the flare loop is 107 cm. Small
flares near this limit correspond to nanoflares observed by the Solar
and Heliospheric Observatory EUV Imaging Telescope. (5) We can plot the
flare evolution track on the EM-T diagram. A flare evolves from the
coronal branch to main-sequence flares, then returns to the coronal
branch eventually. These properties of the EM-T diagram are similar
to those of the H-R diagram for stars, and thus we propose that the
EM-T diagram is quite useful for estimating the physical quantities
(loop length, heating flux, magnetic field strength, total energy,
and so on) of flares and coronae when there are no spatially resolved
imaging observations.
Title: The Large-Scale Coronal Field Structure and Source Region
Features for a Halo Coronal Mass Ejection
Authors: Wang, Tongjiang; Yan, Yihua; Wang, Jialong; Kurokawa, H.;
Shibata, K.
Bibcode: 2002ApJ...572..580W
Altcode:
On 1998 May 2 a class X1/3B flare occurred at 13:42 UT in NOAA Active
Region 8210 near disk center, which was followed by a halo coronal mass
ejection (CME) at 15:03 UT observed by SOHO/LASCO. Using the boundary
element method (BEM) on a global potential model, we reconstruct
the large-scale coronal field structure from a composite boundary by
SOHO/MDI and Kitt Peak magnetograms. The extrapolated large field lines
well model a transequatorial interconnecting loop (TIL) seen in the
soft X-ray (SXR) between AR 8210 and AR 8214, which disappeared after
the CME. The EUV Imaging Telescope (EIT) observed the widely extending
dimmings, which noticeably deviate from the SXR TIL in position. We find
that the major dimmings are magnetically linked to the flaring active
region but some dimmings are not. The spatial relationships of these
features suggest that the CME may be led by a global restructuring of
multipolar magnetic systems due to flare disturbances. Mass, magnetic
energy, and flux of the ejected material estimated from the dimming
regions are comparable to the output of large CMEs, derived from the
limb events. At the CME source region, Huairou vector magnetograms
show that a strong shear was rapidly developed in a newly emerging
flux region (EFR) near the main spot before the flare. Magnetic field
extrapolations reveal the presence of a ``bald patch'' (defined as the
locations where the magnetic field is tangent to the photosphere) at the
edge of the EFR. The preflare features such as EUV loop brightenings
and SXR jets appearing at the bald patch suggest a slow reconnection
between the TIL field system and a preexisting overlying field above
the sheared EFR flux system. High-cadence Yohkoh/SXT images reveal a
fast expanding motion of loops above a bright core just several minutes
before the hard X-ray onset. This may be a precursor for the eruption
of the sheared EFR flux to produce the flare. We propose a scenario,
similar to the ``breakout'' model in principle, that can interpret
many observed features.
Title: A Simple Model for a Magnetic Reconnection-heated Corona
Authors: Liu, B. F.; Mineshige, S.; Shibata, K.
Bibcode: 2002ApJ...572L.173L
Altcode: 2002astro.ph..5257L
We construct a simple model for a magnetic reconnection-heated
corona above a thin accretion disk in active galactic nuclei
(AGNs) and Galactic black hole candidates (GBHCs). The basic
assumptions are that (1) the magnetic reconnection heat is cooled
down overwhelmingly by Compton scattering in the corona and (2)
the thermal conduction is dominantly cooled by evaporation of
the chromospheric plasma in the disk-corona interface before
Compton cooling sets in. With these two basic equations as well
as equipartition of magnetic energy with gas energy in the disk,
we can consistently determine the fraction of accretion energy
dissipated in the corona without free parameters and thus determine
the temperature and all other quantities in both the corona and disk
for a given black hole mass and accretion rate. Then, we calculate
the luminosity contributed from the disk and corona and the coronal
flux-weighted Compton y-parameter. It is found that at low luminosity
(L<0.1LEdd) the spectrum is hard, with a spectral index
of αsp~1 (fν~ν-αsp),
while at high luminosity (L>~0.1LEdd) the spectrum can
be either soft or hard. We also find that the situation is almost the
same for supermassive and stellar mass black holes. These features
are consistent with observations of AGNs and GBHCs.
Title: Relation between a Moreton Wave and an EIT Wave Observed on
1997 November 4
Authors: Eto, Shigeru; Isobe, Hiroaki; Narukage, Noriyuki; Asai, Ayumi;
Morimoto, Taro; Thompson, Barbara; Yashiro, Seiji; Wang, Tongjiang;
Kitai, Reizaburo; Kurokawa, Hiroki; Shibata, Kazunari
Bibcode: 2002PASJ...54..481E
Altcode:
We consider the relationship between two flare-associated waves,
a chromospheric Moreton wave and a coronal EIT wave, based on an
analysis of an X-class flare event in AR 8100 on 1997 November 4. A
Moreton wave was observed in Hα + 0.8 Å, and Hα - 0.8 Å with the
Flare-Monitoring Telescope (FMT) at the Hida Observatory. An EIT wave
was observed in EUV with the Extreme ultraviolet Imaging Telescope
(EIT) on board SOHO. The propagation speeds of the Moreton wave and
the EIT wave were approximately 715 km s-1 and 202 km
s-1, respectively. The times of visibility for the Moreton
wave did not overlap those of the EIT wave, but the continuation of the
former is indicated by a filament oscillation. Data on the speed and
location clearly show that the Moreton wave differed physically from
the EIT wave in this case. The Moreton wave preceded the EIT wave,
which is inconsistent with an identification of the EIT wave with a
fast-mode MHD shock.
Title: Simultaneous Observation of a Moreton Wave on 1997 November
3 in Hα and Soft X-Rays
Authors: Narukage, N.; Hudson, H. S.; Morimoto, T.; Akiyama, S.;
Kitai, R.; Kurokawa, H.; Shibata, K.
Bibcode: 2002ApJ...572L.109N
Altcode:
We report the observation of a Moreton wave in Hα (line center and
+/-0.8 Å) with the Flare Monitoring Telescope at the Hida Observatory
of Kyoto University at 4:36-4:41 UT on 1997 November 3. The same
region (NOAA Active Region 8100) was simultaneously observed in soft
X-rays with the soft X-ray telescope on board Yohkoh, and a wavelike
disturbance (``X-ray wave'') was also found. The position of the wave
front as well as the direction of propagation of the X-ray wave roughly
agree with those of the Moreton wave. The propagation speeds of the
Moreton wave and the X-ray wave are about 490+/-40 and 630+/-100 km
s-1, respectively. Assuming that the X-ray wave is an MHD
fast-mode shock, we can estimate the propagation speed of the shock, on
the basis of MHD shock theory and the observed soft X-ray intensities
ahead of and behind the X-ray wave front. The estimated fast shock
speed is 400-760 km s-1, which is in rough agreement with
the observed propagation speed of the X-ray wave. The fast-mode Mach
number of the X-ray wave is also estimated to be about 1.15-1.25. These
results suggest that the X-ray wave is a weak MHD fast-mode shock
propagating through the corona and hence is the coronal counterpart
of the Moreton wave.
Title: Evidence of EIT and Moreton Waves in Numerical Simulations
Authors: Chen, P. F.; Wu, S. T.; Shibata, K.; Fang, C.
Bibcode: 2002ApJ...572L..99C
Altcode:
Solar coronal mass ejections (CMEs) are associated with many dynamical
phenomena, among which EIT waves have always been a puzzle. In this
Letter MHD processes of CME-induced wave phenomena are numerically
simulated. It is shown that as the flux rope rises, a piston-driven
shock is formed along the envelope of the expanding CME, which sweeps
the solar surface as it propagates. We propose that the legs of the
shock produce Moreton waves. Simultaneously, a slower moving wavelike
structure, with an enhanced plasma region ahead, is discerned, which
we propose corresponds to the observed EIT waves. The mechanism for
EIT waves is therefore suggested, and their relation with Moreton
waves and radio bursts is discussed.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, Ken-Ichi; Koide, Shinji; Shibata, Kazunari; Kudoh,
Takashiro; Sol, Helene; Hughes, John
Bibcode: 2002APS..APRB17093N
Altcode:
We have investigated the dynamics of an accretion disk around
Schwarzschild black holes initially threaded by a uniform poloidal
magnetic field in a non-rotating corona (either in a steady-state
infalling state) around a non-rotating black hole using a 3-D GRMHD with
the ``axisymmetry'' along the z-direction. Magnetic field is tightly
twisted by the rotation of the disk, and plasmas in the shocked region
of the disk are accelerated by J × B force to form bipolar relativistic
jets. In order to investigate variabilities of generated relativistic
jets and magnetic field structure inside jets, we have performed
calculations using the 3-D GRMHD code with a full 3-dimensional system
without the axisymmetry. We have investigated how the third dimension
affects the global disk dynamics and jet generation. We will perform
simulations with various incoming flows from an accompanying star.
Title: Are Jets Ejected from Locally Magnetized Accretion Disks?
Authors: Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 2002PASJ...54..267K
Altcode:
We investigated the jet formation from accretion disks in which a weak
localized poloidal magnetic field is initially embedded. Previous MHD
numerical simulations of jet formation from accretion disks initially
assumed a large-scale vertical uniform magnetic field that threads
the disk, which showed that jets are ejected from accretion disks by
a magneto-centrifugal force and magnetic pressure. In contrast to
a large-scale uniform magnetic field, what happens if the magnetic
field is localized in the disk? Our MHD numerical simulation shows
that the jet structure appears even if the initial magnetic field
is localized in the disk. The disk material is ejected as a poloidal
magnetic loop by magnetic pressure due to a toroidal magnetic field
that is generated by the disk rotation. Though the ejection mechanism
is different from that of the magneto-centrifugally driven jet model,
the rising magnetic loop behaves like a jet; it is collimated by a
pinching force of the t oroidal magnetic field, and its velocity is
on order of the Keplerian velocity of the disk.
Title: Extraction of Black Hole Rotational Energy by a Magnetic
Field and the Formation of Relativistic Jets
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro; Meier,
David L.
Bibcode: 2002Sci...295.1688K
Altcode:
Using numerical simulations, we modeled the general relativistic
magnetohydrodynamic behavior of a plasma flowing into a rapidly rotating
black hole in a large-scale magnetic field. The results show that a
torsional Alfvén wave is generated by the rotational dragging of space
near the black hole. The wave transports energy along the magnetic field
lines outward, causing the total energy of the plasma near the hole to
decrease to negative values. When this negative energy plasma enters the
horizon, the rotational energy of the black hole decreases. Through this
process, the energy of the spinning black hole is extracted magnetically
Title: 2.5-dimensional Nonsteady Magnetohydrodynamic Simulations of
Magnetically Driven Jets from Geometrically Thin Disks
Authors: Kato, Seiichi X.; Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 2002ApJ...565.1035K
Altcode:
We have performed self-consistent 2.5-dimensional nonsteady MHD
numerical simulations of jets from geometrically thin disks including
the dynamics of accretion disks. For the initial rotational velocity
of the disk, we consider two cases, the Keplerian case and the
sub-Keplerian case. We compare our results with the thick-disk case in
detail. The characteristics of nonsteady jets from geometrically thin
disks in our Keplerian case are similar to those of the steady theory
and thick-disk cases: (1) The ejection point of the jets corresponds
to the slow magnetosonic point, which is determined by the effective
potential made by the gravitational and centrifugal forces along the
magnetic field. (2) The dependences of the velocity (Vjet)
and the mass outflow rate (Mw) on the initial magnetic
field strength (B0) are Mw~B0 and
Vjet~(Ω2FB20/Mw)1/3,
therefore Vjet~B1/30, where
ΩF is the angular velocity of a field line. Although
this dependence of the velocity corresponds to Michel's scaling
law, the velocity of our simulated jets still does not reach the
fast magnetosonic velocity. In the sub-Keplerian case, the relation
Vjet~B1/30 is satisfied, but the
other dependences are not necessarily equal to those of the Keplerian
case. The velocity of the jets is larger when the initial rotational
velocity of the disk is smaller. The initial acceleration force on the
jets is the magnetic pressure when the initial magnetic field is weak,
while the centrifugal force is dominant when the initial magnetic
field is strong. Finally, we found two interesting phenomena in the
sub-Keplerian cases: one is knotlike structures around the rotational
axis, and the other is outflow along the disk surface.
Title: Effect of a Magneto-Rotational Instability on Jets from
Accretion Disks
Authors: Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 2002PASJ...54..121K
Altcode:
We present the results of 2.5-dimensional MHD simulations of
jet formation from accretion disks that are unstable for a
magneto-rotational instability. Numerical simulations show that
magnetically driven jets are ejected from magneto-rotationally-unstable
disks. The velocities of the jets are of the order of the Keplerian
velocities of the disks. In this paper, we mainly focus on the effect
of the magneto-rotational instability on magnetically driven jets
from thick disks. For that purpose, we initially impose a sinusoidal
perturbation with finite amplitude on a rotating disk that is threaded
by a vertical uniform magnetic field. The perturbation grows by the
magneto-rotational instability, and the nonlinear development of the
instability leads to a channel flow which causes violent accretion. As
the accretion continues, the accretion flow is partially turned outward
to the outflow that is accelerated by the magnetic force along the
poloidal magnetic field line, i.e., a ma gnetically driven jet is
produced. Models with initial finite amplitude perturbation (i.e.,
|δv|/Vs = 0.1 where δv is the velocity perturbation and is
the sound velocity) are compared with those without any perturbation
that we previously studied. The mass-accretion rate, mass-ejection
rate, and jet velocity are larger when the perturbation is imposed on
in the disk. However, the jet velocity is of the order of the Keplerian
velocity of the disk, almost independent of the perturbation.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.
Bibcode: 2002astro.ph..2396N
Altcode:
We have performed a first fully 3-D GRMHD simulation with Schwarzschild
black hole with a free falling corona. The initial simulation
results show that a jet is created as in previous axisymmetric
simulations. However, the time to generate the jet is longer than in
the 2-D simulations. We expect that due to the additional azimuthal
dimension the dynamics of jet formation can be modified.
Title: Reconnection Rate in the Decay Phase of a Long Duration Event
Flare on 1997 May 12
Authors: Isobe, Hiroaki; Yokoyama, Takaaki; Shimojo, Masumi; Morimoto,
Taro; Kozu, Hiromichi; Eto, Shigeru; Narukage, Noriyuki; Shibata,
Kazunari
Bibcode: 2002ApJ...566..528I
Altcode:
Recent analyses of long duration event (LDE) flares indicate successive
occurrences of magnetic reconnection and resultant energy release
in the decay phase. However, quantitative studies of the energy
release rate and the reconnection rate have not yet been made. In
this paper we focus on the decay phase of an LDE flare on 1997 May
12 and derive the energy release rate H and the reconnection rate
MA=vin/vA, where vin is
the inflow velocity and vA is the Alfvén velocity. For this
purpose, we utilize a method to determine vin and the coronal
magnetic field Bcorona indirectly, using the following
relations:H=2B2corona/4πvinAr,Bcoronavin=Bfootvfoot,where
Ar, Bfoot, and vfoot are the area of
the reconnection region, the magnetic field strength at the footpoints,
and the separation velocity of the footpoints, respectively. Since H,
Ar, vfoot, and Bfoot are obtained from
the Yohkoh Soft X-Ray Telescope data and a photospheric magnetogram,
vin and Bcorona can be determined from these
equations. The results are as follows: H is ~1027 ergs
s-1 in the decay phase. This is greater than 1/10th of
the value found in the rise phase. MA is 0.001-0.01,
which is about 1 order of magnitude smaller than found in previous
studies. However, it can be made consistent with the previous
studies under the reasonable assumption of a nonunity filling
factor. Bcorona is found to be in the range of 5-9 G, which
is consistent with both the potential extrapolation and microwave
polarization observed with the Nobeyama Radioheliograph.
Title: The heliosphere in rising phase and the triple-dipole model
Authors: Saito, Takao; Watanabe, Ta.; Shibata, K.
Bibcode: 2002AdSpR..29.1533S
Altcode:
The 22-year variation of the structure of the heliosphere has been
studied in relation to the triple, dipole model. The variation is well
explained by the model, if coronal streamers, especially in rising
phase of a solar cycle, are classified into two: magnetically bipolar
streamer emanating radially along the heliomagnetic equatorial plane,
and unipolar streamer emanating non-radially from the magnetically
high latitude regions.
Title: Multi-wavelength Observations of a Moreton Wave on 2000 March 3
Authors: Narukage, N.; Morimoto, T.; Kitai, R.; Kurokawa, H.;
Shibata, K.
Bibcode: 2002aprm.conf..449N
Altcode:
Moreton waves are flare-associated waves observed to propagate across
the solar disk in Hα (Moreton, 1960). Such waves have been identified
as the intersections of a coronal fast-mode shock fronts and the
chromosphere (Uchida, 1968). We report the observation of a Moreton
wave in Hα (line center and ± 0.8 Å) with the Flare Monitoring
Telescope (FMT) at the Hida Observatory of Kyoto University on 2000
March 3. The same region (NOAA 8882) was simultaneously observed in
soft X-rays with the soft X-ray Telescope (SXT) on board Yohkoh, and
a coronal wave-like disturbance (``X-ray wave") was also found. The
region (NOAA 8882) is near the solar limb. Hence the chromospheric
Moreton wave propagated on the solar disk at a speed of 1050 km/s,
whereas the coronal X-ray wave propagated towards the outer corona
at 1300 km/s. We identified the X-ray wave as an MHD fast-mode shock,
i.e. a coronal counterpart of the Moreton wave, using MHD shock theory
and the observed soft X-ray intensities (Narukage et al., 2002). On the
basis of this result, the propagation of these two waves indicates the
3-dimensional structure of the flare-associated shock wave. This event
is the first observation of the 3-d structure of the shock. Moreover, a
type II radio burst and a coronal mass ejection (CME) were also observed
simultaneously. The shock speed given by the type II radio burst is 1150
km/s. The CME propagated at a speed of 800 km/s. A basic component of
CME is a density enhancement, and the shock preceding the CME propagates
roughly 1.5 times faster than the CME, in this case at 1200 km/s.
Title: A Further Consideration of the Mechanism for EIT Waves
Authors: Chen, P. F.; Shibata, K.
Bibcode: 2002aprm.conf..421C
Altcode:
EIT waves are observed as blurry fronts spreading almost circularly with
enhanced coronal emission. It is found that they are reliable indicators
of the onset of coronal mass ejections, whereas their mechanism has been
a great puzzle. They are often considered as fast mode waves, though
their velocities are much smaller than the Alfven speed. To reconcile
the discrepancy, we proposed in a previous paper that EIT waves are not
real waves, but that they are formed by successive opening of closed
field lines, and the opening is transferred by wave groups. This model
indicates that EIT waves propagate three times slower than Moreton
waves, consistent with observational results. Based on this model, this
paper further illustrates that the EIT wave fronts are blurry in nature,
and their width is comparable with their distance from the flaring site.
Title: Magnetic reconnection in solar and stellar coronae
Authors: Shibata, K.
Bibcode: 2002cosp...34E3089S
Altcode: 2002cosp.meetE3089S
Recent numerical simulations of magnetic reconnection with heat
conduction and evaporation as a model of solar flares have revealed
that the flare peak temperature is given by the simple scaling law :
TB 7/6 L2/7 (Yokoyama and Shibata). Based on this scaling law, the
following relation was found between peak flare emission measure EM,
peak flare temperature T, preflare coronal density n0, and magnetic
field strength in flare/corona B: E MB -5 n0-3/2 T 17/2 . We argue
how these relations are tested with Yohkoh flare data, and discuss
possible application of these relations to estimate magnetic field
strength and flare loop length in stellar flares.
Title: Evolution of Flare Ribbons and Energy Release
Authors: Asai, A.; Masuda, S.; Yokoyama, T.; Shimojo, M.; Ishii,
T. T.; Isobe, H.; Shibata, K.; Kurokawa, H.
Bibcode: 2002aprm.conf..415A
Altcode:
We estimated the released magnetic energy via magnetic reconnection
in the corona by using photospheric and chromospheric features. We
observed an X2.3 flare, which occurred in active region NOAA9415 on 2001
April 10, in Hα with the Sartorius Telescope at Kwasan Observatory,
Kyoto University. Comparing the Hα images with the hard X-ray (HXR)
images obtained with Yohkoh/HXT, we see only two HXR sources which
are accompanied by Hα kernels. At these Hα kernels the large energy
release is thought to be larger than at other Hα kernels. We estimated
the energy release rates at each Hα kernel by using the photospheric
magnetic field strength and the separation speed of the Hα flare
ribbons at the same location. The estimated energy release rates at
the Hα kernels associated with the HXR sources are locally large
enough to explain the different appearance. Their temporal evolution
also shows peaks corresponding to HXR bursts.
Title: Numerical simulations of solar eruption and the role of
magnetic helicity
Authors: Shibata, K.
Bibcode: 2002cosp...34E3012S
Altcode: 2002cosp.meetE3012S
Recent solar observations suggested that the emergence of twisted
magnetic flux tube and associated dynamics is essential for generation
of strong flare activitiy. Since the emergence of twisted flux tubes is
an important way to inject magnetic helicity into the solar atmosphere,
there is no doubt that magnetic helicity plays an important role for
generation of strong flare activity. However, detailed physical proceses
have not been clarified yet on how magnetic helicity is related to
the trigger and occurrence of solar flares. We will discuss these
questions using numerical simulation results.
Title: Effects of Convective Sweeping on Photospheric Magnetic
Reconnection
Authors: Takeuchi, A.; Shibata, K.
Bibcode: 2002aprm.conf..467T
Altcode:
We investigate effects of convective sweeping on photospheric magnetic
reconnection, performing 2-dimensional magnetohydrodynamic (MHD)
numerical simulations. The gravitational stratification is taken
into account in our calculations. The reconnection is induced by an
encounter of oppositely directed vertical magnetic flux sheets in the
convective downflow region. The initial flux sheets are calculated
adopting the thin flux tube approximation. The convection is modelled
using artificial circular motions in a vertical plane. Beneath the
photosphere we assume a layer having the adiabatic temperature gradient
to maintain the circular convective motions. We adopt a resistivity
model in which the resistivity has a maximum at the photosphere (where
the magnetic Reynolds number = 1000) and decreases with distance
from the photosphere. Owing to the resistivity and the convection,
magnetic reconnection occurs around the photosphere. A wide parameter
range is covered in our calculations, where the parameters are the
magnetic field strength in the flux sheets and the velocity of the
convection. It is shown that the reconnection rate is proportional
to the ratio of the kinetic energy density of the convection to the
magnetic energy density in the flux sheets.
Title: The Effect of Resistivity in Magnetically Driven Mass Accretion
Authors: Kuwabara, T.; Shibata, K.; Kudoh, T.; Matsumoto, R.
Bibcode: 2002aprm.conf..341K
Altcode:
We present the results of 2.5 dimensional global magnetohydrodynamic
(MHD) simulations of an accretion disk initially threaded by
poloidal magnetic fields including the effects of magnetic turbulent
diffusivity. We successfully attained a quasi-steady state by
time-dependent computer simulations (Kuwabara 2000). We compared
these numerical results with analytical solutions obtained by Kaburaki
(2000) for axisymmetric steady-state flows for geometrically thin
accretion disks threaded by poloidal external magnetic fields. The
radial structure of the numerically obtained quasi-steady disk coincides
well with the analytical solution. On the other hand, we found
that the major driving force of jet ejection from disk depends on the
strength of magnetic diffusivity. When the magnetic diffusivity is small
(Rm>2.2), the mass outflow takes place intermittently. In
the mildly diffusive case, the mass outflow rate approaches a constant
value. In a highly diffusive disk (Rm<1.0), no outflow
takes place. In each case, we investigated the acceleration
mechanism of jets by putting Lagrangian test particles near the disk
surface and extracted forces acting on these particles. We found
that in mildly diffusive disks, the pressure gradient force is the major
driving force which turns the accretion flow into outflows. In weakly
diffusive models, the magneto-centrifugal acceleration is essential.
Title: Rayleigh-Taylor Instability Induced by the Collision between
a Magnetic Reconnection Jet and a Magnetic Loop
Authors: Tanuma, S.; Shibata, K.
Bibcode: 2002aprm.conf..469T
Altcode:
The solar corona as well as the interstellar medium are filled with hot
plasma and high energy particles. In the solar flare called ``impulsive
flare'', the hard X-ray emission is observed at the flare loop top and
foot points. In this paper, we propose a model for the origin of high
energy particles in an impulsive flare, by performing 3D resistive
MHD simulations of magnetic reconnection triggered by the secondary
tearing instability under simple assumptions. We assume the current
sheet between a uniform magnetic field and an anti-parallel one,
and perturbed the sheet. As a result, the fast magnetic reconnection
occurs at the current sheet, and heats the gas by releasing magnetic
energy. The Rayleigh-Taylor-like instability occurs at the reconnection
jet, when the reconnection jet collides with both the magnetic loop and
high pressure gas created by the magnetic reconnection. The helical and
turbulent magnetic field can be created in the magnetic loop, because
of the Rayleigh-Taylor instability. In the solar atmosphere, especially
at the loop top of an impulsive flare, high energy particles created by
the reconnection may be confined by this magnetic field. This process is
also applicable to the origin of cosmic rays in the interstellar medium.
Title: Heating Rate of Coronal Active Regions
Authors: Yashiro, S.; Shibata, K.
Bibcode: 2002mwoc.conf...43Y
Altcode:
We study the relation between thermal and magnetic properties of active
regions in the corona observed with the Soft X-ray Telescope aboard
Yohkoh. We derive the mean temperature and pressure of 64 mature
active region using the filter-ratio technique, and examine the
relationship of region size with temperature and pressure. We find
that the temperature T of active regions increases with increasing
region size L as T ~ L0.28, while the pressure P slightly
decreases with the region size as P ~ L-0.16. We confirm
the scaling law T ~ (P cdot L)1/3 for mature active regions
found by Rosner et al. We examined the magnetic properties of active
regions by analyzing 31 active regions observed with SOHO/MDI, and
find the following empirical scaling law between thermal and magnetic
properties, Uth ~ Phi1.33, P ~ B0.78,
where Uth, Phi, and B are total thermal energy content,
total magnetic flux, and average magnetic flux density of active
regions, respectively. The former is consistent with the results of
Golub1980 et al., but the latter is not. Implications of our findings
for coronal heating mechanisms are discussed.
Title: Multi-Wavelength Observation of A Moreton Wave on November
3, 1997
Authors: Narukage, N.; Shibata, K.; Hudson, H. S.; Eto, S.; Isobe,
H.; Asai, A.; Morimoto, T.; Kozu, H.; Ishii, T. T.; Akiyama, S.;
Kitai, R.; Kurokawa, H.
Bibcode: 2002mwoc.conf..295N
Altcode:
No abstract at ADS
Title: Timing and Occurrence Rate of X-Ray Plasma Ejections
Authors: Ohyama, M.; Shibata, K.
Bibcode: 2002mwoc.conf..297O
Altcode:
We examined 126 limb flares between October 1991 and August 1998. X-ray
plasma ejections are found in 54 flares. All X-ray plasmoids are
detected in images taken before the maximum peak of hard X-ray (HXR)
emission or in each first image after the HXR peak. If we choose 57
flares which soft X-ray telescope aboard Yohkoh started to observe
before the HXR peak, X-ray plasma ejections are found in about 63-70% of
these flares. We find X-ray plasma ejections in 100% of X-class flares
and 74-82% of M-class flares, whereas only 31-38% of C-class flares
have X-ray plasma ejections. It is difficult to detect X-ray plasma
ejections in C-class flares, because the scale size is short and the
lifetime of ejections is short. We propose that solar flares including
microflares occur through magnetic reconnection, and that X-ray plasma
ejections are general phenomena associated with solar flares.
Title: Coronal mass ejections and emerging flux
Authors: Chen, P. F.; Fang, C.; Shibata, K.; Tang, Y. H.
Bibcode: 2002AdSpR..30..535C
Altcode:
This paper reviews our recent progress in the numerical study of
coronal mass ejections (CMEs) based on flux rope model, which shows
that when the reconnection-favored emerging flux appears either within
or on the outer edge of the filament channel, the flux rope would lose
its equilibrium, and be ejected, while a current sheet is formed below
the flux rope. For the case with emergence within the filament channel,
even small flux is enough to trigger the loss of equilibrium, however,
there is a threshold for the emerging flux on the outer edge of the
filament channel. Given that anomalous resistivity sets in (e.g. when
the current density exceeds a critical value), fast reconnection is
resulted in, leading to fast eruption of the flux rope and localized
flare (either impulsive-type or LDE-type depending on the height of
the reconnection point) near the solar surface. The numerical results
can well explain why CMEs are not centered on flares and provide hints
for CME-flare spatial and temporal relationships.
Title: Statistical Study of the Reconnection Rate in Solar Flares
Authors: Isobe, H.; Morimoto, T.; Eto, S.; Narukage, N.; Shibata, K.
Bibcode: 2002mwoc.conf..171I
Altcode:
The soft X-ray telescope (SXT) aboard Yohkoh has established that the
driving mechanism of solar flares is magnetic reconnection. However,
the physics of reconnection has not been clarified. One of the
current puzzles is: what determines the reconnection rateNULL The
reconnection rate is defined as reconnected magnetic flux per unit time
or equivalently the ratio of inflow speed into reconnection point to
Alfven velocity in non-dimension, and is one of the most important
physical quantities in reconnection physics. However, observations
have not yet succeeded to statistically determine the reconnection rate
because direct observation of reconnection inflow and coronal magnetic
field is difficult. In this poster we present a method to determine
the reconnection rate from observational data, which use the following
relations: H = frac B2 4pi vinL2
vinB = vfootBfoot. Here H, L,
vfoot and Bfoot are respectively the flare
heating rate, size of the flare arcade, separation velocity of the two
ribbon, and magnetic field strength of the foot points. Since these four
quantities can be obtained from observational data, the relations above
give the inflow velocity vin and coronal magnetic field B,
and thus the reconnection rate can be determined. Appling this method
to many flare observations, we will determine the reconnection rate
in solar flares statistically. A preliminary result is presented.
Title: Fine Structure inside Flare Ribbons and Temporal Evolution
Authors: Asai, A.; Masuda, S.; Yokoyama, T.; Shimojo, M.; Kurokawa,
H.; Shibata, K.; Ishii, T. T.; Kitai, R.; Isobe, H.; Yaji, K.
Bibcode: 2002mwoc.conf..221A
Altcode:
Non-thermal particles generated in the impulsive phase of
solar flares are observed mainly in microwave, hard X-rays, and
gamma-rays. Observations in Hα can also give important informations
about non-thermal particles precipitating into the chromosphere with
a higher spatial resolution than in other wavelengths. We observed an
X2.3 flare which occurred in the active region NOAA 9415 on 10 April
2001, in Hα with Sartorius Telescope at Kwasan Observatory, Kyoto
University. Thanks to the short exposure time given for the flare, the
Hα images show fine structures inside the flare ribbons. In addition
to Hα, we analyze microwave, hard X-ray, and EUV data obtained with
Nobeyama Radioheliograph, Yohkoh/HXT, and TRACE, respectively. In Hα,
several bright kernels are observed in the flare ribbons. On the other
hand, the hard X-ray images show only a single pair of bright sources
which correspond to one of several pairs of Hα kernels. Examining
the difference in the magnetic field strength and in the time profiles
of Hα emission for these kernels, we discuss the reason why only one
pair kernels are bright in the hard X-ray among the other bright Hα
kernels. Comparing the Hα images with EUV images, we also examine
the three-dimensional structure of solar flares. While broad and
network-like ribbons are observed in Hα, the width of EUV ribbons is
relatively narrow, and EUV ribbons are located at the outer edges of
the corresponding Hα ribbon.
Title: 3D Structure of A Magnetic Reconnection Jet: Application to
Looptop Hard X-Ray Emission
Authors: Tanuma, S.; Yokoyama, T.; Kudoh, T.; Shibata, K.
Bibcode: 2002mwoc.conf..177T
Altcode:
We examine the magnetic reconnection triggered by a shock wave
generated by a point explosion, by performing two-dimensional(2D)
resistive magnetohydrodynamic(MHD) numerical simulations with high
spatial resolution, and three-dimensional(3D) simulations with low
spatial resolution. We found that the magnetic reconnection starts long
after the shock wave (fast-mode MHD shock) passes a current sheet. In
2D models, the current sheet evolves as follows: (i) Tearing-mode
instability is excited by the shock wave, and the current sheet
becomes thin in its nonlinear stage. (ii) The current-sheet thinning is
saturated when the current-sheet thickness becomes comparable to that
of Sweet-Parker current sheet. After that, Sweet-Parker type (slow)
reconnection starts, and the current-sheet length increases. (iii)
``Secondary tearing-mode instability'' occurs in the thin Sweet-Parker
current sheet. (iv) As a result, further current-sheet thinning occurs
and anomalous resistivity sets in, because gas the density decreases
in the current sheet just after the plasmoid is ejected. Petschek type
(fast) reconnection starts and heats the gas. The magnetic energy is
released quickly while the magnetic islands are moving in the current
sheet during Petschek type reconnection. The released magnetic energy
is determined by the magnetic field strength, not energy of initial
explosion, nor distance between the explosion and current sheet
(i.e., the initial explosion is only a perturbation). The results such
as reconnection rate are not different between 3D and 2D models. In
actual Sun, the magnetic Reynolds number is much larger than the value
in the numerical simulations so that Petschek type reconnection will
occur after the multiple tearing instability. We suggest that the
multiple tearing instability is a possible mechanism to trigger the
fast reconnection.
Title: Evolution of flare ribbons and energy release
Authors: Asai, A.; Masuda, S.; Yokoyama, T.; Shimojo, M.; Kurokawa,
H.; Ishii, T.; Shibata, K.
Bibcode: 2002cosp...34E1179A
Altcode: 2002cosp.meetE1179A
Non-thermal particles generated in the impulsive phase of
solar flares are observed mainly in microwave, hard X-rays, and g
amma-rays. Observations in Halpha can also give important informations
about non-thermal particles precipitating into the chromosphere with a
higher spatial resolution than in other wavelengths. We observed an X2.3
flare which occurred in the active region NOAA 9415 on 10 April 2001,
in Halpha with the Sartorius Telescope at Kwasan Observatory, Kyoto
University. Thanks to the short exposure time given for the flare, the
Halpha images show fine structures inside the flare ribbons. In Halpha,
several bright kernels are observed in the flare ribbons. On the other
hand, the hard X-ray images show only a single pair of bright sources
which correspond to one of several pairs of Halpha kernels. In this
paper, we examined the magnetic field strength at each H kernel and
the separation speed of the H flare ribbons, and estimated the energy
release rate with the method based on the magnetic reconnection model
(Isobe et al. 2002). We found that the energy release rate is well
correlated with the time profiles of hard X-ray and microwave.
Title: General Relativistic Simulation of Jet Formation by a
Rapidly-Rotating Black Hole
Authors: Koide, S.; Meier, D. L.; Shibata, K.; Kudoh, T.
Bibcode: 2002ASPC..255..251K
Altcode: 2002moag.conf..251K
No abstract at ADS
Title: Hydrodynamic Modeling of a Protostellar Flare Loop Connecting
a Central Star with its Accretion Disk
Authors: Isobe, H.; Shibata, K.; Yokoyama, T.; Imanishi, K.
Bibcode: 2002aprm.conf..433I
Altcode:
We present a hydrodynamical simulation of a protostellar flare loop
which connects a central star with its accretion disk. To reproduce
the observed X-ray light curves and spectra, the heat conduction and
resultant evaporation of the chromosphere of the central star and the
disk are included. We have found that in some cases all the plasma
in the accretion disk is heated to the flare temperature and spreads
over the flare loop. The condition for this ``disk disappearance''
is examined in this paper. We also synthesized the expected X-ray
spectrum when we observe the simulation result with ASCA/GIS.
Title: Simulation of Jet Formation from Magnetized Accretion Disk
Around Kerr Black Hole
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro; Meier,
David L.
Bibcode: 2002luml.conf..456K
Altcode:
We present a general relativistic magnetohydrodynamic simulation of
jet formation from accretion disk around Kerr black hole.
Title: Mechanism of coronal mass ejections triggered by emerging flux
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2002HiA....12..394C
Altcode:
No abstract at ADS
Title: Plasma astrophysics
Authors: Tajima, T.; Shibata, K.
Bibcode: 2002plap.book.....T
Altcode:
No abstract at ADS
Title: MHD Jets, Flares, and Gamma Ray Bursts
Authors: Shibata, Kazunari; Aoki, Seiichiro
Bibcode: 2002bjgr.conf..160S
Altcode:
No abstract at ADS
Title: Simultaneous observations of Moreton waves in Hα and Soft
X-ray
Authors: Narukage, N.; Hudson, H.; Morimoto, T.; Kitai, R.; Kurokawa,
H.; Shibata, K.
Bibcode: 2002cosp...34E1337N
Altcode: 2002cosp.meetE1337N
Moreton waves are flare-associated waves observed to propagate
across the solar disk in H (Moreton, 1960). Such waves have been
identified as the intersections of a coronal fast-mode shock fronts
and the chromosphere (Uchida, 1967). We report the two observations of
Moreton waves in H (line center and +/- 0.8 A) with the Flare Monitoring
Telescope (FMT) at the Hida Observatory of Kyoto University. The both
events were simultaneously observed in soft X-rays with the Soft X-ray
Telescope (SXT) on board Yohkoh, and wave-like disturbances ("X-ray
wave") were also found. One event occurred in solar-disk on November 3,
1997, the other near solar limb on March 3, 2000. Assuming that the
X-ray waves are the MHD fast shocks, we can estimate the propagation
speeds of the shocks, based on the MHD shock theory and the observed
soft X-ray intensities ahead and behind the X-ray wave fronts. It is
found that the estimated fast shock speeds are in rough agreement with
the observed propagation speeds of the X-ray waves. The fast mode Mach
numbers of the X-ray waves are also estimated. These results suggest
that the X-ray waves are MHD fast shocks propagating through the corona
and hence are the coronal counterparts of the Moreton waves.
Title: Observations of Moreton Waves and EIT Waves
Authors: Shibata, K.; Eto, S.; Narukage, N.; Isobe, H.; Morimoto,
T.; Kozu, H.; Asai, A.; Ishii, T.; Akiyama, S.; Ueno, S.; Kitai, R.;
Kurokawa, H.; Yashiro, S.; Thompson, B. J.; Wang, T.; Hudson, H. S.
Bibcode: 2002mwoc.conf..279S
Altcode:
The Moreton wave is a flare-associated wave observed in H alpha, and
is now established to be a fast mode MHD shock emitted from the flare,
but the physical mechanism to create the wave is still puzzling. On
the other hand, the EIT wave is a newly discovered flare-associated
wave observed in EUV with the Extreme ultraviolet Imaging Telescope
(EIT) aboard SOHO, and in this case, not only its origin but also
its physical property are both puzzling. We study the relationship
of these two flare-associated waves, Moreton waves and EIT waves, by
analyzing 4 events observed on Nov. 3 and 4, 1997, Aug. 8, 1998, and
Mar. 3, 2000 (Narukage et al. 2001). The Moreton waves were observed
in Ha, Ha+0.8A and Ha-0.8A with the Flare Monitoring Telescope (FMT)
at the Hida Observatory of Kyoto University, while the EIT waves were
observed with SOHO/EIT. In the typical case associated with an X-class
flare in AR 8100 on 4 November 1997 (Eto et al. 2001) the propagation
speeds of the Moreton wave and the EIT wave were approximately 780
km/s and 200 km/s respectively. The data on speed and location show
clearly that the Moreton wave differs physically from the EIT wave in
this case. The detailed analyses of the other events (Nov. 3, 1997,
Aug. 8, 1998, and Mar. 3, 2000) will also be presented, with Yohkoh/SXT
data in the lucky case.
Title: GRIPS Solar Experiments Intercomparison Project: Initial
Results
Authors: Kodera, K.; Matthes, K.; Haigh, J.; Shindell, D.; Shibata,
K.; Langematz, U.; Rozanov, E.; Kuroda, Y.
Bibcode: 2002cosp...34E.476K
Altcode: 2002cosp.meetE.476K
The GRIPS solar intercomparison project presented here is part of the
"GCM Reality Intercomparison Project for SPARC (GRIPS)" focusing on the
influence of 11-year solar cycle variations on the atmosphere. Each of
the different GCMs uses the same wavelength-dependent solar irradiance
changes as well as the resulting ozone changes calculated with 2-D
chemical models enabling a better intercomparison of the different
GCMs. Five model simulations give a similar response to solar forcing in
temperature field in the low latitudes of the stratosphere. However,
large differences are found among the models in the winter polar
temperatures, due to the difference in dynamical responses in each
model. The nature and the origin of the difference in the dynamical
response will be discussed in the presentation.
Title: Properties of Magnetic Reconnection in a Stratified Atmosphere
Authors: Takeuchi, A.; Shibata, K.
Bibcode: 2002mwoc.conf..205T
Altcode:
We investigate properties of magnetic reconnection in a stratified
atmosphere such as the solar photosphere, performing 2--dimensional
magnetohydrodynamic (MHD) numerical simulations. The reconnection
is caused by an encounter of oppositely directed vertical magnetic
flux sheets, where the initial sheets are calculated assuming the
thin flux tube approximation. We adopt a resistivity model in which
the resistivity is described as a function of height with a maximum
(where the magnetic Reynolds number = 2000) at a middle height of our
simulation box, imitating the solar resistivity distribution which
possesses a maximum at the temperature--minimum region. Owing to the
resistivity, magnetic reconnection occurs at the middle and evolves
into reconnection with slow mode MHD standing shock waves like the
Petschek type reconnection. It is found that the velocity of the
upward reconnection jet is faster than that of the downward jet. We
next investigate mass, momentum, and energy carried by the outflow from
the reconnection region. In these variables, however, the differences
between the upward and the downward quantities are negligibly small
in spite of the stratification. The inflow speed to the reconnection
region (vi ~ 160 m/s) is nearly equal to the speeds implied
by observations of canceling magnetic features on the photosphere. Thus
photospheric magnetic reconnection seems to be a cancellation mechanism.
