Author name code: nitta
ADS astronomy entries on 2022-09-14
author:"Nitta, Nariaki V."
------------------------------------------------------------------------
Title: Galactic Cosmic Rays and Solar Energetic Particles in Cis-Lunar
Space: Need for contextual energetic particle measurements at Earth
and supporting distributed observations
Authors: Corti, Claudio; Whitman, Kathryn; Desai, Ravindra; Rankin,
Jamie; Toit Strauss, Du; Nitta, Nariaki; Turner, Drew; Y Chen, Thomas
Bibcode: 2022arXiv220903635C
Altcode:
The particle and radiation environment in cis-lunar space is becoming
increasingly important as more hardware and human assets occupy
various orbits around the Earth and space exploration efforts turn
to the Moon and beyond. Since 2020, the total number of satellites in
orbit has approximately doubled, highlighting the growing dependence on
space-based resources. Through NASA's upcoming Artemis missions, humans
will spend more time in cis-lunar space than ever before supported
by the expansive infrastructure required for extended missions to the
Moon, including a surface habitat, a communications network, and the
Lunar Gateway. This paper focuses on galactic cosmic rays (GCRs) and
solar energetic particles (SEPs) that create a dynamic and varying
radiation environment within these regions. GCRs are particles of
hundreds of MeV/nucleon (MeV/n) and above generated in highly energetic
astrophysical environments in the Milky Way Galaxy, such as supernovae
and pulsars, and beyond. These particles impinge isotropically on the
heliosphere and are filtered down to 1 AU, experiencing modulation in
energy and intensity on multiple timescales, from hours to decades,
due to the solar magnetic cycle and other transient phenomena. SEPs are
particles with energies up to thousands of MeV/n that are accelerated
in eruptive events on the Sun and flood the inner heliosphere causing
sudden and drastic increases in the particle environment on timescales
of minutes to days. This paper highlights a current and prospective
future gap in energetic particle measurements in the hundreds of
MeV/n. We recommend key observations near Earth to act as a baseline
as well as distributed measurements in the heliosphere, magnetosphere,
and lunar surface to improve the scientific understanding of these
particle populations and sources.
Title: CME Evolution in the Structured Heliosphere and Effects at
Earth and Mars During Solar Minimum
Authors: Palmerio, Erika; Lee, Christina O.; Richardson, Ian G.;
Nieves-Chinchilla, Teresa; Dos Santos, Luiz F. G.; Gruesbeck, Jacob
R.; Nitta, Nariaki V.; Mays, M. Leila; Halekas, Jasper S.; Zeitlin,
Cary; Xu, Shaosui; Holmström, Mats; Futaana, Yoshifumi; Mulligan,
Tamitha; Lynch, Benjamin J.; Luhmann, Janet G.
Bibcode: 2022arXiv220905760P
Altcode:
The activity of the Sun alternates between a solar minimum and a
solar maximum, the former corresponding to a period of "quieter"
status of the heliosphere. During solar minimum, it is in principle
more straightforward to follow eruptive events and solar wind
structures from their birth at the Sun throughout their interplanetary
journey. In this paper, we report analysis of the origin, evolution,
and heliospheric impact of a series of solar transient events that took
place during the second half of August 2018, i.e. in the midst of the
late declining phase of Solar Cycle 24. In particular, we focus on two
successive coronal mass ejections (CMEs) and a following high-speed
stream (HSS) on their way towards Earth and Mars. We find that the
first CME impacted both planets, whilst the second caused a strong
magnetic storm at Earth and went on to miss Mars, which nevertheless
experienced space weather effects from the stream interacting region
(SIR) preceding the HSS. Analysis of remote-sensing and in-situ data
supported by heliospheric modelling suggests that CME--HSS interaction
resulted in the second CME rotating and deflecting in interplanetary
space, highlighting that accurately reproducing the ambient solar
wind is crucial even during "simpler" solar minimum periods. Lastly,
we discuss the upstream solar wind conditions and transient structures
responsible for driving space weather effects at Earth and Mars.
Title: Multi-spacecraft Observations of Heavy Ion Characteristics
in the 28 October 2021 Solar Energetic Particle Event
Authors: Cohen, Christina; Roelof, Edmond; Mitchell, Donald; Ho,
George; Leske, Richard; Labrador, Allan; Cummings, A. C.; McNutt,
Ralph; Stone, E. C.; Hill, Matthew; De Nolfo, Georgia; Matthaeus,
William; Christian, Eric; Nitta, Nariaki; Schwadron, Nathan; Giacalone,
Joe; Desai, Mihir; Mewaldt, Richard; Mason, Glenn M.; Wiedenbeck, Mark;
Szalay, Jamey; Jin, Meng; Allen, Robert; McComas, David; Mitchell,
John Grant; Joyce, Colin; Rankin, Jamie
Bibcode: 2022cosp...44.1170C
Altcode:
The large solar energetic particle (SEP) event on 28 October 2021 was
the first ground level enhancement (GLE) event of solar cycle 25, having
been detected by a number of ground-based neutron monitors. At the time
of the event, Parker Solar Probe (PSP), Solar Orbiter, and STEREO-A
(STA) were located within ~60° of near-Earth spacecraft (e.g., ACE)
and all observed the SEP event. Despite the radial and longitudinal
distribution of the spacecraft, PSP, STA, and ACE measured power law
spectra for O and Fe with the same spectral index and identical and
constant Fe/O abundance ratios over approximately an order of magnitude
in energy. This striking uniformity of the composition and heavy ion
spectra will be discussed in relation to the very symmetric EUV wave
observed by the Solar Dynamics Observatory. The results of modeling
the background solar wind, the erupting coronal mass ejection, and
the related shock properties near the Sun will also be discussed.
Title: Solar Energetic Particle Events with Delayed Onsets
Authors: Kihara, Kosuke; Asai, Ayumi; Nitta, Nariaki; Yashiro, Seiji
Bibcode: 2022cosp...44.1169K
Altcode:
Solar energetic particles (SEPs) give rise to critical radiation
hazards for astronauts and airline passengers and cause damages to
satellites. They have a significant impact on the social infrastructure
and are an important subject in space weather. One of the major
origins of SEPs is considered to be coronal mass ejections (CMEs),
and the shocks at the fronts accelerate the charged particles to be
SEPs. In order to understand how SEPs are accelerated by CMEs and
reach the Earth, we conducted a statistical study of the relation
between CMEs and SEP events. We measured the onset time (TO), defined
as the time from the CME launch to the proton arrival in Earth orbit,
and studied the relation between CME or SEP parameters. Our study
showed that TO tends to be shorter if the source CMEs erupt near the
nominal footpoints of parker spiral magnetic fields connecting to the
instrument. The relation between the speed of CMEs and TO is negative in
that longitude. On the other hand, even if the source and speed of CMEs
are similar, the distribution of TO is slightly dispersed. Therefore,
we now focus on the SEP events that occurred on July 14, 2017, which has
a relatively long TO among those events, and are conducting a detailed
analysis. We extracted a total of 4 events that have long and short
TO originating from similar CMEs, including this event. This analysis
has shown that the characteristics of the accompanying flares were
significantly different between long and short TO events. Furthermore,
we compared the time of type II radio burst, which is considered to
be an indicator of particle acceleration in coronal shock waves, with
that of particle release measured by velocity dispersion analysis and
found that the interval between them is longer in the long TO event
on July 14, 2017.
Title: Coronal Dimming as a Proxy for Solar and Stellar Coronal
Mass Ejections
Authors: Jin, Meng; Nitta, Nariaki; Derosa, Marc; Cheung, Mark; Osten,
Rachel; France, Kevin; Mason, James; Kowalski, Adam; Schrijver, Carolus
Bibcode: 2022cosp...44.1404J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings as well as their relationship
with CMEs. Recent study (e.g., Veronig et al. 2021) also shows promising
dimming signals from distant stars, which suggest the possibility of
using coronal dimming as a proxy to diagnose stellar CMEs. In this
study, we first conduct a comparative study of solar coronal dimming
using MHD simulations and SDO observations. A detailed analysis of
the simulation and observation data reveals how transient dimming
/ brightening are related to plasma heating processes, while the
long-lasting core and remote dimmings are caused by mass loss process
induced by the CME. Using metrics such as dimming depth and dimming
slope, we uncover a relationship between dimmings and CME properties
(e.g., CME mass, CME speed) in the simulation. We further extend the
model for simulating the stellar CMEs and dimmings and compare with
solar cases. Our result suggests that coronal dimmings encode important
information about the associated CMEs, which provides a physical basis
for detecting stellar CMEs from distant solar-like stars.
Title: Understanding the Origin of Iron-rich Gradual Solar Energetic
Particle Events
Authors: Nitta, Nariaki; Cohen, Christina; Jin, Meng
Bibcode: 2022cosp...44.1168N
Altcode:
It is generally thought that gradual solar energetic particle (SEP)
events are accelerated at shock waves driven by corona mass ejections
(CMEs) and that their composition is more or less similar to that of
the solar wind. However, some of gradual SEP events show enhancement
of heavy ions such as iron. One of the competing scenarios links
iron-rich gradual SEP events to quasi-perpendicular shocks, which
may preferentially accelerate suprathermal particles from earlier or
concomitant flares that already have flare-like composition. It is
difficult to test this hypothesis since the magnetic field of the
corona is not directly observed on a routine basis, and CMEs and
related structures are observed only in projection to the plane of
the sky. We have modeled eight eruptions that produced SEPs, using
advanced numerical simulations that yield shock parameters including
the shock angle between the normal to the shock surface and magnetic
field lines that are connected to the observer. Most of these SEP
events were observed at more than one of three locations, namely,
STEREO-B, STEREO-A and near Earth, letting us compare the shock angle
and the Fe/O ratio at a total of 20 locations. We do not find a strong
correlation between Fe-rich SEP events and quasi-perpendicular shocks
but some trend may exist. We discuss the results including alternative
scenarios for the variations of the SEP composition and the limitations
of the simulation results possibly attributable to input magnetic maps.
Title: Assessing the Influence of Input Magnetic Maps on Global
Modeling of the Solar Wind and CME-driven Shock in the 2013 April
11 Event
Authors: Jin, Meng; Cohen, Christina; Nitta, Nariaki
Bibcode: 2022cosp...44.3442J
Altcode:
In the past decade, significant efforts have been made in developing
physics-based solar wind and coronal mass ejection (CME) models,
which have been or are being transferred to national centers (e.g.,
SWPC, CCMC) to enable space weather predictive capability. However,
the input data coverage for space weather forecasting is extremely
limited. One major limitation is the solar magnetic field measurements,
which are used to specify the inner boundary conditions of the global
magnetohydrodynamic (MHD) models. In this study, using the Alfven wave
solar model (AWSoM), we quantitatively assess the influence of the
magnetic field map input (synoptic/diachronic vs. synchronic magnetic
maps) on the global modeling of the solar wind and the CME-driven
shock in the 2013 April 11 solar energetic particle (SEP) event. Our
study shows that due to the inhomogeneous background solar wind and
dynamical evolution of the CME, the CME-driven shock parameters change
significantly both spatially and temporally as the CME propagates
through the heliosphere. The input magnetic map has a great impact
on the shock connectivity and shock properties in the global MHD
simulation. Therefore this study illustrates the importance of taking
into account the model uncertainty due to the imperfect magnetic field
measurements when using the model to provide space weather predictions.
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
Using MHD Simulations and SDO Observations
Authors: Jin, Meng; Cheung, Mark C. M.; DeRosa, Marc L.; Nitta,
Nariaki V.; Schrijver, Carolus J.
Bibcode: 2022ApJ...928..154J
Altcode: 2022arXiv220213034J
Solar coronal dimmings have been observed extensively in recent
years. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings as well as their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-constrained
global magnetohydrodynamics model (Alfvén-wave solar model), we
simulate coronal dimmings resulting from different CME energetics and
flux rope configurations. We synthesize the emissions of different EUV
spectral bands/lines and compare with SDO/AIA and EVE observations. A
detailed analysis of the simulation and observation data suggests
that the transient dimming/brightening are related to plasma heating
processes, while the long-lasting core and remote dimmings are caused
by mass-loss process induced by the CME. Moreover, the interaction
between the erupting flux rope with different orientations and the
global solar corona could significantly influence the coronal dimming
patterns. Using metrics such as dimming depth and dimming slope,
we investigate the relationship between dimmings and CME properties
(e.g., CME mass, CME speed) in the simulation. Our result suggests
that coronal dimmings encode important information about the associated
CMEs, which provides a physical basis for detecting stellar CMEs from
distant solar-like stars.
Title: Assessing the Influence of Input Magnetic Maps on Global
Modeling of the Solar Wind and CME-Driven Shock in the 2013 April
11 Event
Authors: Jin, Meng; Nitta, Nariaki V.; Cohen, Christina M. S.
Bibcode: 2022SpWea..2002894J
Altcode: 2022arXiv220207214J
In the past decade, significant efforts have been made in developing
physics-based solar wind and coronal mass ejection (CME) models,
which have been or are being transferred to national centers (e.g.,
SWPC, Community Coordinated Modeling Center) to enable space weather
predictive capability. However, the input data coverage for space
weather forecasting is extremely limited. One major limitation is
the solar magnetic field measurements, which are used to specify
the inner boundary conditions of the global magnetohydrodynamic
(MHD) models. In this study, using the Alfvén wave solar model, we
quantitatively assess the influence of the magnetic field map input
(synoptic/diachronic vs. synchronic magnetic maps) on the global
modeling of the solar wind and the CME-driven shock in the 11 April
2013 solar energetic particle event. Our study shows that due to the
inhomogeneous background solar wind and dynamical evolution of the CME,
the CME-driven shock parameters change significantly both spatially
and temporally as the CME propagates through the heliosphere. The
input magnetic map has a great impact on the shock connectivity and
shock properties in the global MHD simulation. Therefore this study
illustrates the importance of taking into account the model uncertainty
due to the imperfect magnetic field measurements when using the model
to provide space weather predictions.
Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
Bibcode: 2022ApJ...926...53C
Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
Bibcode: 2021AGUFMSH51A..08C
Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) of the coronal dynamics of solar flares and eruptive
phenomena. The highest resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by IRIS for the low solar atmosphere,
we need high-resolution spectroscopic measurements with simultaneous
imaging to understand the dominant processes. In this paper: (1)
we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
observatory to fill this observational gap by providing high-cadence
(<20 s), sub-arcsecond resolution spectroscopic rasters over an
active region size of the solar transition region and corona; (2)
using advanced numerical models, we demonstrate the unique diagnostic
capabilities of MUSE for exploring solar coronal dynamics, and for
constraining and discriminating models of solar flares and eruptions;
(3) we discuss the key contributions MUSE would make in addressing the
science objectives of the Next Generation Solar Physics Mission (NGSPM),
and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
Daniel K Inouye Solar Telescope (and other ground-based observatories)
can operate as a distributed implementation of the NGSPM. This is a
companion paper to De Pontieu et al. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.
Title: The long period of 3He-rich solar energetic
particles measured by Solar Orbiter 2020 November 17-23
Authors: Bučík, R.; Mason, G. M.; Gómez-Herrero, R.; Lario, D.;
Balmaceda, L.; Nitta, N. V.; Krupař, V.; Dresing, N.; Ho, G. C.;
Allen, R. C.; Carcaboso, F.; Rodríguez-Pacheco, J.; Schuller, F.;
Warmuth, A.; Wimmer-Schweingruber, R. F.; Freiherr von Forstner,
J. L.; Andrews, G. B.; Berger, L.; Cernuda, I.; Espinosa Lara, F.;
Lees, W. J.; Martín, C.; Pacheco, D.; Prieto, M.; Sánchez-Prieto,
S.; Schlemm, C. E.; Seifert, H.; Tyagi, K.; Maksimovic, M.; Vecchio,
A.; Kollhoff, A.; Kühl, P.; Xu, Z. G.; Eldrum, S.
Bibcode: 2021A&A...656L..11B
Altcode: 2021arXiv210905570B
We report observations of a relatively long period of
3He-rich solar energetic particles (SEPs) measured
by Solar Orbiter. The period consists of several well-resolved
ion injections. The high-resolution STEREO-A imaging observations
reveal that the injections coincide with extreme ultraviolet jets and
brightenings near the east limb, not far from the nominal magnetic
connection of Solar Orbiter. The jets originated in two adjacent,
large, and complex active regions, as observed by the Solar Dynamics
Observatory when the regions rotated into the Earth's view. It
appears that the sustained ion injections were related to the
complex configuration of the sunspot group and the long period of
3He-rich SEPs to the longitudinal extent covered by the
group during the analyzed time period.
Title: Multi-spacecraft Observations of Gradual Solar Energetic
Particle Events with Enhanced 3He Abundance
Authors: Bucik, Radoslav; Mason, Glenn; Dayeh, Maher; Desai,
Mihir; Cohen, Christina; Lario, David; Balmaceda, Laura; Krupar,
Vratislav; Wiedenbeck, Mark; Wimmer-Schweingruber, Robert; Ho, George;
Rodriguez-Pacheco, Javier; Nitta, Nariaki; Kuhl, Patrick; Xu, Zigong;
Asfaw, Tilaye
Bibcode: 2021AGUFMNG35B0444B
Altcode:
Flare suprathermal ions with enhanced 3He and heavy-ion abundances are
an essential component of the seed population accelerated by CME-driven
shocks in gradual solar energetic particle (GSEP) events. However,
the mechanisms through which CME-driven shocks gain access to flare
suprathermals and produce spectral and abundance variations in GSEP
events remain largely unexplored. We report two recent GSEP events:
one observed by Solar Orbiter on 2020 Nov 24 (the first GSEP event on
Solar Orbiter) and the other by ACE on 2021 May 29 (the most intense
GOES proton event in the present solar cycle). The events were preceded
by impulsive SEP (ISEP) events. Abundances and energy spectra are
markedly different in the examined events at < 1 MeV/nucleon. For
example, in the May event, Fe/O is typical of ISEP events, a factor
of 100 to 10 higher than Fe/O in the November event. 3He abundance in
the November event is high, typical of ISEP events, while in the May
event, it is much lower, though finite. The May event shows a hard
4He spectrum with a power-law index of 1.6, and the November event a
soft spectrum with an index of 3.5. The events were associated with
halo CMEs with speeds around 900 km/s. The November event was also
measured by Parker Solar Probe and the May event by STEREO-A and
Solar Orbiter. This paper discusses the origin of vastly different
abundances and spectral shapes in terms of variable remnant population
from preceding ISEP events. Furthermore, we discuss a possible direct
contribution from parent flares.
Title: Correction to: Understanding the Origins of Problem Geomagnetic
Storms Associated with "Stealth" Coronal Mass Ejections
Authors: Nitta, Nariaki V.; Mulligan, Tamitha; Kilpua, Emilia K. J.;
Lynch, Benjamin J.; Mierla, Marilena; O'Kane, Jennifer; Pagano, Paolo;
Palmerio, Erika; Pomoell, Jens; Richardson, Ian G.; Rodriguez, Luciano;
Rouillard, Alexis P.; Sinha, Suvadip; Srivastava, Nandita; Talpeanu,
Dana-Camelia; Yardley, Stephanie L.; Zhukov, Andrei N.
Bibcode: 2021SSRv..217...84N
Altcode:
No abstract at ADS
Title: Understanding the Origin of Fe-rich Gradual Solar Energetic
Particle Events
Authors: Nitta, Nariaki; Jin, Meng; Cohen, Christina
Bibcode: 2021AGUFMSH51B..05N
Altcode:
Solar energetic particle (SEP) events are often discussed as if
they clearly belonged to either the impulsive or gradual category,
which is believed to correspond, respectively, to the reconnection
(flare) and shock (coronal mass ejection (CME)) origin. Following
this classification, gradual SEP events should have a solar wind-like
composition, but we sometimes observe gradual SEP events that do not
follow this expectation. For example, gradual SEP events enriched in
Fe ions at high energies (>10 MeV/n) are not uncommon. There are
two ways of explaining these events. One is a direct contribution of
flare-accelerated material, and another is a preferential acceleration
of suprathermal particles at quasi-perpendicular shock waves. Neither
has been universally accepted. Here we specifically test the second
hypothesis, using numerical simulations with the Alfven Wave Solar
Model (AWSoM) that is part of the Space Weather Modeling Framework
(SWMF). This is an approach complementary to the technique that
geometrically constrains CME-driven shocks in combination with
background solar wind from numerical models. We instead simulate CMEs,
tracing the shock wave driven by the CME that evolves dynamically, and
study the properties of the shock wave at the location on the shock
surface traversed by field lines that connect to the observer. We
compare them with 8 multi-spacecraft SEP events sampled at 20
locations by ACE, STEREO-A and STEREO-B. We do not find a correlation
of SEP events that have enhanced Fe/O ratios at 10-25 MeV/n with
quasi-perpendicular shocks. We discuss this in comparison with other
shock and SEP properties as well as with solar source properties.
Title: The Long Period of 3He-rich Solar Energetic Particles Measured
by Solar Orbiter 2020 November 1723
Authors: Bucik, Radoslav; Mason, Glenn; Gomez-Herrero, Raul; Lario,
David; Balmaceda, Laura; Nitta, Nariaki; Krupar, Vratislav; Dresing,
Nina; Ho, George; Allen, Robert; Carcaboso-Morales, Fernando;
Rodriguez-Pacheco, Javier; Schuller, Frederic; Warmuth, Alexander;
Wimmer-Schweingruber, Robert; Freiherr von Forstner, Johan; Andrews,
G.; Berger, Lars; Cernuda, Ignacio; Espinosa Lara, Francisco; Lees,
W.; Martin-Garcia, Cesar; Pacheco, Daniel; Prieto, Manuel; Sanchez
Prieto, Sebastian; Schlemm, Charles; Seifert, Helmut; Tyagi, Kush;
Maksimovic, Milan; Vecchio, Antonio; Kollhoff, Alexander; Kuhl,
Patrick; Xu, Zigong; Eldrum, Sandra
Bibcode: 2021AGUFMSH25B2084B
Altcode:
We report observations of a relatively long period of 3He-rich solar
energetic particles (SEPs) measured by Solar Orbiter. The period
consists of several well-resolved ion injections. The high-resolution
STEREO-A imaging observations reveal that the injections coincide
with extreme ultraviolet jets and brightenings near the east limb,
not far from the nominal magnetic connection of Solar Orbiter. The
jets originated in two adjacent, large, and complex active regions,
as observed by the Solar Dynamics Observatory when the regions rotated
into the Earth's view. It appears that the sustained ion injections
were related to the complex configuration of the sunspot group and
the long period of 3He-rich SEPs to the longitudinal extent covered
by the group during the analyzed time period.
Title: Earth-affecting solar transients: a review of progresses in
solar cycle 24
Authors: Zhang, Jie; Temmer, Manuela; Gopalswamy, Nat; Malandraki,
Olga; Nitta, Nariaki V.; Patsourakos, Spiros; Shen, Fang; Vršnak,
Bojan; Wang, Yuming; Webb, David; Desai, Mihir I.; Dissauer, Karin;
Dresing, Nina; Dumbović, Mateja; Feng, Xueshang; Heinemann, Stephan
G.; Laurenza, Monica; Lugaz, Noé; Zhuang, Bin
Bibcode: 2021PEPS....8...56Z
Altcode: 2020arXiv201206116Z
This review article summarizes the advancement in the studies of
Earth-affecting solar transients in the last decade that encompasses
most of solar cycle 24. It is a part of the effort of the International
Study of Earth-affecting Solar Transients (ISEST) project, sponsored
by the SCOSTEP/VarSITI program (2014-2018). The Sun-Earth is an
integrated physical system in which the space environment of the
Earth sustains continuous influence from mass, magnetic field, and
radiation energy output of the Sun in varying timescales from minutes to
millennium. This article addresses short timescale events, from minutes
to days that directly cause transient disturbances in the Earth's
space environment and generate intense adverse effects on advanced
technological systems of human society. Such transient events largely
fall into the following four types: (1) solar flares, (2) coronal mass
ejections (CMEs) including their interplanetary counterparts ICMEs,
(3) solar energetic particle (SEP) events, and (4) stream interaction
regions (SIRs) including corotating interaction regions (CIRs). In
the last decade, the unprecedented multi-viewpoint observations of
the Sun from space, enabled by STEREO Ahead/Behind spacecraft in
combination with a suite of observatories along the Sun-Earth lines,
have provided much more accurate and global measurements of the size,
speed, propagation direction, and morphology of CMEs in both 3D and over
a large volume in the heliosphere. Many CMEs, fast ones, in particular,
can be clearly characterized as a two-front (shock front plus ejecta
front) and three-part (bright ejecta front, dark cavity, and bright
core) structure. Drag-based kinematic models of CMEs are developed to
interpret CME propagation in the heliosphere and are applied to predict
their arrival times at 1 AU in an efficient manner. Several advanced
MHD models have been developed to simulate realistic CME events from
the initiation on the Sun until their arrival at 1 AU. Much progress
has been made on detailed kinematic and dynamic behaviors of CMEs,
including non-radial motion, rotation and deformation of CMEs, CME-CME
interaction, and stealth CMEs and problematic ICMEs. The knowledge
about SEPs has also been significantly improved. An outlook of how to
address critical issues related to Earth-affecting solar transients
concludes this article.
Title: Understanding the Origins of Problem Geomagnetic Storms
Associated with "Stealth" Coronal Mass Ejections
Authors: Nitta, Nariaki V.; Mulligan, Tamitha; Kilpua, Emilia K. J.;
Lynch, Benjamin J.; Mierla, Marilena; O'Kane, Jennifer; Pagano, Paolo;
Palmerio, Erika; Pomoell, Jens; Richardson, Ian G.; Rodriguez, Luciano;
Rouillard, Alexis P.; Sinha, Suvadip; Srivastava, Nandita; Talpeanu,
Dana-Camelia; Yardley, Stephanie L.; Zhukov, Andrei N.
Bibcode: 2021SSRv..217...82N
Altcode: 2021arXiv211008408N
Geomagnetic storms are an important aspect of space weather
and can result in significant impacts on space- and ground-based
assets. The majority of strong storms are associated with the passage
of interplanetary coronal mass ejections (ICMEs) in the near-Earth
environment. In many cases, these ICMEs can be traced back unambiguously
to a specific coronal mass ejection (CME) and solar activity on the
frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth
from routine observations of CMEs and solar activity currently makes a
major contribution to the forecasting of geomagnetic storms. However,
it is clear that some ICMEs, which may also cause enhanced geomagnetic
activity, cannot be traced back to an observed CME, or, if the CME
is identified, its origin may be elusive or ambiguous in coronal
images. Such CMEs have been termed "stealth CMEs". In this review,
we focus on these "problem" geomagnetic storms in the sense that the
solar/CME precursors are enigmatic and stealthy. We start by reviewing
evidence for stealth CMEs discussed in past studies. We then identify
several moderate to strong geomagnetic storms (minimum Dst <−50
nT) in solar cycle 24 for which the related solar sources and/or
CMEs are unclear and apparently stealthy. We discuss the solar and
in situ circumstances of these events and identify several scenarios
that may account for their elusive solar signatures. These range from
observational limitations (e.g., a coronagraph near Earth may not
detect an incoming CME if it is diffuse and not wide enough) to the
possibility that there is a class of mass ejections from the Sun that
have only weak or hard-to-observe coronal signatures. In particular,
some of these sources are only clearly revealed by considering the
evolution of coronal structures over longer time intervals than is
usually considered. We also review a variety of numerical modelling
approaches that attempt to advance our understanding of the origins
and consequences of stealthy solar eruptions with geoeffective
potential. Specifically, we discuss magnetofrictional modelling of the
energisation of stealth CME source regions and magnetohydrodynamic
modelling of the physical processes that generate stealth CME or
CME-like eruptions, typically from higher altitudes in the solar corona
than CMEs from active regions or extended filament channels.
Title: Unusual enhancement of 30 MeV proton flux in an ICME
sheath region
Authors: Oka, Mitsuo; Obara, Takahiro; Nitta, Nariaki V.; Yashiro,
Seiji; Shiota, Daikou; Ichimoto, Kiyoshi
Bibcode: 2021EP&S...73...31O
Altcode:
In gradual Solar Energetic Particle (SEP) events, shock waves driven
by coronal mass ejections (CMEs) play a major role in accelerating
particles, and the energetic particle flux enhances substantially
when the shock front passes by the observer. Such enhancements are
historically referred to as Energetic Storm Particle (ESP) events,
but it remains unclear why ESP time profiles vary significantly from
event to event. In some cases, energetic protons are not even clearly
associated with shocks. Here, we report an unusual, short-duration
proton event detected on 5 June 2011 in the compressed sheath
region bounded by an interplanetary shock and the leading edge of the
interplanetary CME (or ICME) that was driving the shock. While < 10
MeV protons were detected already at the shock front, the higher-energy
(> 30 MeV) protons were detected about four hours after the shock
arrival, apparently correlated with a turbulent magnetic cavity embedded
in the ICME sheath region.
Title: The unusual widespread solar energetic particle event on 2013
August 19. Solar origin and particle longitudinal distribution
Authors: Rodríguez-García, L.; Gómez-Herrero, R.; Zouganelis,
I.; Balmaceda, L.; Nieves-Chinchilla, T.; Dresing, N.; Dumbović,
M.; Nitta, N. V.; Carcaboso, F.; dos Santos, L. F. G.; Jian, L. K.;
Mays, L.; Williams, D.; Rodríguez-Pacheco, J.
Bibcode: 2021A&A...653A.137R
Altcode: 2021arXiv210710257R
Context. Late on 2013 August 19, STEREO-A, STEREO-B, MESSENGER, Mars
Odyssey, and the L1 spacecraft, spanning a longitudinal range of 222°
in the ecliptic plane, observed an energetic particle flux increase. The
widespread solar energetic particle (SEP) event was associated with a
coronal mass ejection (CME) that came from a region located near the
far-side central meridian from Earth's perspective. The CME erupted
in two stages, and was accompanied by a late M-class flare observed
as a post-eruptive arcade, persisting low-frequency (interplanetary)
type II and groups of shock-accelerated type III radio bursts, all of
them making this SEP event unusual.
Aims: There are two main
objectives of this study, disentangling the reasons for the different
intensity-time profiles observed by the spacecraft, especially at
MESSENGER and STEREO-A locations, longitudinally separated by only 15°,
and unravelling the single solar source related with the widespread SEP
event.
Methods: The analysis of in situ data, such as particle
fluxes, anisotropies and timing, and plasma and magnetic field data,
is compared with the remote-sensing observations. A spheroid model is
applied for the CME-driven shock reconstruction and the ENLIL model
is used to characterize the heliospheric conditions, including the
evolution of the magnetic connectivity to the shock.
Results:
The solar source associated with the widespread SEP event is the shock
driven by the CME, as the flare observed as a post-eruptive arcade
is too late to explain the estimated particle onset. The different
intensity-time profiles observed by STEREO-A, located at 0.97 au,
and MESSENGER, at 0.33 au, can be interpreted as enhanced particle
scattering beyond Mercury's orbit. The longitudinal extent of the
shock does not explain by itself the wide spread of particles in the
heliosphere. The particle increase observed at L1 may be attributed to
cross-field diffusion transport, and this is also the case for STEREO-B,
at least until the spacecraft is eventually magnetically connected to
the shock when it reaches ∼0.6 au. Movies are available at https://www.aanda.org
Title: Investigating Remote-sensing Techniques to Reveal Stealth
Coronal Mass Ejections
Authors: Palmerio, Erika; Nitta, Nariaki V.; Mulligan, Tamitha;
Mierla, Marilena; O'Kane, Jennifer; Richardson, Ian G.; Sinha, Suvadip;
Srivastava, Nandita; Yardley, Stephanie L.; Zhukov, Andrei N.
Bibcode: 2021FrASS...8..109P
Altcode: 2021arXiv210607571P
Eruptions of coronal mass ejections (CMEs) from the Sun are usually
associated with a number of signatures that can be identified in
solar disc imagery. However, there are cases in which a CME that
is well observed in coronagraph data is missing a clear low-coronal
counterpart. These events have received attention during recent years,
mainly as a result of the increased availability of multi-point
observations, and are now known as 'stealth CMEs'. In this work,
we analyse examples of stealth CMEs featuring various levels of
ambiguity. All the selected case studies produced a large-scale CME
detected by coronagraphs and were observed from at least one secondary
viewpoint, enabling a priori knowledge of their approximate source
region. To each event, we apply several image processing and geometric
techniques with the aim to evaluate whether such methods can provide
additional information compared to the study of "normal" intensity
images. We are able to identify at least weak eruptive signatures
for all events upon careful investigation of remote-sensing data,
noting that differently processed images may be needed to properly
interpret and analyse elusive observations. We also find that the
effectiveness of geometric techniques strongly depends on the CME
propagation direction with respect to the observers and the relative
spacecraft separation. Being able to observe and therefore forecast
stealth CMEs is of great importance in the context of space weather,
since such events are occasionally the solar counterparts of so-called
'problem geomagnetic storms'.
Title: The Unusual Widespread Solar Energetic Particle Event on 2013
August 19: Solar origin, CME-driven shock evolution and particle
longitudinal distribution
Authors: Rodríguez-García, Laura; Gómez-Herrero, Raúl; Zouganelis,
Yannis; Balmaceda, Laura; Nieves-Chinchilla, Teresa; Dresing, Nina;
Dumbovic, Mateja; Nitta, Nariaki; Carcaboso, Fernando; Guedes dos
Santos, Luiz Fernando; Jian, Lan; Mays, Leila; Williams, David;
Rodríguez-Pacheco, Javier
Bibcode: 2021EGUGA..23..134R
Altcode:
Context: Late on 2013 August 19, STEREO-A, STEREO-B, MESSENGER, Mars
Odyssey, and L1 spacecraft, spanning a longitudinal range of 222° in
the ecliptic plane, observed an energetic particle flux increase. The
widespread solar energetic particle (SEP) event was associated with
a coronal mass ejection (CME) that came from a region located near
the far-side central meridian from Earth's perspective. The CME
appeared to consist of two eruptions, and was accompanied by a ~M3
flare as a post-eruption arcade, and low-frequency (interplanetary)
type II and shock-accelerated type III radio bursts.Aims: The main
objectives of this study are two, disentangling the reasons of the
different intensity-time profiles observed by MESSENGER and STEREO-A,
longitudinally separated by only 15°, and unravelling the single solar
source related with the SEP event.Results: The solar source associated
with the widespread SEP event is the shock driven by the two-stages CME,
as the flare observed as a posteruptive arcade is too late to explain
the estimated particle onset. The different intensity-time profiles
observed by STEREO-A, located at 0.97 au, and MESSENGER, at 0.33 au,
can be interpreted as enhanced particle scattering beyond Mercury's
orbit. The longitudinal extent of the shock does not explain by itself
the wide spread of particles in the heliosphere. The particle increase
observed at L1 may be attributed to cross-field diffusion transport,
and this is also the case for STEREO-B, at least until the spacecraft
is eventually magnetically connected to the shock at ~0.6 au. The
CME-driven shock may have suffered distortion in its evolution in the
heliosphere, such that the shock flank overtakes the shock nose at 1 au.
Title: Unusual enhancement of 30 MeV proton flux in an ICME sheath
region
Authors: Oka, Mitsuo; Obara, Takahiro; Nitta, Nariaki; Yashiro, Seiji;
Shiota, Daikou; Ichimoto, Kiyoshi
Bibcode: 2021EGUGA..23..421O
Altcode:
In gradual Solar Energetic Particle (SEP) events, shock waves driven
by coronal mass ejections (CMEs) play a major role in accelerating
particles, and the energetic particle flux enhances substantially
when the shock front passes by the observer. Such enhancements are
historically referred to as Energetic Storm Particle (ESP) events,
but it remains unclear why ESP time profiles vary significantly from
event to event. In some cases, energetic protons are not even clearly
associated with shocks. Here we report an unusual, short-duration proton
event detected on 5 June 2011 in the compressed sheath region bounded
by an interplanetary shock and the leading-edge of the interplanetary
CME (or ICME) that was driving the shock. While <10 MeV protons
were detected already at the shock front, the higher-energy (>30
MeV) protons were detected about four hours after the shock arrival,
apparently correlated with a turbulent magnetic cavity embedded in
the ICME sheath region.
Title: Temperature in Solar Sources of 3He-rich Solar
Energetic Particles and Relation to Ion Abundances
Authors: Bučík, Radoslav; Mulay, Sargam M.; Mason, Glenn M.; Nitta,
Nariaki V.; Desai, Mihir I.; Dayeh, Maher A.
