Author name code: yokoyama
ADS astronomy entries on 2022-09-14
=author:"Yokoyama, Takaaki"
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Title: Mean-field Analysis on Large-scale Magnetic Fields at High
Reynolds Numbers
Authors: Shimada, Ryota; Hotta, Hideyuki; Yokoyama, Takaaki
Bibcode: 2022ApJ...935...55S
Altcode: 2022arXiv220701639S
Solar magnetic fields comprise an 11 yr activity cycle, represented
by the number of sunspots. The maintenance of such a solar magnetic
field can be attributed to fluid motion in the convection zone, i.e.,
a dynamo. This study conducts the mean-field analyses of the global
solar dynamo simulation presented by Hotta et al. (2016). Although
the study succeeds in producing coherent large-scale magnetic fields
at high Reynolds numbers, the detailed physics of the maintenance of
these fields have not been fully understood. This study extracts the
α tensor and the turbulent magnetic diffusivity tensor β through
mean-field analyses. The turbulent magnetic diffusivity exhibits a
significant decrease toward high Reynolds numbers. The decrease in
the turbulent magnetic diffusivity suppresses the energy conversion of
large-scale field to small-scale field. This implies that the decrease
in the turbulent magnetic diffusivity contributes to the maintenance of
a large-scale magnetic field at high Reynolds numbers. A significant
downward turbulent pumping is observed; it is enhanced in the weak
phase of the large-scale field. This study proposes a cyclic reversal
process of a large-scale field, which is dominantly driven by the α
effect and is possibly triggered by downward pumping.
Title: Radiative MHD simulations of solar coronal loops considering
the energy injectionfrom intergranular lanes.
Authors: Kuniyoshi, Hidetaka; Yokoyama, Takaaki; Iijima, Haruhisa
Bibcode: 2021AGUFMSH12B..08K
Altcode:
The temperatures of the solar corona are millions of Kelvins greater
than the surface. Over the previous decades, many authors have discussed
the coronal heating problem, the question of how the hot temperatures
of the corona are maintained. The heating mechanism is classified
into static DC heating and wave-like AC heating depending on the
relationship between Alfvén wave travel time along the entire loop
and the timescale of the mechanical motion in and below the photosphere
which displaces the footpoints of coronal magnetic field lines. So far,
many magnetohydrodynamic (MHD) simulations have been conducted to study
the heating process. Using Reduced MHD simulation, van Ballegooijen et
al. (2017) proposed the AC heating process via Alfvén wave turbulence
by modeling intergranular lanes from which Alfvén waves are injected
into the upper atmosphere. However, their Reduced MHD simulation cannot
model the plasma temperature nor the excitation process of Alfvén waves
self-consistently. Rempel (2017) simulated the DC-like heating process
using radiative MHD code including the photosphere, the chromosphere,
and the corona. However, the spatial grid size is not small enough to
resolve the wave excitation in the intergranular lanes. Therefore,
we study the energy transport process in a coronal loop, using
radiative MHD simulation for the realistic modeling from the upper
convection zone to the corona. The sufficiently small grid spacing is
used to resolve the wave excitation in the intergranular lanes. In the
preliminary analysis, we found that the timescales of Alfvén waves that
contribute to the coronal energy transport are shorter than the Alfvén
wave travel time. This result indicates that AC heating contributes to
heat the corona considerably. In other words, small-scale Alfvén waves
injected from intergranular lanes are important for coronal heating.
Title: Fast Magnetic Wave Could Heat the Solar Low-beta Chromosphere
Authors: Wang, Yikang; Yokoyama, Takaaki; Iijima, Haruhisa
Bibcode: 2021ApJ...916L..10W
Altcode: 2021arXiv210713722W
Magnetohydrodynamic (MHD) waves are candidates for heating the solar
chromosphere, although it is still unclear which mode of the wave is
dominant in heating. We perform two-dimensional radiative MHD simulation
to investigate the propagation of MHD waves in the quiet region of the
solar chromosphere. We identify the mode of the shock waves by using
the relationship between gas pressure and magnetic pressure across the
shock front and calculate their corresponding heating rate through the
entropy jump to obtain a quantitative understanding of the wave-heating
process in the chromosphere. Our result shows that the fast magnetic
wave is significant in heating the low-beta chromosphere. The low-beta
fast magnetic waves are generated from high-beta fast acoustic waves
via mode conversion crossing the equipartition layer. Efficient mode
conversion is achieved by large attacking angles between the propagation
direction of the shock waves and the chromospheric magnetic field.
Title: Thermally conductive magnetohydrodynamic simulation of X-ray
spectral states and transitions in black hole binaries
Authors: Nakamura, Kenji; Yokoyama, Takaaki; Kaneko, Takafumi;
Matsumoto, Ryoji; Machida, Mami
Bibcode: 2021cosp...43E1698N
Altcode:
We performed thermally conductive magnetohydrodynamic simulation to
study evolution of an accretion flow during X-ray state transitions
between the hard state and the soft state in a black hole binary. Our
models are two dimensional and axisymmetric. We adopt the Spitzer
thermal conductivity. Thermal conduction in the vertical direction
of magnetic field lines is restricted and treated as zero in our
simulations. We suppose a hot and low-density accretion flow is
surrounded by a hotter halo initially. When the density of evolved
accretion flow is higher than a critical value to cause a cooling
instability, the accretion flow contracts and forms the cool accretion
disk in the equatorial plane. Since heat transferred from the halo by
thermal conduction evaporates the cool accretion disk, the intermediate
region appears between the cool accretion disk and the hot halo. The
Compton y-parameter of the intermediate region is estimated about 2
in our models. The intermediate region works as a corona surrounding
the cool accretion disk. Continuous heating promotes the evaporation
of the cool accretion disk, finally the cool accretion disk disappears
and the hot low-density accretion flow reappears. These results could
be clues to understand the observations of the hard-to-soft state
transition and the soft-to-hard state transition.
Title: The Solar-C (EUVST) mission: the latest status
Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko; Suematsu,
Yoshinori; Hara, Hirohisa; Tsuzuki, Toshihiro; Katsukawa, Yukio; Kubo,
Masahito; Ishikawa, Ryoko; Watanabe, Tetsuya; Toriumi, Shin; Ichimoto,
Kiyoshi; Nagata, Shin'ichi; Hasegawa, Takahiro; Yokoyama, Takaaki;
Watanabe, Kyoko; Tsuno, Katsuhiko; Korendyke, Clarence M.; Warren,
Harry; De Pontieu, Bart; Boerner, Paul; Solanki, Sami K.; Teriaca,
Luca; Schuehle, Udo; Matthews, Sarah; Long, David; Thomas, William;
Hancock, Barry; Reid, Hamish; Fludra, Andrzej; Auchère, Frederic;
Andretta, Vincenzo; Naletto, Giampiero; Poletto, Luca; Harra, Louise
Bibcode: 2020SPIE11444E..0NS
Altcode:
Solar-C (EUVST) is the next Japanese solar physics mission to
be developed with significant contributions from US and European
countries. The mission carries an EUV imaging spectrometer with
slit-jaw imaging system called EUVST (EUV High-Throughput Spectroscopic
Telescope) as the mission payload, to take a fundamental step towards
answering how the plasma universe is created and evolves and how the
Sun influences the Earth and other planets in our solar system. In
April 2020, ISAS (Institute of Space and Astronautical Science) of JAXA
(Japan Aerospace Exploration Agency) has made the final down-selection
for this mission as the 4th in the series of competitively chosen
M-class mission to be launched with an Epsilon launch vehicle in mid
2020s. NASA (National Aeronautics and Space Administration) has selected
this mission concept for Phase A concept study in September 2019 and
is in the process leading to final selection. For European countries,
the team has (or is in the process of confirming) confirmed endorsement
for hardware contributions to the EUVST from the national agencies. A
recent update to the mission instrumentation is to add a UV spectral
irradiance monitor capability for EUVST calibration and scientific
purpose. This presentation provides the latest status of the mission
with an overall description of the mission concept emphasizing on key
roles of the mission in heliophysics research from mid 2020s.
Title: Inverse First Ionization Potential Effects in Giant Solar
Flares Found from Earth X-Ray Albedo with Suzaku/XIS
Authors: Katsuda, Satoru; Ohno, Masanori; Mori, Koji; Beppu, Tatsuhiko;
Kanemaru, Yoshiaki; Tashiro, Makoto S.; Terada, Yukikatsu; Sato,
Kosuke; Morita, Kae; Sagara, Hikari; Ogawa, Futa; Takahashi, Haruya;
Murakami, Hiroshi; Nobukawa, Masayoshi; Tsunemi, Hiroshi; Hayashida,
Kiyoshi; Matsumoto, Hironori; Noda, Hirofumi; Nakajima, Hiroshi;
Ezoe, Yuichiro; Tsuboi, Yohko; Maeda, Yoshitomo; Yokoyama, Takaaki;
Narukage, Noriyuki
Bibcode: 2020ApJ...891..126K
Altcode: 2020arXiv200110643K
We report X-ray spectroscopic results for four giant solar flares
that occurred on 2005 September 7 (X17.0), 2005 September 8 (X5.4),
2005 September 9 (X6.2), and 2006 December 5 (X9.0), obtained from
Earth albedo data with the X-ray Imaging Spectrometer (XIS) on board
Suzaku. The good energy resolution of the XIS (FWHM ∼ 100 eV) enables
us to separate a number of line-like features and detect the underlying
continuum emission. These features include Si Heα, Si Lyα, S Heα,
S Lyα, Ar Heα, and Ca Heα originating from solar flares as well as
fluorescent Ar Kα and Ar Kβ from the Earth's atmosphere. Absolute
elemental abundances (X/H) averaged over the four flares are obtained
to be ∼2.0 (Ca), ∼0.7 (Si), ∼0.3 (S), and ∼0.9 (Ar) at around
flare peaks. This abundance pattern is similar to those of active
stars' coronae showing inverse first ionization potential (I-FIP)
effects, I.e., elemental abundances decrease with decreasing FIP
with a turnover at the low end of the FIP. The abundances are almost
constant during the flares, with the exception of Si which increases by
a factor of ∼2 in the decay phase. The evolution of the Si abundance
is consistent with the finding that the I-FIP plasma originates from
chromospheric evaporation and then mixes with the surrounding low-FIP
biased materials. Flare-to-flare abundance varied by a factor of two,
agreeing with past observations of solar flares. Finally, we emphasize
that Earth albedo data acquired by X-ray astronomy satellites like
Suzaku and the X-Ray Imaging Spectroscopy Mission can significantly
contribute to studies of solar physics.