Title: Three-Dimensional Numerical Magnetohydrodynamic Simulations
of Magnetic Reconnection in the Interstellar Medium
Authors: Tanuma, Syuniti; Yokoyama, Takaaki; Kudoh, Takahiro; Shibata,
Kazunari
Bibcode: 2001JKAS...34..309T
Altcode:
Strong thermal X-ray emission, called Galactic Ridge X-ray Emission,
is observed along the Galactic plane (Koyama et al. 1986). The
origin of hot ( 7 keV) component of GRXE is not known, while cool
( 0.8$ keV) one is associated with supernovae (Kaneda et al. 1997,
Sugizaki et al. 2001). We propose a possible mechanism to explain
the origin; locally strong magnetic fields of B_local 30 micro Gauss
heat interstellar gas to 7 keV via magnetic reconnection (Tanuma et
al. 1999). There will be the small-scale (<10 pc) strong magnetic
fields, which can be observed as _obs 3 micro Gauss by integration of
Faraday Rotation Measure, if it is localized by a volume filling factor
of f 0.1. In order to examine this model, we solved three-dimensional
(3D) resistive magnetohydrodynamic (MHD) equations numerically to
examine the magnetic reconnection triggered by a supernova shock
(fig.1). We assume that the magnetic field is B_x=30 tanh(y/20 pc)
micro Gauss, B_y=B_z=0, and the temperature is uniform, at the initial
condition. We put a supernova explosion outside the current sheet. The
supernova-shock, as a result, triggers the magnetic reconnection,
and the gas is heatd to >7 keV. The magnetic reconnection heats the
interstellar gas to 7 keV in the Galactic plane, if it occurs in the
locally strong magnetic fields of B_local 30 micro Gauss. The heated
plasma is confined by the magnetic field for 1E+5.5 yr. The required
interval of the magnetic reconnections (triggered by anything) is 1-10
yr. The magnetic reconnection will explain the origin of X-rays from the
Galactic ridge, furthermore the Galactic halo, and clusters of galaxies.
Title: Numerical Method for General Relativistic Magnetohydrodynamics
in Kerr Space-Time
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro; Meier,
David L.
Bibcode: 2001JKAS...34..215K
Altcode:
We present the whole basis of numerical method and useful formulae for
general relativistic magnetohydrodynamic simulations in Kerr space-time.
Title: Numerical Simulation of a Protostar Flare Loop between the
Core and Disk
Authors: Isobe, Hiroaki; Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 2001JKAS...34..337I
Altcode:
One-dimensional hydrodynamic modeling of a protostellar flare loop is
presented. The model consists of thermally isolated loop connecting the
central core and the accretion disk. We found that the conductive heat
flux of a flare heated the accretion disk up to coronal temperature
and consequently the disk is evaporated and disappeard. This effect
may explain the ovserved feature of the repeated flare from the young
stellar object YLW 15.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Sol,
H.; Hughes, J. P.
Bibcode: 2001AAS...19913204N
Altcode: 2001BAAS...33.1498N
We have investigated the dynamics of an accretion disk around
Schwarzschild black holes initially threaded by a uniform poloidal
magnetic field in a non-rotating corona (either in a steady-state
infalling state) around a non-rotating black hole using a 3-D GRMHD with
the ``axisymmetry'' along the z-direction. Magnetic field is tightly
twisted by the rotation of the disk, and plasmas in the shocked region
of the disk are accelerated by J x B force to form bipolar relativistic
jets. In order to investigate variabilities of generated relativistic
jets and magnetic field structure inside jets, we have performed
calculations using the 3-D GRMHD code with a full 3-dimensional system
without the axisymmetry. We have investigated how the third dimension
affects the global disk dynamics and jet generation. We will perform
simulations with various incoming flows from an accompanying star.
Title: Metric Radio Emission Associated with X-Ray Plasmoid Ejections
Authors: Kundu, M. R.; Nindos, A.; Vilmer, N.; Klein, K. -L.; Shibata,
K.; Ohyama, M.
Bibcode: 2001ApJ...559..443K
Altcode:
In this paper we report the first detection of metric/decimetric
radio emission associated with two soft X-ray plasmoid ejecta events
that occurred during two limb flares observed by the Yohkoh SXT. In
the first event a loop started to rise slowly (~10 km s-1)
before the beginning of the hard X-ray impulsive phase of the flare. At
about the onset of the impulsive flare, there was acceleration of the
ejecta, resulting in a speed of 130 km s-1 and finally to
~200 km s-1. The associated radio emission was observed with
the Nançay radioheliograph (NRH) in the frequency range of 230-450
MHz. It was an unpolarized continuum that lasted 8-10 minutes. The 410
MHz source was located close to the height where the plasmoid was last
identified in the SXT images. In the second event an eruption resulted
in the expansion of a large-scale, looplike feature and the development
of two plasmoid ejecta which moved in different directions. The speed of
the ejecta was 60-100 km s-1. In this event, the associated
radio emission was a long-lasting (about 2 hr) continuum observed from
450 to 164 MHz. The onset of the low-frequency emission was delayed
with respect to the onset of the high-frequency emission. In both
cases the radio sources were located above the soft X-ray ejecta in
the general direction of the prolongation of the ejecta movement. In
both cases the radio emission comes from nonthermal electrons which
are accelerated in close relationship with the propagation of the X-ray
plasmoid: as the plasmoid reaches higher altitudes, it interacts with
increasingly more extended magnetic field lines and new coronal sites
of production of nonthermal electrons are created.
Title: What Determines the Height of Spicules? I. Alfvén-Wave Model
and Slow-Wave Model
Authors: Saito, Takuya; Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 2001ApJ...554.1151S
Altcode:
We perform numerical simulations for torsional Alfvén waves and
slow waves propagating along an open magnetic flux tube in the solar
atmosphere to elucidate the mechanisms of spicule formation and coronal
heating. We introduce random motions of about 1 km s-1 in
the photosphere as the source of Alfvén and slow waves, and solve the
1.5-dimensional magnetohydrodynamic equations. The waves generated
by the random motions propagate upward and lift up the transition
region. The chromospheric plasma just below the transition region is
thought to be observed as a spicule. We investigate the effect of
the initial height of the transition region, or transition-region
pressure, on spicule formation. Our results agree well with the
observational fact that spicules are absent over plages, where the
transition-region pressure is high, and tall under coronal holes,
where the transition-region pressure is low. We also show that the
dependence of spicule height on the initial transition-region height
(or pressure) is well described by the theoretical relation, which is
for the slow-wave acceleration of spicules even though we input only
Alfvén waves in the photosphere. Although both fast and slow waves are
excited by the nonlinear coupling of Alfvén waves, our results suggest
that the slow waves play a more fundamental role in the generation
of spicules. How much energy flux is transported to the corona is
also estimated. A part of the energy flux carried by the waves that
generate spicules propagates into the corona and contributes to the
heating of the corona. Alfvén waves can transport enough energy flux
to heat the quiet corona, but slow waves cannot.
Title: Global destabilization due to localized reconnection: A
mechanism for coronal mass ejections
Authors: Chen, P. F.; Shibata, K.; Yokoyama, T.
Bibcode: 2001EP&S...53..611C
Altcode:
Solar CMEs are large scale eruptive phenomena, while flux emergence
is a local event on the Sun. Our numerical simulations show that
two categories of reconnection-favored emerging flux can trigger
the destabilization and the ejection of the filament (i.e., CME):
within the filament channel or on the outer edge of the channel, which
confirms recent important observations by Feynman and Martin (1995). In
particular for the latter category, numerical results show that there
is a critical amount for the emerging flux, below which the flux rope
eruption cannot be triggered. Our numerical model, for the first time,
provides a physical explanation for the observed correlation between
CMEs and the reconnection-favored emerging flux.
Title: Plasmoid-induced-reconnection and fractal reconnection
Authors: Shibata, Kazunari; Tanuma, Syuniti
Bibcode: 2001EP&S...53..473S
Altcode: 2001astro.ph..1008S
As a key to understanding the basic mechanism for fast reconnection in
solar flares, plasmoid-induced-reconnection and fractal reconnection
are proposed and examined. We first briefly summarize recent
solar observations that give us hints on the role of plasmoid
(flux rope) ejections in flare energy release. We then discuss
the plasmoid-induced-reconnection model, which is an extention of
the classical two-ribbon-flare model which we refer to as the CSHKP
model. An essential ingredient of the new model is the formation and
ejection of a plasmoid which play an essential role in the storage of
magnetic energy (by inhibiting reconnection) and the induction of a
strong inflow into reconnection region. Using a simple analytical model,
we show that the plasmoid ejection and acceleration are closely coupled
with the reconnection process, leading to a nonlinear instability
for the whole dynamics that determines the macroscopic reconnection
rate uniquely. Next we show that the current sheet tends to have a
fractal structure via the following process path: tearing ⇒ sheet
thinning ⇒ Sweet-Parker sheet ⇒ secondary tearing ⇒ further
sheet thinning ⇒ ⋯. These processes occur repeatedly at smaller
scales until a microscopic plasma scale (either the ion Larmor radius
or the ion inertial length) is reached where anomalous resistivity or
collisionless reconnection can occur. The current sheet eventually has
a fractal structure with many plasmoids (magnetic islands) of different
sizes. When these plasmoids are ejected out of the current sheets,
fast reconnection occurs at various different scales in a highly time
dependent manner. Finally, a scenario is presented for fast reconnection
in the solar corona on the basis of above plasmoid-induced-reconnection
in a fractal current sheet.
Title: Preface
Authors: Hoshino, M.; Stenzel, R. L.; Shibata, K.
Bibcode: 2001EP&S...53..409H
Altcode:
No abstract at ADS
Title: Solar photospheric magnetic reconnection
Authors: Takeuchi, Akitsugu; Shibata, Kazunari
Bibcode: 2001EP&S...53..605T
Altcode:
We investigate photospheric magnetic reconnection due to an encounter
of oppositely directed vertical magnetic flux sheets, performing
2.5-dimensional magnetohydrodynamic (MHD) numerical simulations. We
construct the initial flux sheets adopting the thin flux tube
approximation. Since actual solar resistivity possesses a maximum at
the temperature-minimum region, we adopt a resistivity model in which
the resistivity is described as a function of height with a maximum
(where the magnetic Reynolds number = 1000) at a middle height of
our simulation box. Owing to the resistivity, the Sweet-Parker type
reconnection occurs at the middle. The inflow speed (υ i
∼ 160 m/s) is nearly equal to the speeds implied by observations
of canceling magnetic features on the photosphere. Thus photospheric
reconnection seems to be a cancellation mechanism. It is shown that
upward propagating MHD slow mode waves are generated by an upward
reconnection jet. Moreover, when we incline the initial field lines
30° from the vertical direction in the other flux sheet, Alfvén waves
are also generated as a result of the reconnection. The energy flux
carried by the slow modes and Alfvén waves are 1010 and
108 erg/cm2/s, respectively, and the durations are
40 s. Since in models of solar spicules upward propagating slow waves
or Alfvén waves are usually assumed as the initial perturbations, we
compare the energy of both waves. It is found that the wave energies
due to the reconnection are comparable to those assumed in spicule
models. Thus the photospheric magnetic reconnection might be one of
the causes of solar spicules.
Title: Two-dimensional Magnetohydrodynamic Numerical Simulations
of Magnetic Reconnection Triggered by a Supernova Shock in the
Interstellar Medium: Generation of X-Ray Gas in the Galaxy
Authors: Tanuma, Syuniti; Yokoyama, Takaaki; Kudoh, Takahiro; Shibata,
Kazunari
Bibcode: 2001ApJ...551..312T
Altcode: 2000astro.ph..9088T
We examine magnetic reconnection triggered by a supernova (or a point
explosion) in the interstellar medium by performing two-dimensional
resistive magnetohydrodynamic (MHD) numerical simulations with high
spatial resolution. We find that magnetic reconnection starts long
after a supernova shock (fast-mode MHD shock) passes a current sheet. A
current sheet evolves as follows: (1) Tearing-mode instability is
excited by the supernova shock, and in its nonlinear stage the current
sheet becomes thin. (2) The current-sheet thinning is saturated when the
current-sheet thickness becomes comparable to that of the Sweet-Parker
current sheet. After that, Sweet-Parker reconnection starts, and
the current-sheet length increases. (3) ``Secondary tearing-mode
instability'' occurs in the thin Sweet-Parker current sheet. (4) As a
result, further current-sheet thinning occurs and anomalous resistivity
sets in, because gas density decreases in the current sheet. Petschek
reconnection starts and heats the interstellar gas. Magnetic energy is
released quickly as magnetic islands move in the current sheet during
Petschek reconnection. The released magnetic energy is determined
by the interstellar magnetic field strength, not the energy of the
initial explosion or the distance to the explosion. We suggest that
magnetic reconnection is a possible mechanism to generate X-ray gas
in the Galaxy.
Title: One-dimensional and Pseudo-Two-dimensional Hydrodynamic
Simulations of Solar X-Ray Jets
Authors: Shimojo, Masumi; Shibata, Kazunari; Yokoyama, Takaaki;
Hori, Kuniko
Bibcode: 2001ApJ...550.1051S
Altcode:
We present results of one-dimensional hydrodynamic simulations
of the chromospheric evaporation produced by a microflare in a
large-scale loop as a model of X-ray jets. The initial conditions
of the simulations are based on the observations of X-ray jets. We
deposit thermal energy (~1×1028 ergs) in the corona. The
deposited energy is rapidly transported to the chromosphere by
conduction, which heats the dense plasma in the upper chromosphere. As
a result, the gas pressure is increased and drives a strong upflow of
dense, hot plasma along the magnetic loop. We found the following
features of evaporation in the results of our simulations: (1)
the maximum temperature of the evaporating plasma is determined by
the balance between the conductive flux and the heating flux; (2)
the total mass of evaporating plasma is controlled by the balance
between the conductive flux and enthalpy flux; (3) the relationship
between the density neva, height of energy deposition
sflare, and heating rate Fh is described as
neva~F4/7h/s3/7flare
(4) the X-ray intensity along the evaporation-flow plasma decreases
exponentially with distance from the footpoint, and that exponential
intensity distribution holds from the early phase to the decay phase;
(5) in the single-loop model, the temperature decreases with distance
from the energy deposition site (on the other hand, a hot region is
present in front of the evaporation front in the multiple-loop model);
(6) we compare the physical parameters of the evaporation flow with the
observations of the X-ray jet that occurred on 1992 September 3 and
find that the physical parameters of evaporating plasma are similar
to those of the Yohkoh-observed X-ray jet. Since these properties of
the evaporation flow are similar to the observed properties of X-ray
jets, we suggest that an X-ray jet is the evaporation flow produced
by a flare near the footpoint of a large-scale loop. Furthermore,
according to the X-ray intensity distribution along the evaporation
flow, we suggest that a multiple-loop model based on the magnetic
reconnection mechanism can reproduce the properties of an X-ray jet
better than the single-loop model.
Title: Fast Magnetic Reconnection in Free Space: Self-similar
Evolution Process
Authors: Nitta, S.; Tanuma, S.; Shibata, K.; Maezawa, K.
Bibcode: 2001ApJ...550.1119N
Altcode: 2000astro.ph.11343N
We present a new model for the time evolution of fast
magnetic reconnection in free space, which is characterized by
self-similarity. Reconnection triggered by locally enhanced resistivity
assumed at the center of the current sheet can self-similarly and
unlimitedly evolve until external factors affect the evolution. The
possibility and stability of this type of evolution are verified by
numerical simulations in a very wide spatial dynamic range. Actual
astrophysical reconnection in solar flares and geomagnetospheric
substorms can be treated as an evolutionary process in free space,
because the resultant scale is much larger than the initial scale. In
spite of this fact, most of the previous numerical works focused
on the evolutionary characters strongly affected by artificial
boundary conditions on the simulation boundary. Our new model
clarifies a realistic evolution for such cases. The characteristic
structure around the diffusion region is quite similar to the Petschek
model, which is characterized by a pair of slow-mode shocks and the
fast-mode rarefaction-dominated inflow. However, in the outer region,
a vortex-like return flow driven by the fast-mode compression caused by
the piston effect of the plasmoid takes place. The entire reconnection
system expands self-similarly.
Title: Relation between Thermal and Magnetic Properties of Active
Regions as a Probe of Coronal Heating Mechanisms
Authors: Yashiro, Seiji; Shibata, Kazunari
Bibcode: 2001ApJ...550L.113Y
Altcode:
We study the relation between thermal and magnetic properties of active
regions in the corona observed with the soft X-ray telescope aboard
Yohkoh. We derive the mean temperature and pressure of 64 mature
active regions using the filter ratio technique, and examine the
relationship of region size with temperature and pressure. We find
that the temperature T of active regions increases with increasing
region size L as T~L0.28, while the pressure P slightly
decreases with the region size as P~L-0.16. We confirm the
scaling law T~(PL)1/3 for mature active regions found by
R. Rosner, W. H. Tucker, & G. S. Vaiana. We examined the magnetic
properties of active regions by analyzing 31 active regions observed
with the Solar and Heliospheric Observatory/Michelson Doppler Imager and
find the following empirical scaling law between thermal and magnetic
properties,Uth~Φ1.33,P~B0.78,where
Uth, Φ, and B are the total thermal energy content, total
magnetic flux, and average magnetic flux density of active regions,
respectively. The former is consistent with the results of L. Golub et
al., but the latter is not. Implications of our findings for coronal
heating mechanisms are discussed.
Title: Magnetohydrodynamic Simulation of a Solar Flare with
Chromospheric Evaporation Effect Based on the Magnetic Reconnection
Model
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 2001ApJ...549.1160Y
Altcode:
Two-dimensional magnetohydrodynamic (MHD) simulation of a solar flare
including the effect of anisotropic heat conduction and chromospheric
evaporation based on the magnetic reconnection model is performed. In
the simulation model, the coronal magnetic energy is converted to the
thermal energy of plasma by magnetic reconnection. This energy is
transported to the chromosphere by heat conduction along magnetic
field lines and causes an increase in temperature and pressure
of the chromospheric plasma. The pressure gradient force drives
upward motion of the plasma toward the corona, i.e., chromospheric
evaporation. This enhances the density of the coronal reconnected
flare loops, and such evaporated plasma is considered to be the
source of the observed soft X-ray emission of a flare. The results
show that the temperature distribution is similar to the cusp-shaped
structure of long-duration-event (LDE) flares observed by the soft
X-ray telescope aboard the Yohkoh satellite. The simulation results are
understood by a simple scaling law for the flare temperature described
asTtop~(B3L2πκ0sqrt(4πρ))2/7
,where Ttop, B, ρ, and κ0 are the temperature
at the flare loop top, coronal magnetic field strength, coronal
density, and heat conduction coefficient, respectively. This
formula is confirmed by the extensive parameter survey about
B, κ0, and L in the simulation. The energy release
rate is found to be described as a linearly increasing function of time:
|dEm/dt|~B2/(4π)VinCAt~B2/(4π)0.1C2At,
where Em is the magnetic energy, Vin
is the inflow velocity, and CA is the Alfvén
velocity. Thus, the second time derivative is found to be
|d2Em/dt2|~B4. We also
find that the major feature of the reconnection inflow region is
the expansion wave propagating outward from the magnetic neutral
point. This expanded plasma has very low emission measure, which is
4 orders of magnitude smaller than that of the brightest feature in
a flare. This explains the dimming phenomena associated with flares.
Title: Three-Dimensional Numerical Magnetohydrodynamic Simulations
of Magnetic Reconnection as the Origin of X-ray Gas in the Galaxy
Authors: Tanuma, Syuniti; Yokoyama, Takaaki; Kudoh, Takahiro; Shibata,
Kazunari
Bibcode: 2001ASPC..251..320T
Altcode: 2001ncxa.conf..320T
No abstract at ADS
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Sol, H.
Bibcode: 2001ASPC..250...22N
Altcode: 2001pfrg.conf...22N
No abstract at ADS
Title: Magnetic Reconnection Induced by Convective Intensification
of Solar Photospheric Magnetic Fields
Authors: Takeuchi, Akitsugu; Shibata, Kazunari
Bibcode: 2001ApJ...546L..73T
Altcode:
We investigate photospheric magnetic reconnection induced by
convective intensificd ..ation of solar surface magnetic fields,
performing 2.5-dimensional magnetohydrodynamic (MHD) numerical
simulations. We assume as the initial conditions oppositely
directed vertical weak magnetic fields (Bz) penetrating
a convectively unstable layer. To initiate convective instability,
the unstable layer is perturbed by weak circular motions. As a
result of the growing convective motions, the oppositely directed
magnetic fields are swept up into the convective downflow region,
forming a current sheet between them. At the same time, the field
strength is increased to kilogauss range, and the current density in
the current sheet is strengthened. As a result, magnetic reconnection
is initiated. It is shown that upward-propagating slow-mode MHD waves
are generated by a collision of a reconnection jet with U-shaped field
lines. Moreover, when the initial magnetic fields are not parallel,
i.e., when there is a perpendicular component of the magnetic field
(By=Bz) in the other half of the flux sheet,
Alfvén waves are also generated as a result of reconnection. The
energy flux carried by the slow and Alfvén waves are 1010
and 109 ergs cm-2 s-1 and the durations
are 80 and 100 s, respectively, at the photosphere. The energies of
these waves are sufficient to produce solar spicules. Thus, photospheric
magnetic reconnection might be one of the causes of solar spicules.
Title: VERA (VLBI Exploration of Radio Astrometry) system (invited)
Authors: Kobayashi, H.; Sasao, T.; Kawaguchi, N.; Manabe, S.; Miyaji,
T.; Shibata, K.; Kameya, O.; Honma, M.; Tamura, Y.; Satou, K.; Kuji,
S.; Horiai, K.; Iwadate, K.; Imai, H.; Yokoyama, K.; Omodaka, T.;
Hirota, T.; Nishimo, M.; Kasuga, T.
Bibcode: 2001aprs.conf..243K
Altcode:
No abstract at ADS
Title: Three Dimensional MHD Simulations of Accretion Disks and
Jet Formation
Authors: Matsumoto, R.; Matsuzaki, T.; Tajima, T.; Shibata, K.
Bibcode: 2001ASSL..263..247M
Altcode: 2001nhcs.conf..247M
No abstract at ADS
Title: First results on the origin of universal correlation between
flare temperature and emission measure for solar and stellar flares
Authors: Shibata, K.; Yokoyama, T.
Bibcode: 2001AdSpR..26.1753S
Altcode:
We investigate the correlation between flare temperature T and emission
measure EM = n2V found by Feldman (1995) by extending the
temperature and emission measure range to include solar microflares and
protostellar flares. We found that the correlation is well represented
by the power-law relation, EM ∝ T a with power-law index a
⋍ 8.3 ± 1.0, where n is the electron density and V is the volume. We
then present a theory to explain the observed power-law relation
between T and EM on the basis of a magnetic reconnection model with
heat conduction and chromospheric evaporation, assuming that the gas
pressure of a flare loop is comparable to the magnetic pressure. This
theory predicts the relation ? cm -3, which explains well the
observed correlation in the range of 6 × 10 6 K < T <
10 8 K and 10 44 < EM < 10 55
cm -3 from solar microflares to protostellar flares observed
by ASCA. Here, B is the magnetic field strength, n0 is the
pre-flare proton (electron) number density.
Title: Origin of Universal Correlation between Temperature and
Emision Measure for Solar/Stellar Flares
Authors: Shibata, K.; Yokoyama, T.
Bibcode: 2001IAUS..203..318S
Altcode:
We present a theory to explain the observed universal correlation
between flare temperature T and emission measure EM = n2
V for solar and stellar flares (including solar microflares observed
by Yohkoh as well as protostellar flares observed by ASCA), where n
is the electron density and V is the volume. The theory is based on a
magnetic reconnection model with heat conduction and chromospheric
evaporation, assuming that the gas pressure of a flare loop is
comparable to the magnetic pressure. This theory predicts the relation
EM ∝ B-5 T17/2 which explains well the observed
correlation between EM and T in the range of 6 × 106 K <
T < 108 K and 1044 < EM < 1055
cm-3 from solar microflares to protostellar flares, if the
magnetic field strength of a flare loop, B, is nearly constant for
solar and stellar flares.
Title: An Emerging Flux Trigger Mechanism for Coronal Mass Ejections
Authors: Chen, P. F.; Shibata, K.
Bibcode: 2000ApJ...545..524C
Altcode:
Observations indicate that reconnection-favored emerging flux has
a strong correlation with coronal mass ejectons (CMEs). Motivated
by this observed correlation and based on the flux rope model, an
emerging flux trigger mechanism is proposed for the onset of CMEs,
using two-dimensional magnetohydrodynamic (MHD) numerical simulations:
when such emerging flux emerges within the filament channel, it cancels
the magnetic field below the flux rope, leading to the rise of the flux
rope (owing to loss of equilibrium) and the formation of a current sheet
below it. Similar global restructuring and a resulting rise motion
of the flux rope occur also when reconnection-favored emerging flux
appears on the outer edge of the filament channel. In either case, fast
magnetic reconnection in the current sheet below the flux rope induces
fast ejection of the flux rope (i.e., CME). It is also shown that the
nonreconnecting emerging flux, either within the filament channel or
on the outer edge of the channel, makes the flux rope move down, i.e.,
no CMEs can be triggered. Although the present two-dimensional model
can not provide many details of the largely unknown three-dimensional
processes associated with prominence eruptions, it shows some
observational features such as the height-time profile of erupting
prominences. Most importantly, our model can well explain the observed
correlation between CMEs and the reconnection-favored emerging flux.
Title: Resistive Magnetohydrodynamics of Jet Formation and
Magnetically Driven Accretion
Authors: Kuwabara, Takuhito; Shibata, Kazunari; Kudoh, Takahiro;
Matsumoto, Ryoji
Bibcode: 2000PASJ...52.1109K
Altcode: 2000astro.ph.11165K
We carried out 2.5-dimensional resistive magnetohydrodynamic simulations
to study the effects of magnetic diffusivity on magnetically driven
mass accretion and jet formation. The initial state is a constant
angular-momentum torus threaded by large-scale vertical magnetic
fields. Since the angular momentum of the torus is extracted due
to magnetic braking, the torus medium falls toward the central
region. The infalling matter twists the large-scale magnetic fields
and drives bipolar jets. We found that (1) when the normalized
magnetic diffusivity, bar ηequiv eta /(r_0 V_K0), where V_K0 is
the Keplerian rotation speed at a reference radius r=r_0, is small
(bar η<= 10-3), mass accretion and jet formation
take place intermittently; (2) when 10-3<=bar η<=
10-2, the system evolves toward a quasi-steady state; and
(3) when bar η>= 10-2 the accretion/mass outflow rate
decreases with bar η and approaches 0. The results of these simulations
indicate that in the center of a galaxy which has a super-massive
( ~ 109 MO ) black hole, a massive ( ~ 108
MO ) gas torus and magnetic braking provide a mass accretion rate
which is sufficient to explain the activity of AGNs when bar η<=
5times 10-2.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Sol,
H.; Hughes, J. P.
Bibcode: 2000AAS...197.7501N
Altcode: 2000BAAS...32.1521N
We have investigated the dynamics of an accretion disk around
Schwarzschild black holes initially threaded by a uniform poloidal
magnetic field in a non-rotating corona (either in a steady-state
infalling state) around a non-rotating black hole using a 3-D GRMHD
with the ``axisymmetry'' along the z-direction. Magnetic field is
tightly twisted by the rotation of the disk, and plasmas in the
shocked region of the disk are accelerated by J x B force to form
bipolar relativistic jets. In order to investigate variabilities of
generated relativistic jets and magnetic field structure inside jets,
we have performed calculations using the 3-D GRMHD code with a full
3-dimensional system. We will investigate how the third dimension
affects the global disk dynamics and jet generation.
Title: Solar X-ray Jets
Authors: Shibata, K.
Bibcode: 2000eaa..bookE2272S
Altcode:
X-ray jets have been discovered on the Sun by the soft x-ray telescope
aboard the Japanese YOHKOH satellite as transitory x-ray enhancements in
the solar CORONA with apparent collimated motion. They are associated
with small flares (microflares or subflares), which occur in x-ray
bright points (XBPs, see SOLAR CORONA: X-RAY BRIGHT POINTS), emerging
flux regions (EFRs, see CHROMOSPHERE: EMERGING F...
Title: Timing and occurrence rate of X-ray plasma ejections
Authors: Ohyama, M.; Shibata, K.
Bibcode: 2000JASTP..62.1509O
Altcode: 2000JATP...62.1509O
We examined 126 limb flares formed between October 1991 and August
1998. X-ray plasma ejections were found in 54 flares. All the X-ray
plasmoids were detected in images taken before the maximum peak
of hard X-ray (HXR) emission or in each of the first image after
the HXR peak. In our choice of 57 flares which Yohkoh started to
observe before the HXR peak, with the soft X-ray telescope aboard,
X-ray plasma ejections were found in /~63-70% of these flares. We
found X-ray plasma ejections in 100% of X-class flares and 74-82%
of M-class flares, whereas only 31-38% of C-class flares have X-ray
plasma ejections. It is difficult to detect X-ray plasma ejections
in C-class flares, because the scale size and lifetime of ejections
are short. We propose that solar flares including microflares occur
through magnetic reconnection and that X-ray plasma ejections are
general phenomena associated with solar flares.
Title: Surges
Authors: Shibata, K.
Bibcode: 2000eaa..bookE2053S
Altcode:
Surges are cool plasma jets ejected from small flare-like chromospheric
bright points, such as subflares or Ellerman bombs (moustaches)
near sunspots (see SOLAR CHROMOSPHERE: ELLERMAN BOMBS). They are a
kind of active prominences, usually observed in Hα at ground-based
observatories, although space observations (such as with EUV telescope)
also detect surges. Figure 1 shows a typical example of a...
Title: Physical Parameters of Solar X-Ray Jets
Authors: Shimojo, Masumi; Shibata, Kazunari
Bibcode: 2000ApJ...542.1100S
Altcode:
We derived the physical parameters of X-ray jets and associated flares
using the high-resolution data taken with the soft X-ray telescope
aboard Yohkoh. We analyzed 16 X-ray jets and found the following
properties of the jets and the footpoint flares: (1) the temperatures
and density of the jets, respectively, are 3-8 MK (average: 5.6 MK) and
0.7-4.0×109 cm-3 (average: 1.7×109
cm-3), (2) the temperatures of the jets are similar to
those of the footpoint flares, (3) the thermal energies of the jets are
1027-1029 ergs, which is 1/4 to 1/7 of those of
the footpoint flares, (4) the apparent velocity of the jets is usually
slower than the sound speed, and (5) there is a correlation between
the temperatures of the jets and the sizes (square root of area)
of the footpoint flares. On the basis of these results, we find that
the temperatures of a jet and a footpoint flare are determined by the
balance between heating flux and conductive flux and that the mass
of a jet is comparable to the theoretical value based on the balance
between conductive flux and enthalpy flux carried by the evaporation
flow. These results suggest that X-ray jets are evaporation flows
produced by the reconnection heating.
Title: Generalized Equivalence Principle in Extended New General
Relativity
Authors: Kawai, T.; Shibata, K.; Tanaka, I.
Bibcode: 2000PThPh.104..505K
Altcode: 2000gr.qc.....7025K
In extended new general relativity, which is formulated as a reduction
of a Poincaré gauge theory of gravity whose gauge group is the
covering group of the Poincaré group, we study the problem of
whether the total energy-momentum, total angular momentum and total
charge are equal to the corresponding quantities of the gravitational
source. We examine this problem for charged axi-symmetric solutions
of gravitational field equations. Our main concern is the restriction
on the asymptotic form of the gravitational field variables imposed
by the requirement that physical quantities of the total system are
equivalent to the corresponding quantities of the charged rotating
source body. This requirement can be regarded as an equivalence
principle in a generalized sense.
Title: A Unified Model of Coronal Mass Ejection-related Type II
Radio Bursts
Authors: Magara, Tetsuya; Chen, Pengfei; Shibata, Kazunari; Yokoyama,
Takaaki
Bibcode: 2000ApJ...538L.175M
Altcode:
We present a theoretical model for the shock formation that is related
to coronal and interplanetary type II radio bursts associated with
coronal mass ejections on the basis of the magnetic reconnection
model of eruptive solar flares. Coronal type II bursts are usually
observed in the metric wavelength range (metric type II bursts), and
interplanetary bursts are usually observed in the decametric-hectometric
wavelength range (decametric-hectometric bursts). Our research shows
that the decametric-hectometric type II radio bursts are produced by the
piston-driven fast-mode MHD shock that is formed in front of an eruptive
plasmoid (a magnetic island in the two-dimensional sense or a magnetic
flux rope in the three-dimensional sense), while the metric radio
bursts are produced by the reverse fast-mode MHD shock that is formed
through the collision of a strong reconnection jet with the bottom of
the plasmoid. This reverse shock apparently moves upward as long as the
reconnection jet is sufficiently strong and dies away when the energy
release of the reconnection stops or weakens significantly. On the other
hand, the piston-driven fast shock continues to exist when the plasmoid
moves upward. Our model succeeds in explaining the observational result
that the piston-driven fast shock that produces decametric-hectometric
type II bursts moves faster and survives longer than the other shock.
Title: Dynamics of the Parker-Jeans Instability in a Galactic
Gaseous Disk
Authors: Chou, Wenchien; Matsumoto, Ryoji; Tajima, Toshiki; Umekawa,
Michihisa; Shibata, Kazunari
Bibcode: 2000ApJ...538..710C
Altcode:
Linear analysis and nonlinear three-dimensional simulation of
magnetohydrodynamics of a gas layer are carried out in local corotating
coordinates, taking into account the effect of self-gravitational
force. The gas is subject to the Jeans instability when there is no
magnetic field, or to the Parker-Jeans instability when a magnetic
field is present. We study the evolution of these instabilities in
several cases which vary in factors such as the rotational speed,
strength of magnetic fields, external pressure, external gravitational
force, and directions of perturbations. We find that the growth rate
of the instabilities and the shape of the dense blobs that aggregate
in the nonlinear stage depend sensitively on these factors. Our result
shows that the Jeans instability is stabilized by fast rotation while
the Parker-Jeans mode may still be unstable. When the gas rotation is
negligible, our nonlinear simulations show that the gas may form dense
blobs or filaments that are perpendicular or parallel to the magnetic
field, depending on the strength of external pressure and the direction
of initial perturbations. When rotation is included, the gas forms
coherent long filaments with their major axes perpendicular to the
magnetic field. When we adopt typical parameters of nearby molecular
clouds, the separation of these filaments is about 5 pc, consistent
with the observation. This suggests that molecular clouds with a
coherent filamentary structure may be attributed to the Parker-Jeans
instability of a gaseous disk under the influence of rotation. This
study sheds light on how interstellar/intergalactic gas aggregates to
form molecular clouds and seeds of stars, and thus on the star-forming
process at its very early stage.
Title: HALCA: the radio telescope for the space VLBI mission
Authors: Murata, Y.; Hirabayashi, Hisashi; Kobayashi, Hideyuki;
Hirosawa, H.; Edwards, P.; Kii, Tsuneo; Asaki, Yoshiharu; Wajima,
K.; Inoue, M.; Miyaji, T.; Shibata, K.; Fujisawa, Kenta; Okayasu,
R.; Kawaguchi, Noriyuki; Kameno, S.
Bibcode: 2000SPIE.4015..204M
Altcode:
The Institute of Space and Astronautical Science (ISAS) launched
the first space VLBI (Very Long Baseline Interferometry) satellite,
HALCA, in February 1997. After completing a series of engineering
experiments to verify space-VLBI observations, the first VLBI fringes
and images were obtained in May and in June, respectively. HALCA has
now been operated for science observations at 1.6 and 5 GHz for the
VSOP (VLBI Space Observatory Programme) project in cooperation with
many organizations and radio telescopes around the world. In this
paper the current science activities of the mission are reviewed and
results presented.
Title: General Relativistic Simulations of Early Jet Formation in
a Rapidly Rotating Black Hole Magnetosphere
Authors: Koide, Shinji; Meier, David L.; Shibata, Kazunari; Kudoh,
Takahiro
Bibcode: 2000ApJ...536..668K
Altcode: 1999astro.ph..7435K
To investigate the formation mechanism of relativistic jets in
active galactic nuclei and microquasars, we have developed a new
general relativistic magnetohydrodynamic code in Kerr geometry. Here
we report on the first numerical simulations of jet formation in a
rapidly rotating (a=0.95) Kerr black hole magnetosphere. We study
cases in which the Keplerian accretion disk is both corotating
and counter-rotating with respect to the black hole rotation, and
investigate the first ~50 light-crossing times. In the corotating disk
case, our results are almost the same as those in Schwarzschild black
hole cases: a gas pressure-driven jet is formed by a shock in the disk,
and a weaker magnetically driven jet is also generated outside the
gas pressure-driven jet. On the other hand, in the counter-rotating
disk case, a new powerful magnetically driven jet is formed inside the
gas pressure-driven jet. The newly found magnetically driven jet in
the latter case is accelerated by a strong magnetic field created by
frame dragging in the ergosphere. Through this process, the magnetic
field extracts the energy of the black hole rotation.
Title: Numerical Simulations of Solar Flares
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 2000IAUS..195..445Y
Altcode:
No abstract at ADS
Title: General-Relativistic MHD Simulation of Jets from a
Geometrically Thin Accretion Disk Around a Schwarzschild Black Hole
Authors: Aoki, S. I.; Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 2000IAUS..195..373A
Altcode:
We have performed a 2.5D, nonsteady, general-relativistic MHD
simulation. Initially, we assumed a uniform magnetic field, a
geometrically thin accretion disk rotating at Keplerian velocity,
and a hydrostatic corona around a Schwarzschild black hole. We have
investigated the formation mechanism of gas-pressure driven jets
expected by Koide et al. and found the strong dependence of jet
velocities (γj = 1 / √1 - Vj2;
Lorentz factor of jets) on the ratio of the density of the accretion
disk to that of the corona (ρd / ρc),
where γj2 - γj ~(ρd
/ ρc)0.75.
Title: Jets from Black Hole Magnetospheres
Authors: Shibata, K.; Koide, S.; Kudoh, T.; Aoki, S.
Bibcode: 2000IAUS..195..265S
Altcode:
Recent general-relativistic MHD simulations of jets ejected from
black-hole magnetospheres (for both Schwarzschild and Kerr holes)
have revealed that (1) strong shock waves are formed in the accretion
flow inside 3 rs, (2) jets show two-layered shell structure
consisting of a gas-pressure driven jet and a magnetically driven jet,
the former being accelerated from a high-pressure region heated by
strong shocks, and (3) in the case of a Kerr hole, magnetically driven
jets are produced from the ergosphere by the effect of frame dragging.
Title: Soft X-ray flares and magnetic configuration in a solar active
region in February 1992
Authors: Zhang, H. Q.; Sakurai, T.; Shibata, K.; Shimojo, M.;
Kurokawa, H.
Bibcode: 2000A&A...357..725Z
Altcode:
In this paper, we examine the observational soft X-ray flares and
the relationship with photospheric vector magnetograms in the active
region (NOAA 7070). We analyze the soft X-ray flare on Feb. 24-25,
1992, especially the pre-flare and the relationship with the highly
sheared photospheric vector magnetic field near the photospheric
magnetic neutral line. We find that the initial reconnection of the
magnetic field in the flare on Feb. 24-25, 1992 probably occurs near
the magnetic neutral line in the lower atmosphere of the active region,
where the highly sheared magnetic flux erupts up and triggers the
reconnection of the large-scale magnetic field. The possible process
of the magnetic reconnection of the limb flare on Feb. 20-21, 1992
in this active region is proposed also based on the analogy with the
flare on Feb. 24-25 near the center of the solar disk.
Title: Which Forces Accelerate Jets?
Authors: Kato, S. X.; Kudoh, T.; Shibata, K.
Bibcode: 2000IAUS..195..401K
Altcode:
We performed 2.5-dimensional, nonsteady MHD simulations of jets from
geometrically thin accretion disks and investigated the acceleration
forces of jets in detail.
Title: Collimation of Magnetically Driven Outflows from Accretion
Disks
Authors: Kudoh, T.; Matsumoto, R.; Shibata, K.
Bibcode: 2000IAUS..195..407K
Altcode:
We performed 2.5-dimensional, nonsteady MHD numerical simulations to
investigate the acceleration and collimation of magnetically driven
outflows from accretion disks, including the accretion process itself,
consistently. As an initial condition, we used a paraboloidal magnetic
field line that is produced by electric current on the equatorial
plane. We found that the outflow ejected from the accretion disk
is collimated by the pinch effect of the toroidal component of the
magnetic field that is produced by the rotation of the disk.
Title: Temporal 1/fα Fluctuations from Fractal Magnetic
Fields in Black-Hole Accretion Flow
Authors: Kawaguchi, Toshihiro; Mineshige, Shin; Machida, Mami;
Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 2000PASJ...52L...1K
Altcode: 1999astro.ph.12510K
Rapid fluctuation with a frequency dependence of 1/fα
(with alpha =~ 1-2) is characteristic of radiation from black-hole
objects. Its origin remains poorly understood. We examined
three-dimensional magnetohydrodynamical simulation data, finding that
a magnetized accretion disk exhibits both 1/fα fluctuation
(with alpha =~ 2) and a fractal magnetic structure (with the fractal
dimension of D ~ 1.9). The fractal field configuration leads to
reconnection events with a variety of released energy and duration,
thereby producing 1/fα fluctuations.
Title: MHD Simulations of Jets from Magnetized Accretion Disk
Authors: Shibata, K.
Bibcode: 2000aprs.conf...29S
Altcode:
In the last 10 years, magnetohydrodynamic (MHD) simulations of jets
ejected from magnetized accretion disks, have developed significantly
through the rapid development of supercomputers --these advances are
briefly reviewed.
Title: The VSOP Survey I: Description and Participation
Authors: Fomalont, E.; Hirabayashi, H.; Murata, Y.; Kobayashi,
H.; Inoue, M.; Burke, B.; Dewdney, P.; Gurvits, L.; Jauncey, D.;
McCulloch, P.; Preston, R.; Horiuchi, S.; Lovell, J.; Moellenbrock,
G.; Edwards, P.; Asaki, Y.; Avruch, I.; Nicolson, G.; Quick, J.;
Costa, M.; Dodson, R.; Reynolds, J.; Tzioumis, A.; Tingay, S.; Hong,
X.; Liang, S.; Trigilio, C.; Tuccari, G.; Nakajima, J.; Kawai, E.;
Fujisawa, K.; Kawaguchi, N.; Miyaji, T.; Kus, A.; Ghigo, F.; Salter,
C.; Slysh, V.; Cannon, W.; Carlson, B.; Dougherty, S.; del Rizzo, D.;
Scott, W.; Taylor, R.; Kameno, S.; Shibata, K.; Umemoto, T.; Benson,
J.; Flatters, C.; Hale, A.; Lewis, C.; Romney, J.; Miller, K.; Smith,
J.; Wietfeldt, R.; Meier, D.; Murphy, D.; Langston, G.; Minter, A.;
Popov, M.; Schilizzi, R.; Shen, Z. -Q.