Bibcode: 2021ApJ...908..243B
Altcode: 2020arXiv201215390B
3He-rich solar energetic particles (SEPs) are believed to
be accelerated in solar flares or jets by a mechanism that depends on
the ion charge-to-mass (Q/M) ratio. It implies that the flare plasma
characteristics (e.g., temperature) may be effective in determining the
elemental abundances of 3He-rich SEPs. This study examines
the relation between the suprathermal (≲0.2 MeV nucleon-1)
abundances of the He-Fe ions measured on the Advanced Composition
Explorer and temperature in the solar sources for 24 3He-rich
SEP events in the period 2010-2015. The differential emission measure
technique is applied to derive the temperature of the source regions
from the extreme ultraviolet imaging observations on the Solar Dynamics
Observatory. The obtained temperature distribution peaks at 2.0-2.5
MK that is surprisingly consistent with earlier findings based on in
situ elemental abundance or charge state measurements. We have found a
significant anticorrelation between 3He/4He ratio
and solar source temperature with a coefficient -0.6. It is most likely
caused by non-charge-stripping processes, as both isotopes would be
fully ionized in the inferred temperature range. This study shows that
the elemental ratios 4He/O, N/O, Ne/O, Si/O, S/O, Ca/O, Fe/O
generally behave with temperature as expected from abundance enhancement
calculations at ionization equilibrium. The C and Mg, the two species
with small changes in the Q/M ratio in the obtained temperature range,
show no such behavior with temperature and could be influenced by
similar processes as for the 3He/4He ratio.
Title: Understanding the Origin of Variable Compositions of Gradual
Solar Energetic Particle Events by Combining Observations and
Numerical Simulations
Authors: Nitta, Nariaki; Cohen, Christina; Jin, Meng
Bibcode: 2021cosp...43E.904N
Altcode:
One of the mysteries of solar energetic particle (SEP) events is the
compositional variability in those events that are clearly shock-related
and may be called gradual events. In particular, the reason for the
enhancement of Fe with respect to O or C at high energies has been
debated over the past two decades, and yet it is still unsettled. One
hypothesis relates the compositional variability with whether the
CME-driven shock is quasi-parallel or quasi-perpendicular near the Sun,
but this may not be easily tested using remote-sensing data alone. In
recent years, however, CME-driven shock waves have been modelled
by fitting shock-like features in EUV and white-light images with
relatively simple shapes, and in combination with magnetic field models,
it is possible to compute shock parameters at the shock surface. In
this presentation, we simulate a few CMEs whose associated SEP events
show widely different ratios of Fe/O, using the Alfven wave Solar Model
(AWSoM) that is part of the Space Weather Modeling Framework (SWMF). We
constrain the input parameters of the simulations so that the observed
pre-eruption corona, eruption and CME are well-reproduced. The shock
surface, across which the shock parameters are highly non-uniform,
is carefully traced, and the time-dependent connectivity of the shock
surface with the observers at multiple spacecraft is compared with
the SEP properties including composition. We discuss how much about
the compositional variability of SEP events can be learned with this
technique.
Title: The Elusive Origins of Magnetic Flux Ropes Observed in
Near-Earth Space
Authors: Nitta, Nariaki; Skov, Tamitha
Bibcode: 2021cosp...43E1032N
Altcode:
A subset of interplanetary coronal mass ejections (ICMEs) observed in
near-Earth space or at 1 AU from the Sun are characterized by smoothly
rotating magnetic field over large angles. They may be regarded as
magnetic flux ropes at least in the first approximation. They may cause
strong geomagnetic storms if they contain sustained southward magnetic
field. Such organized structures observed in situ may be thought to come
from eruptions that contain clearly identifiable flux ropes in the solar
corona. But this link between the ICME and the eruption in the corona
is often hard to establish. First, the CME responsible for the ICME
may not be unambiguously isolated. Second, even if the CME is found,
its low coronal signatures of the eruption may be far from clear, and
we do not know what magnetic structures in the corona may be involved
in the CME. These two aspects give major challenges in space weather
prediction. We report on a recent progress in our understanding of
stealthy CMEs, some of which result in flux-rope ICMEs, how common they
are in different phases of solar cycle, whether they can distinguish
themselves from normal CMEs, and where they may possibly come from,
on the basis of analyses of a wide range of data.
Title: Probing the Puzzle of Fermi Long-Duration Gamma-Ray Flares
by Data-driven Global MHD Simulations
Authors: Jin, Meng; Manchester, Ward, IV; Effenberger, Frederic;
Petrosian, Vahe; Nitta, Nariaki; Liu, Wei; Omodei, Nicola; Li, Gang;
Pesce-Rollins, Melissa; Allafort, Alice
Bibcode: 2021cosp...43E.906J
Altcode:
With the increasing number of long-duration gamma-ray solar flares
>100 MeV observed by Fermi/LAT, it poses a puzzle on the particle
acceleration and transport mechanisms. The recent detections of
behind-the-limb (BTL) solar flares (e.g., 2014 September 1 event),
in which the gamma-ray emission region is located away from the
BTL flare site by up to tens of degrees in heliographic longitude,
and on-disk flares with migration of gamma-ray emission centroid
hours past the impulsive phase (e.g., 2012 March 7 event), present
further new challenges on the theoretical models for interpreting the
observations. Since most of the long-duration events are associated
with fast CMEs, it is therefore intriguing to understand the role
of CMEs and CME-driven shocks in these events. To probe this puzzle,
we perform data-driven, global magnetohydrodynamics simulations of the
CMEs associated with the long-duration gamma-ray flares and investigate
the magnetic connectivity and evolution of the CME-driven shock, and
their relationship, in both space and time, with the observed gamma-ray
emission. Specifically, we derive and track the time-varying shock
parameters over the area that is magnetically connected to gamma-ray
emission region. Based on the modeling results, we discuss the causes
and implications of Fermi long-duration gamma-ray events, in the
framework of a potential shift of paradigm on particle acceleration
in solar flares and CMEs.
Title: Probing the Puzzle of Fermi Long-Duration Gamma-Ray Flares
by Data-driven Global MHD Simulations
Authors: Jin, M.; Petrosian, V.; Liu, W.; Nitta, N.; Omodei, N.;
Effenberger, F.; Li, G.; Pesce-Rollins, M.; Allafort, A.; Manchester,
W.
Bibcode: 2020AGUFMSH008..03J
Altcode:
With the ever growing number of long-duration, >100 MeV gamma-ray
solar flares observed by Fermi/LAT, it poses a puzzle on the underlying
particle acceleration and transport mechanisms. Further challenges
come from (i) recent detection of gamma-rays in behind-the-limb (BTL)
flares (e.g., the 2014 September 1 event), in which the gamma-ray
emission region is located away from the BTL flare site by tens of
degrees in heliographic longitude, and (ii) migration of gamma-ray
emission centroids on the solar disk hours past the impulsive phase
(e.g., the 2012 March 7 event). Most of the long-duration events are
associated with fast CMEs, it is thus necessary to understand the role
of CMEs and CME-driven shocks in these events. To probe this puzzle,
we perform data-driven, global magnetohydrodynamics simulations of CMEs
associated with the long-duration gamma-ray flares. We investigate
the magnetic connectivity and evolution of the CME-driven shocks,
and their relationship, in both space and time, with the observed
gamma-ray emission. Specifically, we derive and track the time-varying
shock parameters over the area that is magnetically connected to the
gamma-ray emission region. Based on the modeling results, we discuss
the causes of Fermi long-duration gamma-ray events. In particular, we
address the possibility of CME shock-accelerated particles traveling
back to the Sun to produce gamma-rays, a scenario that bears potentially
paradigm-shifting implications on particle acceleration and transport
in solar eruptive events including flares and CMEs.
Title: Tracking a CME and SIR to Earth and Mars during the deep
minimum of Solar Cycle 24
Authors: Palmerio, E.; Lee, C. O.; Richardson, I. G.; Nitta, N.;
Mays, M. L.; Halekas, J. S.; Zeitlin, C.; Luhmann, J. G.
Bibcode: 2020AGUFMSH0180001P
Altcode:
Interplanetary coronal mass ejections (ICMEs) and stream interaction
regions (SIRs) are large-scale solar wind structures that can often
be tracked at widely-separated locations in the heliosphere. Their
occurrence is strongly dependent on the phase of the solar cycle,
with ICMEs being more frequent during solar maximum and SIRs being
dominant during solar minimum. Nevertheless, a few ICMEs are present
during solar minimum, and coordinated studies of these ICMEs, from their
origin throughout their evolution in the heliosphere, are facilitated by
the simpler solar wind structure, a near absence of other transients,
and low levels of solar activity. We present a detailed analysis
of an ICME and SIR that were observed during the solar minimum between
Solar Cycles 24 and 25. To achieve this, we combine remote-sensing solar
data with in-situ measurements at Earth and Mars, which were separated
by ~10° in longitude at the time of the events under analysis. The
aim of this study is to determine how the two structures evolve and
interact as they travel away from the Sun, as well as to evaluate and
compare the space weather response from the same chain of events on
two different planets.
Title: Statistical Analysis of the Relation between Coronal Mass
Ejections and Solar Energetic Particles
Authors: Kihara, K.; Nitta, N.; Yashiro, S.; Asai, A.; Ichimoto, K.
Bibcode: 2020AGUFMSH012..02K
Altcode:
To improve the forecasting capability of impactful solar energetic
particle (SEP) events, the relation between coronal mass ejections
(CMEs) and SEP events needs to be better understood. Here we present
a statistical study of SEP occurrences and timescales with respect
to the CME source locations and speeds, considering all 257 fast
(vCME ≥ 900 km/s) and wide (angular width ≥ 60°) CMEs
that occurred between December 2006 and October 2017. We associate
them with SEP events at energies above 10 MeV. Examination of the
source region of each CME reveals that CMEs more often accompany a
SEP event if they originate from the longitude of E20-W100 relative
to the observer. However, a SEP event could still be absent if the
CME is < 2000 km/s. For the associated CME-SEP pairs, we compute
three timescales for each of the SEP events, following Kahler (2005,
2013); namely the timescale of the onset (TO), the rise time (TR),
and the duration (TD). They are correlated with the longitude of
the CME source region relative to the footpoint of the Parker spiral
(ΔΦ) and vCME. The TO tends to be short for |ΔΦ| <
60° . This trend is weaker for TR and TD. The SEP timescales are only
weakly correlated with vCME. Positive correlations of both
TR and TD with vCME are seen in poorly connected (large
|ΔΦ|) events. Additionally, TO appears to be negatively correlated
with vCME for events with small |ΔΦ|.
Title: Statistical Analysis of the Relation between Coronal Mass
Ejections and Solar Energetic Particles
Authors: Kihara, Kosuke; Huang, Yuwei; Nishimura, Nobuhiko; Nitta,
Nariaki V.; Yashiro, Seiji; Ichimoto, Kiyoshi; Asai, Ayumi
Bibcode: 2020ApJ...900...75K
Altcode: 2020arXiv200708062K
To improve the forecasting capability of impactful solar energetic
particle (SEP) events, the relation between coronal mass ejections
(CMEs) and SEP events needs to be better understood. Here we present
a statistical study of SEP occurrences and timescales with respect
to the CME source locations and speeds, considering all 257 fast
(vCME ≥ 900 km s-1) and wide (angular width
≥60°) CMEs that occurred between 2006 December and 2017 October. We
associate them with SEP events at energies above 10 MeV. Examination of
the source region of each CME reveals that CMEs more often accompany a
SEP event if they originate from the longitude of E20-W100 relative to
the observer. However, an SEP event could still be absent if the CME is
<2000 km s-1. For the associated CME-SEP pairs, we compute
three timescales for each of the SEP events, namely the timescale of
the onset (TO), the rise time (TR), and the duration (TD). They are
correlated with the longitude of the CME source region relative to
the footpoint of the Parker spiral (ΔΦ) and vCME. The
TO tends to be short for $| {\rm{\Delta }}{\rm{\Phi }}| \ \lt $
60°. This trend is weaker for TR and TD. The SEP timescales are only
weakly correlated with vCME. Positive correlations of both
TR and TD with vCME are seen in poorly connected (large $|
{\rm{\Delta }}{\rm{\Phi }}| $ ) events. Additionally, TO appears to
be negatively correlated with vCME for events with small $|
{\rm{\Delta }}{\rm{\Phi }}| $ .
Title: Understanding the Origin of Variable Compositions of Gradual
Solar Energetic Particle Events by Combining Observations and
Numerical Simulations
Authors: Nitta, Nariaki; Jin, Meng; Cohen, Christina
Bibcode: 2020EGUGA..2221333N
Altcode:
One of the mysteries of solar energetic particle (SEP) events is the
compositional variability in those events that are clearly shock-related
and may be called gradual events. In particular, the reason for the
enhancement of Fe with respect to O or C at high energies has been
debated over the past two decades, and yet it is still unsettled. One
hypothesis relates the compositional variability with whether the
CME-driven shock is quasi-parallel or quad-perpendicular near the Sun,
but this may not be easily tested using remote-sensing data alone. In
recent years, however, CME-driven shock waves have been modelled
by fitting shock-like features in EUV and white-light images with
relatively simple shapes, and in combination with magnetic field models,
ir is possible to compute shock parameters at the shock surface. In this
presentation, we simulate a few CMEs whose associated SEP events show
widely different Fe/O, using the Alfven wave Solar Model (AWSoM) that is
part of the Space Weather Modeling Framework (SWMF). We constrain the
input parameters of the simulations so that the observed pre-eruption
corona, eruption and CME are well-reproduced. The shock surface, across
which the shock parameters are highly non-uniform, is carefully traced,
and the time-dependent connectivity of the shock surface with the
observer at multiple spacecraft is compared with the SEP properties
including composition. We discuss how much about the compositional
variability of SEP events can be learned with this technique.
Title: Comprehensive Characterization of Solar Eruptions with Remote
and In-Situ Observations, and Modeling: The Major Solar Events on
4 November 2015
Authors: Cairns, Iver H.; Kozarev, Kamen A.; Nitta, Nariaki V.;
Agueda, Neus; Battarbee, Markus; Carley, Eoin P.; Dresing, Nina;
Gómez-Herrero, Raúl; Klein, Karl-Ludwig; Lario, David; Pomoell,
Jens; Salas-Matamoros, Carolina; Veronig, Astrid M.; Li, Bo; McCauley,
Patrick
Bibcode: 2020SoPh..295...32C
Altcode: 2019arXiv191003319C
Solar energetic particles (SEPs) are an important product of solar
activity. They are connected to solar active regions and flares,
coronal mass ejections (CMEs), EUV waves, shocks, Type II and III
radio emissions, and X-ray bursts. These phenomena are major probes
of the partition of energy in solar eruptions, as well as for the
organization, dynamics, and relaxation of coronal and interplanetary
magnetic fields. Many of these phenomena cause terrestrial space
weather, posing multiple hazards for humans and their technology
from space to the ground. Since particular flares, shocks, CMEs,
and EUV waves produce SEP events but others do not, since propagation
effects from the low corona to 1 AU appear important for some events
but not others, and since Type II and III radio emissions and X-ray
bursts are sometimes produced by energetic particles leaving these
acceleration sites, it is necessary to study the whole system with
a multi-frequency and multi-instrument perspective that combines
both in-situ and remote observations with detailed modeling of
phenomena. This article demonstrates this comprehensive approach
and shows its necessity by analyzing a trio of unusual and striking
solar eruptions, radio and X-ray bursts, and SEP events that occurred
on 4 November 2015. These events show both strong similarities and
differences from standard events and each other, despite having
very similar interplanetary conditions and only two flare sites and
CME genesis regions. They are therefore major targets for further
in-depth observational studies, and for testing both existing and
new theories and models. We present the complete suite of relevant
observations, complement them with initial modeling results for the
SEPs and interplanetary magnetic connectivity, and develop a plausible
scenario for the eruptions. Perhaps controversially, the SEPs appear to
be reasonably modelled and evidence points to significant non-Parker
magnetic fields. Based on the very limited modeling available, we
identify the aspects that are and are not understood, and we discuss
ideas that may lead to improved understanding of the SEP, radio,
and space-weather events.
Title: Spectral Structures of Type II Solar Radio Bursts and Solar
Energetic Particles
Authors: Iwai, Kazumasa; Yashiro, Seiji; Nitta, Nariaki V.; Kubo, Yûki
Bibcode: 2020ApJ...888...50I
Altcode: 2019arXiv191105897I
We investigated the relationship between the spectral structures of
type II solar radio bursts in the hectometric and kilometric wavelength
ranges and solar energetic particles (SEPs). To examine the statistical
relationship between type II bursts and SEPs, we selected 26 coronal
mass ejection (CME) events with similar characteristics (e.g., initial
speed, angular width, and location) observed by the Large Angle and
Spectrometric Coronagraph, regardless of the characteristics of the
corresponding type II bursts and the SEP flux. Then, we compared
associated type II bursts observed by the Radio and Plasma Wave
Experiment on board the Wind spacecraft and the SEP flux observed by
the Geostationary Operational Environmental Satellite orbiting around
the Earth. We found that the bandwidth of the hectometric type II
bursts and the peak flux of the SEPs has a positive correlation (with
a correlation coefficient of 0.64). This result supports the idea that
the nonthermal electrons of type II bursts and the nonthermal ions of
SEPs are generated by the same shock and suggests that more SEPs may be
generated for a wider or stronger CME shock with a longer duration. Our
result also suggests that considering the spectral structures of type
II bursts can improve the forecasting accuracy for the peak flux of
gradual SEPs.
Title: Coronal dimming as a proxy for stellar coronal mass ejections
Authors: Jin, M.; Cheung, M. C. M.; DeRosa, M. L.; Nitta, N. V.;
Schrijver, C. J.; France, K.; Kowalski, A.; Mason, J. P.; Osten, R.
Bibcode: 2020IAUS..354..426J
Altcode: 2020arXiv200206249J
Solar coronal dimmings have been observed extensively in the past
two decades and are believed to have close association with coronal
mass ejections (CMEs). Recent study found that coronal dimming
is the only signature that could differentiate powerful flares
that have CMEs from those that do not. Therefore, dimming might be
one of the best candidates to observe the stellar CMEs on distant
Sun-like stars. In this study, we investigate the possibility of using
coronal dimming as a proxy to diagnose stellar CMEs. By simulating a
realistic solar CME event and corresponding coronal dimming using a
global magnetohydrodynamics model (AWSoM: Alfvén-wave Solar Model),
we first demonstrate the capability of the model to reproduce solar
observations. We then extend the model for simulating stellar CMEs
by modifying the input magnetic flux density as well as the initial
magnetic energy of the CME flux rope. Our result suggests that with
improved instrument sensitivity, it is possible to detect the coronal
dimming signals induced by the stellar CMEs.
Title: Global Magnetohydrodynamics Simulation of EUV Waves and Shocks
from the X8.2 Eruptive Flare on 2017 September 10
Authors: Jin, M.; Liu, W.; Cheung, C. M. M.; Nitta, N.; DeRosa,
M. L.; Manchester, W.; Ofman, L.; Downs, C.; Petrosian, V.; Omodei,
N.; Moschou, S. P.; Sokolov, I.
Bibcode: 2019AGUFMSH32A..01J
Altcode:
As one of the largest flare-CME eruptions during solar cycle 24, the
2017 September 10 X8.2 flare event is associated with spectacular
global EUV waves that transverse almost the entire visible solar
disk, a CME with speed > 3000 km/s, which is one of the fastest
CMEs ever recorded, and >100 MeV Gamma-ray emission lasting for
more than 12 hours. All these unique observational features pose new
challenge on current numerical models to reproduce the multi-wavelength
observations. To take this challenge, we simulate the September 10
event using a global MHD model (AWSoM: Alfven Wave Solar Model) within
the Space Weather Modeling Framework and initiate CMEs by Gibson-Low
flux rope. We assess several important observed and physical inputs
(e.g., flux rope properties, polar magnetic field) in the model to
better reproduce the multi-wavelength observations. We find that the
simulated EUV wave morphology and kinematics are sensitive to the
orientation of the initial flux rope introduced to the source active
region. An orientation with the flux-rope axis in the north-south
direction produces the best match to the observations, which suggests
that EUV waves may potentially be used to constrain the flux-rope
geometry for such limb or behind-the-limb eruptions that lack good
magnetic field observations. By further combining with the white
light and radio observations, we demonstrate the flux rope-corona
interaction can greatly impact the early phase shock evolution (e.g.,
geometry and shock parameters) therefore plays a significant role
for particle acceleration near the Sun in this event. By propagating
the CMEs into the heliosphere and beyond the Earth and Mars orbits, we
compare the model results with the in-situ measurements and demonstrate
the importance of input polar magnetic field on the realistic CME
modeling therefore space weather forecasting.
Title: Stealthy Coronal Mass Ejections and High Speed Streams:
A Recipe for Surprisingly Geoeffective Events at 1 AU
Authors: Mulligan Skov, T.; Nitta, N.
Bibcode: 2019AGUFMSH42A..05M
Altcode:
One of the underlying problems in the investigation of CME genesis
and evolution is relating remote-sensing observations of coronal
mass ejections (CMEs) to in-situ observations of interplanetary CMEs
(ICMEs). Typically, remote-sensing observations of an eruption are
first observed in the low corona, followed by coronagraph observations
of the global structure of the CME projected onto the plane of the sky,
and then finally local, highly-quantitative measurements of an ICME are
made in situ along a spacecraft trajectory. However, the dramatic change
in solar activity in recent years has raised awareness of "stealth"
CMEs, which are CMEs that may or may not be observed in coronagraph
data, and typically are not observed in coronal images, especially
in disk view. These events often cause problem geomagnetic storms
because they go unnoticed until they hit Earth. Largely identified
during the deep minimum of cycle 23/24, stealth CMEs appear to be on
the rise. Since solar cycle 25 brings with it the possibility of yet
another low activity cycle, it is very likely that the number of stealth
CMEs will remain a significant fraction of ejecta. We investigate the
properties of stealth CMEs, paying special attention to their proximity
to coronal holes. We note the existence of mismatched polarity reversals
in the magnetic field and electron strahl measured in situ within ICMEs
associated with stealth CMEs and discuss the plausibility of interaction
with solar wind emanating from coronal holes as a key element of stealth
CME eruption. That stealth ICME arrival at Earth is often accompanied
by fast solar wind streams, which enhances their geoeffectiveness,
means that these events often cause strong geomagnetic storms at Earth
with little to no warning. Thus these near-invisible events represent
a large gap in our ability to provide accurate storm predictions and
have defined a new challenge when it comes to space weather forecasting.
Title: The Origin of Magnetic Flux Ropes Observed at 1 AU from the Sun
Authors: Nitta, N.; Skov, T. M.
Bibcode: 2019AGUFMSH32A..05N
Altcode:
It is not always easy to know the origin of geomagnetic
storms. Interplanetary coronal mass ejections (ICMEs) may be responsible
for most of non-recurrent geomagnetic storms. In particular, those
containing a flux rope could cause stronger geomagnetic disturbances,
depending on the occurrence of enhanced southward magnetic field. We
have known examples of such ICMEs that are not clearly connected to
low coronal signatures (LCSs). In order to find out how common this
phenomenon is, we have isolated in in situ data periods of enhanced
magnetic field that is seen to smoothly rotate with time, which can,
in the first approximation, indicate the presence of a flux rope. We
rank the difficulty of associating these flux rope ICMEs with possible
LCSs of the parent CME. When the CME is identified, we more often fail
to locate its source region without processing EUV images. Even after
image processing, it is sometimes difficult to find the CME source
region. To our surprise, some flux rope ICMEs may not be traced back
to a CME close to the Sun. We discuss these findings in terms of where
and how the eruption starts and evolves into an organized structure,
referring to recent simulation results. In some of our examples,
another explanation may be possioble for the apparent field rotation
found at 1 AU.
Title: Temperature in Solar Sources of 3He-rich SEP Events
Authors: Bucik, R.; Mulay, S. M.; Mason, G. M.; Nitta, N.
Bibcode: 2019AGUFMSH23C3339B
Altcode:
Impulsive (or 3He-rich) solar energetic particle (SEP) events show a
peculiar elemental composition, markedly different from solar corona or
solar wind, with several orders of magnitude enhanced abundances of 3He
and ultra-heavy ions. 3He-rich SEPs are believed to be accelerated in
solar flares or jets by a mechanism based on ions charge state. Thus,
the plasma characteristics (like electron temperature and density)
in the source flares may be effective in determining the abundances
in 3He-rich SEP events. This study examines the relation between
the temperature of the source plasma and the abundances of elements
for several 3He-rich SEP events measured by ULEIS on the Advanced
Composition Explorer. The differential emission measure technique is
applied to derive the temperature of the source regions observed by
AIA on board the Solar Dynamics Observatory. Preliminary results show
that 3He-rich SEP solar sources are relatively cool (median 1.6 MK)
compared to typical flare temperatures. This work greatly benefits
from the discussions at the ISSI Team meeting - Origins of 3He-rich
Solar Energetic Particles.
Title: Polar magnetic field in solar cycle 24
Authors: Nitta, Nariaki
Bibcode: 2019AAS...23440103N
Altcode:
Coronal holes often sit in Sun's polar regions, and they are believed
to be responsible for the fast solar wind. The magnetic field therein is
open to the heliosphere. It is possible that such field may emanate from
small patches with enhanced field strength, which were first revealed
in data from the Spectropolarimeter (SP) of the Hinode Solar Optical
Telescope. It has recently been shown that very similar patches can be
found in vector magnetograms from the Helioseismic and Magnetic Imager
(HMI) on the Solar Dynamics Observatory (SDO), even though SP has better
spatial resolution and vector magnetography. An advantage of SDO/HMI is
constant availability of full-disk vector data. This allows us to make
movies of radial field in polar regions with varying cadences, which can
be used to study the dynamical evolution of the patchy magnetic field
regions in high latitudes, such as poleward of 60 degrees, generally
difficult to observe from an ecliptic vantage point. We discuss the
general distribution of magnetic field in polar regions over solar
cycle 24 using such movies. The patches of enhanced radial field are
compared with certain types of solar activity such as coronal jets. We
emphasize the limitation of the existing data for studying polar field.
Title: 3He-rich Solar Energetic Particles from Sunspot Jets
Authors: Bučík, Radoslav; Wiedenbeck, Mark E.; Mason, Glenn M.;
Gómez-Herrero, Raúl; Nitta, Nariaki V.; Wang, Linghua
Bibcode: 2018ApJ...869L..21B
Altcode: 2018arXiv181207735B
Solar sources of suprathermal (<1 MeV nucleon-1)
3He-rich solar energetic particles (SEPs) have been
commonly associated with jets originating in small, compact active
regions at the periphery of near-equatorial coronal holes. Sources
of relatively rare, high-energy (>10 MeV nucleon-1)
3He-rich SEPs remain unexplored. Here we present two of the
most intense 3He-rich (3He/4He >
1) SEP events of the current solar cycle 24 measured on the Advanced
Composition Explorer at energy >10 MeV nucleon-1. Although
3He shows high intensities, Z > 2 ions are below the
detection threshold. The events are accompanied by type-III radio
bursts, but no type-II emission as typically seen for suprathermal
3He-rich SEPs. The corresponding solar sources were analyzed
using high-resolution, extreme-ultraviolet imaging and photospheric
magnetic field observations on the Solar Dynamics Observatory. We
find the sources of these events associated with jets originating
at the boundary of large sunspots with complex βγδ magnetic
configuration. Thus, details of the underlying photospheric field
apparently are important to produce 3He to high energies
in the examined events.
Title: Global Magnetohydrodynamics Simulation of EUV Waves and Shocks
from the X8.2 Eruptive Flare on 2017 September 10
Authors: Jin, Meng; Liu, Wei; Cheung, Mark; Nitta, Nariaki; Manchester,
Ward; Ofman, Leon; Downs, Cooper; Petrosian, Vahe; Omodei, Nicola
Bibcode: 2018csc..confE..66J
Altcode:
As one of the largest flare-CME eruptions during solar cycle 24, the
2017 September 10 X8.2 flare event is associated with spectacular
global EUV waves that transverse almost the entire visible solar
disk, a CME with speed > 3000 km/s, which is one of the fastest
CMEs ever recorded, and >100 MeV Gamma-ray emission lasting for
more than 12 hours. All these unique observational features pose new
challenge on current numerical models to reproduce the multi-wavelength
observations. To take this challenge, we simulate the September 10 event
using a global MHD model (AWSoM: Alfven Wave Solar Model) within the
Space Weather Modeling Framework and initiate CMEs by Gibson-Low flux
rope. We conduct detailed comparisons of the synthesized EUV images with
SDO/AIA observations of global EUV waves. We find that the simulated
EUV wave morphology and kinematics are sensitive to the orientation
of the initial flux rope introduced to the source active region. An
orientation with the flux-rope axis in the north-south direction
produces the best match to the observations, which suggests that EUV
waves may potentially be used to constrain the flux-rope geometry for
such limb or behind-the-limb eruptions that lack good magnetic field
observations. We also compare observed and simulated EUV intensities
in multiple AIA channels to perform thermal seismology of the global
corona. Furthermore, we track the 3D CME-driven shock surface in the
simulation and derive the time-varying shock parameters together with
the dynamic magnetic connectivity between the shock and the surface
of the Sun, with which we discuss the role of CME-driven shocks in
the long-duration Gamma-ray events.
Title: A Truly Global Extreme Ultraviolet Wave from the SOL2017-09-10
X8.2+ Solar Flare-Coronal Mass Ejection
Authors: Liu, Wei; Jin, Meng; Downs, Cooper; Ofman, Leon; Cheung,
Mark C. M.; Nitta, Nariaki V.
Bibcode: 2018csc..confE..40L
Altcode:
We report SDO/AIA observations of an extraordinary global extreme
ultraviolet (EUV) wave triggered by the X8.2+ flare-CME eruption on 2017
September 10. This was one of the best EUV waves ever observed with
modern instruments, yet it was likely the last one of such magnitudes
of Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable
characteristics include the following. (1) The wave was observed,
for the first time, to traverse the full-Sun corona over the entire
visible solar disk and off-limb circumference, manifesting a truly
global nature, owing to its exceptionally large amplitude, e.g., with
EUV enhancements by up to 300% at 1.1 Rsun from the eruption. (2)
This leads to strong transmissions (in addition to commonly observed
reflections) in and out of both polar coronal holes, which are usually
devoid of EUV waves. It has elevated wave speeds >2000 km/s within
them, consistent with the expected higher fast-mode magnetosonic wave
speeds. The coronal holes essentially serve as new "radiation centers"
for the waves being refracted out of them, which then travel toward the
equator and collide head-on, causing additional EUV enhancements. (3)
The wave produces significant compressional heating to local plasma
upon its impact, indicated by long-lasting EUV intensity changes and
differential emission measure increases at higher temperatures (e.g.,
log T=6.2) accompanied by decreases at lower temperatures (e.g.,
log T=6.0). These characteristics signify the potential of such EUV
waves for novel magnetic and thermal diagnostics of the solar corona
on global scales.
Title: Properties of Coronal Mass Ejections as Inferred from Coronal
Dimming
Authors: Nitta, Nariaki; Jin, Meng
Bibcode: 2018csc..confE..31N
Altcode:
Observations suggest that coronal dimming in solar EUV data
originates from evacuation of the coronal mass due to coronal
mass ejections (CMEs). Dimming may ultimately be used as one of
the most powerful tools to detect CMEs from stars. However, it is
at present not straightforward to make a quantitative link between
the two phenomena. In spatially-integrated EUV data from SDO/EVE,
dimming can be overshadowed by concomitant flares especially in those
lines sensitive to high temperatures. Here we study several energetic
CMEs from limb-occulted regions that accompanied significant dimming
but almost no flare emission in order to explore empirical relations
between the dimming parameters and CME properties. We try to understand
coronal dimming in spatially-integrated time series from EVE and AIA
with references to the development of dimming in EUV images from AIA
and STEREO EUVI and to the early evolution of CMEs in coronagraph data
from SOHO/LASCO and STEREO/COR1 and COR2. We show different patterns
of how dimming is observed with respect to the CME even though the
intense flare emission from low-lying loops is blocked. Emission measure
maps at different temperatures and numerical simulations of CMEs are
employed to discuss how to infer the properties of CMEs from coronal
dimming in spatially-integrated EUV data.
Title: Probing the Puzzle of Behind-the-limb γ-Ray Flares:
Data-driven Simulations of Magnetic Connectivity and CME-driven
Shock Evolution
Authors: Jin, Meng; Petrosian, Vahe; Liu, Wei; Nitta, Nariaki V.;
Omodei, Nicola; Rubio da Costa, Fatima; Effenberger, Frederic; Li,
Gang; Pesce-Rollins, Melissa; Allafort, Alice; Manchester, Ward, IV
Bibcode: 2018ApJ...867..122J
Altcode: 2018arXiv180701427J
Recent detections of high-energy γ-rays from behind-the-limb (BTL)
solar flares by the Fermi Gamma-ray Space Telescope pose a puzzle and
challenge on the particle acceleration and transport mechanisms. In
such events, the γ-ray emission region is located away from the BTL
flare site by up to tens of degrees in heliographic longitude. It
is thus hypothesized that particles are accelerated at the shock
driven by the coronal mass ejection (CME) and then travel from the
shock downstream back to the front side of the Sun to produce the
observed γ-rays. To test this scenario, we performed data-driven,
global magnetohydrodynamics simulations of the CME associated with a
well-observed BTL flare on 2014 September 1. We found that part of the
CME-driven shock develops magnetic connectivity with the γ-ray emission
region, facilitating transport of particles back to the Sun. Moreover,
the observed increase in γ-ray flux is temporally correlated with (1)
the increase of the shock compression ratio and (2) the presence of a
quasi-perpendicular shock over the area that is magnetically connected
to the γ-ray emitting region, both conditions favoring the diffusive
shock acceleration (DSA) of particles. These results support the above
hypothesis and can help resolve another puzzle, i.e., long-duration
(up to 20 hr) γ-rays flares. We suggest that, in addition to DSA,
stochastic acceleration by plasma turbulence may also play a role,
especially in the shock downstream region and during the early stage
when the shock Alfvén Mach number is small.
Title: A Truly Global Extreme Ultraviolet Wave from the SOL2017-09-10
X8.2+ Solar Flare-Coronal Mass Ejection
Authors: Liu, Wei; Jin, Meng; Downs, Cooper; Ofman, Leon; Cheung,
Mark C. M.; Nitta, Nariaki V.
Bibcode: 2018ApJ...864L..24L
Altcode: 2018arXiv180709847L
We report Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) observations of an extraordinary global extreme ultraviolet
(EUV) wave triggered by the X8.2+ flare-CME eruption on 2017 September
10. This was one of the best EUV waves ever observed with modern
instruments, yet it was likely the last one of such magnitudes of
Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable
characteristics include the following. (1) The wave was observed,
for the first time, to traverse the full-Sun corona over the entire
visible solar disk and off-limb circumference, manifesting a truly
global nature, owing to its exceptionally large amplitude, e.g.,
with EUV enhancements by up to 300% at 1.1 {R}⊙ from
the eruption. (2) This leads to strong transmissions (in addition to
commonly observed reflections) in and out of both polar coronal holes
(CHs), which are usually devoid of EUV waves. It has elevated wave
speeds >2000 {km} {{{s}}}-1 within the CHs, consistent
with the expected higher fast-mode magnetosonic wave speeds. The CHs
essentially serve as new “radiation centers” for the waves being
refracted out of them, which then travel toward the equator and collide
head-on, causing additional EUV enhancements. (3) The wave produces
significant compressional heating to local plasma upon its impact,
indicated by long-lasting EUV intensity changes and differential
emission measure increases at higher temperatures (e.g., {log}T=6.2)
accompanied by decreases at lower temperatures (e.g., {log}T=6.0). These
characteristics signify the potential of such EUV waves for novel
magnetic and thermal diagnostics of the solar corona on global scales.
Title: The origin of electrons in solar energetic particle events
and their relation to protons
Authors: Nitta, Nariaki; Gomez-Herrero, Raul; Wang, Linghua
Bibcode: 2018cosp...42E2456N
Altcode:
One of the observables that is used to distinguish gradual and
impulsive solar energetic particle (SEP) events is the electron to
proton ratio. The ratio is higher in impulsive SEP events that are
often enriched in 3He and/or heavy ions and are generally attributed to
magnetic reconnection in solar flares or jets. But electrons are always
present even in gradual SEP events that are due to acceleration at shock
waves driven out by coronal mass ejections (CMEs). A question arises as
to whether electrons in gradual SEP events have the same (CME-driven
shock) origin and injection as protons or they are still produced in
the flare site through magnetic reconnection. If CME-driven shocks are
responsible for both protons and electrons in gradual SEP events, are
they accelerated at the same or different portions of the shock? If not,
what is the relation between magnetic reconnection in flares and shock
waves driven by CMEs? We study electron events that have different
levels of similarities/proportionalities to and differences from
concurrent proton events in terms of the peak flux and onset and peak
times, using L1 and STEREO observations. We note the comparisons depend
on energy ranges both electrons and protons. The characteristics of
the source regions and associated flares and CMEs are investigated for
SEP events that have different relations between protons and electrons,
with additional information such as metric and decametric radio bursts.