Title: Simulation of Alfvén Wave Propagation in the Magnetic
Chromosphere with Radiative Loss: Effects of Nonlinear Mode Coupling
on Chromospheric Heating
Authors: Wang, Yikang; Yokoyama, Takaaki
Bibcode: 2020ApJ...891..110W
Altcode: 2020arXiv200305796W
We perform magnetohydrodynamic simulations to investigate the
propagation of Alfvén waves in the magnetic chromosphere. We use the
1.5D expanding flux tube geometry setting and transverse perturbation at
the bottom to generate the Alfvén wave. Compared with previous studies,
our expansion is that we include the radiative loss term introduced
by Carlsson & Leenaarts. We find that when an observation-based
transverse wave generator is applied, the spatial distribution of the
time-averaged radiative loss profile in our simulation is consistent
with that in the classic atmospheric model. In addition, the energy flux
in the corona is larger than the required value for coronal heating in
the quiet region. Our study shows that the Alfvén wave-driven model
has the potential to simultaneously explain chromospheric heating and
how energy is transported to the corona.
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: The Solar-C_EUVST mission
Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko;
Ichimoto, Kiyoshi; Suematsu, Yoshinori; Hara, Hirohisa; Katsukawa,
Yukio; Kubo, Masahito; Toriumi, Shin; Watanabe, Tetsuya; Yokoyama,
Takaaki; Korendyke, Clarence M.; Warren, Harry P.; Tarbell, Ted; De
Pontieu, Bart; Teriaca, Luca; Schühle, Udo H.; Solanki, Sami; Harra,
Louise K.; Matthews, Sarah; Fludra, A.; Auchère, F.; Andretta, V.;
Naletto, G.; Zhukov, A.
Bibcode: 2019SPIE11118E..07S
Altcode:
Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a
solar physics mission concept that was selected as a candidate for
JAXA competitive M-class missions in July 2018. The onboard science
instrument, EUVST, is an EUV spectrometer with slit-jaw imaging
system that will simultaneously observe the solar atmosphere from the
photosphere/chromosphere up to the corona with seamless temperature
coverage, high spatial resolution, and high throughput for the first
time. The mission is designed to provide a conclusive answer to the
most fundamental questions in solar physics: how fundamental processes
lead to the formation of the solar atmosphere and the solar wind, and
how the solar atmosphere becomes unstable, releasing the energy that
drives solar flares and eruptions. The entire instrument structure
and the primary mirror assembly with scanning and tip-tilt fine
pointing capability for the EUVST are being developed in Japan, with
spectrograph and slit-jaw imaging hardware and science contributions
from US and European countries. The mission will be launched and
installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in
2025. ISAS/JAXA coordinates the conceptual study activities during the
current mission definition phase in collaboration with NAOJ and other
universities. The team is currently working towards the JAXA final
down-selection expected at the end of 2019, with strong support from
US and European colleagues. The paper provides an overall description
of the mission concept, key technologies, and the latest status.
Title: Diffusion regions and 3D energy mode development in spontaneous
reconnection
Authors: Wang, Shuoyang; Yokoyama, Takaaki
Bibcode: 2019PhPl...26g2109W
Altcode: 2019arXiv190603964W
The understanding of magnetic reconnection in three-dimensions
(3D) is far shallower than its counterpart in two-dimensions due
to its potential complication, not to mention the evolving of
the spontaneously growing turbulence. We investigate the reason
for reconnection acceleration on the characters and development of
diffusion regions and sheared 3D energy modes (energy modes that are
not parallel to the antiparallel magnetic fields) during the turbulence
building stage. We found that multiple reconnection layers emerge due
to the growth of 3D sheared tearing instability. Diffusion regions
in adjacent reconnection layers form an inflow-outflow coupling that
enhances the local reconnection. Further coupling of the existing
energy modes breeds new energy modes near the current sheet edge. As
reconnection layers span and interact with each other across the whole
current sheet, global magnetic energy consumption accelerates. The
significant contribution of 3D energy modes and their interaction to
the reconnection rate enhancement seems to be independent of magnetic
diffusivity. On the other hand, the global guide field changes the
layout of the 3D reconnection layer and thus determines whether the
system is fast-reconnection-preferable.
Title: Three-dimensional Simulation of the Fast Solar Wind Driven
by Compressible Magnetohydrodynamic Turbulence
Authors: Shoda, Munehito; Suzuki, Takeru Ken; Asgari-Targhi, Mahboubeh;
Yokoyama, Takaaki
Bibcode: 2019ApJ...880L...2S
Altcode: 2019arXiv190511685S
Using a three-dimensional compressible magnetohydrodynamic (MHD)
simulation, we have reproduced the fast solar wind in a direct
and self-consistent manner, based on the wave/turbulence-driven
scenario. As a natural consequence of Alfvénic perturbations at
the coronal base, highly compressional and turbulent fluctuations
are generated, leading to heating and acceleration of the solar
wind. The analysis of power spectra and structure functions reveals
that the turbulence is characterized by its imbalanced (in the sense
of outward Alfvénic fluctuations) and anisotropic nature. The density
fluctuation originates from the parametric decay instability (PDI)
of outwardly propagating Alfvén waves and plays a significant role in
the Alfvén-wave reflection that triggers turbulence. Our conclusion
is that the fast solar wind is heated and accelerated by compressible
MHD turbulence driven by PDI and resultant Alfvén-wave reflection.
Title: Observations of photospheric magnetic structure below a dark
filament using the Hinode Spectro-Polarimeter
Authors: Yokoyama, Takaaki; Katsukawa, Yukio; Shimojo, Masumi
Bibcode: 2019PASJ...71...46Y
Altcode: 2019arXiv190110695Y; 2019PASJ..tmp...26Y
The structure of the photospheric vector magnetic field below a
dark filament on the Sun is studied using the observations of the
Spectro-Polarimeter attached to the Solar Optical Telescope onboard
Hinode. Special attention is paid to discriminating between two
suggested models, a flux rope or a bent arcade. "Inverse polarity"
orientation is possible below the filament in a flux rope, whereas
"normal polarity" can appear in both models. We study a filament in the
active region NOAA 10930, which appeared on the solar disk during 2006
December. The transverse field perpendicular to the line of sight has a
direction almost parallel to the filament spine with a shear angle of
30°, the orientation of which includes the 180° ambiguity. To know
whether it is in the normal orientation or in the inverse one, the
center-to-limb variation is used for the solution under the assumption
that the filament does not drastically change its magnetic structure
during the passage. When the filament is near the east limb, we found
that the line-of-site magnetic component below the filament is positive,
while it is negative near the west limb.This change of sign indicates
that the horizontal photospheric field perpendicular to the polarity
inversion line beneath the filament has an "inverse-polarity", which
indicates a flux-rope structure of the filament supporting field.
Title: Impact of Dynamic State on the Mass Condensation Rate of
Solar Prominences
Authors: Kaneko, Takafumi; Yokoyama, Takaaki
Bibcode: 2018ApJ...869..136K
Altcode: 2018arXiv181100828K
The interiors of quiescent prominences are filled with turbulent
flows. The evolution of upflow plumes, descending pillars,
and vortex motions has been clearly detected in high-resolution
observations. The Rayleigh-Taylor instability is thought to be a
driver of such internal flows. Descending pillars are related to
the mass budgets of prominences. There is a hypothesis of dynamic
equilibrium where the mass drainage via descending pillars and the
mass supply via radiative condensation are balanced to maintain the
prominence mass; however, the background physics connecting the two
different processes is poorly understood. In this study, we reproduced
the dynamic interior of a prominence via radiative condensation
and the mechanism similar to the Rayleigh-Taylor instability
using a three-dimensional magnetohydrodynamic simulation including
optically thin radiative cooling and nonlinear anisotropic thermal
conduction. The process to prominence formation in the simulation
follows the reconnection-condensation model, where topological change
in the magnetic field caused by reconnection leads to radiative
condensation. Reconnection is driven by converging motion at the
footpoints of the coronal arcade fields. In contrast to the previous
model, by randomly changing the speed of the footpoint motion along
a polarity inversion line, the dynamic interior of prominence is
successfully reproduced. We find that the mass condensation rate of
the prominence is enhanced in the case with dynamic state. Our results
support the observational hypothesis that the condensation rate is
balanced with the mass drainage rate and suggest that a self-induced
mass maintenance mechanism exists.
Title: ALMA Observations of the Solar Chromosphere on the Polar Limb
Authors: Yokoyama, Takaaki; Shimojo, Masumi; Okamoto, Takenori J.;
Iijima, Haruhisa
Bibcode: 2018ApJ...863...96Y
Altcode: 2018arXiv180701411Y
We report the results of the Atacama Large Millimeter/sub-millimeter
Array (ALMA) observations of the solar chromosphere on the southern
polar limb. Coordinated observations with the Interface Region Imaging
Spectrograph (IRIS) are also conducted. ALMA provided unprecedented
high spatial resolution in the millimeter band (≈2.″0) at 100
GHz frequency with a moderate cadence (20 s). The results are as
follows. (1) The ALMA 100 GHz images show saw-tooth patterns on the
limb, and a comparison with Solar Dynamics Observatory/Atmospheric
Imaging Assembly 171 Å images shows a good correspondence of the limbs
with each other. (2) The ALMA animation shows a dynamic thorn-like
structure elongating from the saw-tooth patterns on the limb, with
lengths reaching at least 8″, thus suggesting jet-like activity in
the ALMA microwave range. These ALMA jets are in good correspondence
with the IRIS jet clusters. (3) A blob-ejection event is observed. By
comparing with the IRIS Mg II slit-jaw images, the trajectory of the
blob is located along the spicular patterns.