Bibcode: 2000aprs.conf..167F
Altcode:
The VSOP mission is a Japanese-led project to image radio sources with
sub-milliarcsec resolution by correlating the signal from the orbiting
8-m telescope, HALCA, with a global array of telescopes. Twenty-five
percent of the scientific time of this mission is devoted to a survey
of the 400 brightest, small-diameter extra-galactic radio sources at
5GHz. The major goals of the VSOP survey are statistical in nature:
to determine the brightness temperature and approximate structure;
to provide a source list for use with future VLBI space missions;
and to compare radio properties with other data throughout the EM
spectrum. This paper describes: The compilation of the entire source
list; the selection of sources to be observed with VSOP; the extensive
ground resources; and the status of the observations as of March
2000. In these proceedings Paper II describes the reduction methods
and Paper III gives initial statistical results.
Title: An Emerging Flux Region Observed With TRACE
Authors: Yashiro, S.; Shibata, K.
Bibcode: 2000ASPC..205..133Y
Altcode: 2000ltse.conf..133Y
We study the early evolution of an active region in the corona observed
with the Transition Region and Coronal Explorer (TRACE). This region
appeared around 8-Jun-98 02:11 UT near the west limb, and grew into
the large active region. In the TRACE data, we can clearly see many
individual emerging magnetic loops during 1 day after emergence, and we
examine their apparent velocities. We found that the rise velocity of a
bright loop is about 5 - 15 km/s, which is almost the same as those of
Hα arch filaments as well as the prediction from theory. In the present
Yohkoh data, we can not see individual emerging loops directly. For
that reason, we calculate the time variation of size of this active
region using Yohkoh data, and obtained the average apparent expansion
velocity 1 km/s. We conclude that the apparent expansion of EFRs in
soft X-rays does not reflect the actual rise motion of magnetic loops.
Title: Three-Dimensional MHD Simulations of the Emergence of Twisted
Flux Tubes
Authors: Matsumoto, R.; Tonooka, H.; Tajima, T.; Chou, W.; Shibata, K.
Bibcode: 2000AdSpR..26..543M
Altcode:
We carried out three-dimensional magnetohydrodynamic (MHD) simulations
of the emergence of a twisted magnetic flux tube initially imbedded
in the convection zone. When the magnetic twist exceeds a threshold
value, the flux tube deforms itself into a helical structure through
the kink instability. For sufficiently long wavelength perturbations
(λp > 10H, where H is the local scale height), the
flux tube also subjects to the undular mode of the magnetic buoyancy
instability (Parker instability). The emergent portions of such
helically deformed, undulating flux tube show a series of S-shaped
X-ray emitting regions as observed by the Yohkoh satellite
Title: Non-Radial Unipolar Coronal Streamers in Magnetically High
Latitudes and Radial Bipolar Streamers at the Magnetic Equator of
the Sun
Authors: Saito, Takao; Shibata, K.; Dere, K. P.; Numazawa, S.
Bibcode: 2000AdSpR..26..807S
Altcode:
The complex structure of coronal streamers can be better understood by
their classification into two classes: non-radial unipolar streamers
emanating from magnetically high latitudes, and radial bipolar streamers
emanating from the magnetic equator. The coronal streamers observed
during the total solar eclipses of July 11,1991 and February 26, 1998
are classified into the two types. Various characteristics of the two
types are discussed and summarized. Important relations of the two to
solar, interplanetary, and terrestrial phenomena are discussed
Title: Recurrent Magnetic Reconnection in Protostellar Magnetospheres
Authors: Hayashi, Mitsuru; Shibata, K.; Matsumoto, R.
Bibcode: 2000AdSpR..26..567H
Altcode:
The solar X-ray satellite Yohkoh, has found extensive evidence
for magnetic reconnection in the solar corona. Here we extend the
magnetic reconnection model of solar flares to hard X-ray flares
observed in star forming regions. A new ingredient is a protostellar
disk which can inject helicity into the magnetosphere if the disk is
threaded by the dipole magnetic field of the protostar. We carried out
2.5-dimensional magnetohydrodynamical (MHD) simulations of the disk-star
interaction. The closed magnetic loops connecting the central star
and the disk are twisted by the rotation of the disk. In the presence
of resistivity, magnetic reconnection takes place in the current sheet
formed inside the expanding loops. A hot, outgoing plasmoid and hot post
flare loops are formed as a result of the reconnection. Numerical
results are consistent with the observed plasma temperature
(107 - 108K), the length of the flaring loop
(1011 - 1012cm), and the velocity of optical jets
(200 - 400km/s). We use high-resolution numerical simulations to show
that multiple magnetic islands are created in the current sheet due to
the growth of the tearing mode instability. The magnetic islands are
ejected quasi-periodically. Intermittent flaring activity continues
as long as the disk matter twists the dipole magnetic field
Title: Formation of Localized Strongly Magnetized Regions in Galaxies
and Clusters of Galaxies
Authors: Matsumoto, R.; Valinia, A.; Tajima, T.; Makishima, K.;
Shibata, K.
Bibcode: 2000AdSpR..25..499M
Altcode:
Through three-dimensional global magnetohydrodynamic (MHD) simulations,
we show that localized strongly magnetized regions are created in
galaxies and clusters of galaxies. We carried out simulations of (1)
dynamical evolution of magnetic fields in differentially rotating
disks and (2) amplification of magnetic fields through the rotation
and motions of individual galaxies in clusters of galaxies. In either
case, we find that even though the mean magnetic energy is smaller
than the thermal energy (β = Pgas/Pmag >=
1), magnetic pressure dominated, low-β (β <= 1) filaments are
created. Magnetic reconnection taking place in the strongly magnetized
regions heats the plasma typically up to 10 keV
Title: Magnetic Reconnection Model of X-ray Plasmas in the Galactic
Center
Authors: Yokoyama, T.; Tanuma, S.; Kudoh, T.; Shibata, K.
Bibcode: 2000AdSpR..25..505Y
Altcode:
A new model, the magnetic reconnection model, is proposed to
explain the heating of the superhot plasmas of Galactic center. In
this model, the magnetic field is amplified by the rotation of the
Galactic gas disk, and expand from the disk to outside by the Parker
instability. The expanding magnetic loop collides with ambient field
lines and induces magnetic reconnection in which the magnetic energy
is efficiently converted into the thermal energy of the plasmas. MHD
numerical simulations of the amplification process by the nonlinear
magnetorotational instability and of the heating process by magnetic
reconnection driven by the Parker instability are shown
Title: X-Ray Plasma Ejection and Magnetic Reconnection
Authors: Ohyama, Masamitsu; Shibata, Kazunari
Bibcode: 2000AdSpR..26..461O
Altcode:
X-ray plasma ejections are discovered above soft X-ray loops in
impulsive flares through Yohkoh observations. An X-ray plasmoid in
November 11, 1993 flare formed after its footpoint brightened long
before the impulsive phase, and was already heated to about 10 MK before
the impulsive phase. Before the impulsive phase the electron density
of the X-ray pasmoid was already an order of magnitude larger than the
typical density of the active-region corona. From these observational
results we propose that the ejected plasma was supplied owing to
chromospheric evaporation caused by the preflare heating. The kinetic
energy of the plasmoid was smaller than the thermal energy content
of the flare loop. This indicates that the plasmoid ejection is not
the energy source of the flare. A strong acceleration of the plasmoid
occurred around the onset of the hard X-ray emission, and the velocity
of the X-ray plasmoid and the hard X-ray emission are increased. This
indicates that plasmoid ejection and reconnection are closely coupled
Title: Resistive Processes in the Preflare Phase of Eruptive Flares
Authors: Magara, T.; Shibata, K.
Bibcode: 2000AdSpR..26..521M
Altcode:
It is now widely accepted that fast magnetic reconnection based on
locally enhanced resistivity plays an important role in such violent
phenomena as solar flares. Here we study how such localization of
resistivity occurs in flare evolution. We start with a 2.5-dimensional
force-free current sheet under a uniformly distributed resistivity,
which is subject to a very small random velocity perturbation. Then the
evolution enters the linear stage of the tearing instability and later a
sufficient amount of thermal energy is produced in the nonlinear stage,
which is considered to have a relation with the preflare heating. As
the non-linear evolution proceeds, the magnetic fields perpendicular
to current sheet (perpendicular magnetic fields) flow away from the
X-points formed in the current sheet and eventually the current-sheet
collapse occurs at these points. This collapse greatly reduced the
thickness of current sheet into the range of microscopic values if
the magnetic Reynolds number is quite large and the plasma beta is
quite low. Since the formation of thin current sheet leads to the
occurrence of a locally enhanced resistivity (anomalous resistivity),
the transition from the gradual energy-release phase under a uniformly
distributed resistivity to the rapid phase with a locally enhanced
anomalous resistivity can be accomplished. This transition is
responsible for various explosive phenomena in the sun
Title: Time Dependent Ionization Balance in Solar Flares
Authors: Kato, T.; Kato, M.; Shibata, K.
Bibcode: 2000AdSpR..25.1781K
Altcode:
We have analyzed the time evolution of solar flare X-ray spectra from
He-like Fe, Ca and S ions measured for a 1992 September 6 flare by the
Bragg Crystal Spectrometer (BCS) on the Yohkoh satellite. We derived
time-dependent electron temperature, ion temperature, electron density
as well as the ion density ratios such as n(Li-like)/n(He-like),
n(Be-like)/n(He-like), n(B-like)/n(He-like) for Fe ions and
n(He-like)/n(H) for Fe and Ca ions. The results show a deviation from
ionization equilibrium. In this paper we investigate the origin of
the deviation and consider time dependent ionization models assuming
material flow in solar flares or multiple loops occurring with a short
time period
Title: Observational Evidence of Magnetic Reconnection in Solar
X-Ray Jets
Authors: Shimojo, M.; Shibata, K.
Bibcode: 2000AdSpR..26..449S
Altcode:
The solar X-ray jets are one of the most interesting findings of
Soft X-Ray Telescope(SXT) aboard Yohkoh. They are transitory X-ray
enhancements with an apparent collimated motion. In this paper,
we present the observational evidence of magnetic reconnection of
solar X-ray jets.From the morphological study of solar X-ray jets
using SXT, we found the following properties of solar X-ray jets. 1)
Most X-ray jets are associated with flares (microflares - flares)
at their footpoints. 2) When the active regions at the footpoint
of jets can be resolved well, it is found that morphology changes
significantly during jets. 3) 27% of the jets show a gap ( >
104 km) between the exact footpoint of the jet and the
brightest part of the associated flare. Furthermore, as a result of
the co-alignment between magnetograms and SXT images, we found that
the jet-producing region are the mixed polarity region and the region
of evolving magnetic flux (increasing or decreasing). Canfield et
al. (1996) investigated some Hα surges which were associated with
X-ray jets and found some new Ha phenomena (moving-blueshift feature,
converging footpoint motion). They suggested that these phenomena are
the results of magnetic reconnection. As the results, we propose that
the solar X-ray jets are produced by energy input from the (micro)flares
which are generated by magnetic reconnection
Title: Computational Magnetohydrodynamics of Turbulence, Dynamos,and
Jet Formation in Differentially Rotating Astrophysical Disks
Authors: Matsumoto, R.; Machida, M.; Hayashi, M.; Shibata, K.
Bibcode: 2000PThPS.138..632M
Altcode:
We present the results of three-dimensional global magnetohydrodynamic
(MHD) simulations of differentially rotating astrophysical disks. We
simulate the time evolution of the disk by using a parallelized
three-dimensional MHD code. Typical number of grid points is
(Nr, Nϕ, Nz)=(200,64,240) in a
cylindrical coordinate system. We found that when the initial magnetic
field is toroidal and weak (β=Pgas/Pmag ≫ 1),
magnetic energy is amplified exponentially due to the dynamo action
driven by the magnetorotational instability. In the nonlinear stage,
magnetic turbulence excited in the disk tangles magnetic field lines. We
found that the amplification of magnetic energy saturates when β
∼ 10 and that the system approaches a quasi-steady state. Inside
the disk, filamentary shaped, magnetic pressure dominated (β <
1) regions appear. Magnetic energy release in low-β regions leads
to violent time variations of X-ray emission from the disk. When the
initial magnetic field is poloidal, magnetically driven collimated
jet emanates from the surface of the disk.
Title: Thermal Evolution of Coronal Active Regions Observed with
the Yohkoh Soft X-Ray Telescope
Authors: Yashiro, S.; Shibata, K.; Shimojo, M.
Bibcode: 2000AdSpR..25.1773Y
Altcode:
We study the thermal evolution of active regions in the corona by
analyzing 51 emerging flux regions (EFRs) observed with the soft
X-ray telescope aboard Yohkoh. We derive the mean temperature and
pressure of active regions as a function time using filter ratio
technique, and found that the mean temperature increases with its
expansion. We also studied the relationship between the region
size and the temperature, and found that the temperature (T) and
the pressure (P) of EFRs increase with increasing region size (L);
TαL00.42PαL00.25
Title: 2D Numerical MHD Simulation of Magnetic Reconnection As the
Origin of Interstellar Hot Plasma
Authors: Tanuma, S.; Yokoyama, T.; Kudoh, T.; Shibata, K.
Bibcode: 2000AdSpR..25..509T
Altcode:
X-ray astronomy satellites detected strong X-ray emission along Galactic
ridge (Galactic Ridge X-ray Emission, GRXE). Origin and nature of
the GRXE are not known. In the present paper, we propose a possible
mechanism to explain the origin of GRXE. We solved two-dimensional
resistive MHD (magnetohydrodynamics) equation numerically to examine
the magnetic reconnection triggered by a supernova and the interstellar
magnetic structure in Galaxy. Especially we find that the magnetic
reconnection heats X-ray plasma in Galactic plane if reconnection
occurs in locally strong magnetic fields of ~ 30 μG. The released
magnetic energy by the reconnection is determined by the magnetic field
strength. The magnetic islands are generated by the reconnection. Heated
plasma is confined by magnetic fields for a long time
Title: Magnetic reconnection processes in the solar
atmosphere. Proceedings. E2.3 Symposium of COSPAR Scientific
Commission E held during the 32nd COSPAR Scientific Assembly, Nagoya
(Japan), 12 - 19 Jul 1998.
Authors: Shibata, K.; Hoshino, M.
Bibcode: 2000AdSpR..26.....S
Altcode:
The following topics were dealt with: reconnection in the magnetosphere,
reconnection in planetary magnetospheres, reconnection in the solar
atmosphere, observations, theory, simulations, reconnection in
astrophysical objects, comparative study of solar and magnetospheric
reconnection.
Title: Imaging Capability of the Mitaka VSOP Correlator
Authors: Horiuchi, S.; Kameno, S.; Nan, R.; Shibata, K.; Inoue, M.;
Kobayashi, H.; Murata, Y.; Fomalont, E.; Carlson, B.
Bibcode: 2000AdSpR..26..625H
Altcode:
We have made a comparison between data between the Mitaka and Penticton
correlators, using VSOP observations of NRAO530 at 1.6 GHz. After
calibrating the data with AIPS we find excellent agreement with the
closure phase between the data processed at the two correlators. The
images made from both data sets also agree well. We have made other
VSOP images of OQ208 at 1.6 GHz, and 3C395, J2011-15 at 5 GHz from data
correlated by the Mitaka correlator. These images are all consistent
with ground-only VLBI results at higher frequencies, and demonstrate
the capability of the Mitaka correlator as a VSOP correlator
Title: Commission 10: Solar Activity: (Activite Solaire)
Authors: Ai, G.; Benz, A.; Dere, K. P.; Engvold, O.; Gopalswamy, N.;
Hammer, R.; Hood, A.; Jackson, B. V.; Kim, I.; Marten, P. C.; Poletto,
G.; Rozelot, J. P.; Sanchez, A. J.; Shibata, K.; van Driel-Geztelyi, L.
Bibcode: 2000IAUTA..24...67A
Altcode:
No abstract at ADS
Title: Theory and Observations of X-ray Jets
Authors: Shimojo, M.; Shibata, K.
Bibcode: 2000IAUJD...7E...7S
Altcode:
X-ray jets were discovered by soft X-ray telescope aboard Yohkoh
in 1991 as transitory X-ray enhancements with apparent collimated
motion. They are ejected from small flares or microflares, and could
be related to coronal heating mechanism and acceleration of high speed
solar wind. Since their discovery, both theory (especially magnetic
reconnection model) and observations of X-ray jets have been greatly
advanced. We review the current status of the understanding of X-ray
jets from both theoretical and observational points of view, and discuss
the relation to other jet-like phenomena on the Sun, coronal heating,
and acceleration of high speed solar wind.
Title: General relativistic simulations of jet formation by a rapidly
rotating black hole
Authors: Koide, S.; Meier, D. L.; Shibata, K.; Kudoh, T.
Bibcode: 2000NuPhS..80C0115K
Altcode: 1999astro.ph..7434K
Recent observations of Galactic Black Hole Candidates (BHCs) suggest
that those that are superluminal jet sources have more rapid black
hole spin rates than otherwise normal BHCs. This provides observational
support for models of relativistic jet formation that extract rotational
energy of the central black hole. To investigate this mechanism, we
have developed a new general relativistic magnetohydrodynamic code
in Kerr geometry. Here we report on the first numerical simulation
of the formation of a relativistic jet in a rapidly-rotating (a=0.95)
Kerr black hole magnetosphere. We assume that the initial velocity of
the disk is zero. We find that the maximum velocity of the jet reaches
0.93c (Lorentz factor, 2.7) and the terminal velocity of the jet is
0.85c (Lorentz factor, 1.9). On the other hand, for a non-rotating
(a=0) Schwarzschild black hole, the maximum outflow velocity is less
than 0.6c for initial magnetospheric conditions similar to to those of
the Kerr black hole case. These numerical results show the importance
of the rapidly rotating black hole for the relativistic jet formation.
Title: Comparative Study of Flares and Substorms
Authors: Terasawa, T.; Shibata, K.; Scholer, M.
Bibcode: 2000AdSpR..26..573T
Altcode:
Comparative studies of solar flares and substorms are useful
in obtaining comprehensive understanding of magnetic reconnection
processes, since flare studies give us better information on macroscopic
processes such as global changes of magnetic field configuration
while substorm studies give us more detailed knowledge on microscopic
processes. Recent spacecraft observations both for magnetospheric
substorms and solar flares have dramatically enriched our understanding
for physical processes occurring during these interesting phenomena. We
first summarize keys of these new observations from the magnetotail
studies, and try to compare them with those from the solar flare
studies on a common physical basis
Title: Simultaneous Hα and X-ray Observations of Prominence Eruption
and Disappearance
Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.;
Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K.
Bibcode: 2000AdSpR..26..473T
Altcode:
Prominence eruptions or disappearances observed with an Hα limb
filtergraph can be classified into 3 categories, the eruptive
prominence, the quasi-eruptive prominence, and the disappearing
prominence. We investigated their mechanism by comparing the results
of simultaneous observations by Yohkoh SXT and Hα. We found that soft
X-ray features change in both eruptive prominences and quasieruptive
prominences, whereas no significant change takes place in disappearing
prominences.In one prominence eruption event soft X-ray cusp structure
suggests that the reconnection point is just below the Hα prominence
Title: Evolution of Magnetic Field and CMEs
Authors: Fang, C.; Chen, P. F.; Tang, Y. H.; Shibata, K.
Bibcode: 2000IAUJD...7E..17F
Altcode:
Observations show that some evolving magnetic structures occur in
solar active regions before the onset of CMEs. In 2D framework, through
changing the bottom boundary of magnetic field, we simulate numerically
the dynamics of the coronal plasma with a magnetic configuration similar
to a filament. In some cases, we find the flux rope (or filament) loses
its equilibrium, and moves upward. A current sheet is formed below the
filament. If there is no reconnection or the reconnection is not so
fast, the filament will finally stop motion and fall down, while when
fast reconnection occurs, the filament erupts. The characteristics of
its motion are well consistent with filament eruptions and CMEs. After
the reconnection, a cusp shaped hot X-ray loop is formed as the
signature of solar flares. This model provides deep insight on the
relation between the photospheric magnetic field variation and the
onset of CMEs, and on the relation between CMEs and solar flares.
Title: Resistive Magnetohydrodynamic Simulations of Jet Formation
Authors: Kuwabara, T.; Shibata, K.; Matsumoto, R.
Bibcode: 1999sf99.proc..284K
Altcode:
We carried out 2.5-dimensional resistive magnetohydrodynamic
simulations to study the effects of magnetic diffusivity on
the magnetically driven mass accretion and jet formation. When a
constant angular momentum torus is threaded by large-scale vertical
magnetic fields, the angular momentum of the torus is extracted due
to magnetic braking and the torus medium falls toward the central
region. The large-scale magnetic fields twisted by the infalling
matter drives bipolar jets. We studied the dependence of the mass
accretion rate and the mass out flow rate on magnetic diffusivity
η0=η/(vK0r0), where r0 is
the reference radius and vK0 is the Keplerian rotation speed
at r=r0. We found that (1) in non-diffusive or low diffusive
model (η0 < 10-3), the mass accretion and the
jet formation take place intermittently, (2) in mildly diffusive model
(10-3< η0 <10-2), the system
evolves toward a quasi-steady state, (3) in highly diffusive model
(η0>10-2), accretion/mass outflow rate
reduces with η0 and approaches 0.
Title: Magnetohydrodynamic Simulations of Hard X-Ray Emission and
Mass Outflows from Protostars
Authors: Hayashi, M. R.; Shibata, K.; Matsumoto, R.
Bibcode: 1999sf99.proc..288H
Altcode:
We carried out 2.5 dimensional magnetohydrodynamic (MHD) simulations
of the magnetic interaction between a protostar and its surrounding
disk. The results show that high-velocity and high-temperature plasma
flows can be created via magnetic reconnection. In our model which
can explain protostellar flares and mass outflows both qualitatively
and quantitatively, magnetic reconnection plays an essential role
in heating the plasmas. We analyse the one-dimensional distribution
across the shock around the reconnection site to clarify the heating
machanism in protostellar flares. Around the reconnection point, we
found two peaks of peaks of toroidal current density and a pressure
increase with decreasing the magnetic field strength, indicating that
these shocks are slow-mode shocks
Title: The Dependence on the Initial Magnetic Field Strength in
MHD Jets
Authors: Kato, X. S.; Kudoh, T.; Shibata, K.
Bibcode: 1999sf99.proc..282K
Altcode:
We performed 2.5 dimensional nonsteady MHD numerical simulations of jets
from geometrically thin disks including the dynamics of accretion disks
self-consistently. For the initial rotational velocity of the disk,
we consider two cases, Keplerian cases and sub-Keplerian cases. In
Keplerian cases, we found the following results: (1) The ejection
point of jets is determined by the effective potential made by the
gravitational and centrifugal forces along the magnetic field (Blandford
and Payne 1982). (2) The initial acceleration forces of jets is magnetic
pressure when the initial magnetic field strength(B0) is
weak, and magneto-centrifugal force is effective when B0
is strong. (3) The dependences of the velocity (Vjet)
and the mass outflow rate (dot{M}w) on B0
are dot{M}w propto Emg1/2
and Vjet propto left( frac{ΩF2
Emg}{dot{M}w} right)1/3, therefore
Vjet propto Emg1/6 where ΩF
is the angular velocity of a field line (essentially the Keplerian
angular velocity where the jet is ejected), Emg= (magnetic
energy/gravitational energy) propto B02. In
sub-Keplerian cases, Vjet propto Emg1/6
is satisfied but the other dependences are not always equal to those
of Keplerian cases.
Title: Non-Linear Evolution of Self-Gravitational MHD Instabilities
in Rotating Disks
Authors: Chou, W.; Matsumoto, R.; Tajima, T.; Umekawa, M.; Shibata, K.
Bibcode: 1999sf99.proc...40C
Altcode:
We carried out linear analysis and non-linear 3D simulations of
self-gravitational magnetohydrodynamics of gas layer in a local
co-rotating coordinates. We studied the evolution of Parker-Jeans
instability in several cases which are different in factors such
as rotational speed, magnetic field, external pressure, external
gravitational force, and directions of perturbations. We found that
the growth rate of the instabilities and the shape of the dense
blobs aggregated in the nonlinear stage sensitively depend on these
factors. This study provides us hints on how interstellar/intergalactic
gas aggregates to form dense blobs/filaments, which correspond to
molecular clouds, under the circumstance of rotation and magnetic field.
Title: Thermal Evolution of Coronal Active Regions
Authors: Yashiro, S.; Shibata, K.; Shimojo, M.
Bibcode: 1999spro.proc...99Y
Altcode:
We study the thermal evolution of active regions in the corona by
analyzing 51 emerging flux regions (EFRs) observed with the soft
X-ray telescope aboard Yohkoh. We derive the mean temperature and
pressure of active regions as a function of time using the filter
ratio technique, and found that the mean temperature increases with
its expansion. We also studied the relationship between the region
size and the temperature, and found that the temperature (T) and the
pressure (P) of EFRs increase with increasing region size (L); T ∝
L0.42, P ∝ L0.25. The relation between pressure
and region size, however, is strongly influenced by the assumption of
the region's thickness along the line of sight.
Title: A Microwave Study of Coronal and Chromospheric Ejecta
Authors: Nindos, A.; Kundu, M. R.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999spro.proc..135N
Altcode:
We have studied the radio properties of 18 X-ray coronal jets (observed
by the Yohkoh SXT) using Nobeyama 17 GHz data. We also searched for
chromospheric ejecta (Hα surges) during the time intervals that the
X-ray images were available. Microwave emission was associated with the
majority of the X-ray jets. The radio emission came from the base or
the lower part of the jets. We detected radio emission from almost all
jets which showed flare-like activity at their footpoints. The 17 GHz
time profiles were gradual and unpolarized, implying that the emission
was thermal. When possible, we computed the physical properties of the
X-ray-emitting ejected plasma. In one two-sided-loop type jet and one
anemone-type jet, the observed microwave fluxes from the lower part of
the jets were well above the fluxes predicted from the computed electron
temperatures and emission measures of the soft X-ray-emitting material
on the basis of thermal free-free emission. We interpreted the large
discrepancies in terms of the presence of lower temperature material
which cannot be detected by the SXT but produces strong microwave
free-free emission. This is the first time that such material is
observed in two-sided-loop type jets. Thus our observations confirm the
theoretical prediction by Yokoyama and Shibata (1996). We detected no
cool material at the base of the jets. We also observed an Hα surge
which was not associated with an X-ray jet and showed no signatures on
the SXT images but was detected with the Nobeyama Radioheliograph. The
emission of the microwave surge-associated source was free-free from the
chromospheric plasma. Constraints for the surge density were derived.
Title: Magnetically Driven Outflows from Accretion Disks
Authors: Kudoh, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1999sf99.proc..286K
Altcode:
This is the first magnetohydrodynamic (MHD) numerical simulation
including accretion of a disk, ejection of a jet, acceleration and
collimation of the jet, and formation of a bow shock. We extend the
the computational region to about 100 times larger than the radius of
central star to investigate the collimation of jet and a formation of
a bow shock. (It is about 10 times larger than those of the previous
works.) We found that both the magnetic field lines and stream lines
are collimated by the pinch effect of the toroidal component of the
magnetic field that is generated by the differential rotation in the
disk. Finally, the bow shock is formed at the head of the jet by the
'piston effect' of the toroidal component of the magnetic field. The
result of the large scale simulation should be compared to the high
resolution observation by VLBI.
Title: Formation and Collimation of Jets by Magnetic Forces
Authors: Shibata, K.; Kudoh, T.
Bibcode: 1999sf99.proc..263S
Altcode:
Recent development of theory and numerical simulations of magnetically
driven jets from young stellar objects is reviewed. Topics to be
discussed are: 1) Acceleration of jets: Magnetically driven jets are
accelerated by both magneto-centrifugal force and magnetic pressure
force. The former (latter) becomes important when magnetic field is
strong (weak). The basic properties (i.e., terminal velocity and mass
flux) of jets accelerated by these two forces is discussed in detail. We
also discuss the condition of production of jets, which is applied to
answer the following question: When do jets begin to be accelerated in
the course of star formation ? 2) Collimation of jets: Magnetically
driven jets can in principle be collimated by pinching effect of
toroidal magnetic fields. Recently, some controvertial arguments have
been put forward: Are all field lines (and jets) really collimated by
pinching effect ? The current status of this issue is discussed. 3)
Protostellar flares: Based on theory and numerical simulations,
it has recently been recognized that the formation of jets has a
close connection with occurrence of flares (possibly due to magnetic
reconnection). We discuss how and when magnetic reconnection occurs
in relation to jets.
Title: Reconnection Models of Flares
Authors: Shibata, K.
Bibcode: 1999spro.proc..381S
Altcode:
Yohkoh observations of solar flares have revealed various types
of evidence of magnetic reconnection, not only for large scale
flares but also for small scale flares. Observations have also
revealed that the association of mass ejections (plasmoids) with
these flares is much more common than previously thought. On the
basis of these new Yohkoh observations, we briefly (but critically)
review various reconnection models of flares, and then discuss the
plasmoid-induced-reconnection model, which is an extension of the
CSHKP model but includes the following points as essential ingredients
of nonsteady fast reconnection. Plasmoid formation and ejections are
not simple by-products of reconnection, but play an essential role
in storing energy (by inhibiting reconnection in the preflare phase)
and by inducing strong inflow into reconnection region (by ejecting a
huge amount of plasma in the impulsive phase). We shall also discuss how
plasmoids are accelerated by global magnetic pressure and reconnection
jets. It is stressed that the plasmoid-induced-reconnection model
naturally explains both large and small scale flares, forming a basis
of a unified model of flares.
Title: X-ray Plasma Ejection Associated with Hα Filament Eruption
Authors: Ohyama, M.; Shibata, K.
Bibcode: 1999spro.proc..367O
Altcode:
The 1993 May 14 flare was associated with both an X-ray plasma
ejection and an eruption of an active region filament. The flare
proceeded through two stages. In the first stage the X-ray plasma
ejection, Hα filament eruption, and a chain of point-like Hα
brightenings occurred. In the second stage, an Hα two-ribbon flare
and X-ray arcade structure were seen in Hα and soft X-ray images,
respectively. The X-ray plasma ejection and the eruptive Hα filament
in the first stage started to rise before the main peak of the hard
X-ray emission. The ejected X-ray plasma was a loop-like feature that
started to rise with a speed of ~270 km s-1 from below and
temporally after the Hα filament eruption. The ejected X-ray loop
appeared to be decelerated when it approached the Hα filament, and
then rose with the eruptive filament at an apparent velocity of ~ 100 km
s-1. The temperature of the ejected loop was 9.5±2.3 MK. The
mass of the ejected X-ray loop and of the pre-eruption Hα filament
were estimated to be ~ 1014 g and <= 1015 g,
respectively. Even if all the material of the pre-eruption filament
was ejected, the total kinetic energy (<= 1.5 × 1029
erg) of both the ejected X-ray loop and the eruptive Hα filament was
smaller than the thermal energy content of the flare loops (~ 1.3 ×
1030 erg). This result implies that the energy involved in
the ejected material was not the energy source of the flare, although
they were closely related each other. The rising motion of the ejected
X-ray loop was, presumably, one of the causes triggering the flare.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Frank,
J.; Sol, H.
Bibcode: 1999AAS...195.6902N
Altcode: 1999BAAS...31.1471N
We have investigated the dynamics of an accretion disk around
Schwarzschild black holes initially threaded by a uniform poloidal
magnetic field in a non-rotating corona (either in a steady-state
infalling state or in hydrostatic equilibrium) around a non-rotating
black hole using a 3-D GRMHD with the ``axisymmetry'' along the
z-direction. Magnetic field is tightly twisted by the rotation of the
disk, and plasmas in the shocked region of the disk are accelerated by
J x B force to form bipolar relativistic jets. In order to investigate
variabilities of generated relativistic jets and magnetic field
structure inside jets, we have performed calculations using the 3-D
GRMHD code with a full 3-dimensional system. We will investigate how
the third dimension affects the global disk dynamics and jet generation.
Title: Jet Formation and Angular Momentum Problem of YSOs
Authors: Hirose, S.; Uchida, Y.; Shibata, K.; Matsumoto, R.
Bibcode: 1999sf99.proc..290H
Altcode:
We performed 2.5 dimensional magnetohydrodynamic numerical simulation
of disk accretion onto magnetized YSOs. Our numerical model can explain
both jets from YSOs and the angular momentum problem.
Title: Origin of the Universal Correlation between the Flare
Temperature and the Emission Measure for Solar and Stellar Flares
Authors: Shibata, Kazunari; Yokoyama, Takaaki
Bibcode: 1999ApJ...526L..49S
Altcode:
We present a theory to explain the observed universal correlation
between flare temperature T and emission measure EM=n2V
for solar and stellar flares (including solar microflares observed by
Yohkoh as well as protostellar flares observed by ASCA), where n is
the electron density and V is the volume. The theory is based on a
magnetic reconnection model with heat conduction and chromospheric
evaporation, assuming that the gas pressure of a flare loop is
comparable to the magnetic pressure. This theory predicts the relation
EM~B-5T17/2, which explains well the observed
correlation between EM and T in the range of 6×106 K <
T<108 K and 1044<EM<1055
cm-3 from solar microflares to protostellar flares, if the
magnetic field strength B of a flare loop is nearly constant for solar
and stellar flares.
Title: Relativistic Jet Formation from Black Hole Magnetized
Accretion Disks: Method, Tests, and Applications of a General
RelativisticMagnetohydrodynamic Numerical Code
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro
Bibcode: 1999ApJ...522..727K
Altcode:
Relativistic jets are observed in both active galactic nuclei (AGNs) and
``microquasars'' in our Galaxy. It is believed that these relativistic
jets are ejected from the vicinity of black holes. To investigate the
formation mechanism of these jets, we have developed a new general
relativistic magnetohydrodynamic (GRMHD) code. We report on the basic
methods and test calculations to check whether the code reproduces some
analytical solutions, such as a standing shock and a Keplerian disk
with a steady state infalling corona or with a corona in hydrostatic
equilibrium. We then apply the code to the formation of relativistic
MHD jets, investigating the dynamics of an accretion disk initially
threaded by a uniform poloidal magnetic field in a nonrotating corona
(either in a steady state infall or in hydrostatic equilibrium) around
a nonrotating black hole. The numerical results show the following: as
time goes on, the disk loses angular momentum as a result of magnetic
braking and falls into the black hole. The infalling motion of the
disk, which is faster than in the nonrelativistic case because of
general relativistic effects below 3rS (rS is the
Schwarzschild radius), is strongly decelerated around r=2rS
by centrifugal force to form a shock inside the disk. The magnetic
field is tightly twisted by the differential rotation, and plasma
in the shocked region of the disk is accelerated by the JXB force
to form bipolar relativistic jets. In addition, and interior to,
this magnetically driven jet, we also found a gas-pressure-driven
jet ejected from the shocked region by the gas-pressure force. This
two-layered jet structure is formed not only in the hydrostatic corona
case but also in the steady state falling corona case.
Title: Evidence of Magnetic Reconnection in Solar Flares and a
Unified Model of Flares
Authors: Shibata, Kazunari
Bibcode: 1999sffg.conf...74S
Altcode:
The solar X-ray observing satellite Yohkoh has discovered various new
dynamic features in solar flares and corona, e.g., cusp-shaped flare
loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections
from impulsive flares, transient brightenings (spatially resolved
microflares), X-ray jets, large scale arcade formation associated
with filament eruption or coronal mass ejections, and so on. It has
soon become clear that many of these features are closely related to
magnetic reconnection. We can now say that Yohkoh established (at least
phenomenologically) the magnetic reconnection model of flares. In this
paper, we review various evidence of magnetic reconnection in solar
flares and corona, and present unified model of flares on the basis
of these new Yohkoh observations.
Title: Erratum: Magnetically Driven Jets from Accretion
Disks. III. 2.5-dimensional Nonsteady Simulations for Thick Disk Case
Authors: Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 1999ApJ...521..934K
Altcode:
In the paper ``Magnetically Driven Jets from Accretion
Disks. III. 2.5-dimensional Nonsteady Simulations for Thick Disk Case''
by Takahiro Kudoh, Ryoji Matsumoto, and Kazunari Shibata (ApJ, 508,
186 [1998]), Figure 2 was printed in black-and-white instead of color
as the result of an error in the printing process. The original color
version of Figure 2 appears below. The Press sincerely apologizes for
this error.
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999ApJ...520..391K
Altcode:
Using Nobeyama 17 GHz data, we have studied the radio properties of
19 coronal jets identified in Yohkoh soft X-ray imaging telescope
(SXT) X-ray observations. The radio data provide information on the
physical conditions in the jets, which complements the data from the
X-ray surveys. Microwave emission was associated with the majority of
the X-ray jets in our sample. The radio emission typically came from
the base or the base and lower part of the jets. We detected radio
emission from almost all jets that showed flarelike activity at their
bases. The jets that were not associated with radio emission did not
show any significant increase in X-ray emission at their bases. The
strongest radio emission came from two of the largest jets in our
sample. Our data show a general correlation between the X-ray jet
fluxes and the associated radio fluxes. The 17 GHz time profiles were
gradual and unpolarized, implying that the emission was thermal. In a
two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993
February 9 event), the observed microwave fluxes from the lower part
of the jets were well above the fluxes calculated from the computed
physical parameters of the soft X-ray-emitting material on the basis
of thermal free-free emission. We interpret the large discrepancies
in terms of the presence of lower temperature material, which cannot
be detected by the SXT (the SXT is most sensitive to hot plasma above
2×106 K), but which produces strong microwave free-free
emission. This is the first time that such material has been observed
in two-sided-loop-type jets. We also observed motion of a jet-associated
microwave source with a velocity of 55 km s-1. The microwave
motion occurred after the appearance of the X-ray jet. There is clear
evidence that the microwave emission of that source was associated
with the jet and not with the associated small flare.
Title: Theory of coronal X-ray jets
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1999AIPC..471...61Y
Altcode: 1999sowi.conf...61Y
We review our magnetic reconnection model of the solar coronal
X-ray jets. The plasma of an X-ray jet is accelerated and heated by
reconnection between the emerging flux and a pre-existing coronal
field. Many observed characteristics of the X-ray jets could be
successfully reproduced through the two-dimensional MHD numerical
simulations based on this model. We also discuss the Alfvén wave
generated by the reconnection process. The simulation results show that
the amount of Alfvén wave energy is ~3% of the total energy released
by the magnetic reconnection.
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999AAS...194.1704K
Altcode: 1999BAAS...31..853K
Using Nobeyama 17 GHz data, we have studied the radio properties
of 19 coronal jets identified in Yohkoh SXT X-ray observations. The
radio data provide information on the physical conditions in the jets
which complements the data from the X-ray surveys. Microwave emission
was associated with the majority of the X-ray jets in our sample. The
radio emission typically came from the base or the base and lower part
of the jets. We detected radio emission from almost all jets which
showed flare-like activity at their bases. The jets which were not
associated with radio emission did not show any significant increase
in X-ray emission at their bases. The strongest radio emission came
from two of the largest jets in our sample. Our data show a general
correlation between the X-ray jet fluxes and the associated radio
fluxes. The 17 GHz time profiles were gradual and unpolarized, implying
that the emission was thermal. In a two-sided-loop jet (July 22, 1992
event) and one anemone-type jet (February 9, 1993 event), the observed
microwave fluxes from the lower part of the jets were well above the
fluxes calculated from the computed physical parameters of the soft
X-ray-emitting material on the basis of thermal free-free emission. We
interpret the large discrepancies in terms of the presence of lower
temperature material which cannot be detected by the SXT (the SXT is
most sensitive to hot plasma above 2 x 10(6) K) but which produces
strong microwave free-free emission. This is the first time that
such material has been observed in two-sided-loop type jets. We also
observed motion of a jet-associated microwave source with a velocity
of 55 km/sec. The microwave motion occurred after the appearance of
the X-ray jet. There is clear evidence that the microwave emission of
that source was associated with the jet and not with the associated
small flare.
Title: 3-D General Relativistic MHD Simulations of Generating Jets
Authors: Nishikawa, K. -I.; Koide, S.; Shibata, K.; Kudoh, T.; Frank,
J.; Sol, H.
Bibcode: 1999AAS...194.7317N
Altcode: 1999BAAS...31..952N
Koide et al have investigated the dynamics of an accretion disk
initially threaded by a uniform poloidal magnetic field in a
non-rotating corona (either in a steady-state infalling state or in
hydrostatic equilibrium) around a non-rotating black hole using a
3-D GRMHD with the ``axisymmetry'' along the z-direction. Magnetic
field is tightly twisted by the rotation of the disk, and plasmas in
the shocked region of the disk are accelerated by J x B force to form
bipolar relativistic jets. In order to investigate variabilities of
generated relativistic jets and magnetic field structure inside jets,
we have performed calculations using the 3-D GRMHD code on a full
3-dimensional system. We will investigate how the third dimension
affects the global disk dynamics. 3-D RMHD simulations wil be also
performed to investigate the dynamics of a jet with a helical mangetic
field in it.
Title: Occurrence Rate of Microflares in an X-Ray-bright Point within
an Active Region
Authors: Shimojo, Masumi; Shibata, Kazunari
Bibcode: 1999ApJ...516..934S
Altcode:
We examine the occurrence rate of microflares (transient brightenings)
in an X-ray bright point (XBP) during the lifetime of the XBP observed
with the Yohkoh soft X-ray telescope (SXT). The XBP with a size of
~30'' appeared near a preceding spot of NOAA 7270 on 1992
September 1 and disappeared on 1992 September 4. The XBP produced
92 microflares during the observation time of the SXT. We found that
the occurrence rate (number of events per hour) did not change much
during the lifetime of the XBP, although the magnetic flux of the XBP
changed. We also found that the frequency distribution of microflares
as a function of the soft X-ray peak intensity shows a power law with
index 1.7+/-0.4, which is consistent with the previously known index for
flares and microflares based on the data for the whole Sun or a whole
active region. This result suggests that the power-law distribution
of flares holds not only for larger areas but also for smaller areas.