Title: Global Magnetohydrodynamics Simulation of EUV Waves and Shocks
from the X8.2 Eruptive Flare on 2017 September 10
Authors: Jin, Meng; Liu, Wei; Cheung, Mark; Nitta, Nariaki; Manchester,
Ward; Ofman, Leon; Downs, Cooper; Petrosian, Vahe; Omodei, Nicola
Bibcode: 2018shin.confE.207J
Altcode:
As one of the largest flare-CME eruptions during solar cycle 24, the
2017 September 10 X8.2 flare event is associated with spectacular
global EUV waves that transverse almost the entire visible solar
disk, a CME with speed > 3000 km/s, which is one of the fastest
CMEs ever recorded, and >100 MeV Gamma-ray emission lasting for
more than 12 hours. All these unique observational features pose new
challenge on current numerical models to reproduce the multi-wavelength
observations. To take this challenge, we simulate the September 10 event
using a global MHD model (AWSoM: Alfven Wave Solar Model) within the
Space Weather Modeling Framework and initiate CMEs by Gibson-Low flux
rope. We conduct detailed comparisons of the synthesized EUV images with
SDO/AIA observations of global EUV waves. We find that the simulated
EUV wave morphology and kinematics are sensitive to the orientation
of the initial flux rope introduced to the source active region. An
orientation with the flux-rope axis in the north-south direction
produces the best match to the observations, which suggests that EUV
waves may potentially be used to constrain the flux-rope geometry for
such limb or behind-the-limb eruptions that lack good magnetic field
observations. We also compare observed and simulated EUV intensities
in multiple AIA channels to perform thermal seismology of the global
corona. Furthermore, we track the 3D CME-driven shock surface in the
simulation and derive the time-varying shock parameters together with
the dynamic magnetic connectivity between the shock and the surface
of the Sun, with which we discuss the role of CME-driven shocks in
the long-duration Gamma-ray events.
Title: High-Energy Aspects of Solar Flares: Recent Advances in
Observations and Models
Authors: Liu, Wei; Effenberger, Frederic; Petrosian, Vahe; Nitta,
Nariaki; Omodei, Nicola; Rubio da Costa, Fatima; Pesce-Rollins,
Melissa; Jin, Meng; Allafort, Alice
Bibcode: 2018cosp...42E2050L
Altcode:
Solar flares are one of the most spectacular manifestations of
solar activity with significant space-weather impacts. They also
serve as a unique laboratory for probing the underlying physics of
wide-ranging processes, including magnetic reconnection, generation
of plasma turbulence and shocks, particle acceleration, and plasma
heating. Solar flares thus bear important implications for physically
similar phenomena elsewhere in the universe, such as laboratory plasmas,
planetary magnetospheres, and flares on other stars or near compact
objects. In this presentation, we review recent observational and
modeling advances in solar flare research, with a focus on high-energy
aspects. Special attention will be paid to imaging and spectroscopic
observations, e.g., of long-duration and behind-the-limb gamma-ray
flares detected by Fermi, of coronal reconnection sites seen by
RHESSI, SDO, Hinode, and IRIS, and of the flaring lower atmosphere
(chromosphere and transition region) seen by IRIS. We will discuss
broad physical connections of flares to other phenomena on the Sun
(to a much greater extent than conventionally thought), such as the
correlation between quasi-periodic flare pulsations (seen from radio to
hard X-rays) and various MHD waves (some are directly imaged). We will
also review relevant modeling efforts to explain these observations,
focusing on a hybrid model of stochastic (by turbulence) plus shock
acceleration of particles, as well as combined kinetic and radiative
hydrodynamic simulations.
Title: Recent Progress in Understanding Earth-Affecting Solar
Eruptive Events
Authors: Nitta, Nariaki; Webb, David
Bibcode: 2018cosp...42E2455N
Altcode:
We report on our effort of the Campaign Events Working Group of ISEST
("International Study of Earth-affecting Solar Transients"), which
is one of the four elements of the SCOSTEP/VarSITI program. Our main
task is to integrate theory, simulations and observations to better
understand the chain of cause-effect activities from the Sun to Earth
for a handful of carefully selected events. The events in our list show
varied degrees of difficulty to understand the link between the solar
origin and the heliospheric consequence. At one end of the spectrum,
there are "textbook" events in which we can unambiguously associate the
solar eruption with the interplanetary CME (ICME). At another end of the
spectrum, there are "problem" events in which it is extremely difficult
to find the progenitor of the ICME at 1 AU, either as a CME close to the
Sun or as an eruption that is expected to leave low coronal signatures
(LCSs). In this talk, using representative examples, we summarize recent
progress in understanding the LCS-ICME link, and re-forecasting the
shock/CME arrival time and the magnitude of the geomagnetic storm. We
show how the progress has been made by coordinated analysis of new data
(especially from SDO and STEREO) and advanced numerical modeling.
Title: On the Relation of EUV Waves with Coronal Mass Ejections
Authors: Nitta, Nariaki
Bibcode: 2018shin.confE.139N
Altcode:
EUV waves are often associated with coronal mass ejections (CMEs). In
a widely supported scenario, a lateral over-expansion of the CME
bubble drives a fast-mode MHD wave, which may initially be a shock but
later propagates freely as the driver slows down. After the nearly
two-decade-long debate, there are still different ideas published
about the nature of EUV waves. For example, Howard and Pizzo (2016)
argued that many EUV waves are blast waves, not related to CMEs. Chen
(2016) still seems to stick to his old idea that the EUV wave reflects
the radial stretching of CME structures (Chen et al. 2002). I will
discuss how observations are consistent with different views about
EUV waves, on the basis of hundreds of examples observed by SDO/AIA
and STEREO/EUVI. I will also refresh the association of EUV waves with
other phenomena such as coronal dimming and type II radio bursts in the
context of their relations to CMEs and CME-driven shocks, and speculate
on a possible solar-cycle dependence of the manifestation of EUV waves.
Title: Suprathermal flare ions in gradual solar energetic particle
events: remnant and/or instantaneous?
Authors: Bucik, Radoslav; Park, Jinhye; Mason, Glenn M.; Nitta,
Nariaki V.; Wiedenbeck, Mark E.
Bibcode: 2018shin.confE..99B
Altcode:
Suprathermal ions from impulsive (or 3He-rich) solar energetic
particle (SEP) events have been thought to affect the size and
elemental abundances in gradual SEP events. There is a concept of the
remnant suprathermal ion population from previous flares residing in
the heliospheric reservoirs that are available for acceleration by
shocks in gradual SEP events. Interplanetary type III radio bursts
have been firmly associated with impulsive SEP events and thus may
serve as a proxy for flare suprathermals. We examine how prior type
III radio burst occurrence rate relates to the size and the abundance
variations in gradual SEP events. We further explore how suprathermal
ions for gradual SEP events may arise from an instantaneous activity
in the corona. With an unprecedented resolution of current imaging
observations, we can resolve details of flaring activity in gradual
SEP events as never before. For instance, coronal jets in gradual
events may indicate acceleration in magnetic reconnection on open
field lines. Furthermore, simultaneous jets in the remote places may
be visited by propagating coronal shocks or divergent coronal field
lines may facilitate access to the shocks.
Title: The Best and Last of Solar Cycle 24 - The Global EUV Wave from
the X8 Flare-CME Eruption on 2017-Sept-10: SDO/AIA Observations and
Data-constrained Simulations
Authors: Liu, Wei; Ofman, Leon; Nitta, Nariaki; Cheung, Mark; Downs,
Cooper; Jin, Meng
Bibcode: 2018cosp...42E2051L
Altcode:
Global extreme ultraviolet (EUV) waves are commonly associated with
coronal mass ejections (CMEs) and flares. One particular EUV wave
that was triggered by the X8 flare-CME eruption on 2017 September
10 was extraordinary - one of the best EUV waves ever observed with
modern instruments (e.g., SDO/AIA and GOES/SUVI), yet likely the last
one of such magnitudes in Cycle 24 as the Sun heads toward the solar
minimum. We present here detailed analysis of SDO/AIA observations
of this event and comparison with high-fidelity, data-constrained MHD
simulations using the University of Michigan Alfven Wave Solar Model
(AWSoM). Observational highlights include: (1) The EUV wave traverses
almost the entire visible solar disk and circumference, manifesting its
truly global nature. This vast range is mainly due to the exceptionally
large wave amplitude, with EUV intensity changes by up to a factor of
3 (as opposed to, e.g., 30% for moderate events). (2) The large wave
amplitude also leads to the novel detection of strong transmission
components (in addition to commonly observed reflections) into and
through both polar coronal holes, at elevated apparent wave speeds up
to 2600 km/s. (3) The wave also produces significant heating, indicated
by long-lasting EUV intensity changes. As such, this EUV wave offers
unique magnetic and thermal diagnostics of the global, CME-spawning
corona. Our MHD simulations have largely reproduced the observed
features. We find that the simulated EUV wave morphology and kinematics
are sensitive to the orientation of the initial flux rope introduced to
the host active region. An orientation with the flux-rope axis in the
north-south direction produces the best match to the observations. This
suggests that EUV waves may potentially be used to constrain the
flux-rope geometry for such limb or behind-the-limb eruptions, whose
source-region magnetic fields cannot be directly observed, and thus
offer useful implications for space-weather predictions.
Title: Surprise geoeffectiveness of stealthy coronal mass ejections
from the Sun to 1 AU
Authors: Skov, Tamitha; Nitta, Nariaki
Bibcode: 2018cosp...42E3167S
Altcode:
One of the underlying problems in the investigation of CME genesis
and evolution is relating remote- sensing observations of coronal
mass ejections (CMEs) to in-situ observations of interplanetary CMEs
(ICMEs). Typically, remote-sensing observations of an eruption are first
observed in the low corona, followed by coronagraph observations of the
global structure of the CME projected onto the plane of the sky, and
then finally local, highly-quantitative measurements of an ICME are made
in situ along a spacecraft trajectory. However, the dramatic change in
solar activity in recent years has raised awareness of "stealth" CMEs,
which are CMEs observed in coronagraph data but not in coronal images,
especially in disk view. Largely identified during the deep minimum
of cycle 23/24, stealth CMEs appear to be on the rise. Since solar
cycle 25 brings with it the possibility of yet another low activity
cycle, it is very likely that the number of stealth CMEs will remain a
significant fraction of ejecta. We investigate the properties of stealth
CMEs during the rise of solar cycle 24 through solar maximum, paying
special attention to their proximity to coronal holes. We investigate
the existence of mismatched polarity reversals in the magnetic field
and electron strahl measured in situ within ICMEs associated with
stealth CMEs. We discuss the plausibility of interaction with solar wind
emanating from coronal holes as a key element of stealth CME eruption
as evidenced by expanding coronal hole boundaries during eruption and
how continued interaction with the fast solar wind emanating from
these coronal holes during interplanetary transit influences the
geoeffectivness of the corresponding ICMEs at 1AU.
Title: Possible Scenario to Effectively Improve Space Weather
Predictions from Space-based Observations
Authors: Nitta, Nariaki; Hurlburt, Neal E.; Jin, Meng
Bibcode: 2018tess.conf41605N
Altcode:
We discuss how distributed remote-sensing and in situ observations
from space will alleviate the two factors below that pose severe
limitations on today's space weather forecasting capability. First, we
only have incomplete knowledge of the photospheric magnetic field that
has been used for computing the coronal magnetic field. This impacts
the capability of accurately modeling solar wind, which then makes it
hard to know how and when interplanetary coronal mass ejections (ICMEs)
affect the geo-space. A number of efforts have been made to correct
the polar field measurements, but we may not know its properties unless
we directly and routinely measure it clear of foreshortening. EUV and
microwave images suggest that the polar regions may be more dynamic than
usually assumed. At present we still need to wait for the technology
to mature that facilitates high-inclination heliocentric orbits, but
eventually we should place several small satellites with a compact
magnetograph in such orbits to constantly observe both poles. Second,
we have only limited understanding of the solar wind structures at
1 AU that directly cause geomagnetic disturbances. After the arrival
of the shock wave from a CME that occurred typically 2-3 days earlier,
we cannot predict how the solar wind will evolve during the next day or
two. This is because we characterize the solar wind structures on the
basis of single measurements, i.e. at L1. Multiple cubesats or small
satellites in sub-L1 orbits (covering ranges of distance from the Sun
and angle from the Sun-Earth line) will not only improve our nowcasting
capability but also advance our understanding of the structure of
the ICME and its interaction with solar wind, as shown in examples
of state-of-the-arts numerical simulations such as the University of
Michigan Alfven Wave Solar Model (AWSoM). This sub-L1 concept may be
easily executed even now, and it should represent an important first
step toward larger constellations that include a polar mission.
Title: Further Study of the Origin of Impulsive Solar Energetic
Particle Events in Solar Cycle 24
Authors: Nitta, Nariaki; Mason, Glenn M.; Cohen, Christina; Wiedenbeck,
Mark E.
Bibcode: 2018tess.conf40546N
Altcode:
Impulsive solar energetic particle (SEP) events are characterized
by enhancement of 3He and heavy ions (such as Fe), typically in
energies below a few MeV per nucleon. These ions will lose energy into
"suprathermal" ranges as they escape from the solar atmosphere. It is
possible that they provide seed particles for acceleration at the shock
wave driven by a subsequent coronal mass ejection (CME) that results
in a gradual SEP event. Nitta et al. (2015) searched for the solar
sources of 3He-rich SEP events in solar cycle 24 and found a few types
of transient activities in the corona, including jets. They limited
to 26 relatively stronger events with a clear discrete injection,
but there are a larger number of weaker impulsive SEP events. Some
of them lack a clear injection and last longer. In this study, using
remote-sensing data from SDO and STEREO, we try to trace the origins
of these 3He or Fe-rich periods as found in ACE and STEREO data, and
to examine the properties of the regions that produced the activities
likely responsible for compositional anomalies.
Title: Editorial: Earth-affecting Solar Transients
Authors: Zhang, Jie; Blanco-Cano, Xochitl; Nitta, Nariaki; Srivastava,
Nandita; Mandrini, Cristina H.
Bibcode: 2018SoPh..293...80Z
Altcode:
No abstract at ADS
Title: Global Magnetohydrodynamics Simulation of EUV Waves and Shocks
from the X8.2 Eruptive Flare on 2017 September 10
Authors: Jin, Meng; Liu, Wei; Cheung, Chun Ming Mark; Nitta, Nariaki;
Manchester, Ward; Ofman, Leon; Downs, Cooper; Petrosian, Vahe;
Omodei, Nicola
Bibcode: 2018tess.conf31905J
Altcode:
As one of the largest flare-CME eruptions during solar cycle 24, the
2017 September 10 X8.2 flare event is associated with spectacular
global EUV waves that transverse almost the entire visible solar
disk, a CME with speed > 3000 km/s, which is one of the fastest
CMEs ever recorded, and >100 MeV Gamma-ray emission lasting for
more than 12 hours. All these unique observational features pose new
challenge on current numerical models to reproduce the multi-wavelength
observations. To take this challenge, we simulate the September 10 event
using a global MHD model (AWSoM: Alfven Wave Solar Model) within the
Space Weather Modeling Framework and initiate CMEs by Gibson-Low flux
rope. We conduct detailed comparisons of the synthesized EUV images with
SDO/AIA observations of global EUV waves. We find that the simulated
EUV wave morphology and kinematics are sensitive to the orientation
of the initial flux rope introduced to the source active region. An
orientation with the flux-rope axis in the north-south direction
produces the best match to the observations, which suggests that EUV
waves may potentially be used to constrain the flux-rope geometry for
such limb or behind-the-limb eruptions that lack good magnetic field
observations. We also compare observed and simulated EUV intensities
in multiple AIA channels to perform thermal seismology of the global
corona. Furthermore, we track the 3D CME-driven shock surface in the
simulation and derive the time-varying shock parameters together with
the dynamic magnetic connectivity between the shock and the surface
of the Sun, with which we discuss the role of CME-driven shocks in
the long-duration Gamma-ray events.
Title: The Relation of Electron Events with EUV Waves Revisited for
Solar Cycle 24 Events
Authors: Nitta, Nariaki; Jian, Lan; Gomez-Herrero, Raul
Bibcode: 2018EGUGA..2012362N
Altcode:
Thanks to data from the Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory (SDO), we can characterize EUV waves
more thoroughly than before. They are correlated with SEP events
including electrons, especially with those that show wide longitudinal
distributions. We study the onset phase of electron events during
solar cycle 24 in comparison with EUV waves observed by AIA and STEREO
EUVI. Our interest is whether the EUV wave reaches the footpoint
of the field line that connects to the observer (connection point)
around the time at which electrons are injected. The connection point
is derived by the Parker spiral plus either the PFSS model or Predictive
Science's MHD model. In several cases, the wave may never arrives at the
connection point, or it may arrive either too early or too late. The
distance between the connection point and the wave front is measured
along a great circle at the particle injection. We investigate how the
3D structure of the EUV wave (as rendered using STEREO observations)
changes the timing relations. We check the consistency of the polarity
at the connection point and at 1 AU. Solar wind data are examined
to detect disturbances and discontinuities that may alter the Parker
spiral. We also discuss how adequate are the input synoptic maps for
global magnetic field extrapolations.
Title: 3He-rich Solar Energetic Particles in Helical Jets
on the Sun
Authors: Bučík, Radoslav; Innes, Davina E.; Mason, Glenn M.;
Wiedenbeck, Mark E.; Gómez-Herrero, Raúl; Nitta, Nariaki V.
Bibcode: 2018ApJ...852...76B
Altcode: 2017arXiv171109394B
Particle acceleration in stellar flares is ubiquitous in the universe;
however, our Sun is the only astrophysical object where energetic
particles and their source flares can both be observed. The acceleration
mechanism in solar flares, tremendously enhancing (up to a factor of
10,000) rare elements like 3He and ultra-heavy nuclei, has
been puzzling for almost 50 years. Here we present some of the most
intense 3He- and Fe-rich solar energetic particle events
ever reported. The events were accompanied by nonrelativistic electron
events and type-III radio bursts. The corresponding high-resolution,
extreme-ultraviolet imaging observations have revealed for the first
time a helical structure in the source flare with a jet-like shape. The
helical jets originated in relatively small, compact active regions,
located at the coronal-hole boundary. A mini-filament at the base of
the jet appears to trigger these events. The events were observed with
the two Solar Terrestrial Relations Observatories on the backside of
the Sun, during the period of increased solar activity in 2014. The
helical jets may be a distinct feature of these intense events that
is related to the production of high 3He and Fe enrichments.
Title: Earth-Affecting Coronal Mass Ejections Without Obvious Low
Coronal Signatures
Authors: Nitta, N.; Mulligan Skov, T.
Bibcode: 2017AGUFMSH51E..04N
Altcode:
We present a study of the origins of coronal mass ejections (CMEs)
that were not accompanied by obvious low coronal signatures (LCSs)
and yet were responsible for appreciable disturbances at 1 AU. These
CMEs characteristically start slowly. In several examples, EUV images
taken by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory reveal coronal dimming and a post-eruption arcade when we
make difference images with long enough temporal separations, which
are commensurate with the slow initial development of the CME. Data
from the EUV imager and COR coronagraphs of the Sun Earth Connection
Coronal and Heliospheric Investigation on the Solar Terrestrial
Relations Observatory, which provide the limb views of Earth-bound
CMEs, greatly help us limit the time interval in which the CME forms
and undergoes initial acceleration. For other CMEs, we find similar
dimming, although only with lower confidence as to its link to the
CME. It is noted that even these unclear events result in unambiguous
flux rope signatures in in situ data at 1 AU. There is a tendency that
the CME source regions are located near coronal holes or open field
regions. This may have implications for both the initiation of the
stealthy CME in the corona and its outcome in the heliosphere.
Title: Evidence for Alfvén Waves in Source Flares of Impulsive
Solar Energetic Particle Events
Authors: Bucik, R.; Innes, D.; Mason, G. M.; Wiedenbeck, M. E.;
Gomez-Herrero, R.; Nitta, N.
Bibcode: 2017AGUFMSH44B..05B
Altcode:
Impulsive solar energetic particle events, characterised by a
peculiar elemental composition with the rare elements like 3He and
ultra-heavy ions enhanced by factors up to ten thousand above their
thermal abundance, have been puzzling for almost 50 years. The solar
sources of these events have been commonly associated with coronal
jets, believed to be a signature of magnetic reconnection involving
field lines open to interplanetary space. Here we present some of the
most intense events, highly enriched in both 3He and heavier ions. The
corresponding high-resolution, extreme-ultraviolet imaging observations
have revealed for the first time a helical structure in the source
flare with a jet-like shape. A mini-filament at the base of the jet
appears to trigger these events. The events were observed with the two
Solar Terrestrial Relations Observatories on the backside of the Sun,
during the period of increased solar activity in 2014. During the last
decade, it has been established that the helical motions in coronal jets
represent propagating Alfvén waves. Revealing such magnetic-untwisting
waves in the solar sources of highly enriched events in this study is
consistent with a stochastic acceleration mechanism. An examination of
jets in previously reported impulsive solar energetic particle events
indicates that they tend to be large-scale blowout jets, sometimes
cleanly showing a twisted configuration.The work of R. Bucik is
supported by the Deutsche Forschungsgemeinschaft grant BU 3115/2-1.
Title: Data-driven Simulations of Magnetic Connectivity in
Behind-the-Limb Gamma-ray Flares and Associated Coronal Mass Ejections
Authors: Jin, M.; Petrosian, V.; Liu, W.; Nitta, N.; Omodei, N.;
Rubio da Costa, F.; Effenberger, F.; Li, G.; Pesce-Rollins, M.
Bibcode: 2017AGUFMSH44B..03J
Altcode:
Recent Fermi detection of high-energy gamma-ray emission from the
behind-the-limb (BTL) solar flares pose a puzzle on the particle
acceleration and transport mechanisms in such events. Due to
the large separation between the flare site and the location of
gamma-ray emission, it is believed that the associated coronal
mass ejections (CMEs) play an important role in accelerating and
subsequently transporting particles back to the Sun to produce obseved
gamma-rays. We explore this scenario by simulating the CME associated
with a well-observed flare on 2014 September 1 about 40 degrees behind
the east solar limb and by comparing the simulation and observational
results. We utilize a data-driven global magnetohydrodynamics model
(AWSoM: Alfven-wave Solar Model) to track the dynamical evolution
of the global magnetic field during the event and investigate the
magnetic connectivity between the CME/CME-driven shock and the Fermi
emission region. Moreover, we derive the time-varying shock parameters
(e.g., compression ratio, Alfven Mach number, and ThetaBN) over the
area that is magnetically connected to the visible solar disk where
Fermi gamma-ray emission originates. Our simulation shows that the
visible solar disk develops connections both to the flare site and to
the CME-driven shock during the eruption, which indicate that the CME's
interaction with the global solar corona is critical for understanding
such Fermi BTL events and gamma-ray flares in general. We discuss
the causes and implications of Fermi BTL events, in the framework
of a potential shift of paradigm on particle acceleration in solar
flares/CMEs.
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
using MHD Simulations and SDO Observations
Authors: Jin, M.; Cheung, C. M. M.; DeRosa, M. L.; Nitta, N.;
Schrijver, K.
Bibcode: 2017AGUFMSH41A2758J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings and determine their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-driven global
magnetohydrodynamics model (AWSoM: Alfven-wave Solar Model), we simulate
coronal dimmings resulting from different CME energetics and flux rope
configurations. We synthesize the emissions of different EUV spectral
bands/lines and compare with SDO/AIA and EVE observations. A detailed
analysis of simulation and observation data suggests that although
the transient dimming / brightening patterns could relate to plasma
heating processes (either by adiabatic compression or reconnection),
the long-lasting "core" and "remote" (also known as "secondary")
dimmings both originate from regions with open/quasi-open fields and
are caused by mass loss process. The mass loss in the remote dimming
region is induced by CME-driven shock. Using metrics such as dimming
depth, dimming slope, and recovery time, we investigate the relationship
between dimmings and CME properties (e.g., CME mass, CME speed) in the
simulation. Our result suggests that coronal dimmings encode important
information about CME energetics, CME-driven shock properties, and
magnetic configuration of erupting flux ropes. We also discuss how our
knowledge about solar coronal dimmings could be extended to the study
of stellar CMEs, which may prove important for exoplanet atmospheres
and habitability but which are currently not observable.
Title: Diagnosing the Heliospheric Magnetic Field with
Mildly-Relativistic Electrons
Authors: Nitta, N.; Jian, L. K.; Gomez-Herrero, R.
Bibcode: 2017AGUFMSH33C..06N
Altcode:
In solar energetic particle (SEP) events, electrons in the 30-500 keV
range are universal, accompanying both small impulsive (3He-rich)
and large gradual SEP events. They are often thought to propagate
scatter-free, and they can accordingly serve as a reliable tracer of the
magnetic field between the observer and the source region. In order to
evaluate how well the heliospheric magnetic field is modeled, we study
the onset time of these electrons with respect to the associated type
III radio burst in comparison with the distance between the source
region and the footpoint of the magnetic field that connects to the
observer, which is computed using simplified assumptions such as
the potential field source surface model inside the source surface
and the Parker spiral outside. This study includes many of the SEP
events observed at L1 and STEREO spacecraft after the SDO launch in
2010. We use EUV images from SDO/AIA and STEREO/SECCHI to find the
source region. In gradual SEP events that are attributed to shock waves
driven out by coronal mass ejections (CMEs), we examine the associated
EUV waves and identify as the source region the wave front that is
closest to the footpoint of the well-connected field line. This applies
to some of the events labeled impulsive. Three factors that may affect
the onset time are considered: release of particles at higher altitudes
that may be modeled by fitting the CME shock geometry in 3D, anisotropy
of electrons that may be linked to cross-field diffusion, and deviations
of the observed interplanetary magnetic field from the Parker spiral. We
discuss how limited our understanding of the heliospheric magnetic
field may be due to the lack of routine simultaneous observations of
magnetic field over the full sphere of the Sun including the both poles.
Title: Understanding Problem Forecasts of ISEST Campaign Flare-CME
Events
Authors: Webb, David; Nitta, Nariaki
Bibcode: 2017SoPh..292..142W
Altcode:
The goal of the International Study of Earth-affecting Solar Transients
(ISEST) project as part of the Variability of the Sun and Its
Terrestrial Impact (VarSITI) program is to understand the origin,
evolution, and propagation of solar transients through the space
between the Sun and Earth, and to improve our prediction capability
for space weather. A goal of ISEST Working Group 4 (Campaign Events)
is to study a set of well-observed Sun-to-Earth events to develop an
understanding of why some events are successfully forecast (textbook
cases), whereas others become problem or failed forecasts. In this
article we study six cases during the rise of Solar Cycle 24 that
highlight forecasting problems. Likely source coronal mass ejections
(CMEs) were identified in all six cases, but the related solar surface
activity ranged from uncertain or weak to X-class flares. The geoeffects
ranged from none to severe as in the two Sun-Earth events in 2015 that
caused severe storms. These events were chosen to illustrate some key
problems in understanding the chain from cause to geoeffect.
Title: Earth-Affecting Coronal Mass Ejections Without Obvious Low
Coronal Signatures
Authors: Nitta, Nariaki V.; Mulligan, Tamitha
Bibcode: 2017SoPh..292..125N
Altcode:
We present a study of the origin of coronal mass ejections (CMEs)
that were not accompanied by obvious low coronal signatures (LCSs)
and yet were responsible for appreciable disturbances at 1 AU. These
CMEs characteristically start slowly. In several examples, extreme
ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly
onboard the Solar Dynamics Observatory reveal coronal dimming and a
post-eruption arcade when we make difference images with long enough
temporal separations, which are commensurate with the slow initial
development of the CME. Data from the EUV imager and COR coronagraphs of
the Sun Earth Connection Coronal and Heliospheric Investigation onboard
the Solar Terrestrial Relations Observatory, which provide limb views
of Earth-bound CMEs, greatly help us limit the time interval in which
the CME forms and undergoes initial acceleration. For other CMEs, we
find similar dimming, although only with lower confidence as to its
link to the CME. It is noted that even these unclear events result in
unambiguous flux rope signatures in in situ data at 1 AU. There is a
tendency that the CME source regions are located near coronal holes or
open field regions. This may have implications for both the initiation
of the stealthy CME in the corona and its outcome in the heliosphere.
Title: Stealthy but Geoeffective Coronal Mass Ejections
Authors: Nitta, Nariaki; Mulligan, Tamitha
Bibcode: 2017SPD....4811407N
Altcode:
We have long known about the existence of "problem" geomagnetic storms
whose origins are elusive. In more general terms, not all the 1 AU
disturbances can be clearly attributed to coronal mass ejections
(CMEs), high speed streams (HSSs) or corotation interaction regions
(CIRs.) When interplanetary CME (ICME) signatures are found in in
situ data, there is not always a flare or filament eruption on the
Sun or even an obvious CME observed close to the Sun that correlates
with the ICME within a reasonable time range. These ICMEs sometimes
result in intense storms. Furthermore, there is a possibility that
some of the more severe storms could be partly contributed by such
ICMEs of unclear origin. Therefore space weather prediction will remain
incomplete without properly understanding these ICMEs. Even if the ICME
is paired with a CME, it is sometimes difficult to find where the latter
comes from. This is often called the “stealth CME” that apparently
lacks low coronal signatures (LCSs). STEREO's second and third view
points have tremendously helped us determine its front-side origin
and find when and where it forms and accelerates, which is important
for isolating possible LCSs. Although SDO/AIA has been continuously
taking full-disk EUV images in a wide temperature range since 2010,
there are still a number of stealthy CMEs whose LCSs are unclear or
ambiguous. It is assumed that they start at high altitudes, leaving
weak or negligible LCSs. Some of them seem to involve multiple magnetic
domains, and weak or open field regions. We present AIA observations
of several stealthy CMEs, including recent ones, that were responsible
for geomagnetic storms, emphasizing the need to compare images with
long time differences and to find the periods at which the CME forms
and accelerates. We also discuss uncertainties in interpreting in situ
data as to whether a CME is present when data are dominated by other
solar wind features, such as HSS and CIR.
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
using MHD Simulations and SDO Observations
Authors: Jin, Meng; Cheung, Mark; DeRosa, Marc L.; Nitta, Nariaki;
Schrijver, Karel
Bibcode: 2017SPD....4820602J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings and determine their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-driven
global magnetohydrodynamics model (AWSoM: Alfven-wave Solar Model), we
simulate coronal dimmings resulting from different CME energetics and
flux rope configurations. We synthesize the emissions of different EUV
spectral bands/lines and compare with SDO/AIA and EVE observations. A
detailed analysis of simulation and observation data suggests that the
“core” dimming is mainly caused by the mass loss from the CME, while
the “remote” dimming could have a different origin (e.g., plasma
heating). Moreover, the interaction between the erupting flux rope with
different orientations and the global solar corona could significantly
influence the coronal dimming patterns. Using metrics such as dimming
depth, dimming slope, and recovery time, we investigate the relationship
between dimmings and CME properties (e.g., CME mass, CME speed) in the
simulation. Our result suggests that coronal dimmings encode important
information about CMEs. We also discuss how our knowledge about solar
coronal dimmings could be extended to the study of stellar CMEs.
Title: Study of EUV Waves and Type II Bursts in Relation to Energetic
Particles in Space
Authors: Nitta, Nariaki
Bibcode: 2017shin.confE.119N
Altcode:
Both EUV waves and metric type II bursts are occasionally thought
to signify CME-driven shocks in their early stages. In order to
better understand shock acceleration close to the Sun, we study the
two phenomena both statistically and in detail as individual events,
using data from SDO/AIA and USAF/RSTN. They are typically not observed
before the peak of the soft X-ray flux of the associated flare - by
then the impulsive phase is largely over. The association rates are
50-70% in both directions, depending on how they are defined. There
is a tendency that fast EUV waves are associated with a type II burst,
but a number of fast EUV waves lack a type II burst and several type II
bursts do not accompany an EUV wave. The relation of the two phenomena
may hold important information as to what contributions magnetic
reconnection may make in triggering a CME, that is, the yet unsettled
relationship between flares and CMEs. We present an updated study of
EUV waves and type II bursts, and their relation to flare-associated
ejections and coronal dimming. Even if particles are accelerated in
CME-driven shocks close to the Sun, an additional condition seems to be
needed for particles to be observed in the interplanetary space. That
is the presence of open field lines (revealed in DH type III bursts)
in the regions strongly compressed as diagnosed with EUV waves.
Title: The Relationship between Stealth CMEs and Coronal Holes:
Origin, Interaction, and Geoeffectiveness
Authors: Mulligan Skov, T.; Nitta, N.
Bibcode: 2016AGUFMSH13B2294M
Altcode:
The current solar cycle is unique in that levels of activity are the
lowest for any cycle since the space age in terms of a shortage
of strong geomagnetic storms and high-energy particle events
(e.g. ground-level events (GLEs)). This low level of activity has
brought about unique challenges. One of these is the rise in "stealth
CMEs," which are CMEs observed in coronagraph data but not in coronal
images, especially in disk view. These events were identified during the
deep minimum of cycle 23/24 and appear to be increasing. Since solar
cycle 25 brings with it the possibility of yet another low activity
cycle, it is very likely that the number of stealth CMEs will remain a
significant fraction of ejecta. Thus a better understanding of their
eruption mechanisms, the timescale over which on-disk observations
must be made for unambiguous identification, and a reliable association
with ICME events at 1 AU are of paramount importance to space weather
prediction in the coming cycle. It is not yet clear if stealth CMEs
represent a new class of CMEs that cannot not be explained in terms
of standard models. However, they cannot be ignored. Under certain
circumstances these events have been known to drive large geomagnetic
storms. We investigate the existence of mismatched polarity reversals
in the magnetic field and electron strahl measured in situ within ICMEs
associated with stealth CMEs. We discuss the plausibility of interaction
with solar wind emanating from coronal holes as a key element of stealth
CME eruption as evidenced by expanding coronal hole boundaries during
eruption and the presence of interchange reconnection within ICMEs.
Title: Coronal Disturbances and the Early Evolution of SEP Events
Authors: Nitta, N.; Jian, L.; Gomez-Herrero, R.
Bibcode: 2016AGUFMSH41B2532N
Altcode:
Many solar energetic particle (SEP) events have been observed at
widely separate heliographic longitudes during solar cycle 24,
on the basis of data taken around the Sun-Earth line and at twin
STEREO spacecraft. Earlier studies of this phenomenon emphasized the
possible role of coronal disturbances in the form of EUV waves in the
acceleration and transport of the particles in the low corona. However,
more recent studies tend to downplay EUV waves because they are first
of all not seen to propagate to the footpoints of the magnetic field
lines that connect to the observer. It is therefore important to model
coronal disturbances in 3D by combining EUV images and coronagraph data,
instead of relying only on EUV waves, which are often considered to
be the low-altitude manifestations of shock waves driven by coronal
mass ejections (CMEs). We note a wide variety of spatial and temporal
characteristics of SEP events in their early evolution even though
the associated coronal disturbances appear to be similar. In extreme
cases, we see no SEPs even from strong eruptions that occur around
well-connected longitudes, accompanying type II bursts and EUV waves and
resulting in fast CMEs. We give a correlative study of a large number of
solar eruptions that produced fast CMEs/type II bursts/EUV waves with
or without SEP events and discuss the properties of the eruptions that
could influence particle acceleration and transport. We also discuss
the caveat of using standard magnetic field extrapolation and field
line tracing in the studies of the spatial and temporal behaviors of
SEP events.
Title: Timing signatures of solar flares
Authors: Balasubramaniam, K. S.; Lynch, C.; Henry, T.; Nitta, N.;
Hurlburt, N. E.; Slater, G. L.
Bibcode: 2016AGUFMSH43E..02B
Altcode:
We compare the timing signatures of solar flares observed with the GOES
X-ray and the SDO/AIA instruments between the years 2010-2015. From
this comparison we find that: (i) the rise-time of flares (time
difference from the background to peak) is inversely correlated with
the solar cycle, i.e. longer lasting rise times occur during the
solar minimum. This implies that a higher thermal state of the outer
solar atmosphere, during solar maximum, is far more receptive to being
heated than during a solar minimum. (ii) From an analysis of rise-times,
statistically, we find that 171 A appears to detect the earliest flares,
providing clues to fact that this might be layer where reconnections
are first triggered. We discuss the implications of these and other
statistical results in terms of forecasting of solar flares.
Title: Observations of solar X-ray and EUV jets and their related
phenomena
Authors: Innes, D. E.; Bučík, R.; Guo, L. -J.; Nitta, N.