Title: Numerical modeling of prominence formation from reconnection
to radiative condensation
Authors: Kaneko, Takafumi; Yokoyama, Takaaki
Bibcode: 2018cosp...42E1673K
Altcode:
We briefly review recent progress in numerical modeling for
prominence formation and introduce our model, reconnection-condensation
model. Origin of cool dense plasmas and mechanism of mass maintenance in
the hot tenuous corona is one of the most important subjects in studies
of solar prominences. Radiative cooling condensation is a promising
process to supply mass for prominences. The formation mechanism of
fine structures and turbulence in prominence and their physical role
for mass condensation are also unclear.Numerical modeling is useful to
investigate these issues. In previous numerical studies, it is known
that chromospheric evaporation driven by parameterized footpoint heating
leads to in-situ coronal condensation. The evaporation-condensation
model was demonstrated in a three-dimensional flux rope structure using
magnetohydrodynamic (MHD) simulations including thermal conduction
and radiative cooling, and succeeded in reproducing prominences
with fine structures by fragmented condensations. Despite these
efforts, the issue on unclear origin of the footpoint heating still
remains.We attempt to consider a different process leading to radiative
condensation. In observations, prominences always appear along polarity
inversion lines, suggesting that cancelation or reconnection must
be related to radiative condensation. In the previous simulations on
radiative condensation, self-consistent multi-dimensional reconnection
process were absent. We propose reconnection-condensation model and
demonstrate it using three-dimensional MHD simulations including
nonlinear anisotropic thermal conduction and optically thin radiative
cooling. In our model, a flux rope is created by reconnection via
converging footpoint motion. By elevation of dense coronal plasmas and
topological change in coronal magnetic fields, radiative condensation
is triggered inside the flux rope. Our results show clear link between
reconnection and radiative condensation, and suggest that evaporation
is not always necessary.Recently, we improved the model to include
dynamic fine structures by the Rayleigh-Taylor instability. We found
that mass condensation rate is enhanced to balance with mass drainage
rate by coupling with the Rayleigh-Taylor instability. We compare the
simulation results with observations and discuss remained issues in
numerical modeling for prominence formation.
Title: Frequency-dependent Alfvén-wave Propagation in the Solar Wind:
Onset and Suppression of Parametric Decay Instability
Authors: Shoda, Munehito; Yokoyama, Takaaki; Suzuki, Takeru K.
Bibcode: 2018ApJ...860...17S
Altcode: 2018arXiv180302606S
Using numerical simulations we investigate the onset and suppression
of parametric decay instability (PDI) in the solar wind, focusing on
the suppression effect by the wind acceleration and expansion. Wave
propagation and dissipation from the coronal base to 1 au is solved
numerically in a self-consistent manner; we take into account the
feedback of wave energy and pressure in the background. Monochromatic
waves with various injection frequencies, f 0, are injected
to discuss the suppression of PDI, while broadband waves are applied
to compare the numerical results with observation. We find that
high-frequency ({f}0≳ {10}-3 {Hz}) Alfvén
waves are subject to PDI. Meanwhile, the maximum growth rate of the
PDI of low-frequency ({f}0≲ {10}-4 {Hz})
Alfvén waves becomes negative due to acceleration and expansion
effects. Medium-frequency ({f}0≈ {10}-3.5
{Hz}) Alfvén waves have a positive growth rate but do not show the
signature of PDI up to 1 au because the growth rate is too small. The
medium-frequency waves experience neither PDI nor reflection so they
propagate through the solar wind most efficiently. The solar wind
is shown to possess a frequency-filtering mechanism with respect to
Alfvén waves. The simulations with broadband waves indicate that the
observed trend of the density fluctuation is well explained by the
evolution of PDI while the observed cross-helicity evolution is in
agreement with low-frequency wave propagation.
Title: Anisotropic Magnetohydrodynamic Turbulence Driven by
Parametric Decay Instability: The Onset of Phase Mixing and Alfvén
Wave Turbulence
Authors: Shoda, Munehito; Yokoyama, Takaaki
Bibcode: 2018ApJ...859L..17S
Altcode: 2018arXiv180500285S
We conduct a 3D magnetohydrodynamic (MHD) simulation of the parametric
decay instability of Alfvén waves and resultant compressible MHD
turbulence, which is likely to develop in the solar wind acceleration
region. Because of the presence of the mean magnetic field, the
nonlinear stage is characterized by filament-like structuring and
anisotropic cascading. By calculating the timescales of phase mixing
and the evolution of Alfvén wave turbulence, we have found that the
early nonlinear stage is dominated by phase mixing, while the later
phase is dominated by imbalanced Alfvén wave turbulence. Our results
indicate that the regions in the solar atmosphere with large density
fluctuation, such as the coronal bottom and wind acceleration region,
are heated by phase-mixed Alfvén waves, while the other regions are
heated by Alfvén wave turbulence.
Title: A Self-consistent Model of the Coronal Heating and Solar
Wind Acceleration Including Compressible and Incompressible Heating
Processes
Authors: Shoda, Munehito; Yokoyama, Takaaki; Suzuki, Takeru K.
Bibcode: 2018ApJ...853..190S
Altcode: 2017arXiv171207760S
We propose a novel one-dimensional model that includes both shock
and turbulence heating and qualify how these processes contribute
to heating the corona and driving the solar wind. Compressible MHD
simulations allow us to automatically consider shock formation and
dissipation, while turbulent dissipation is modeled via a one-point
closure based on Alfvén wave turbulence. Numerical simulations
were conducted with different photospheric perpendicular correlation
lengths {λ }0, which is a critical parameter of Alfvén wave
turbulence, and different root-mean-square photospheric transverse-wave
amplitudes δ {v}0. For the various {λ }0,
we obtain a low-temperature chromosphere, high-temperature corona,
and supersonic solar wind. Our analysis shows that turbulence heating
is always dominant when {λ }0≲ 1 {Mm}. This result does
not mean that we can ignore the compressibility because the analysis
indicates that the compressible waves and their associated density
fluctuations enhance the Alfvén wave reflection and therefore the
turbulence heating. The density fluctuation and the cross-helicity are
strongly affected by {λ }0, while the coronal temperature
and mass-loss rate depend weakly on {λ }0.
Title: High-frequency Spicule Oscillations Generated via Mode
Conversion
Authors: Shoda, Munehito; Yokoyama, Takaaki
Bibcode: 2018ApJ...854....9S
Altcode: 2018arXiv180101254S
Spicule oscillations involve high-frequency components with a
typical period approximately corresponding to 40-50 s. The typical
timescale of the photospheric oscillation is a few minutes, and
thus, the origin of this high-frequency component is not trivial. In
this study, a one-dimensional numerical simulation is performed to
demonstrate that the observed spicule oscillations originate from
longitudinal-to-transverse mode conversion that occurs around the
equipartition layer in the chromosphere. Calculations are conducted
in a self-consistent manner with the exception of additional heating
to maintain coronal temperature. The analyses indicate the following
features: (1) mode conversion efficiently excites high-frequency
transverse waves; (2) the typical period of the high-frequency waves
corresponds to the sound-crossing time of the mode conversion region;
and (3) simulated root-mean-square velocity of the high-frequency
component is consistent with the observed value. These results indicate
that the observation of spicule oscillation provides direct evidence
of mode conversion in the chromosphere.
Title: Effects of the enhanced subadiabatic layer in effectively
high-Prandtl number thermal convection
Authors: Bekki, Yuto; Hotta, Hideyuki; Yokoyama, Takaaki
Bibcode: 2017SPD....4840302B
Altcode:
It has been recently suggested, both from theoretical and observational
points of view, that the convective velocities achieved in global solar
convection simulations might be over-estimated (e.g., Hanasoge et
al. 2016). The effects of the prevailing small-scale magnetic field
generated by small-scale dynamo which cannot be fully resolved in
the current MHD simulations may contain promising solutions to this
problem (Hotta et al. 2015). The small-scale magnetic fields can
reduce the convective amplitude not only through the Lorentz-force
feedback but also via the increase in the effective Prandtl number,
as recently pointed out by O'Mara et al (2016). In this talk, we
propose and numerically confirm another suppression mechanism of the
convective velocities that can also be achieved in high-Prandtl number
thermal convection. This mechanism can be understood as follows. If
the effective horizontal thermal diffusivity decreases due to the
existence of small-scale magnetic fields, the subadiabatic layer
which is formed near the base by depositions of low entropy fluids of
adiabatically downflowing cold plumes is enhanced and extended. The
global convective amplitude in high-Prandtl thermal convection is thus
decreased via the change in the mean entropy profile, which is more
subadiabatic near the base and less superadiabatic in the bulk.
Title: MHD simulations of formation and eruption of a magnetic flux
rope in an active region with a delta-sunspot
Authors: Yokoyama, Takaaki; Oi, Yoshiaki; Toriumi, Shin
Bibcode: 2017SPD....4840002Y
Altcode:
Active regions holding a delta-sunspot are known to produce the largest
class of solar flares. How, where, and when such large flares occur
above a delta-sunspot are still under debate. For studying this,
3D MHD simulations of the emergence of a subsurface flux tube at two
locations in a simulation box modeling the convection zone to the corona
were conducted. We found that a flux rope is formed as a consequence
of magnetic reconnection of two bipolar loops and sunspot rotation
caused by the twist of the subsurface flux tube. Moreover, the flux
rope stops ascending when the initial background is not magnetized,
whereas it rises up to the upper boundary when a reconnection favorably
oriented pre-existing field is introduced to the initial background.
Title: Reconnection-Condensation Model for Solar Prominence Formation
Authors: Kaneko, Takafumi; Yokoyama, Takaaki
Bibcode: 2017ApJ...845...12K
Altcode: 2017arXiv170610008K
We propose a reconnection-condensation model in which topological
change in a coronal magnetic field via reconnection triggers radiative
condensation, thereby resulting in prominence formation. Previous
observational studies have suggested that reconnection at a
polarity inversion line of a coronal arcade field creates a flux
rope that can sustain a prominence; however, they did not explain the
origin of cool dense plasmas of prominences. Using three-dimensional
magnetohydrodynamic simulations, including anisotropic nonlinear thermal
conduction and optically thin radiative cooling, we demonstrate that
reconnection can lead not only to flux rope formation but also to
radiative condensation under a certain condition. In our model, this
condition is described by the Field length, which is defined as the
scale length for thermal balance between radiative cooling and thermal
conduction. This critical condition depends weakly on the artificial
background heating. The extreme ultraviolet emissions synthesized
with our simulation results have good agreement with observational
signatures reported in previous studies.
Title: Non-kinematic Flux-transport Dynamos Including the Effects
of Diffusivity Quenching
Authors: Ichimura, Chiaki; Yokoyama, Takaaki
Bibcode: 2017ApJ...839...18I
Altcode:
Turbulent magnetic diffusivity is quenched when strong magnetic
fields suppress turbulent motion in a phenomenon known as diffusivity
quenching. Diffusivity quenching can provide a mechanism for amplifying
magnetic field and influencing global velocity fields through
Lorentz force feedback. To investigate this effect, we conducted mean
field flux-transport dynamo simulations that included the effects of
diffusivity quenching in a non-kinematic regime. We found that toroidal
magnetic field strength is amplified by up to approximately 1.5 times
in the convection zone as a result of diffusivity quenching. This
amplification is much weaker than that in kinematic cases as a
result of Lorentz force feedback on the system’s differential
rotation. While amplified toroidal fields lead to the suppression of
equatorward meridional flow locally near the base of the convection
zone, large-scale equatorward transport of magnetic flux via meridional
flow, which is the essential process of the flux-transport dynamo,
is sustainable in our calculations.