Title: Evolution of Eruptive Flares. II. The Occurrence of Locally
Enhanced Resistivity
Authors: Magara, Tetsuya; Shibata, Kazunari
Bibcode: 1999ApJ...514..456M
Altcode:
In this paper we study resistive processes in the preflare phase
of eruptive flares by means of the 2.5-dimensional MHD numerical
simulation. According to many detailed observations of solar flares,
their evolution is characterized by several phases, each of which
has a distinct nature. In the first phase, some kinds of radiation
begin to be enhanced gradually, which implies the occurrence of the
preflare heating. Then, at a certain time, that gradual energy-release
phase is replaced by the violent energy-release phase in which a huge
amount of energy is released in various forms. So far, the nature of
this violent energy-release phase has been well studied by using a
flare model based on the fast magnetic reconnection, although those
problems of the preflare heating and the transition from the gradual
energy-release phase to the violent one have not been sufficiently
discussed yet. In this paper, in order to tackle these problems,
we start with a 2.5-dimensional force-free current sheet under a
uniformly distributed resistivity, which is subject to a very small
random velocity perturbation. At first the evolution enters on the
linear stage of tearing instability and later a sufficient amount of
thermal energy is produced in the nonlinear stage, which is considered
to have a relation with the preflare heating. In this nonlinear
stage, the component of magnetic fields perpendicular to the sheet
(perpendicular magnetic fields) flows away from X-points formed in the
sheet and eventually the current sheet collapses at these points. This
collapse strongly reduces the thickness of the sheet if the magnetic
Reynolds number is quite large and the plasma beta is quite low. Since
the formation of thin current sheet leads to the occurrence of locally
enhanced resistivity (anomalous resistivity), the transition from the
gradual energy-release phase under a uniformly distributed resistivity
to the violent one under a locally enhanced anomalous resistivity can
be accomplished, which causes the fast magnetic reconnection responsible
for various explosive phenomena in the Sun.
Title: Alfvén Wave Model of Spicules and Coronal Heating
Authors: Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 1999ApJ...514..493K
Altcode:
Magnetohydrodynamic simulations are performed for torsional
Alfvén waves propagating along an open magnetic flux tube in the
solar atmosphere. It is shown that, if the root mean square of the
perturbation is greater than ~1 km s-1 in the photosphere,
(1) the transition region is lifted up to more than ~5000 km
(i.e., the spicule is produced), (2) the energy flux enough for
heating the quiet corona (~3.0×105 ergs s-1
cm-2) is transported into the corona, and (3) nonthermal
broadening of emission lines in the corona is expected to be ~20 km
s-1. We assumed that the Alfvén waves are generated by
random motions in the photosphere. As the Alfvén waves propagate
upward in the solar atmosphere, longitudinal motions are excited by
the nonlinear couplings. The longitudinal motions propagate upward
as slow or fast waves and lift up the transition region (i.e., the
spicule is produced). A part of the Alfvén waves are reflected in
the transition region, but the remaining waves propagate upward to
the corona and contribute both to the heating of the corona and the
nonthermal broadening of emission lines. The result of our simulation
would suggest that the quiet hot corona, nonthermal broadening of
lines, and spicules are caused by Alfvén waves that are generated in
the photosphere.
Title: Dynamics of Local Isolated Magnetic Flux Tubes in a Rapidly
Rotating Stellar Atmosphere
Authors: Chou, Wenchien; Tajima, Toshiki; Matsumoto, Ryoji; Shibata,
Kazunari
Bibcode: 1999PASJ...51..103C
Altcode:
The dynamics of magnetic flux tubes in a rapidly rotating stellar
atmosphere is considered. We focus on the effects and signatures of
the instability of the flux tube emergence influenced by the Coriolis
force. We present the results from a linear stability analysis and
discuss its possible signatures during the course of the evolution
of G-type and M-type stars. We carried out three-dimensional
magnetohydrodynamical simulations of local isolated magnetic flux
tubes under a magnetic buoyancy instability in co-rotating Cartesian
coordinates. We have found that the combination of the buoyancy
instability and the Coriolis effect gives rise to a mechanism to twist
the emerging magnetic flux tube into a helical structure. The tilt
angle, east-west asymmetry, and magnetic helicity of the twisted flux
tubes in the simulations have been studied in detail. The linear and
nonlinear analyses provide hints as to what kind of pattern of large
spots in young M-type main-sequence stars might be observed. We have
found that young and old G-type stars may have different distributions
of spots, while M-type stars may always have low-latitude spots. The
size of stellar spots may decrease when a star becomes older, due to
a decrease in the magnetic field. A qualitative comparison with solar
observations is also presented.
Title: Magnetic Reconnection as the Origin of Galactic-Ridge X-Ray
Emission
Authors: Tanuma, Syuniti; Yokoyama, Takaaki; Kudoh, Takahiro;
Matsumoto, Ryoji; Shibata, Kazunari; Makishima, Kazuo
Bibcode: 1999PASJ...51..161T
Altcode:
We present a scenario for the origin of the hot plasma in our Galaxy as
a model of strong X-ray emission [~3-10 keV; LX(2-10 keV)
~1038 erg s^{-1}], called Galactic Ridge X-ray Emission
(GRXE), which has been observed near to the galactic plane. GRXE is
thermal emission from a hot component (~7 keV) and a cool component
(~0.8 keV). Observations suggest that the hot component is diffuse, and
that it is not escaping away freely. Both what heats the hot component
and what confines it in the galactic ridge still remain puzzling,
while the cool component is believed to be created by supernovae. We
propose a new scenario: the hot component is heated by magnetic
reconnection, and confined by a helical magnetic field produced by
magnetic reconnection. We solved two-dimensional magnetohydrodynamic
equations numerically to study how magnetic reconnection, triggered
by a supernova explosion, creates hot plasmas and magnetic islands
(helical tubes), and how the magnetic islands confine the hot plasmas
in the Galaxy. This is one of the possible mechanisms to trigger
reconnection in the Galaxy. We conclude that magnetic reconnection is
able to heat the GRXE plasma if the magnetic field is localized in an
intense flux tube with Blocal ~30 mu G.
Title: X-ray emission caused by MHD Process in star forming region
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1999hea..work...40H
Altcode:
No abstract at ADS
Title: Magneto hydrodynamic simulations of magnetically driven mass
accretion in active galactic nuclei
Authors: Kuwabara, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1999AdSpR..23.1105K
Altcode:
When a nuclear gas torus in active galactic nuclei (AGN) is threaded
by large-scale magnetic fields, magnetic braking can drive mass
accretion from the torus to the central black hole. Through two
dimensional resistive MHD simulations, we studied the dependence
of the mass accretion rate on magnetic diffusivity η0 =
η/(vK0r0), where r0 is the reference
radius and vK0 is the Keplerian rotation speed at r =
r0. We found three regimes, (a) when 0 <= η0
<= 0.003 accretion takes place episodically, (b) when 0.003 <=
η0 <= 0.01 accretion rate increases monotonically
with time and approaches to a constant value, (c) when η0
> 0.01 the saturation level of the accretion rate decreases with
η0. If we assume the mass of the central black hole as
109Msolar and the mass of the circum-nuclear gas torus as
108Msolar, the mass accretion rate is larger than 1Msolar/yr
when η0 <= 0.02.
Title: Global three-dimensional MHD simulations of accretion disks
and jet formation in AGNS
Authors: Matsumoto, R.; Shibata, K.
Bibcode: 1999AdSpR..23.1109M
Altcode:
We present the results of three-dimensional global magnetohydrodynamic
(MHD) simulations of a gas torus in active galactic nuclei. When
the torus is threaded by large-scale vertical magnetic fields, a
magnetically driven jet is ejected from the torus. Since magnetic
braking extracts angular momentum from the torus, the surface layer
of the torus accretes like an avalanche. Due to the growth of global
non-axisymmetric MHD modes, the avalanching flow breaks up into
spiral channels and creates a helical structure inside the jet. We
found that when the torus is initially threaded by weak toroidal
magnetic fields, it evolves toward a quasi-steady state with β =
Pgas/Pmag ~ 10. As the angular momentum is
re-distributed, the torus becomes flattened and the angular momentum
distribution approaches that of Keplerian disks.
Title: 2. 5D Nonsteady MHD Simulations of Magnetically Driven Jets
from Accretion Disks by Using the CIP-MOCCT MethodCD
Authors: Kudoh, T.; Shibata, K.; Matsumoto, R.
Bibcode: 1999ASSL..240..203K
Altcode: 1999numa.conf..203K
No abstract at ADS
Title: Evidence of Magnetic Reconnection in Solar Flares and a
Unified Model of Flares
Authors: Shibata, Kazunari
Bibcode: 1999Ap&SS.264..129S
Altcode: 1998Ap&SS.264..129S
The solar X-ray observing satellite Yohkoh has discovered various new
dynamic features in solar flares and corona, e.g., cusp-shaped flare
loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections
from impulsive flares, transient brightenings (spatially resolved
microflares), X-ray jets, large scale arcade formation associated
with filament eruption or coronal mass ejections, and so on. It has
soon become clear that many of these features are closely related to
magnetic reconnection. We can now say that Yohkoh established (at least
phenomenologically) the magnetic reconnection model of flares. In this
paper, we review various evidence of magnetic reconnection in solar
flares and corona, and present unified model of flares on the basis
of these new Yohkoh observations.
Title: Are blazar jets magnetically driven outflows ?
Authors: Kudoh, T.; Aoki, S.; Koide, S.; Shibata, K.
Bibcode: 1999AN....320..311K
Altcode:
No abstract at ADS
Title: Magnetohydrodynamic Simulations of Recurrent X-Ray Flares
in Protostars
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1999ASSL..240..231H
Altcode: 1999numa.conf..231H
No abstract at ADS
Title: Two-Dimensional Numerical MHD Simulation of Magnetic
Reconnection in GalaxyCD
Authors: Tanuma, S.; Yokoyama, T.; Kudoh, T.; Shibata, K.
Bibcode: 1999ASSL..240..119T
Altcode: 1999numa.conf..119T
No abstract at ADS
Title: MHD Simulation of a Solar Flare Based on a Magnetic
Reconnection ModelCD
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1999ASSL..240..327Y
Altcode: 1999numa.conf..327Y
No abstract at ADS
Title: Three-Dimensional MHD Simulations of the Emergence of Twisted
Magnetic Flux Tubes in the Solar Atmosphere
Authors: Tonooka, H.; Matsumoto, R.; Chou, W.; Tajima, T.; Shibata, K.
Bibcode: 1999ASSL..240..343T
Altcode: 1999numa.conf..343T
No abstract at ADS
Title: The 3-Dimensional Numerical Simulation of Solar Flares
Authors: Magara, T.; Shibata, K.
Bibcode: 1999ASSL..240..341M
Altcode: 1999numa.conf..341M
No abstract at ADS
Title: General Relativistic Jet Formation from Black-Hole Magnetized
Accretion Disks
Authors: Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 1999ASSL..240..215K
Altcode: 1999numa.conf..215K
No abstract at ADS
Title: Numerical simulations of magnetic reconnection triggered by
a supernova
Authors: Tanuma, S.; Yokoyama, T.; Kudoh, T.; Shibata, K.
Bibcode: 1999AN....320..341T
Altcode:
No abstract at ADS
Title: MHD Simulation of Active Galactic Nuclei JetsCD
Authors: Kato, S.; Kudoh, T.; Shibata, K.
Bibcode: 1999ASSL..240..213K
Altcode: 1999numa.conf..213K
No abstract at ADS
Title: Dynamics of a Magnetic Flux Tube in Differentially Rotating
DisksCD
Authors: Kuwabara, A.; Matsumoto, R.; Shibata, K.; Chou, W.
Bibcode: 1999ASSL..240..233K
Altcode: 1999numa.conf..233K
No abstract at ADS
Title: Magnetically driven jets from accretion disks: the effect of
magneto-rotational instability
Authors: Kudoh, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1999AdSpR..23.1101K
Altcode:
We present the results of 2.5-dimensional MHD simulations for
jet formation from accretion disks in a situation such that the
magneto-rotational instability is occurring in the disk. When there is
no initial perturbation in the disk, the surface layer falls faster
than the equatorial part to make a surface avalanche and associated
jets. However, if we input an initially large perturbation (> 10 %
of sound speed) in the disk, the dense region of the disk falls on an
orbital time scale to make a more violent accretion and jet. In this
case, the accretion rate and mass loss rates are an order of magnitude
larger than those in the case without initial perturbation. The speed
of the jet is of order the Keplerian velocity of the disk regardless
of the instability.
Title: Resistive MHD Simulations of Magnetically Driven Mass Accretion
in Active Galactic Nuclei
Authors: Kuwabara, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1999ASSL..240..229K
Altcode: 1999numa.conf..229K
No abstract at ADS
Title: Magnetohydrodynamic Simulations of High-Beta Disks and
Low-Beta Disks
Authors: Matsuzaki, T.; Shibata, K.; Tajima, T.; Matsumoto, R.
Bibcode: 1999ASSL..240..235M
Altcode: 1999numa.conf..235M
No abstract at ADS
Title: MHD Simulations of Magnetic Reconnection and Solar
FlaresCD
Authors: Shibata, K.; Yokoyama, T.
Bibcode: 1999ASSL..240..303S
Altcode: 1999numa.conf..303S
No abstract at ADS
Title: Planet Formation by Parker Instability
Authors: Chou, Wenchien; Tajima, T.; Noguchi, K.; Shibata, K.
Bibcode: 1998AAS...193.9810C
Altcode: 1998BAAS...30.1395C
More than a dozen of extra-solar planets have been found so far. These
discoveries lead us to believe that planetary systems, like our
own solar system, are common in the universe. One difficulty with
the current model for the origin of planets and the solar system,
the so-called core-accumulation model, is that the time to produce
a giant protoplanet in the outer nebula would be too long. Here
we suggest a new scenario that may solve this difficulty: in
a magnetized protostellar nebula the Parker-Jeans instability
(self-gravitional magnetic buoyancy instability) creates dense gas
blobs (which may be regarded as planetesimals) in a relativily short
time scale. This magnetic instability may act in concert with the
standard core-accumulation scenario to build up the masses of giant
planets. Our 3D self-garvitational MHD simulation will elucidate the
process we raise.
Title: Magnetohydrodynamic Simulation of Jet Formation in Close
Vicinity of Rotating Black Hole
Authors: Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 1998tx19.confE.413K
Altcode:
Astrophysical jets from AGNs and microquasars are separated into two
classes; relativistic (Lorentz factor ~2-20) and sub-relativistic one
(velocity ~0.1-0.3c). It is believed that the jets of both classes
are formed near the central highly condensed objects: black holes or
neutron stars. Some observations suggest that the rotation of the
central objects determine the speed of the jets. In fact, general
relativistic MHD simulations of Schwarzshild black hole magnetosphere
with falling corona never show the highly relativistic jets (Koide,
Shibata, & Kudoh 1998). The fast rotating black hole is expected
to form the highly relativistic jets. In this paper, we report a first
numerical simulations of relativistic jet formation in the Kerr black
hole magnetosphere.
Title: Magnetically Driven Jets from Accretion
Disks. III. 2.5-dimensional Nonsteady Simulations for Thick Disk Case
Authors: Kudoh, Takahiro; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 1998ApJ...508..186K
Altcode:
We present the results of 2.5-dimensional MHD simulations of jet
formation by magnetic accretion disks in which both ejection and
accretion of disk plasma are included self-consistently. Although the
jets in nonsteady MHD simulations have often been described as transient
phenomena resulting from a particular choice of initial conditions, we
found that the characteristics of the nonsteady jets are very similar
to those of steady jets: (1) The ejection point of the jet, which
corresponds to the slow magnetosonic point in steady MHD jet theory,
is determined by the effective potential resulting from gravitational
and centrifugal forces along a field line. (2) The dependences of the
velocity (vz) and mass outflow rate (Ṁw)
on the initial magnetic field strength are approximately
Ṁw~B0andvz~((Ω2FB20)/(Ṁw))1/3~B1/30
, where B0 is the initial poloidal magnetic field
strength and ΩF is the ``angular velocity of the
field line'' (essentially the Keplerian angular velocity where
the jet is ejected). These are consistent with the results of
one-dimensional steady solutions, although their explanation is a
little more complicated in the 2.5-dimensional case, because of an
avalanche-like accretion flow that is present. The dependence of
the accretion rate (Ṁa) on the initial field strength
is given by Ṁa~Bb0 where b ~ 1.4
from the simulations and b ~= 2 from the semianalytical results. We
also confirm that the velocity of the jet is of order the Keplerian
velocity of the disk for a wide range of parameters. We conclude that
the ejection mechanism of nonsteady jets found in the 2.5-dimensional
simulations can be understood using the steady state theory even when
nonsteady avalanche-like accretion occurs along the surface of the
disk. Nevertheless, it must be stressed that the jet and accretion never
reach a steady state in our simulations, in which the back-reaction
of the jet on the disk is included self-consistently.
Title: X-Ray Variability and Mass Outflows in Class I Protostar
Authors: Hayashi, Mitsuru; Shibata, Kazunari; Matsumoto, Hyoji
Bibcode: 1998plth.conf..211H
Altcode:
Recent X-ray astronomical observations show that magnetohydrodynamic
(MHD) view is of vital importance in understanding the dynamics of the
X-ray emitting hot plasmas in the universe. Especially they show that
magnetic reconnection plays a key role. The Solar X-ray satellite,
Yohkoh, showed various evidences of magnetic reconnection in solar
corona. Here we extend the magnetic reconnection model of solar flares
to hard X-ray flares observed in star forming regions. We carried out
2.5-dimensional magnetohydrodynamic (MHD) simulations of the disk-star
interaction. The closed magnetic loops connecting the central star
and the disk are twisted by the rotation of the disk. In the presence
of resistivity, magnetic reconnection takes place in the current sheet
formed inside the expanding loops. Hot, outgoing plasmoid and post flare
loops are formed as a result of the reconnection. Numerical results
are consistent with the observed plasma temperature (107
- 108)K, the length of the flaring loop (1011 -
1012)cm, and the velocity of optical jets (200 - 400km/s). We
showed by high-resolution numerical simulations using parallel computers
that multiple magnetic islands are created in the current sheet due
to the growth of the tearing mode instability. The magnetic islands
are ejected quasi-periodically. the intermittent flaring activity
continues so long as the disk matter twists the post-flare loops.
Title: Single and Multiple Solar Flare Loops: Hydrodynamics and Ca
XIX Resonance Line Emission
Authors: Hori, Kuniko; Yokoyama, Takaaki; Kosugi, Takeo; Shibata,
Kazunari
Bibcode: 1998ApJ...500..492H
Altcode:
Studies made so far with one-dimensional hydrodynamic simulations have
shown that it is difficult to reproduce the soft X-ray spectral line
profile observed in the early phase of solar flares. Simulated line
profiles predict a dominant emission from a large blueshifted component,
while observations show persistently strong stationary components. We
resolve these discrepancies by utilizing a multiple-loop system instead
of just a single loop for conductively heated flare simulations. Under a fixed heat input rate, we examine how the heating duration
τheat affects the Ca XIX resonance (w) line emission
from single and multiple flare loops. In the multiple-loop model,
the flare energy is released into individual loops with a specified
time delay, which implicitly mimics the successive formation of flare
loops due to continuous reconnection. We find that whether or not
τheat is longer than τc affects the hydrodynamic
response in an individual flare loop, where τc corresponds
to the time when the loop is filled with evaporated plasma. The Ca
XIX spectral line shape is characterized by an intensity ratio of
emission from evaporated plasma to emission from accumulated plasma
after evaporation. This ratio is mainly determined by the parameter
τheat/τc. Our findings suggest that
the following scenario can naturally explain the observed spectral
line features. Flare energy is injected into a bundle of loops
successively in two steps: in the preflare stage, τheat
<= τc for the inner loops, and then in the main flare
stage, τheat > τc for the outer loops. A
large initial coronal density is not necessary in this scenario.
Title: X-Ray Plasma Ejection Associated with an Impulsive Flare on
1992 October 5: Physical Conditions of X-Ray Plasma Ejection
Authors: Ohyama, Masamitsu; Shibata, Kazunari
Bibcode: 1998ApJ...499..934O
Altcode:
The 1992 October 5 flare was associated with an X-ray plasma
ejection. Although the ejected plasma looks like a blob (or plasmoid)
in short-exposure images, in long-exposure images it appears to be
penetrated by or connected to the top of a large-scale loop. The ejecta
had started to rise with a speed of ~250 km s-1 before the
main peak of the hard X-ray emission and was accelerated during the
impulsive phase (to ~500 km s-1). We derived the physical
parameters of the ejected plasma and obtained the following results:
(1) The temperature of the ejected plasma was 10.6 +/- 3.6 MK. (2)
Its density was (8-16) × 109 cm-3 and was
an order of magnitude larger than that of the typical active-region
corona. (3) The mass of the ejected plasma was (2-4) × 1013
g. (4) The kinetic energy of the ejecta was smaller than the thermal
energy content of the flare loop. The overall features and evolution
of the hot plasma ejection and flare are in rough agreement with
those expected from the reconnection model, and the reconnection rate
(MA = Vin/VA) is estimated to be ~0.02,
where Vin is the speed of the inflow into the reconnection
region, and VA is the Alfvén speed. Result 4, however, is
not consistent with the assumption in some reconnection models that
an ejected plasma stretches the overlying magnetic fields to form
a current sheet and hence leads to magnetic reconnection. Instead,
our results suggest that both X-ray plasma ejection and reconnection
are a consequence of a common dynamical process such as the global
MHD instability.
Title: General Relativistic Magnetohydrodynamic Simulations of
Jets from Black Hole Accretions Disks: Two-Component Jets Driven by
Nonsteady Accretion of Magnetized Disks
Authors: Koide, Shinji; Shibata, Kazunari; Kudoh, Takahiro
Bibcode: 1998ApJ...495L..63K
Altcode:
The radio observations have revealed the compelling evidence of
the existence of relativistic jets not only from active galactic
nuclei but also from ``microquasars'' in our Galaxy. In the cores
of these objects, it is believed that a black hole exists and that
violent phenomena occur in the black hole magnetosphere, forming the
relativistic jets. To simulate the jet formation in the magnetosphere,
we have newly developed the general relativistic magnetohydrodynamic
code. Using the code, we present a model of these relativistic jets,
in which magnetic fields penetrating the accretion disk around a
black hole play a fundamental role of inducing nonsteady accretion and
ejection of plasmas. According to our simulations, a jet is ejected
from a close vicinity to a black hole (inside 3rS, where
rS is the Schwarzschild radius) at a maximum speed of ~90%
of the light velocity (i.e., a Lorentz factor of ~2). The jet has a
two-layered shell structure consisting of a fast gas pressure-driven
jet in the inner part and a slow magnetically driven jet in the outer
part, both of which are collimated by the global poloidal magnetic
field penetrating the disk. The former jet is a result of a strong
pressure increase due to shock formation in the disk through fast
accretion flow (``advection-dominated disk'') inside 3rS,
which has never been seen in the nonrelativistic calculations.
Title: Magnetic Field Properties of Solar X-Ray Jets
Authors: Shimojo, Masumi; Shibata, Kazunari; Harvey, Karen L.
Bibcode: 1998SoPh..178..379S
Altcode:
From a list of X-ray jets made by Shimojo et al. (1996), we selected
events for which there were magnetic field data from NSO/Kitt
Peak. Using co-aligned SXT and magnetograms, we examined the magnetic
field properties of X-ray jets. We found that 8% of the jets studied
occurred at a single pole (SP), 12% at a bipole (BP), 24% in a mixed
polarity (MP) and 48% in a satellite polarity (ST). If the satellite
polarity region is the same as the mixed polarity region, 72% of the
jets occurred at the (general) mixed polarity region.
Title: Two-Sided-Loop Type X-ray Jets and Metric Radio Bursts
Authors: Kundu, M. R.; Raulin, J. -P.; Nitta, N.; Shibata, K.;
Shimojo, M.
Bibcode: 1998SoPh..178..173K
Altcode: 1998SoPh..178..611K
We have searched for nonthermal radio signatures in the form of metric
type III bursts in conjunction with two-sided-loop-type X-ray jets
observed by the Yohkoh/SXT experiment. We have found no evidence of
type III bursts in association with this particular type of X-ray jets
in contrast to the positive evidence of type III's in association
with anemone-type X-ray jets. This result is consistent with the
simulation results of Yokoyama and Shibata (1995), which show that
anemone-type jets are produced by vertical/oblique plasma flow whereas
the two-sided-loop-type jets are produced by horizontal plasma flow.
Title: A Two-dimensional Magnetohydrodynamic Simulation of
Chromospheric Evaporation in a Solar Flare Based on a Magnetic
Reconnection Model
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1998ApJ...494L.113Y
Altcode:
A two-dimensional simulation of a solar flare is performed using
a newly developed magnetohydrodynamic (MHD) code that includes
a nonlinear anisotropic heat conduction effect. The numerical
simulation starts with a vertical current sheet that is line-tied at
one end to a dense chromosphere. The flare energy is released by the
magnetic reconnection mechanism that is stimulated initially by the
resistivity perturbation in the corona. The released thermal energy
is transported into the chromosphere by heat conduction and drives
chromospheric evaporation. Owing to the heat conduction effect, the
adiabatic slow-mode MHD shocks emanated from the neutral point are
dissociated into conduction fronts and isothermal slow-mode shocks. We
discovered two new features, i.e., (1) a pair of high-density humps
on the evaporated plasma loops that are formed at the collision
site between the reconnection flow and the evaporation flow,
and (2) a loop-top dense blob behind the fast-mode MHD shock. We
also derived a simple scaling law for the flare temperature described
asTA~((B3L)/(2πκ0sqrt(4πρ)))2/7~B6/7,
where TA, B, ρ, and κ0 are the temperature at
the flare loop apex, the coronal magnetic field strength, the coronal
density, and the heat conduction coefficient, respectively. This
formula is confirmed by the numerical simulations. Temperature and
derived soft X-ray distributions are similar to the cusplike structure
of long-duration-event (LDE) flares observed by the soft X-ray telescope
aboard Yohkoh. Density and radio free-free intensity maps show a simple
loop configuration that is consistent with the observation with the
Nobeyama Radio Heliograph.
Title: Professor Fukushima (1910 - 1997 November 16).
Authors: Gotoh, H.; Oba, Y.; Ikuta, S.; Takeishi, M.; Shibata, K.;
Kitao, K.; Yabu, Y.
Bibcode: 1998Heavn..79....8G
Altcode:
No abstract at ADS
Title: Early Evolution of Coronal Active Regions Observed with the
YOHKOH Soft X-Ray Telescope. I. Expansion Velocity
Authors: Yashiro, Seiji; Shibata, Kazunari; Shimojo, Masumi
Bibcode: 1998ApJ...493..970Y
Altcode:
We study the early evolution of active regions in the corona by
analyzing 33 emerging flux regions (EFRs) observed with the soft X-ray
telescope (SXT) aboard Yohkoh during the period from 1992 February to
1996 May. We examine the time variation of the size (projected area)
and the total soft X-ray intensity of the EFRs, and find that the
time-averaged apparent velocity of the expansion of most of the EFRs
is less than 2 km s-1 during the very early phase (t <
6-14 hr after the birth of the EFRs). The average expansion velocity
of 33 EFRs is 1.5 km s-1. This expansion velocity is much
lower than the rise velocity of emerging magnetic loop in the upper
chromosphere, inferred from Hα observations of arch filament systems
as well as MHD simulation and theory. Some possibilities to account
for this discrepancy are discussed.
Title: Physical parameters of solar X-ray jets
Authors: Shimojo, M.; Shibata, K.; Yokoyama, T.; Hori, K.
Bibcode: 1998ESASP.421..163S
Altcode: 1998sjcp.conf..163S
No abstract at ADS
Title: Evolution of Coronal Active Regions Observed with the YOHKOH
Soft X-Ray Telescope
Authors: Yashiro, S.; Shibata, K.; Shimojo, M.
Bibcode: 1998ASSL..229..379Y
Altcode: 1998opaf.conf..379Y
No abstract at ADS
Title: A Unified Model of Solar Flares
Authors: Shibata, K.
Bibcode: 1998ASSL..229..187S
Altcode: 1998opaf.conf..187S
No abstract at ADS
Title: Formation of a Kinked Alignment of Solar Active Regions
Authors: Matsumoto, Ryoji; Tajima, Toshiki; Chou, Wenchien; Okubo,
Akane; Shibata, Kazunari
Bibcode: 1998ApJ...493L..43M
Altcode:
Images of the solar corona as observed by the soft X-ray telescope
aboard the Yohkoh satellite sometimes show a sequence of S-shaped
active regions in the low latitudes of the northern or southern
hemisphere. We suggest that the genesis of such a global structure is
the emergence of a large-scale twisted magnetic flux tube embedded in
the convection zone. When the magnetic twist exceeds a threshold value,
the flux tube deforms itself into a helical structure through the kink
instability. We present first results from a nonlinear three-dimensional
magnetohydrodynamic (MHD) simulations of a twisted flux tube initially
located in a gravitationally stratified, convectively unstable
atmosphere. The effects of gravity, buoyancy, and the layered structure
of the solar atmosphere (convection zone, cool photosphere/chromosphere,
and hot corona) are taken into account. As the instability develops,
the top portions of the helical structure rise by buoyancy and finally
emerge into the corona, forming a sequence of strongly sheared magnetic
loops. Intense vertical magnetic fields corresponding to sunspots are
created at the footpoints of these coronal magnetic loops.
Title: Magnetohydrodynamics of Accretion Disks
Authors: Matsumoto, R.; Matsuzaki, T.; Tajima, T.; Shibata, K.
Bibcode: 1998ASSL..229..115M
Altcode: 1998opaf.conf..115M
No abstract at ADS
Title: Wave Activity and Prominence Eruption
Authors: Baudin, F.; Bocchialini, K.; Delannee, C.; Koutchmy, S.;
Stellmacher, G.; Shibata, K.; Veselovsky, I. S.; Panasenko, O. A.;
Zhukov, A. N.
Bibcode: 1998ASPC..150..314B
Altcode: 1998IAUCo.167..314B; 1998npsp.conf..314B
No abstract at ADS
Title: Outflows and Jets from Magnetized Disks
Authors: Matsumoto, R.; Kudoh, T.; Shibata, K.; Hayashi, M. R.
Bibcode: 1998ASPC..137..286M
Altcode: 1998wsow.conf..286M
No abstract at ADS
Title: MHD simulations of jets from accretion disks: nonsteady jets
vs. steady jets
Authors: Kudoh, T.; Shibata, K.; Matsumoto, R.
Bibcode: 1998IAUS..184..361K
Altcode:
We present the results of 2.5-dimensional MHD simulations for jet
formation from accretion disks. Although the jets in nonsteady
MHD simulations (e.g, Uchida & Shibata 1985) have often been
referred to as a transient phenomena resulting from a special choice
of initial conditions (Hawley 1995), we found that the basic physics
of these nonsteady jets are able to be understood with a theory
of steady jets. (1) The velocity dependence on the field strength
(Vjet propto B1/3 for weak magnetic field) is
in rough agreement with that of the one-dimensional steady solutions
(Kudoh & Shibata 1995, 1997a, 1997b). The velocity of the jet
is an order of a Keplerian velocity of the disk for a wide range of
parameters. (2) The ejection point of the jet is determined by the
effective potential which results from the gravitational force and the
centrifugal force along a field line (Blandford & Payne 1982). We
also performed the simulation assuming that the initial magnetic field
is localized inside the disk in order to investigate the effect of the
initial global magnetic field assumed in many MHD simulations of jets.
Title: Resistive Processes in the Prefiare Phase of Eruptive Flares
Authors: Magara, T.; Shibata, K.
Bibcode: 1998IAUS..188..207M
Altcode:
In order to study how the resistive processes proceed in the preflare
phase of solar eruptive flares, we perform 2.5-D MHD simulations and
obtain those results which suggest that there are three stages in the
resistive processes. As an initially perturbation, we assign a random
velocity field in a current sheet and induce a tearing instability into
the system, which produces many small magnetic islands (stage I). These
magnetic islands then begin to coalesce with each other and through
this coalescent process, a lot of magnetic energy are converted into
thermal energy (stage II). Finally one large magnetic island appears
and a rapid increase of kinetic energy is found, showing the occurrence
of violent flow around the magnetic island (stage III). We then try to
relate these results to the preflare evolution of eruptive flares. At
stage I a spontaneous tearing instability occurs in a current sheet
and many small magnetic islands are produced within a coronal magnetic
arcade. At stage II the heat generated through a coalescent process
with magnetic islands is flowed along the helical magnetic fields
toward the chromosphere, causing the chromospheric evaporation. After
this coalescent process proceeds sufficiently, a large magnetic island
(magnetic flux tube in 3D sense) filled with the evaporated gases is
formed within the magnetic arcade (stage III).
Title: Study of Solar X-Ray Jets Observed by YOHKOH Soft X-Ray
Telescope
Authors: Shimojo, M.; Shibata, K.
Bibcode: 1998ASSL..229..357S
Altcode: 1998opaf.conf..357S
No abstract at ADS
Title: Magnetic Reconnection in the Active Region Inferred by
Homologous Soft X-ray Flares in February 1992
Authors: Zhang, H. Q.; Sakurai, T.; Shibata, K.; Shimojo, M.; Kurokawa,
H.; Morita, S.; Uchida, Y.
Bibcode: 1998ASSL..229..391Z
Altcode: 1998opaf.conf..391Z
No abstract at ADS
Title: Alfvén wave model of spicules
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1998ESASP.421..281K
Altcode: 1998sjcp.conf..281K
No abstract at ADS
Title: Flares and MHD Jets in Protostar
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1998IAUS..188..232H
Altcode:
We present a magnetic reconnection model for hard X-ray emission and
flare-like hard X-ray variabilities associated with protostars detected
by ASCA. The energy released by protostellar flares is 102 -
105 times larger than solar flares. Moreover, the spectrum
is harder. A new ingredient in protostellar flare is the existence
of a protostellar disk which can twist the magnetic fields threading
the protostellar disk. We carried out magnetohydrodynamic (MHD)
simulations of the disk-star interaction. The closed magnetic loops
connecting the central star and the disk are twisted by the rotation of
the disk. In the presence of resistivity, magnetic reconnection takes
place in the current sheet formed inside the expanding loops. Hot,
outgoing plasmoid and post flare loops are formed as a result of the
reconnection. Numerical results are consistent with the observed
plasma temperature (107 - 108K), the length
of the flaring loop (1011-1012cm), the total
energy of X-ray flares (~1035-36erg). Furthermore, along
the opening magnetic loops, hot jet is ejected in bipolar directions
with speed 200-400 km/s. The speed and mass flow rate of the jet is
consistent with those of optical jets. Our model can explain both the
X-ray flare-like variability and mass outflow in star forming region.
Title: MHD Simulation of Chromospheric Evaporation in a Solar Flare
Based on Magnetic Reconnection Model
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1998IAUS..188..213Y
Altcode:
Two-dimensional magnetohydrodynamic simulation of chromospheric
evaporation associated with a solar flare is performed. The
thermal energy driving the evaporation is first supplied from
the coronal magnetic field through the magnetic reconnection
mechanism and is transported into the chromosphere by heat
conduction. Nonlinear anisotropic heat conduction and radiative cooling
of optically-thin plasma are taken into account. The results show
that temperature distribution is similar to the cusp-like structure of
long-duration-event (LDE) flares observed by the soft X-ray telescope
aboard Yohkoh satellite.
Title: Magnetic Reconnection and Flares in Star Forming Regions:
Flares in Protostars
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1998ASSL..229..137H
Altcode: 1998opaf.conf..137H
No abstract at ADS
Title: SXR Coronal Polar Jets and Recurrent Flashes
Authors: Koutchmy, S.; Hara, H.; Shibata, K.; Suematsu, Y.; Reardon, K.
Bibcode: 1998ASSL..229...87K
Altcode: 1998opaf.conf...87K
No abstract at ADS
Title: X-ray jets and X-ray plasmoids
Authors: Shibata, K.
Bibcode: 1998ESASP.421..137S
Altcode: 1998sjcp.conf..137S
No abstract at ADS
Title: Foreword
Authors: Koutchmy, S.; Martens, P.; Shibata, K.
Bibcode: 1998ESASP.421...14K
Altcode: 1998sjcp.conf...14K
No abstract at ADS
Title: Soft X-Ray Features of Prominence Eruption and Disappearance
Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.;
Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K.
Bibcode: 1998ASSL..229..371T
Altcode: 1998opaf.conf..371T
No abstract at ADS
Title: Theory of Flares ; MHD Jets
Authors: Shibata, K.
Bibcode: 1998IAUS..188....9S
Altcode:
Recently, the understanding of solar flares and related mass
ejections such as jets has greatly advanced both observationally
and theoretically. That is, Yohkoh has revealed a lot of evidence
of magnetic reconnection in flares, and discovered a new class of
jets, X-ray jets, which are also found to be related to magnetic
reconnection. On the other hand, recent development of supercomputer
has enabled us to simulate magnetic reconnection in various physical
situations, e.g., reconnection driven by shear motion or emerging
flux, reconnection coupled with heat conduction, reconnection in three
dimensional space, etc. These simulation studies have developed not
only fundamental theory of magnetic reconnection but also modeling
of solar flares and jets. We first review these theoretical studies
of solar flares/jets with emphasis on the fundamental physics of bulk
plasma acceleration and heating through magnetic reconnection. We next
discuss application of these MHD theories to related astrophysical
phenomena revealed by ASCA, such as protostellar flares, galactic
ridge X-ray emission, and jets ejected from accretion disks.
Title: Magnetically Driven Jets from Accretion Disks
Authors: Kudoh, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1998ASSL..229..143K
Altcode: 1998opaf.conf..143K
No abstract at ADS
Title: Plasmoid Formation in Eruptive Flares
Authors: Magara, T.; Shibata, K.; Yokoyama, T.
Bibcode: 1998ASSL..229..173M
Altcode: 1998opaf.conf..173M
No abstract at ADS
Title: Local Three-Dimensional MHD Simulations of the Parker
Instability in Differentially Rotating Disks
Authors: Matsuzaki, T.; Matsumoto, R.; Shibata, K.; Tajima, T.
Bibcode: 1998ASSL..229..321M
Altcode: 1998opaf.conf..321M
No abstract at ADS
Title: X-Ray Plasma Ejections and Jets from Solar Compact Flares
Observed with the YOHKOH Soft X-Ray Telescope
Authors: Ohyama, M.; Shibata, K.; Shimojo, M.; Yokoyama, T.
Bibcode: 1998ASSL..229..333O
Altcode: 1998opaf.conf..333O
No abstract at ADS
Title: The Supply of Magnetic Fields from a cD Galaxy to Intra-Cluster
Space
Authors: Hirashita, H.; Mineshige, S.; Shibata, K.; Matsumoto, R.
Bibcode: 1998IAUS..188..306H
Altcode:
Recent observations have revealed the existence of magnetic fields
in clusters of galaxies. Since the field strength is of the order of
several mu G, magnetic energy is comparable to the thermal energy of
intra-cluster medium. Thus the magnetic field is important for dynamical
and thermodynamical processes in clusters. However, the origin of the
magnetic fields in clusters remains to be understood. We consider, here,
that fields may originate from cD galaxies, which exist at the center
of rich clusters. We conjecture that the magnetic fields in cD galaxies
can be lifted up to intra-cluster space by MHD instabilities. The most
possible instability is the Parker instability, which is a global
MHD instability in a gravitational field. The timescales of the
Parker instability in cD galaxies are estimated as 108yr,
which is shorter than the timescales of galaxy evolutions. In the
realistic situation, the magnetic tension due to the curved geometry
of a cD galaxy may stabilize such an instability. We, however, found
by linear analysis that the instability indeed takes place in spite
of the curvature effect.
Title: Three-Dimensional Local MHD Simulations of High States and
Low States in Magnetic Accretion Disks
Authors: Matsuzaki, T.; Matsumoto, R.; Tajima, T.; Shibata, K.
Bibcode: 1998IAUS..188..400M
Altcode:
We present the results of local three-dimensional magnetohydrodynamic
simulations of Keplerian accretion disks. The effects of vertical
gravity are included. Initial magnetic field is assumed to be in the
azimuthal direction. We confirmed the model proposed by Shibata et
al. (1990) that magnetic accretion disks fall into two types: gas
pressure dominated (high-beta) disk and magnetic pressure dominated
(low-beta) disk, where beta is the ratio of gas pressure to the magnetic
pressure. In high-beta disks, magnetic fields are amplified due to the
Balbus-Hawley instability. The system approaches to a quasi-steady state
with beta ~30. The effective value of the viscosity parameter alpha is
the order of 0.01. We found that low-beta disks stay in the low-beta
state for time scale much longer than the rotation period. Although
magnetic flux escapes from the disk by buoyancy, the amplification of
magnetic fields overcomes the magnetic flux loss unless beta is low
enough to cutoff the Balbus and Hawley instability. The effective
magnetic viscosity in low-beta state is the order of 0.1. When the
magnetic energy stored in the low-beta disk is released, we expect
large amplitude sporadic time variations as observed in low-states in
black hole candidates.
Title: Magnetic Reconnection as the Origin of Galactic Ridge X-Ray
Emission
Authors: Tanuma, S.; Yokoyama, T.; Kudoh, T.; Shibata, K.; Matsumoto,
R.; Makishima, K.
Bibcode: 1998IAUS..188..269T
Altcode:
We present a scenario for the origin of the hot plasma in our Galaxy, as
a model of a strong X-ray emission, called Galactic Ridge X-ray Emission
(GRXE), observed near the Galactic plane. Observations suggest that the
hot plasma is diffuse, and is not escaping away freely, but we don't
know why. We propose a new scenario: We conclude that when there is
a hot (T ~106K) phase of the ISM heated by the supernovae
near the Galactic plane, the magnetic reconnection is able to heat
it up to T ~108K comparable to the GRXE, if the local
magnetic field strength is B ~30 mu G (volume filling factor f ~0.1)
and that the helical magnetic field confines it. To test this scenario,
we solved the two-dimensional magnetohydrodynamic equations numerically
and studied the magnetic reconnection in our Galaxy.