Bibcode: 2016AN....337.1024I
Altcode: 2016arXiv160303258I
Solar jets are fast-moving, elongated brightenings related to
ejections seen in both images and spectra on all scales from barely
visible chromospheric jets to coronal jets extending up to a few
solar radii. The largest, most powerful jets are the source of type
III radio bursts, energetic electrons and ions with greatly enhanced
3He and heavy element abundances. The frequent coronal jets
from polar and equatorial coronal holes may contribute to the solar
wind. The primary acceleration mechanism for all jets is believed to
be release of magnetic stress via reconnection; however the energy
buildup depends on the jets' source environment. In this review,
we discuss how certain features of X-ray and EUV jets, such as their
repetition rate and association with radio emission, depends on their
underlying photospheric field configurations (active regions, polar
and equatorial coronal holes, and quiet Sun).
Title: Stealth CMEs and Stealthy Geomagnetic Storms
Authors: Nitta, Nariaki; Mulligan, Tamitha
Bibcode: 2016usc..confE..63N
Altcode:
We sometimes encounter coronal mass ejections (CMEs) whose low
coronal signatures are apparently missing, especially when viewed on
the disk. They are called stealth CMEs, which are usually slow and
diffuse. Some of them result in medium geomagnetic storms. Similarly,
there are orphan interplanetary CMEs (ICMEs) that lack a parent CME
in coronagraph data but could cause geomagnetic storms when strong
and sustained southward magnetic field is present. In addition,
some geomagnetic storms may be attributable to a fast solar wind
and stream interaction region, but it is often hard to rule out a
trace of ICME (coming from a solar eruption) in in situ data. These
events present a major challenge not only in heliophysics research but
also in space weather prediction. We summarize our recent attempt to
understand the origins of stealth CMEs and stealthy geomagnetic storms,
making extensive use of SDO/AIA data in comparison with SOHO/LASCO
and STEREO/EUVI/COR data. In situ data from Wind, ACE and STEREO are
also examined. We discuss the relations of these events with coronal
holes and sector boundaries.
Title: The Riddles of the 2011 November 3 Circumsolar SEP Event and
Limited Knowledge of the Coronal Magnetic Field
Authors: Nitta, Nariaki; Jian, Lan; Gomez-Herrero, Raul
Bibcode: 2016shin.confE..25N
Altcode:
The 2011 November 3 SEP event was observed at all the three locations of
STEREO A and B and L1 within 40 minutes of the onset of the eruption
that is thought to be the single cause (Gomez-Herrero et al. 2015). The
CME was fast, but not extremely fast ( 1000 km/s), and the associated
flare was estimated to be only X1. The source region at E152 was
best connected to STEREO-A, but was poorly connected to STEREO-B and
L1. AIA/EUVI images suggest that the EUV wave from the eruption did not
reach the footpoints of magnetic field lines from STEREO-B or L1. We
suggest instead that particles were released as the coronal disturbances
in the form of an EUV wave intersected field lines from the south
polar region. We discuss how such a scenario could be possible, given
the level of our knowledge of the coronal magnetic field at present.
Title: a Numerical Study of Long-Range Magnetic Impacts during
Coronal Mass Ejections
Authors: Jin, Meng; Schrijver, Karel; Cheung, Mark; DeRosa, Marc;
Nitta, Nariaki; Title, Alan
Bibcode: 2016shin.confE..38J
Altcode:
With the global view and high-cadence observations from SDO/AIA and
STEREO, many spatially separated solar eruptive events appear to be
coupled. However, the mechanisms for 'sympathetic' events are still
largely unknown. In this study, we investigate the impact of an erupting
flux rope on surrounding solar structures through large-scale magnetic
coupling. We build a realistic environment of the solar corona on
2011 February 15 using a global magnetohydrodynamics (MHD) model and
initiate coronal mass ejections (CMEs) in active region (AR) 11158
by inserting Gibson-Low analytical flux ropes. We show that a CME's
impact on the surrounding structures depends not only on the magnetic
strength of these structures and their distance to the source region,
but also on the interaction between the CME with the large-scale
magnetic field. Within the CME expansion domain where the flux rope
field directly interacts with the solar structures, expansion-induced
reconnection often modifies the overlying field, thereby increasing
the decay index. This effect may provide a primary coupling mechanism
underlying the sympathetic eruptions. The magnitude of the impact
is found to depend on the orientation of the erupting flux rope,
with the largest impacts occurring when the flux rope is favorably
oriented for reconnecting with the surrounding regions. Outside the
CME expansion domain, the influence of the CME is mainly through field
line compression or post-eruption relaxation. Based on our numerical
experiments, we discuss a way to quantify the eruption impact, which
could be useful for forecasting purposes.
Title: Evidence for a Common Acceleration Mechanism for Enrichments
of 3He and Heavy Ions in Impulsive SEP Events
Authors: Mason, Glenn M.; Nitta, Nariaki V.; Wiedenbeck, Mark E.;
Innes, Davina E.
Bibcode: 2016ApJ...823..138M
Altcode:
We have surveyed the period 1997-2015 for a rare type of
3He-rich solar energetic particle (SEP) event, with
enormously enhanced values of the S/O ratio, that differs from the
majority of 3He-rich events, which show enhancements of
heavy ions increasing smoothly with mass. Sixteen events were found,
most of them small but with solar source characteristics similar to
other 3He-rich SEP events. A single event on 2014 May 16
had higher intensities than the others, and curved Si and S spectra
that crossed the O spectrum above ∼200 keV nucleon-1. Such
crossings of heavy-ion spectra have never previously been reported. The
dual enhancement of Si and S suggests that element Q/M ratio is
critical to the enhancement since this pair of elements uniquely has
very similar Q/M ratios over a wide range of temperatures. Besides
3He, Si, and S, in this same event the C, N, and Fe spectra
also showed curved shape and enhanced abundances compared to O. The
spectral similarities suggest that all have been produced from the
same mechanism that enhances 3He. The enhancements are
large only in the high-energy portion of the spectrum, and so affect
only a small fraction of the ions. The observations suggest that the
accelerated plasma was initially cool (∼0.4 MK) and was then heated
to a few million kelvin to generate the preferred Q/M ratio in the
range C-Fe. The temperature profile may be the distinct feature of
these events that produces the unusual abundance signature.
Title: Comparing Dynamics in Eruptive and Non-Eruptive Flares
Authors: Nitta, Nariaki; Tarbell, Theodore D.; Slater, Gregory L.;
Frank, Zoe Anne
Bibcode: 2016SPD....47.0620N
Altcode:
Close comparison of EUV and coronagraph data suggests that there may
not be clear distinction between eruptive and non-eruptive flares as
far as the coronal and chromospheric signatures are concerned. Here we
define eruptive and non-eruptive flares in terms of the presence and
absence of the associated coronal mass ejection (CME). We have studied
several flares in both categories using Hinode/SOT and IRIS data. The
pointing of the Hinode/SOT data has been updated by correlating
them with AIA 1700 A images. We show our initial results about how
the flare development compares in eruptive and non-eruptive flares,
including the reconnection rate as derived from the magnetic field
swept over by flare ribbons (in SOT Ca images), and the line-of-sight
velocities at different locations and temperatures (in IRIS spectral
data). We also discuss large-scale disturbances and related CMEs in
SDO/AIA and SOHO/LASCO data as context information.
Title: A Numerical Study of Long-range Magnetic Impacts during
Coronal Mass Ejections
Authors: Jin, M.; Schrijver, C. J.; Cheung, M. C. M.; DeRosa, M. L.;
Nitta, N. V.; Title, A. M.
Bibcode: 2016ApJ...820...16J
Altcode: 2016arXiv160304900J
With the global view and high-cadence observations from Solar Dynamics
Observatory/Atmospheric Imaging Assembly and Solar TErrestrial RElations
Observatory, many spatially separated solar eruptive events appear
to be coupled. However, the mechanisms for “sympathetic” events
are still largely unknown. In this study, we investigate the impact
of an erupting flux rope on surrounding solar structures through
large-scale magnetic coupling. We build a realistic environment of the
solar corona on 2011 February 15 using a global magnetohydrodynamics
model and initiate coronal mass ejections (CMEs) in active region
11158 by inserting Gibson-Low analytical flux ropes. We show that a
CME’s impact on the surrounding structures depends not only on the
magnetic strength of these structures and their distance to the source
region, but also on the interaction between the CME and the large-scale
magnetic field. Within the CME expansion domain where the flux rope
field directly interacts with the solar structures, expansion-induced
reconnection often modifies the overlying field, thereby increasing
the decay index. This effect may provide a primary coupling mechanism
underlying the sympathetic eruptions. The magnitude of the impact
is found to depend on the orientation of the erupting flux rope,
with the largest impacts occurring when the flux rope is favorably
oriented for reconnecting with the surrounding regions. Outside the
CME expansion domain, the influence of the CME is mainly through field
line compression or post-eruption relaxation. Based on our numerical
experiments, we discuss a way to quantify the eruption impact, which
could be useful for forecasting purposes.
Title: Coronal Waves and Solar Energetic Particle Events Observed
at Widely Separate Locations
Authors: Nitta, N.; Jian, L.; Gomez-Herrero, R.
Bibcode: 2015AGUFMSH33B2466N
Altcode:
During solar cycle 24, thanks largely to the Solar Terrestrial
Relations Observatory (STEREO), many solar energetic particle
(SEP) events have been observed at widely separate locations in the
heliosphere, even including impulsive events that are usually assumed
to reflect localized acceleration and injection. It is found that
many of these wide SEP events accompany coronal waves that typically
appear in extreme-ultraviolet (EUV) images. The EUV wave phenomenon
has been observed much more closely than before by the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory that
continuously produces full-disk EUV images with unprecedentedly fast
cadence and high sensitivity in multiple wavelength bands covering
a broad temperature range. This is complemented by the EUV Imager
on STEREO that traces the wave front into regions inaccessible from
Earth. Several authors have attempted to explain wide SEP events in
terms of EUV waves, especially comparing the SEP release times with
how and when the EUV wave fronts traverse the magnetic footprints of
the locations of SEPs. They have come to mixed results. The primary
reason for the mixed results may be that they tend to overlook or
underestimate the uncertainties inherent in the works. For example,
how well do we model magnetic field connection in the corona and
heliosphere? Do we adequately take into account the evolving solar
wind conditions? Here we study a number of SEP events with various
angular spreads in comparison with newly analyzed EUV waves. We
discuss the importance of including the above-mentioned uncertainties
as well as understanding EUV waves as part of the 3d propagation of
CME-driven shock waves into the coronagraph fields of view. Without
these approaches, it may remain ambiguous how much of the angular
spread of SEP events is attributable to coronal shock waves.
Title: The Role of Large-scale Magnetic Coupling for Solar Corona
Sympathy
Authors: Jin, M.; Schrijver, K.; Cheung, C. M. M.; DeRosa, M. L.;
Nitta, N.; Title, A. M.
Bibcode: 2015AGUFMSH23A2425J
Altcode:
With the comprehensive view and high cadence observations from
SDO/AIA and STEREO in solar cycle 24, a large number of spatially
separated solar eruptive events are found to be coupled. However,
compared with the established initiation mechanisms for "isolated"
events, the mechanisms for "sympathetic" events are still largely
unknown, and nascent theories are untested. In this study, we build
a realistic environment of solar corona on 2011 February 15 using
a global MHD model and investigate how an eruption can impact the
surrounding solar structures. Our result shows that the solar eruption's
impact on the different structures can be quite different. Within
the CME expansion domain, it is possible to trigger an eruption by
overlaying field removal through expansion induced reconnection. The
magnitude of impact is found to be dependent on the orientation
of the erupting flux rope. Outside the CME expansion domain, the
post-eruption reconfiguration could play an important role for solar
sympathy. Based on the modeling results, we discuss the possibility of
using observable/estimable parameters to quantify the eruption impact
therefore providing an useful parameter for forecasting sympathy.
Title: Linking Stealthy Signatures of Coronal Mass Ejections at the
Sun to 1 AU
Authors: Mulligan, T.; Nitta, N.
Bibcode: 2015AGUFMSH33D..02M
Altcode:
One of the underlying problems in the investigation of CME genesis
and evolution is relating remote- sensing observations of coronal
mass ejections (CMEs) to in-situ observations of interplanetary CMEs
(ICMEs). Typically, remote-sensing observations of an eruption are first
observed in the low corona, followed by coronagraph observations of the
global structure of the CME projected onto the plane of the sky, and
then finally local, highly-quantitative measurements of an ICME are made
in situ along a spacecraft trajectory. However, the dramatic change in
solar activity in recent years has raised awareness of "stealth" CMEs,
which are CMEs observed in coronagraph data but not in coronal images,
especially in disk view. Largely identified during the deep minimum
of cycle 23/24, stealth CMEs appear to be on the rise. Since solar
cycle 25 brings with it the possibility of yet another low activity
cycle, it is very likely that the number of stealth CMEs will remain
a significant fraction of ejecta. We investigate the properties of
stealth CMEs during the rise of solar cycle 24 and through the current
solar maximum, paying special attention to their proximity to coronal
holes. We investigate the existence of mismatched polarity reversals
in the magnetic field and electron strahl measured in situ within ICMEs
associated with stealth CMEs. We discuss the plausibility of interaction
with solar wind emanating from coronal holes as a key element of stealth
CME eruption as evidenced by expanding coronal hole boundaries during
eruption and the presence of interchange reconnection within ICMEs.
Title: Low Solar Wind Density Causing the Fast Coronal Mass Ejection
from 23 July 2012
Authors: Nitta, N.; Temmer, M.
Bibcode: 2015AGUFMSH53A2458N
Altcode:
The fast coronal mass ejection (CME) from July 23, 2012 raised special
attention due to its short propagation time of less than 21hrs from
Sun to 1 AU. In-situ data from STEREO-A revealed the arrival of a
fast forward shock having a velocity of more than 2200 km/s followed
by a magnetic structure with a speed of almost 1900 km/s. We study the
evolution of the CME in interplanetary (IP) space using the drag based
model to reproduce the short propagation time and high impact speed
as derived from in-situ data. We find that the ambient density must
have been exceptionally low due to which the drag force is reduced
such that the CME experienced almost no deceleration. The density is
found to be rather low due to the weak solar activity and was lowered
even more by a previous CME event.
Title: Comparison between the eruptive X2.2 flare on 2011 February
15 and confined X3.1 flare on 2014 October 24
Authors: Jing, Ju; Xu, Yan; Lee, Jeongwoo; Nitta, Nariaki V.; Liu,
Chang; Park, Sung-Hong; Wiegelmann, Thomas; Wang, Haimin
Bibcode: 2015RAA....15.1537J
Altcode:
We compare two contrasting X-class flares in terms of magnetic free
energy, relative magnetic helicity and decay index of the active regions
(ARs) in which they occurred. The events in question are the eruptive
X2.2 flare from AR 11158 accompanied by a halo coronal mass ejection
(CME) and the confined X3.1 flare from AR 12192 with no associated
CME. These two flares exhibit similar behavior of free magnetic energy
and helicity buildup for a few days preceding them. A major difference
between the two flares is found to lie in the time-dependent change
of magnetic helicity of the ARs that hosted them. AR 11158 shows a
significant decrease in magnetic helicity starting ∼4 hours prior
to the flare, but no apparent decrease in helicity is observed in AR
12192. By examining the magnetic helicity injection rates in terms
of sign, we confirmed that the drastic decrease in magnetic helicity
before the eruptive X2.2 flare was not caused by the injection of
reversed helicity through the photosphere but rather the CME-related
change in the coronal magnetic field. Another major difference we find
is that AR 11158 had a significantly larger decay index and therefore
weaker overlying field than AR 12192. These results suggest that the
coronal magnetic helicity and the decay index of the overlying field
can provide a clue about the occurrence of CMEs.
Title: Constraints on Mechanisms for Longitudinal Spreading of
Impulsive SEPs from Multispacecraft Observations of Scatter-free Even
Authors: Wiedenbeck, M.; Cohen, C.; Klassen, A.; Leske, R.; Liewer,
P.; Mason, G.; Nitta, N.
Bibcode: 2015ICRC...34..106W
Altcode: 2015PoS...236..106W
No abstract at ADS
Title: Solar Sources of 3He-rich Solar Energetic Particle
Events in Solar Cycle 24
Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wang, Linghua; Cohen,
Christina M. S.; Wiedenbeck, Mark E.
Bibcode: 2015ApJ...806..235N
Altcode: 2015arXiv150506804N
Using high-cadence EUV images obtained by the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory, we investigate
the solar sources of 26 3He-rich solar energetic particle
events at ≲1 MeV nucleon-1 that were well-observed by the
Advanced Composition Explorer during solar cycle 24. Identification of
the solar sources is based on the association of 3He-rich
events with type III radio bursts and electron events as observed by
Wind. The source locations are further verified in EUV images from the
Solar and Terrestrial Relations Observatory, which provides information
on solar activities in the regions not visible from the Earth. Based on
AIA observations, 3He-rich events are not only associated
with coronal jets as emphasized in solar cycle 23 studies, but
also with more spatially extended eruptions. The properties of the
3He-rich events do not appear to be strongly correlated
with those of the source regions. As in the previous studies, the
magnetic connection between the source region and the observer is not
always reproduced adequately by the simple potential field source
surface model combined with the Parker spiral. Instead, we find a
broad longitudinal distribution of the source regions extending well
beyond the west limb, with the longitude deviating significantly from
that expected from the observed solar wind speed.
Title: Solar Sources of 3He-rich Solar Energetic Particle Events in
Solar Cycle 24
Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wang, Linghua; Cohen,
Christina; Wiedenbeck, Mark E.
Bibcode: 2015TESS....140101N
Altcode:
We still do not understand the origin of impulsive SEP events enriched
in 3He and heavy ions. A major impediment may be the difficulty to
observe them in the corona, apart from the common knowledge that
3He-rich SEP events are correlated with longer-than-metric type III
radio bursts and <100 keV electron events. This is because their
X-ray and EUV signatures tend to be tiny and short-lived. Using
high-cadence and high-sensitivity EUV images obtained by SDO/AIA,
we investigate the solar sources of 26 3He-rich SEP events during
solar cycle 24 that were well-observed by ACE. The source locations
are further confirmed in data from STEREO/EUVI, which capture solar
activities in the regions inaccessible from the Earth. We confirm
that 3He-rich events have a broad longitudinal distribution (including
locations well behind the west limb) and that a frequent association
with coronal jets and narrow CMEs. Some events were seen in association
with eruptions of closed structures and large-scale coronal propagating
fronts (LCPFs, aka EUV waves). While these LCPFs may account for the
occasional mismatching polarities at the source region and L1 in such
a way that the particles are transported to and released at a region
that has the opposite polarity, their associated CMEs may not be fast
enough to drive shock waves for particle acceleration. Moreover, open
field lines from PFSS models may not be correct for the entire Sun
although they often look reasonable in discrete locations. We also
discuss the apparent lack of correlation between the solar sources
and the basic properties of 3He-rich SEP events.
Title: What Do EUV Dimmings Tell Us About CME Topology
Authors: Thompson, Barbara J.; DeRosa, Marc L.; Fisher, Richard R.;
Krista, Larisza D.; Kwon, Ryun Young; Mason, James P.; Mays, Mona L.;
Nitta, Nariaki V.; Webb, David F.; West, Matthew J.
Bibcode: 2015TESS....121201T
Altcode:
Large-scale coronal EUV dimmings develop on timescales of hours in
association with a flare or filament eruption, and are known to be
well correlated with coronal mass ejections (CMEs). However, it is not
clear why some CMEs have dimmings and some do not, nor is it clear how
these dimmings relate to CME topology. The inner coronal coverage of
SDO AIA and STEREO EUVI, combined with the extended field of view of
PROBA2's SWAP imager, allow us the opportunity to map the topology of
a dimming region in three dimensions into an erupting CME. Although
the location and extent of a dimming region appears to be the best
indicator of the inner "footprint" of a CME, the correlation is far
from perfect. However, dimmings can provide vital clues about the
development and 3D kinematics of CMEs. This is particularly important
as we are currently in an extended period where the STEREO coronagraph
images are not always available and are increasingly "mirroring" LASCO
images, and therefore the 3D properties of a CME will be difficult
to deduce. Thus, understanding the inner coronal manifestations of
a CME can provide clues to its structure and dynamics, even without
multi-viewpoint coronagraph observations. We present the results of
this combined analysis effort, along with a discussion of how dimmings
can be used to forecast CME trajectories.
Title: Scatter-free Impulsive SEP Events Observed at Multiple
Spacecraft
Authors: Wiedenbeck, Mark E.; Cohen, Christina; Klassen, Andreas;
Leske, Richard; Liewer, Paulett C.; Mason, Glenn; Nitta, Nariaki
Bibcode: 2015TESS....140103W
Altcode:
Impulsive solar energetic particle (ISEP) events are thought to result
from particle acceleration associated with magnetic reconnection
in relatively compact regions in the solar corona and subsequent
escape of accelerated particles along open field lines involved in the
reconnection. In some cases, ISEP events measured at 1 AU are observed
to have clear velocity dispersion, strongly anisotropic outward flows,
and/or flux drop-outs, which indicate that minimal particle scattering
has occurred between the Sun and the observer. We have investigated
several such "scatter-free" ISEP events that were observed using
the energetic particle instruments of the IMPACT suite on NASA's two
STEREO spacecraft in April-May 2014 when the spacecraft separation
was 38 degrees in heliographic longitude. We will report on the event
characteristics and on the solar and heliospheric context in which
they occurred and will argue that these events having scatter-free
characteristics at two separated spacecraft are providing evidence
either for longitudinal transport occurring close to the Sun or for
random walk of magnetic field lines that extends over a wide range
of longitudes.
Title: Interplanetary Propagation Behavior of the Fast Coronal Mass
Ejection on 23 July 2012
Authors: Temmer, M.; Nitta, N. V.
Bibcode: 2015SoPh..290..919T
Altcode: 2015SoPh..tmp....2T; 2014arXiv1411.6559T
The fast coronal mass ejection (CME) on 23 July 2012 caused attention
because of its extremely short transit time from the Sun to 1 AU,
which was shorter than 21 h. In situ data from STEREO-A revealed the
arrival of a fast forward shock with a speed of more than 2200 km
s−1 followed by a magnetic structure moving with almost
1900 km s−1. We investigate the propagation behavior of the
CME shock and magnetic structure with the aim to reproduce the short
transit time and high impact speed as derived from in situ data. We
carefully measured the 3D kinematics of the CME using the graduated
cylindrical shell model and obtained a maximum speed of 2580±280 km
s−1 for the CME shock and 2270±420 km s−1
for its magnetic structure. Based on the 3D kinematics, the drag-based
model (DBM) reproduces the observational data reasonably well. To
successfully simulate the CME shock, the ambient flow speed needs
to have an average value close to the slow solar wind speed (450
km s−1), and the initial shock speed at a distance of
30 R⊙ should not exceed ≈ 2300 km s−1,
otherwise it would arrive much too early at STEREO-A. The model results
indicate that an extremely small aerodynamic drag force is exerted
on the shock, smaller by one order of magnitude than average. As
a consequence, the CME hardly decelerates in interplanetary space
and maintains its high initial speed. The low aerodynamic drag can
only be reproduced when the density of the ambient solar wind flow,
in which the fast CME propagates, is decreased to ρsw=1 -
2 cm−3 at the distance of 1 AU. This result is consistent
with the preconditioning of interplanetary space by a previous CME.
Title: The Relation Between Large-Scale Coronal Propagating Fronts
and Type II Radio Bursts
Authors: Nitta, Nariaki V.; Liu, Wei; Gopalswamy, Nat; Yashiro, Seiji
Bibcode: 2014SoPh..289.4589N
Altcode: 2014arXiv1409.4754N
Large-scale, wave-like disturbances in extreme-ultraviolet (EUV) and
type II radio bursts are often associated with coronal mass ejections
(CMEs). Both phenomena may signify shock waves driven by CMEs. Taking
EUV full-disk images at an unprecedented cadence, the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory has
observed the so-called EIT waves or large-scale coronal propagating
fronts (LCPFs) from their early evolution, which coincides with the
period when most metric type II bursts occur. This article discusses the
relation of LCPFs as captured by AIA with metric type II bursts. We show
examples of type II bursts without a clear LCPF and fast LCPFs without
a type II burst. Part of the disconnect between the two phenomena may
be due to the difficulty in identifying them objectively. Furthermore,
it is possible that the individual LCPFs and type II bursts may reflect
different physical processes and external factors. In particular,
the type II bursts that start at low frequencies and high altitudes
tend to accompany an extended arc-shaped feature, which probably
represents the 3D structure of the CME and the shock wave around it,
and not just its near-surface track, which has usually been identified
with EIT waves. This feature expands and propagates toward and beyond
the limb. These events may be characterized by stretching of field
lines in the radial direction and may be distinct from other LCPFs,
which may be explained in terms of sudden lateral expansion of the
coronal volume. Neither LCPFs nor type II bursts by themselves serve
as necessary conditions for coronal shock waves, but these phenomena
may provide useful information on the early evolution of the shock
waves in 3D when both are clearly identified in eruptive events.
Title: Improvement of Space Weather Forecasting in Solar Cycle 24
Authors: Nitta, N.
Bibcode: 2014AGUFMSH51E..08N
Altcode:
Solar Cycle 24 has not produced extreme space weather events at
Earthcomparable to the Halloween 2003 events. However, there have been
anumber of geomagnetic storms more intense than Dst of 100 nT as wellas
several major solar energetic particle (SEP) events at Earth.Before
predicting geomagnetic storms and radiation storms well inadvance,
it is necessary to make a firm link of solar activity,notably coronal
mass ejections (CMEs), with interplanetary CMEs(ICMEs) and shock
waves. This cycle has benefitted from the SolarDynamics Observatory
that provides uninterrupted and high-qualityfull-disk images at Earth,
and the Solar Terrestrial RelationsObservatory that has observed CMEs
away from the Sun-Earth line andunambiguously isolated those that were
directed toward Earth. Thispresentation aims at evaluating how these
observations have refinedour understanding of the origins of ICMEs
and helped models reproducethe arrival times of the disturbances and
the occurrence and magnitudeof SEP events. We also discuss what may be
critically missing and yetessential for achieving useful predictions
in the future. A review isgiven as to how the forecasts on the basis
of solar and near-Sunobservations have fared against the actual
ICMEs and shocks, and howmany of the latter have not been properly
handled because of noobvious CMEs. A similar attempt is made for the
occurrence andmagnitude of SEP events. It is important to critically
analyze theinadequate forecasts (or just expectations) in terms of
uncertaintiesfrom observations and modeling.
Title: What Do EUV Dimmings Tell Us About CME Topology?
Authors: Thompson, B. J.; DeRosa, M. L.; Fisher, R. R.; Krista, L. D.;
Kwon, R. Y.; Mason, J. P.; Mays, M. L.; Nitta, N.; Savani, N.; West,
M. J.
Bibcode: 2014AGUFMSH43B4202T
Altcode:
Large-scale coronal EUV dimmings, developing on timescales of minutes
to hours in association with a flare or filament eruption, are known to
exhibit a high correlation with coronal mass ejections. However, it is
not clear why some CMEs have dimmings and some do not, nor is it clear
how these dimmings relate to CME topology. The inner coronal coverage
of SDO AIA and STEREO EUVI, combined with the extended field of view
of PROBA2's SWAP imager, allow us the opportunity to map the topology
of a dimming region in three dimensions into an erupting CME. Although
the location and extent of a dimming region appears to be the best
indicator of the inner "footprint" of a CME, the correlation is far
from perfect. However, dimmings can provide vital clues about the
development and 3D kinematics of a CME. This is particularly important
as we are entering an extended period of time where STEREO coronagraph
images will not always be available, and therefore the 3D properties of
a CME will be difficult to deduce. Therefore, understanding the inner
coronal manifestations of a CME can provide clues to its structure and
dynamics, even without multi-viewpoint coronagraph observations. We
present the results of this combined analysis effort, along with a
discussion of how dimmings can be used in forecasting CME directions.
Title: A Comparison of the Characteristics of 3He-Rich Solar Energetic
Particle Events Observed at Single and at Multiple Spacecraft
Authors: Wiedenbeck, M. E.; Mason, G. M.; Klassen, A.; Cohen, C. M.;
Nitta, N.
Bibcode: 2014AGUFMSH21B4116W
Altcode:
An important objective of NASA's STEREO mission has been to understand
the mechanisms responsible for the transport of solar energetic
particles (SEPs) between the solar corona and 1 AU. For 3He-rich SEP
events, in which particle acceleration is believed to be associated with
a relatively small magnetic-reconnection region, it had been thought
that the longitudinal distribution of particles arriving at 1 AU should
be relatively narrow and single-spacecraft studies had been used to
infer a standard deviation in longitude of about 20 degrees. However,
measurements on ACE and STEREO showed that some 3He-rich SEP events
are observable over a much broader range of longitudes. Whether the
longitudinal spreading of the particles in these events occurs in the
solar corona or in interplanetary space has not yet been conclusively
established. It was originally anticipated that the twin STEREO
spacecraft would be able to investigate the longitudinal transport
problem as they began separating from one another shortly after their
2006 launch. However, during the long, quiet minimum between solar
cycles 23 and 24 there were hardly any 3He-rich events on which to base
such studies. The longitudinal separation of the two spacecraft on
the back side of the Sun is now less than 35 degrees and decreasing,
making it possible to carry out the studies that could not be done in
2007-2009. Although frequent detections of individual 3He-rich events
at both spacecraft have become common, there are still instances where
an event is clearly observed at one spacecraft but not detected at the
other. We will report results from a study in which we investigate the
particle transport by determining the characteristics of events observed
at both spacecraft and comparing with the corresponding characteristics
of events that are observable at only one of the two. These results
are expected to aid in the development of SEP transport models that
can be used for predicting the characteristics of events that will be
observed by the Solar Probe Plus and Solar Orbiter missions.
Title: Homologous flare-CME events and their metric type II radio
burst association
Authors: Yashiro, S.; Gopalswamy, N.; Mäkelä, P.; Akiyama, S.;
Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
Bibcode: 2014AdSpR..54.1941Y
Altcode:
Active region NOAA 11158 produced many flares during its disk
passage. At least two of these flares can be considered as homologous:
the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February
14, 2011. Both flares occurred at the same location (eastern edge of
the active region) and have a similar decay of the GOES soft X-ray
light curve. The associated coronal mass ejections (CMEs) were slow
(334 and 337 km/s) and of similar apparent widths (43° and 44°), but
they had different radio signatures. The second event was associated
with a metric type II burst while the first one was not. The COR1
coronagraphs on board the STEREO spacecraft clearly show that the
second CME propagated into the preceding CME that occurred 50 min
before. These observations suggest that CME-CME interaction might be
a key process in exciting the type II radio emission by slow CMEs.
Title: The Relation between Type II Radio Bursts and Large-scale
Coronal Propagating Fronts
Authors: Nitta, Nariaki
Bibcode: 2014AAS...22430304N
Altcode:
Both type II radio bursts and chromospheric Moreton-Ramsey waves
are believed to signify shock waves that propagate in the solar
corona. Large-scale coronal propagating fronts (LCPFs), which are
also called EIT waves, EUV waves or coronal bright fronts in the
literature, were initially thought to be coronal counterparts of
Moreton-Ramsey waves, and thus they were expected to be correlated
with type II bursts. At present, the prevailing view seems to be that
both type II bursts and LCPFs are more closely linked with CMEs than
with flares. Here we revisit the relation between type II bursts and
LCPFs, by examining radio dynamic spectra (180-25 MHz) as obtained
by USAF/RSTN and analyzing EUV and white-light data from SDO and
STEREO. In the sample of about 140 type II bursts and LCPFs between
April 2010 and January 2013, we find the correlation of 50-60 %. Type II
bursts could be associated with eruptions without significant lateral
expansion, and fast LCPFs could show no presence in the metric radio
spectral range. Using data from STEREO COR-1 that observed the CME as
a limb event, in 42 cases we directly measure the height of the CME
at the onset of the type II burst. As expected, the height tends to
be lower when the type II burst starts at a higher frequency. It is
found that those type II bursts that start at higher altitudes and
lower frequencies tend to have weaker EUV fronts. This may indicate
multiple ways of how LCPFs and type II bursts are related with CMEs.
Title: Necessity to Include Weak Events and to Review Results
Authors: Nitta, Nariaki
Bibcode: 2014shin.confE.170N
Altcode:
As is clear this cycle has not seen extreme geomagnetic storms, however,
several minor ones with Dst -100 nT occurred. One thing that can be
wrong is that we tend to look for spectacular events on the Sun to
predict major impact on geo-space. In particular, the occurrence of an
intense flare should not automatically wake up the forecasters unless
a wide CME is associated. But then what is the significance of a wide
or halo CME? This presentation tries to provide a scoreboard of what
fraction of geomagnetic storms since 2013 was successfully predicted
and satisfactorily explained in terms of solar activity. We emphasize
the possibility of stealth CMEs without convincing disk signatures
and filament eruptions near central meridian not observed as CMEs
could cause the level of storms we have seen. It is also important to
critically analyze false positives from the predictions based only on
clear halo CMEs.
Title: Solar Sources of 3He-rich SEP Events
Authors: Nitta, Nariaki; Mason, Glenn; Cohen, Christina; Wiedenbeck,
Mark
Bibcode: 2014shin.confE.106N
Altcode:
During solar cycle 23, it became clear that 3He-rich SEP events
are often associated with coronal jets and narrow CMEs involving
open field regions. The basic technique to find the solar sources
of these weak events observed around 1 MeV/n is to find a type
III radio burst and to find possibly related activity in EUV/X-ray
images. Sometimes an electron event is seen around the time of the type
III burst. Furthermore, it is a common practice to confirm the presence
of open field lines close to the source region using magnetic field
extrapolations. We have repeated these procedures for 3He-rich SEP
events after 2010 using SDO/AIA data, which are a huge enhancement to
SOHO/EIT data, meaning we can now observe weaker and shorter-duration
activities. We discuss the source regions and possible association of
3He-rich SEP events with large-scale activities.
Title: Large-Scale Coronal Propagating Fronts and Their Relation
with SEP Events Observed at Widely Separate Locations
Authors: Nitta, Nariaki
Bibcode: 2014shin.confE..90N
Altcode:
Thanks to STEREO/IMPACT, many SEP events are observed at widely
separate longitudes. One hypothesis is that particles are already
widely transported in the corona, say, within a few solar radii from
the photosphere. A relevant agent is the MHD waves in the corona that
can steepen into shock waves depending on the ambient magneto-acoustic
speeds. Some workers use EUV waves or large-scale coronal propagating
fronts (LCPFs) as proxies for coronal shock waves and explain the SEP
early temporal behavior in terms of how the waves intersect with field
lines that are connected to the observer. We have gone through multiple
examples and now try to understand the potential and limitation of
this approach. Based on our extensive analysis of LCPFs, we ask how to
distinguish the fronts capable of accelerating particles from those
that are not, how much do we have to rely on theory/simulations to
make up for what is not visible, how differently the coronal waves
act on electrons and ions, etc.
Title: The Association of Solar Flares with Coronal Mass Ejections
During the Extended Solar Minimum
Authors: Nitta, N. V.; Aschwanden, M. J.; Freeland, S. L.; Lemen,
J. R.; Wülser, J. -P.; Zarro, D. M.
Bibcode: 2014SoPh..289.1257N
Altcode: 2013arXiv1308.1465N
We study the association of solar flares with coronal mass ejections
(CMEs) during the deep, extended solar minimum of 2007 - 2009, using
extreme-ultraviolet (EUV) and white-light (coronagraph) images from the
Solar Terrestrial Relations Observatory (STEREO). Although all of the
fast (v>900 km s−1), wide (θ>100∘) CMEs
are associated with a flare that is at least identified in GOES soft
X-ray light curves, a majority of flares with relatively high X-ray
intensity for the deep solar minimum (e.g. ≳1×10−6
W m−2 or C1) are not associated with CMEs. Intense
flares tend to occur in active regions with a strong and complex
photospheric magnetic field, but the active regions that produce
CME-associated flares tend to be small, including those that have no
sunspots and therefore no NOAA active-region numbers. Other factors
on scales similar to and larger than active regions seem to exist that
contribute to the association of flares with CMEs. We find the possible
low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves,
and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of
the 35 flares that we associate with CMEs. None of these observables
can fully replace direct observations of CMEs by coronagraphs.
Title: STEREO/ Extreme Ultraviolet Imager (EUVI) Event Catalog 2006
- 2012
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; Freeland, Sam; Thompson, William T.
Bibcode: 2014SoPh..289..919A
Altcode: 2013arXiv1306.3180A
We generated an event catalog with an automated detection algorithm
based on the entire EUVI image database observed with the two Solar
Terrestrial Relations Observatory (STEREO)-A and -B spacecraft over
the first six years of the mission (2006 - 2012). The event catalog
includes the heliographic positions of some 20 000 EUV events,
transformed from spacecraft coordinates to Earth-based coordinates,
and information on associated GOES flare events (down to the level
of GOES A5-class flares). The 304 Å wavelength turns out to be the
most efficient channel for flare detection (79 % of all EUVI event
detections), while the 171 Å (4 %), 195 Å (10 %), and the 284 Å
channel (7 %) retrieve substantially fewer flare events, partially
due to the suppressing effect of EUV dimming, and partially due
to the lower cadence in the later years of the mission. Due to the
Sun-circling orbits of STEREO-A and -B, a large number of flares have
been detected on the farside of the Sun, invisible from Earth, or seen
as partially occulted events. The statistical size distributions of
EUV peak fluxes (with a power-law slope of αP=2.5±0.2)
and event durations (with a power-law slope of αT=2.4±0.3)
are found to be consistent with the fractal-diffusive self-organized
criticality model. The EUVI event catalog is available on-line at
secchi.lmsal.com/EUVI/euvi_autodetection/euvi_events.txt and may serve
as a comprehensive tool to identify stereoscopically observed flare
events for 3D reconstruction and to study occulted flare events.