Title: Observational signatures of transverse MHD waves and associated
dynamic instabilities
Authors: Antolin, Patrick; De Moortel, Ineke; Van Doorsselaere, Tom;
Yokoyama, Takaaki
Bibcode: 2017arXiv170200775A
Altcode:
MHD waves permeate the solar atmosphere and constitute potential
coronal heating agents. Yet, the waves detected so far may be but a
small subset of the true existing wave power. Detection is limited by
instrumental constraints, but also by wave processes that localise the
wave power in undetectable spatial scales. In this study we conduct 3D
MHD simulations and forward modelling of standing transverse MHD waves
in coronal loops with uniform and non-uniform temperature variation in
the perpendicular cross-section. The observed signatures are largely
dominated by the combination of the Kelvin-Helmholtz instability (KHI),
resonant absorption and phase mixing. In the presence of a cross-loop
temperature gradient we find that emission lines sensitive to the
loop core catch different signatures than those more sensitive to the
loop boundary and the surrounding corona, leading to an out-of-phase
intensity modulation produced by the KHI mixing. Common signatures to
all considered models include an intensity and loop width modulation
at half the kink period, fine strand-like structure, a characteristic
arrow-shaped structure in the Doppler maps, overall line broadening in
time but particularly at the loop edges. For our model, most of these
features can be captured with a spatial resolution of $0.33\arcsec$ and
spectral resolution of 25~km~s$^{-1}$, although severe over-estimation
of the line width is obtained. Resonant absorption leads to a
significant decrease of the observed kinetic energy from Doppler
motions over time, which is not recovered by a corresponding increase
in the line width from phase mixing and the KHI motions. We estimate
this hidden wave energy to be a factor of $5-10$ of the observed value.
Title: Three-dimensional MHD Magnetic Reconnection Simulations with
a Finite Guide Field: Proposal of the Shock-evoking Positive-feedback
Model
Authors: Wang, Shuoyang; Yokoyama, Takaaki; Isobe, Hiroaki
Bibcode: 2015ApJ...811...31W
Altcode: 2015arXiv150803140W
Using a three-dimensional (3D) magnetohydrodynamic model, we simulate
the magnetic reconnection in a single current sheet. We assume a
finite guide field, a random perturbation on the velocity field, and
uniform resistivity. Our model enhances the reconnection rate relative
to the classical Sweet-Parker model in the same configuration. The
efficiency of magnetic energy conversion is increased by interactions
between the multiple tearing layers coexisting in the global current
sheet. This interaction, which forms a positive-feedback system,
arises from coupling of the inflow and outflow regions in different
layers across the current sheet. The coupling accelerates the elementary
reconnection events, thereby enhancing the global reconnection rate. The
reconnection establishes flux tubes along each tearing layer. Slow-mode
shocks gradually form along the outer boundaries of these tubes, further
accelerating the magnetic energy conversion. Such a positive-feedback
system is absent in two-dimensional simulations, 3D reconnection without
a guide field, and reconnection under a single perturbation mode. We
refer to our model as the “shock-evoking positive-feedback” model.
Title: Resonant Absorption of Transverse Oscillations and Associated
Heating in a Solar Prominence. I. Observational Aspects
Authors: Okamoto, Takenori J.; Antolin, Patrick; De Pontieu, Bart;
Uitenbroek, Han; Van Doorsselaere, Tom; Yokoyama, Takaaki
Bibcode: 2015ApJ...809...71O
Altcode: 2015arXiv150608965O
Transverse magnetohydrodynamic waves have been shown to be ubiquitous
in the solar atmosphere and can, in principle, carry sufficient energy
to generate and maintain the Sun’s million-degree outer atmosphere
or corona. However, direct evidence of the dissipation process of these
waves and subsequent heating has not yet been directly observed. Here we
report on high spatial, temporal, and spectral resolution observations
of a solar prominence that show a compelling signature of so-called
resonant absorption, a long hypothesized mechanism to efficiently
convert and dissipate transverse wave energy into heat. Aside
from coherence in the transverse direction, our observations show
telltale phase differences around 180° between transverse motions
in the plane-of-sky and line-of-sight velocities of the oscillating
fine structures or threads, and also suggest significant heating from
chromospheric to higher temperatures. Comparison with advanced numerical
simulations support a scenario in which transverse oscillations trigger
a Kelvin-Helmholtz instability (KHI) at the boundaries of oscillating
threads via resonant absorption. This instability leads to numerous
thin current sheets in which wave energy is dissipated and plasma is
heated. Our results provide direct evidence for wave-related heating
in action, one of the candidate coronal heating mechanisms.
Title: Simulation Study of Solar Plasma Eruptions Caused by
Interactions between Emerging Flux and Coronal Arcade Fields
Authors: Kaneko, Takafumi; Yokoyama, Takaaki
Bibcode: 2014ApJ...796...44K
Altcode: 2014arXiv1410.0189K
We investigate the triggering mechanisms of plasma eruptions in the
solar atmosphere due to interactions between emerging flux and coronal
arcade fields by using two-dimensional MHD simulations. We perform
parameter surveys with respect to arcade field height, magnetic field
strength, and emerging flux location. Our results show that two possible
mechanisms exist, and which mechanism is dominant depends mostly
on emerging flux location. One mechanism appears when the location
of emerging flux is close to the polarity inversion line (PIL) of an
arcade field. This mechanism requires reconnection between the emerging
flux and the arcade field, as pointed out by previous studies. The other
mechanism appears when the location of emerging flux is around the edge
of an arcade field. This mechanism does not require reconnection between
the emerging flux and the arcade field but does demand reconnection in
the arcade field above the PIL. Furthermore, we found that the eruptive
condition for this mechanism can be represented by a simple formula.
Title: Statistical Analysis of the Horizontal Divergent Flow in
Emerging Solar Active Regions
Authors: Toriumi, Shin; Hayashi, Keiji; Yokoyama, Takaaki
Bibcode: 2014ApJ...794...19T
Altcode: 2014arXiv1408.2383T
Solar active regions (ARs) are thought to be formed by magnetic fields
from the convection zone. Our flux emergence simulations revealed that
a strong horizontal divergent flow (HDF) of unmagnetized plasma appears
at the photosphere before the flux begins to emerge. In our earlier
study, we analyzed HMI data for a single AR and confirmed presence of
this precursor plasma flow in the actual Sun. In this paper, as an
extension of our earlier study, we conducted a statistical analysis
of the HDFs to further investigate their characteristics and better
determine the properties. From SDO/HMI data, we picked up 23 flux
emergence events over a period of 14 months, the total flux of which
ranges from 1020 to 1022 Mx. Out of 23 selected
events, 6 clear HDFs were detected by the method we developed in our
earlier study, and 7 HDFs detected by visual inspection were added
to this statistic analysis. We found that the duration of the HDF is
on average 61 minutes and the maximum HDF speed is on average 3.1 km
s-1. We also estimated the rising speed of the subsurface
magnetic flux to be 0.6-1.4 km s-1. These values are
highly consistent with our previous one-event analysis as well as our
simulation results. The observation results lead us to the conclusion
that the HDF is a rather common feature in the earliest phase of AR
emergence. Moreover, our HDF analysis has the capability of determining
the subsurface properties of emerging fields that cannot be directly
measured.
Title: Magnetothermal instability in the solar corona
Authors: Yokoyama, Takaaki
Bibcode: 2014cosp...40E3724Y
Altcode:
By in situ measurements of spacecrafts, it is known that there
exists a turbulence in the solar wind plasmas. It is composed of a
superposition of Alfven waves whose source is believed to be the solar
lower atmosphere. The energy spectrum of the turbulence ranges froma low
frequency around 0.01 mHz. In order to explain the generation of such
low frequency waves, we discussed an application of the magnetothermal
instability (MTI) to the solar atmosphere. This instability proposed by
Balbus (2000) occurs in weakly collisionless plasmas where non-isotropic
thermal conduction plays a role in a magnetized atmosphere. The time
scale of the maximum growth is given as approximately sqrt(H/g)
where H is the scale height, and g is the gravity. The magnetic
field must be weak enough since its tension force contributes as a
restoring force. The solar corona is a dilute hot atmosphere where the
thermal conduction is non-isotropic. The MTI is possible to work in the
upper corona around a few solar radii above the photosphere where the
temperature is decreasing outward and the scale height is about one
solar radius. The condition for weak horizontal magnetic field might
be satisfied above a closed loop in the lower corona. If the MTI is
effective in such regions, it might contribute to generate the waves
or perturbations in the solar wind. We found that the MTI is unlikely
to work in the upper corona because of its strong magnetic field that
suppress the growth of the geometrically possible wavelength modes. It
is found that when the field strength is 0.1 times the real corona, the
wavelength for the maximum growth is comparable with the geometrical
radius. The growth time for this setup can be consistent with the low
frequency fluctuations in the solar wind.
Title: Fine strand-like structure in the corona from MHD transverse
oscillations
Authors: Antolin, Patrick; Yokoyama, Takaaki; Van Doorsselaere, Tom
Bibcode: 2014cosp...40E.104A
Altcode:
Current analytical and numerical modelling suggest the existence
of ubiquitous thin current sheets in the corona that could explain
the observed line broadening and heating requirements. On the other
hand, new high resolution observations of the corona indicate that
its magnetic field may tend to organise itself in fine strand-like
structures of a few hundred kilometres widths. The link between small
structure in models and the observed widths of strand-like structure
several orders of magnitude larger is still not clear. A popular
theoretical scenario is the nanoflare model, in which each strand
is the product of an ensemble of heating events. Here, we suggest an
alternative mechanism for strand generation. Through forward modelling
of 3D MHD simulations we show that if a loop has initially a monolithic
structure, even a small amplitude transverse MHD wave can lead in a
few periods time to strand-like structure in EUV intensity images. Our
model is based on previous numerical work showing that transverse MHD
oscillations can lead to Kelvin-Helmholtz instabilities that deform the
cross-sectional area of loops. While previous work has focused on large
amplitude oscillations, here we show that the instability can occur
even for low wave amplitudes, matching those presently observed in the
corona. Through forward modelling we show that the roll-ups generated
from the instability are velocity sheared regions with enhanced
emissivity and line broadening hosting current sheets. Strand-like
structure results as a complex combination of the roll-ups and the
line-of-sight angle, can last over relatively long timescales and can
be observed for spatial resolutions discerning a tenth of a loop radius.