Title: Magnetic reconnection as the origin of superhot plasmas in
the Galactic center
Authors: Yokoyama, T.; Tanuma, S.; Kudoh, T.; Shibata, K.
Bibcode: 1998IAUS..184..355Y
Altcode:
Recent X-ray astronomy satellite (e.g., Ginga, ASCA) has revealed that
the center of our Galaxy is filled with a large amount of very hot
plasmas (a few - 10 keV) on a scale of 100 pc, which are referred to
as superhot plasmas. These plasmas are similar to the Galactic Ridge
X-ray Emission (GRXE), but with larger gas pressure, and their formation
mechanism has been a big puzzle. Here we propose a new model, magnetic
reconnection model, to explain the heating as well as the confinement
of the Galactic center superhot plasmas, by performing MHD numerical
simulations of magnetic reconnection in the situation suitable for the
Galactic center. In our model, the magnetic field is amplified by the
rotation of the Galactic gas disk, and inflate from the disk to outside
by the Parker instability. The inflating magnetic loop collides with
ambient field lines, thus inducing the magnetic reconnection.
Title: Three-dimensional global MHD simulations of jet formation in
active galactic nuclei
Authors: Matsumoto, R.; Shibata, K.
Bibcode: 1998IAUS..184..363M
Altcode:
Three-dimensional global magnetohydrodynamic simulations have been
carried out to simulate the jet formation from an accretion disk
threaded by large scale magnetic fields. The initial magnetic field is
assumed to be uniform and parallel to the rotational axis. We assume
that the disk is initially in rotational equilibrium. Outside the disk,
hot, non-rotating spherical halo is assumed. Small, non-axisymmetric
perturbations are imposed on the rotation speed of the disk. We
confirmed our previous 2.5-dimensional results that the surface layer of
the disk accretes faster than the equatorial region like an avalanche
because magnetic braking extracts angular momentum. Due to the growth
of non-axisymmetric instabilities, spiral patterns appear in the
avalanching flow. Accretion proceeds along these spiral channels. The
non-axisymmetric accreting flow bunches the large-scale magnetic
fields into helical filaments. The radial magnetic fields created
by the avalanching motion are further twisted by the differential
rotation. Spirally shaped, magnetic pressure dominated regions appear
in the disk. When the accumulated twists relax as torsional Alfven
waves, they incorporate the matter in the surface layer of the disk
and form bipolar jets. Magnetic fields and density distribution show
helical structure in the jet. Numerical results will be demonstrated
by animation.
Title: Numerical Simulation of Relativistic Jet Formation in Black
Hole Magnetosphere
Authors: Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 1998ASSL..229..149K
Altcode: 1998opaf.conf..149K
No abstract at ADS
Title: Numerical Simulation of Relativistic Jet Formation in Black
Hole Magnetosphere
Authors: Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 1998IAUS..188..415K
Altcode:
The radio jets not only from active galactic nuclei (AGNs) but also
from the microquasars in our Galaxy such as GRS 1915+105 sometimes
appear to move faster than light (Miley 1980; Mirabel & Rodriguez
1994). These phenomena, which are called superluminal motion, are
explained as relativistic jets propagating almost toward us with Lorentz
factor more than 2. These relativistic jets are thought to arise from
processes associated with black holes. For the formation of such a
relativistic jet, magnetically driven mechanism around a black hole is
most promising. To simulate the process, we have developed a general
relativistic magnetohydrodynamic (GRMHD) numerical code. We have found
the formation of magnetically driven jets with ~90% of light velocity
(i.e. Lorentz factor ~2.3) near the black hole. The result shows that
the relativistic jet is formed nearer to the black hole than expected
by the present theory. We also newly find the difference of the jet
formation mechanism between the relativistic and nonrelativistic cases.
Title: Magnetic Field Amplification ; Intergalactic Plasma Heating
through Magnetic Twist Injection from Rotating Galaxies
Authors: Matsumoto, R.; Valinia, A.; Tajima, T.; Mineshige, S.;
Shibata, K.
Bibcode: 1998IAUS..188..315M
Altcode:
We propose a mechanism of amplification of magnetic fields and plasma
heating in clusters of galaxies. Recent observations indicate the
existence of several mu G magnetic fields in clusters of galaxies. There
should be some mechanism which locally amplify magnetic fields. In
weakly magnetized (high-beta) plasma, magnetic fields tend to be
concentrated in filaments. In clusters of galaxies, individual motions
of galaxies may create locally strong field region by stretching and
tangling the magnetic fields threading the galaxies. Here we present
the results of three-dimensional magnetohydrodynamic simulations of
large-scale magnetic fields threading the rotating galaxies. Since
torsional Alfven waves generated by the rotation of galaxies extract
angular momentum, the rotating disk begins to infall toward the galactic
center. The infalling gas further twists the magnetic fields and bunch
them into a twisted filament along the rotational axis. Numerical
results show that the magnetic pressure of this filament is comparable
to the thermal pressure of the intergalactic medium. When the magnetic
twist accumulates, the flux tube deforms itself into a helical structure
due to the kink instability. Magnetic reconnection taking place after
this stage creates hot X-ray emitting plasma.
Title: Magnetic avalanche model of mass supply in active galactic
nuclei
Authors: Kuwabara, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1998IAUS..184..365K
Altcode:
Global magnetohydrodynamic(MHD) simulations of accretion disks
threaded by large-scale magnetic fields have shown that in fully
ionized disks, when magnetic field is weak, the surface layer of the
disk accretes like an avalanche because magnetic braking efficiently
extracts angular momentum from that layer. In the avalanche, the
radial flow speed can be the order of the free-fall speed. The mass
accretion rate of the avalanche is a decreasing function of β =
Pgas/Pmag in the disk. In active galactic nuclei,
in the region 1-100pc from the central black hole, the disk may not be
fully ionized. Since the resistivity increases when the ionization rate
decreases, we carried out 2.5-dimensional MHD simulations including
resistivity. The magnetic avalanche ceases when resistivity is
sufficiently large. In such resistive disks, accretion proceed mainly
in the equatorial plane. Even when the initial state is a torus with
constant angular momentum distribution, slowly accreting Keplerian
disk is formed. When the surface of such resistive disk is ionized by
external UV radiation, magnetic avalanche will start and enhance the
mass supply to the central black hole.
Title: Pseudo-Two-dimensional Hydrodynamic Modeling of Solar Flare
Loops
Authors: Hori, Kuniko; Yokoyama, Takaaki; Kosugi, Takeo; Shibata,
Kazunari
Bibcode: 1997ApJ...489..426H
Altcode:
We have developed a ``pseudo-two-dimensional'' model of solar
flare loops from one-dimensional hydrodynamic calculations. The
model consists of thermally isolated and fixed semicircular loops
with different lengths and constant cross sections. To simulate
a magnetic reconnection process, flare energy release is assumed
to take place as heat depositions at the top portion of each loop,
proceeding successively from the innermost loop to the outermost. In
addition to temperature, density, and pressure distributions of the
pseudo-two-dimensional flare loops, we compute surface brightness
distributions of soft X-ray (SXR) emissions seen in bandpass filters
of the Yohkoh Soft X-Ray Telescope (SXT). We find the following: (1)
SXR brightening starts from the footpoints and expands into the upper
corona as a result of chromospheric evaporation in the multiple-loop
system. The resulting SXR-emitting structure has an apparent uniform
width, which does not necessarily trace the underlying field lines. The
outer edge of the structure corresponds to higher temperature regions
(13-17 MK). (2) If the flare heating ceases before the structure is
completely filled with evaporated plasma, a transient high-pressure
region is produced at the top, which appears as a compact bright
loop-top source in the SXT Be 119 μm filter, but not in the Al 0.1
μm filter.
Title: Parker Instability in Nonuniform Gravitational Fields. III. The
Effect of a Corona
Authors: Kamaya, Hideyuki; Horiuchi, Toshiro; Matsumoto, Ryoji;
Hanawa, Tomoyuki; Shibata, Kazunari; Mineshige, Shin
Bibcode: 1997ApJ...486..307K
Altcode:
Magnetized gas layers in gravitational fields (e.g., accretion disks and
galactic disks) can be subject to the Parker instability, an undular
mode of the magnetic buoyancy instabilities. By means of a linear
stability analysis, we examined the effects of hot, tenuous regions
(``coronae'') on the growth of the Parker instability in the underlying
magnetized gas layers. As an unperturbed state, we consider the
magnetized gas layers in static equilibrium. The stratified gas layers
are threaded by horizontal magnetic fields in the x-direction. The
temperature varies almost discontinuously at the coronal base in the
z-direction. The ratio of magnetic pressure to gas pressure, α, is
assumed to be constant. Our analysis has confirmed that the presence of
a corona reduces the growth rate of the Parker instability and increases
the critical wavelength. It is found that the growth of the Parker
instability is more sensitive to the height of the coronal base than the
temperature ratio between the disk and the corona is. In particular,
the Parker instability is stabilized substantially when the coronal
base lies below the height of maximum gravitational acceleration. When
the wavenumber vector of the perturbation is parallel to the magnetic
field (ky = 0), the growth rates of all modes in the disk
are reduced considerably in the limit of the vanishing coronal base
height. The first harmonic mode (1h-mode with odd symmetric velocity
eigenfunctions with respect to the equatorial plane) is more easily
stabilized by coronae than the fundamental mode is (f-mode with even
symmetric velocity eigenfunctions). This is because global convective
motion across the equatorial plane is allowed for the f-mode even when
ky = 0, whereas it is not allowed for the 1h-mode. For the
f-mode, furthermore, we find that the smallest possible γ (critical
gamma) against the instability is γcrit = 1 + α, regardless
of the value of ky. The reason for this is discussed briefly.
Title: Evolution of Eruptive Flares. I. Plasmoid Dynamics in
Eruptive Flares
Authors: Magara, Tetsuya; Shibata, Kazunari; Yokoyama, Takaaki
Bibcode: 1997ApJ...487..437M
Altcode:
We investigate the resistive processes of plasmoid dynamics in
eruptive flares by performing 2.5-dimensional resistive MHD numerical
simulations. We start with a linear force-free field arcade and
impose the localized resistive perturbation on the symmetry axis of
the arcade. Then the magnetic fields begin to dissipate, producing
inflows toward this region. These inflows make the magnetic fields
convex to the symmetry axis and hence a neutral point is formed on this
axis, leading to a formation of a magnetic island around the symmetry
axis. At the first stage, the magnetic island slowly rises by the upflow
produced by the initial resistive perturbation. Then, once the anomalous
resistivity sets in, the magnetic island begins to be accelerated. This
acceleration stops after the fast MHD shock is formed at the bottom
of the magnetic island, which implies that the upflow around the
central part of the magnetic island is no longer strong. These three
stages in the evolution of the plasmoid are confirmed to exist in the
observational results. Moreover, a time lag between the start time
when the magnetic island begins to be accelerated and the peak time of
the neutral-point electric field can be explained by the inhibition of
magnetic reconnection by the perpendicular magnetic field. We also study
the difference of the initial rise motion of the plasmoid between the
simulation results and the observational ones, and we conclude that,
in actual situations, the initial resistive perturbation proceeds very
weakly and at many positions inside the arcade.
Title: Statistics of Fluctuations in the Solar Soft X-Ray Emission
Authors: UeNo, S.; Mineshige, S.; Negoro, H.; Shibata, K.; Hudson,
H. S.
Bibcode: 1997ApJ...484..920U
Altcode:
X-ray emission from the Sun fluctuates as a result of occasional flare
events. We have calculated the power spectra of the solar soft X-ray
variations using the photometric data of the GOES 6 satellite. The data
cover the period 1991 September to 1994 April, about 32 months in total,
and we have worked with 10 minute averages. We find that the total power
spectral densities (PSDs) have three distinct components; a flat or
slightly decreasing low-frequency section, a 1/f-like moderate decline
medium-frequency section, and steep decline high-frequency parts. The
break frequencies separating three parts are fbreak ~=
10-4.7 Hz and 10-3.8 Hz , respectively. Such
downward breaks are expected from the shot-noise (flarelike)
character of solar X-ray emission, and we interpret the lower break
frequency as indicating an upper limit on flare timescales. These
break frequencies do not vary appreciably with activity level. This
suggests the existence of a universal mechanism for triggering flares
in the solar corona. Moreover, the power-law index (where we assume
PSD ~ f-β, f being frequency) of each part does not vary
appreciably with the level of activity either; its average is β ~=
0.45, 0.95, and 1.5, respectively. The overall shape of the PSD is
quite similar to those of other astrophysical objects such as black
hole candidate stars and active galactic nuclei, albeit on a vastly
different scale.
Title: Linear and Nonlinear Evolution of the Parker Instability of
Magnetic-Flux Sheets in Co-Rotating Coordinates
Authors: Chou, Wenchien; Tajima, Toshiki; Matsumoto, Ryoji; Shibata,
Kazunari
Bibcode: 1997PASJ...49..389C
Altcode:
We present a study of the magnetic buoyancy instability (or Parker
instability) of stellar atmospheres, in particular that of the effect of
the Coriolis force, via a linear stability analysis and nonlinear 3-D
MHD simulations. We find that fast rotation stabilizes this mode for a
toroidal flux sheet located near to the equator: magnetic flux sheets in
low-latitude regions are locally stable, while the growth rate of the
flux sheet near to the polar zone is not much reduced by the Coriolis
effect. This may explain the observational result that stellar spots
of fast rotating stars appear toward the polar region. Another effect
of the Coriolis force is that there is a cut-off rotational angular
velocity above which the long-wavelength transverse perturbations are
more unstable than the short wavelength ones. This may explain why large
stellar patches are observed in fast rotating stars. 3-D simulations of
magnetic flux sheets under buoyancy instability have also been carried
out in co-rotating Cartesian coordinates. The simulation results show
that the combination of the buoyancy instability and the Coriolis effect
gives rise to a mechanism to twist the magnetic-flux sheet embedded in
the solar (or stellar) atmosphere of a slowly rotating star (such as
the Sun), emerging to the surface with a helical structure. We suggest
that this can be a model to explain the S-shaped active regions in the
low latitudes of the Sun shown in the soft X-ray picture taken by the
Yohkoh satellite.
Title: Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet
Forming Disks
Authors: Matsumoto, R.; Matsuzaki, T.; Tajima, T.; Shibata, K.
Bibcode: 1997AAS...190.2711M
Altcode: 1997BAAS...29..813M
We carried out three-dimensional magnetohydrodynamic simulations of
differentially rotating disks. First, we present the results of local
simulations of a gravitationally stratified, Keplerian disk initially
threaded by azimuthal magnetic field. Numerical results indicate that
magnetic accretion disks have two states; a gas pressure dominated
quasi-steady state and a magnetic pressure dominated cataclysmic
state. In weakly magnetized disks, we confirmed the results by
Stone et al. (1996) that the disk approaches to a quasi-steady
state with beta =Pgas/Pmag ~ 30 and alpha_B
=-<B_x B_y/(4pi P)right > ~ 0.01. Once the magnetic pressure
is comparable to the gas pressure, however, the Balbus & Hawley
instability coupled with the Parker instability further amplifies
magnetic fields. We found that the disk stays in a low-beta (beta
<= 1) state for time scale longer than the rotation period. Large
amplitude sporadic time variations in low-state disks may be due to the
magnetic energy release in low-beta disks. By including the effects of
radial boundaries and curvature of azimuthal magnetic field lines,
we also carried out global three-dimensional simulations of the
whole disk. We assumed a differentially rotating polytropic plasma
threaded by azimuthal magnetic field (B_ϕ ~ 1/r). In a cylindrical
model neglecting vertical stratification, we found that when the
rotation law is nearly Keplerian, non-axisymmetric high-m (azimuthal
wave number) modes grow. The essential results of local simulations of
high-beta disks are confirmed by global simulations. When the initial
magnetic field is parallel to the rotation axis, since the rotating
cylinder tends to become low-beta, we included vertical gravity and
simulated the evolution of a torus from which plasma can expand along
the rotation axis. We show that a bipolar jet is ejected from the torus
and that non-axisymmetric, helical structures are created in the jet.
Title: Preflare Heating and Mass Motion in a Solar Flare Associated
with Hot Plasma Ejection: 1993 November 11 C9.7 Flare
Authors: Ohyama, Masamitsu; Shibata, Kazunari
Bibcode: 1997PASJ...49..249O
Altcode:
X-ray plasma ejection associated with an impulsive flare was observed
near to the solar limb on 1993 November 11 with the Yohkoh soft
X-ray telescope. We examined the physical condition as well as the
morphological evolution of the ejecta and flare. The results are
the following: (1) The shape of the ejecta was a loop before the
start of ejection. (2) The ejected loop appeared after its footpoint
brightened, and started to slowly rise long before the impulsive phase
(at $\sim 10$~km~s$^{-1}$, the preflare slow rise), but was suddenly
accelerated to $\sim 130$~km~s$^{-1}$ just before or at about the
onset of the impulsive phase ({\it the main rise}). (3) The ejected
material was already heated to $\sim 11.3 \pm4$~MK {\it before the
main rise. (4) The electron density of the ejecta ($\sim 4.5 \times
10^9-1.4 x 1010$~cm$^{-3}$) was larger than the typical
density of the active-region corona. The mass of the ejecta was $\sim
1014$~g. These results suggest chromospheric evaporation
due to preflare heating. It is suggested that magnetic reconnection
may already be occurring in the preflare phase, leading to heating of
the ejected material (possibly plasmoids) and causing chromospheric
evaporation. The reconnection rate (or the speed of the inflow into
the neutral point) is also estimated for the preflare, the impulsive,
and the gradual phases. }
Title: Disk Accretion onto a Magnetized Young Star and Associated
Jet Formation
Authors: Hirose, Shigenobu; Uchida, Yutaka; Shibata, Kazunari;
Matsumoto, Ryoji
Bibcode: 1997PASJ...49..193H
Altcode:
We investigated disk accretion onto a magnetized young star by
performing MHD numerical simulations. We considered the case in which
the stellar magnetosphere truncates the accretion disk carrying the
interstellar magnetic field, and the star is magnetized in the same
direction as the disk. In such a case, the interface between the
accretion disk and the magnetosphere contains a magnetically neutral
ring in the equatorial plane. The numerical results show that the disk
accretion drives the magnetic reconnection between the magnetospheric
field and the disk magnetic field, which allows mass transfer from the
disk to the magnetosphere. Most of the transferred mass accretes to the
star along the reconnected magnetospheric field, while the rest of the
mass is accelerated to the bipolar directions by a Lorentz force along
the stellar open magnetic field. This ``reconnection-driven'' jet is
further accelerated magneto-centrifugally due to stellar rotation, and
corresponds to optical jets from young stars. The magnetic braking as a
reaction of the magneto-centrifugal acceleration of the jet may explain
the observed slow rotations of young stars in the disk-accretion stage.
Title: Shrinkage of Coronal X-Ray Loops
Authors: Wang, J.; Shibata, K.; Nitta, N.; Slater, G. L.; Savy, S. K.;
Ogawara, Y.
Bibcode: 1997ApJ...478L..41W
Altcode:
We present the first set of examples of the shrinkage of large-scale
nonflare loops in the solar corona, observed by the Yohkoh Soft X-Ray
Telescope in 1993 February. A large and isolated active region exhibited
an unusual south-north asymmetry in coronal dynamics and heating. The
northern part, referred to the main magnetic axis, showed episodic
expansion and heating. In contrast, the southern part displayed obvious
shrinking and cooling. This asymmetry was correlated with a severe
asymmetry in the surface magnetic activity revealed by Huairou vector
magnetograms. Observations suggest that this shrinkage is not an
apparent motion, but a real contraction of coronal loops that brighten
as a result of heating at footpoints followed by gradual cooling.
Title: Magnetically Driven Jets from Accretion Disks. II. Nonsteady
Solutions and Comparison with Steady Solutions
Authors: Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 1997ApJ...476..632K
Altcode:
We perform time-dependent one-dimensional (1.5-dimensional)
magnetohydrodynamic numerical simulations of astrophysical jets that
are magnetically driven from Keplerian disks, in order to study the
origin and structure of jets ejected from star-forming regions,
close binary systems, and active galactic nuclei. We study the
initial-value problem, in which the Keplerian disk threaded by the
poloidal magnetic field suddenly begins to rotate and twists the field
line, generating ``nonsteady jets'' by the J X B force. This is similar
to the problem treated by Shibata & Uchida in their two-dimensional
(2.5-dimensional) simulations. The main purpose of this study is to
clarify the physical relation between such ``nonsteady jets'' and steady
jets, by using one-dimensional (1.5-dimensional) simulations. The
one-dimensional (1.5-dimensional) simulation has the merit that we
can perform simulations over many disk orbital periods with large
computational regions in a wide range of parameters. We find that
the jets, which are ejected from the disk, have the same properties as
the steady magnetically driven jets: (1) The mass flux of the nonsteady
jet strongly depends on the angle between the disk's surface and the
magnetic field line. (2) The scaling law known as Michel's solution is
also satisfied by the nonsteady jets. (3) The magnetic energy dependence
of the mass flux of the nonsteady jet is consistent with that of the
steady one. One of the most important findings in this study is that,
even when the initial poloidal magnetic field is very weak in the
disk [e.g., Emg = (magnetic energy/gravitational energy)
~ 10-6], a jet with a speed on the order of the Keplerian
velocity is produced by the effect of magnetic pressure force in the
toroidal fields generated from the poloidal fields by the rotation of
the disk. We also find several new nonsteady phenomena, which cannot
be found from the steady models but may be important for application:
MHD fast and slow shocks inside the jets, and quasi-periodic mass
ejections from the disk by large-amplitude Alfvén waves.
Title: Magnetically Driven Jets from Accretion Disks: Comparison
Between 2.5D Nonsteady Simulations and 1.5D Nonsteady/Steady Solutions
Authors: Kudoh, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1997ASPC..121..753K
Altcode: 1997IAUCo.163..753K; 1997apro.conf..753K
No abstract at ADS
Title: X-Ray Flares and Outflows Driven by Magnetic Interaction
Between a Protostar and its Surrounding Disk
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1997ASPC..121..717H
Altcode: 1997IAUCo.163..717H; 1997apro.conf..717H
No abstract at ADS
Title: Mass Flux and Toroidal Magnetic Field in Magnetically Driven
Jets from Accretion Disks: Steady Solutions and Application to
Jet/Wind in YSO
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1997ASPC..121..751K
Altcode: 1997IAUCo.163..751K; 1997apro.conf..751K
No abstract at ADS
Title: Rapidly Time Variable Phenomena: Jets, Explosive Events,
and Flares
Authors: Shibata, K.
Bibcode: 1997ESASP.404..103S
Altcode: 1997soho....5..103S; 1997cswn.conf..103S
No abstract at ADS
Title: Two-Dimensional MHD Simulation of Chromospheric Evaporation
Driven by Magnetic Reconnection in Solar Flares
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1997ESASP.404..745Y
Altcode: 1997cswn.conf..745Y
No abstract at ADS
Title: Three Dimensional MHD Simulations of Parker Instability in
Differentially Rotating Disk
Authors: Matsuzaki, T.; Matsumoto, R.; Tajima, T.; Shibata, K.
Bibcode: 1997ASPC..121..766M
Altcode: 1997IAUCo.163..766M; 1997apro.conf..766M
No abstract at ADS
Title: Plasma astrophysics
Authors: Tajima, T.; Shibata, K.
Bibcode: 1997plas.conf.....T
Altcode:
No abstract at ADS
Title: Magnetic Reconnection Coupled with Heat Conduction
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1997ApJ...474L..61Y
Altcode:
Magnetic reconnection coupled with nonlinear anisotropic heat
conduction is studied by using a two-dimensional magnetohydrodynamic
(MHD) simulation. Owing to the heat conduction effect, the adiabatic
slow-mode MHD shocks that emanate from the neutral point are dissociated
into conduction fronts and isothermal shocks. The dependence on heat
conductivity of the physical variables in the outflow region, such as
temperature, density, and velocity, are studied. We also discuss the
energy release and the reconnection rate.
Title: General Relativistic Simulation of Jet Formation from
Magnetized Accretion Disk
Authors: Koide, S.; Shibata, K.; Kudoh, T.
Bibcode: 1997ASPC..121..667K
Altcode: 1997IAUCo.163..667K; 1997apro.conf..667K
No abstract at ADS
Title: Magnetically Driven Jets from Accretion Disks. I. Steady
Solutions and Application to Jets/Winds in Young Stellar Objects
Authors: Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 1997ApJ...474..362K
Altcode:
We solve one-dimensional steady and axisymmetric magnetohydrodynamic
(MHD) equations to study basic properties of astrophysical jets
from accretion disks. Assuming the configuration of the poloidal
magnetic field, we solve for a wide range of parameters of the
poloidal magnetic field strength in the disk. We include a thermal
energy in the solution, although the jet is mainly accelerated by
the magnetic force, so that we are able to obtain the mass flux of
the jet and physical quantities, such as temperature, in the disk. We find that the mass flux (Ṁ) depends on the poloidal magnetic
field strength of the disk (Bp0) when the toroidal
component of the magnetic field (Bφ0) is dominant
near the disk surface, although it is independent of the magnetic
field when the poloidal component is dominant there:Ṁ~const,if
|Bφ/Bp|0<<1,Bp0,if
|Bφ/Bp|0>>1. Since
Michel's minimum energy solution
[v∞~(B2p0/Ṁ)1/3]
is almost satisfied in the magnetically driven jets, the
terminal velocity (v∞) depends on Bp0
as v∞~B1/3p0 when |
Bφ/Bp |0 >> 1,
and as v∞~B2/3p0 when |
Bφ/Bp |0 << 1. When the
toroidal component of the magnetic field is dominant near the disk
surface (| Bφ/Bp |0 >> 1),
the acceleration mainly takes place after the flow speed exceeds the
Alfvén speed. This means that the magnetic pressure largely contributes
to the acceleration of these jets. We also study the dependence of mass
flux on the other parameters, such as inclination angle of the poloidal
field, the rotational velocity of the disk, and the r-dependence of
the poloidal magnetic field strength along the field line, where r is
the distance from the axis. We discuss the application of these
models, i.e., the MHD jets from accretion disks, to jets/winds observed
in young stellar objects (such as optical jets, T Tauri winds, and fast
neutral winds). The mass-loss rates observed in these jets/winds will
constrain the physical quantities in the disks. When the mass-loss rate
is Ṁ~10-8 M⊙ yr-1 and the velocity
is vjet ~ 100 km s-1, the physical quantities
in the disk are n0 ~ 1013 cm-3,
Bp0 ~ 10 G, and T0 ~ 1000 K for a jet/wind
that is produced around r0 ~ 15 R⊙, and the
mass of the central star is 1 M⊙, where n0
is the number density, T0 is the temperature in the disk,
and r0 is the radial distance from the central star.
Title: Propagation of Nonlinear Alfvén Waves in the Solar Atmosphere:
Production of Spicules and Coronal Heating
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1997ESASP.404..477K
Altcode: 1997cswn.conf..477K
No abstract at ADS
Title: Statistical Properties of Microflares in an X-Ray Bright
Point within an Active Region
Authors: Shimojo, M.; Shibata, K.
Bibcode: 1997IAUJD..19E..45S
Altcode:
An X-ray Bright Point (XBP) is a relatively small scale (< 1^{'})
and small lifetime (about 2 days) emission feature. XBPs can be found
at practically all solar latitudes and they are associated with small
magnetic bipoles whose average total magnetic flux is 2 -- 3 times
1019 Mx. Harvey (1985) concluded that about one-third of
XBPs are associated with emerging magnetic flux (ephemeral regions)
while the other two-thirds are associated with canceling magnetic
feature. In this paper we examine the evolution of an XBP in an active
region observed with Yohkoh Soft X-ray Telescope (SXT). An XBP appeared
to be related to a new small emerging flux region near a preceding
spot of NOAA 7270 on September 1, 1992. We studied this "active region"
XBP using high resolution data of SXT. The XBP produced 92 microflares
(transient brightenings) during observation time of SXT, whose frequency
distribution as a function of the soft X-ray peak intensity shows a
single power-law. This result suggests that the power-law distribution
of microflares (Shimizu 1995) is universal and holds even in a small
emerging flux region.
Title: Solar Coronal Dynamics and Flares as a Cause of Interplanetary
Disturbances
Authors: Kosugi, Takeo; Shibata, Kazunari
Bibcode: 1997GMS....98...21K
Altcode: 1997mast.book...21K
Flares and coronal mass ejectios (CMEs) are the most energetic and
eruptive among various types of solar coronal magnetic activity, and
as such they might be responsible for major geomagnetic storms. It
has not yet been fully understood, however, how these two major
types of solar activity are interrelated. This article is aimed to
clarify this point, first, by summarizing observations over the past
several decades of various types of eruptive phenomena, and then, by
presenting recent observations from the Yohkoh satellite. The Yohkoh
observations show that the solar corona is intermittently expanding
and restructuring itself through magnetic reconnection, that flares
occur in association with rapid expansion and restructuring of the
surrounding active-region corona, and further that both flares
(irrespective of whether they are of impulsive or long-duration
type) and large-scale arcade formations (believed to be intimately
related to CMEs) show features such as "loop-with-a-cusp" structure
and "plasmoid/filament ejection" in common. We conclude that flares,
CMEs, and possibly some other related phenomena can be interpreted as
manifesting different aspects of a common process. Several fundamental
subprocesses are involved in this process, in which plasmoid ejection
and magnetic reconnection play dominant roles. A schematic model is
presented for understanding flares and CMEs in a unified way.
Title: X-ray plasma ejections and jets from solar compact flares
observed with the YOHKOH soft X-ray telescope
Authors: Ohyama, Masamitsu; Shibata, Kazunari; Yokoyama, Takaaki;
Shimojo, Masumi
Bibcode: 1997AdSpR..19.1849O
Altcode:
Yohkoh soft X-ray observations have revealed coronal X-ray plasma
ejections and jets associated with solar flares. We have studied an
X-ray plasma ejection on 1993 November 11 in detail, as a typical
example of X-ray plasma ejections (possibly plasmoids expected from
the reconnection model). The results are as follows: (1) The shape
of the ejected material is a loop before it begins to rise. (2) The
ejecta are already heated to 5 - 16 MK before rising. (3) The kinetic
energy of the ejecta is smaller than the thermal energy content of the
ejecta. (4) The thermal energy of the ejecta is smaller than that of
the flare regions. (5) The acceleration occurs during the impulsive
phase. These results are compared with the characteristics of X-ray
jets, and a possible interpretation (for both plasmoids and jets)
based on the magnetic reconnection model is briefly discussed.
Title: Magnetic Reconnection Coupled with Heat Conduction
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1997xisc.conf..579Y
Altcode:
No abstract at ADS
Title: A Numerical Model of the Existence of Two States in Magnetic
Accretion Disks
Authors: Matsumoto, R.; Matsuzaki, T.; Tajima, T.; Shibata, K.
Bibcode: 1997asxo.proc...87M
Altcode:
We present the results of three-dimensional magnetohydrodynamic
simulations of Keplerian accretion disks under local shearing box
approximation. The effect of vertical gravity is included. Initial
magnetic field is assumed to be in the azimuthal direction. We confirmed
the model proposed by Shibata et al. (1990) that magnetic accretion
disks fall into two types: gas pressure dominated (high-beta) disk and
magnetic pressure dominated (low-beta) disk. When the initial state
is high-beta, magnetic field is amplified due to the Balbus-Hawley
instability. The growth of magnetic fluctuations saturates when
beta=10-30. Angular momentum is transported outward due to the presence
of off-diagonal components of magnetic stress. The effective value of
alpha in the quasi-steady state in high-beta disk is 0.01. When the
initial state is low-beta, we found that the disk stays in the low-beta
state for time scale much longer than the rotation period. Unless
beta is extremely low, magnetorotational instability coupled with
the Parker instability generates fluctuating magnetic fields which
transport angular momentum. We discuss the possibility that large
amplitude sporadic time variations in low-state disks are due to the
magnetic energy release in low-beta disks.
Title: Magnetic reconnection coupled with heat conduction
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1997AdSpR..19.1801Y
Altcode:
Magnetic reconnection coupled with anisotropic nonlinear heat conduction
is studied by using an MHD simulation. Due to the heat conduction
effect, the adiabatic slow-mode MHD shocks emanating from the neutral
point are dissociated into conduction fronts and isothermal shocks.
Title: A Model of X-Ray Flares and Outflows in Star Forming Regions
Authors: Hayashi, M.; Shibata, K.; Matsumoto, R.
Bibcode: 1997xisc.conf..563H
Altcode:
No abstract at ADS
Title: Three-Dimensional Global MHD Simulations of Magnetised
Accretion Disks and Jets
Authors: Matsumoto, R.; Shibata, K.
Bibcode: 1997ASPC..121..443M
Altcode: 1997apro.conf..443M; 1997IAUCo.163..443M
No abstract at ADS
Title: Nonlinear Alfven wave propagation in the solar atmosphere:
production of spicules and coronal heating
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1997IAUJD..19E..29K
Altcode:
We present the results of 1.5-dimensional MHD simulations for solar
spicule formation and heating of the corona. The MHD simulations
are performed for torsional Alfven waves which propagate in an open
magnetic flux tube in the solar atmosphere. The Alfven waves are
assumed to be generated by random motions in the photosphere. As
the Alfven wave propagate up in the solar atmosphere, a part of the
Alfven wave is reflected at the transition region and produces a
slow mode magneto-hydrodynamic wave. Then, the slow mode wave lift
up the transition region and produces a spicule. The remaining Alfven
wave propagate up to the corona and will contribute to heating of the
corona. Our simulation shows that the enough energy flux for heating
the quiet corona (~3.0 times 10^5 erg s^{-1} cm^{-2}) is transported if
the root mean square of the random motion is greater than ~1 km s^{-1}
in the photosphere. Moreover, the transition region is lifted up to more
than ~7000 km when the the enough energy flux for coronal heating can be
transported to the corona. We expect that the velocity fluctuation of ~1
km s^{-1} in the photosphere could be observed by the Solar B satellite.
Title: Evolution of Coronal Active Regions Observed with the YOHKOH
Soft X-ray Telescope
Authors: Yashiro, S.; Shibata, K.; Shimojo, M.
Bibcode: 1997IAUJD..19E..58Y
Altcode:
We study the early evolution of active regions in the corona by
analyzing 56 emerging flux regions (EFRs) observed with the soft X-ray
telescope (SXT) aboard Yohkoh during the period from 1992 January to
1996 March. We examine the time variation of the size (projected area)
and the total soft X-ray intensity of the EFRs, and find that the
initial apparent velocity of the expansion of the EFRs is 0.4 - 3.6
km/s, which is much lower than that inferred from H alpha observations
of arch filament systems or from theory. We also find that there are
two types of evolution of EFRs. In one type (which we call the single
expansion type), the size and the total X-ray intensity of the EFR
monotonically increase for a few days after birth, and then slows or
stops. In the other type (multiple expansion type), the size and the
total X-ray intensity of an EFR show a non-monotonic or a multi-step
increase. We also study the thermal evolution of active regions in the
corona, and found that the temperature of EFRs increase with increasing
the region size.
Title: Plasmoid formation in eruptive flares
Authors: Magara, T.; Shibata, K.
Bibcode: 1997AdSpR..19.1903M
Altcode:
One phenomena Yohkoh has observed is plasmoid eruption in flares. Thus
this is a key factor that must be explained in any flare mechanism. In
order to understand the dynamics of a plasmoid, we performed a numerical
MHD simulation and investigated the evolution of the coronal magnetic
field, which is initially a force-free configuration. The main results
are as follows. At first, small amount of dissipation, induced by the
initial perturbation, occurs in the current sheet where the plasmoid
forms. This plasmoid is slowly going upward by magnetic tension force
of the reconnected magnetic fields produced by initial dissipation. The
crucial point comes when the perpendicular magnetic fields are washed
away from the reconnection point, after that the reconnection proceeds
effectively so that the magnetic tension force of the reconnected
fields becomes strong, which make the plasmoid be rapidly erupted
upward. These are consistent with the observational results, which say
that before the main energy release the plasmoid slowly rises and when
the flare sets in it is rapidly accelerated upward. In this paper,
we emphasize on the role that the perpendicular magnetic fields play
in the evolution of flare.
Title: X-ray Plasma Ejections Associated with Impulsive Solar Flares
Authors: Ohyama, M.; Shibata, K.
Bibcode: 1997IAUJD..19E..35O
Altcode:
Yohkoh discovered X-ray plasma ejections in some impulsive
flares. Although impulsive flares have previously been thought not to
be explained by a classical two ribbon flare reconnection model, these
observations suggest that impulsive flares occur through a magnetic
reconnection process similar to that proposed in a classical two-ribbon
flare model since the plasma ejection is a key feature predicted
by the reconnection model. In order to confirm this suggestion, we
analyzed X-ray plasma ejections quantitatively in detail and found
the following. (1) The X-ray ejects showed a loop-like structure. (2)
The temperature of the ejects was about 10 MK, and its density was
about 1010 cm^{-3}, which is about an order of magnitude
larger than the typical density of the active-region corona. (3)
The ejections started to rise before the main peak of the hard X-ray
emission and was accelerated during the impulsive phase.
Title: Simultaneous Acceleration of X-ray Jets and Surges by Magnetic
Reconnection Accompanied by Twist Injection
Authors: Okubo, A.; Matsumoto, R.; Miyaji, S.; Yokoyama, T.;
Shibata, K.
Bibcode: 1996AAS...189.1806O
Altcode: 1996BAAS...28.1298O
Many X-ray jet-like features, i.e., transient X-ray enhancements
with an apparently collimated motion are observed with soft X-ray
telescope aboard Yohkoh satellite. Many of these X-ray jets are
associated with flares in X-ray bright points, emerging flux regions,
or active regions. Shibata et al.(1992) reported that one of the
X-ray jets in NOAA7070 associated with a flaring bright point at
the footpoint of the H-alpha surge. An integrated analysis of the
observations of NOAA7070 by SXT, H-alpha, and vector magnetogram
showed that both the X-ray jets and H-alpha surges grow in the same
direction almost perpendicular to the emerging loops (okubo et al.,
1996). This indicates that the pre-existing oblique filed in this
region is almost perpendicular to the emerging loops. We extended the
numerical simulations of coronal X-ray jets by Yokoyama and Shibata
(1995) to the case when the emerging magnetic fields cross the
pre-existing oblique field (i.e., magnetic twists are injected into
the oblique field as a result of magnetic reconnection). We carried
out 2.5D MHD simulation including the three components of velocity
and magnetic fileds. A new factor introduced in our model is the shear
Alfven wave generated by the magnetic reconnection. We found that cool
plasmas on emerging magnetic loops are more efficiently accelerated than
those in previous 2D simulations. These cool plasmas may correspond
to the H-alpha surges simultaneously ejected with the X-ray jets. The
kinetic energy of jets, reconnection rate and the energy flux carried
by Alfven waves increase with the angle between the emerging magnetic
fields and the pre-existing field.
Title: A Filament Eruption and Accompanying Coronal Field Changes
on November 5, 1992
Authors: McAllister, A. H.; Kurokawa, H.; Shibata, K.; Nitta, N.
Bibcode: 1996SoPh..169..123M
Altcode:
An Hα filament eruption on November 5, 1992 was fully observed in Hα
with the Hida Flare Monitoring Telescope, while Yohkoh's Soft X-ray
Telescope observed the pre- and post-eruption evolution of the coronal
magnetic fields. From the Hα data, including the red and blue wings,
we have reconstructed the rise of the filament, including trajectory,
velocity, and acceleration. In combination with the Yohkoh data this
reconstruction suggests that the filament had several interactions
with other coronal magnetic fields during the eruption. The Yohkoh
data also shows pre-eruption changes in the coronal fields and several
post-eruption bright coronal structures. The pre-eruption changes are
interpreted as a partial opening of the corona, indicating that it is
not necessary to have a complete opening of the corona in order for a
filament to erupt and we discuss the several possible contributions
from emerging flux. The post-event bright coronal structures are
compared with theory and with a cleaner filament eruption event on
July 31, 1992. These comparisons suggest that, although there are many
similarities, it is hard to completely reconcile the observations with
the existing theory.
Title: X-Ray Flares and Mass Outflows Driven by Magnetic Interaction
between a Protostar and Its Surrounding Disk
Authors: Hayashi, M. R.; Shibata, K.; Matsumoto, R.
Bibcode: 1996ApJ...468L..37H
Altcode: 1996astro.ph..6157H
We propose a model of hard X-ray flares in protostars observed by
the ASCA satellite. We assumed that the dipole magnetic field of
the protostar threads the protostellar disk and carried out 2.5
dimensional magnetohydrodynamic (MHD) simulations of the disk-star
interaction. The closed magnetic loops connecting the central star
and the disk are twisted by the rotation of the disk. As the twist
accumulates, magnetic loops expand and finally approach the open field
configuration. A current sheet is formed inside the expanding loops. In
the presence of resistivity, magnetic reconnection takes place in the
current sheet. Outgoing magnetic island and postflare loops are formed
as a result of the reconnection. The timescale of this "flare" is the
order of the rotation period of the disk. The released magnetic energy
partly goes into the thermal energy and heats up the flaring plasma up
to 108 K. The length of the flaring loop is several times the radius
of the central star, consistent with observations. The speed of the
hot plasmoid ejected by the reconnection is 200--400 km s-1 when the
footpoint of the loop is at 0.03 AU from 1 Msolar protostar. The hot
plasma outflow can explain the speed and mass flow rate of optical
jets. Dense, cold, magnetically accelerated wind (v ~ 150--250 km s-1)
emanates from the surface of the disk along the partially open magnetic
field lines threading the disk. This dense, cold wind may correspond
to high-velocity neutral winds.