Title: Observations of an extreme storm in interplanetary space
caused by successive coronal mass ejections
Authors: Liu, Ying D.; Luhmann, Janet G.; Kajdič, Primož; Kilpua,
Emilia K. J.; Lugaz, Noé; Nitta, Nariaki V.; Möstl, Christian;
Lavraud, Benoit; Bale, Stuart D.; Farrugia, Charles J.; Galvin,
Antoinette B.
Bibcode: 2014NatCo...5.3481L
Altcode: 2014arXiv1405.6088L; 2014NatCo...5E3481L
Space weather refers to dynamic conditions on the Sun and in the
space environment of the Earth, which are often driven by solar
eruptions and their subsequent interplanetary disturbances. It has
been unclear how an extreme space weather storm forms and how severe
it can be. Here we report and investigate an extreme event with
multi-point remote-sensing and in situ observations. The formation
of the extreme storm showed striking novel features. We suggest that
the in-transit interaction between two closely launched coronal mass
ejections resulted in the extreme enhancement of the ejecta magnetic
field observed near 1 AU at STEREO A. The fast transit to STEREO A
(in only 18.6 h), or the unusually weak deceleration of the event,
was caused by the preconditioning of the upstream solar wind by an
earlier solar eruption. These results provide a new view crucial to
solar physics and space weather as to how an extreme space weather
event can arise from a combination of solar eruptions.
Title: Multiwavelength diagnostics of the precursor and main phases
of an M1.8 flare on 2011 April 22
Authors: Awasthi, A. K.; Jain, R.; Gadhiya, P. D.; Aschwanden, M. J.;
Uddin, W.; Srivastava, A. K.; Chandra, R.; Gopalswamy, N.; Nitta,
N. V.; Yashiro, S.; Manoharan, P. K.; Choudhary, D. P.; Joshi, N. C.;
Dwivedi, V. C.; Mahalakshmi, K.
Bibcode: 2014MNRAS.437.2249A
Altcode: 2013arXiv1310.6029A; 2013MNRAS.tmp.2720A
We study the temporal, spatial and spectral evolution of the M1.8 flare,
which occurred in the active region 11195 (S17E31) on 2011 April 22,
and explore the underlying physical processes during the precursor
phase and their relation to the main phase. The study of the source
morphology using the composite images in 131 Å wavelength observed by
the Solar Dynamics Observatory/Atmospheric Imaging Assembly and 6-14
keV [from the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI)] revealed a multiloop system that destabilized systematically
during the precursor and main phases. In contrast, hard X-ray emission
(20-50 keV) was absent during the precursor phase, appearing only from
the onset of the impulsive phase in the form of foot-points of emitting
loops. This study also revealed the heated loop-top prior to the loop
emission, although no accompanying foot-point sources were observed
during the precursor phase. We estimate the flare plasma parameters,
namely temperature (T), emission measure (EM), power-law index (γ)
and photon turn-over energy (ɛto), and found them to be
varying in the ranges 12.4-23.4 MK, 0.0003-0.6 × 1049
cm-3, 5-9 and 14-18 keV, respectively, by forward fitting
RHESSI spectral observations. The energy released in the precursor
phase was thermal and constituted ≈1 per cent of the total energy
released during the flare. The study of morphological evolution of
the filament in conjunction with synthesized T and EM maps was carried
out, which reveals (a) partial filament eruption prior to the onset of
the precursor emission and (b) heated dense plasma over the polarity
inversion line and in the vicinity of the slowly rising filament during
the precursor phase. Based on the implications from multiwavelength
observations, we propose a scheme to unify the energy release during
the precursor and main phase emissions in which the precursor phase
emission was originated via conduction front that resulted due to the
partial filament eruption. Next, the heated leftover S-shaped filament
underwent slow-rise and heating due to magnetic reconnection and finally
erupted to produce emission during the impulsive and gradual phases.
Title: Solar Cycle Variations of the Radio Brightness of the Solar
Polar Regions as Observed by the Nobeyama Radioheliograph
Authors: Nitta, Nariaki V.; Sun, Xudong; Hoeksema, J. Todd; DeRosa,
Marc L.
Bibcode: 2014ApJ...780L..23N
Altcode:
We have analyzed daily microwave images of the Sun at 17 GHz obtained
with the Nobeyama Radioheliograph (NoRH) in order to study the solar
cycle variations of the enhanced brightness in the polar regions. Unlike
in previous works, the averaged brightness of the polar regions is
obtained from individual images rather than from synoptic maps. We
confirm that the brightness is anti-correlated with the solar cycle and
that it has generally declined since solar cycle 22. Including images
up to 2013 October, we find that the 17 GHz brightness temperature
of the south polar region has decreased noticeably since 2012. This
coincides with a significant decrease in the average magnetic field
strength around the south pole, signaling the arrival of solar maximum
conditions in the southern hemisphere more than a year after the
northern hemisphere. We do not attribute the enhanced brightness
of the polar regions at 17 GHz to the bright compact sources that
occasionally appear in synthesized NoRH images. This is because they
have no correspondence with small-scale bright regions in images
from the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory with a broad temperature coverage. Higher-quality radio
images are needed to understand the relationship between microwave
brightness and magnetic field strength in the polar regions.
Title: Solar energetic particle events during the rise phases of
solar cycles 23 and 24
Authors: Chandra, R.; Gopalswamy, N.; Mäkelä, P.; Xie, H.; Yashiro,
S.; Akiyama, S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Jain,
R.; Awasthi, A. K.; Manoharan, P. K.; Mahalakshmi, K.; Dwivedi, V. C.;
Choudhary, D. P.; Nitta, N. V.
Bibcode: 2013AdSpR..52.2102C
Altcode:
We present a comparative study of the properties of coronal mass
ejections (CMEs) and flares associated with the solar energetic particle
(SEP) events in the rising phases of solar cycles (SC) 23 (1996-1998)
(22 events) and 24 (2009-2011) (20 events), which are associated
with type II radio bursts. Based on the SEP intensity, we divided the
events into three categories, i.e. weak (intensity < 1 pfu), minor
(1 pfu < intensity < 10 pfu) and major (intensity ⩾ 10 pfu)
events. We used the GOES data for the minor and major SEP events and
SOHO/ERNE data for the weak SEP event. We examine the correlation
of SEP intensity with flare size and CME properties. We find that
most of the major SEP events are associated with halo or partial halo
CMEs originating close to the sun center and western-hemisphere. The
fraction of halo CMEs in SC 24 is larger than the SC 23. For the minor
SEP events one event in SC23 and one event in SC24 have widths <
120° and all other events are associated with halo or partial halo
CMEs as in the case of major SEP events. In case of weak SEP events,
majority (more than 60%) of events are associated with CME width <
120°. For both the SC the average CMEs speeds are similar. For major
SEP events, average CME speeds are higher in comparison to minor and
weak events. The SEP event intensity and GOES X-ray flare size are
poorly correlated. During the rise phase of solar cycle 23 and 24,
we find north-south asymmetry in the SEP event source locations: in
cycle 23 most sources are located in the south, whereas during cycle
24 most sources are located in the north. This result is consistent
with the asymmetry found with sunspot area and intense flares.
Title: Soft X-ray Fluxes of Major Flares Far Behind the Limb as
Estimated Using STEREO EUV Images
Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P. F.; Freeland,
S. L.; Lemen, J. R.; Wuelser, J. -P.
Bibcode: 2013SoPh..288..241N
Altcode: 2013arXiv1304.4163N
With increasing solar activity since 2010, many flares from the backside
of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI)
on either of the twin STEREO spacecraft. Our objective is to estimate
their X-ray peak fluxes from EUVI data by finding a relation of the EUVI
with GOES X-ray fluxes. Because of the presence of the Fe XXIV line at
192 Å, the response of the EUVI 195 Å channel has a secondary broad
peak around 15 MK, and its fluxes closely trace X-ray fluxes during
the rise phase of flares. If the flare plasma is isothermal, the EUVI
flux should be directly proportional to the GOES flux. In reality,
the multithermal nature of the flare and other factors complicate
the estimation of the X-ray fluxes from EUVI observations. We discuss
the uncertainties, by comparing GOES fluxes with the high cadence EUV
data from the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can
provide estimates of the X-ray peak fluxes of intense flares (e.g.,
above M4 in the GOES scale) to small uncertainties. Lastly we show
examples of intense flares from regions far behind the limb, some of
which show eruptive signatures in AIA images.
Title: Large-scale Coronal Propagating Fronts in Solar Eruptions
as Observed by the Atmospheric Imaging Assembly on Board the Solar
Dynamics Observatory—an Ensemble Study
Authors: Nitta, Nariaki V.; Schrijver, Carolus J.; Title, Alan M.;
Liu, Wei
Bibcode: 2013ApJ...776...58N
Altcode: 2013arXiv1308.3544N
This paper presents a study of a large sample of global disturbances
in the solar corona with characteristic propagating fronts as
intensity enhancement, similar to the phenomena that have often
been referred to as Extreme Ultraviolet Imaging Telescope (EIT)
waves or extreme-ultraviolet (EUV) waves. Now EUV images obtained by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory provide a significantly improved view of these large-scale
coronal propagating fronts (LCPFs). Between 2010 April and 2013 January,
a total of 171 LCPFs have been identified through visual inspection of
AIA images in the 193 Å channel. Here we focus on the 138 LCPFs that
are seen to propagate across the solar disk, first studying how they
are associated with flares, coronal mass ejections (CMEs), and type II
radio bursts. We measure the speed of the LCPF in various directions
until it is clearly altered by active regions or coronal holes. The
highest speed is extracted for each LCPF. It is often considerably
higher than EIT waves. We do not find a pattern where faster LCPFs
decelerate and slow LCPFs accelerate. Furthermore, the speeds are
not strongly correlated with the flare intensity or CME magnitude,
nor do they show an association with type II bursts. We do not find
a good correlation either between the speeds of LCPFs and CMEs in a
subset of 86 LCPFs observed by one or both of the Solar and Terrestrial
Relations Observatory spacecraft as limb events.
Title: Heliospheric Imaging of 3D Density Structures During the
Multiple Coronal Mass Ejections of Late July to Early August 2010
Authors: Webb, D. F.; Möstl, C.; Jackson, B. V.; Bisi, M. M.; Howard,
T. A.; Mulligan, T.; Jensen, E. A.; Jian, L. K.; Davies, J. A.; de
Koning, C. A.; Liu, Y.; Temmer, M.; Clover, J. M.; Farrugia, C. J.;
Harrison, R. A.; Nitta, N.; Odstrcil, D.; Tappin, S. J.; Yu, H. -S.
Bibcode: 2013SoPh..285..317W
Altcode:
It is usually difficult to gain a consistent global understanding
of a coronal mass ejection (CME) eruption and its propagation
when only near-Sun imagery and the local measurements derived from
single-spacecraft observations are available. Three-dimensional (3D)
density reconstructions based on heliospheric imaging allow us to
"fill in" the temporal and spatial gaps between the near-Sun and in
situ data to provide a truly global picture of the propagation and
interactions of the CME as it moves through the inner heliosphere. In
recent years the heliospheric propagation of dense structures has been
observed and measured by the heliospheric imagers of the Solar Mass
Ejection Imager (SMEI) and on the twin Solar TErrestrial RElations
Observatory (STEREO) spacecraft. We describe the use of several 3D
reconstruction techniques based on these heliospheric imaging data sets
to distinguish and track the propagation of multiple CMEs in the inner
heliosphere during the very active period of solar activity in late July
- early August 2010. We employ 3D reconstruction techniques used at the
University of California, San Diego (UCSD) based on a kinematic solar
wind model, and also the empirical Tappin-Howard model. We compare
our results with those from other studies of this active period,
in particular the heliospheric simulations made with the ENLIL model
by Odstrcil et al. (J. Geophys. Res., 2013) and the in situ results
from multiple spacecraft provided by Möstl et al. (Astrophys. J.758,
10 - 28, 2012). We find that the SMEI results in particular provide
an overall context for the multiple-density flows associated with
these CMEs. For the first time we are able to intercompare the 3D
reconstructed densities with the timing and magnitude of in situ
density structures at five spacecraft spread over 150° in ecliptic
longitude and from 0.4 to 1 AU in radial distance. We also model the
magnetic flux-rope structures at three spacecraft using both force-free
and non-force-free modelling, and compare their timing and spatial
structure with the reconstructed density flows.
Title: A multiwavelength study of eruptive events on January 23,
2012 associated with a major solar energetic particle event
Authors: Joshi, N. C.; Uddin, W.; Srivastava, A. K.; Chandra, R.;
Gopalswamy, N.; Manoharan, P. K.; Aschwanden, M. J.; Choudhary, D. P.;
Jain, R.; Nitta, N. V.; Xie, H.; Yashiro, S.; Akiyama, S.; Mäkelä,
P.; Kayshap, P.; Awasthi, A. K.; Dwivedi, V. C.; Mahalakshmi, K.
Bibcode: 2013AdSpR..52....1J
Altcode: 2013arXiv1303.1251J
We use multiwavelength data from space and ground based instruments
to study the solar flares and coronal mass ejections (CMEs) on January
23, 2012 that were responsible for one of the largest solar energetic
particle (SEP) events of solar cycle 24. The eruptions consisting of two
fast CMEs (≈1400 km s-1 and ≈2000 km s-1) and
M-class flares that occurred in active region 11402 located at ≈N28
W36. The two CMEs occurred in quick successions, so they interacted
very close to the Sun. The second CME caught up with the first one
at a distance of ≈11-12 Rsun. The CME interaction may be
responsible for the elevated SEP flux and significant changes in the
intensity profile of the SEP event. The compound CME resulted in a
double-dip moderate geomagnetic storm (Dst∼-73nT). The two dips are
due to the southward component of the interplanetary magnetic field in
the shock sheath and the ICME intervals. One possible reason for the
lack of a stronger geomagnetic storm may be that the ICME delivered
a glancing blow to Earth.
Title: SDO AIA Observations of Large-Scale Coronal Propagating Fronts
Authors: Nitta, Nariaki; Schrijver, C. J.; Title, A. M.; Liu, W.
Bibcode: 2013SPD....44...40N
Altcode:
The discovery of "EIT waves" rekindled interests in what used to be
called flare waves, which had been typically observed in H-alpha. In
addition to Moreton waves, first observed at the Lockheed Solar
Observatory, other manifestations of shock waves propagating in the
corona include type II radio bursts and filament oscillations away from
flare sites. Identification of EIT waves with the postulated fast-mode
MHD shock waves in the corona has been questioned, however, largely
because of their low speeds (e.g., 200-400 km/s). EIT's 10-20 minute
cadence could be a contributing factor for this, and we need to find how
fast large-scale coronal propagating fronts are in higher-cadence EUV
images. It is clear that AIA on SDO is the best instrument at the moment
for this type of work. With the availability of high-cadence full-disk
images, we now can compare propagating fronts in different directions,
and determine the highest speed of each event on AIA images more
objectively and accurately than on EIT (and STEREO EUVI) images. In a
large number of EIT wave events, we have measured speeds of propagating
fronts using AIA's 193 A images. Before the fronts are deflected by the
discontinuities, e.g., active regions and coronal holes, the mean and
median speeds are 620 km/s and 600 km/s, respectively, and many exceed
800 km/s. Higher speeds are often seen in events that accompany a type
II burst, strong flare or energetic CME, but the distribution of the
speed with these attributes is broad. We also find that the speeds of
the large-scale coronal propagating fronts are not well correlated
with those of the associated CMEs. Given that large-scale coronal
propagating fronts at large distances represent freely propagating MHD
waves, we discuss how to understand their nature close to their origins.
Title: Height of shock formation in the solar corona inferred from
observations of type II radio bursts and coronal mass ejections
Authors: Gopalswamy, N.; Xie, H.; Mäkelä, P.; Yashiro, S.; Akiyama,
S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
Bibcode: 2013AdSpR..51.1981G
Altcode: 2013arXiv1301.0893G
Employing coronagraphic and EUV observations close to the solar surface
made by the Solar Terrestrial Relations Observatory (STEREO) mission,
we determined the heliocentric distance of coronal mass ejections
(CMEs) at the starting time of associated metric type II bursts. We
used the wave diameter and leading edge methods and measured the CME
heights for a set of 32 metric type II bursts from solar cycle 24. We
minimized the projection effects by making the measurements from a
view that is roughly orthogonal to the direction of the ejection. We
also chose image frames close to the onset times of the type II bursts,
so no extrapolation was necessary. We found that the CMEs were located
in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs),
with mean and median values of 1.43 and 1.38 Rs, respectively. We
conclusively find that the shock formation can occur at heights
substantially below 1.5 Rs. In a few cases, the CME height at type
II onset was close to 2 Rs. In these cases, the starting frequency
of the type II bursts was very low, in the range 25-40 MHz, which
confirms that the shock can also form at larger heights. The starting
frequencies of metric type II bursts have a weak correlation with the
measured CME/shock heights and are consistent with the rapid decline
of density with height in the inner corona.
Title: SHILLELAgh: A data-driven solar wind model for studying solar
energetic particle events
Authors: Higgins, Paul Anthony; Perez-Suarez, David; Nitta, Nariaki;
Gallagher, Peter T.
Bibcode: 2013shin.confE.140H
Altcode:
A method for estimating the properties of the solar wind in the
equatorial plane is presented. OMNI and STEREO in situ solar wind data
is used as input. The solar wind is assumed to propagate radially at
a constant velocity and the corona is assumed to remain static. An
empirical density model is applied to the ballistically propagated
solar wind. A 2D map of solar wind properties is obtained that is
well suited for use in investigations of heliospheric features (e.g.,
co-rotating interaction regions). In this work, the propagation of solar
energetic particle (SEP) events is investigated using the model. Our
method provides a convenient vehicle for assessing the directionality
and hence, the geo-effectiveness of SEP events.
Title: What are the solar source regions of energetic 3He-rich SEPs?
Authors: Mason, Glenn M.; Nitta, N. V.; Cohen, C. M. S.; Wiedenbeck,
M. E.
Bibcode: 2013shin.confE.131M
Altcode:
One of the surprising observations from the ACE mission has been
the detection of energetic 3He emission occurring over multi-day
periods. Previously observations of solar energetic 3He had detected
short-lived "impulsive" energetic particle events which were associated
with type III bursts and energetic electrons. The ACE observations
were able to detect 3He at very low levels (<1% of 4He compared to
10% in most earlier work) and this showed that the impulsive events
often occurred during seemingly continuous multi-day periods of 3He
emission. During solar active periods, 3He was present at 1 AU the
majority of the time, giving evidence for either semi-continuous
processes or else unresolved multiple small injections. The obvious
injections during such periods were strongly associated with jet
activity. By adding STEREO and SDO observations we are seeking to
extend the observational picture for these events. First, by following
single 3He emitting regions from STEREO-B to ACE to STEREO-A we seek
to examine for how long the 3He emission can continue, since any single
spacecraft can be magnetically connected to a single region for only a
few days and ACE often sees emission periods of that length. Second, by
using SDO-AIA we seek to probe further the properties of the emitting
regions to see if the previously reported association with jets is
seen in events which we can now observe with greater resolution,
sensitivity, and cadence than previously possible.
Title: Large-Scale Coronal Propagating Fronts and Solar Energetic
Particle Events
Authors: Nitta, Nariaki V.; Higgins, Paul A.; Gomez Herrero, R.;
Jian, Lan
Bibcode: 2013shin.confE.138N
Altcode:
A correlation of EIT waves and solar energetic particle (SEP) events
was shown in solar cycle 23. In particular, EIT waves were found when
an electron event was delayed with respect to the associated type III
radio burst. SEP events observed at widely separate locations in recent
years have sparked renewed interest in the possible role of large-scale
coronal propagating fronts (LCPFs) in the acceleration and transport
of the observed particles. It is indeed a right time to study this
topic, thanks to SDO's AIA that reveals LCPFs in unprecedented details
and STEREO's SECCHI that contribute to the 360-degree view of LCPFs
and show their relations with CMEs. In this presentation, we analyze
these data for SEP events with and without wide angular spread and
discuss how relevant are LCPFs for explaining the SEP observations,
including possible limitations from the uncertainties in magnetic
field connections.
Title: The Expected Sensitivity of Solar Probe Plus and Solar Orbiter
to 3He-rich Solar Energetic Particle Events
Authors: Wiedenbeck, M. E.; Mason, G. M.; Cohen, C. M.; Nitta, N. V.;
Gómez-Herrero, R.; Haggerty, D. K.
Bibcode: 2013AGUSMSH41A..05W
Altcode:
Studies combining data from the two STEREO spacecraft and ACE
have shown that 3He-rich solar energetic particle (SEP)
events are frequently detectable over a wide range of heliographic
longitudes. However, particle fluences can decrease rapidly with
separation from the longitude that is magnetically best connected to
the solar source. Thus detectability depends on both that separation
and on instrument threshold sensitivity. One of the objectives
of the NASA Solar Probe Plus (SPP) and ESA Solar Orbiter (SolO)
missions is to explore the acceleration and transport of SEPs
close to the Sun. Information will be obtained about the radial
as well as the longitudinal and, in the case of Solar Orbiter, the
latitudinal dependences of particle intensities. Using the STEREO and
ACE measurements of longitudinal distributions in 3He-rich
events together with reasonable assumptions about radial and latitudinal
dependences we assess expected distribution of 3He-rich
event fluences to be sampled by instruments on SPP and SolO. These
estimates lead to predictions of the occurrence rate of events that
can be detected at both spacecraft and, in some cases, at spacecraft
operating near 1 AU.
Title: Recent Advances in Observations of Coronal EUV Waves
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
Bibcode: 2013enss.confE..67L
Altcode:
MHD waves can be used as seismological tools to decipher otherwise
elusive physical parameters of the solar corona, such as the magnetic
field strength and plasma density. Recent high cadence, high resolution,
full-disk imaging observations from SDO/AIA have opened a new chapter
in understanding these waves. Various types of EUV waves associated with
flares/CMEs have been discovered or observed in unprecedented detail. In
this talk, we will review such new observations, focusing on the
following topics and their interrelationships: (1) quasi-periodic fast
waves traveling along coronal funnels within CME bubbles at speeds up
to 2000 km/s, associated with flare pulsations at similar frequencies;
(2) quasi-periodic wave trains within broad, diffuse pulses of global
EUV waves (so-called "EIT waves") running ahead of CME fronts; (3)
interactions of global EUV waves with local coronal structures on
their paths, such as flux-rope coronal cavities and their embedded
filaments (kink oscillations) and coronal holes or active regions
(deflections). We will discuss the implications of these observations
on coronal seismology, on their roles in transporting energy through
different parts of the solar atmosphere, and on understanding their
associated eruptive flares/CMEs.
Title: SDO AIA Observations of Large-Scale Coronal Disturbances in
the Form of Propagating Fronts
Authors: Nitta, Nariaki V.; Schrijver, Carolus J.; Title, Alan M.;
Liu, Wei
Bibcode: 2013enss.confE.111N
Altcode:
One of the most spectacular phenomena detected by SOHO EIT
was the large-scale propagating fronts associated with solar
eruptions. Initially these 'EIT' waves were thought to be coronal
counterparts of chromospheric Moreton waves. However, different
spatial and kinematic properties of the fronts seen in H-alpha and
EUV images, and far more frequent occurrences of the latter have
led to various interpretations that are still actively debated by
a number of researchers. A major factor for the lack of closure was
the various limitation in EIT data, including the cadence that was
typically every 12 minutes. Now we have significantly improved data
from SDO AIA, which have revealed some very interesting phenomena
associated with EIT waves. However, the studies so far conducted
using AIA data have primarily dealt with single or a small number of
events, where selection bias and particular observational conditions
may prevent us from discovering the general and true nature of EIT
waves. Although automated detection of EIT waves was promised for
AIA images some time ago, it is still not actually implemented in the
data pipeline. Therefore we have manually found nearly 200 examples
of large-scale propagating fronts, going through movies of difference
images from the AIA 193 A channel up to January 2013. We present our
study of the kinematic properties of the fronts in a subset of about
150 well-observed events in relation with other phenomena that can
accompany EIT waves. Our emphasis is on the relation of the fronts
with the associated coronal eruptions often but not always taking
the form of full-blown CMEs, utilizing STEREO data for a subset of
more than 80 events that have occurred near the limb as viewed from
one of the STEREO spacecraft. In these events, the availability of
data from the STEREO inner coronagraph (COR1) as well as from the EUVI
allows us to trace eruptions off the solar disk during the times of our
propagating fronts. The representative relations between the fronts and
CMEs will be discussed in terms of the evolution of EIT waves observed
in different channels of AIA, which provide information of the thermal
properties of the fronts. Our study will further clarify the variety
of solar eruptions and their associated manifestations in the corona.
Title: Furthering Our Understanding of Wide Longitude 3He-rich
SEP Events
Authors: Cohen, C. M. S.; Wiedenbeck, M. E.; Mason, G. M.;
Gomez-Herrero, R.; Haggerty, D. K.; Nitta, N. V.
Bibcode: 2013ICRC...33.1420C
Altcode:
With the significant separation between ACE and the twin STEREO
spacecraft solar energetic particle (SEP) events can be observed
simultaneously from substantially different longitudinal locations
relative to the solar source. One of the surprising results from such
multi-spacecraft studies is the observation of 3He-rich SEP events
over a wide longitude range. As these events are generally believed
to originate from relatively small solar source regions (e.g.,
5-10°), it was unexpected to observe them on multiple spacecraft
spanning up to '- 130 ° of longitude. Although the study of these
events is ongoing, it is still not clear how the large longitudinal
distribution is created. In an effort to further our understanding of
these events, we have used the ACE and STEREO SEP data to identify seven
single-spacecraft 3He-rich events similar in peak intensity to those
observed over wide longitudes. Given the absence of detection on two
of the three spacecraft, we determined maximum standard deviations
for the longitudinal spread of these events and found them to be
significantly smaller than those of the multi-spacecraft 3He-rich
events. This indicates that the mechanism responsible for distributing
the SEPs in longitude is variable and able to produce longitudinal
distributions with a range of widths.
Title: Multi-wavelength diagnostics of thermal and non-thermal
characteristics in 22 April 2011 confined flare
Authors: Awasthi, Arun K.; Jain, Rajmal; Aschwanden, Markus J.; Uddin,
Wahab; Srivastava, Abhishek K.; Chandra, Ramesh; Gopalswamy, Nat;
Nitta, Nariaki; Yashiro, Seiji; Manoharan, P. K.; Prasad Choudhary,
Debi; Joshi, N. C.; Dwivedi, Vidya Charan; Mahalakshmi, K.
Bibcode: 2013ASInC...9...71A
Altcode:
We study the spatial, spectral and temporal characteristics of thermal
and non-thermal emission in an M1.8 flare, which occurred in NOAA
AR 11195 (S17E31) on 22 April 2011. This study quantifies spatial
and temporal correlation of thermal and non-thermal emissions in
precursor, impulsive as well as gradual phase of energy release
employing multi-wavelength observation from SDO, HESSI and SOXS
missions. Based on spectral fitting analysis performed on the X-ray
emission observed by RHESSI as well as SOXS missions in low energy
and high energy respectively, we define that <20 keV emission
corresponds to thermal and >20 keV emission to be non-thermal
counterpart of the emission. Therefore, we construct X-ray images
employing RHESSI observation in energy bands 6-20 and 20-100 keV
over the time integration of 30s. We report co-spatial X-ray emission
in various phases of emission. We also report absence of non-thermal
counterpart in the X-ray emission in precursor phase however visible at
the commencement of main phase. To characterize thermal and non-thermal
signatures, we overlay the X-ray image contours on the Hα and EUV
observations from GONG and SDO/AIA respectively. We report thermal
emission in the precursor phase to be co-spatial to UV counterpart. In
contrast, we report absence of emission in the EUV wavebands i.e. 1600
and 1700 Å which, in principle, correspond to temperature minimum
zone and photosphere during the precursor phase. This confirms the
absence of non-thermal emission as appeared in X-ray emission during
the precursor phase. Further, during the impulsive as well as in
gradual phase, thermal and non-thermal emissions have been found to
be originated from a compact source, co-spatial in nature. Analysis
of Line of sight (LOS) magnetic field observations from SDO/HMI does
not reveal noticeable changes in the positive and negative fluxes
as well as magnetic-field gradient during this event. In contrast,
Hα emission observed by GONG has revealed the filament eruption as
the trigger of flare. This suggests filament eruption to be driver of
this event, consistent with the CSHKP model of solar flare.
Title: Observations of Solar Energetic Particles from
3He-rich Events over a Wide Range of Heliographic Longitude
Authors: Wiedenbeck, M. E.; Mason, G. M.; Cohen, C. M. S.; Nitta,
N. V.; Gómez-Herrero, R.; Haggerty, D. K.
Bibcode: 2013ApJ...762...54W
Altcode:
A prevailing model for the origin of 3He-rich solar energetic
particle (SEP) events attributes particle acceleration to processes
associated with the reconnection between closed magnetic field lines
in an active region and neighboring open field lines. The open field
from the small reconnection volume then provides a path along which
accelerated particles escape into a relatively narrow range of angles
in the heliosphere. The narrow width (standard deviation <20°)
of the distribution of X-ray flare longitudes found to be associated
with 3He-rich SEP events detected at a single spacecraft
at 1 AU supports this model. We report multispacecraft observations
of individual 3He-rich SEP events that occurred during
the solar minimum time period from 2007 January through 2011 January
using instrumentation carried by the two Solar Terrestrial Relations
Observatory spacecraft and the Advanced Composition Explorer. We find
that detections of 3He-rich events at pairs of spacecraft are
not uncommon, even when their longitudinal separation is >60°. We
present the observations of the 3He-rich event of 2010
February 7, which was detected at all three spacecraft when they spanned
136° in heliographic longitude. Measured fluences of 3He in
this event were found to have a strong dependence on longitude which is
well fit by a Gaussian with standard deviation ~48° centered at the
longitude that is connected to the source region by a nominal Parker
spiral magnetic field. We discuss several mechanisms for distributing
flare-accelerated particles over a wide range of heliographic longitudes
including interplanetary diffusion perpendicular to the magnetic field,
spreading of a compact cluster of open field lines between the active
region and the source surface where the field becomes radial and opens
out into the heliosphere, and distortion of the interplanetary field by
a preceding coronal mass ejection. Statistical studies of additional
3He-rich events detected at multiple spacecraft will be
needed to establish the relative importance of the various mechanisms.
Title: Quantifying Coronal Dimming as Observed in EUV and X-ray
Images in Eruptive Events
Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P.; Hill, S. M.;
Lemen, J. R.; Liu, W.; Schrijver, C.; Wuelser, J.
Bibcode: 2012AGUFMSH41A2097N
Altcode:
Data from SOHO have shown that coronal dimming is closely related
with coronal mass ejections (CMEs). In particular, dimming areas in
EIT 195 A images often match the lateral extension of the associated
CMEs. In this presentation, we summarize how CMEs compare with dimming
as identified at different wavelengths and by other instruments, such as
Yohkoh SXT, TRACE, GOES (12-15) SXI, STEREO EUVI and SDO AIA. Emphasis
is placed on recent data, since the combination of AIA and STEREO
data can lead us to better characterize CMEs and to more accurately
estimate how much mass is ejected. We discuss technical issues that
arise when quantifying dimming as a proxy for a CME. The issues include
instrument calibration, effects of heating and cooling and integration
along the line of sight. We also touch on the relation of dimming with
globally propagating coronal fronts, which are routinely isolated in
running difference images, and its implications on the magnitudes of
the associated CMEs.
Title: Longitudinal dependences of SEP events: a 360-degree view
by STEREO
Authors: Gomez-Herrero, R.; Dresing, N.; Klassen, A.; Heber, B.;
Rodriguez-Pacheco, J.; Blanco, J. J.; Wiedenbeck, M. E.; Cohen, C. M.;
Nitta, N. V.; Mason, G. M.; Malandraki, O.
Bibcode: 2012AGUFMSH23B..05G
Altcode:
The STEREO mission is particularly well-suited for the study of
longitudinal dependences of Solar Energetic Particle (SEP) events at
1 AU near the ecliptic plane. The twin nearly-identical spacecraft
follow a heliocentric orbit around the Sun with increasing angular
separation with respect to the Earth. Their heliocentric distances
are always very close to 1 AU, minimizing the effects of radial
dependences. After reaching 180-degree separation in February 2011,
the two STEREO spacecraft combined with the constellation of near-Earth
observatories provided for the first time a 360-degree view of the solar
surface. The combination of in-situ and remote sensing observations
makes possible an unambiguous identification of the solar sources of
SEP events showing large azimuthal spreads. This global coverage was
not possible in previous multi-point studies based on earlier missions
like Helios, not equipped with remote-sensing instruments. Different
physical mechanisms have been proposed to explain the broad angular
particle distribution observed during some SEP events. These processes
can take place near the source (broad acceleration or injection regions
or quick angular spread close to the Sun) or in the interplanetary
medium (perpendicular diffusion). The relative importance of these
physical mechanisms is still under debate. We present an overview
of the new insights provided by STEREO, based on the study of the
longitudinal dependences of different SEP event properties.
Title: Coordinated Observations of Energetic Particles from
3He-rich Events by STEREO and ACE
Authors: Wiedenbeck, M. E.; Mason, G. M.; Cohen, C. M.; Nitta, N. V.;
Gomez-Herrero, R.; Haggerty, D. K.
Bibcode: 2012AGUFMSH24A..01W
Altcode:
The two STEREOs, together with ACE and other near-Earth spacecraft,
have produced data sets that are well suited for investigating
how 3He-rich energetic particles that are accelerated
in reconnection events on the Sun get distributed in heliographic
longitude. Our earlier study[1] of one such 3He-rich event
(7 February 2010), which was detected at ACE and both STEREOs when
they spanned 136o, showed that accelerated particles can be
distributed over a wide range of longitudes even when they originate
from a localized solar source. In addition, the particle fluences were
found to decrease strongly with increasing distance from the longitude
having the best magnetic connection to the source region. Based on
data from the first four years of the STEREO mission, when solar
activity was very low, we have used additional 3He-rich
events detected by one or both of the STEREO/LET instruments and, in
some cases also by ACE/ULEIS and/or SIS, to investigate the conditions
under which the accelerated particles can gain access to a wide range of
heliographic longitudes and to determine the longitudinal dependences
of particle fluences in such events. During 2011 and 2012, when the
level of solar activity has been significantly greater than in the
preceding four years, unambiguous association of events detected at
different spacecraft has been hampered by the possibility of chance
coincidences between detections of particles from separate solar events
as well as by the increased energetic particle background levels. We
have investigated the possibility that event characteristics such as
composition can be use to confirm that some events detected at widely
spaced locations are associated with the same injection at the Sun. Such
associations have the potential to extend the longitude range over
which 3He-rich events can be studied and also to investigate
the solar-cycle dependence of longitudinal spreading. We will report
results from the statistical study of events that occurred between
January 2007 and January 2011 and discuss selected event associations
that have made over the subsequent two years. [Ref 1. Wiedenbeck et
al. 2012, ApJ, in press]
Title: Solar Source Regions of Energetic 3He Emission
Authors: Mason, G. M.; Nitta, N. V.; Cohen, C. M.; Wiedenbeck, M. E.
Bibcode: 2012AGUFMSH21A2163M
Altcode:
One of the surprising observations from the ACE mission has been
the detection of energetic 3He emission occurring over multi-day
periods. Previously observations of solar energetic 3He had detected
short-lived "impulsive" energetic particle events which were associated
with type III bursts and energetic electrons. The ACE observations
were able to detect 3He at very low levels (<1% of 4He compared to
~10% in most earlier work) and this showed that the impulsive events
often occurred during seemingly continuous multi-day periods of 3He
emission. During solar active periods, 3He was present at 1 AU the
majority of the time, giving evidence for either semi-continuous
processes or else unresolved multiple small injections. The obvious
injections during such periods were strongly associated with jet
activity By adding STEREO and SDO observations we are seeking to extend
the observational picture for these events. First, by following single
3He emitting regions from STEREO-B to ACE to STEREO-A we seek to examine
for how long the 3He emission can continue, since any single spacecraft
can be magnetically connected to a single region for only a few days
and ACE often sees emission periods of that length. Second, by using
SDO-AIA we seek to probe further the properties of the emitting regions
to see if the previously reported association with jets is seen in
events which we can now observe with greater resolution, sensitivity,
and cadence than previously possible.