Title: Imaging, spectroscopic and stereoscopic observations of the
bi-directional reconnection inflow
Authors: Matsui, Yuki; Yokoyama, Takaaki
Bibcode: 2014cosp...40E2037M
Altcode:
The standard model of solar flares based on the magnetic
reconnection includes bi-directional inflow toward the reconnection
point. Corresponding to the bi-directional inflow, high temperature
loops like a cusp shape are formed due to the magnetic reconnection. By
combination of imaging, spectroscopic and stereoscopic observations,
we succeeded in capture the three-dimensional structure of a
bi-directional reconnection inflow of a solar flare. We analyzed a
C-class flare that occurred on 2012 September 11 beyond the solar
limb. The bi-directional inflow was found in the images of coronal
temperature filter taken by AIA onboard SDO. Hinode EUV Imaging
Spectrometer (EIS) also observed this flare and provide the Doppler
velocity of the bi-directional inflows. At the same time, cusp loops
were observed with the raster scans of FeXXIV emission line (over 10
MK) at the region surrounded by the bi-directional inflow. This is
clear evidence that 1MK loops are heated over 10MK by the magnetic
reconnection. STEREO A/SECCHI was observing this flow from a different
line of site. Inflowing angle in STEREO A/SECCHI images is consistent
with the angle speculated by apparent velocity of SDO/AIA and line
of sight velocity of Hinode/EIS. By combining these data sets, we
constructed a self-consistent three-dimensional picture of the flows.
Title: Probing the Shallow Convection Zone: Rising Motion of
Subsurface Magnetic Fields in the Solar Active Region
Authors: Toriumi, Shin; Ilonidis, Stathis; Sekii, Takashi; Yokoyama,
Takaaki
Bibcode: 2013ApJ...770L..11T
Altcode: 2013arXiv1305.3023T
In this Letter, we present a seismological detection of a rising motion
of magnetic flux in the shallow convection zone of the Sun, and show
estimates of the emerging speed and its decelerating nature. In order to
evaluate the speed of subsurface flux that creates an active region, we
apply six Fourier filters to the Doppler data of NOAA AR 10488, observed
with the Solar and Heliospheric Observatory/Michelson Doppler Imager,
to detect the reduction of acoustic power at six different depths from
-15 to -2 Mm. All the filtered acoustic powers show reductions, up to 2
hr before the magnetic flux first appears at the visible surface. The
start times of these reductions show a rising trend with a gradual
deceleration. The obtained velocity is first several km s-1
in a depth range of 15-10 Mm, then ~1.5 km s-1 at 10-5 Mm,
and finally ~0.5 km s-1 at 5-2 Mm. If we assume that the
power reduction is actually caused by the magnetic field, the velocity
of the order of 1 km s-1 is well in accordance with previous
observations and numerical studies. Moreover, the gradual deceleration
strongly supports the theoretical model that the emerging flux slows
down in the uppermost convection zone before it expands into the
atmosphere to build an active region.
Title: Temporal and Spatial Analyses of Spectral Indices of Nonthermal
Emissions Derived from Hard X-Rays and Microwaves
Authors: Asai, Ayumi; Kiyohara, Junko; Takasaki, Hiroyuki; Narukage,
Noriyuki; Yokoyama, Takaaki; Masuda, Satoshi; Shimojo, Masumi;
Nakajima, Hiroshi
Bibcode: 2013ApJ...763...87A
Altcode: 2012arXiv1212.1806A
We studied electron spectral indices of nonthermal emissions seen in
hard X-rays (HXRs) and microwaves. We analyzed 12 flares observed by
the Hard X-Ray Telescope aboard Yohkoh, Nobeyama Radio Polarimeters,
and the Nobeyama Radioheliograph (NoRH), and compared the spectral
indices derived from total fluxes of HXRs and microwaves. Except
for four events, which have very soft HXR spectra suffering from the
thermal component, these flares show a gap Δδ between the electron
spectral indices derived from HXRs δ X and those from
microwaves δμ (Δδ = δ X - δμ)
of about 1.6. Furthermore, from the start to the peak times of the
HXR bursts, the time profiles of the HXR spectral index δ
X evolve synchronously with those of the microwave spectral
index δμ, keeping the constant gap. We also examined the
spatially resolved distribution of the microwave spectral index by
using NoRH data. The microwave spectral index δμ tends
to be larger, which means a softer spectrum, at HXR footpoint sources
with stronger magnetic field than that at the loop tops. These results
suggest that the electron spectra are bent at around several hundreds
of keV, and become harder at the higher energy range that contributes
the microwave gyrosynchrotron emission.
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: 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: Three-dimensional MHD Simulations Of Magnetic Reconnection
With Finite Fluctuations
Authors: Yokoyama, Takaaki; Isobe, H.
Bibcode: 2010AAS...21640802Y
Altcode: 2010BAAS...41R.815Y
The magnetic reconnection is one of the fundamental processes for
the heating, bulk flow acceleration, and magnetic topology change
in the solar/stellar atmospheres and other astrophysical energetic
phenomena. For the quantitative understanding of these phenomena,
it is crucially important to determine the energy release rate or,
equivalently, the reconnection rate from numerical studies. Owing
to the enormously large magnetic Reynolds number, it is expected
that the MHD turbulence or some stochastic process may play a role
for the magnetic diffusion in the reconnection region. By performing
three-dimensional MHD simulations, we are working on this issue. The
temporal evolution of a simple current sheet with initially imposed
fluctuations in the resistivity is studied. A substantial increase of
energy release ratewas found by adding the guide field, i.e. the field
parallelto the electric current. It is interpreted that this is due
to the mutual interactions of magnetic islands formed in a spatially
separated parallel resonant layers.
Title: Magnetic reconnection with finite fluctuations
Authors: Yokoyama, Takaaki; Isobe, Hiroaki
Bibcode: 2010cosp...38.1935Y
Altcode: 2010cosp.meet.1935Y
The magnetic reconnection is one of the fundamental processes for
the heating, bulk flow acceleration, and magnetic topology change in
the solar atmosphere. For the quantitative understanding of these
phenomena, it is crucially important to determine the reconnection
rate from theoretical/numerical studies. Owing to the enormously large
magnetic Reynolds number, it is expected that the MHD turbulence or
some stochastic process may play a role for the magnetic diffusion
in the reconnection region. In this talk, we would like to review the
studies related on this idea and show our recent works. In our study,
by performing three-dimensional MHD simulations, the temporal evolution
of a simple current sheet with initially imposed fluctuations in the
resistivity is investigated. Although the enhancement is limited only
by a few percent beyond the Sweet-Parker rate in cases with initially
anti-parallel fields, a substantial increase of energy release rate
was found by adding the guide field, i.e. the field parallelto the
electric current. It is interpreted that this is due to the mutual
interactions of magnetic islands formed in a spatially separated
parallel resonant layers.
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: Imaging Spectroscopy on Preflare Coronal Nonthermal Sources
Associated with the 2002 July 23 Flare
Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; Yokoyama,
Takaaki; Masuda, Satoshi; Krucker, Säm
Bibcode: 2009ApJ...695.1623A
Altcode: 2009arXiv0901.3591A
We present a detailed examination on the coronal nonthermal emissions
during the preflare phase of the X4.8 flare that occurred on 2002 July
23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama
Radioheliograph, at Nobeyama Solar Radio Observatory and the hard
X-ray (HXR) data taken with RHESSI obviously showed nonthermal sources
that are located above the flare loops during the preflare phase. We
performed imaging spectroscopic analyses on the nonthermal emission
sources both in microwaves and in HXRs, and confirmed that electrons
are accelerated from several tens of keV to more than 1 MeV even in this
phase. If we assume the thin-target model for the HXR emission source,
the derived electron spectral indices (~4.7) is the same value as that
from microwaves (~4.7) within the observational uncertainties, which
implies that the distribution of the accelerated electrons follows a
single power law. The number density of the microwave-emitting electrons
is, however, larger than that of the HXR-emitting electrons, unless
we assume low-ambient plasma density of about 1.0 × 109
cm-3 for the HXR-emitting region. If we adopt the
thick-target model for the HXR emission source, on the other hand,
the electron spectral index (~6.7) is much different, while the gap
of the number density of the accelerated electrons is somewhat reduced.
Title: Formation of Solar Magnetic Flux Tubes with Kilogauss Field
Strength Induced by Convective Instability
Authors: Nagata, Shin'ichi; Tsuneta, Saku; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Katsukawa, Yukio; Shimizu, Toshifumi; Yokoyama,
Takaaki; Tarbell, Theodore D.; Lites, Bruce W.; Shine, Richard A.;
Berger, Thomas E.; Title, Alan M.; Bellot Rubio, Luis R.; Orozco
Suárez, David
Bibcode: 2008ApJ...677L.145N
Altcode:
Convective instability has been a mechanism used to explain
the formation of solar photospheric flux tubes with kG field
strength. However, the turbulence of the Earth's atmosphere has
prevented ground-based observers from examining the hypothesis
with precise polarimetric measurement on the subarcsecond scale
flux tubes. Here we discuss observational evidence of this scenario
based on observations with the Solar Optical Telescope (SOT) aboard
Hinode. The cooling of an equipartition field strength flux tube
precedes a transient downflow reaching 6 km s-1 and the
intensification of the field strength to 2 kG. These observations
agree very well with the theoretical predictions.
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: Specific Cutting Resistance of Lunar Regolith Simulant under
Low Gravity Conditions
Authors: Nakashima, Hiroshi; Shioji, Yasuyuki; Tateyama, Kazuyoshi;
Aoki, Shigeru; Kanamori, Hiroshi; Yokoyama, Takaaki
Bibcode: 2008JSpEn...1...58N
Altcode:
Specific cutting resistance was determined through airplane experiments
under low relative gravity conditions such as μ G, 0.15 G, 0.3 G,
0.5 G, and 1 G. Results showed that the relationship between specific
cutting resistance and relative gravity could be expressed as a linear
function. As for numerical analysis by discrete element method (DEM),
the data of spring constant in a contact model of DEM could be treated
as constant in the analysis of specific cutting resistance under low
gravity conditions from the viewpoint of stress-oriented soil-machine
interaction. Moreover, the numerical analysis by DEM with change of
relative gravity and the corresponding modification of consolidation
time is found to be sufficient to obtain a specific cutting resistance
at a given low gravity condition below 1 G.