Title: Numerical Simulation of Magnetic Reconnection in Eruptive
Flares
Authors: Magara, Tetsuya; Mineshige, Shin; Yokoyama, Takaaki; Shibata,
Kazunari
Bibcode: 1996ApJ...466.1054M
Altcode:
Prompted by the Yohkoh observations of solar flares, which have
established the essential role of magnetic reconnection in the
release of energy, we have studied the evolution of eruptive
flares in some detail based on the reconnection model by means of
the two-dimensional magnetohydrodynamic (MHD) simulations. We are
interested in what factor affects the time evolution of solar flares
and how the related phenomena, particularly observed loop-top source
and plasmoid eruption, can be explained by this model. We have studied
the dependence of the structure and evolution of the system on plasma
β (ratio of gas pressure to magnetic pressure), adiabatic index, γ,
and ρc (initial density in the current sheet). If the time
scale and velocity are normalized by Alfvén time and Alfvén speed,
respectively, we find that the main results (e.g., reconnection rate,
plasmoid velocity, etc.) are rather insensitive to the plasma β. The
ρc value, on the other hand, crucially affects the motion
of a plasmoid: the ejection velocity of plasmoid grows in proportion
to ρc½ in the early phase, which suggests
that the observed plasmoid velocity can be reproduced when we assign
ρc ≃ 40 ρ0 (initial density outside the
current sheet). When adiabatic index y is small, corresponding to the
case of efficient thermal conduction, plasma heating will be generally
suppressed, but the compression effect can be rather enhanced, which
plays an important role in forming the high-density loop-top source. We
discuss loop-top sources, plasmoid eruption, and the rise motion of
a loop in comparison with the observations. Our simulations can well
account for the existence of the loop-top, hard X-ray sources discovered
in the impulsive flares. We concluded that both the impulsive flares
and the LDE (long duration event) flares can be generally understood
by the reconnection model for the cusp-type flares.
Title: Coronal X-ray jets
Authors: Shibata, Kazunari
Bibcode: 1996AIPC..382...18S
Altcode:
The soft X-ray telescope (SXT) aboard Yohkoh has discovered coronal
X-ray jets associated with small flares (microflares-subflares) in X-ray
bright points (XBPs), emerging flux regions (EFRs), or active regions
(ARs). These newly discovered jets may provide clues to solving the
coronal heating mechanism and acceleration of high speed solar wind. The
recent development of observations and theoretical modeling of X-ray
jets are reviewed with emphasis upon the role of magnetic reconnection
in generating these jets.
Title: H alpha Surges and X-Ray Jets in AR 7260
Authors: Canfield, Richard C.; Reardon, Kevin P.; Leka, K. D.; Shibata,
K.; Yokoyama, T.; Shimojo, M.
Bibcode: 1996ApJ...464.1016C
Altcode:
We discuss nine events, observed simultaneously as jets in X-rays and
surges in Hα, which are associated with moving magnetic bipoles. The
X-ray jets share many features with those discovered by Yohkoh in active
regions, emerging flux regions, and X-ray bright points (see paper by
Shibata et al.); in particular, they originate near one end of a pair of
small flaring loops. The Hα surges are adjacent to the X-ray jets. At
the bases of these surges we observe both blueshifts (initially) and
redshifts (1-2 minutes later). All the observed surges spin in a sense
consistent with the relaxation of the twist stored in the magnetic
fields of the moving magnetic bipoles. Newly discovered phenomena
include footpoint convergence and moving-blueshift features. We develop a model of the role of magnetic reconnection in these
events. This model explains the temporal and spatial relationship
between the jets and surges, the role of the moving bipoles, the
flaring X-ray loops and their converging Hα footpoints, the Hα
moving-blueshift features, the direction and amount of spin of the
surges, and the relative temporal development of the Hα redshifts
and blueshifts.
Title: Radio Jets and the Formation of Active Galaxies: Accretion
Avalanches on the Torus by the Effect of a Large-Scale Magnetic Field
Authors: Matsumoto, R.; Uchida, Y.; Hirose, S.; Shibata, K.; Hayashi,
M. R.; Ferrari, A.; Bodo, G.; Norman, C.
Bibcode: 1996ApJ...461..115M
Altcode:
We present the results of 2.5-dimensional MHD simulations for
a magnetohydrodynamic model of jet formation associated with the
formation of active galaxies. We also study the enhanced accretion near
the central object of active galactic nuclei. A new factor introduced in
our model is the presence of a large-scale poloidal magnetic field that
may correspond to either the primordial magnetic field swept into the
central region during the galaxy formation process or the central part
of a dynamo- generated magnetic field. The differentially rotating disk
produced around a central massive object such as a black hole interacts
with the large-scale magnetic field, and produces spinning bipolar jets
through the production and escape of the magnetic twists propagating
into bipolar directions (a form of large-amplitude torsional Alfven
waves). The production and escape of these removes angular momentum from
the disk material and allows an enhanced accretion rate in the disk. The
surface layers of disks accrete faster than the equatorial part because
the magnetic braking most effectively affects that layer. The infalling
gas spins up, and jet formation strengthens until magnetic reconnection
occurring at the inner edge of the disk saturates the process.
Title: Numerical Simulation of Solar Coronal X-Ray Jets Based on
the Magnetic Reconnection Model
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1996PASJ...48..353Y
Altcode:
We performed two-dimensional numerical simulations of solar coronal
X-ray jets by solving the resistive magnetohydrodynamic (MHD)
equations. The simulations were based on the magnetic reconnection
model, in which the plasma of an X-ray jet is accelerated and
heated by reconnection between the emerging flux and a pre-existing
coronal field. Many observed characteristics of X-ray jets could
be successfully reproduced. Morphologically, the two observed
types of jets, two-sided-loop type and anemone-jet type, were well
reproduced. Here, the two-sided-loop type is a pair of horizontal jets
(or loops), which occurs when an emerging flux appears in a quiet region
where the coronal field is approximately horizontal. In contrast, the
anemone-jet type is a vertical jet, which takes place when an emerging
flux appears in a coronal hole where the coronal field is vertical or
oblique. Quantitatively, the velocity, temperature, thermal energy,
kinetic energy, and other parameters obtained in the simulation are in
good agreement with the observations. Furthermore, the simulations
reveal new features which might be associated with X-ray jets:
(1) A fast-mode MHD shock is produced at the collision site of each
reconnection jet with the ambient magnetic field. (2) Reconnection
produces a cool jet as well as a hot jet (X-ray jet). The hot and cool
jets are adjacent to each other, which is consistent with the observed
simultaneous coexistence of X-ray jets and {Hα } surges in the sun.
Title: New observational facts about solar flares from YOHKOH studies
- evidence of magnetic reconnection and a unified model of flares
Authors: Shibata, K.
Bibcode: 1996AdSpR..17d...9S
Altcode: 1996AdSpR..17....9S
Recent discoveries on flares and related phenomena with the soft
X-ray telescope aboard Yohkoh are discussed with emphasis on evidence
of magnetic reconnection. These include also the big discovery of a
hard X-ray loop top source by Masuda et al. (1994) using the hard
X-ray telescope. It is emphasized that LDE (long duration events)
flares, large scale arcade formation, and (simple loop) impulsive
flares show many common features, such as plasmoid/filament ejection,
in Yohkoh images. I will discuss that many of them are interpreted
as due to magnetic reconnection, or to be related to reconnection,
and present a unified model to explain LDE flares and impulsive flares
with a single physical mechanism, in which a plasmoid ejection plays
a key role to induce fast reconnection. It is further discussed that
the same physical mechanism may be applied to smaller ``flares'',
such as microflares and X-ray jets, in a grand unified scheme.
Title: Coronal X-ray jets observed with Yohkoh/SXT
Authors: Shibata, K.; Yokoyama, T.; Shimojo, M.
Bibcode: 1996AdSpR..17d.197S
Altcode: 1996AdSpR..17..197S
The soft X-ray telescope (SXT) aboard Yohkoh has discovered coronal
X-ray jets associated with small flares in X-ray bright points
(XBPs), emerging flux regions (EFRs), or active regions (ARs). The
common observed characteristics of these jets are discussed mainly from
morphological points of view. It is suggested that magnetic reconnection
between emerging magnetic flux and the overlying coronal/chromospheric
magnetic field is a key physical process for producing these jets.
Title: Chromospheric ejections and their signatures in X-ray observed
by YOHKOH
Authors: Schmieder, B.; Mein, N.; Shibata, K.; van Driel-Gesztelyi,
L.; Kurokawa, H.
Bibcode: 1996AdSpR..17d.193S
Altcode: 1996AdSpR..17..193S
Surges, or condensations of chromospheric material, are commonly
observed in active regions when new emerging magnetic flux (EMF)
occurs. In order to study EMF-related phenomena in the corona, observing
campaigns were organized with the Yohkoh X-ray satellite and ground
based observatories. EMFs could be detected in magnetograms made in
Potsdam and Hawaii, surge and arch filament system (AFS) events (on
Oct 7 1991, May 1 1993, respectively) with the Multichannel Subtractive
Double Pass instruments operating at Meudon and on the Canary Islands
and with the Hida telescope at Kyoto University. Bright X-ray loops
coincident with chromospheric surge activity were detected in the high
resolution Yohkoh SXT images. We also found coincident brightness
variations between an X-ray bright point (XBP) and the underlying
chromospheric plage related to EMF. We interpret the appearance of
AFS as condensation of material among magnetic field lines, the X-ray
bright points as the result of a magnetic reconnection process, and
surges as ejection of cold plasma bubbles squeezed between field lines
as a result of reconnection.
Title: Emerging flux and flares in NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Shibata, K.
Bibcode: 1996AdSpR..17d.201N
Altcode: 1996AdSpR..17..201N
We have studied the relation between flux emergence and flare
activity in the active region NOAA 7260, using images from the Soft
X-ray Telescope aboard the Yohkoh spacecraft and other supporting
ground-based data. It is found that microflares start around the time
of flux emergence as recorded in white-light data, which generally
precedes a major flare by several hours. We interpret the microflares as
due to fast reconnection that takes place intermittently in the slow
reconnection stage while more energy is accumulated in preparation
for a larger flare.
Title: Chromospheric Evaporation in Four Solar Flares Observed
by YOHKOH
Authors: Ding, M. D.; Watanabe, T.; Shibata, K.; Sakurai, T.; Kosugi,
T.; Fang, C.
Bibcode: 1996ApJ...458..391D
Altcode:
We present an analysis of the Ca XIX soft X-ray spectra of four
flares observed by the Yohkoh spacecraft. These spectra show evident
blueshifted signatures. Using a two-component fitting technique,
we obtain the time variations of the fitting parameters. The
peak line-of-sight upflow velocities range from 180 to 320 km
s-1. The durations of upflows with observable velocities
(≥ 100 km s-1) are found to be 4-10 minutes. Particularly,
for two of the four flares, the emission measure of the blueshifted
component is larger than that of the stationary component at early
times. This fact agrees qualitatively with the predictions of numerical
simulations of chromospheric evaporation. There is a rough correlation
between the nonthermal turbulent velocity and the upflow velocity,
consistent with the results of Fludra et al. We also find that most Ca
xix resonance profiles show no significant shift of the profile peak,
regardless of the blue asymmetry which sometimes appears. The possible
causes of this phenomenon are discussed.
Title: Interstellar Mass Outflow to Galactic Halos by the
Supernova-driven Parker Instability
Authors: Kamaya, Hideyuki; Mineshige, Shin; Shibata, Kazunari;
Matsumoto, Ryoji
Bibcode: 1996ApJ...458L..25K
Altcode:
We here demonstrate by means of two-dimensional magnetohydrodynamic
simulations that even a single supernova (SN) explosion can easily
trigger a Parker instability. When SNs occur in the galactic disk
with horizontal magnetic fields, an outgoing blast wave will lift up
the fields, thus forming an Omega -shaped field structure. Hence,
the interstellar medium (ISM) can effectively be confined inside
the Omega -shaped, bent fields because of their decelerating J{X}B
force. It is thus usually thought that unless unrealistically large
numbers of SNs occur, it is difficult for the ISM to blow out into the
halo. We find, however, that this is the case only before the Parker
instability is excited; namely, for the first several megayears after
the explosion. The gas frozen into the Omega -shaped magnetic fields
will eventually slide down along the fields to the galactic disk by
gravity, so the magnetic fields can move upward by an enhanced magnetic
buoyancy. In roughly tens of megayears after the explosions the Parker
instability will be triggered, giving rise to a blowout of the disk
material into the halo. Importantly, a nonlinear Parker instability
can be triggered quite generally; i.e., even when the underlying gas
layer is stable against infinitesimal perturbations.
Title: Statistical Study of Solar X-Ray Jets Observed with the YOHKOH
Soft X-Ray Telescope
Authors: Shimojo, Masumi; Hashimoto, Shizuyo; Shibata, Kazunari;
Hirayama, Tadashi; Hudson, Hugh S.; Acton, Loren W.
Bibcode: 1996PASJ...48..123S
Altcode:
We have found 100 X-ray jets in the database of full Sun images taken
with the Soft X-ray Telescope (SXT) aboard Yohkoh during the period
from 1991 November through 1992 April. A statistical study for these
jets results in the following characteristics: 1)\ Most are associated
with small flares (microflares--subflares) at their footpoints. 2)\ The
lengths lie in the range of a few times 10(4) --4 times 10(5) km. 3)\
The widths are 5 times 10(3) --10(5) km. 4)\ The apparent velocities are
10--1000 km s(-1) with an average velocity of about 200 km s(-1) . 5)\
The lifetime of the jet extends to ~ 10 hours and the distribution
of the observed lifetime is a power law with an index of ~ 1.2. 6)\
76% of the jets show constant or converging shapes; the width of the
jet is constant or decreases with distance from the footpoint. The
converging type tends to be generated with an energetic footpoint
event and the constant type by a wide energy range of the footpoint
event. 7)\ Many jets ( ~ 68%) appear in or near to active regions
(AR). Among the jets ejected from bright-point like features in ARs,
most ( ~ 86%) are observed to the west of the active region. 8)\ 27%
of the jets show a gap ( > 10(4) km) between the exact footpoint of
the jet and the brightest part of the associated flare. 9)\ The X-ray
intensity distribution along an X-ray jet often shows an exponential
decrease with distance from the footpoint. This exponential intensity
distribution holds from the early phase to the decay phase.
Title: Observations and Numerical Studies of Coronal X-Ray Jets and
H alpha Surges Associated with Emerging Magnetic Fields
Authors: Okubo, A.; Matsumoto, R.; Miyaji, S.; Akioka, M.; Shibata,
K.; Yokoyama, T.
Bibcode: 1996ASPC..111...39O
Altcode: 1997ASPC..111...39O
Observations of the simultaneous ejections of an X-ray jet and Hα
surges in NOAA 7070 indicate that they are ejected in a direction
almost perpendicular to the emerging loop. The authors extended
numerical simulation of reconnection between an emerging loop and
overlying oblique magnetic field (Yokoyama and Shibata, 1995, 1996)
to the case where they are not in the same plane. Shear Alfvén waves
generated by the twist injection associated with magnetic reconnection
can accelerate cool plasma more efficiently than the shock compression.
Title: Formation of Reconnection-Driven Jets with Disk Accretion
onto Magnetized Young Stars
Authors: Hirose, S.; Uchida, Y.; Shibata, K.; Matsumoto, R.
Bibcode: 1996mpsa.conf..509H
Altcode: 1996IAUCo.153..509H
No abstract at ADS
Title: MHD Simulation of X-ray Jets Based on Magnetic Reconnection
Model
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1996mpsa.conf..315Y
Altcode: 1996IAUCo.153..315Y
No abstract at ADS
Title: 3D MHD Simulation of Magnetic Flux Tubes in Co-Rotating
Coordinates
Authors: Chou, W.; Tajima, T.; Matsumoto, R.; Shibata, K.
Bibcode: 1996mpsa.conf..613C
Altcode: 1996IAUCo.153..613C
No abstract at ADS
Title: X-ray Plasma Ejection in an Eruptive Flare
Authors: Ohyama, M.; Shibata, K.
Bibcode: 1996mpsa.conf..525O
Altcode: 1996IAUCo.153..525O
No abstract at ADS
Title: Three dimensional structure of magnetized disks.
Authors: Matsumoto, R.; Matsuzaki, T.; Hayashi, M.; Tajima, T.;
Mineshige, S.; Shibata, K.
Bibcode: 1996bpad.conf..305M
Altcode: 1996pada.conf..305M
The authors present the results of two-dimensional and three-dimensional
simulations of magnetohydrodynamic processes in accretion disks such
as (1) the escape of magnetic flux by the Parker instability, (2)
generation of fluctuating magnetic fields due to the Balbus-Hawley
instability, (3) jet formation in accretion disks threaded by
large-scale magnetic fields, and (4) plasma heating and plasmoid
ejection by magnetic reconnection. Large amplitude 1/f fluctuations
in X-rays from black hole candidates may be explained by sporadic
magnetic reconnections in advection-dominated, low-β disks in which
the magnetic pressure exceeds the gas pressure.
Title: Formation of a Kinked Alignment of Solar Active Region
Authors: Matsumoto, R.; Tajima, T.; Chou, W.; Shibata, K.
Bibcode: 1996mpsa.conf..355M
Altcode: 1996IAUCo.153..355M
No abstract at ADS
Title: Magnetic Reconnection Coupled with Heat Conduction
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1996ASPC..111..274Y
Altcode: 1997ASPC..111..274Y
Magnetic reconnection coupled with anisotropic nonlinear heat conduction
is studied by using MHD simulation. Due to the heat conduction effect,
the adiabatic slow-mode MHD shocks emanating from the neutral point
are dissociated into conduction fronts and isothermal shocks. An MHD
simulation of chromospheric evaporation is also performed.
Title: Magnetically Constricted Plasmas in Clusters of Galaxies
Authors: Valinia, Azita; Tajima, Toshiki; Matsumoto, Ryoji; Makishima,
Kazuo; Shibata, Kazunari
Bibcode: 1996ASPC...88..205V
Altcode: 1996clfu.conf..205V
We have studied the role of magnetic fields on the evolution and
dynamics of Intracluster Medium (ICM) through magnetohydrodynamics
simulations. We expect that the magnetic fields that are anchored in
the gravitational potential of galaxies can be considerably twisted,
braided and stretched due to the motion of galaxies, and reconnect
and heat the plasma trapped in the tangled fields. To this end, we
have performed a simple calculation of two counter-rotating galactic
disks in an ambient magnetic field parallel to the rotation axes
of the disks. Through the rotation of the galactic disks, toroidal
magnetic fields are produced leading to the amplification of the
initial fields. The evolution creates an intermittent plasma in the
intergalactic medium with hot plasma surrounded by cooler gas regions,
thus causing a two-phase medium and nonuniform temperature in the ICM.
Title: Numerical Simulation of Magnetic Reconnection in Eruptive
Flares
Authors: Magara, T.; Mineshige, S.; Yokoyama, T.; Shibata, K.
Bibcode: 1996mpsa.conf..585M
Altcode: 1996IAUCo.153..585M
No abstract at ADS
Title: Challenge to the riddle of solar flares. To what extent did
Yohkoh resolve solar flares?
Authors: Shibata, K.
Bibcode: 1996AstHe..89...60S
Altcode:
Since its launch on August 30, 1991, the solar X-ray observing satellite
Yohkoh has been successfully observing the Sun and obtained data of more
than 1000 flares. From extensive analysis of these vast amount of data,
the riddle of flares has begun to be solved. Yohkoh data has finally
given various evidence supporting the magnetic reconnection hypothesis,
which has been considered to be a promising model for flares but has
often been questioned due to the lack of clear observational evidence.
Title: Magnetically Driven Jets from Accretion Disks: Nonsteady and
Steady Solutions
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1996ApL&C..34..339K
Altcode:
No abstract at ADS
Title: MHD Simulation of Solar Coronal X-ray Jets: Emerging Flux
Reconnection Model
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1996ApL&C..34..133Y
Altcode:
No abstract at ADS
Title: Pseudo-Two-Dimensional Hydrodynamic Modeling of Flare Loops
Authors: Hori, K.; Yokoyama, T.; Kosugi, T.; Shibata, K.
Bibcode: 1996mpsa.conf..519H
Altcode: 1996IAUCo.153..519H
No abstract at ADS
Title: Magnetically Driven Jets from Accretion Disks: Nonsteady and
Steady Solutions
Authors: Kudok, T.; Shibata, K.
Bibcode: 1996mpsa.conf..511K
Altcode: 1996IAUCo.153..511K
No abstract at ADS
Title: Loop-Top Hard X-ray Source in Solar Flares
Authors: Masuda, S.; Kosugi, T.; Shibata, K.; Hara, H.; Sakao, T.
Bibcode: 1996mpsa.conf..203M
Altcode: 1996IAUCo.153..203M
No abstract at ADS
Title: Detection of Nonthermal Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Raoult, A.; Shibata, K.; Shimojo, M.
Bibcode: 1996mpsa.conf..445K
Altcode: 1996IAUCo.153..445K
No abstract at ADS
Title: H alpha Surges and X-ray Jets in AR7260
Authors: Canfield, R. C.; Reardon, K. P.; Leka, K. D.; Shibata, K.;
Yokoyama, T.; Shimojo, M.
Bibcode: 1996mpsa.conf...49C
Altcode: 1996IAUCo.153...49C
No abstract at ADS
Title: Numerical simulation of solar and astrophysical MHD
flows. Jets, loops, and flares.
Authors: Shibata, K.
Bibcode: 1996ASIC..481..217S
Altcode:
The author summarizes the difficulties and richness intrinsic to solar
and similar astrophysical MHD simulations, and then shows several
examples of simulation results which answer various questions presented
by old and new observations of solar and astrophysical MHD flows. In
particular, solar jets (spicules and surges) accelerated by nonlinear
MHD waves, astrophysical jets ejected from accretion disks via magnetic
forces, the nonlinear evolution of the Parker instability in galactic
and accretion disks, emerging magnetic loops in the solar atmosphere,
magnetic reconnection driven by the Parker instability as a model of
solar X-ray jets and compact flares, and finally magnetic reconnection
in protostellar magnetospheres.
Title: Theory and Observations of X-Ray Jets (Invited)
Authors: Shibata, K.; Shimojo, M.; Yokoyama, T.; Ohyama, M.
Bibcode: 1996ASPC..111...29S
Altcode: 1997ASPC..111...29S
The soft X-ray telescope (SXT) aboard Yohkoh has discovered coronal
X-ray jets associated with small flares (microflares - subflares). The
recent development of observations and theoretical modeling of
X-ray jets are reviewed with emphasis upon the role of magnetic
reconnection. The relation to X-ray plasma ejections (plasmoids)
from large flares is also discussed.
Title: Dynamical Processes in the Solar Corona
Authors: Shibata, K.
Bibcode: 1996mpsa.conf...13S
Altcode: 1996IAUCo.153...13S
No abstract at ADS
Title: X-ray and Magnetic Features of H alpha Surges
Authors: Okubo, A.; Matsumoto, R.; Miyaji, S.; Akioka, M.; Zhang,
H.; Shimojo, M.; Nishino, Y.; Ichimoto, K.; Shibata, K.; Sakurai, T.
Bibcode: 1996mpsa.conf..437O
Altcode: 1996IAUCo.153..437O
No abstract at ADS
Title: Mass flux and terminal velocities of magnetically driven jets
from accretion disks: steady and nonsteady solutions.
Authors: Kudoh, T.; Shibata, K.
Bibcode: 1996bpad.conf..311K
Altcode: 1996pada.conf..311K
The authors solve 1.5-dimensional steady and axisymmetric MHD equations
to study basic properties of astrophysical jets from accretion
disks. They find that the mass flux depends on the poloidal magnetic
field of the disk when the toroidal component of the magnetic field
is dominant at the disk, although it is independent of the magnetic
field when the poloidal component is dominant there. The authors
also perform time-dependent 1.5D MHD numerical simulations of the
astrophysical jets to understand the relation between the nonsteady
and steady jets. It is found that the nonsteady jets have the same
properties of the steady magnetically driven jets.
Title: YOHKOH SXT Observations of Prominence Eruption and
Disappearance
Authors: Tonooka, H.; Matsumoto, R.; Miyaji, S.; Martin, S. F.;
Canfield, R. C.; Reardon, K.; McAllister, A.; Shibata, K.
Bibcode: 1996mpsa.conf..493T
Altcode: 1996IAUCo.153..493T
No abstract at ADS
Title: Statistical Study of Solar X-ray Jets Observed with the YOHKOH
Soft X-ray Telescope
Authors: Shimojo, M.; Hashimoto, T.; Shibata, K.; Hirayama, T.;
Harvey, K. L.
Bibcode: 1996mpsa.conf..449S
Altcode: 1996IAUCo.153..449S
No abstract at ADS
Title: X-ray flares in protostars
Authors: Hayashi, M. R.; Shibata, K.; Matsumoto, R.
Bibcode: 1995AAS...187.2202H
Altcode: 1995BAAS...27.1319H
Origin of X-ray flares in protostars and the formation of
magnetohydrodynamical jets are studied by numerically simulating
the interaction between the disk material and the dipole magnetic
field of the central protostar.At the initial state, we assume that
a thin Keplerian disk is threaded by the dipole magnetic fields of
the central star. The closed magnetic loops connecting the central
star and the disk are twisted by the rotation of the disk. As the
twist accumulates, magnetic loops expand and finally approach to the
open field configuration. In the presence of resistivity, magnetic
reconnection takes place in the current sheet developed along the
expanding magnetic loops. Outgoing magnetic island and 'post flare
loops' are formed as a result of the reconnection.This process can
be regarded as a bifurcation to the lower energy state triggered by
continuous helicity injection (e.g., Kusano 1995). The time scale
of this flare is the order of the rotation period of the disk. The
released magnetic energy (typically 10(35) erg in protostars) goes into
the thermal energy of the plasma in the flaring loop and the thermal
and kinetic energies of the ejected plasmoids. The maximum speed of the
streamer is the order of the Keplerian rotation speed around the inner
edge of the disk. High energy particles created by the reconnection
by bremsstrahlung emission at the footpoints of the flaring loop. The
length of the flaring loop is several times larger than the radius of
the central star. The magnetic reconnection accompanying this mechanism
can explain hard X-ray flares in protostars observed by ASCA (Koyama
et al. 1995).
Title: Coordinated OVRO, BATSE, Yohkoh, and BBSO Observations of
the 1992 June 25 M1.4 Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Schwartz, R. A.; Sakao,
T.; Kosugi, T.; Shibata, K.
Bibcode: 1995ApJ...453..505W
Altcode:
We compare 1-14 GHz microwave images observed at the Owens Valley Radio
Observatory (OVRO), 16- and 256-channel hard X-ray spectra obtained by
the Burst and Transient Source Experiment (BATSE) onboard the Compton
Gamma Ray Observatory (CGRO), soft and and hard X-ray images obtained
by Yohkoh, and Hα images and magnetograms observed at the Big Bear
Solar Observatory (BBSO) for the 1992 June 25 M1.4 flare. We find the
following unique properties for this flare: (1) Soft X-ray emissions
connect two foot- points, the primary microwave source is located at
one footpoint, and hard X-ray emissions are concentrated in the other
footpoint The radio footpoint is associated with an umbra and may
have stronger magnetic field. (2) During the period that 256-channel
BATSE data are available, the hard X-ray photon spectrum consists
of two components: a superhot component with a temperature of 8.4
× 107 K and emission measure of 2.5 × 1046
cm-3 and a power-law component with a photon index of
4.2. This is the first time that such a high temperature is reported
for the hard X-ray thermal components. It is even more interesting
that such a superhot component is identified before the peak of the
flare. The microwave brightness temperature spectra during the same
period also demonstrate two components: a thermal component near the
loop top and a nonthermal component at the footpoint of the loop. The
microwave thermal component has the similar temperature as that of
the hard X-ray superhot component. These measurements are consistent
with the theory that the microwaves and hard X-rays are due to the
same group of electrons, despite the fact that they are separated by
35,000 km. (3) The soft X-ray emissions brighten the existing loops and
co-align with Hα emissions throughout the entire duration of the flare.
Title: Hard X-Ray Sources and the Primary Energy-Release Site in
Solar Flares
Authors: Masuda, Satoshi; Kosugi, Takeo; Hara, Hirohisa; Sakao, Taro;
Shibata, Kazunari; Tsuneta, Saku
Bibcode: 1995PASJ...47..677M
Altcode:
Accurately coaligned hard and soft X-ray images, taken simultaneously
with the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope
(SXT) aboard Yohkoh, of impulsive solar flares on 1992 January 13
(17:29 UT), 1992 October 4 (22:21 UT), and 1993 February 17 (10:35
UT), occurring near the limb, clearly reveal that, in addition to
double-footpoint sources, a hard X-ray source exists well above the
corresponding soft X-ray loop structure around the peak time of the
impulsive phase. This hard X-ray source shows an intensity variation
similar to double-footpoint sources and a spectrum that is relatively
hard compared with that of loop-top gradual source which appeared later
in the flare. We believe that this is the first clear evidence that
magnetic reconnection, which is responsible for the primary flare energy
release, is under progress above the soft X-ray flaring loop. Maybe
this ``loop-top'' hard X-ray source represents the reconnection site
itself or the site where the downward plasma stream, ejected from
the reconnection point far above the hard X-ray source, collides with
the underlying closed magnetic loop. The characteristics of this hard
X-ray source are quantitatively discussed in the schemes of thermal
(T >~ 10(8) K) and nonthermal interpretations of hard X-ray emission.
Title: Hot-Plasma Ejections Associated with Compact-Loop Solar Flares
Authors: Shibata, K.; Masuda, S.; Shimojo, M.; Hara, H.; Yokoyama,
T.; Tsuneta, S.; Kosugi, T.; Ogawara, Y.
Bibcode: 1995ApJ...451L..83S
Altcode:
Masuda et al. found a hard X-ray source well above a soft X-ray loop
in impulsive compact-loop flares near the limb. This indicates that
main energy release is going on above the soft X-ray loop, and suggests
magnetic reconnection occurring above the loop, similar to the classical
model for two ribbon flares. If the reconnection hypothesis is correct,
a hot plasma (or plasmoid) ejection is expected to be associated
with these flares. Using the images taken by the soft X-ray telescope
aboard Yohkoh, we searched for such plasma ejections in eight impulsive
compact-loop flares near the limb, which are selected in an unbiased
manner and include also the Masuda flare, 1992 January 13 flare. We
found that all these flares were associated with X-ray plasma ejections
high above the soft X-ray loop and the velocity of ejections is within
the range of 50--400 km s-1. This result gives further support for
magnetic reconnection hypothesis of these impulsive compact-loop flares.
Title: Mass Flux and Terminal Velocities of Magnetically Driven Jets
from Accretion Disks
Authors: Kudoh, Takahiro; Shibata, Kazunari
Bibcode: 1995ApJ...452L..41K
Altcode:
In order to investigate astrophysical jets from accretion disks,
we solve 1.5-dimensional steady MHD equations for a wide range of
parameters, assuming the shape of poloidal magnetic field lines. We
include a thermal effect to obtain the relation between the mass flux
of the jet and the magnetic energy at the disk, although the jet is
mainly accelerated by the magnetic force. It is found that the mass
flux of the jets ( M dot ) is dependent on the magnetic energy at the
disk surface, i.e., M dot ~ (rho Aa|Bp/B|)_{{slow}} ~ (rho
Aa|Bp/Bphi|)_{{slow}} ~ Ealpha_{{mg}}
[where rho is the density, a is the sound velocity, A is the cross
section of the magnetic flux, B = (B2p +
B2phi)^{1/2} , Bp and B phi are the poloidal and
toroidal magnetic field strength, respectively, Emg is the magnetic
energy in unit of the gravitational energy at the disk surface, and the
suffix "slow" denotes the value at a slow point], when the magnetic
energy is not too large. The parameter alpha increases from 0 to 0.5
with decreasing magnetic energy. Since the scaling law of Michel's
minimum energy solution nearly holds in the magnetically driven flows,
the dependence of the terminal velocity on the magnetic energy becomes
weaker than had been expected, i.e., v_∞ ~ E^{(1-alpha)/3}_{{mg}}
. It is shown that the terminal velocity of the jet is an order of
Keplerian velocity at the footpoint of the jets for a wide range of
values of Emg expected for accretion disks in star-forming regions and
active galactic nuclei. We argue that the mass-loss rates observed in
the star-forming regions would constrain the magnetic energies at the
disk surfaces.
Title: Detection of Nonthermal Radio Emission from Coronal X-Ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Shimojo, M.; Shibata, K.; Raoult, A.
Bibcode: 1995ApJ...447L.135K
Altcode:
We report the detection of a type III burst in association with a
dynamic X-ray coronal jet observed by Yohkoh/SXT. The type III burst
observed with the Nancay (France) multifrequency radioheliograph is
spatially and temporally coincident with the X-ray jet. The radio
locations at different frequencies (236.6 and 164 MHz) are aligned
along the length of the jet. The observation of the type III burst in
association with the X-ray jet implies the acceleration of electrons
to several tens of keV, along with the heating responsible for the
production of soft X-rays. This association implies the existence of
open field lines in dense coronal structures identified on the Sun's
disk. This is the first observation of dense coronal structures on the
disk, along which type III emitting nonthermal electrons propagate. We
find that this structure begins to form before the type III emission. At
the time of the type III burst we estimate a density of 6--10 x 108
cm-3 for a temperature of ~5--6 MK at an altitude of 20,000 km.
Title: Coronal x-ray jets: Observation and theory
Authors: Shibata, Kazunari
Bibcode: 1995sowi.conf...28S
Altcode:
The Soft X-ray Telescope (SXT) aboard Yohkoh has revealed that the solar
corona is much more dynamic that had been thought. Among various newly
discovered dynamic phenomena, one of the most surprising findings is
the discovery of coronal x-ray jets. The length of these jets is a few
103 - a few 105 km, their (apparent) velocity is a
few 10 - a few 100 km/s (some reached 1000 km/s), and the corresponding
kinetic energy is estimated to be 1025 - 1028
erg. They occur in association with small flares in active regions,
emerging flux regions, and x-ray bright points, and show the following
common characteristics: recurrency, whip-like motion, change in
morphology at the footpoint ARs, and often converging (or inverted-Y)
shape. Large scale loop brightenings observed by SXT seem to correspond
to jets occurring in closed loop systems. These observations suggest
that the magnetic reconnection between the emerging magnetic flux
(or expanding loop) and the overlying coronal/chromospheric magnetic
field is a key physical process for producing these jets. We shall
summarize characteristics of these coronal X-ray jets observed by SXT,
and also discuss the theoretical interpretation of them, especially
in the framework of a magnetic reconnection model.
Title: Magnetic reconnection as the origin of X-ray jets and Hα
surges on the Sun
Authors: Yokoyama, Takaaki; Shibata, Kazunari
Bibcode: 1995Natur.375...42Y
Altcode:
THE solar corona (the outermost portion of the Sun's atmosphere)
is far hotter than the 'surface' (the photosphere). Recent
observations of X-ray jets1á€-4 (collimated flows
of plasma at temperatures of a few million degrees) suggest that
magnetic reconnectioná€"the cutting of stressed magnetic field
lines, which is associated with a violent release of energy,
and their subsequent reconnectioná€"may be responsible for
heating the corona5. But the physical relationship
between the X-ray jets, microflares (localized impulsive bursts
whose total energy is below the level of the standard flares)
and cooler Hα surges6 (jets of gas at a temperature
of about 10,000 K) has been unclear. In particular, it has been
thought7 that Ha surges and X-ray jets must arise from
independent processes, on the grounds that reconnection would heat any
plasma to X-ray-emitting temperatures. Here we present the results of
magnetohydrody-namic simulations of the reconnection process, which
show that X-ray jets and Ha surges can be ejected simultaneously
from microflares8,9. This suggests that the total energy
associated with the microflares is much greater than previously thought,
and may be significant in heating the corona.
Title: Hα surges and associated Soft X-ray loops
Authors: Schmieder, B.; Shibata, K.; van Driel-Gesztelyi, L.;
Freeland, S.
Bibcode: 1995SoPh..156..245S
Altcode:
A recurrent Hα surge was observed on 7 October, 1991 on the western
solar limb with the Meudon MSDP spectrograph. The GOES satellite
recorded X-ray subflares coincident with all three events. During
two of the surges high-resolutionYohkoh Soft X-ray Telescope (SXT)
images have been taken. Low X-ray loops overlying the active region
where the surges occurred were continuously restructuring. A flare
loop appeared at the onset of each surge event and somewhat separated
from the footpoint of the surge. The loops are interpreted as causally
related to the surges. It is suggested that surges are due to magnetic
reconnection between a twisted cool loop and open field lines. Cold
plasma bubbles or jets squeezed among untwisting magnetic field
lines could correspond to the surge material. No detection was made
of either X-ray emission along the path of the surges or X-ray jets,
possibly because of the finite detection threshold of theYohkoh SXT.
Title: Comparison of Synoptic Maps of Solar Soft X-Ray Features,
Photospheric Magnetic Fields, and Helium 1083 NM
Authors: Harvey, J.; Slater, G.; Nitta, N.; Shibata, K.; Tsuneta,
S.; Sakurai, T.; Hara, H.
Bibcode: 1994AAS...18512308H
Altcode: 1994BAAS...26Q1523H
We studied the wealth of structural features visible in Yohkoh/SXT
and NSO/KP synoptic maps of the Sun that cover Carrington rotations
1847 through 1879. In order to do this comparison, various methods to
reduce soft X-ray maps to simple structural elements were explored. In
the end, the best way of comparing the various data sets turned out to
be to filter the X-ray maps to emphasize high-spatial frequencies and
then to either simply blink the various images or to make colorized
composite maps that distinctively assign different colors to various
quantities. Among the results are: 1. Active regions exhibit normal or
"anemone" (fountain-like) X-ray loop structure tendency depending on
whether the surrounding large-scale unipolarity of the magnetic field
is small or large. 2. There is a systematic twist of the coronal loops
around magnetic concentrations in the southern hemisphere and vice-versa
in the north. The sense is the same as one would expect from the action
of differential rotation. 3. Dark lanes in the X-ray images are centered
over large-scale polarity patterns of one sign or the other. 4. The
X-ray loops at the boundaries between large-scale opposite polarity
patterns are frequently strongly sheared. The presence or absence of
a filament in these locations may be related in a complicated way to
the amount of shear. 5. At the resolution of the synoptic maps, the
footpoints of X-ray loops are almost always rooted in locally strong
magnetic concentrations and also in extra-dark 1083 nm elements.
Title: What is the Condition for Fast Magnetic Reconnection?
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1994ApJ...436L.197Y
Altcode:
The magnetic reconnection driven by magnetic buoyancy instability
is studied using two-dimensional MHD simulations for two resistivity
models; uniform resistivity, and anomalous resistivity. It is found
that the reconnection rate is not uniquely determined by the driving
process but strongly dependent on the resistivity model, i.e., the local
plasma condition near the neutral point. The uniform resistivity case
becomes steady Sweet-Parker type, while the anomalous resistivity case
tends to nonsteady Petschek type. In the latter case, the reconnection
rate increases with increasing threshold of anomalous resistivity. The
formation of magnetic islands (plasmoids) and their subsequent ejection
from the current sheet is found to be a key physical process leading
to fast reconnection.
Title: Jets and brightenings generated by energy deposition in the
middle and upper solar chromosphere
Authors: Sterling, Alphonse C.; Shibata, Kazunari; Mariska, John T.
Bibcode: 1994SSRv...70...77S
Altcode:
Numerical simulations of energy depositions in the middle and upper
solar chromosphere result in ejection of chromospheric material into
the corona and heating of the chromospheric gas. These simulations may
be capable of describing some of the features seen by the soft X-ray
telescope on board theYohkoh satellite.
Title: A Gigantic Coronal Jet Ejected from a Compact Active Region
in a Coronal Hole
Authors: Shibata, K.; Nitta, N.; Strong, K. T.; Matsumoto, R.;
Yokoyama, T.; Hirayama, T.; Hudson, H.; Ogawara, Y.
Bibcode: 1994ApJ...431L..51S
Altcode:
A gigantic coronal jet greater than 3 x 105 km long (nearly
half the solar radius) has been found with the soft X-ray telescope
(SXT) on board the solar X-ray satellite, Yohkoh. The jet was ejected
on 1992 January 11 from an 'anemone-type' active region (AR) appearing
in a coronal hole and is one of the largest coronal X-ray jets observed
so far by SXT. This gigantic jet is the best observed example of many
other smaller X-ray jets, because the spatial structures of both the jet
and the AR located at its base are more easily resolved. The range of
apparent translational velocities of the bulk of the jet was between
90 and 240 km s-1, with the corresponding kinetic energy
estimated to be of order of 1028 ergs. A detailed analysis
reveals that the jet was associated with a loop brightening (a small
flare) that occurred in the active region. Several features of this
observation suggest and are consistent with a magnetic reconnection
mechanism for the production of such a 'jet-loop-brightening' event.
Title: Coronal X-ray Jets
Authors: Shibata, K.; Yokoyama, T.; Shimojo, M.
Bibcode: 1994kofu.symp...75S
Altcode:
The soft X-ray telescope (SXT) aboard Yohkoh has discovered coronal
X-ray jets associated with small flares in XBPs, EFRs, or ARs. The
common observed characteristics of these jets are discussed mainly from
morphological points of view. It is suggested that magnetic reconnection
between emerging magnetic flux and the overlying coronal/chromospheric
magnetic field is a key physical process for producing these jets.
Title: Numerical Simulation of Reconnection Between Emerging Flux
and Coronal Field
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1994kofu.symp..367Y
Altcode:
Two dimensional resistive MHD numerical simulation is performed for
the reconnection between emerging flux and overlying coronal field. Two
types of reconnection are investigated. The `two-sided-loop' type occurs
when the coronal field is horizontal, and a pair of horizontal hot
jets and cool magnetic island ejection is produced. The `anemone-jet'
type reconnection occurs when the coronal field is vertical or oblique,
and both a vertical hot jet and a cool jet are generated.
Title: A Morphological Study of Magnetic Shear Development in a
Flare-Productive Region NOAA 7270
Authors: Kurokawa, H.; Kitai, R.; Kawai, G.; Shibata, K.; Yaji, K.;
Ichimoto, K.; Nitta, N.; Zhang, H.
Bibcode: 1994kofu.symp..283K
Altcode:
The evolutional changes of a flare-productive region NOAA 7270 were
examined in details with high resolution H_alpha images, magnetograms
and soft X-ray images to study the process of the magnetic shear
development and its relation to the strong flare activity of the
region. This study led us to the following results and a conclusion:(1)
Several new bipolar pairs simultaneously and or successively emerged in
NOAA 7270 from 5 through 7 September. (2) Magnetic shear configurations
developed at three locations, where most of flares occurred from 5
through 7 , September. (3)These magnetic shear configurations were
formed by successive emergences of twisted magnetic ropes from below
the photosphere.