Title: Hot Precursor Ejecta and Other Peculiarities of the 2012 May
17 Ground Level Enhancement Event
Authors: Gopalswamy, N.; Xie, H.; Nitta, N. V.; Usoskin, I.; Davila,
J. M.
Bibcode: 2012AGUFMSH21A2180G
Altcode:
We report on the first Ground Level Enhancement (GLE) event of Solar
Cycle 24, which occurred on May 17, 2012 from a well-connected region
(NOAA AR 11476, N11W76) on the Sun. There has been a real dearth
of GLE events during cycle 24: even though the Sun has reached its
solar maximum phase, it has produced only this one GLE event. Over the
first 4.5 years of solar cycle 23, there were 5 GLE events, which is
roughly a third of all the events of that cycle. The recent GLE event
was associated with a moderate flare with an X-ray size of only M5.1,
well below the median flare size (X3.8) of cycle 23 GLE events. On
the other hand, the associated CME was very fast (~2000 km/s),
typical of GLE events. During cycle 23, the CME speeds in GLE events
ranged from 1203 km/s to 3675 km/s with an average value of 2083 km/s
(Gopalswamy et al., 2012). The speed of the cycle 24 GLE was measured
accurately because it was a limb event in the SOHO coronagraphic field
of view. The CME was also observed by the STEREO coronagraphs, which
helped derive the initial acceleration as 1.5 km/s/s, which is also
typical of GLE-producing CMEs. We were also able to directly determine
the heliocentric distance of the CME (2.3 solar radii (Rs)) at the
time of the release of GLE particles because there was a STEREO/COR1
image precisely at the time of the particle release. This result
is consistent with what was obtained for the cycle 23 GLE events,
including the distance of the CME at the time of metric type II burst
onset (1.3 Rs), indicating shock formation very close to the Sun
( ~0.3 Rs above the solar surface). We infer that the shock had to
travel an additional 1 Rs before the GLE particles were released. The
CME had a precursor in the form of a hot ejecta some tens of minutes
before the main eruption. The preceding ejecta is termed hot because
it was observed only in the 94 A images obtained by the Solar Dynamics
Observatory (SDO). The 94 A images correspond to coronal a temperature
of ~6MK. The lower temperature images such as at 193 A did not show
the ejecta. The hot ejecta was accelerating and attained a speed
of ~70 km/s before it was blasted by the big GLE-producing CMEs. We
suggest that the hot material of the precursor ejecta might have been
further accelerated by the CME-driven shock resulting in the GLE
event. Reference Gopalswamy, N.,Xie, H., Yashiro, S., Akiyama, S.,
Mäkelä, P., Usoskin, I. G., Properties of Ground Level Enhancement
Events and the Associated Solar Eruptions During Solar Cycle 23,
Space Science reviews, DOI: 10.1007/s11214-012-9890-4
Title: Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from
Small (6∘) to Large (170∘) Spacecraft
Separation Angles
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki;
Lemen, James
Bibcode: 2012SoPh..281..101A
Altcode: 2012SoPh..tmp..197A; 2012arXiv1207.2787A
We performed for the first time stereoscopic
triangulation of coronal loops in active regions
over the entire range of spacecraft separation angles
(αsep≈6∘,43∘,89∘,127∘,and
170∘). The accuracy of stereoscopic correlation depends
mostly on the viewing angle with respect to the solar surface
for each spacecraft, which affects the stereoscopic correspondence
identification of loops in image pairs. From a simple theoretical model
we predict an optimum range of αsep≈22∘ -
125∘, which is also experimentally confirmed. The best
accuracy is generally obtained when an active region passes the central
meridian (viewed from Earth), which yields a symmetric view for both
STEREO spacecraft and causes minimum horizontal foreshortening. For
the extended angular range of αsep≈6∘
- 127∘ we find a mean 3D misalignment angle
of μPF≈21∘ - 39∘ of
stereoscopically triangulated loops with magnetic potential-field
models, and μFFF≈15∘ - 21∘
for a force-free field model, which is partly caused by stereoscopic
uncertainties μSE≈9∘. We predict optimum
conditions for solar stereoscopy during the time intervals of 2012 -
2014, 2016 - 2017, and 2021 - 2023.
Title: What Are Special About Ground-Level Events?. Flares, CMEs,
Active Regions and Magnetic Field Connection
Authors: Nitta, N. V.; Liu, Y.; DeRosa, M. L.; Nightingale, R. W.
Bibcode: 2012SSRv..171...61N
Altcode: 2012arXiv1203.5777N; 2012SSRv..tmp...21N
Ground level events (GLEs) occupy the high-energy end of gradual solar
energetic particle (SEP) events. They are associated with coronal
mass ejections (CMEs) and solar flares, but we still do not clearly
understand the special conditions that produce these rare events. During
Solar Cycle 23, a total of 16 GLEs were registered, by ground-based
neutron monitors. We first ask if these GLEs are clearly distinguishable
from other SEP events observed from space. Setting aside possible
difficulties in identifying all GLEs consistently, we then try to find
observables which may unmistakably isolate these GLEs by studying the
basic properties of the associated eruptions and the active regions
(ARs) that produced them. It is found that neither the magnitudes of
the CMEs and flares nor the complexities of the ARs give sufficient
conditions for GLEs. It is possible to find CMEs, flares or ARs that
are not associated with GLEs but that have more extreme properties than
those associated with GLEs. We also try to evaluate the importance of
magnetic field connection of the AR with Earth on the detection of GLEs
and their onset times. Using the potential field source surface (PFSS)
model, a half of the GLEs are found to be well-connected. However,
the GLE onset time with respect to the onset of the associated flare
and CME does not strongly depend on how well-connected the AR is. The
GLE onset behavior may be largely determined by when and where the
CME-driven shock develops. We could not relate the shocks responsible
for the onsets of past GLEs with features in solar images, but the
combined data from the Solar TErrestrial RElations Observatory (STEREO)
and the Solar Dynamics Observatory (SDO) have the potential to change
this for GLEs that may occur in the rising phase of Solar Cycle 24.
Title: Multi-point Shock and Flux Rope Analysis of Multiple
Interplanetary Coronal Mass Ejections around 2010 August 1 in the
Inner Heliosphere
Authors: Möstl, C.; Farrugia, C. J.; Kilpua, E. K. J.; Jian, L. K.;
Liu, Y.; Eastwood, J. P.; Harrison, R. A.; Webb, D. F.; Temmer, M.;
Odstrcil, D.; Davies, J. A.; Rollett, T.; Luhmann, J. G.; Nitta, N.;
Mulligan, T.; Jensen, E. A.; Forsyth, R.; Lavraud, B.; de Koning,
C. A.; Veronig, A. M.; Galvin, A. B.; Zhang, T. L.; Anderson, B. J.
Bibcode: 2012ApJ...758...10M
Altcode: 2012arXiv1209.2866M
We present multi-point in situ observations of a complex
sequence of coronal mass ejections (CMEs) which may serve as
a benchmark event for numerical and empirical space weather
prediction models. On 2010 August 1, instruments on various
space missions, Solar Dynamics Observatory/Solar and Heliospheric
Observatory/Solar-TErrestrial-RElations-Observatory (SDO/SOHO/STEREO),
monitored several CMEs originating within tens of degrees from the
solar disk center. We compare their imprints on four widely separated
locations, spanning 120° in heliospheric longitude, with radial
distances from the Sun ranging from MESSENGER (0.38 AU) to Venus
Express (VEX, at 0.72 AU) to Wind, ACE, and ARTEMIS near Earth and
STEREO-B close to 1 AU. Calculating shock and flux rope parameters at
each location points to a non-spherical shape of the shock, and shows
the global configuration of the interplanetary coronal mass ejections
(ICMEs), which have interacted, but do not seem to have merged. VEX
and STEREO-B observed similar magnetic flux ropes (MFRs), in contrast
to structures at Wind. The geomagnetic storm was intense, reaching
two minima in the Dst index (≈ - 100 nT), and was caused by the
sheath region behind the shock and one of two observed MFRs. MESSENGER
received a glancing blow of the ICMEs, and the events missed STEREO-A
entirely. The observations demonstrate how sympathetic solar eruptions
may immerse at least 1/3 of the heliosphere in the ecliptic with their
distinct plasma and magnetic field signatures. We also emphasize the
difficulties in linking the local views derived from single-spacecraft
observations to a consistent global picture, pointing to possible
alterations from the classical picture of ICMEs.
Title: Preface
Authors: Gopalswamy, N.; Nitta, N. V.
Bibcode: 2012SSRv..171....1G
Altcode: 2012SSRv..tmp...79G
No abstract at ADS
Title: First Three-dimensional Reconstructions of Coronal Loops
with the STEREO A+B Spacecraft. IV. Magnetic Modeling with Twisted
Force-free Fields
Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; DeRosa, Marc L.; Malanushenko, Anna
Bibcode: 2012ApJ...756..124A
Altcode: 2012arXiv1207.2790A
The three-dimensional coordinates of stereoscopically triangulated
loops provide strong constraints for magnetic field models of active
regions in the solar corona. Here, we use STEREO/A and B data from some
500 stereoscopically triangulated loops observed in four active regions
(2007 April 30, May 9, May 19, and December 11), together with SOHO/MDI
line-of-sight magnetograms. We measure the average misalignment angle
between the stereoscopic loops and theoretical magnetic field models,
finding a mismatch of μ = 19°-46° for a potential field model,
which is reduced to μ = 14°-19° for a non-potential field model
parameterized by twist parameters. The residual error is commensurable
with stereoscopic measurement errors (μSE ≈ 8°-12°). We
developed a potential field code that deconvolves a line-of-sight
magnetogram into three magnetic field components (Bx ,
By , Bz ), as well as a non-potential field
forward-fitting code that determines the full length of twisted loops
(L ≈ 50-300 Mm), the number of twist turns (median N twist
= 0.06), the nonlinear force-free α-parameter (median α ≈ 4 ×
10-11 cm-1), and the current density (median
jz ≈ 1500 Mx cm-2 s-1). All twisted
loops are found to be far below the critical value for kink instability,
and Joule dissipation of their currents is found to be far below the
coronal heating requirement. The algorithm developed here, based on an
analytical solution of nonlinear force-free fields that is accurate to
second order (in the force-free parameter α), represents the first
code that enables fast forward fitting to photospheric magnetograms
and stereoscopically triangulated loops in the solar corona.
Title: On the Oscillatory and Non-oscillatory Loop Systems and
Dynamical Processes during the X2.1 Solar Flare on 06 September 2011
Authors: Srivastava, Abhishek K.; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy,
Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Norris, Max; Makela, Pertti; Mahalakshmi,
K.; Elamathi, E.
Bibcode: 2012cosp...39.1882S
Altcode: 2012cosp.meet.1882S
No abstract at ADS
Title: Magnetic Flux Imbalance in Active Regions NOAA 11283 and
NOAA 11302
Authors: Prasad Choudhary, Debi; Jain, Rajmal; Charan Dwivedi, Vidya;
Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun
Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Manoharan, P. K.;
Norris, Max; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro,
Seiji
Bibcode: 2012cosp...39..334P
Altcode: 2012cosp.meet..334P
We investigate the magnetic flux imbalance of two active regions NOAA
11302 and NOAA 11283 during their disk passage. The active region NOAA
11302 appeared in the east limb on September 23, 2011 as beta-gamma
complexity and produced 73 c-class, 27 M-class and 2 X-class flares
many of which were associated with CMEs during the disk passage. The
active region NOAA 11283 appeared on the east limb on September 1,
2011 as beta-gamma complexity and produced 16 c-class, 9 m-class and 2
x-class flares and CMEs. Both these active regions were of similar size
but the evolution of magnetic complexity during their disk passage
was very different. None of them made second disk passage. These
two active regions represent two different class of activity. Among
several reasons, the magnetic flux imbalance of the active regions
result due to the presence of electric current with in the active
regions. The high cadence full disk magnetograms obtained using the
GONG and SDO-HMI instruments serve as the primary data source of this
investigation. We relate the change in the magnetic flux imbalance
with the flare occurrence in these two contrasting active regions.
Title: Height of Shock Formation in the Solar Corona Inferred from
Observations of Type II Radio Bursts and Coronal Mass Ejections
Authors: Gopalswamy, Nat; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Awasthi,
Arun Kumar; Srivastava, Abhishek K.; Joshi, N. C.; Manoharan, P. K.;
Makela, Pertti; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro,
Seiji; Akiyam, Sachiko
Bibcode: 2012cosp...39..653G
Altcode: 2012cosp.meet..653G
No abstract at ADS
Title: Quasi-periodic Fast-mode Wave Trains within a Global EUV Wave
and Sequential Transverse Oscillations Detected by SDO/AIA
Authors: Liu, Wei; Ofman, Leon; Nitta, Nariaki V.; Aschwanden, Markus
J.; Schrijver, Carolus J.; Title, Alan M.; Tarbell, Theodore D.
Bibcode: 2012ApJ...753...52L
Altcode: 2012arXiv1204.5470L
We present the first unambiguous detection of quasi-periodic wave
trains within the broad pulse of a global EUV wave (so-called EIT wave)
occurring on the limb. These wave trains, running ahead of the lateral
coronal mass ejection (CME) front of 2-4 times slower, coherently
travel to distances >~ R ⊙/2 along the solar surface,
with initial velocities up to 1400 km s-1 decelerating to
~650 km s-1. The rapid expansion of the CME initiated at
an elevated height of 110 Mm produces a strong downward and lateral
compression, which may play an important role in driving the primary
EUV wave and shaping its front forwardly inclined toward the solar
surface. The wave trains have a dominant 2 minute periodicity that
matches the X-ray flare pulsations, suggesting a causal connection. The
arrival of the leading EUV wave front at increasing distances produces
an uninterrupted chain sequence of deflections and/or transverse (likely
fast kink mode) oscillations of local structures, including a flux-rope
coronal cavity and its embedded filament with delayed onsets consistent
with the wave travel time at an elevated (by ~50%) velocity within
it. This suggests that the EUV wave penetrates through a topological
separatrix surface into the cavity, unexpected from CME-caused magnetic
reconfiguration. These observations, when taken together, provide
compelling evidence of the fast-mode MHD wave nature of the primary
(outer) fast component of a global EUV wave, running ahead of the
secondary (inner) slow component of CME-caused restructuring.
Title: Multi-wavelength diagnostics of thermal and non-thermal
sources in the 22 April 2011 flare event
Authors: Awasthi, Arun Kumar; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy,
Nat; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Norris, Max; Mahalakshmi, K.; Elamathi, E.;
Uddin, Wahab
Bibcode: 2012cosp...39...75A
Altcode: 2012cosp.meet...75A
No abstract at ADS
Title: Coronal Mass Ejections and Type II Radio Bursts from Active
Region 11158
Authors: Yashiro, Seiji; Jain, Rajmal; Prasad Choudhary, Debi; Charan
Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat;
Awasthi, Arun Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap,
Pradeep; Joshi, N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi,
K.; Elam, E.
Bibcode: 2012cosp...39.2205Y
Altcode: 2012cosp.meet.2205Y
The NOAA active region (AR) 11158 emerged at around S20E50 on 2011
February 10 as two bipoles and quickly developed into a large complex
region. During 2011 February 13-17, AR 11158 produced 48 flares
(>C1 level) including the first X-class flare of 15 February,
2011 in solar cycle 24. The 48 flares can be divided into four groups
based on their location within the AR. We examined their associations
of coronal mass ejections (CMEs) and metric type II radio bursts in
order to find preferred locations of both the phenomena. We found that,
out of 48 flares, 15 had associated CMEs, occurring frequently at the
eastern edge of the AR. We also found that six flares were associated
with type II radio bursts and all of them were associated with CMEs
also. No type II was associated with the CME-less flare. This suggests
that the CME association is a necessary condition for a flare to be
associated with a metric type II burst.
Title: A Study of the 12 June 2010 C6.1/SF flare associated with a
CME, surge and energetic particles
Authors: Uddin, Wahab; Jain, Rajmal; Manoharan, P. K.; Prasad
Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta,
Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh;
Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Norris, Max;
Makela, Pertti; Mahalaksh, K.
Bibcode: 2012cosp...39.2026U
Altcode: 2012cosp.meet.2026U
In this paper, we present the multiwavelength analysis of the C6.1/SF
flare on 12 June 2010 from NOAA AR 11081. The flare was observed by
various ground based (ARIES H-alpha; HIRAS Radio) and space borne
observatories (SDO, STEREO, SOHO, GOES). The flare was accompanied by
a spray/surge and a slow coronal mass ejection (CME) that propagated
with a speed of ~382 km/s. The eruption was associated with a weak
solar energetic particle (SEP) event. The solar source of the eruption
was a rapid emerging flux region. The eruption was also associated
with the three major types of radio bursts (type II, III and IV). The
interesting observation is the shock production (type II burst and
SEP event) by a relatively slow CME. We interpret the results in the
light of existing theories.
Title: Large-Scale Coronal Disturbances as Observed by SDO AIA
Authors: Nitta, Nariaki; Schrijver, Carolus; Title, Alan; Lemen,
James; Liu, Wei
Bibcode: 2012cosp...39.1378N
Altcode: 2012cosp.meet.1378N
With increasing solar activity, the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory (SDO) has observed a number
of large-scale coronal disturbances, which may correspond to what we
have generally known as "EIT waves." Although their nature is still
actively debated, these disturbances usually accompany CMEs. In certain
cases, the fronts of the disturbances may signify CME-related shock
waves important for particle acceleration. Using the unprecedented
temporal resolution and broad temperature coverage of the AIA, we have
studied more than 100 such events. Here we discuss their kinematics
characterized by faster fronts than EIT waves in Solar Cycle 23, and
spatial relations with CMEs using STEREO data that provide triangulation
of the fronts. We also try plasma diagnostic using images in different
filters. Association of these disturbances with CMEs, flares and type
II bursts is discussed on a statistical basis. Lastly, we explore the
possible relation of the larger-scale coronal disturbances with SEP
events observed at widely separate longitudes and their onset times.
Title: Solar Energetic Particle Events and Associated CMEs during
the Rising Phases of Solar Cycle 23 and 24 - A Comparative study
Authors: Chandra, Ramesh; Jain, Rajmal; Prasad Choudhary, Debi; Charan
Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat;
Awasthi, Arun Kumar; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi, K.; Elamathi,
E.; Uddi, Wahab
Bibcode: 2012cosp...39..303C
Altcode: 2012cosp.meet..303C
No abstract at ADS
Title: Environmental Conditions of Multi-spacecraft 3He-rich Events
Authors: Cohen, Christina; Gomez-Herrero, R.; Haggerty, D. K.; Mason,
G. M.; Nitta, N.; Wiedenbeck, M. E.
Bibcode: 2012shin.confE..22C
Altcode:
In a study of 17 3He-rich SEP events, Kocharov et al [2008] found
such events were typically associated with slow or declining solar
wind periods, often following a high speed stream (consistent with
the presence of a coronal hole near the solar source of the SEP
event). Here we present the interplanetary and solar conditions of
3He-rich events observed simultaneously by multiple spacecraft. These
events are observed even when the spacecraft are separated by more
than 60 degrees in solar longitude, contrary to expectations assuming
a small, localized solar source region. We compare the environment
in which these multi-spacecraft 3He-rich events occur to that of the
results of studies such as Kocharov et al. and our own examination of
single-spacecraft 3He-rich events.
Title: Multi-point shock and flux rope analysis of multiple ICMEs
around 2010 August 1 in the inner heliosphere
Authors: Moestl, Christian; Farrugia, C. J.; Kilpua, E. K. J.; Jian,
L.; Liu, Y.; Jensen, L.; Mulligan, T.; Eastwood, J.; Rollett, T.;
Temmer, M.; Luhmann, J. G.; Harrison, R.; Davies, J. A.; Webb, D.;
Forsyth, R.; Lavraud, B.; Odstrcil, D.; de Koning, C. A.; Nitta, N.;
Veronig, A. M.; Galvin, A. B.; Zhang, T. L.
Bibcode: 2012shin.confE..77M
Altcode:
We present multi-point in situ observations of a complex sequence
of coronal mass ejections which may serve as a benchmark event for
numerical and empirical space weather prediction models. On 2010 August
1, instruments on various space missions (SDO/SOHO/STEREO) monitored
repeated coronal mass ejections originating within tens of degrees from
solar disk center. We compare their imprints on four widely separated
locations, covering 120 degree in heliospheric longitude, with radial
distances from the Sun ranging from MESSENGER (0.38 AU) to Venus Express
(VEX, at 0.72 AU) to Wind, ACE and ARTEMIS near Earth and STEREO-B close
to 1 AU. Calculating shock and flux rope parameters at each location
points to a non-spherical shape of the shock, and shows the global
configuration of the interplanetary coronal mass ejections (ICMEs),
which have interacted but not merged, making individual identifications
still possible. VEX and STEREO-B observed similar magnetic flux ropes,
in contrast to the structures at Wind. The geomagnetic storm was
moderate to major, reaching two minima in the Dst index, caused by the
sheath region behind the shock and one of two observed magnetic flux
ropes. MESSENGER received a glancing blow of the ICMEs, and the events
missed STEREO-A entirely. The observations demonstrate how sympathetic
solar eruptions may immerse at least 1/3 of the heliosphere in the
ecliptic with their distinct plasma and magnetic field signatures
and emphasize the difficulties in linking the local views derived
from single-spacecraft observations to a consistent global picture,
pointing to possible alterations from the classical picture of ICMEs.
Title: SDO/AIA Detection of Quasi-periodic Wave Trains Within Global
EUV ("EIT") Waves and Their Coronal Seismology Implications
Authors: Liu, Wei; Ofman, L.; Aschwanden, M. J.; Nitta, N.; Schrijver,
C. J.; Title, A. M.; Tarbell, T. D.
Bibcode: 2012AAS...22051501L
Altcode:
The nature of global EUV waves (so-called "EIT waves") has long
been under debate because of instrumental limitations and projection
effects when viewed on the solar disk. We present here high cadence
SDO/AIA observations of global EUV waves occurring on the limb. We
report newly discovered quasi-periodic wave trains located in the low
corona within a broad, diffuse pulse of the global EUV wave ahead of
the lateral CME front/flank. These waves coherently travel to large
distances on the order of 1 solar radii with initial velocities up
to 1400 km/s. They have dominant 1-3 minute periodicities that often
match the X-ray pulsations of the accompanying flare, suggestive of
a causal connection. In addition, recently discovered quasi-periodic
fast propagating (QFP) waves of 1000-2000 km/s (Liu, Title, Zhao et
al. 2011 ApJL) are found in the funnel of coronal loops rooted at the
flare kernel. These waves are spatially confined within the CME bubble
and rapidly disappear while approaching the CME front, suggestive
of strong damping and/or dispersion. These observations provide new
evidence of the fast-mode wave nature of the primary, fast component
of a global EUV wave, running ahead of a secondary, slow component
of CME-caused restructuring of the coronal magnetic field. We suggest
that the two types of quasi-periodic waves are both integral parts of
global coronal dynamics manifested as a CME/flare eruption, and they
have important implications for global and local coronal seismology.
Title: Force-Free Magneto-Stereoscopy of Coronal Loops
Authors: Aschwanden, Markus J.; Malanushenko, A.; Wuelser, J.; Nitta,
N.; Lemen, J. R.; DeRosa, M.
Bibcode: 2012AAS...22041103A
Altcode:
We derive an analytical approximation of nonlinear force-free
magnetic field solutions (NLFFF) that can efficiently be used for
fast forward-fitting to solar magnetic data, constrained either by
observed line-of-sight magnetograms and stereoscopically triangulated
coronal loops, or by 3D vector-magnetograph data. We test the code by
forward-fitting to simulated data, to force-free solutions derived by
Low and Lou (1990), and to active regions observed with STEREO/EUVI and
SOHO/MDI. The forward-fitting tests demonstrate: (i) a satisfactory
convergence behavior (with typical misalignment angles of 1-10 deg),
(ii) a high fidelity of retrieved force-free alpha-parameters, and
(iii) relatively fast computation times (from seconds to minutes). The
novel feature of this NLFFF code is the derivation of a quasi-forcefree
field based on coronal constraints, which bypasses the non-forcefree
photosphere of standard magnetograms. Applications range from magnetic
modeling of loops to the determnination of electric currents, twist,
helicity, and free (non-potential) energy in active regions.
Title: An Analysis of the Origin and Propagation of the Multiple
Coronal Mass Ejections of 2010 August 1
Authors: Harrison, R. A.; Davies, J. A.; Möstl, C.; Liu, Y.; Temmer,
M.; Bisi, M. M.; Eastwood, J. P.; de Koning, C. A.; Nitta, N.; Rollett,
T.; Farrugia, C. J.; Forsyth, R. J.; Jackson, B. V.; Jensen, E. A.;
Kilpua, E. K. J.; Odstrcil, D.; Webb, D. F.
Bibcode: 2012ApJ...750...45H
Altcode:
On 2010 August 1, the northern solar hemisphere underwent significant
activity that involved a complex set of active regions near central
meridian with, nearby, two large prominences and other more distant
active regions. This activity culminated in the eruption of four major
coronal mass ejections (CMEs), effects of which were detected at Earth
and other solar system bodies. Recognizing the unprecedented wealth of
data from the wide range of spacecraft that were available—providing
the potential for us to explore methods for CME identification and
tracking, and to assess issues regarding onset and planetary impact—we
present a comprehensive analysis of this sequence of CMEs. We show that,
for three of the four major CMEs, onset is associated with prominence
eruption, while the remaining CME appears to be closely associated
with a flare. Using instrumentation on board the Solar Terrestrial
Relations Observatory spacecraft, three of the CMEs could be tracked
out to elongations beyond 50° their directions and speeds have been
determined by various methods, not least to assess their potential for
Earth impact. The analysis techniques that can be applied to the other
CME, the first to erupt, are more limited since that CME was obscured
by the subsequent, much faster event before it had propagated far from
the Sun; we discuss the speculation that these two CMEs interact. The
consistency of the results, derived from the wide variety of methods
applied to such an extraordinarily complete data set, has allowed
us to converge on robust interpretations of the CME onsets and their
arrivals at 1 AU.
Title: Large-scale Coronal Disturbances As Observed By SDO AIA
Authors: Nitta, Nariaki; Schrijver, C.; title, A.; Liu, W.; Lemen, J.
Bibcode: 2012AAS...22051502N
Altcode:
With increasing solar activity, the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory (SDO) has observed a number
of large-scale coronal disturbances, which may correspond to what
we have generally known as "EIT waves." Their nature is still
actively debated. In certain cases, the fronts of the disturbances
may signify CME-related shock waves that are important for particle
acceleration. Using the unprecedented temporal resolution and broad
temperature coverage of the AIA, we have studied more than 100 such
events. Here we discuss their kinematics characterized by faster fronts
than EIT waves in Solar Cycle 23, and spatial relations with CMEs
using STEREO data that provide triangulation of the fronts. We also
try plasma diagnostic using images in different filters. Association
of these disturbances with other phenomena such as CMEs, flares and
type II bursts, is discussed on a statistical basis.
Title: Magnetic field connection and large scale coronal disturbances
in the context of gradual SEP events
Authors: Nitta, Nariaki V.
Bibcode: 2012AIPC.1436..259N
Altcode:
In order to be able to predict extremely hazardous energetic particles
from the Sun, it is important to understand the factors that control
the onset behavior and peak flux of solar energetic particle (SEP)
events. The shock properties should be naturally important for so-called
gradual SEP events, which are believed to result from acceleration at
shocks driven by coronal mass ejections (CMEs). Magnetic connection of
the CME source region and adjacent regions to the observer may also
play a role in the early development of SEP events. Now with STEREO,
SEP events can be observed at three widely separated locations. We
study such SEP events that occurred in August 2010. Although their
proton flux was modest, they also lack the anomaly in abundance that is
characteristic of impulsive SEP events. Therefore they are likely to
be shock-accelerated gradual SEP events. In this paper we take up one
of those events and try to explain their different temporal profiles
of near-relativistic electrons at Earth and two STEREO spacecraft in
terms of the proposition that first arriving particles are released
when the acceleration region intersects well-connected field lines. We
discuss how wave-like motions seen in EUV images from SDO and STEREO
could indicate the shock waves responsible for SEP events.
Title: CME-CME interaction during the 2010 August 1 events
Authors: Temmer, M.; Vrsnak, B.; Rollett, T.; Bein, B.; deKoning,
C. A.; Liu, Y.; Bosman, E.; Davies, J. A.; Möstl, C.; Zic, T.;
Veronig, A. M.; Bothmer, V.; Harrison, R.; Nitta, N.; Bisi, M.; Flor,
O.; Eastwood, J.; Odstrcil, D.; Forsyth, R.
Bibcode: 2012EGUGA..14.1677T
Altcode:
We study a CME-CME interaction that occurred during the 2010 August 1
events using STEREO/SECCHI data (COR and HI). The CMEs were Earth
directed where clear signatures of magnetic flux ropes could be
measured from in situ Wind data. To give evidence of the actual
interaction we derive the direction of motion for both CMEs applying
several independent methods. From this we obtain that both CMEs head
into similar directions enabling us to actually observe the merging
in the HI1 field-of-view (and rule out the possibility that this is
just a line of sight effect). The full de-projected kinematics of the
faster CME from Sun to Earth is derived when combining data points from
remote observations with in situ parameters of the ICME measured at
1 AU. We study the evolution of the kinematical profile of the faster
CME by applying a drag based model.
Title: Characteristics of Kinematics of a Coronal Mass Ejection
during the 2010 August 1 CME-CME Interaction Event
Authors: Temmer, Manuela; Vršnak, Bojan; Rollett, Tanja; Bein, Bianca;
de Koning, Curt A.; Liu, Ying; Bosman, Eckhard; Davies, Jackie A.;
Möstl, Christian; Žic, Tomislav; Veronig, Astrid M.; Bothmer, Volker;
Harrison, Richard; Nitta, Nariaki; Bisi, Mario; Flor, Olga; Eastwood,
Jonathan; Odstrcil, Dusan; Forsyth, Robert
Bibcode: 2012ApJ...749...57T
Altcode: 2012arXiv1202.0629T
We study the interaction of two successive coronal mass ejections (CMEs)
during the 2010 August 1 events using STEREO/SECCHI COR and heliospheric
imager (HI) data. We obtain the direction of motion for both CMEs by
applying several independent reconstruction methods and find that the
CMEs head in similar directions. This provides evidence that a full
interaction takes place between the two CMEs that can be observed in the
HI1 field of view. The full de-projected kinematics of the faster CME
from Sun to Earth is derived by combining remote observations with in
situ measurements of the CME at 1 AU. The speed profile of the faster
CME (CME2; ~1200 km s-1) shows a strong deceleration over
the distance range at which it reaches the slower, preceding CME (CME1;
~700 km s-1). By applying a drag-based model we are able
to reproduce the kinematical profile of CME2, suggesting that CME1
represents a magnetohydrodynamic obstacle for CME2 and that, after
the interaction, the merged entity propagates as a single structure
in an ambient flow of speed and density typical for quiet solar wind
conditions. Observational facts show that magnetic forces may contribute
to the enhanced deceleration of CME2. We speculate that the increase
in magnetic tension and pressure, when CME2 bends and compresses the
magnetic field lines of CME1, increases the efficiency of drag.
Title: SDO/AIA Observations of Various Coronal EUV Waves Associated
with Flares/CMEs and Their Coronal Seismology Implications
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
Bibcode: 2012decs.confE..87L
Altcode:
MHD waves can be used as diagnostic tools of coronal seismology to
decipher otherwise elusive critical physical parameters of the solar
corona, such as the magnetic field strength and plasma density. They
are analogous to acoustic waves used in helioseismology, but with
complexities arising from the magnetic field and nonlinearity. Recent
high cadence, high resolution, full-disk imaging observations from
SDO/AIA have opened a new chapter in understanding these waves. Various
types of EUV waves associated with flares/CMEs have been discovered
or observed in unprecedented detail. In this presentation, we will
review such new AIA observations, focusing on the following topics and
their interrelationships: (1) quasi-periodic fast waves traveling along
coronal funnels within CME bubbles at speeds up to 2000 km/s, associated
with flare pulsations at similar frequencies; (2) quasi-periodic wave
trains within broad, diffuse pulses of global EUV waves (so-called
EIT waves) running ahead of CME fronts; (3) interactions of global EUV
waves with local coronal structures on their paths, such as flux-rope
coronal cavities and their embedded filaments (kink oscillations)
and coronal holes/active regions (deflections). We will discuss the
implications of these observations on coronal seismology, on their roles
in transporting energy through different parts of the solar atmosphere,
and on understanding their associated eruptive flares/CMEs.
Title: Coronal Magnetic Field Measurement from EUV Images Made by
the Solar Dynamics Observatory
Authors: Gopalswamy, Nat; Nitta, Nariaki; Akiyama, Sachiko; Mäkelä,
Pertti; Yashiro, Seiji
Bibcode: 2012ApJ...744...72G
Altcode: 2011arXiv1109.2925G
By measuring the geometrical properties of the coronal mass ejection
(CME) flux rope and the leading shock observed on 2010 June 13 by
the Solar Dynamics Observatory (SDO) mission's Atmospheric Imaging
Assembly we determine the Alfvén speed and the magnetic field strength
in the inner corona at a heliocentric distance of ~1.4 Rs. The basic
measurements are the shock standoff distance (ΔR) ahead of the CME
flux rope, the radius of curvature of the flux rope (R c),
and the shock speed. We first derive the Alfvénic Mach number (M) using
the relationship, ΔR/R c = 0.81[(γ-1) M 2 +
2]/[(γ+1)(M 2 - 1)], where γ is the only parameter that
needed to be assumed. For γ = 4/3, the Mach number declined from
3.7 to 1.5 indicating shock weakening within the field of view of the
imager. The shock formation coincided with the appearance of a type II
radio burst at a frequency of ~300 MHz (harmonic component), providing
an independent confirmation of the shock. The shock compression ratio
derived from the radio dynamic spectrum was found to be consistent
with that derived from the theory of fast-mode MHD shocks. From the
measured shock speed and the derived Mach number, we found the Alfvén
speed to increase from ~140 km s-1 to 460 km s-1
over the distance range 1.2-1.5 Rs. By deriving the upstream plasma
density from the emission frequency of the associated type II radio
burst, we determined the coronal magnetic field to be in the range
1.3-1.5 G. The derived magnetic field values are consistent with other
estimates in a similar distance range. This work demonstrates that the
EUV imagers, in the presence of radio dynamic spectra, can be used as
coronal magnetometers.
Title: The Dependence of Solar Energetic Particle Event
Characteristics on Heliographic Longitude
Authors: Wiedenbeck, M. E.; Cohen, C. M.; Cummings, A. C.;
Gómez-Herrero, R.; Haggerty, D. K.; Leske, R. A.; Mason, G. M.;
Mewaldt, R. A.; Nitta, N. V.; Stone, E. C.; von Rosenvinge, T. T.
Bibcode: 2011AGUFMSH12A..04W
Altcode:
Characteristics of solar energetic particle (SEP) events observed
at 1 AU from the Sun are known to depend on the magnetic connection
of the observer to the acceleration region as well as on whether
the acceleration is associated with a shock ("gradual" SEP events)
or with flare reconnection ("impulsive" events). Much of our present
knowledge of the dependence of SEP event characteristics on heliographic
longitude has been the result of single-spacecraft statistical studies
that identified systematic correlations of various event properties
(e.g., rise time, peak intensity, composition) with the location of
the observer relative to the magnetic field line having the best
connection to the solar source region. Several studies combining
SEP observations made near Earth with data from spacecraft at other
heliographic longitudes (Helios and various planetary missions) or
latitudes (Ulysses) have also contributed. The two STEREO spacecraft,
in conjunction with ACE and Wind operating near Earth, have enabled
the first systematic study of the longitudinal dependence of SEP
characteristics in individual events at 1 AU. The years 2007 through
2010, during which longitudinal separations of the STEREOs from
Earth increased from 0o to nearly 90o, were
characterized by a very low level of solar activity. These conditions
allowed unusually sensitive measurements of a number of impulsive
SEP events and led to the realization that particles accelerated in
these events are frequently detectable over a range of longitudes much
greater than expected from simple models in which flare-accelerated
particles escape into the heliosphere along small clusters of open
field lines involved in a reconnection event. Particle fluences,
however, were found to have a strong dependence on separation from the
well-connected longitude. Since early 2011, as solar activity has been
on the increase, a number of gradual SEP events have been observed
over the >180o range of heliographic longitudes now
spanned by the STEREOs and ACE, enabling multipoint studies of this
type of event as well. The interpretation of the SEP data, which are
sensitive to both acceleration and transport effects, is benefitting
significantly from the 3D observations of coronal activity and
CMEs that are now being provided by STEREO, SDO, and SOHO. We will
present recent observations of the longitudinal dependence of SEP
event characteristics and discuss how they are beginning to modify
our understanding of these events. This work was supported by NASA
at Caltech (under grants NNX08AI11G, NNX10AQ68G, and NNX10AQ68G, and
through UC Berkeley under contract NAS5-03131), JPL, APL (under grant
NNX10AT75G), and LMSAL (under grants NNX07AN13G and NNX08AB23G).