Title: Comparative Study of Non-Thermal Emissions and Electron
Transport in a Solar Flare
Authors: Minoshima, Takashi; Yokoyama, Takaaki; Masuda, Satoshi
Bibcode: 2008cosp...37.2050M
Altcode: 2008cosp.meet.2050M
It is well known that a large amount of non-thermal electrons are
produced in a solar flare. To understand their acceleration and
transport mechanisms, hard X-ray (HXR) and microwave observations are
the most powerful means. HXRs are emitted primarily by electrons with
energy below several hundred keV via bremsstrahlung (Brown 1971),
while microwaves are by electrons with energy above several hundred
keV via gyrosynchrotron radiation (e.g., Ramaty 1969). Therefore
these two sources of emissions provide information on electrons in
two different energy ranges. A comparative study by using both HXR
and microwave observations is useful for understanding the physics of
electrons over a wide range of energies. We observed a solar flare
occurred on 2003 May 29 with HXRs taken by the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI), and microwaves by the
Nobeyama Radio Polarimeters (NoRP) and the Nobeyama Radioheliograph
(NoRH). In particular, we focus on characteristics of higher energy
(>100 keV) HXRs. They are emitted from both footpoints of the flare
loop in the same manner as the lower energy (<100 keV) HXRs, while
microwaves are emitted primarily at the top of the loop. On the other
hand, we found that the time profile of the spectral index of the higher
energy HXRs is more similar to that of the microwaves than to that of
the lower energy HXRs. To understand the observed characteristics in
terms of an energy-dependent transport effect of electrons, we develop
a more general treatment of trap-plus-precipitation (TPP; Melrose and
Brown, 1976) by using the gyro-averaged Fokker-Planck equation. We model
the time evolution of the electron phase space distribution under the
influence of Coulomb collisions and magnetic mirror, and then calculate
the resulting HXR and microwave emissions for comparison with the
observation. It is found that the TPP model in the weak diffusion regime
well explains the observed characteristics. Further, we conclude from
both the observation and the modelling that the observed time profile
of the spectral index of the higher energy HXRs can be explained if
the pitch-angle distribution of the parent electrons is concentrated
perpendicular to the magnetic field line when they are injected into
the loop. This indicates that the non-thermal electrons are accelerated
more perpendicular to than parallel to the magnetic field line.
Title: Temporal evolution of a Current Sheet with Initial Finite
Perturbations by Three-dimensional MHD Simulations
Authors: Yokoyama, Takaaki
Bibcode: 2008cosp...37.3554Y
Altcode: 2008cosp.meet.3554Y
Temporal evolution of a current sheet with initial perturbations is
studied by using the threedimensional resistive magnetohydrodynamic
(MHD) simulations. The magnetic reconnection is considered to be the
main engine of the energy rele ase in solar flares. The structure
of the diffusion region is, however, not stil l understood under the
circumstances with enormously large magnetic Reynolds num ber as the
solar corona. In particular, the relationship between the flare's
macroscopic physics and the microscopic ones are unclear. It is
generally believed that the MHD turbulence s hould play a role in the
intermediate scale. The initial current sheet is in an approximately
hydromagnetic equilibrium with anti-parallel magnetic field in the
y-direction. We imposed a finite-amplitude perturbations (=50ee what
happens. Special attention is paid upon the evolution of a three-dimens
ional structure in the direction along the initial electric current
(z-direction ). Our preliminary results are as follows: (1) In the
early phase of the evolut ion, high wavenumber modes in the z-direction
are excited and grow. (2) Many "X "-type neutral points (lines) are
generated along the magnetic neutral line (pla ne) in the current
sheet. When they evolve into the non-linear phase, three-dime nsional
structures in the z-direction also evolve. The spatial scale in the z-di
rection seems to be almost comparable with that in the xy-plane. (3)
The energy release rate is reduced in case of 3D simulations compared
with 2D ones probably because of the reduction of the inflow cross
sections by the formation of pattc hy structures in the current sheet.
Title: Initial Results on Line-of-Sight Field Calibrations of SP/NFI
Data Taken by SOT/Hinode
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Shimizu, Toshifumi; Shine, Richard A.; Tarbell,
Theodore D.; Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata,
Shin'ichi; Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.619C
Altcode:
We present initial results on the line-of-sight field calibration
of the two kinds of Stokes I and V data taken by the Solar Optical
Telescope on the satellite Hinode: spectral profiles of Stokes I and V
parameters recorded on the Spectro-polarimeter (SP), and monochromatic
images of the same parameters recorded on the Narrow-band Filter Imager
(NFI). By applying the center-of-gravity method to the SP data of
AR10930 taken on 2006 December 11, we determined the line-of-sight field
at every location in the active region. As a result, we found that the
line-of-sight field strength ranges up to 2kG in plages, even without
taking into account the filling factor, and up to 3.5kG or higher values
inside the umbra of the major sunspot. We calibrated the NFI data in
reference to the field determined from the SP data. In regions outside
the sunspots and the penumbral regions, we adopted a linear relation,
B|| = βV / I, between the circular polarization, V / I,
and the line-of-sight field strength, B||, and obtained β =
23.5kG in regions outside the sunspots, and β = 12.0kG in penumbral
regions. In umbral regions of sunspots, a first-order polynomial was
adopted to model the reversal of the polarization signal over the
field strength.
Title: Hinode Observations of a Vector Magnetic Field Change
Associated with a Flare on 2006 December 13
Authors: Kubo, Masahito; Yokoyama, Takaaki; Katsukawa, Yukio; Lites,
Bruce; Tsuneta, Saku; Suematsu, Yoshinori; Ichimoto, Kiyoshi; Shimizu,
Toshifumi; Nagata, Shin'ichi; Tarbell, Theodore D.; Shine, Richard A.;
Title, Alan M.; Elmore David
Bibcode: 2007PASJ...59S.779K
Altcode: 2007arXiv0709.2397K
Continuous observations of the flare productive active region 10930
were successfully carried out with the Solar Optical Telescope aboard
the Hinode spacecraft during 2006 December 6 to 19. We focused on the
evolution of photospheric magnetic fields in this active region, and the
magnetic field properties at the site of the X3.4 class flare, using
a time series of vector field maps with high spatial resolution. The
X3.4 class flare occurred on 2006 December 13 at the apparent
collision site between the large, opposite polarity umbrae. Elongated
magnetic structures with alternatingly positive and negative polarities
resulting from flux emergence appeared one day before the flare in the
collision site penumbra. Subsequently, the polarity inversion line
at the collision site became very complicated. The number of bright
loops in CaII H increased during the formation of these elongated
magnetic structures. Flare ribbons and bright loops evolved along
the polarity inversion line and one footpoint of the bright loop was
located in a region having a large departure of the field azimuth angle
with respect to its surroundings. SOT observations with high spatial
resolution and high polarization precision revealed temporal change in
the fine structure of magnetic fields at the flare site: some parts of
the complicated polarity inversion line then disappeared, and in those
regions the azimuth angle of the photospheric magnetic field changed by
about 90°, becoming more spatially uniform within the collision site.
Title: Fine-Scale Structures of the Evershed Effect Observed by the
Solar Optical Telescope aboard Hinode
Authors: Ichimoto, Kiyoshi; Shine, Richard A.; Lites, Bruce; Kubo,
Masahito; Shimizu, Toshifumi; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Tarbell, Theodore D.; Title, Alan M.; Nagata,
Shin'ichi; Yokoyama, Takaaki; Shimojo, Masumi
Bibcode: 2007PASJ...59S.593I
Altcode:
The small-scale structure of the Evershed effect is being studied
using data obtained by the Spectropolarimeter and the Broadband Filter
Imager of the Solar Optical Telescope aboard Hinode. We find that the
Evershed flow starts at the leading edge of inwardly migrating bright
penumbral grains, and turns to nearly a horizontal flow preferentially
in the dark lanes of the penumbra. A number of small elongated regions
that have an upward motion of ∼ 1kms-1 are found in the
deep photosphere distributed over the penumbra. They are cospatial
with bright grains and have relatively horizontal magnetic fields. A
number of patches having a strong downward motion associated with the
opposite magnetic polarity from the sunspot are also found in the mid
and outer penumbra. They could be identified as foot points of the
Evershed flow channels, though the identification of individual pairs
is not straightforward. Our results provide strong support for some
recent findings from ground-based high-resolution observations, and
are in general agreement with the well-known picture of the uncombed
structure of the penumbra, in which the penumbrae consist of rising
flux tubes carrying nearly horizontal Evershed flows embedded in more
vertical background magnetic fields.
Title: Hinode SP Vector Magnetogram of AR10930 and Its
Cross-Comparison with MDI
Authors: Moon, Yong-Jae; Kim, Yeon-Han; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Chae, Jongchul; Cho, Kyung Suk; Bong,
Suchan; Suematsu, Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Shimojo,
Masumi; Shimizu, Toshifumi; Shine, Richard A.; Tarbell, Theodore D.;
Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata, Shin'ichi;
Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.625M
Altcode:
We present one Hinode Spectropolarimeter (SP) magnetogram of AR 10930
that produced several major flares. The inversion from Stokes profiles
to magnetic field vectors was made using the standard Milne-Eddington
code. We successfully applied the Uniform Shear Method for resolving
the 180° ambiguity to the magnetogram. The inversion gave very strong
magnetic field strengths (near 4500 gauss) for a small portion of area
in the umbra. Considering that the observed V-profile of 6301.5Å was
well-fitted as well as a direct estimation of the Zeeman splitting
results in 4300-4600 gauss, we think that the field strengths
should not be far from the actual value. A cross-comparison of the
Hinode SP and SOHO MDI high resolution flux densities shows that the
MDI flux density could be significantly underestimated by about a
factor of two. In addition, it has a serious negative correlation
(the so-called Zeeman saturation effect) with the Hinode SP flux
density for umbral regions. Finally, we could successfully obtain
a recalibrated MDI magnetogram that has been corrected for the
Zeeman saturation effect using not only a pair of MDI intensity and
magnetogram data simultaneously observed, but also the relationship
from the cross-comparison between the Hinode SP and MDI flux densities.