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
Authors: Nitta, N.; Driel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1994kofu.symp..385N
Altcode:
Active region NOAA 7260 exhibited remarkable flare activity as an
emerging flux region appeared in the following part and evolved into
the delta configuration. While it is difficult to associate an emerging
bipole with a flare both temporally and spatially, there is an overall
correlation of the total darkness integrated over of the sunspot area,
as measured in the Yohkoh/SXT white-light images, with the soft X-ray
flux and flare occurrence. It appears that the flares in the emerging
flux region occurred preferentially at locations close to the spot of
preceding polarity that emerged in the earliest evolution of the region.
Title: Flares on September 6, 1992
Authors: Kitai, R.; Kurokawa, H.; Funakoshi, Y.; Nakai, Y.; Shibata,
K.; Yaji, K.; Nitta, N.; YOHKOH Team; NAOJ Flare Telescope Team
Bibcode: 1994kofu.symp..147K
Altcode:
We present some preliminary results of our observational
study of typical eruptive flares in NOAA7270 on September 6,
1992. (1)Magnetic shear and flux emergence are strongly related to
flare production. (2)Observed flares showed a common temporal relation
between cool plasma dynamics seen in H_alpha and coronal energy releases
seen in YOHKOH data. (3)At the pre-heating stage of flares, there were
some indications of slow reconnection of adjacent magnetic loops.
Title: Computational Magnetohydrodynamics of Twisted Flux Tubes in
Gravitationally Stratified Atmosphere
Authors: Matsumoto, R.; Tajima, T.; Chou, W.; Shibata, K.
Bibcode: 1994AAS...184.4009M
Altcode: 1994BAAS...26..923M
We carried out three-dimensional nonlinear MHD simulations of
the evolution of twisted flux tubes in gravitationally stratified
atmosphere. The initial magnetic field is assumed to be concentrated in
a uniformly twisted, force-free horizontal flux tube in convectively
unstable layer. The origin of the magnetic twist is either by the
convective motion or by the Coriolis force. The nonlinear evolution of
this system has been simulated by the MHD code based on the modified
Lax-Wendroff scheme with artificial viscosity in rectangular Cartesian
coordinate. The number of grid points for typical model is 64 times 64
times 150. When the magnetic twist exceeds a threshold, the flux tube
forms itself into a helical structure by the kink instability. After
this stage, the particular portion of the helical structure rises by
buoyancy, and forms a sequence of sheared magnetic loops. Emergence
of such helical flux tube may account for the global structure of the
kinked series of active regions prominent in the soft X-ray image of
the solar corona as observed by the Yohkoh satellite. Furthermore,
numerical results reproduce the separation and the drifting motion of
f- and p-spots. We also studied the interaction between twisted flux
tubes through three-dimensional resistive MHD simulation. We found
(1) the reconnection time scale for counter-helicity flux tubes is
much shorter than that for co-helicity flux tubes, (2) reconnection
jets with speed comparable to the Alfven speed are ejected along the
magnetic loops. Numerical results will be compared with the observations
of interacting magnetic loops in the solar corona.
Title: X-ray solar physics from YOHKOH
Authors: Uchida, Yutaka; Watanabe, Tetsuya; Shibata, Kazunari; Hudson,
Hugh S.
Bibcode: 1994xspy.conf.....U
Altcode:
No abstract at ADS
Title: Two Types of Interaction Between Emerging Flux and Coronal
Magnetic Field
Authors: Shibata, K.; Nitta, N.; Matsumoto, R.; Tajima, T.; Yokoyama,
T.; Hirayama, T.; Hudson, H.
Bibcode: 1994xspy.conf...29S
Altcode:
No abstract at ADS
Title: A Compact SIS Receiver Cooled by 4 K GM Refrigerator
Authors: Kuriyama, T.; Hatakeyama, H.; Takahashi, M.; Nakagare, H.;
Kawabe, R.; Iwashita, H.; McCulloch, G.; Shibata, K.; Ukita, N.
Bibcode: 1994ASPC...59..113K
Altcode: 1994amsw.conf..113K; 1994IAUCo.140..113K
No abstract at ADS
Title: Structure of the Soft X-Ray Corona and its Effect to the
Earth Quadrupole Type and Sea Anemone Type
Authors: Takahashi, Ta.; Saito, T.; Shibata, K.; Kozuka, Y.; Minami,
S.; Mori, Y.
Bibcode: 1994xspy.conf..305T
Altcode:
No abstract at ADS
Title: The Variable Structure of the X-ray Corona as Viewed by YOHKOH
Authors: Shibata, K.
Bibcode: 1994svsp.coll...89S
Altcode: 1994IAUCo.143P..89S
No abstract at ADS
Title: An Antennacluster-Antennacluster VLBI Project VERA
Authors: Sasao, T.; Kawano, N.; Hara, T.; Kuji, S.; Shibata, K.;
Lwadate, K.; Sato, K.; Kameya, 0.; Tsuruta, S.; Asari, K.; Tamura,
Y.; Horiai, K.; Sato, K.; Hanada, H.; Tsubokawa, T.; Yokoyama, K.;
Manabe, S.; Sakai, S.
Bibcode: 1994vtpp.conf..254S
Altcode:
No abstract at ADS
Title: Flares in Active Region NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Sakurai,
T.; Shibata, K.; Ichimoto, K.; Canfield, R. C.; Wülser, J. -P.;
Metcalf, T. R.; Mickey, D. L.
Bibcode: 1994xspy.conf..111N
Altcode:
No abstract at ADS
Title: X-Ray Activity in Coronal Loops and its
Photospheric/chromospheric Signatures
Authors: Sakurai, T.; Shibata, K.; Ichimoto, K.; Takata, M.
Bibcode: 1994xspy.conf...37S
Altcode:
No abstract at ADS
Title: A Model of X-Ray Jets and Loop Brigtenings Associated with
Emerging Flux
Authors: Yokoyama, T.; Shibata, K.
Bibcode: 1994xspy.conf..317Y
Altcode:
No abstract at ADS
Title: Coronal Magnetic Fields Parallel to Magnetic Polarity
Inversion Lines
Authors: McAllister, A. H.; Uchida, Y.; Khan, J. I.; Shibata, K.
Bibcode: 1994xspy.conf..189M
Altcode:
No abstract at ADS
Title: 3D-MHD simulation of emerging magnetic flux in the solar
atmosphere
Authors: Kaisig, M.; Matsumoto, R.; Tajima, T.; Shibata, K.
Bibcode: 1994smf..conf..142K
Altcode:
No abstract at ADS
Title: A New Advanced One-Unit VLBI Correlator (NAOCO)
Authors: Shibata, K.; Sasao, T.; Kawaguchi, N.; Tamura, Y.; Kameno,
S.; Miyoshi, M.; Asari, K.; Manabe, S.; Hara, T.; Kuji, S.; Sato,
K.; Miyaji, T.; Matsumoto, K.; Asaki, Y.; Yasuda, S.; Nakamura, S.
Bibcode: 1994vtpp.conf..327S
Altcode:
No abstract at ADS
Title: Three-dimensional Magnetohydrodynamics of the Emerging Magnetic
Flux in the Solar Atmosphere
Authors: Matsumoto, R.; Tajima, T.; Shibata, K.; Kaisig, M.
Bibcode: 1993ApJ...414..357M
Altcode:
The nonlinear evolution of an emerging magnetic flux tube or sheet in
the solar atmosphere is studied through 3D MHD simulations. In the
initial state, a horizontal magnetic flux sheet or tube is assumed
to be embedded at the bottom of MHD two isothermal gas layers, which
approximate the solar photosphere/chromosphere and the corona. The
magnetic flux sheet or tube is unstable against the undular mode of
the magnetic buoyancy instability. The magnetic loop rises due to the
linear and then later nonlinear instabilities caused by the buoyancy
enhanced by precipitating the gas along magnetic field lines. We find
by 3D simulation that during the ascendance of loops the bundle of
flux tubes or even the flux sheet develops into dense gas filaments
pinched between magnetic loops. The interchange modes help produce a
fine fiber flux structure perpendicular to the magnetic field direction
in the linear stage, while the undular modes determine the overall
buoyant loop structure. The expansion of such a bundle of magnetic
loops follows the self-similar behavior observed in 2D cases studied
earlier. Our study finds the threshold flux for arch filament system
(AFS) formation to be about 0.3 x 10 exp 20 Mx.
Title: Coronal Arcade-like Structures Over Magnetic Inversion Lines
Authors: McAllister, A. H.; Uchida, Y.; Khan, J. I.; Shibata, K.
Bibcode: 1993BAAS...25R1178M
Altcode:
No abstract at ADS
Title: Large-Scale Explosions and Superbubbles in the Galactic Disk
and Halo. I. Magnetohydrodynamic Simulations
Authors: Mineshige, Shin; Shibata, Kazunari; Shapiro, Paul R.
Bibcode: 1993ApJ...409..663M
Altcode:
The evolution of the interstellar superbubbles arising from sequential
supernova explosions or winds from OB associations in the Galactic disk
is studied numerically, using a two-dimensional magnetohydrodynamics
(MHD) code. We find that in the presence of horizontal magnetic fields
of strength comparable to that in the Galactic disk, B = 5 micro G,
the vertical expansion of the superbubbles (the contact surface) can,
under some conditions, be significantly inhibited by the effect of a
decelerating J x B force. At the same time, the outermost effect of the
disturbance actually propagates somewhat faster than in nonmagnetic
cases, as an MHD fast shock or nonlinear wave. The implications of
our results for galactic supershells, the galactic fountain, observed
activity in starburst galaxies, and supernova remnants are briefly
discussed.
Title: Classification of Active Regions Based on X-ray Images
1. Active Regions appearing in Coronal Holes
Authors: Nitta, N.; Shibata, K.; Hara, H.
Bibcode: 1993BAAS...25.1187N
Altcode:
No abstract at ADS
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
and Reconnection
Authors: Nitta, N.; Drel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1993BAAS...25.1223N
Altcode:
No abstract at ADS
Title: Flares on 1992 September 6
Authors: Shibata, K.; Nitta, N.; Kitai, R.; Kurokawa, H.; Yaji, K.;
Kato, T.; Zarro, D. M.
Bibcode: 1993BAAS...25R1187S
Altcode:
No abstract at ADS
Title: X ray jets in the solar corona: Observations with YOHKOH Soft
X Ray Telescope
Authors: Shibata, Kazunari; Ishido, Y.; Acton, L.; Strong, K.;
Hirayama, T.; Uchida, Yutaka; McAllister, A.; Matsumoto, R.; Tsuneta,
Saku; Shimizu, T.
Bibcode: 1993ppcn.conf..207S
Altcode:
The discovery of x-ray jet like features using the soft x-ray telescope
onboard Yohkoh satellite is reported. The origin of newly discovered
x-ray jets and the relation to known solar jets at other wavelengths
are discussed. The jets are associated with flare like bright points,
flaring emerging flux regions, or flaring active regions. They appear
to recur at the same place. The following characteristics are found:
in some cases, a dark void appears after ejection at the footpoint of
the jet; some jets show structure which suggests a helical magnetic
field configuration along the jet; one of the jets associated with a
flaring bright point was found to be identified as an H alpha surge. In
this case, the x-ray bright point is situated just on H alpha bright
point at the footpoint of the surge. The top of the surge is not bright
in x-rays.
Title: Emergence of Twisted Flux Tubes in the Solar Atmosphere
Authors: Matsumoto, R.; Tajima, T.; Shibata, K.
Bibcode: 1993AAS...182.4806M
Altcode: 1993BAAS...25..880M
Solar coronal loops observed in soft X-rays often show twisted
structures. Sometimes, magnetic loops unwind after the topological
change of magnetic field lines (McAllister et al. 1992). Since
twisted flux tubes have more free energy than untwisted loop, when
they emerge into the corona and interact with other loop or overlying
magnetic fields, the release of magnetic energy will lead to energetic
events such as flares, jets, and prominence eruptions. We performed
three-dimensional nonlinear MHD simulation of the evolution of twisted
flux tubes. The initial state consists of hydrostatic atmosphere
with a cold dense layer and a hot corona. In the cold layer, magnetic
fields are concentrated in the horizontal flux tube. Fluid rotation
at both ends of the flux tube is continuously applied. Such twisting
motion generates torsional Alfven waves which propagate along the flux
tube. After several rotations, the flux tube becomes kink unstable
around the midpoint where the magnetic twists are accumulated. The flux
tube is deformed into a super-coil structure. Later, twisted magnetic
loops rise by the magnetic force and the buoyancy force created by
sliding the gas along the flux tube. The length of each rising loop
is about 10H in the cold layer, where H is the scale height. We will
further study the subsequent evolution of twisted flux tubes in the
corona, and their interaction with overlying magnetic fields.
Title: Solar Chromospheric and Transition Region Response to Energy
Deposition in the Middle and Upper Chromosphere
Authors: Sterling, Alphonse C.; Shibata, Kazunari; Mariska, John T.
Bibcode: 1993ApJ...407..778S
Altcode:
A series of numerical simulations modeling the chromosphere and
transition region response to deposition of thermal energy ranging
from about 5 x 10 exp 24 to 5 x 10 exp 28 ergs in the middle or upper
chromosphere is reported. The dissipative effects of heat conduction,
optically thin radiation losses in the corona, and an approximate
expression for the radiation losses of lower temperature plasma are
calculated. In response to the energy deposition, chromospheric material
is ejected into the corona in the form of pressure gradient generated
jets, jets with pressure-gradient, and shock-generated components,
or high-speed gas plugs. Category of ejection depends on the spatial
and temporal distribution and the magnitude of the input energy source.
Title: MHD Numerical Simulations of Magnetic Reconnection Associated
with Emerging Flux
Authors: Shibata, K.; Nozawa, S.; Matsumoto, R.
Bibcode: 1993ASPC...46..500S
Altcode: 1993mvfs.conf..500S; 1993IAUCo.141..500S
No abstract at ADS
Title: Observations of X-ray Jets Using YOHKOH Soft X-Ray Telescope
Authors: Shibata, K.; Ishido, Y.; Acton, L.; Strong, K.; Hirayama,
T.; Uchida, Y.; McAllister, A.; Matsumoto, R.; Tsuneta, S.; Shimizu,
T.; Hara, H.; Sakurai, T.; Ichimoto, K.; Nishino, Y.; Ogawara, Y.
Bibcode: 1993ASPC...46..343S
Altcode: 1993IAUCo.141..343S; 1993mvfs.conf..343S
No abstract at ADS
Title: Comparison between YOHKOH Soft X-ray Images and 3D MHD
Simulations of Solar Emerging Flux Regions
Authors: Matsumoto, R.; Tajima, T.; Kaisig, M.; Shibata, K.; Ishido,
Y.; Tsuneta, S.; Shimizu, T.; Kawai, G.; Kurokawa, H.; Akioka, M.;
Acton, L.; Strong, K.; Nitta, N.
Bibcode: 1992AAS...181.8109M
Altcode: 1992BAAS...24.1253M
The soft X-ray telescope on the Yohkoh mission enabled us to observe
the evolution of emerging flux regions (EFR) in coronal X-rays with
high spatial and temporal resolution. Furthermore, we now have enough
computing capability to perform three-dimensional MHD simulation
of EFRs with sufficient spacial resolution to study details of the
flux emergence process. These new tools provide the opportunity to
investigate the physics involved in the formation of coronal loops
in much more detail. We carried out 3D MHD simulations of emerging
magnetic flux regions under various intial conditions; (1) a horizontal
magnetic flux sheet, (2) a bundle of horizontal flux tubes, and (3)
a flux sheet with sheared magnetic fields. Numerical results show that
coronal magnetic loops are formed due to the enhanced buoyancy resulting
from gas precipitating along magnetic field lines. The interchange modes
help to produce a fine fibrous structure perpendicular to the magnetic
field direction in the linear stage, while the undular modes determine
the overall loop structure. We observe in 3D simulations that during the
ascendance of loops the bundle of flux tubes, or even the flux sheet,
developes into dense filaments pinched between magnetic loops. We
also find that magnetic field lines are twisted by the vortex motion
produced by the horizontal expansion of magnetic loops. Our numerical
results may explain the observed signatures such as (1) the spacial
relation between soft X-ray loops and Hα arch filaments obtained by
coordinated observation between Yohkoh and ground-based observatories
(Kawai et al. 1992), (2) the rate of increase in size of soft X-ray
loops in EFRs (Ishido et al. 1992), (3) emergence of twisted magnetic
loops, and (4) the threshold flux for formation of chromospheric arch
filament systems (AFS).
Title: Flare-Related Relaxation of Magnetic Shear as Observed with
the Soft X-Ray Telescope of YOHKOH and with Vector Magnetographs
Authors: Sakurai, Takashi; Shibata, Kazunari; Ichimoto, Kiyoshi;
Tsuneta, Saku; Acton, Loren W.
Bibcode: 1992PASJ...44L.123S
Altcode:
The soft X-ray Telescope of Yohkoh observed an M-class flare on 1992
February 6. As the flare progressed, an initially sheared coronal loop
structure was seen to evolve toward a relaxed magnetic configuration. In
association with this evolution, the vector magnetograph observations
detected a decrease in the electric currents.
Title: Observations of X-Ray Jets with the YOHKOH Soft X-Ray Telescope
Authors: Shibata, Kazunari; Ishido, Yoshinori; Acton, Loren W.; Strong,
Keith T.; Hirayama, Tadashi; Uchida, Yutaka; McAllister, Alan H.;
Matsumoto, Ryoji; Tsuneta, Saku; Shimizu, Toshifumi; Hara, Hirohisa;
Sakurai, Takashi; Ichimoto, Kiyoshi; Nishino, Yohei; Ogawara, Yoshiaki
Bibcode: 1992PASJ...44L.173S
Altcode:
Time series of Soft X-ray Telescope images have revealed many X-ray
jets in the solar corona. The typical size of a jet is 5 times 10(3)
--4 times 10(5) km, the translational velocity is 30--300 km s(-1) ,
and the corresponding kinetic energy is estimated to be 10(25) --10(28)
erg. Many of the jets are associated with flares in X-ray bright points,
emerging flux regions, or active regions. They sometimes occur several
times from the same X-ray feature. In some cases, a dark void appears
after ejection at the footpoint of the jet. The void seems to be
the result of a change in the topology of the X-ray emitting plasma,
perhaps due to magnetic reconnection. Some jets show a structure which
suggests a helical magnetic field configuration along the jet. One
of the jets associated with a flaring bright point was identified as
being an Hα surge. In this case, the X-ray bright point is situated
just on the Hα bright point at the footpoint of the surge. The top
of the surge is not bright in X-rays. We briefly discuss the origin
of these newly discovered X-ray jets.
Title: The Structure of the Coronal Soft X-Ray Source Associated
with the Dark Filament Disappearance of 1991 September 28 Using the
YOHKOH Soft X-Ray Telescope
Authors: McAllister, Alan; Uchida, Yutaka; Tsuneta, Saku; Strong,
Keith T.; Acton, Loren W.; Hiei, Eijiro; Bruner, Marilyn E.; Watanabe,
Takashi; Shibata, Kazunari
Bibcode: 1992PASJ...44L.205M
Altcode:
The internal structure of an X-ray emitting elongated object
appearing in association with Hα -dark filament disappearance of 1991
September 28 was analyzed with the help of a fine-structure enhancing
technique. We present a description of the soft X-ray structures and
their evolution, while focusing on the central, brightest part of the
structure, which is also the most difficult to resolve. We start with
the idea of applying the standard ``eruption-reconnection'' models
of Hα double-ribbon flares with filament disappearences in order to
explain this event as the appearence of an arcade of loops across the
initial dark filament position, with a row of hot spots at reconnection
sites along the loop tops. Our study of the Yohkoh Soft X-ray images,
including their fine-structure enhancement, the making of an accurately
aligned movie, and a preliminary comparison with Hα data, however,
has led us to question the applicability of these models to this type
of event. It seems, rather, that much of the bright structure comprises
heated pre-existing loops, which interact where they cross each other
in a complex ``knot'' at the northern end of the disappearing dark
filament. The bright part turns out to comprise highly sheared loops,
perhaps being pushed up by the slowly rising axial field of the dark
filament, which is, however, contained under the arcade, rather than
having broken through it. We conclude that there is a strong possibility
that much of the dark filament mass remains in the heated unwinding
axial field and briefly discuss the theoretical implications.
Title: Comparison between Hα and YOHKOH Soft X-Ray Images of Emerging
Flux Regions
Authors: Kawai, Goro; Kurokawa, Hiroki; Tsuneta, Saku; Shimizu,
Toshifumi; Shibata, Kazunari; Acton, Loren W.; Strong, Keith T.;
Nitta, Nariaki
Bibcode: 1992PASJ...44L.193K
Altcode:
We carried out a detailed comparison between Hα and Yohkoh Soft X-ray
(SXR) images of three emerging flux regions. The main results are:
(1) In general, SXR bright features coincide well in space with Hα
arch filament systems in the emerging flux regions (EFR). (2) Some
young and active parts of EFRs are especially bright in SXR. (3)
The SXR structures related to EFR show fairly rapid changes in both
brightness and shape. These results are consistent with the model that
the emerging cool loops of EFRs evolve into hot coronal loops through
some heating processes.
Title: Giant Molecular Cloud Formation through the Parker Instability
in a Skewed Magnetic Field
Authors: Hanawa, Tomoyuki; Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 1992ApJ...393L..71H
Altcode: 1992astro.ph..5002H
The effect of the magnetic skew on the Parker instability is
investigated by means of the linear stability analysis for a
gravitationally stratified gas layer permeated by a horizontal
magnetic field. When the magnetic field is skewed (i.e., the field
line direction is a function of the height), the wavelength of the
most unstable mode is $ \lambda \; \sim \; 10 H $ where $ H $ is the
pressure scale height. The growth rate of the short wavelength modes
is greatly reduced when the gradient in the magnetic field direction
exceeds 0.5 radian per scale height. Our results indicate that the
Parker instability in a skewed magnetic field preferentially forms
large scale structures like giant molecular clouds.
Title: Three-Dimensional MHD Simulation of the Parker Instability
in Galactic Gas Disks and the Solar Atmosphere
Authors: Matsumoto, Ryoji; Shibata, Kazunari
Bibcode: 1992PASJ...44..167M
Altcode:
Three-dimensional (3D) magnetohydrodynamic simulations were performed in
a study of the nonlinear evolution of the Parker instability in galactic
gas disks and/or accretion disks, as well as in emerging flux regions
(EFR) of the Sun. The initial magnetic fields are parallel to one of
the horizontal coordinates in magnetostatic equilibrium. The effect of
coupling between the Parker (undular) instability and the interchange
instability was mainly studied. In both the galactic and solar cases,
adjacent flux tubes move independently, so that a highly interleaved
structure is created, although the expansion of magnetic loops induced
by the Parker mode is similar to that found in 2D models. In galactic
gas disks magnetic loops evacuate regions of interstellar space by
accumulating interstellar gas in magnetic pockets. The accumulated
gas is compressed by both infalling gas and horizontally expanding
magnetic loops, forming dense, thin spurs. That is, the 3D nonlinear
Parker instability creates a large-scale void-shell-spur structure,
which is very similar to the large-scale structure of the universe as
well as the interstellar gas and/or dust distribution. In the solar EFR
model, magnetic loops expand into the corona, while the gas slides down
along the magnetic field lines. An approximate self-similar expansion
and shock wave formation at the loop footpoints (which were previously
found in 2D loops) were observed in 3D loops. A horizontal expansion
of the rising flux tubes produces vortex motions, which then generate
torsional Alfven waves.
Title: Magnetic Reconnection Associated with Emerging Magnetic Flux
Authors: Shibata, Kazunari; Nozawa, Satoshi; Matsumoto, Ryoji
Bibcode: 1992PASJ...44..265S
Altcode:
Two-dimensional (2D) magnetohydrodynamic (MHD) numerical simulations
have been performed in order to study magnetic reconnection between
an emerging flux and an overlying coronal magnetic field, while
taking into account the effect of gravity, high spatial resolution,
and a sufficient time span. The reconnection starts when most of the
chromospheric mass in the current sheet between the emerging flux and
the coronal field has drained down along the loop due to gravity. This
implies that the start of a compact flare, or an X-ray bright point,
follows the disappearance of an arch filament. It is also found that
multiple magnetic islands are created in the sheet, which confine a
cool, dense chromospheric plasma. These islands dynamically coalesce
with each other and are ejected along the sheet at the Alfven speed. To
check whether these phenomena occur in association with emerging flux
would be an interesting subject for a cooperative study between optical
and soft X-ray observations using the Yohkoh (Solar A) satellite.
Title: A Magnetohydrodynamic Model for Herbig-Haro Objects:
Magnetically Guided Shocked Flows Associated with Optical Jets from
Young Stellar Objects
Authors: Uchida, Yutaka; Todo, Yasushi; Rosner, Robert; Shibata,
Kazunari
Bibcode: 1992PASJ...44..227U
Altcode:
We propose a magnetohydrodynamic (MHD) model for Herbig-Haro (HH)
objects in which a large-scale interstellar magnetic field plays
a basic role. The model contains two essential, distinct elements:
First, we assume that the large-scale interstellar magnetic field is
spatially constricted during the contraction phase of star formation,
and becomes helical due to production of a toroidal field component
by the rotation of the central star + disk system. Second, we assume
that this large-scale field acts as a guide field for the high-velocity
flows associated with the optical jets emanating from the star forming
at the center. In that case, we show that the impinging optical jet
accompanies four distinct shock fronts, namely, MHD fast and slow
shocks ahead, and reverse fast and slow shocks propagating backward
into the moving jet medium behind the contact discontinuity. The result
of our simplified 1.5-D MHD simulations strongly indicate that our
``magnetically-guided stream" picture can account for the high Doppler
and proper-motion velocities of the HH objects, which are generally
different from each other. Our results suggest that the more complex
morphological characteristics of these objects [such as the ``jet with
bright-spot + bow shock" type structures (e.g., HH34) and those with
``wiggled filamentary structures" (e.g., HH46/47) of some HH objects]
could be explained by MHD models in 2.5-D or 3-D.
Title: Study of Flare Productive Active Regions
Authors: Nitta, N.; Harvey, K. L.; Shibata, K.; Strong, K. T.
Bibcode: 1992AAS...180.1807N
Altcode: 1992BAAS...24..755N
The Soft X-ray Telescope (SXT) on the Yohkoh spacecraft has made it
possible for us for the first time to monitor the evolution of active
regions in soft X-rays with a good cadence and spatial resolution
over an extended period of time. One of our interests is what makes
an active region produce large flares. Since the SXT started its
operation, we have identified more than 40 active regions that have
produced flares whose GOES class are > M1. Although many such active
regions appear active when they first become visible on the eastern
limb, we have succeeded in observing some of them from their birth
through activation. We compare X-ray images taken from the SXT with
magnetograms and other groundbased data. We discuss similarities and
differences of such regions from other more quiet regions in terms of
morphology and physical parameters.
Title: On the Origin of Cosmological Magnetic Fields
Authors: Tajima, T.; Cable, S.; Shibata, K.; Kulsrud, R. M.
Bibcode: 1992ApJ...390..309T
Altcode:
The paper shows that a plasma with temperature T sustains fluctuations
of electromagnetic fields and particle density even if it is assumed to
be in a thermal equilibrium. The level of fluctuations in the plasma
for a given wavelength and frequency of electromagnetic fields is
rigorously computed by the fluctuation-dissipation theorem. A large
zero-frequency peak of electromagnetic fluctuations is discovered. It
is shown that the energy contained in this peak is complementary to the
energy 'lost' by the plasma cutoff effect. The present computer particle
simulation confirms the theory and exhibits a peaking of the magnetic
energy spectrum at zero frequency. The level of magnetic fields is
significant at the early radiation epoch of the universe. Such magnetic
fluctuations provide seed fields for later evolution. Implications of
these magnetic fields in the early universe are discussed.
Title: Non-linear evolution of synchrotron thermal instabilities.
Authors: Bodo, G.; Ferrari, A.; Massaglia, S.; Rossi, P.; Shibata,
K.; Uchida, Y.
Bibcode: 1992A&A...256..689B
Altcode:
We present a time-dependent, nonlinear study of thermal instability in
a magnetized plasma whose relativistic component supports the pressure
and undergoes synchrotron losses while the thermal matter provides
the inertia of the medium. We follow the temporal evolution of the
instability in the nonlinear regime by means of a 1D finite-difference
MHD numerical code employing either free or periodic boundary
conditions. We find that, in a plasma subject to constant heating,
after an initial phase in which the instability growth rate follows
the linear model, the instability reaches a quasi-equilibrium state on
timescales of the order of several synchrotron timescales. An essential
condition for this instability to be efficient is that the plasma is
out of equipartition with the relativistic particle energy exceeding
the magnetic field energy. The mechanism considered can interpret the
formation of filaments of enhanced emission observed at high resolution
in lobes and jets of several extragalactic radio sources.
Title: Emergence of Magnetic Flux from the Convection Zone into the
Solar Atmosphere. I. Linear and Nonlinear Adiabatic Evolution of
the Convective-Parker Instability
Authors: Nozawa, S.; Shibata, K.; Matsumoto, R.; Sterling, A. C.;
Tajima, T.; Uchida, Y.; Ferrari, A.; Rosner, R.
Bibcode: 1992ApJS...78..267N
Altcode:
The linear and nonlinear properties of the evolution of emerging
magnetic flux from the solar convection zone into the photosphere,
chromosphere, and corona are studied. A linear stability analysis
of the partially magnetized convection zone is presented. The growth
rate of this combined convective-Parker instability is found to differ
significantly from that of the Parker instability in the absence of
convection. When beta(=pg/pm) is greater than 10 in the initial flux
sheet, the growth rate increases with horizontal wavenumber, and there
is no maximum growth rate. A local maximum can occur when the flux
is initially located near the top of the convection zone. When beta
is less than 10, the convective-Parker instability behaves like the
Parker instability for long-wavelength modes, and like the convective
instability for short-wavelength modes. A 2D MHD code is used to study
the nonlinear evolution of the system. When the initial flux sheet has
beta less than 10, the long-wavelength mode dominates the nonlinear
evolution of the system, independently of the initial perturbation
wavelength.
Title: A Unified Model of the MHD Activity in the Galactic Center
Authors: Shibata, K.
Bibcode: 1992fxra.conf..419S
Altcode:
No abstract at ADS
Title: Numerical Simulations of Microflare Evolution in the Solar
Transition Region and Corona
Authors: Sterling, Alphonse C.; Mariska, John T.; Shibata, Kazunari;
Suematsu, Yoshinori
Bibcode: 1991ApJ...381..313S
Altcode:
Several observers report transient ultraviolet brightenings, often
referred to as microflares, in the solar atmosphere. In this paper,
the results are presented of a series of one-dimensional numerical
simulations examining possible relationships between microflares
and the generation of dynamical chromospheric and transition region
features. Low-energy and medium-energy microflares eject long-lived
cool, dense gas plugs into the corona, with the gas plug traversing
the loop apex in the medium energy case. In the case of high-energy
microflares, the gas plug is rapidly heated to the temperature of the
surrounding corona, and the results resemble the dynamics occurring
in standard solar flare thick-target electron beam models.
Title: Formation of giant molecular clouds and helical magnetic
fields by the Parker instability
Authors: Shibata, Kazunari; Matsumoto, Ryoji
Bibcode: 1991Natur.353..633S
Altcode:
USING the Nagoya telescope1, Uchida et al.2
found an unusual helical filamentary structure, spinning about its long
axis, in the L1641 cloud in the Orion cloud complex. Noting that this
structure is consistent with a helically twisted magnetic field inferred
from optical polarization observations3,4, they argued that
the helical filament is a manifestation of torsional magnetohydrodynamic
(Alfvén) waves draining angular momentum from a nearby massive
cloud, thus promoting collapse and star formation. Here we present
an alternative interpretation. We suggest that the Orion molecular
cloud complex formed through the Parker instability5
(the buoyancy of a magnetic field entrained in matter), and that
the helical filament is the result of spinning gas falling along the
magnetic field and twisting it. The twisted magnetic field, unlike a
purely planar field, suppresses the Parker instability on small scales,
allowing the generation of finite clouds rather than general turbulence.
Title: Numerical Studies of Atmospheric Dynamics Driven by Energy
Deposition in the Chromosphere
Authors: Sterling, A. C.; Mariska, J. T.; Shibata, K.
Bibcode: 1991BAAS...23.1029S
Altcode:
No abstract at ADS
Title: Magnetic Pressure-Driven Jets from a Torus
Authors: Fukue, Jun; Shibata, Kazunari; Okada, Rika
Bibcode: 1991PASJ...43..131F
Altcode:
We examine steady, wind-type magnetohydrodynamical (MHD) jets
accelerated in a funnel formed along the rotational axis of a
geometrically thick torus around a central object, incorporating
the effects of mass and toroidal magnetic field injections from
the torus. When the mass and magnetic field are injected from only
the base of the jets (without injection from the funnel wall),
the magnetic pressure of the toroidal magnetic fields accelerates
the gas, whereas the magnetic tension decelerates it as long as the
cross-sectional area of the jets increases. In the present case, the
locations of the trans-magnetosonic points of jet flow are determined
by the thermal properties of the flow and are independent of the
magnetic properties. Trans-magnetosonic points shift to infinity in
the cold limit, similar to the case of a spherical approximation,
although this is not the general case. The terminal speed, v_∞,
of MHD jets in the funnel becomes the order of [{mit Phi }(2}/(4pi
dot {M})]({1/3)) , where dot {M} is the mass flux and {mit Phi } is
the transfer rate of the toroidal magnetic flux. When the mass and
magnetic field injections from the funnel wall are taken into account,
trans-magnetosonic points are located at larger distances than the scale
length of mass injection. We found that the injection of magnetic fields
from the funnel wall enchances the accelaration of jets. In this case,
v_∞ is the order of [{mit Phi }(2}_∞/(4pi dot {M}_∞)]({1/3)) ,
where the mass flux and the magnetic flux are those at infinity.
Title: Magnetic Reconnection Associated with Emerging Magnetic Flux
Authors: Shibata, K.; Nozawa, S.; Matsumoto, R.
Bibcode: 1991saaj.conf..169S
Altcode:
No abstract at ADS
Title: Nonlinear Evolution of the Parker Instability
Authors: Matsumoto, R.; Shibata, K.
Bibcode: 1991IAUS..144..429M
Altcode:
Two-dimensional MHD simulations are performed to study the nonlinear
equation of the Parker instability in galactic gas disks. When the most
unstable mode grows, magnetic field lines kink across the equatorial
plane of the disk and thin spur-like structures are formed above dense
regions in magnetic pockets. In low beta disks, shock waves are produced
at the footpoint of magnetic loops, while in high beta disks, nonlinear
oscillations are excited and the loop length increases with time.
Title: On mechanisms of solar flares —some observational tests by
using Solar-A
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1991LNP...387..230U
Altcode: 1991fpsa.conf..230U
The possibilities for using Solar-A observations to discriminate between
the thus-far proposed flare mechanisms are discussed. In this context,
we point out some thus-far unnoticed restrictions of the standard
models, and suggest that by relaxing these restrictions we can avoid the
difficulties these conventional models encountered. These theoretical
pionts indicate what Solar-A observations would be particularly useful
for clarifying this topic.
Title: Three Dimensional MHD Simulation of the Parker Instability
Authors: Matsumoto, R.; Shibata, K.
Bibcode: 1991saaj.conf..177M
Altcode:
No abstract at ADS
Title: Magnetic Accretion Disks Fall Into Two Types.
Authors: Shibata, K.; Tajima, T.; Matsumoto, R.; Fukue, J.; Okada, R.
Bibcode: 1991sepa.conf..517S
Altcode: 1991IAUCo.129..517S
No abstract at ADS
Title: Theoretical Models of Solar Flares
Authors: Shibata, K.
Bibcode: 1991LNP...387..205S
Altcode: 1991LNP...387..203S; 1991fpsa.conf..205S
Recent progress in the understanding of the basic physical picture
of solar flares is discussed from a theoretical point of view,
with emphasis on magnetohydrodynamic processes, such as magnetic
reconnection. Several models of CME (Coronal Mass Ejection related
flare and compact flare models are critically reviewed. The role of the
successive emergence of twisted flux tubes is stressed, not only for
modeling compact flares, but also for understanding CME related flares.
Title: Formation of Bipolar Radio Jets and Lobes from Accretion Disk
around Forming Blackhole at the Center of Protogalaxies
Authors: Uchida, Y.; Matsumoto, R.; Hirose, S.; Shibata, K.
Bibcode: 1991ASSL..169..409U
Altcode: 1991pnee.proc..409U
We propose that radio jets and lobes from QSO's are 'magnetic bipolar
jets from forming blackholes', physically analogous to those of
star-formation bipolar flows, but with very much greater energy due
to very much greater depth in gravitational potential. We perform 2.5D
MHD simulations for the situation in which the condensing mass of the
accretion disk associated with the blackhole brought the magnetic flux
with it, deforming the magnetic field into an hourglass shape. The
differential rotation of the disk rotating at its neck continuously
produces magnetic twists and sends them out in the form of nonlinear
torsional Alfven waves to the bipolar directions. The gas of the disk
atmosphere and the halo is accelerated helically when these nonlinear
torsional Alfven waves (NTAWs) propagate through them. These NTAWs,
at the same time, dynamically pinch the initially hourglass-shaped
field into a collimated rod-shaped structure, and in some cases cause
helical instability to make it into a winding structure.
Title: Numerical Simulations of Ultraviolet and X-ray Microflares
Authors: Sterling, A. C.; Shibata, K.; Mariska, J. T.
Bibcode: 1991LNP...387...71S
Altcode: 1991fpsa.conf...71S
A series of numerical simulations indicates that thermal energy
releases of 1025 - 1027 ergs in the middle chromosphere can produce
ejections into the corona in the form of pressure gradient generated
jets, jets with pressure gradient and shock generated components, or
high speed gas plugs. Heating of the chromosphere to X-ray emitting
temperatures occurs in association with gas plugs, perhaps generating
X-ray microflares observable by Solar-A. Chromospheric UV-microflares
can occur in association with some jets, but do not generally occur
with spicules.
Title: Atmospheric Heating in Emerging Flux Regions (With 2 Figures)
Authors: Shibata, K.; Nozawa, S.; Matsumoto, R.; Tajima, T.; Sterling,
A. C.
Bibcode: 1991mcch.conf..609S
Altcode:
No abstract at ADS
Title: Observations of circumstellar envelopes with the NMA.
Authors: Shibata, K.; Kasuga, T.; Deguchi, S.; Ukita, N.; Izumiura,
H.; Tsuji, T.
Bibcode: 1991AstHe..84....5S
Altcode:
No abstract at ADS
Title: A study on citations of PASP papers.
Authors: Shibata, K.; Hirano, R.
Bibcode: 1991AstHe..84...86S
Altcode:
No abstract at ADS
Title: On the Origin of Cosmological Magnetic Fields
Authors: Tajima, T.; Cable, S.; Shibata, K.
Bibcode: 1991ASSL..169..423T
Altcode: 1991pnee.proc..423T
No abstract at ADS
Title: Self-similar evolution of the nonlinear magnetic buoyancy
instability
Authors: Shibata, K.; Tajima, T.; Matsumoto, R.
Bibcode: 1990PhFlB...2.1989S
Altcode:
A new type of self-similar solution of ideal magnetohydrodynamics
(MHD) in the nonlinear stage of the undular model (k parallel to B)
of the magnetic buoyancy instability (the ballooning instability in
fusion plasma physics or the Parker instability in astrophysics)
is found through MHD simulation and theory. The linear theory
developed agrees well with the simulation in the early (linear)
stage. The nonlinear stages of the instability in the simulation
show the self-similar evolution. One of the solutions obtained
from the nonlinear analysis has the characteristics of nonlinear
instability in Lagrangian coordinates; the fluid velocity and the
Alfven speed on each magnetic loop increase exponentially with time,
because the loop is evacuated by the field-aligned motion of matter
resulting from gravitational acceleration. In the later stage of the
nonlinear evolution, the solution property changes from exponential
to power-law time dependence. The latter corresponds to a force-free
expansion solution. The later saturation of the velocity increment is
also discussed.
Title: Nonlinear Excitation of Magnetic Undular Instability by
Convective Motion
Authors: Kaisig, Michael; Tajima, Toshiki; Shibata, Kazunari; Nozawa,
S.; Matsumoto, Ryoji
Bibcode: 1990ApJ...358..698K
Altcode:
The influence of convective motions on the stabilty of a Parker
stable magnetic flux sheet is numerically investigated. The general
characteristics of the Parker instability with convective motions in the
nonlinear stage are studied. The possibility that a destabilization of
stable flux can be realized by either horizontal photospheric shearing
motions and/or by vertical convective flows is examined.
Title: Magnetic Model for Asymmetric Supernova Remnants
Authors: Mineshige, Shin; Shibata, Kazunari
Bibcode: 1990ApJ...355L..47M
Altcode:
Results are reported from two-dimensional MHD simulations for the
asymmetric SNRs such as VRO 42.05.01. Two models are compared: one
is the hot tunnel model, in which the hot elongated cavity (tunnel)
between two warm interclouds is responsible for the observed asymmetric
structure. The other is the magnetic model, in which material from the
warm intercloud is compresed by the SN shock at the magnetic wall,
creating an arc-shaped dense structure. The shapes of the density
contours are quite similar. In contrast, the velocity vectors differ
significantly between the two models.
Title: Nonlinear Parker Instability in Nonuniform Gravitational
Fields: Nonlinear Oscillations and Shock Waves
Authors: Matsumoto, Ryoji; Horiuchi, Toshiro; Hanawa, Tomoyuki;
Shibata, Kazunari
Bibcode: 1990ApJ...356..259M
Altcode:
The nonlinear evolution of the 'Parker' instability in accretion
disks and galactic gas disks is studied by using a two-dimensional
magnetohydrodynamic (MHD) code. The gas layer is assumed to be located
at some distance from a point mass which is the origin of gravity. The
magnetic fields are assumed to be parallel to the disk plane in
magnetostatic equilibrium. The sound and Alfven speeds are taken to be
spatially uniform in the initial state. The most unstable mode as well
as other modes are examined in detail. It is found that nonlinear stages
of the instability are generally classified into two cases: nonlinear
oscillation and shock wave formation. Applications to magnetic loops
in accretion disks and in galactic disks are briefly discussed.