Title: Observables Indicating Two Major Coronal Mass Ejections During
the WHI
Authors: Nitta, N. V.
Bibcode: 2011SoPh..274..219N
Altcode:
Two of the five fast (v≳900 km s−1) coronal mass
ejections (CMEs) between January 2007 and December 2009 were
observed during the Whole Heliosphere Interval (WHI: 20 March - 16
April 2008). The main purpose of this article is to discuss possible
observational signatures that could have been used to predict these
CMEs. During the WHI, there were three active regions aligned almost
East-West in a longitudinal span of about 60°. They were NOAA active
region (AR) 10987, 10988, and 10989. In terms of the sunspot area,
AR 10988 was the largest. However, the fast CMEs were launched from
AR 10989 on 25 March and from AR 10987 on 5 April. One explanation
for this may be that AR 10988, unlike the other two regions, emerged
underneath a predominantly closed magnetic-field environment, as shown
by global magnetic-field extrapolations. Around the times of these
CMEs, however, magnetic-field observations of the source regions
were essentially missing, because they were close to, or behind,
the limb as viewed from Earth. Therefore, we explore an extended
view in longitude of the regions from the Solar Terrestrial Relations
Observatory (STEREO). The two STEREO spacecraft were located ≈ 24°
East and West of the Sun-Earth line during the period of interest. We
study the frequency of microflares in the three regions and changes in
large-scale structures including streamers, but the CMEs do not seem
to be correlated with either of them. Instead, activation of filaments
or prominences may directly signal subsequent eruptions.
Title: Large-scale Coronal Propagating Fronts During the Rising
Phase of Solar Cycle 24
Authors: Nitta, N. V.; Liu, W.; Schrijver, C. J.; Title, A. M.; Lemen,
J. R.
Bibcode: 2011AGUFMSH23A1941N
Altcode:
With increasing solar activity, the AIA on SDO has observed a number
of large-scale coronal propagating fronts, which are often called
"EIT waves." Although their nature is still actively debated,
these propagating fronts usually accompany CMEs, and, in certain
cases, may signify CME-related shock waves important for particle
acceleration. Using the unprecedented temporal resolution and broad
temperature coverage of the AIA, it is possible to characterize
the propagating fronts in the corona far better than before, as
demonstrated in the literature for a yet small number of cases. We
study the properties of more than 40 propagating fronts as observed
by AIA, and discuss the key properties for them to be associated with
other phenomena such as type II radio bursts, flares, CMEs, ICMEs,
and SEP events. We make use of data, both remote-sensing and in-situ,
from STEREO which provides two additional vantage points, to make the
associations more solid. For the associated phenomena, their basic
properties are correlated with those of the propagating fronts. We
also revisit the association of EIT waves with other phenomena during
the similar phase of Solar Cycle 23 and discuss possible differences
in terms of global magnetic field. Understanding their relation with
other phenomena, we can have a more complete picture of the coronal
propagating fronts in the context of CME acceleration and deceleration.
Title: Coronal Seismology in the SDO Era: AIA Observations of Various
Coronal Waves Associated with CMEs/Flares
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
Bibcode: 2011sdmi.confE..49L
Altcode:
MHD waves, as critical diagnostic tools of coronal seismology, can be
used to decipher otherwise elusive physical parameters of the solar
corona, such as the magnetic field strength and plasma density. They
are analogous to acoustic waves used in helioseismology. Recent high
cadence, high resolution, full-disk imaging observations from SDO/AIA
have opened a new chapter in understanding these waves. Various types
of waves associated with flares and/or CMEs have been discovered. In
this presentation, we will review such new AIA observations, focusing
on the following topics: (1) fine structures in CME-related global EUV
waves (so-called EIT waves), including a diffuse pulse superimposed
with multiple sharp fronts or "ripples" (Liu et al. 2010, ApJL); (2)
quasi-periodic fast waves traveling in coronal funnels at speeds up to
2000 km/s and associated with flares pulsating at similar frequencies
(Liu et al. 2011, ApJL); (3) interaction of global EUV waves with local
coronal structures on their paths, such as flux-rope coronal cavities
(triggered kink oscillations, Liu et al. in preparation) and coronal
holes/active regions (deflection). We will discuss the implications
of these observations on coronal seismology and on understanding their
associated flares and CMEs. We also anticipate to exchange ideas with
helioseismologists at this workshop, in a hope to bring together coronal
seismology and helioseismology techniques to advance our understanding
of solar oscillations from the interior to the upper atmosphere.
Title: Coronal magnetic field measurement using CME-driven shock
observations
Authors: Gopalswamy, N.; Nitta, N.; Yashiro, S.; Mäkelä, P.; Xie,
H.; Akiyama, S.
Bibcode: 2011sdmi.confE..22G
Altcode:
Collisionless shocks form ahead of coronal mass ejections (CMEs) when
the CME speed exceeds the Alfven speed of the ambient plasma in the
corona and interplanetary medium. The shock stands at a distance from
the CME flux rope that depends on the shock Mach number, the geometry
of the driver, and the adiabatic index. While the shock ahead of the
CME has been observed for a long time in the in situ data, it has
been identified recently near the Sun in the coronagraphic and EUV
images. Unlike in situ observations, the imaging observations are two
dimensional, so one can better discern the CME-shock relationship
near the Sun. Gopalswamy and Yashiro (2011) demonstrated that the
coronal magnetic field can be derived from the shock standoff distance
measured in coronagraphic images. The method involves measuring the
standoff distance, the radius of curvature of the flux rope, and
assuming the value of the adiabatic index and deriving the Alfvénic
Mach number. The next step is to derive the Alfvénic Mach number
from the measured shock speed and an estimate of the local solar
wind speed. The final step involves deriving the magnetic field from
the Alfven speed by measuring the local plasma density either from
coronagraphic (polarized brightness) images (Gopalswamy and Yashiro
2011) or from the band-splitting of type II radio bursts (Gopalswamy
et al., 2011). In this paper, we derive the combined magnetic field
profile from near the Sun to the edge of the LASCO field of view
(1.5 to 30 solar radii) and compare it with the current model profiles.
Title: Propagation and impact of multiple coronal mass ejections
events on August 1 2010 in the heliosphere
Authors: Möstl, Christian; Farrugia, Charles J.; Harrison, Richard
A.; Davies, J. A.; Kilpua, Emilia K. J.; Odstrcil, Dusan; Rollett,
Tanja; Temmer, Manuela; Veronig, Astrid; Jian, Lan; Liu, Ying;
Eastwood, Jonathan; Forsyth, Robert; Webb, David; Bisi, Mario M.;
Jackson, Bernard V.; Mulligan, Tamitha; Jensen, Liz; Lavraud, Benoit;
de Koning, Curt A.; Nitta, Nariaki; Luhmann, Janet; Galvin, Antoinette
B.; Zhang, Tielong
Bibcode: 2011sdmi.confE..69M
Altcode:
On August 1 2010 a large region of the solar northern hemisphere
displayed major activity involving a complex set of central meridian and
remote active regions, and two large prominence channels (Schrijver
and Title, JGR, 2011). We witnessed the eruption of four coronal
mass ejections (CMEs) which partly impacted Earth and lead to one
of the first geomagnetic storms of the new solar cycle. We present an
overview of the results of several analyses exploiting the extraordinary
completeness of the imaging data (SDO/STEREO/SOHO) in combination with
numerical simulations (ENLIL) and in situ observations. The imprints of
the CMEs, including a prior event on July 30, were observed in situ in
an almost laboratory-like configuration at 4 widely separated locations
spanning over 120 degrees of heliospheric longitude (STEREO-B, Venus
Express, ACE/Wind, ARTEMIS, and MESSENGER). The CME density enhancements
could be followed with the STEREO-A/HI and Coriolis/SMEI instruments
continuously from the Sun to 1 AU. Evidences of CME-CME interactions
and resulting overlapping tracks in Jmaps make the analysis complex,
but nevertheless we find robust interpretations for linking two magnetic
flux ropes at Earth, one of them geo-effective and including elevated
alpha particles related to possible filament material, to their solar
counterparts. Additionally, we discuss the relationship between the
in situ observations and the global picture given by the ENLIL model.
Title: SDO/AIA Observations of a Global EUV Disturbance Traveling
into a Coronal Cavity and Its Subsequent Oscillations: New Evidence
of Fast Mode MHD Waves
Authors: Liu, Wei; Aschwanden, M. J.; Ofman, L.; Nitta, N. V.; Tarbell,
T. D.
Bibcode: 2011SPD....42.0906L
Altcode: 2011BAAS..43S.0906L
We report new SDO/AIA observations of a global EUV disturbance
that propagates at 600 km/s and sweeps through a coronal cavity,
instigating its bodily transverse oscillations. The high temporal
resolution and large FOV of AIA allow us to clearly see, for the first
time, the timing coincidence between the onsets of the oscillations
and the arrival of the disturbance at increasing distances covering
300 Mm in the neighborhood of the cavity. There is a time delay of
the oscillations from the near side to the far side of the cavity,
which is consistent with the travel time of the global perturbation. In
addition, we find a fine structure consisting of evenly spaced pulses
of periods 100-120 s within the global disturbance. In contrast, the
CME loop expansion falls behind the global disturbance at a smaller
velocity of 200 km/s. These observations suggests that this global
disturbance is a real fast mode MHD wave that continues propagating
into the cavity, rather than an apparent wave caused by CME expulsion
that is not expected to penetrate through a topological separatrix,
including the flux rope cavity boundary here. The cavity and its
hosted prominence have oscillation amplitudes of 20 km/s and periods
of 20-30 minutes. Such unusually long periods, compared with a few
minutes commonly observed in coronal loops, likely reflect kink mode
oscillations of the long cavity flux rope of a large length (a fraction
of the solar radius).
Title: Case Study of Four Homologous Large-scale Coronal Waves
Observed on 2010 April 28 and 29
Authors: Kienreich, I. W.; Veronig, A. M.; Muhr, N.; Temmer, M.;
Vršnak, B.; Nitta, N.
Bibcode: 2011ApJ...727L..43K
Altcode: 2011arXiv1101.5232K
On 2010 April 28 and 29, the Solar TErrestrial Relations Observatory
B/Extreme Ultraviolet Imager observed four homologous large-scale
coronal waves, the so-called EIT-waves, within 8 hr. All waves emerged
from the same source active region, were accompanied by weak flares and
faint coronal mass ejections, and propagated into the same direction
at constant velocities in the range of ~220-340 km s-1. The
last of these four coronal wave events was the strongest and fastest,
with a velocity of 337 ± 31 km s-1 and a peak perturbation
amplitude of ~1.24, corresponding to a magnetosonic Mach number of M
ms ~ 1.09. The magnetosonic Mach numbers and velocities of
the four waves are distinctly correlated, suggestive of the nonlinear
fast-mode magnetosonic wave nature of the events. We also found a
correlation between the magnetic energy buildup times and the velocity
and magnetosonic Mach number.
Title: Toward better understanding of the origin of CMEs using
combined SDO/AIA and STEREO/SECCHI data (Invited)
Authors: Nitta, N. V.
Bibcode: 2010AGUFMSH21C..06N
Altcode:
Thanks to uninterrupted observations by SOHO, solar cycle 23 has seen
great progress in characterizing coronal mass ejections (CMEs). But the
way CMEs are dramatically accelerated is still not well understood,
letting most of the proposed models remain as possibilities. This is
primarily because of the lack of high cadence observations in the low
corona, and also of mapping between EUV or X-ray and coronagraphic
manifestations of CMEs. Now these problems are largely ameliorated
by continuous data from SDO/AIA with ~12 second cadence (and 4k x 4k
0.5" pixels to cover more than the full disk in seven EUV channels),
which can be compared in detail with white-light observations. As
solar activity started to increase this year, a number of large
CMEs occurred and their associated phenomena were well observed by
AIA. This talk gives examples of CMEs jointly observed by AIA and
STEREO/SECCHI, and discusses how to clarify the sequence of physical
processes responsible for CME initiation and acceleration. Emphasis
is given to those disk events in Earth view that were well observed by
the COR-1 telescope(s) on STEREO as limb events, in which the mapping
of the eruption in the low corona and at larger distance can be made
in a relativelystraightforward manner.
Title: Large-scale coronal disturbances and angular spread of
SEP events
Authors: Nitta, N. V.; Mason, G. M.; Haggerty, D. K.; Cohen, C. M.;
Wiedenbeck, M. E.; Gómez-Herrero, R.
Bibcode: 2010AGUFMSH33B1847N
Altcode:
We discuss the currently available observations and tools to understand
the angular spread of solar energetic particle (SEP) events in the
corona. Following the widely used classification scheme of SEP events,
large gradual SEP events, accelerated by CME-driven shocks, are
expected to have much larger angular spreads than small impulsive SEP
events that are thought to be due to impulsive solar flares. But some
SEP events, labeled as impulsive, are observed at locations several
tens of degrees away from the longitudes that are usually conceived
as well-connected. One possibility is that large-scale disturbances
from the flare/CME site can transport energetic particles to locations
that are well-connected to Earth. Another possibility is that magnetic
field in the corona undergoes significant excursions before it reaches
the height above which it approximately follows the Parker spiral. In
order to evaluate relative importance of the two possibilities, we
re-examine the archive of SOHO EIT data to study the association
of SEP events during cycle 23 with EIT waves, and conduct global
magnetic field extrapolation using the potential field source surface
(PFSS) model. We also study recent SEP events whose source regions were
observed by STEREO EUVI and/or SDO AIA. Images from AIA, in particular,
have revealed more EUV waves from flares and CMEs than EIT would have,
largely because of the improvement in cadence and sensitivity. This
makes it possible for us to clarify the role of large-scale coronal
disturbances in the angular spread of SEP events.
Title: First SDO/AIA Observations of Global Coronal EUV "Waves":
Multiple Components and "Ripples"
Authors: Liu, W.; Nitta, N. V.; Schrijver, C. J.; Title, A. M.;
Tarbell, T. D.
Bibcode: 2010AGUFMSH13A..07L
Altcode:
Global coronal EUV disturbances (so-called "EIT waves") are useful
diagnostics for physical conditions on the Sun. Major drawbacks that
hindered our understanding of this phenomenon were previous instruments'
single view point, low cadence (e.g., 12 minutes of EIT), and limited
wavelength coverage. The Atmospheric Imaging Assembly (AIA) on board
the Solar Dynamics Observatory (SDO) observes the full-sun corona
at 10 EUV and UV wavelengths, covering a wide range of temperatures,
with high resolution (1.4") and cadence (up to 12 s). It thus offers a
great chance to end the decade long debate on the nature of global EUV
"waves". We present here the first AIA observations of such phenomenon
on 2010 April 8 revealed in unprecedented detail and discuss their
physical implications. The disturbance exhibits two components: one
weak, diffuse pulse superimposed by multiple strong, sharp fronts,
which again have slow and fast components. The disturbance originates
in front of erupting coronal loops and the slow sharp front undergoes
acceleration, both implying the disturbance being driven by the
coronal mass ejection (CME). Even at a 20 s cadence, the diffuse pulse
propagates at a surprisingly constant velocity of ~200 km/s, weakly
dependent on direction. The fast sharp front overtakes the slow front,
producing multiple "ripples" and steepening of the local pulse, and both
fronts propagate independently afterwards. These resemble the nature
of real waves. Unexpectedly, the amplitude and FWHM of the diffuse
pulse decreases linearly with distance. The diffuse pulse appears as
emission enhancement at hotter 193 Å but reduction at cooler 171 Å,
suggestive of heating, while the sharp fronts appear as enhancement at
both wavelengths, indicating density increase. As evidence for both
wave and non-wave models of "EIT waves" has been found, we suggest
that a hybrid model combining both mechanisms (e.g., Cohen et al. 2010)
may best explaine the data. In addition to the global EUV disturbance,
we found fast (600-1100 km/s) features repeated at 100 s intervals
as tentative evidence of fast mode MHD waves. Discoveries of the fast
features, multiple ripples, and two-component fronts were made possible
for the first time thanks to AIA's high cadences and sensitivities.
Title: Testing the PFSS Model Using Coronal Streamer Locations
Derived From LASCO, STEREO, EIT, and AIA Imagery
Authors: Slater, G. L.; Nitta, N. V.
Bibcode: 2010AGUFMSH31B1800S
Altcode:
A systematic comparison of the empirically measured root locations of
coronal streamers at the photosphere is compared with the corresponding
locations as derived by PFSS model extrapolations from the source
surface back to the photosphere. The method of empirically deriving
the root location of observed streamers is an adaptation of the method
applied by Liewer et al (2001) to a limited sample of Carrington
rotations using LASCO observations. We automate this procedure and make
use of LASCO, STEREO, EIT, and AIA image archives to produce a much
larger set of comparisons between PFSS extrapolations and empirical
observations. Using this expanded database of PFSS to actual streamer
locations, we draw conclusions about the reliability of the PFSS method
in modeling the large scale magnetic structure of the corona far from
the photosphere.
Title: An Alternative View of the "Masuda" Flare
Authors: Nitta, Nariaki V.; Freeland, Samuel L.; Liu, Wei
Bibcode: 2010ApJ...725L..28N
Altcode:
The limb flare on 1992 January 13, the so-called Masuda flare,
has stimulated scientists to refine theory of solar flares based on
two-dimensional magnetic reconnection. This is primarily because of
the hard X-ray (HXR) source seen above the clearly defined flare loop,
and the outward motions in soft X-rays (SXRs) interpreted as "plasmoid"
ejections. We have revisited Yohkoh HXR and SXR data for this and other
limb flares and found that the Masuda flare is still unique in terms of
the location and spectral properties of the coronal HXR source. However,
the outward motions in SXR outside the flare loop may not be as simply
characterized as plasmoid ejections as in other flares, nor are they
particularly fast. The motions appear complex partly because we also see
trans-equatorial loops in motion, one of whose legs anchors close to the
main flare loop. It is possible that these large-scale loops represent
post-flare loops, and that the flare may also be explained in terms of
three-dimensional quadrupolar reconnection, similar to those flares
where a pair of two loops exchange their footpoints through magnetic
reconnection. It appears that expansion and brightening of large-scale
loops offset from the main flare loop are not common, possibly providing
a reason for the unusual coronal HXR source in the Masuda flare.
Title: End-to-End Observations and Modeling of the 17-21 January
2010 CME/ICME
Authors: Webb, D. F.; Cliver, E. W.; Nitta, N. V.; Attrill, G. D.;
Marubashi, K.; Howard, T. A.; Tappin, J.; Jackson, B. V.
Bibcode: 2010AGUFMSH41A1778W
Altcode:
On 17 January 2010, before it rotated onto the Earth-facing disk and
produced a series of M-class X-ray flares, active region 11041 was
associated with an energetic CME with a coronal wave and dimming, radio
type II and III emission. During launch the CME revealed an unusual
circular profile viewed from STEREO-B with EUVI and extending into
the COR1 field. It was also observed over the southeast limb from SOHO
EIT and LASCO from which it appeared as a partial halo. The views from
STEREO and SOHO near the Sun and HI-A and SMEI at 1 AU suggest that at
least part of the CME traveled toward STEREO-B, where a small magnetic
cloud was observed on 21 January. The importance of this event lies in
the multiwavelength observations with high time cadences of near-limb
observations of a CME, its manifestations in the low corona, its passage
through the heliosphere and its appearance as an ICME/magnetic cloud
in-situ at STEREO-B 3.5 days later. This event permits us to study the
origin and driving of the wave because the flanks of the CME and its
relationship to the wave can be studied in detail. Our interpretation
is that the wave is initially driven by the CME and then becomes freely
propagating after the CME lateral expansion ends. Several models are
used to understand the 3-D geometry and propagation of the CME, and
two flux rope models are compared with the launch observations and
magnetic field orientations.
Title: Precursors of CMEs in coronal images
Authors: Freeland, S. L.; Slater, G. L.; Nitta, N. V.
Bibcode: 2010AGUFMSH43B1824F
Altcode:
For many years we have tried to find key observational signatures
primarily in low coronal images (such as SOHO EIT) that are useful for
predicting coronal mass ejections (CMEs). The signatures are isolated
in an automated way. It is also important to estimate the spatial and
kinematic properties of the white-light CME on the basis of its earlier
signatures as captured in EUV and X-ray images. In this presentation,
we give examples to demonstrate how low coronal observations help
us predict or understand the spatial and kinematic properties of
CMEs. We discuss how SDO AIA observations, either on their own or
in combination with data from other experiments (such as STEREO),
may improve our capability to predict CMEs and their properties.
Title: First SDO AIA Observations of a Global Coronal EUV "Wave":
Multiple Components and "Ripples"
Authors: Liu, Wei; Nitta, Nariaki V.; Schrijver, Carolus J.; Title,
Alan M.; Tarbell, Theodore D.
Bibcode: 2010ApJ...723L..53L
Altcode: 2012arXiv1201.0815L
We present the first Solar Dynamics Observatory Atmospheric Imaging
Assembly (AIA) observations of a global coronal EUV disturbance
(so-called "EIT wave") revealed in unprecedented detail. The disturbance
observed on 2010 April 8 exhibits two components: one diffuse pulse
superimposed, on which are multiple sharp fronts that have slow and
fast components. The disturbance originates in front of erupting coronal
loops and some sharp fronts undergo accelerations, both effects implying
that the disturbance is driven by a coronal mass ejection. The diffuse
pulse, propagating at a uniform velocity of 204-238 km s-1
with very little angular dependence within its extent in the south,
maintains its coherence and stable profile for ~30 minutes. Its arrival
at increasing distances coincides with the onsets of loop expansions and
the slow sharp front. The fast sharp front overtakes the slow front,
producing multiple "ripples" and steepening the local pulse, and both
fronts propagate independently afterward. This behavior resembles
the nature of real waves. Unexpectedly, the amplitude and FWHM of
the diffuse pulse decrease linearly with distance. A hybrid model,
combining both wave and non-wave components, can explain many, but
not all, of the observations. Discoveries of the two-component fronts
and multiple ripples were made possible for the first time thanks to
AIA's high cadences (<=20 s) and high signal-to-noise ratio.
Title: Coronal Dimming And Waves Observed In Flare-Associated CMEs
Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.;
Wuelser, J.; Zarro, D.
Bibcode: 2010AAS...21640614N
Altcode: 2010BAAS...41..882N
The relationship between solar flares and coronal mass ejections
(CMEs) is still an active area of research. It is studied from
various aspects. Our goal is to understand the importance of magnetic
reconnection in launching CMEs and that of magnetic field environment
of the flaring regions to determine how eruptive flares are. We have
studied the association of solar flares during 2007-2009 with CMEs,
using primarily extreme-ultraviolet (EUV) and inner coronagraphic
images from the Solar Terrestrial Relations Observatory (STEREO). While
energetic CMEs tend to accompany a flare, flares with relatively high
soft X-ray intensity for the extended solar minimum conditions are often
found without an associated CME, even though the underlying photospheric
magnetic field is strong and complex. In contrast, some of the regions
hosting flare-associated CMEs have weak photospheric field, sometimes
not even classified as active regions because of no sunspots. Out of
several signatures in low coronal images previously raised as proxies
for CMEs, large-scale dimming that persists for at least an hour is
found to be a sufficient condition. Waves in EUV images, on the other
hand, may not necessarily signal an appreciable CME that is still
clearly observed beyond, for example, 5 Rsun, unless the concurrent
dimming is substantial. This suggests that waves detected in EUV images
may have more than one origins. We present the result of the survey
and discuss a number of well-observed cases focusing on the properties
of the flares and CMEs with respect to the extents of dimming and wave.
Title: New Insights to Global Coronal EUV Waves: First Double
Quadrature Observations by SDO/AIA and STEREO/EUVI
Authors: Liu, Wei; Nitta, N. V.; Schrijver, C. J.; Title, A. M.
Bibcode: 2010AAS...21640230L
Altcode:
Global coronal EUV waves are useful diagnostic tools for physical
conditions on the Sun. Major drawbacks that hindered our understanding
of EUV waves were previous instruments' low cadence (e.g., 12
minutes for SoHO/EIT) and limited spatial resolution and wavelength
coverage. The Atmospheric Imaging Assembly (AIA) on board the recently
launched Solar Dynamics Observatory (SDO), joined by the STEREO EUV
Imager (EUVI), offers a great chance to end the decade long debate
on the nature of global EUV waves. AIA observes the corona at 10 EUV
and UV wavelengths, covering a wide range of temperatures. It has high
resolution (0.6") and cadence (20 s, 7 times faster than EUVI). These
capabilities allow us to study the thermal structure and kinematics
of EUV waves in unprecedented detail. We present here the first AIA
observations of an EUV wave occurring on 2010 April 8. AIA observed
this event on the solar disk, while the STEREO Ahead (A) and Behind
(B) spacecraft, which were 67 degree ahead and 72 degree behind the
Earth, respectively, provided side views of both the EUV wave and the
halo coronal mass ejection (seen by SoHO/LASCO) near the limb. This
formed a double quadrature configuration with great advantages to
infer the 3D structure. Initial analysis indicates that this wave
exhibited strong anisotropy, propagating primarily toward the south,
on the same side of the erupting loop system. We will examine its
spatial and temporal relationship with the erupting loop and CME and
discuss physical implications.
Title: What Do High-resolution EIT Waves Tell Us About CMEs?
Authors: Thompson, Barbara; Biesecker, D. A.; Nitta, N.; Ofman, L.;
West, M. J.
Bibcode: 2010AAS...21640229T
Altcode:
Although many studies have demonstrated that some coronal waves are
not generated by coronal mass ejections, we have learned a great
deal about the ability of coronal mass ejections to drive large-scale
coronal waves, also called "EIT waves." We present new results based
on EIT wave amplitude, timing, speed, and direction of propagation,
with respect to their correlation with CME-related dimmings, speeds,
locations and widths. Furthermore, we demonstrate the ability to
correlate different aspects of EIT waves with some of the observed
structure of CMEs observed in coronagraph data. Finally, we expand
on the discussion of the types of wave modes that can be generated
by a coronal mass ejection, and how these observations can serve as a
diagnostic of the type of impulse a CME can deliver to the surrounding
corona. These diagnostics are obtained by examining the motion of
individual field lines, requiring high-resolution observations like
those provided by TRACE and SDO/AIA.
Title: Observations of a 3He-rich SEP Event over a Broad
Range of Heliographic Longitudes: Results from STEREO and ACE
Authors: Wiedenbeck, M. E.; Mason, G. M.; Gómez-Herrero, R.; Haggerty,
D.; Nitta, N. V.; Cohen, C. M. S.; Chollet, E. E.; Cummings, A. C.;
Leske, R. A.; Mewaldt, R. A.; Stone, E. C.; von Rosenvinge, T. T.;
Müller-Mellin, R.; Desai, M.; Mall, U.
Bibcode: 2010AIPC.1216..621W
Altcode:
Observations of energetic ions and electrons from STEREO and ACE have
been used to investigate the longitudinal extent of particle emissions
from 3He-rich solar energetic particle (SEP) events. In
the event of 3-4 Nov 2008, ions and electrons were detected 20° ahead
and behind the nominal connection from the source region to 1 AU, and
electrons were also detected 60° ahead. The results are consistent with
those of earlier studies that correlated data from near-Earth spacecraft
with Helios data or with observations of source regions on the Sun.
Title: CME-related Phenomena and Solar Flares
Authors: Nitta, Nariaki; Aschwanden, Markus; Freeland, Samuel; Lemen,
James; Wuelser, Jean-Pierre; Zarro, Dominic
Bibcode: 2010cosp...38.1792N
Altcode: 2010cosp.meet.1792N
The relationship between solar flares and coronal mass ejections
(CMEs) is still an active area of research. It is studied from
various aspects. Our goal is to understand the importance of magnetic
reconnection in launching CMEs and that of magnetic field environment
of the flaring regions to determine how eruptive flares are. We have
studied the association of solar flares dur-ing 2007-2009 with CMEs,
using primarily extreme-ultraviolet (EUV) and inner coronagraphic
images from the Solar Terrestrial Relations Observatory (STEREO). While
energetic CMEs tend to accompany a flare, flares with relatively high
soft X-ray intensity for the extended solar minimum conditions are often
found without an associated CME, even though the underlying photospheric
magnetic field is strong and complex. In contrast, some of the regions
hosting flare-associated CMEs have weak photospheric field, sometimes
not even classified as active regions because of no sunspots. Out of
several signatures in low coronal images previously raised as proxies
for CMEs, large-scale dimming that persists for at least an hour is
found to be a sufficient condition. Waves in EUV images, on the other
hand, may not necessarily signal an appreciable CME that is still
clearly observed beyond, for example, 5 Rsun, unless the concur-rent
dimming is substantial. This suggests that waves detected in EUV images
may have more than one origins. We present the result of the survey
and discuss a number of well-observed cases focusing on the properties
of the flares and CMEs with respect to the extents of dimming and wave.
Title: Joint STEREO-Hinode Observations of Coronal Dimming and Waves
Associated with a CME/Flare Event
Authors: Nitta, Nariaki; Aschwanden, Markus; Frank, Zoe; Slater,
Gregory; Tarbell, Theodore; Zarro, Dominic
Bibcode: 2010cosp...38.2928N
Altcode: 2010cosp.meet.2928N
During the solar minimum between cycles 23 and 24, a number of
minor flares from unimpressive active regions were associated with
large-scale dimming and waves as observed by the EUV Imager (EUVI) on
STEREO. We present a detailed study on one of the CME/flare events that
was observed also by the instruments on Hinode. We analyze SOT Ca II
data to explore the origin of the disturbances and EIS slit spectra to
determine the timings of the upflow and associated turbulence that are
presumably correlated with coronal dimming. The sequence of phenomena
as captured by different instruments on STEREO, Hinode and SOHO helps
us identify the key physical processes that gave rise to the event.
Title: First Measurements of the Mass of Coronal Mass Ejections from
the EUV Dimming Observed with STEREO EUVI A+B Spacecraft
Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser,
Jean-Pierre; Lemen, James R.; Sandman, Anne; Vourlidas, Angelos;
Colaninno, Robin C.
Bibcode: 2009ApJ...706..376A
Altcode:
The masses of coronal mass ejections (CMEs) have traditionally been
determined from white-light coronagraphs (based on Thomson scattering
of electrons), as well as from extreme ultraviolet (EUV) dimming
observed with one spacecraft. Here we develop an improved method
of measuring CME masses based on EUV dimming observed with the dual
STEREO/EUVI spacecraft in multiple temperature filters that includes
three-dimensional volume and density modeling in the dimming region
and background corona. As a test, we investigate eight CME events with
previous mass determinations from STEREO/COR2, of which six cases are
reliably detected with the Extreme Ultraviolet Imager (EUVI) using our
automated multi-wavelength detection code. We find CME masses in the
range of m CME = (2-7) × 1015 g. The agreement
between the two EUVI/A and B spacecraft is mA /mB
= 1.3 ± 0.6 and the consistency with white-light measurements by COR2
is m EUVI/m COR2 = 1.1 ± 0.3. The consistency
between EUVI and COR2 implies no significant mass backflows (or inflows)
at r < 4 R sun and adequate temperature coverage for the
bulk of the CME mass in the range of T ≈ 0.5-3.0 MK. The temporal
evolution of the EUV dimming allows us to also model the evolution
of the CME density ne (t), volume V(t), height-time h(t),
and propagation speed v(t) in terms of an adiabatically expanding
self-similar geometry. We determine e-folding EUV dimming times of
tD = 1.3 ± 1.4 hr. We test the adiabatic expansion model
in terms of the predicted detection delay (Δt ≈ 0.7 hr) between
EUVI and COR2 for the fastest CME event (2008 March 25) and find good
agreement with the observed delay (Δt ≈ 0.8 hr).
Title: Solar Energetic Particle 3He-Rich Events from the
Nearly Quiet Sun in 2007-2008
Authors: Mason, G. M.; Nitta, N. V.; Cohen, C. M. S.; Wiedenbeck, M. E.
Bibcode: 2009ApJ...700L..56M
Altcode:
We have used Advanced Composition Explorer instruments to survey the
period 2007 March through the end of 2008 for 3He-rich
solar energetic particle (SEP) events occurring during near solar
minimum conditions. Four events were found, all associated with
single solar active regions in the western hemisphere. They all
show 3He:4He ratios of a few percent (i.e.,
gsim100 times solar system abundances), low intensities, and spectra
extending up to at least 1 MeV nucleon-1. Two events, on
2008 February 4 and 2008 June 16, were devoid of signatures associated
with 3He-rich SEPs, namely they lacked associations with
energetic electrons and type-III bursts. In addition, there were no
clear coronal mass ejections and X-ray flare activity was very low
or absent. We take this as evidence that the irreducible requirement
for 3He-rich SEPs is a western hemisphere solar active
region where magnetic and plasma processes preferentially energize
3He and heavy ions.
Title: Properties of Solar Active Regions and Their Relationship
with Solar Eruption: a Statistical Study
Authors: Liu, Yang; Akiyama, S.; Gopalswamy, N.; Mason, J.; Nitta,
N.; Tylka, A.; Yashiro, S.; Yurchyshyn, V.
Bibcode: 2009SPD....40.0920L
Altcode:
Using magnetograms taken by SOHO/MDI, we have calculated some parameters
for solar active regions, and explored possible relationships between
them and solar eruptions. The parameters of active regions we studied
are magnetic flux, net flux, potential field energy, orientation
and separation. We also estimated decay index of magnetic field
overlying the neutral line, and the configuration of ambient field
under which the active region sits. The data used were taken from
1996 to 2005. With these results as a reference, we studied the active
regions that produced the large solar energetic particle (SEP) events,
or produced ground level enhancement (GLE) events. Comparison is also
made between the active regions that produced full eruption and confine
eruption (based on an event list published by Yashiro et al 2005, JGR,
11012S05Y). We present our results here, together with a discussion.
Title: Tracking The 3-d Structure Of Erupting Filaments With The
Stereo/secchi Euvi
Authors: Wuelser, Jean-Pierre; Aschwanden, M.; Lemen, J.; Nitta, N.;
Sandman, A.
Bibcode: 2009SPD....40.2605W
Altcode:
In May 2007, the STEREO/SECCHI EUVI observed several erupting filaments
that were associated with a CME. The stereoscopic observations in both
He II and Fe IX allow a detailed study of the 3-D geometry and dynamics
of the filaments as they erupt. Deconvolution of the EUVI imagery with
the instrument point spread function enhances image contrast and detail,
and improves the accuracy of the 3-D analysis. The quantitative results
of this analysis may potentially lead to a better understanding of
the early CME process.
Title: Solar Flare and CME Observations with STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J. P.; Nitta, N. V.; Lemen, J. R.
Bibcode: 2009SoPh..256....3A
Altcode:
STEREO/EUVI observed 185 flare events (detected above the GOES class C1
level or at > 25 keV with RHESSI) during the first two years of the
mission (December 2006 - November 2008), while coronal mass ejections
(CMEs) were reported in about a third of these events. We compile a
comprehensive catalog of these EUVI-observed events, containing the peak
fluxes in soft X rays, hard X rays, and EUV, as well as a classification
and statistics of prominent EUV features: 79% show impulsive EUV
emission (coincident with hard X rays), 73% show delayed EUV emission
from postflare loops and arcades, 24% represent occulted flares, 17%
exhibit EUV dimming, 5% show loop oscillations or propagating waves,
and at least 3% show erupting filaments. We analyze an example of each
EUV feature by stereoscopic modeling of its 3D geometry. We find that
EUV emission can be dominated by impulsive emission from a heated,
highly sheared, noneruptive filament, in addition to the more common
impulsive EUV emission from flare ribbons or the delayed postflare
EUV emission that results from cooling of the soft-X-ray-emitting
flare loops. Occulted flares allow us to determine CME-related coronal
dimming uncontaminated from flare-related EUV emission. From modeling
the time evolution of EUV dimming we can accurately quantify the
initial expansion of CMEs and determine their masses. Further, we
find evidence that coronal loop oscillations are excited by the rapid
initial expansion of CMEs. These examples demonstrate that stereoscopic
EUV data provide powerful new methods to model the 3D aspects in the
hydrodynamics of flares and kinematics of CMEs.