Title: Formation Process of a Light Bridge Revealed with the Hinode
Solar Optical Telescope
Authors: Katsukawa, Yukio; Yokoyama, Takaaki; Berger, Thomas E.;
Ichimoto, Kiyoshi; Kubo, Masahito; Lites, Bruce; Nagata, Shin'ichi;
Shimizu, Toshifumi; Shine, Richard A.; Suematsu, Yoshinori; Tarbell,
Theodore D.; Title, Alan M.; Tsuneta, Saku
Bibcode: 2007PASJ...59S.577K
Altcode: 2007arXiv0709.2527K
The Solar Optical Telescope (SOT) on-board Hinode successfully and
continuously observed the formation process of a light bridge in a
matured sunspot of the NOAA active region 10923 for several days with
high spatial resolution. During its formation, many umbral dots were
observed to be emerging from the leading edges of penumbral filaments,
and rapidly intruding into the umbra. The precursor of the light bridge
formation was also identified as a relatively slow inward motion of
the umbral dots, which emerged not near the penumbra, but inside the
umbra. The spectro-polarimeter on SOT provided physical conditions in
the photosphere around the umbral dots and the light bridges. We found
that the light bridges and the umbral dots had significantly weaker
magnetic fields associated with upflows relative to the core of the
umbra, which implies that there was hot gas with weak field strength
penetrating from the subphotosphere to near the visible surface inside
those structures. There needs to be a mechanism to drive the inward
motion of the hot gas along the light bridges. We suggest that the
emergence and the inward motion are triggered by a buoyant penumbral
flux tube as well as subphotospheric flow crossing the sunspot.
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: Imaging Spectroscopy of a Gradual Hardening Flare on 2000
November 25
Authors: Takasaki, Hiroyuki; Kiyohara, Junko; Asai, Ayumi; Nakajima,
Hiroshi; Yokoyama, Takaaki; Masuda, Satoshi; Sato, Jun; Kosugi, Takeo
Bibcode: 2007ApJ...661.1234T
Altcode:
We present an examination of multiwavelength observations of an M8.2
long-duration flare which occurred on 2000 November 25. During the
flare, we can see a hard X-ray (HXR) source on one Hα flare ribbon in
the HXR images obtained with the Hard X-ray Telescope aboard Yohkoh,
and a compact microwave emission source on the other flare ribbon
in the data taken with the Nobeyama Radioheliograph, while we can
also see an extended microwave emission source that connects both of
these emission sources. The compact microwave and HXR sources clearly
showed gradual hardening tendencies in their spectra. In addition,
we found energy-dependent delays of the peak times in the HXR bursts
and concluded that almost all of the accelerated electrons are trapped
in magnetic loops to generate the extended microwave source and are
dripping into the chromosphere at the compact microwave and the HXR
emission sites. We then performed imaging spectroscopic analyses to
the microwave emission sources. The temporal evolutions of the flux
and the spectral index of the compact microwave footpoint source are
quite similar to those of the HXR source, which is mainly emitted at
the other footpoint, while those at the loop-top extended source do
not show this similarity. Moreover, there is a constant gap between
the electron spectral index derived from the microwave footpoint source
and that from the HXR source. We also discuss the constant gap, based
on the trapped and dripping model.
Title: The Origin of Ripples in Cool Cores of Galaxy Clusters:
Heating by Magnetohydrodynamic Waves?
Authors: Fujita, Yutaka; Suzuki, Takeru K.; Kudoh, Takahiro; Yokoyama,
Takaaki
Bibcode: 2007ApJ...659L...1F
Altcode: 2007astro.ph..3053F
We consider MHD waves as a heating source of cool cores of galaxy
clusters. In particular, we focus on transverse waves (Alfvén waves),
because they can propagate a longer distance than longitudinal waves
(sound waves). Using MHD simulations, we found that the transverse
waves can stably heat a cool core if the wave period is large enough
(>~108 yr). Moreover, the longitudinal waves that are
created as a by-product of the nonlinear evolution of the transverse
waves could be observed as the ``ripples'' found in cool cores.
Title: Estimate of Impact Force at Landing on Lunar Surface by
SPH Method
Authors: Yokoyama, Takaaki; Higuchi, Ken
Bibcode: 2007SpT.....6....9Y
Altcode:
To estimate accurately the impact force at landing on the moon,
an experimental study considering the influences of Regolith and of
lunar environment is required. By a numerical model which represents
the phenomena by semi-empirical formula with experimental results,
we can predict the impact force. In this paper, we introduce the
experimental study to estimate the force and the numerical results with
the semi-empirical formula by the Apollo ground model. Furthermore,
we describe the way to predict the force by the computational analysis
with smoothed particle hydrodynamics (SPH) method, and the SPH method
showed good agreement with the numerical and experimental results.
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: Statistical Study of the Reconnection Rate in Solar Flares
Observed with Yohkoh SXT
Authors: Nagashima, Kaori; Yokoyama, Takaaki
Bibcode: 2006ApJ...647..654N
Altcode: 2006astro.ph..5712N
We report a statistical study of flares observed with the Soft X-Ray
Telescope (SXT) on board Yohkoh in the year 2000. We measure physical
parameters of 77 flares, such as the temporal scale, size, and magnetic
flux density, and find that the sizes of flares tend to be distributed
more broadly as the GOES class becomes weaker and that there is a lower
limit of magnetic flux density that depends on the GOES class. We
also examine the relationships among these parameters and find weak
correlation between the temporal and spatial scales of the flares. We
estimate reconnection inflow velocity, coronal Alfvén velocity, and
reconnection rate using the observed values. The inflow velocities
are distributed from a few km s-1 to several tens of km
s-1, and the Alfvén velocities in the corona are in the
range from 103 to 104 km s-1. Hence,
the reconnection rate is 10-3 to 10-2. We find
that the reconnection rate in a flare tends to decrease as the GOES
class of the flare increases. This value is within 1 order of magnitude
of the theoretical maximum value predicted by the Petschek model,
although the dependence of the reconnection rate on the magnetic
Reynolds number tends to be stronger than that in the Petschek model.
Title: Two-dimensional Magnetohydrodynamic Simulations of Relativistic
Magnetic Reconnection
Authors: Watanabe, Naoyuki; Yokoyama, Takaaki
Bibcode: 2006ApJ...647L.123W
Altcode: 2006astro.ph..7285W
It has been recognized that the magnetic reconnection process is
of great importance in high-energy astrophysics. We develop a
new two-dimensional relativistic resistive magnetohydrodynamic
(R2MHD) code and carry out numerical simulations of
magnetic reconnection. We find that the outflow velocity reaches
the Alfvén velocity in the inflow region and that a higher Alfvén
velocity provides a higher reconnection rate. We also find that
Lorentz contraction plays an important role in enhancement of the
reconnection rate.
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: One Solar-Cycle Observations of Prominence Activities Using
the Nobeyama Radioheliograph 1992-2004
Authors: Shimojo, Masumi; Yokoyama, Takaaki; Asai, Ayumi; Nakajima,
Hiroshi; Shibasaki, Kiyoto
Bibcode: 2006PASJ...58...85S
Altcode:
We newly developed a method of limb-event detection for the Nobeyama
Radiograph, and show the results over one solar-cycle, 1992 July-2004
December. We detected 785 prominence activities and 31 flares on the
limb by this method. We investigated the relationship between the
distributions of the prominence activities and the solar cycle. As a
result, we found the following facts: 1) The variation in the number of
prominence activities is similar to that of sunspots during one solar
cycle. 2) There are differences between the peak times of prominence
activities and sunspots. 3) The frequency distribution as a function
of the magnitude of the prominence activities (the size of activated
prominences) at each phase shows a power-law distribution. The power-law
index of the distribution does not change, except around the solar
minimum. 4) The number of prominence activities has a dependence on the
latitude. On the other hand, the average magnitude is independent of the
latitude. 5) During the rise phase of the solar cycle, the location of
the high-latitude prominence activities migrates to the pole region. 6)
After a solar polarity reversal, the location of the prominence
activities in the northern hemisphere migrates to the equator. On
the other hand, the prominence activities in the southern hemisphere
occurred in the high-latitude region until the decay phase of Cycle 23.
Title: Magnetic field variations in the Jovian magnetotail induced
by solar wind dynamic pressure enhancements
Authors: Tao, Chihiro; Kataoka, Ryuho; Fukunishi, Hiroshi; Takahashi,
Yukihiro; Yokoyama, Takaaki
Bibcode: 2005JGRA..11011208T
Altcode:
In order to understand the response of the Jovian magnetosphere to
solar wind dynamic pressure enhancements, we investigate magnetic
field variations observed by the Galileo spacecraft. The lack of
solar wind monitoring just upstream of the Jovian magnetosphere is
overcome by simulating a one-dimensional magnetohydrodynamic (MHD)
propagation of the solar wind from the Earth. We identify the events
with an increase of the solar wind dynamic pressure >0.25 nPa at the
Jovian orbit. Characteristic magnetic field variations are found in
the Jovian magnetosphere for all of the nine events. The rectangular
waveform due to the Jovian rotation disappears for eight of the nine
events. Magnetic field disturbances in the frequency range from 0.3
to 10 mHz are enhanced simultaneously. The maximum amplitude of the
disturbances is in proportional to the maximum amplitude of the solar
wind dynamic pressure. We suggest that the current sheet is greatly
deformed and reconnection bursts are induced under the compressed
magnetosphere.
Title: Repeated injections of energy in the first 600ms of the giant
flare of SGR1806 - 20
Authors: Terasawa, Toshio; Tanaka, Yasuyuki T.; Takei, Yasuhiro;
Kawai, Nobuyuki; Yoshida, Atsumasa; Nomoto, Ken'ichi; Yoshikawa,
Ichiro; Saito, Yoshifumi; Kasaba, Yasumasa; Takashima, Takeshi; Mukai,
Toshifumi; Noda, Hirotomo; Murakami, Toshio; Watanabe, Kyoko; Muraki,
Yasushi; Yokoyama, Takaaki; Hoshino, Masahiro
Bibcode: 2005Natur.434.1110T
Altcode: 2005astro.ph..2315T
The massive flare of 27 December 2004 from the soft γ-ray repeater
SGR1806-20, a possible magnetar, saturated almost all γ-ray
detectors, meaning that the profile of the pulse was poorly
characterized. An accurate profile is essential to determine
physically what was happening at the source. Here we report the
unsaturated γ-ray profile for the first 600ms of the flare, with
a time resolution of 5.48ms. The peak of the profile (of the order
of 107photonscm-2s-1) was reached
~50ms after the onset of the flare, and was then followed by a gradual
decrease with superposed oscillatory modulations possibly representing
repeated energy injections with ~60-ms intervals. The implied total
energy is comparable to the stored magnetic energy in a magnetar (~
1047erg) based on the dipole magnetic field intensity (~
1015G), suggesting either that the energy release mechanism
was extremely efficient or that the interior magnetic field is much
stronger than the external dipole field.