Title: Emergence of Solar Magnetic Flux from the Convection Zone
into the Photosphere and Chromosphere
Authors: Shibata, K.; Nozawa, S.; Matsumoto, R.; Sterling, A. C.;
Tajima, T.
Bibcode: 1990ApJ...351L..25S
Altcode:
A two-dimensional MHD code is used to study the nonlinear dynamics
of solar magnetic flux emerging from the convection zone into the
photosphere and chromosphere. An isolated horizontal magnetic flux with
beta of about 4 is initially located in a convectively unstable layer
(solar convection zone) beneath a two-temperature layered atmosphere
(solar corona-chromosphere/photosphere). The combined effects of
convection and magnetic buoyancy carry the magnetic flux from the
convection zone into the photosphere, where it then expands through
the photosphere and chromosphere. Gas slides down the expanding loop,
resulting in its evacuation and subsequent rise due to enhanced
magnetic buoyancy. Initially, weak convection zone magnetic flux (B
of about 600 G) is amplified up to 1000 G or more after emerging into
the photosphere. The resulting velocity fields are similar to those
observed in arch filament systems.
Title: Interaction of Molecular Bipolar Flows with Interstellar
Condensations: Sweeping Magnetic Twist Mechanism and the Blobs in
Lynds 1551 Molecular Flow
Authors: Shibata, Kazunari; Uchida, Yutaka
Bibcode: 1990PASJ...42...39S
Altcode:
The interaction of a bipolar molecular flow with interstellar
condensations is examined (using the sweeping-magnetic-twist model) by
performing 2.5-dimensional MHD numerical simulations. A torsional Alfven
wave propagating with a hollow cylindrical bipolar flow interacts with
the condensation, and the magnetic twist accumulates in the region
between the flow and the condensation, since the Alfven velocity
in the condensation is smaller than that in the ambient medium. The
stored magnetic twist increases with time, causing various nonlinear
effects, such as a pinching of the gas in the near-axis region, or an
upward-acceleration of the gas in the condensation. Two cases for the
condensation configuration are examined: spherical shape and ring-like
shape. In the spherical case, the condensation, itself, is squeezed by
the pinch effect. In the ring-shaped case, the hot gas near the axis
is pinched and flow is generated along the axis due to an enhanced
gas pressure resulting from the pinch. The velocity of the hot flow is
comparable to that of the original cold bipolar flow. The results in
the ring-shaped case may explain the observed characteristics in the
velocity field around the blob observed in the L1551 flow, supporting
the interpretation (Uchida et al. 1987; AAA 44.131.270) that the mass
in the dense blob lying in the L1551 flow did not come from the source
of the flow, but pre-existed there in the molecular cloud.
Title: Magnetic Accretion Disks Fall into Two Types
Authors: Shibata, K.; Tajima, T.; Matsumoto, R.
Bibcode: 1990ApJ...350..295S
Altcode:
It is demonstrated through nonlinear numerical simulations that
there are two kinds of magnetic accretion disks. One is a high-beta
disk, called a solar-type disk, where magnetic flux escapes from the
disk due to magnetic buoyancy (or Parker) instability to produce a
(low-beta) hot corona surrounding the disk. The second type of disk
is a low-beta disk, called a magnetically cataclysmic disk, where
magnetic flux cannot fully escape from the disk because the nonlinear
escape time of magnetic flux is longer than the amplification time
of magnetic flux by Keplerian shear motions. In this disk magnetic
fields are increased enormously by the shear motion up to a point,
beyond which explosive (or cataclysmic) phenomena occur which suddenly
release magnetic energy from the disk. Possible transitions between
the two types of disks and implications for cataclysmic variables,
X-ray binaries, and active galactic nuclei are discussed.
Title: Computational Magnetohydrodynamic Jets
Authors: Shibata, Kazunari
Bibcode: 1990IAUS..140..419S
Altcode:
By using two-dimensional magnetohydrodynamic (MHD) code, the following
nonsteady MHD jets and outflows are studied in relation to jets ejected
from central regions of galaxies: (1) a jet driven by gas pressure
gradient, collimated by vertical magnetic fields (as a model of bipolar
flows ejected from a hot bubble created by starbursts); (2) a magnetic
twist jet which is accelerated and collimated by J x B force in relaxing
magnetic twists generated by the interaction of poloidal fields with
a rotating disk (as a model of jets from active galactic nuclei);
(3) a magnetic-loop-outflow resulting from the Parker instability,
which may account for the initial acceleration of the magnetic twist
jet and nonthermal emissions near nuclear accretion disks.
Title: Magnetohydrodynamic Models of Galactic Center Lobes Shells
and Filaments
Authors: Shibata, Kazunari; Matsumoto, Ryoji
Bibcode: 1990IAUS..140..379S
Altcode:
Magnetohydrodynamic (MHD) mechanisms producing radio lobes, shells,
and filaments in the Galactic center as well as in the gas disk of the
Galaxy are studied by using two-dimensional MHD code: (1) the explosion
in a magnetized disk, (2) the interaction of a rotating disk with
vertical fields, and (3) the nonlinear Parker instability in toroidal
magnetic fields in a disk. In all cases, dense shells or filaments are
created along magnetic field lines in a transient state, in contrast
to the quasi-equilibrium filaments perpendicular to magnetic fields.
Title: Magnetohydrodynamical simulations in astrophysics - jets,
loops, and flares.
Authors: Uchida, Y.; Shibata, K.; Matsumoto, R.
Bibcode: 1990AstHe..83..181U
Altcode:
No abstract at ADS
Title: Large-scale Explosions and Superbubbles in the Galactic Disk
and Halo: MHD Simulations and Analytical Approximations
Authors: Shapiro, P. R.; Mineshige, S.; Shibata, K.
Bibcode: 1990IAUS..144P.107S
Altcode:
No abstract at ADS
Title: On the Origin of Cosmic Magnetic Fields
Authors: Tajima, T.; Cable, S.; Shibata, K.; Kulsrud, R. M.
Bibcode: 1990IAUS..140..531T
Altcode: 1990STIN...9111634T
It is shown that a plasma with temperature T sustains fluctuations of
electromagnetic fields and particle density even if it is assumed to
be in a thermal equilibrium. The level of fluctuations in the plasma
for a given wavelength and frequency of electromagnetic fields is
rigorously computed by the fluctuation-dissipation theorem. A large
zero frequency peak of electromagnetic fluctuations is discovered. It
is shown that the energy contained in this peak is complementary to
the energy lost by the plasma cutoff effect. The level of the zero
frequency magnetic fields is computed. The theoretical minimum magnetic
field strength is also computed, as no turbulence is assumed. The size
of the fluctuations is shown. These results are not in contradiction
with the conventional black body radiation spectra but its extension,
and as such, do not contradict the observed lack of structure in the
cosmic microwave background. The level of these is computed. The
computer particle simulation shows the support of the theory and
in fact exhibits a peaking of the magnetic energy spectrum at zero
frequency. The level of magnetic fields is significant at the early
radiation epoch of the Universe. Implications of these magnetic fields
in the early Universe are discussed.
Title: MHD Simulation of Halo Cloud Formation by Thermal Instability
Authors: Mineshige, S.; Shapiro, P. R.; Shibata, K.; Tajima, T.
Bibcode: 1990IAUS..144P..99M
Altcode:
No abstract at ADS
Title: Two-dimensional magnetohydrodynamic model of emerging magnetic
flux in the solar atmosphere
Authors: Shibata, K.; Tajima, T.; Steinolfson, R. S.; Matsumoto, R.
Bibcode: 1989ApJ...345..584S
Altcode:
The nonlinear undular mode of the magnetic buoyancy instability in an
isolated horizontal magnetic flux embedded in a two-temperature layered
atmosphere (solar corona-chromosphere/photosphere) is investigated
using a two-dimensional magnetohydrodynamic code. The results show
that the flux sheet with beta of about 1 is initially located at the
bottom of the photosphere, and that the gas slides down the expanding
loop as the instability develops, with the evacuated loop rising as
a result of enhanced magnetic buoyancy. The expansion of the magnetic
loop in the nonlinear regime displays self-similar behavior. The rise
velocity of the magnetic loop in the high chromosphere (10-15 km/s)
and the velocity of downflow noted along the loop (30-50 km/s) are
consistent with observed values for arch filament systems.
Title: On the Origin of Cosmic Magnetic Fields
Authors: Tajima, T.; Shibata, K.
Bibcode: 1989BAAS...21.1173T
Altcode:
No abstract at ADS
Title: Emergence of Solar Magnetic Flux from the Convection Zone
into the Photosphere aand Chromosphere
Authors: Sterling, A. C.; Shibata, K.; Nozawa, S.; Matsumoto, R.;
Tajima, T.
Bibcode: 1989BAAS...21.1179S
Altcode:
No abstract at ADS
Title: Nonlinear Parker Instability of Isolated Magnetic Flux in
a Plasma
Authors: Shibata, K.; Tajima, T.; Matsumoto, R.; Horiuchi, T.; Hanawa,
T.; Rosner, R.; Uchida, Y.
Bibcode: 1989ApJ...338..471S
Altcode:
The nonlinear evolution of the Parker instability in an isolated
horizontal magnetic-flux sheet embedded in a two-temperature layer
atmosphere is studied by using a two-dimensional MHD code. In the solar
case, this two-layer model is regarded as a simplified abstraction
of the sun's photosphere/chromosphere and its overlying much hotter
(coronal) envelope. The horizontal flux sheet is initially located
in the lower temperature atmosphere so as to satisfy magnetostatic
equilibrium under a constant gravitational acceleration. Ideal
MHD is assumed, and only perturbations with k parallel to the
magnetic-field lines are investigated. As the instability develops,
the gas slides down the expanding loop, and the evacuated loop rises as
a result of enhanced magnetic buoyancy. In the nonlinear regime of the
instability, both the rise velocity of a magnetic loop and the local
Alfven velocity at the top of the loop increase linearly with height
and show self-similar behavior with height as long as the wavelength
of the initial perturbation is much smaller than the horizontal size
of the computing domain.
Title: MHD Mechanisms for the Formation of Galactic Center Lobes
Authors: Shibata, K.
Bibcode: 1989IAUS..136..313S
Altcode:
No abstract at ADS
Title: Unusual emission line profiles of M1-1.
Authors: Shibata, K.; Tamura, S.
Bibcode: 1989IAUS..131..188S
Altcode:
No abstract at ADS
Title: Nonlinear evolution of Parker instability of isolated magnetic
flux sheet and its application to emerging magnetic flux in the
solar atmosphere.
Authors: Shibata, K.; Tajima, T.; Steinolfson, R.; Matsumoto, R.
Bibcode: 1989sasf.confP.281S
Altcode: 1988sasf.conf..281S; 1989IAUCo.104P.281S
A two dimensional MHD code is used to study the nonlinear evolution of
the Parker instability in isolated horizontal magnetic flux imbedded
in (or below) the solar photosphere. It is found that the magnetic
loop expands self-similarly in the nonlinear stage. Numerical results
explain many features observed in emerging flux regions.
Title: Nonsteady MHD Jets from Magnetized Accretion Disks -
Sweeping-Magnetic Mechanism -
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1989ASIC..290...65S
Altcode: 1989tad..conf...65S
No abstract at ADS
Title: Expansion Velocities of NII and OIII from Compact Planetary
Nebulae
Authors: Shibata, K.; Tamura, S.
Bibcode: 1989IAUS..131..190S
Altcode:
No abstract at ADS
Title: A Magnetodynamic Mechanism for the Heating of Emerging Magnetic
Flux Tubes and Loop Flares
Authors: Uchida, Yutaka; Shibata, Kazunari
Bibcode: 1988SoPh..116..291U
Altcode:
A new magnetodynamic model for loop flares is proposed to explain the
following observational facts obtained from space during the last solar
activity maximum: (i) Blueshifted lines of Ca XIX and Fe XXV appear in
some cases a minute or so before the initiation of impulsive bursts
and relax into the unshifted lines with large width by the time of
the onset of impulsive bursts, (ii) the hot source is formed by that
time at the top of a loop-like structure, and confined there for a
considerable time, and (iii) γ-ray line enhancement occurs at about
the same time as hard X-ray spikes.
Title: Parker Instability in Nonuniform Gravitational
Fields. II. Nonlinear Time Evolution
Authors: Matsumoto, Ryoji; Horiuchi, Toshiro; Shibata, Kazunari;
Hanawa, Tomoyuki
Bibcode: 1988PASJ...40..171M
Altcode:
The nonlinear evolution of the Parker instability in magnetized
gas disks (e.g., accretion disks, galactic gas disks) is studied
by performing two-dimensional MHD numerical simulations, taking
account of the spatial variation in the vertical gravitational
acceleration. Unperturbed states consist of isothermal, magnetostatic
gas layers with the spatially constant α (== the ratio of magnetic
to gas pressure). The modes with the most unstable wavenumbers
are examined. As the instability develops, the gas slides down the
expanding magnetic loop, forming a dense structure in the valley. The
growth of the perturbation is saturated when the maximum horizontal
velocity of the downflow becomes comparable to the initial Alfvén
speed. The maximum velocity of the rising motion of the magnetic loop
is 0.3-0.5 times that of the initial Alfvén speed. When α ≳ 1, (1)
shock waves are formed in the downflow near the footpoint of the loop,
(2) the dense spur or sheet, whose maximum compression rate is 2-80
for 1 ≤ α ≤ 10, is created in the valley, and (3) the outward
gas motion occurs just above dense spurs. When α < 1, the shock
wave is not formed and the system shows an oscillatory motion.
Title: Two-Dimensional MHD Model of Emerging Magnetic Flux in the
Solar Atmosphere
Authors: Shibata, K.; Tajima, T.; Matsumoto, R.
Bibcode: 1988BAAS...20R.714S
Altcode:
No abstract at ADS
Title: Structure of magnetized accretion disks and origin of
astrophysical jets
Authors: Hanawa, Tomoyuki; Kamahori, Hirotaka; Maruyama, Tetsuji;
Shibata, Kazunari
Bibcode: 1988PASJ...40..729H
Altcode:
A model is presented for an accretion disk with large-scale magnetic
fields. The radial, vertical, and overall structure of the disk
are examined numerically. The gas distribution and magnetic field
configuration are obtained. It is found that the upper part of
the model accretion disk is unstable against the magnetic buoyancy
instability. The role of this instability is discussed in relation to
the formation of bipolar jets.
Title: Parker instability in nonuniform gravitational fields. I -
Linear stability analysis. II - Nonlinear time evolution
Authors: Horiuchi, Toshiro; Matsumoto, Ryoji; Hanawa, Tomoyuki;
Shibata, Kazunari
Bibcode: 1988PASJ...40..147H
Altcode:
The linear and nonlinear dynamical evolution of a magnetized gas
disk rotating at a fixed radial distance around a point mass is
investigated theoretically by means of numerical simulations, with
a focus on the Parker instability. The simulations begin from an
equilibrium state in which the gas and magnetic fields are assumed to
be in magnetohydrostatic equilibrium and symmetric with respect to the
equatorial plane. The derivation of the governing equations is outlined,
and the results are presented in extensive graphs and characterized
in detail. Particular attention is given to the implications of the
present results for theoretical models of stellar accretion disks and
galactic gas disks.
Title: MHD shock waves through a galactic nuclear disk with a vertical
magnetic field.
Authors: Umemura, Satoshi; Iki, Kazuo; Shibata, Kazunari; Sofue,
Yoshiaki
Bibcode: 1988PASJ...40...25U
Altcode:
Effects of a vertical magnetic field on the propagation of shock waves
originating from the center of the nuclear disk in galaxies are examined
by using a two-dimensional MHD code. A single adiabatic explosion is
assumed to occur in the center of a pressureless gas disk with a uniform
magnetic field penetrating the disk perpendicularly, and the presence
of a low density gaseous halo is assumed. For a moderate field strength
a flow with a hollow cylindrical shell structure appears with the shell
surface being a contact surface of the mass ejected from the disk due
to the shock propagation. For a higher field strength the mass motion
is highly collimated and produces a jet with a narrow opening angle
along vertical field lines. The hollow shell structure in the former
case resembles the shapes of the radio lobes found in our galactic
center and in M82. The well-collimated jet in the latter case may be
similar to radio jets ejected from more active galactic nuclei.
Title: Rotating eruption of an untwisting filament triggered by the
3B flare of 25 April, 1984
Authors: Kurokawa, Hiroki; Hanaoka, Yoichiro; Shibata, Kazunari;
Uchida, Yutaka
Bibcode: 1987SoPh..108..251K
Altcode:
A great 3B flare, whose X-ray class was X13, occurred over a
delta-sunspot at 00: 01 UT on April 25, 1984. Before the flare, a strong
magnetic shear was found to be formed along the neutral line in the
delta-sunspot with shear motions of umbrae. The shear motions of the
umbrae were caused by the successive emergence of a magnetic flux rope.
Title: Nonlinear Parker Instability of an Isolated Magnetic Flux
Sheet in Astrophysical Plasmas
Authors: Shibata, K.; Tajima, T.; Matsuomoto, R.; Horiuchi, T.;
Hanawa, T.; Rosner, R.; Uchida, Y.
Bibcode: 1987BAAS...19.1030S
Altcode:
No abstract at ADS
Title: Sweeping Magnetic Twist Mechanism and Molecular Bipolar Flows
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1987IAUS..115..385S
Altcode:
By using the non-steady 2.5-dimensional axisymmetric MHD numerical
simulations, the authors have studied the detailed dynamical processes
in the formation of jets by the "sweeping-magnetic-twist" mechanism,
and have applied the results to the molecular bipolar flows in star
forming regions.
Title: Observations of Molecular Flows in S:140 and L:723
Authors: Hayashi, S. S.; Hayashi, M.; Uchida, Y.; Kaifu, N.; Hasegawa,
T.; Shibata, K.
Bibcode: 1987IAUS..115..348H
Altcode:
The authors have made 15arcsec resolution observations of CO J =
1-0 emission toward L723 and S140 using the Nobeyama 45-m radio
telescope. The maps resolved the molecular flow structures clearly. The
outflow in the S140 molecular cloud was resolved to be a bipolar
structure with its axis being nearly perpendicular to the elongation
of the dense core observed in CS emission and to the direction of
the infrared polarization. The blueshifted and redshifted components
in L723 were resolved into two pairs of bipolar outflows with a
point-symmetric structure.
Title: A magnetodynamical model for the Galactic Center lobes.
Authors: Shibata, Kazunari; Uchida, Yutaka
Bibcode: 1987PASJ...39..559S
Altcode:
An MHD model is developed for the Galactic center lobes (GCLs) by
using an axisymmetric 2.5-dimensional MHD simulation. According to
this model, a GCL is a low-energy jet emanating from the H II gas
disk extending beyond r=100 pc from the Galactic center. The model is
based on the sweeping-magnetic-twist mechanism developed by Uchida and
Shibata (1985 and 1986) for the production of cosmic jets, where the
gas in the surface layer of the contracting disk is lifted up by the
J x B force in the relaxing magnetic twist, which is generated by the
interaction of the rotation of the contracting disk with the poloidal
magnetic field. A realistic gravitational potential is adopted for
the Galactic center region, in which the rotational velocities are
approximately constant for r = 20-100 pc. The difference between the
models with this realistic potential and those with the potential due
to a point mass is examined in detail. On the basis of the numerical
results, a scenario for the formation of the GCL is presented.
Title: Observations of the detailed structure and velocity field in
the CO bipolar flows associated with L 1551 IRS-5.
Authors: Uchida, Y.; Kaifu, N.; Shibata, K.; Hayashi, S. S.; Hasegawa,
T.; Hamatake, H.
Bibcode: 1987PASJ...39..907U
Altcode:
The detailed structure and velocity field in the L1551 CO bipolar
flows were observed by using the 45-m millimetric-wave telescope at
Nobeyama. The observations were made in January and April of 1985 and
supplemented in January of 1986 in the 115 GHz 12CO J =
1-0 line with spatial and spectral resolutions of 18" and 250 kHz
(0.65 km s-1 in velocity), respectively. It was revealed as
the result that the bipolar flow lobes have a clear hollow cylindrical
structure and that both lobes are likely to be spinning with a velocity
of 1-2 km s-1 in the same direction as that of the disklike
object claimed by Kaifu et al. (1984; AAA 37.131.128) in the CS 49-GHz
line. The longitudinal velocity of the flow increases with distance
along the axis up to 0.15 pc from IRS-5, the central object. These
characteristics coincide well with those predicted by the magnetodynamic
theory proposed by Uchida and Shibata (1985; AAA 40.131.174), and
indicate the essential importance of the magnetic field in producing
such flows. It is also suggested that the angular momentum loss due to
the magneto dynamic process is important in the star formation itself.
Title: Evidence for Helical Velocity Field in Molecular Bipolar
Flows - Support for Magnetodynamic Model
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1987IAUS..122...77U
Altcode:
A search for the helical velocity field that had been predicted in a
magnetodynamic theory of Uchida and Shibata was made in the bipolar
flows L1551 by using 12CO 115 GHz line, and evidence was
obtained for it in the low velocity maps as the skew inter-invasion of
the root part of the blue- and red shifted lobes into different sides
of the opposite lobes. Theoretical implications of this and other
findings are discussed, and the advantage of models with magnetic
field is stressed.
Title: Hollow cylindrical lobes with a helical velocity field of
the L1551 bipolar flow.
Authors: Uchida, Yutaka; Kaifu, Norio; Hayashi, Saeko S.; Hasegawa,
Tetsuo; Shibata, Kazunari
Bibcode: 1987IAUS..115..287U
Altcode:
Observations of the structure and the velocity field in the L1551
bipolar flow were made with the 45-m telescope at Nobeyama in the
115-GHz (C-12)O J = (1-0) line with high spatial resolution. It was
found that the bipolar flow lobes have a clear hollow cylindrical
structure and show evidence of a helical velocity field. They appear
to rotate in the same direction as the CS disk found by Kaifu at
al. (1984). The velocity of the flow in the bipolar directions increases
with distance up to about 3 arcmin from the central object. IRS 5. These
characteristics coincide with those predicted by the magnetodynamic
theory proposed by Uchida and Shibata (1984 and 1985) and indicate the
essential importance of the magnetic field in producing such flows and
also in the star-formation process itself through the enhancement of
angular-momentum loss.
Title: Magnetodynamical acceleration of cosmic jets:
sweeping-magnetic-twistmechanism.
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1986CaJPh..64..507U
Altcode:
Characteristic behavior of cosmic jets predicted by a magnetodynamic
mechanism proposed by Uchida and Shibata is discussed in terms of
recent observational results of bipolar flows from star-forming regions
as examples of low-energy cases. The theoretical model considers the
twisting-up of part of the large-scale magnetic field with the driving
mechanism being the contracting rotation of the accretion disk around
the gravitating center. The twisted field screws out the mass from
the surface layers of the disk along the large-scale external field,
explaining the observed tuning-fork type of distribution of the cold
CO bipolar flows, gradual acceleration of the flows from the vicinity
of the disk, and the helical velocity field in the outflows, all of
which are not easy to explain by previous hypotheses assuming the wind
and blast from the central object. Prospects of application of this
mechanism to high-energy jets from active galactic nuclei or such
peculiar objects in the galaxy like SS 433 or Sco X-1 are discussed
from the point of view of the similarity inherent in the mechanism.
Title: Water in Ancient Moon?: Possible Oxic Alteration of 14310
Authors: Tanaka, T.; Shimizu, H.; Shibata, K.; Masuda, A.
Bibcode: 1986LPI....17..867T
Altcode:
No abstract at ADS
Title: Sweeping Magnetic Twist Mechanism for the Acceleration of
Jets in the Solar Atmosphere
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1986SoPh..103..299S
Altcode:
A magnetodynamic mechanism for the acceleration of jets in the solar
atmosphere (surges, Brueckner's EUV jets, and so on) is proposed, and a
2.5-dimensional MHD simulation is performed to show how this mechanism
operates in the situation of the chromosphere-corona region of the solar
atmosphere. It is seen from the result of simulation that together with
the release of the magnetic twist, e.g., into a reconnected open flux
tube, the mass in the high density twisted loop is driven out into the
open flux tube due both to the pinch effect progressing with the packet
of the magnetic twist into the open flux tube, and to the j × B force
at the front of the packet of the unwinding twist in the off-axis part
of the tube. The former, the progressing pinch, is accompanied by an
accelerated hot blob, while the latter, the unwinding front of the
magnetic twist, drives a cool cylindrical flow, both with velocities
of the order of the local Alfvén velocity. One of the characteristic
properties of the jet in our model is that the jet, consisting of hot
core and cool sheath, has a helical velocity field in it, explaining
the thus-far unexplained observed feature.
Title: New Polar Ring Galaxies in Rich Clusters of Galaxies
Authors: Taniguchi, Y.; Shibata, K.; Wakamatsu, K. -I.
Bibcode: 1986Ap&SS.118..529T
Altcode:
Four polar ring galaxies discovered in rich clusters of galaxies are
presented. Brief comments on their structural properties are given.
Title: A magnetohydrodynamic mechanism for the formation of
astrophysical jets. II. Dynamical processes in the accretion of
magnetized mass in rotation.
Authors: Shibata, Kazunari; Uchida, Yutaka
Bibcode: 1986PASJ...38..631S
Altcode:
The paper presents a nonsteady magnetodynamic mechanism for the
formation of astrophysical jets in a magnetized accretion disk
system. The dynamical processes in the contraction of a rotating disk,
which is penetrated by a magnetic field parallel to the rotation
axis, are investigated by using axially symmetric 2.5-dimensional MHD
numerical simulations. As the rotating disk contracts, it pulls the
magnetic field toward the center as well as in the azimuthal direction,
producing a helically twisted magnetic field, and as the magnetic
twist is accumulated and begins to relax along the poloidal field,
the gas in the surface layers of the disk is pushed out to the polar
directions by the J x B force with the relaxing magnetic twist. It is
shown that the accelerated gas is collimated by the magnetic field
and forms a supersonic bipolar jet which has a hollow cylindrical
shell structure with helical motion in it. A considerable fraction
of the gravitational potential energy released in the contraction of
the disk is transformed to the kinetic energy of the jet through the
action of the magnetic field.
Title: Formation of Astrophysical Jets by a Contracting Magnetic
Accretion Disk
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1986Ap&SS.118..443S
Altcode:
In the present paper, we discuss an MHD model for the formation of
astrophysical jets, in which the directed flows are ejected along
the rotation axis of an accretion disk formed from a cloud having a
large scale magnetic field parallel to the angular momentum axis of the
disk. The acceleration of jets is due to thej×B force in the relaxing
magnetic twist which is produced by the rotation of the disk. The
characteristic features of the jets, predicted by our mechanism and
hopefully to be proven by observations, are the helical velocity and
the hollow cylindrical shape of the jet, with a diameter of roughly
the size of the region from which the acceretion disk collected its
mass. Justification for the assumption of the perpendicular orientation
of the disk, or the parallelism of the jets, to the external magnetic
field may be provided by the fact that the component of rotation whose
axis is perpendicular to the field may have been damped in the earlier
phase of the cloud contraction.
Title: Origin of the galactic Centre lobes
Authors: Uchida, Y.; Shibata, K.; Sofue, Y.
Bibcode: 1985Natur.317..699U
Altcode:
Recent observations of the 10-GHz continuum1 have indicated
that there is a pair of lobes having ridges of intensity extending from
the galactic centre region in a direction perpendicular to the galactic
plane. This observation has attracted considerable attention as it is
clearly an indication of dynamic processes occurring at the centre of
our Galaxy that are similar to those occurring in the nuclei of radio
galaxies, though different in scale and in strength2. Here
we interpret these lobes as being due to a mag-netodynamic acceleration
mechanism in which the production and relaxation of the magnetic twist
induced by the rotation of the contracting gas disk play a part. The
plasma is accelerated in a conical cylinder with a helical velocity
field, reproducing the observed feature of radio lobes.
Title: Positive Cerium Anomaly of Lunar 14310: Examination by
138CE/142CE
Authors: Tanaka, T.; Shimizu, H.; Shibata, K.; Masuda, A.
Bibcode: 1985LPI....16..847T
Altcode:
No abstract at ADS
Title: A magnetodynamic mechanism for the formation of astrophysical
jets. I - Dynamical effects of the relaxation of nonlinear magnetic
twists
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1985PASJ...37...31S
Altcode:
A magnetodynamical mechanism for the formation of jets in astrophysical
situations is presented. The effect of the relaxation of the accumulated
nonlinear magnetic twist is calculated in an axisymmetric quasi- three
dimensional simulation, and it is shown that a jet is formed in the
direction of the axis parallel to the large-scale magnetic field along
which the packet of the magnetic twist relaxes. The driving force of
the jet is due both to the pinching front progressing along the axis and
to the j x B force in the front of the unwinding twist in the off-axis
region. The effect is strengthened in the stratified atmosphere in
which the relaxing packet of the magnetic twist emerges into the low-Β
(≡ρg/ρm) region. The jet has a high contrast
in density and has a conspicuously high velocity, because the velocity
of the relaxing front which carries the material is determined by the
Alfven velocity which is supersonic in the low-Β region. One of the
characteristic features of the jet produced in the present mechanism
is its helical motion which is favorable in explaining some of the
observations of astrophysical jets. As to the creation of the magnetic
twist, which is to be discussed in detail in paper II of the series,
it is argued to be due to the winding-up effect of the rotational
velocity component in the stellar surface convection in the case of
the substellar jets, and of the rotational velocity in the contracting
accretion disk in the cases of bipolar flows from young stellar objects
and of bipolar lobes ejected from active galaxies, respectively.
Title: Magnetodynamical acceleration of CO and optical bipolar flows
from the region of star formation.
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1985PASJ...37..515U
Altcode:
We propose a comprehensive model which explains the activities in
the region of star formation recently found in optical, X-ray, and
radio observations. The large-scale cold bipolar flow is accelerated,
according to our model, through the interaction of the rotating mass of
the contracting accretion disk with the magnetic field which is brought
into the disk in the process of the condensation of the mass from the
nebulosity. The magnetic twist, created by the rotation of the disk,
pushes out the mass by the j x B force as it relaxes towards the polar
directions, explaining the production of the large-scale cold bipolar
flows. On the other hand, the action of the magnetic field very close
to the star, including the mass transfer by magnetic reconnection
and angular momentum loss along the magnetic field lines, controls
the final fall of the accreted mass from the innermost edge of the
disk to the star. The free fall converts the potential energy into
kinetic energy of the infall and the latter into heat in the crash at
the polar crowns of the star, explaining the observed inflow of the
gas and the presence of the X-ray emitting regions near the stellar
surface. The recoiling shock may blow off the tail of the infalling
mass along the field lines toward the polar directions, explaining the
observed outflow and the small-scale warm high-velocity jets from the
vicinity of the central star.
Title: Sweeping pinch mechanism and the acceleration of jets in
astrophysics
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1985IAUS..107..287U
Altcode:
A magnetodynamic mechanism of jet formation, in which a packet of the
toroidal component of the magnetic field Bφ plays a role,
is proposed. Such a packet of toroidal field, produced by the rotational
motion in the β = pg/pm very large 1 region,
relaxes itself in the β very low 1 region when brought up into such a
region, for example, by the process of flux emergence due to magnetic
buoyancy. In the β very low 1 region, a progressive pinch is caused by
this relaxation and the mass is swept out by the pinch near the axis and
also by the j×B force in the twisted field region surrounding the axis.
Title: Acceleration of Surges by Sweeping Pinch Mechanism
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1985spit.conf..332U
Altcode:
No abstract at ADS
Title: Bipolar flows and X-ray emission from young stellar objects.
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1985NASCP2358..169U
Altcode: 1985onhm.rept..169U
The production of both large scale CO bipolar flows and small scale
optical bipolar jets from the star-forming regions is interpreted in
terms of a magnetic mechanism operating in an accretion model. It is
shown by an axisymmetric 2.5-dimensional simulation that a large scale
cold bipolar flow may be produced in the relaxation of the magnetic
twist which is created by the rotational winding-up of the magnetic
field in the contracting disk. In contrast, small scale warm bipolar
jets may be driven by the recoiling shocks produced at the stellar
surface by the infalling material, which is released from the inner
edge of the disk through magnetic reconnections.
Title: A magnetodynamic mechanism for the acceleration of
astrophysical jets. Application to bipolar flows in star forming
regions.
Authors: Shibata, K.; Uchida, Y.
Bibcode: 1985AstHe..78..240S
Altcode:
No abstract at ADS
Title: Positive cerium anomaly of lunar 14310 and ALH-765: Examination
by 138Ce/142Ce.
Authors: Tanaka, T.; Shimizu, H.; Shibata, K.; Masuda, A.
Bibcode: 1985anme...10..129T
Altcode:
No abstract at ADS
Title: Chemical abundances of stellar planetary nebula, M1-9, near
the galactic periphery.
Authors: Shibata, K.; Tamura, S.
Bibcode: 1985PASJ...37..325S
Altcode:
Chemical abundances of a stellar image planetary nebula, M 1-9,
are presented. In the diagram of N(N)/N((O) vs N(He)/N(H), prominent
helium and nitrogen enrichment is clearly found. This object may be at
the Galactic periphery in the direction of the anticenter. Its radial
velocity for the local standard of rest is larger than the assumed
circular motion in the Schmidt (1965) model. The chemical abundances
are compared with those of other Galactic planetary nebulae.
Title: Magnetohydrodynamical Activities of Very Young Stars
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1984apoa.conf..297U
Altcode:
No abstract at ADS
Title: Magnetically buffered accretion to a young star and the
formation of bipolar flows.
Authors: Uchida, Y.; Shibata, K.
Bibcode: 1984PASJ...36..105U
Altcode:
A magnetodynamic process involving a magnetized nebular mass accreted
onto a star with magnetic properties is considered. Attention is given
to the magnetic field configuration present in the magnetic formation
of a star. A magneto-equilibrium model of Uchida and Low (1980, 1981)
is shown to be applicable to the magnetic accretion scenario. The
accretion in turn is found to possibly continue even after the star
has formed. The infalling material crashes onto the stellar surface
and produces a shock that propagates to the tail of the inflow, where
material is blown off in the magnetic field direction, i.e., is bent
in the polar directions. The process is a possible model for the old
Herbig-Haro objects being observed near and interacting with young T
Tauri stars.
Title: Dynamical interpretation of the very hot region appearing at
the top of the loop
Authors: Shibata, K.; Uchida, Y.; Sakurai, T.
Bibcode: 1983SoPh...86..345S
Altcode:
In order to explain the appearance of a hard X-ray source at the
top of a loop, we present a model in which the dynamical effects
of the dark filament mass infallng along the loop in association
with the "disparition brusque" plays an important role. The crash
of the infalling mass produces high temperature regions in the low
corona above the two footpoints of the loop, and the up-going shocks,
created in the crash and strengthened in propagating upwards along the
steep density gradient in the tail of the infalling mass, produce a
very high temperature (108 K) region upon colliding with
each other near the top of the loop. Successive occurrence of this
process in successively higher loops in magnetic arcade may account
for the sources of gradual hard X-ray bursts appearing at the top of
the loop-like structure.
Title: Strontium, Cerium and Neodymium Isotopes on ALH 765 Meteorite.
Authors: Tanaka, T.; Shibata, K.; Masuda, A.
Bibcode: 1983anme....8...71T
Altcode:
No abstract at ADS
Title: Nonlinear MHD wave propagation in the solar chromosphere. I
The case of a uniform vertical magnetic field
Authors: Shibata, K.
Bibcode: 1983PASJ...35..263S
Altcode:
Nonlinear MHD wave propagation in the solar chromosphere with a
uniform vertical magnetic field is investigated. Typical cases of weak
and strong field strengths are studied in detail, and a summary of
other cases and a comparison with other one-dimensional hydrodynamic
simulations are given. The energy transfer by slow and fast mode MHD
waves is studied by a rough approximation method. It is found that an
acoustic wave propagating upward in the high beta media splits into a
magnetic fast mode and an acoustic slow mode at the beta = 1 level. If
an initial pressure enhancement occurs isotropically in a medium with
beta of 0.198 or less, the acoustic slow mode generated directly from
this pressure enhancement can be approximated well by one-dimensional
hydrodynamics. When the initial pressure enhancement is located in
the high beta media, hydrodynamic buoyancy causes a vortex motion of
the gas.
Title: Nonlinear MHD wave propagation in the solar chromosphere,
I. The case of uniform vertical magnetic field
Authors: Shibata, Kazunari
Bibcode: 1983PhDT.......234S
Altcode:
No abstract at ADS
Title: Two Types of Jets and Origin of Macrospicules
Authors: Shibata, K.
Bibcode: 1982SoPh...81....9S
Altcode:
Comparison between observations of macrospicules and numerical jet
models obtained in our previous one-dimensional hydrodynamic simulations
suggests that Hα macrospicules and EUV macrospicules unseen in Hα
have different physical origins. Hα macrospicules are produced by
pressure gradient force at the bright point in the middle or in the
upper chromosphere. On the other hand, EUV macrospicules unseen in Hα
are produced by shock waves which originate from network bright-points
in the photosphere or in the low chromosphere and have propagated
through the chromosphere. An essential difference between the two types
of macrospicules is the density ratio between macrospicules and the
corona. The critical parameter causing this difference is the height
of bright points. The general relation between the density in jets
(including also regular spicules) and the height of bright points
is discussed.
Title: Why are spicules absent over plages and long under coronal
holes?
Authors: Shibata, K.; Suematsu, Y.
Bibcode: 1982SoPh...78..333S
Altcode:
One-dimensional hydrodynamic simulations are performed in order
to examine the influence of initial atmospheric structures on the
dynamics of spicules. This is an extended version of our previous
spicule theory: spicules are produced by the shock wave (MHD slow
mode shock) which originates from a bright point appearance (sudden
pressure increase) at the network in the photosphere or in the low
chromosphere. Simulation results well reproduce the observational
facts that spicules are absent over plages and long under coronal
holes. The physical reason is that the growth of a shock wave during
its propagation through the chromosphere is small in plage regions and
large in coronal hole regions, since the growth of a shock is determined
by the density ratio (ϱh0/ϱc) between
the bright point and the corona. An empirical formula ΔHmax
∼ (ϱh0/ϱc)0.46 is
obtained, where ΔHmax is the maximum height of spicules
above the transition region. The cross-section of the vertical magnetic
flux tube is assumed to be constant in the numerical simulations.
Title: Numerical Hydrodynamics of the Jet Phenomena in the Solar
Atmosphere - Part Two - Surges
Authors: Shibata, K.; Nishikawa, T.; Kitai, R.; Suematsu, Y.
Bibcode: 1982SoPh...77..121S
Altcode:
One-dimensional hydrodynamic simulations of surges are performed in
order to make clear their origin and structure. Surges are regarded as
the jets resulting from a sudden pressure increase at the base of the
model atmosphere. The height of the explosion (h0), which
is measured from the level of τ5000 = 1, is regarded as a
free parameter. Another free parameter is the strength of the sudden
pressure increase (p/p0) at h0. Simulations
are performed for values in the ranges of 540 km ≤ h0
≤ 1920 km and 3 ≤ p/p0 ≤ 30. It was found that for a
fixed p/p0 there exists a critical height (hc)
in h0, which separates the jet (surge) models into two
types. For h0 > hc, jets are produced directly
by the pressure gradient force near h0, and made of the
matter ejected from the explosion itself. The essential hydrodynamic
structure of this type is the same as that in a shock tube (this type
is called `shock tube' type). For h0 < hc,
jets are not the direct results of the pressure enhancement, but
are produced by the shock wave which are generated by the pressure
enhancement and which has propagated through the chromosphere (this type
is called the `crest shock' type). It is shown that the critical height
(hc) ranges from 1000 km to 1500 km for 3 ≤ p/p0
≤ 30. General properties of both types are investigated in detail. The
results are compared with observations and it is concluded that small
surges associated with Ellerman bombsbelong to the `crest shock' type,
i.e. they are produced by the shock wave.
Title: Numerical Hydrodynamics of the Jet Phenomena in the Solar
Atmosphere - Part One - Spicules
Authors: Suematsu, Y.; Shibata, K.; Neshikawa, T.; Kitai, R.
Bibcode: 1982SoPh...75...99S
Altcode:
We present a spicule model whose eruption occurs as a result of the
sudden pressure enhancement at the bright point located at the root of
the spicule. To show this, one dimensional (constant cross sectional)
and time dependent hydrodynamic equations are solved numerically in
the realistic solar atmosphere extending from the photosphere to
the corona. Adiabatic motion is assumed. The pressure enhancement
by a bright point at the base of the model atmosphere generates a
shock wave. The shock gets stronger as it passes upward through the
chromosphere and eventually collides with the chromosphere-corona
interface which is a kind of a contact discontinuity. As the result,
the interface begins to move upward. We identify the matter following
behind this interface as the solar spicule. The model explains many
observed features, such as the height and the density of the spicules,
although such features have been hitherto considered not to be explained
easily by shock theories.
Title: Jet phenomena in the solar atmosphere.
Authors: Shibata, K.
Bibcode: 1981AstHe..74...10S
Altcode:
No abstract at ADS
Title: On the Origin of Strong Downdrafts Associated with the Birth
of Sunspots
Authors: Shibata, K.
Bibcode: 1980SoPh...66...61S
Altcode:
During the emerging phase of sunspot pores strong downdrafts are
observed in photospheric lines, whereas the upward flow is not detected
within the observational accuracy. For the study of the origin of
these downdrafts, we present the numerical solutions of non-steady
hydrodynamic equations in one dimension along the rising magnetic flux
tubes. In these solutions, it is assumed that the solar gas inside
the tube is initially in the hydrostatic HSRA and then the tube rises
with a given velocity and configuration. The results reproduce well
the observed radial velocity distributions, hence it is concluded that
the observed strong downdrafts originate from the sliding matter along
the rising magnetic flux tubes.
Title: Numerical Simulations of Active Phenomena in the Solar
Atmosphere
Authors: Shibata, K.
Bibcode: 1980jfss.conf...50S
Altcode:
No abstract at ADS
Title: Rb-Sr whole-rock ages of Precambrian metamorphic rocks in
the Kamiaso conglomerate from central Japan
Authors: Shibata, K.; Adachi, M.
Bibcode: 1973E&PSL..21..277S
Altcode:
No abstract at ADS