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: 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: 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: Magnetohydrodynamic Simulation of Solar Coronal Chromospheric
Evaporation Jets Caused by Magnetic Reconnection Associated with
Magnetic Flux Emergence
Authors: Miyagoshi, Takehiro; Yokoyama, Takaaki
Bibcode: 2004ApJ...614.1042M
Altcode:
We studied solar coronal X-ray jets by MHD numerical simulations with
heat conduction effects based on a magnetic reconnection model. Key
physical processes are included, such as the emergence of magnetic
flux from the convection zone, magnetic reconnection with the
coronal magnetic fields, heat conduction to the chromosphere,
and chromospheric evaporation. Radiation, however, has been
neglected. High-density evaporation jets were successfully reproduced
in the simulations. The mass of the evaporation jets M is described as
M=6.8×1012g(B/10G)15/7(Tcor/106K)5/14(L/5000km)12/7(t/400s),
where B is the strength of magnetic fields, Tcor is the
coronal temperature, L is the loop height, and t is the duration of
ejection, respectively. We also derived a theoretical model of the
Mach number of the reconnection jets as a function of ambient coronal
variables. Numerical simulations also show that two different types
of jets (evaporation jets and low-density jets) exist simultaneously
around the emerging flux region, and the energy of evaporation jets
is somewhat larger than that of the low-density jets.
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: 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: Three-Dimensional Magnetohydrodynamic Numerical Simulations
of Coronal Loop Oscillations Associated with Flares
Authors: Miyagoshi, Takehiro; Yokoyama, Takaaki; Shimojo, Masumi
Bibcode: 2004PASJ...56..207M
Altcode:
We performed three-dimensional MHD numerical simulations for
solar coronal magnetic loop oscillations and found: (1) The loop
oscillation period is determined by its Alfvén time. (2) The
amplitude of oscillation decreases exponentially in time. This is
explained as energy transport by fast-mode MHD waves. The damping
rate, ωdamp, is described as ωdamp ∝
Va/R, where Va is the Alfvén speed around
loops and R is the radius of the loop. Because of computer resources
limitations, the plasma β value is much larger than that of the real
corona. We thus applied a scaling law derived from numerical simulation
results to the real corona parameter ranges and analyzed the results.
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: Measurement of magnetic helicity injection and free energy
loading into the solar corona
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Yokoyama, Takaaki;
Sakurai, Takashi
Bibcode: 2004naoj.book...47K
Altcode:
No abstract at ADS
Title: Microwave imaging observation of high-energy electron
propagation in a solar flare
Authors: Shibasaki, Kiyoto; Nakajima, Hiroshi; Yokoyama, Takaaki;
Melnikov, V. F.; Stepanov, A. V.
Bibcode: 2004naoj.book...15S
Altcode:
No abstract at ADS
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: 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: Magnetohydrodynamic Numerical Simulations of Solar X-Ray Jets
Based on the Magnetic Reconnection Model That Includes Chromospheric
Evaporation
Authors: Miyagoshi, Takehiro; Yokoyama, Takaaki
Bibcode: 2003ApJ...593L.133M
Altcode:
We studied solar coronal X-ray jets by MHD numerical simulations
with thermal conduction effects based on the magnetic reconnection
model. Key physical processes are included, such as the emergence of
magnetic fluxes from the convection zone, magnetic reconnection with
the coronal magnetic fields, heat conduction to the chromosphere,
and chromospheric evaporation. High-density evaporation jets
were successfully reproduced in the simulations. The mass of the
evaporation jets M is described as M=6.8×1012 g(B/10
G)15/7(Tcor/106 K)5/14 ×
(sflare/5000 km)12/7(t/400 s), where B is the
magnetic field strength, Tcor is the coronal temperature,
sflare is the loop height, and t is the duration of the
ejection.
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: The Spatially Resolved Spectrum Analysis of Gradual Hardening
Flare
Authors: Takasaki, H.; Kiyohara, Junko; Yokoyama, Takaaki; Nakajima,
Hiroshi; Masuda, Sotoshi; Sato, Jun, Kosugi, Takeo
Bibcode: 2003ICRC....6.3371T
Altcode: 2003ICRC...28.3371T
We present examination of the multi-wavelength observation of a M8.2
flare which occurred on 2000 November 25. This flare gives us more
detailed pictures of the gradual hard flare amd high energy particles
than before the previous studies.We mainly discussed the magnetic
trapping effect for them and the spatial distribution and the temporal
variation of the indices of the electron energy spectrum inferred from
hard X-ray(HXR) and microwave.The main results are as follows. (1)
In this flare, the HXR emission is mainly produced by electrons which
precipitate into choromosphere after magnetic mirroring in flare loops
and their energy is under 1 MeV. (2) The microwave emission at flare
loop top is produced by trapped electrons and their energy is over
1 MeV. (3) There are a break in the electron spectral index between
lower energy electrons which have over 1 MeV and higher energy ones
under 1 MeV, that is, it is possible that the initial acceleration
mechanism of their electrons at flare are not same.
Title: Evolution of Conjugate Footpoints inside Flare Ribbons during
a Great Two-Ribbon Flare on 2001 April 10
Authors: Asai, Ayumi; Ishii, Takako T.; Kurokawa, Hiroki; Yokoyama,
Takaaki; Shimojo, Masumi
Bibcode: 2003ApJ...586..624A
Altcode:
We report a detailed examination of the fine structure inside flare
ribbons and the temporal evolution of such structure during an
X2.3 solar flare, which occurred on 2001 April 10. We examined fine
structures, such as systems of conjugate footpoints, inside flare
ribbons by using the Hα images obtained with the Sartorius telescope
at Kwasan Observatory, Kyoto University. We identified the conjugate
footpoints of each Hα kernel in both flare ribbons by a new method
that uses cross-correlation functions of the light curves. We also
compared the sites of the Hα kernels with the spatial configurations
of flare loops seen in the extreme-ultraviolet images obtained with
the Transition Region and Coronal Explorer. We found that the highly
correlated pairs of Hα kernels were connected by flare loops seen
in the 171 Å images. Investigating such fine structures inside the
flare ribbons, we can follow the history of energy release and perhaps
acquire key information about particle acceleration.
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: Generation and Annihilation of Helicity in Active Regions
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Miike, Haruka; Yokoyama,
Takaaki; Sakurai, Takashi
Bibcode: 2003IAUJD...3E..32K
Altcode:
Magnetic helicity in active regions is investigated based on the vector
magnetograph observations and the three-dimensional magnetohydrodynamic
simulations. First we measured the helicity flux through the photosphere
into active regions using the magnetic data and the numerical technique
to solve the induction equation inversely and found that the helicity
flux forms a complicated structure in which the sign of helicity is
easily changed within an active region. Secondly from the statistical
analyses for various active regions it was shown that the absolute
value of helicity flux rather than the net flux well correlate the
coronal activity. Thirdly we revealed that for many flares the initial
brightening in H-alpha and Trace 1600 A image located at a region where
the helicity sign was sharply changed on the photosphere. Finally using
the high resolution simulation we demonstrated that the explosive
nonlinear reconnection can arise at the helicity inversion layer
when the counter helicity is gradually injected due the photospheric
motion. Based on the all results we conclude that the annihilation
of both signs of magnetic helicity could be a key mechanism to drive
coronal activity
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: 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: 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 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: 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: 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: Three Dimensional Numerical Study of Solar Coronal Magnetic
Field Based on the Magnetograph Observations
Authors: Kusano, Kanya; Maeshiro, Tomohiro; Yokoyama, Takaaki; Sakurai,
Takashi; Kageyama, Akira
Bibcode: 2000APS..DPPYP1021K
Altcode:
Magnetohydrodynamic (MHD) properties of active regions in solar corona
is investigated by combining the numerical analysis technique and the
vector magnetograph observations. First, we numerically construct the
potential magnetic field of several major active regions, and compare
them with the tangential component of the magnetic field observed by
Solar Flare Telescope in National Astronomical Observatory of Japan. As
a result, we found that the magnetic free energy, which is formed by
the the gap between the observed field and the potential field, as well
as the current helicity, which is produced by the electric current and
the magnetic field across the photosphere, is reduced in some big flare
events. It suggests that the solar flares are processes to reduce the
magnetic helicity contained in the coronal magnetic field. Secondly,
we develop the numerical model to construct the three dimensional
(3D) nonlinear equilibria in active regions using the vector magnetic
field observed by magnetograph. Furthermore, the MHD stability of
the practical solar corona will be discussed based on the numerical
calculations using the observed data.
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: 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: 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: 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: 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: 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: 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: 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: 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, 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: 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: 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: 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: 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: Magnetohydrodynamic simulation of solar coronal X-ray jets
based on magnetic reconnection model
Authors: Yokoyama, Takaaki
Bibcode: 1995PhDT........85Y
Altcode:
No abstract at ADS
Title: Numerical simulation of magnetic reconnection associated with
emerging flux in the solar atmosphere.
Authors: Yokoyama, Takaaki; Shibata, Kazumari
Bibcode: 1993ppcn.conf..203Y
Altcode:
Some of solar compact flares are known to be caused by emerging
flux, for which the magnetic-reconnection-model is suggested. The
authors studied this model using a numerical simulation method. Their
simulation is performed with the two dimensional MHD code. Initially
they consider a two-temperature layered plasma similar to the solar
photosphere/chromosphere and corona in magnetostatic equilibrium. Their
results show that magnetic flux in the photosphere emerges by magnetic
buoyancy instability (Parker instability) as rising loops. When the
rising loops reach the coronal level, magnetic reconnection starts
between the loops and the coronal field, creating a magnetic island,
which confines cool, dense plasma. The magnetic island as well as the
ambient hot plasma are ejected toward both sides of the emerging loop
as a jet-like flow. It is found that, the magnetic reconnection is
more violent, when the resistivity is smaller, or when the specific
heat ratio is smaller. It is also found that there are four types of
jet-like flow associated with the reconnection.