Title: First Measurements of the Mass of Coronal Mass Ejections from
the EUV Dimming Observed with Stereo EUVI A and B Spacecraft
Authors: Aschwanden, Markus J.; Nitta, N. V.; Wuelser, J.; Lemen,
J. R.; Sandman, A.; Vourlidas, A.; Colaninno, R. C.
Bibcode: 2009SPD....40.2116A
Altcode:
The masses of Coronal Mass Ejections (CMEs) have traditionally
been determined from white-light coronagraphs, based on the Thomson
scattering of electrons. Here we develop a new method of measuring CME
masses from the EUV dimming seen with EUV imaging telescopes in multiple
temperature filters. As a test we compare the CME masses measured by
STEREO/EUVI A and B with those previously determined by STEREO/COR2, for
a set of 8 CME events of which we detected 7 with EUVI and determined
the masses in 6 cases. We find CME masses in the range of m = (2-7)
x 10(15) g. The agreement between the two EUVI/A and B spacecraft is
mA/mB =1.3 +/- 0.6 and the consistency with white-light measurements
by COR2 is mEUVI/mCOR2 = 1.1 +/- 0.3. The consistency between EUVI
and COR2 implies no significant mass backflows (or inflows) at r <
4 R and adequate temperature coverage for the bulk of the CME mass
in the range of T = 0.5-3.0 MK. The temporal evolution of the EUV
dimming allows us also to model the evolution of the CME density,
volume, height-time, and propagation speed in terms of an adiabatically
expanding self-similar geometry. We test this model with the predicted
detection delay between EUVI and COR2 for the 2008-Mar-25 event.
Title: The CME-Flare Relation Revisited With STEREO Observations
Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.;
Wuelser, J.; Zarro, D.
Bibcode: 2009SPD....40.2105N
Altcode:
We study the association of solar flares since March 2007 with coronal
mass ejections (CMEs), using images taken by the EUV Imager (EUVI),
COR1 and COR2 coronagraphs on board STEREO. This is done by searching
EUVI data for low coronal signatures attributable to CMEs, such as
dimming, EUV waves and eruptions, following them to COR1 and COR2
fields of view. Base and running difference images (after correcting
for differential rotation) as well as raw images in all the four
filters of EUVI on STEREO A and STEREO B are viewed as movies to find
the CME-related signatures. The COR1 data are particularly helpful for
connecting the EUVI signatures with CMEs observed by COR2. Only 2 (out
of 11) M-class flares and 7 (out of 64) C-class flares are convincingly
associated with CMEs traceable beyond 5 Rs. There are also a handful
of less intense (B-class and A-class) flares associated with CMEs. We
discuss the "calibration" of the low coronal signatures with actual
CMEs, quantitatively re-defining them to be used as reliable proxies
for CMEs. Radio observations are also found to be of use to distinguish
flares associated and not associated with CMEs. Lastly we consider the
CME association of flares in terms of the the following items about the
flaring active regions: their basic properties, their relations with
more global field, and local (spatial or temporal) changes therein. This
study may help us understand the effect of (reconnection-driven)
flare processes on the initiation and subsequent dynamics of CMEs.
Title: First Three-Dimensional Reconstructions of Coronal Loops with
the STEREO A+B Spacecraft. III. Instant Stereoscopic Tomography of
Active Regions
Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; Sandman, Anne
Bibcode: 2009ApJ...695...12A
Altcode:
Here we develop a novel three-dimensional (3D) reconstruction method
of the coronal plasma of an active region by combining stereoscopic
triangulation of loops with density and temperature modeling of
coronal loops with a filling factor equivalent to tomographic volume
rendering. Because this method requires only a stereoscopic image
pair in multiple temperature filters, which are sampled within ≈1
minute with the recent STEREO/EUVI instrument, this method is about
four orders of magnitude faster than conventional solar rotation-based
tomography. We reconstruct the 3D density and temperature distribution
of active region NOAA 10955 by stereoscopic triangulation of 70 loops,
which are used as a skeleton for a 3D field interpolation of some
7000 loop components, leading to a 3D model that reproduces the
observed fluxes in each stereoscopic image pair with an accuracy
of a few percents (of the average flux) in each pixel. With the
stereoscopic tomography we infer also a differential emission
measure distribution over the entire temperature range of T ≈
104-107, with predictions for the transition
region and hotter corona in soft X-rays. The tomographic 3D model
provides also large statistics of physical parameters. We find that
the extreme-ultraviolet loops with apex temperatures of Tm
lsim 3.0 MK tend to be super-hydrostatic, while hotter loops with
Tm ≈ 4-7 MK are near-hydrostatic. The new 3D reconstruction
model is fully independent of any magnetic field data and is promising
for future tests of theoretical magnetic field models and coronal
heating models.
Title: Great geomagnetic storm of 9 November 1991: Association with
a disappearing solar filament
Authors: Cliver, E. W.; Balasubramaniam, K. S.; Nitta, N. V.; Li, X.
Bibcode: 2009JGRA..114.0A20C
Altcode: 2009JGRA..11400A20C
We attribute the great geomagnetic storm on 8-10 November 1991 to a
large-scale eruption that encompassed the disappearance of a ~25° solar
filament in the southern solar hemisphere. The resultant soft X-ray
arcade spanned ~90° of solar longitude. The rapid growth of an active
region lying at one end of the X-ray arcade appears to have triggered
the eruption. This is the largest geomagnetic storm yet associated with
the eruption of a quiescent filament. The minimum hourly Dst value of
-354 nT on 9 November 1991 compares with a minimum Dst value of -161
nT for the largest 27-day recurrent (coronal hole) storm observed
from 1972 to 2005 and the minimum -559 nT value observed during the
flare-associated storm of 14 March 1989, the greatest magnetic storm
recorded during the space age. Overall, the November 1991 storm ranks
15th on a list of Dst storms from 1905 to 2004, surpassing in intensity
such well-known storms as 14 July 1982 (-310 nT) and 15 July 2000
(-317 nT). We used the Cliver et al. and Gopalswamy et al. empirical
models of coronal mass ejection propagation in the solar wind to
provide consistency checks on the eruption/storm association.
Title: Coronal Mass Ejections Associated With Impulsive Solar Flares -
Observations With SECCHI EUVI On STEREO
Authors: Nitta, N. V.; Lemen, J. R.; Wuelser, J.; Aschwanden, M. J.;
Freeland, S. L.; Zarro, D. M.
Bibcode: 2008AGUFMSH13B1538N
Altcode:
Long-duration flares, sometimes referred to as Long Decay Events (LDEs),
are known to be unmistakable signatures of coronal mass ejections
(CMEs), and often of fast and large ones. Short-duration or impulsive
flares, on the other hand, do not as frequently accompany CMEs,
even though X-ray plasmoid ejections seen in some of these flares may
suggest that all flares are eruptive irrespective of durations. Some
of these ejections in X-ray or EUV images could be failed ejections,
however, meaning that they do not move into interplanetary medium. A
complementary, and perhaps more reliable signature of a CME in the
low corona may be large-scale dimming typically observed at 1-2
MK. We report on high cadence observations of SECCHI EUVI on STEREO
that show this phenomenon in weak impulsive flares more frequently
than expected. We systematically study flare periods with good data
coverage. In order to avoid false dimming, we use both base and running
difference images after carefully co-aligning the image pairs. Some
of the dimming events were observed in more than one channel and
at two widely separated view angles, letting us better understand
the nature of dimming especially in terms of the associated CME. We
discuss how the properties of dimming are reflected in CME parameters,
how to distinguish the impulsive flares with large- scale effects from
those that are confined, and whether similar events could account for
orphan ICMEs without a clearly associated CME near the Sun.
Title: Tracking the 3-D Structure of an Erupting Filament With the
STEREO/SECCHI EUVI
Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N.;
Sandman, A.
Bibcode: 2008AGUFMSH12A..01W
Altcode:
On 2007 May 19, the STEREO/SECCHI EUVI observed an erupting filament
that was associated with a CME. The stereoscopic observations in both He
II and Fe IX allow a detailed study of the 3-D geometry and dynamics of
the filament as it erupts. Deconvolution of the EUVI imagery with the
instrument point spread function enhances image contrast and detail,
and improves the accuracy of the 3-D analysis. The quantitative results
of such an analysis may lead to a better understanding of the early
CME process.
Title: Instant Stereoscopic Tomography of Active Regions with
STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J.;
Sandman, A.
Bibcode: 2008AGUFMSH13B1555A
Altcode:
We develop a novel 3D reconstruction method of the coronal plasma of
an active region by combining stereoscopic triangulation of loops with
density and temperature modeling of coronal loops with a filling factor
equivalent to tomographic volume rendering. Because this method requires
only a stereoscopic image pair in multiple temperature filters, which
are sampled within ~1 minute with the recent STEREO/EUVI instrument,
this method is about 4 orders of magnitude faster than conventional
solar rotation-based tomography. We reconstruct the 3D density and
temperature distribution of active region NOAA 10955 by stereoscopic
triangulation of 70 loops, which are used as a skeleton for a 3D field
interpolation of some 7000 loop components, leading to a 3D model that
reproduces the observed fluxes in each stereosocpic image pair with an
accuracy of a few percent (of the average flux) in each pixel. With the
stereoscopic tomography we infer also a differential emission measure
(DEM) distribution over the entire temperature range of T~0.01-10 MK,
with predictions for the transition region and hotter corona in soft
X-rays. The tomographic 3D model provides also large statistics of
physical parameters. We find that the EUV loops with apex temperatures
of T = 1- 3 MK tend to be super-hydrostatic, while hotter loops with T =
4-7 MK are near-hydrostatic. The new 3D reconstruction model is fully
independent of any magnetic field data and is promising for future
tests of theoretical magnetic field models and coronal heating models.
Title: First 3D Reconstructions of Coronal Loops with the STEREO
A+B Spacecraft. II. Electron Density and Temperature Measurements
Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser,
Jean-Pierre; Lemen, James R.
Bibcode: 2008ApJ...680.1477A
Altcode:
Using the stereoscopically derived three-dimensional (3D) geometry
of 30 loops observed with STEREO EUVI (described in Paper I) we
determine here the electron density profiles ne(s) and
electron temperature profiles Te(s) from a triple-filter
analysis of the stereoscopic images taken in the wavelengths of λ =
171, 195, and 284 Å. The statistical results of our analysis of
seven complete loops are: observed loop widths wobs =
2.6 +/- 0.1 Mm, corresponding to effective loop widths of w = 1.1
+/- 0.3 Mm if corrected for the instrumental point-spread function;
loop flux ratios floop/ftotal = 0.11 +/- 0.04;
mean loop (DEM peak) temperatures Tp = 1.1 +/- 0.2 MK;
DEM temperature Gaussian widths σDEM = 0.35 +/- 0.04 MK;
temperature variations along loops σT/Tp = 0.24
+/- 0.05; (resolution-corrected) loop base densities ne =
(2.2 +/- 0.5) × 109 cm-3 loop lengths of L = 130
+/- 67 Mm; and all quantities are found to agree between STEREO A and
B within a few percent. The temperature profiles T(s) along loops are
found to be nearly constant, within the uncertainties of the background
subtraction. The density profiles ne(s) are consistent with
the gravitational stratification of hydrostatic loops, ne(h)
= nbaseexp (- h/λT) , defined by the temperature
scale heights λT and stereoscopically measured from the
height profiles h(s) . The stereoscopic 3D reconstruction allows us for
the first time to accurately measure the loop length L and to test loop
scaling laws. We find that the observations are not consistent with
equilibrium solutions, but rather display the typical overpressures
of loops that have been previously heated to higher temperatures and
cool down in a nonequilibrium state, similar to earlier EIT and TRACE
measurements.
Title: First Three-Dimensional Reconstructions of Coronal Loops with
the STEREO A and B Spacecraft. I. Geometry
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.
Bibcode: 2008ApJ...679..827A
Altcode:
We present one of the first triangulations and 3D reconstructions
of coronal loops, using the EUVI telescopes of the two STEREO A and
B spacecraft. The first triangulation of coronal loops was performed
in an active region, observed with STEREO A and B on 2007 May 9 with a
spacecraft separation angle of αsep = 7.3°, at a wavelength
of 171 Å. We identify 30 loop structures (7 complete loops and 23
partial segments) and compute their 3D coordinates (x,y,z) (the full
3D coordinates are available as an electronic file). We quantify the
height range, the stereoscopic height measurement errors, the loop
plane inclination angles, and the coplanarity and circularity of the
analyzed loops. The knowledge of the exact 3D geometry of a loop with
respect to the observer's line of sight has important consequences
for determining the correct vertical density scale height (used in
hydrostatic models), the aspect angle of loop cross sections (used in
inferring electron densities from optically thin emission measures),
the absolute flow speeds (used in siphon flow models), the correct
loop length (used in loop scaling laws), and the 3D vectors of the
coronal magnetic field (used in testing theoretical magnetic field
extrapolation models). The hydrodynamic and magnetic modeling of the
analyzed loops will be described in subsequent papers.
Title: Exploring large-scale coronal magnetic field over extended
longitudes by STEREO/EUVI and its effect on solar wind prediction
Authors: Nitta, N. V.; De Rosa, M. L.; Zarro, D. M.; Wuelser, J.;
Aschwanden, M. J.; Lemen, J. R.
Bibcode: 2008AGUSMSH23A..06N
Altcode:
The potential field source surface (PFSS) model forms the basis of
a wide range of heliospheric science and applications, including
prediction of the solar wind speed near Earth. Experience shows that
the model sometimes works quite well, but not always. Possible reasons
for failure include deviation of the interplanetary magnetic field from
the nominal Parker spiral, violation of the assumptions used in the
model such as the discontinuity at the source surface, and the lack of
simultaneous full-surface magnetograms. Here we study the impact of the
lack of simultaneous full-surface magnetograms, using observations of
the corona over an extended longitude range made possible by the EUVI
on board the STEREO mission. In spite of the lack of magnetographs on
STEREO, EUVI data with a growing separation angle between spacecraft
A and B at least allow us to locate major active regions and coronal
holes in the area not seen from Earth. The PFSS extrapolations and their
input synoptic maps are compared with EUVI data to measure how well the
model fits the observations. These comparisons are discussed in terms of
the solar wind speed predicted by the model and observed at L1 by ACE.
Title: Tracing the 3-D coronal structure during CMEs with
STEREO/SECCHI EUVI observations
Authors: Wuelser, J.; Aschwanden, M.; De Rosa, M.; Lee, C.; Lemen,
J.; Nitta, N.; Sandman, A.
Bibcode: 2008AGUSMSH31A..05W
Altcode:
STEREO/SECCHI EUVI observations of solar coronal loops, filaments,
and dimming regions provide unique information on the 3-D topology
of the coronal magnetic field above active regions and its evolution
during coronal mass ejections (CMEs). Active Region #10956 produced
several CMEs during its passage across the solar disk in May 2007,
some of them showing filament eruptions and dimming. The SECCHI/EUVI
instrument on STEREO obtained high cadence observations in multiple
lines simultaneously from both STEREO spacecraft. 3-D reconstructions
of coronal features over the course of a CME show significant changes
of the field topology. Comparisons with the potential field topology
from magnetic field extrapolations show the degree of non-potentiality
of the real field and changes in the vicinity of the CME onset. We
present initial results of this study.
Title: Solar Flares and CMEs observed with STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N. V.; Lemen, J. R.
Bibcode: 2008AGUSMSP44A..01A
Altcode:
STEREO/EUVI observed a respectable number of small flares and CMEs
during the first year of its mission. A comprehensive survey between
Dec 2006 and Jan 2008 yields: 10 GOES >M1-class flares, 20 GOES
>C5-class flares, 80 >C1-class flares, 180 flares with >25 keV
hard X-ray emission observed by RHESSI, at least 35 flare events with
associated CME reports by LASCO or STEREO COR-1, and 25 flare events
are occulted by either STEREO A or B. Some flares associated with CMEs
clearly show an EUV dimming in the active region below the launched
CME, for which we determine for the first time the 3D geometry with
the two STEREO A and B spacecraft, allowing us to model the volume,
density, and ejected mass from the corona. Some flares show impulsive
signatures in EUV, simultaneously occurring with hard X-rays pulses,
indicative of the primary heating of the chromospheric plasma. In
larger flares, the bulk of the EUV emission is always substantially
delayed to the soft X-ray emission, as a result of the plasma cooling
from soft X-ray to EUV temperatures, from which we can estimate the
total thermal flare energy using hydrodynamic models.
Title: Coronal Jet Observed by Hinode as the Source of
a3He-rich Solar Energetic Particle Event
Authors: Nitta, Nariaki V.; Mason, Glenn M.; Wiedenbeck, Mark E.;
Cohen, Christina M. S.; Krucker, Säm; Hannah, Iain G.; Shimojo,
Masumi; Shibata, Kazunari
Bibcode: 2008ApJ...675L.125N
Altcode:
We study the solar source of the 3He-rich solar
energetic particle (SEP) event observed on 2006 November 18. The
SEP event showed a clear velocity dispersion at energies below 1 MeV
nucleon-1, indicating its solar origin. We associate the SEP
event with a coronal jet in an active region at heliographic longitude
of W50°, as observed in soft X-rays. This jet was the only noticeable
activity in full-disk X-ray images around the estimated release time of
the ions. It was temporally correlated with a series of type III radio
bursts detected in metric and longer wavelength ranges and was followed
by a nonrelativistic electron event. The jet may be explained in terms
of the model of an expanding loop reconnecting with a large-scale
magnetic field, which is open to interplanetary space for the particles
to be observed at 1 AU. The open field lines appear to be anchored at
the boundary between the umbra and penumbra of the leading sunspot,
where a brightening is observed in both soft and hard X-rays during
the jet activity. Other flares in the same region possibly associated
with 3He-rich SEP events were not accompanied by a jet,
indicative of different origins of this type of SEP event.
Title: A Comparison of Solar Open Field Regions Found by Type III
Radio Bursts and the Potential Field Source Surface Model
Authors: Nitta, Nariaki V.; DeRosa, Marc L.
Bibcode: 2008ApJ...673L.207N
Altcode:
For heliophysics research and applications, the potential field
source surface (PFSS) model is often employed to extrapolate the
photospheric magnetic field to the corona. In an attempt to evaluate
the performance of the PFSS model, we compare the computed footpoints
of the heliospheric magnetic field with the locations of flares
associated with type III radio bursts, which are a good indicator
of open field lines that extend to interplanetary space. Consistent
with past experiences, the agreement is not satisfactory. We discuss
possible reasons for the discrepancy, including the model's inadequacy
to reproduce the coronal magnetic field above evolving active regions
and the lack of a simultaneous full-surface magnetic map. It is argued
that the performance of the PFSS model needs to be quantified further
against solar observations, including type III bursts, before it is
applied to heliospheric models.
Title: Characteristics of Anemone Active Regions Appearing in Coronal
Holes Observed with the Yohkoh Soft X-Ray Telescope
Authors: Asai, Ayumi; Shibata, Kazunari; Hara, Hirohisa; Nitta,
Nariaki V.
Bibcode: 2008ApJ...673.1188A
Altcode: 2008arXiv0805.4474A
Coronal structure of active regions appearing in coronal holes is
studied, using data that were obtained with the Soft X-Ray Telescope
(SXT) aboard Yohkoh between 1991 November and 1993 March. The following
characteristics are found. Many of the active regions (ARs) appearing
in coronal holes show a structure that looks like a sea anemone. Such
active regions are called anemone ARs. About one-fourth of all active
regions that were observed with SXT from their births showed the
anemone structure. For almost all the anemone ARs, the order of the
magnetic polarities is consistent with the Hale-Nicholson polarity
law. These anemone ARs also showed, to a greater or lesser extent,
an east-west asymmetry in the X-ray intensity distribution, such that
the following (eastern) part of the AR was brighter than its preceding
(western) part. This, as well as the anemone shape itself, is consistent
with the magnetic polarity distribution around the anemone ARs. These
observations also suggest that an active region appearing in coronal
holes has a simpler (less sheared) and more preceding-spot-dominant
magnetic structure than those appearing in other regions.
Title: Cross calibration of soft X-ray telescopes between Hinode/XRT
and GOES13/SXI
Authors: Narukage, N.; Sakao, T.; Kano, R.; Shimojo, M.; Cirtain,
J.; Deluca, E.; Nitta, N.; Lemen, J.
Bibcode: 2007AGUFMSH53A1050N
Altcode:
The X-Ray Telescope (XRT) aboard Hinode satellite is a grazing
incidence telescope to observe all the coronal features with a wide
temperature range from less than 1MK to more than 10MK. And the XRT
has 9 X-ray analysis filters which are optimized to observed the
almost whole coronal plasma and to derived the coronal temperature
distribution. Meanwhile, the GOES13 satellite carries a Solar X-ray
Imager (SXI) to monitor the solar X-rays. The SXI is also a grazing
incidence telescope and has 7 X-ray filters. The XRT and SXI are
similar telescopes to observe the dynamic solar corona. On 24 Nov 2006,
the XRT and SXI-team performed the simultaneous observation for the
cross calibration between XRT and SXI. In this study, we analyzed
this data set and checked the actual characteristics of each X-ray
analysis filter.
Title: 3D Geometry of Coronal Loops Measured with STEREO / EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J.
Bibcode: 2007AGUFMSH41B..04A
Altcode:
Using images from the STEREO/EUVI A and B spacecraft we developed
an accurate method that performs stereoscopic triangulation and
reconstruction of the 3D geometry of curvi-linear structures in
the solar corona, such as loops, filaments, prominence threads, or
wave features. We test the coalignment of stereoscopic images and
establish an accuracy of better than <0.1 pixels in east-west
direction, <0.3 pixels in north-south direction, and <0.05
degrees in roll angle. We reconstruct the 3D geometry of some 100
coronal loops loops in active regions in May 2007, when the spacecraft
had a separation angle of ~10 degrees. We find that complete loops or
incomplete segments of loops can only be reconstructed up to altitudes
of about one hydrostatic scale height, which is h<50 Mm at a coronal
temperature of T=1 MK. The determination of the 3D geometry of coronal
loops is an important and necessary step to model their hydrodynamic
structure. We show also that this method can be used to determine
quantitatively the eigen-motion, oscillation, twisting, expansion,
acceleration, or other dynamics of coronal loops, erupting filaments,
and MHD wave fronts, in particular in association with flares and CMEs.
Title: Stereoscopic Observations of Low Coronal Ejections With and
Without CMEs
Authors: Nitta, N. V.; Wülser, J.; Aschwanden, M. J.; Lemen, J. R.
Bibcode: 2007AGUFMSH32A0775N
Altcode:
Yohkoh soft X-ray images of solar flares have frequently shown
characteristic ejections during the impulsive phase. They are
thought to be plasmoids that hold important information on magnetic
reconnection. These ejections are intimately associated with coronal
mass ejections (e.g., Nitta & Akiyama 1999; Kim et al. 2005). They
probably represent internal structures of CMEs, i.e., high-temperature
counterparts of filament eruptions. However, their relation with
ejections seen at low temperatures has not been studied systematically,
although TRACE has revealed many beautiful examples. In this work we
study ejections observed by the SECCHI EUVI on STEREO. Some of them are
associated with CMEs, and others are not. Using pairs of EUVI images
from spacecraft A and B, the trajectories of ejecta in individual
channels (with representative temperatures 0.1-2 MK) are reconstructed
in 3D. We discuss these ejections at different temperatures in the
context of CMEs. Specifically, we ask what kinetic properties are
correlated with CMEs and how they are related with CME manifestations
in the low corona such as dimming and waves.
Title: 3-D reconstruction of CME related transient coronal phenomena
observed with the STEREO/SECCHI Extreme Ultraviolet Imager
Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N.
Bibcode: 2007AGUFMSH32A0772W
Altcode:
One of the primary objectives of the SECCHI investigation on STEREO
is to study the initiation of CMEs in the low corona, and to better
understand CME related changes of the three-dimensional coronal
structure. The SECCHI Extreme Ultraviolet Imagers (EUVI) have been
observing the solar corona from two significantly different vantage
points since about March 2007. They have since captured several
CMEs, including a few during the SECCHI campaign in May 2007, at an
observatory separation angle of about 7-8 degrees. Observations at
relatively small separation angles allow for easier identification of
features in two views, which is critical for visually aided tie- point
tools, as well as for more automated 3-D reconstruction methods. EUVI
movies taken during the early onset of a CME show a range of transient
phenomena, including coronal ejecta that can be tracked into the
coronagraph fields of view, erupting filaments that trail the coronal
ejecta, displacement of active region loops, coronal dimming, and "EIT"
waves. We present preliminary results of our first 3-D reconstruction
attempts on a selection of such CME related phenomena, with emphasis
on coronal ejecta and active region loop displacements.
Title: Solar Origin of 3He-rich Solar Energetic Particle Events
Authors: Nitta, N. V.; Mason, G. M.; Wiedenbeck, M. E.; Cohen, C. M.
Bibcode: 2007AGUFMSH53B..05N
Altcode:
For more than three decades, we have known solar energetic particle
(SEP) events that are characterized by enrichment of 3He and heavy
ions. In order to understand the phenomenon, we first need to know where
these ions come from. It is not straightforward to identify the solar
source of the 3He-rich SEP event, since it often leaves electromagnetic
signatures too small to be detected as a flare in spatially-integrated
GOES X-ray measurements. Another ambiguity arises from the fact that
3He-rich SEP events are more often observed at low energies, meaning
large uncertainties in the particle injection time. In this work we
concentrate on 3He-rich SEP events whose injection times are relatively
well determined by the ULEIS and SIS experiments on ACE. We show the
association of these 3He-rich events with type III radio bursts and
non- or mildly-relativistic electron events, and with solar flares,
jets and CMEs. We compare our results with those in the literature,
and also discuss the possible origin of problem events where we fail
to find any solar signatures in the corona.
Title: Early Evolution of CMEs as Observed by SECCHI EUVI on STEREO
Authors: Lemen, J. R.; Nitta, N. V.; Wülser, J.; Aschwanden, M. J.
Bibcode: 2007AGUFMSH32A0768L
Altcode:
We report on two eruptions associated with small (C1 and B8) flares that
occurred in AR 956 on 2007 May 19 and 20, as observed stereoscopically
by EUVI. The separation of the two spacecraft was approximately 9
degrees. Pairs of images in 171~Å\ and 304~Å\ from two view angles
are used to constrain the trajectories of the ejecta or filaments
that appear to be responsible for the associated CMEs; they were not
homologous. We study how the 3D motions of the ejecta in the low corona
correspond to the CMEs at higher altitudes. We also discuss the possible
relation between the early CME propagation and the coronal magnetic
field topology inferred from EUV loops in EUVI and TRACE 171~Å\ images.
Title: Correction to ``Solar and interplanetary sources of major
geomagnetic storms (Dst <= -100 nT) during 1996-2005''
Authors: Zhang, J.; Richardson, I. G.; Webb, D. F.; Gopalswamy, N.;
Huttunen, E.; Kasper, J.; Nitta, N. V.; Poomvises, W.; Thompson,
B. J.; Wu, C. -C.; Yashiro, S.; Zhukov, A. N.
Bibcode: 2007JGRA..11212103Z
Altcode:
Abstract Available
from http://www.agu.org
Title: Observations of Simultaneous Coronal Loop Shrinkage and
Expansion during the Decay Phase of a Solar Flare
Authors: Khan, J. I.; Fletcher, L.; Nitta, N. V.
Bibcode: 2007ASPC..369..485K
Altcode:
We report what we believe are the first direct and unambiguous
observations of simultaneous coronal magnetic flux loop shrinkage
and expansion during the decay phase of a solar flare. The retracting
and expanding loops were observed nearly face-on (i.e., with the loop
major axis approximately orthogonal to the line of sight) in emission in
imaging data from the Yohkoh Soft X-ray Telescope (SXT). The retracting
loop is observed to shrink with a speed of 165±26 km s^{-1}. The
faint outward moving loop-like feature occurred ∼200 arcsec above
the shrinking loop during the time of the shrinking loop. We estimate
the speed of the outward moving loop was ∼280±130 km s^{-1}. We
interpret the shrinking loop and simultaneous outward moving loop as
direct evidence for reconnected magnetic field lines during a flare.
Title: Solar and interplanetary sources of major geomagnetic storms
(Dst <= -100 nT) during 1996-2005
Authors: Zhang, J.; Richardson, I. G.; Webb, D. F.; Gopalswamy, N.;
Huttunen, E.; Kasper, J. C.; Nitta, N. V.; Poomvises, W.; Thompson,
B. J.; Wu, C. -C.; Yashiro, S.; Zhukov, A. N.
Bibcode: 2007JGRA..11210102Z
Altcode:
We present the results of an investigation of the sequence of
events from the Sun to the Earth that ultimately led to the 88 major
geomagnetic storms (defined by minimum Dst ≤ -100 nT) that occurred
during 1996-2005. The results are achieved through cooperative
efforts that originated at the Living with a Star (LWS) Coordinated
Data-Analysis Workshop (CDAW) held at George Mason University in
March 2005. On the basis of careful examination of the complete array
of solar and in situ solar wind observations, we have identified
and characterized, for each major geomagnetic storm, the overall
solar-interplanetary (solar-IP) source type, the time, velocity, and
angular width of the source coronal mass ejection (CME), the type and
heliographic location of the solar source region, the structure of the
transient solar wind flow with the storm-driving component specified,
the arrival time of shock/disturbance, and the start and ending times
of the corresponding IP CME (ICME). The storm-driving component,
which possesses a prolonged and enhanced southward magnetic field
(Bs), may be an ICME, the sheath of shocked plasma (SH)
upstream of an ICME, a corotating interaction region (CIR), or a
combination of these structures. We classify the Solar-IP sources
into three broad types: (1) S-type, in which the storm is associated
with a single ICME and a single CME at the Sun; (2) M-type, in which
the storm is associated with a complex solar wind flow produced by
multiple interacting ICMEs arising from multiple halo CMEs launched
from the Sun in a short period; (3) C-type, in which the storm is
associated with a CIR formed at the leading edge of a high-speed stream
originating from a solar coronal hole (CH). For the 88 major storms,
the S-type, M-type, and C-type events number 53 (60%), 24 (27%), and 11
(13%), respectively. For the 85 events for which the surface source
regions could be investigated, 54 (63%) of the storms originated in
solar active regions, 11 (13%) in quiet Sun regions associated with
quiescent filaments or filament channels, and 11 (13%) were associated
with coronal holes. Remarkably, nine (11%) CME-driven events showed no
sign of eruptive features on the surface or in the low corona (e.g.,
no flare, no coronal dimming, and no loop arcade, etc.), even though
all the available solar observations in a suitable time period were
carefully examined. Thus while it is generally true that a major
geomagnetic storm is more likely to be driven by a frontside fast
halo CME associated with a major flare, our study indicates a broad
distribution of source properties. The implications of the results
for space weather forecasting are briefly discussed.
Title: The STEREO SECCHI/EUVI EUV coronal imager
Authors: Wülser, Jean-Pierre; Lemen, James R.; Nitta, Nariaki
Bibcode: 2007SPIE.6689E..05W
Altcode: 2007SPIE.6689E...3W
The SECCHI Extreme Ultraviolet Imager (EUVI) is a pair of EUV telescopes
on the NASA STEREO mission that was successfully launched in October
2006. The two telescopes, one on each STEREO spacecraft observe the
solar corona in three dimensions out to 1.7 solar radii, and with a
pixel resolution of 1.59 arcsec. The focus of the EUVI observations
is the initiation and early evolution of coronal mass ejections
(CMEs). This paper presents a preliminary assessment of the on-orbit
performance of the EUVI.
Title: First 3d Triangulation Of Coronal Loops With Stereo/euvi
Authors: Aschwanden, Markus J.; Wuelser, J.; Lemen, J.; Nitta, N.
Bibcode: 2007AAS...210.2810A
Altcode: 2007BAAS...39..137A
The orbits of the two STEREO spacecraft A(head) and B(ehind) move to
progressively larger stereoscopic separation angles a, with a=1.2 deg
on March 1, a=3.0 deg on April 1, a=6.0 deg on May 1, and a=10.5 deg
on June 1. This range of small-angle separation enables the first 3D
triangulation of coronal features. Active region loops at 1 MK have
a scale height of 50 Mm, for which the parallax effect amounts up
to 7 EUVI pixels at a 10 deg separation angle. We present the first
triangulations of active region loops, with the goal to reconstruct
the 3D geometry along entire loop lengths. Such 3D reconstructions
yield the inclination angles of loop planes, which allow us to test
the relation between projected and vertical hydrostatic density scale
heights. Another important application is how the 3D geometry of the
stereoscopically reconstructed loops relates to theoretical (potential,
linear and nonlinear force-free) magnetic field extrapolations. We
attempt also to reconstruct the 3D geometry of filaments and to
track their motion in 3D before eruption and onset of flares and
CMEs. - This work is supported by the NASA STEREO under NRL contract
N00173-02-C-2035.
Title: The Early Development Phase Of An EIT Wave Observed With
GOES-13 SXI
Authors: Biesecker, Douglas A.; Reinard, A.; Hill, S.; Pizzo, V.;
Aulanier, C.; Lemen, J.; Nitta, N.
Bibcode: 2007AAS...210.9119B
Altcode: 2007BAAS...39..207B
On December 5th, the GOES-13 SXI imager observed an EIT wave
initiated at the solar limb. The cadence of full Sun EUV images was
approximately 30 seconds, providing unprecedented detail of an EIT wave
on both small and large scales, simultaneously. The wave was observed to
propagate across almost the full solar disk and to interact with coronal
holes and active regions. The 3-dimensional nature of the EIT wave is
clearly evident in the SXI images. We focus on the earliest phase of
the EIT wave, examining several outstanding questions, including how
and where an EIT wave is initiated and whether the wave begins from a
local or extended source. In particular, we will focus on discerning
the driver of the EIT wave.
Title: First Assessments Of EUVI Performance On STEREO SECCHI
Authors: Lemen, James; Wuelser, J. P.; Nitta, N.; Aschwanden, M.
Bibcode: 2007AAS...210.2801L
Altcode: 2007BAAS...39..135L
The SECCHI investigation on the STEREO mission contains two Extreme
Ultraviolet Imagers (EUVI), one on the ahead spacecraft and one on the
behind spacecraft. EUVI views the solar disk using multilayer-coated
normal-incidence optics that image onto 1.6 arcsec-per-pixel
back-thinned CCD detectors. Four wavelength bandpasses are observed
in series, 17.1 nm (Fe IX), 19.5 nm (Fe XII), 28.4 nm (Fe XV),
and 30.4 nm (He II), covering the chromospheric and coronal plasma
temperatures.Science operations began in January 2007 and both EUVIs
are working very well. The fine pointing system effectively removes
low frequency spacecraft pointing jitter, so the image resolution
quality is very good, approaching 3.5 arcsec. We present early on-orbit
assessments of the performance of both EUVIs and updated predictions
for the temperature dependent instrument response functions, which
are compared to early observations. The lunar transit observed by
the EUVI on the behind spacecraft is used to assess the point spread
function. This work is supported by the NASA STEREO mission under
NRL contract N00173-02-C-2035.
Title: Low Coronal Manifestations Of Coronal Mass Ejections As
Observed By STEREO EUVI
Authors: Nitta, Nariaki; Wuelser, J.; Lemen, J.; Aschwanden, M.;
Attrill, G.
Bibcode: 2007AAS...210.2805N
Altcode: 2007BAAS...39..136N
Data from SOHO/EIT have tremendously advanced our knowledge about
the initiation of coronal mass ejections (CMEs) as observed by
SOHO/LASCO. The unequivocal EUV manifestations of CMEs include
dimming and waves, typically observed at EIT's 195 A channel. In
this presentation, we report mainly on these two observables in two
similar events as observed on 2007 January 24 and 25 by the Extreme
Ultraviolet Imagers (EUVI), which are part of SECCHI on STEREO. The
source region of the two events was most likely AR 0940, which was
located 20 degrees and 10 degrees behind the east limb at the times of
the events. Images in 171 A and 195 A were taken at a basic cadence of
10 minutes, slightly better than that of the EIT movie sequence. But
what is remarkable is the availability of nearly simultaneous (dt =
11 sec) full-disk images in the two wavelengths. We give detailed
comparisons of the wave propagations and dimming regions as observed
at 171 A and 195 A, and discuss their relations with the white-light
CMEs and their associated flares. This work has been supported by NASA
STEREO mission contract N00173-02-C-2035 through NRL.
Title: SEP Properties and Magnetic Field Connection of the Source
Region
Authors: Nitta, N. V.; De Rosa, M. L.
Bibcode: 2006AGUFMSH41B..06N
Altcode:
There seems to be no clear explanation as to why some CME/flare events
produce major SEP events whereas others do not. Furthermore, we still
cannot reliably predict the peak flux and rise time of an SEP event
using remote sensing data. Even though CME shocks are the primary
accelerator for gradual SEP events and they are likely to have wide
angular extensions, we suggest that the magnetic field connection of the
source region to the observer may be an important factor for determining
their occurrence and basic properties. In order to address this issue,
we need to go beyond the common assumption that the longitudes around
W60 have the strongest connection to the Earth. For a number of
SEP-productive active regions, we compare the properties of intense
flares and energetic CMEs that originated from them over their disk
passage with the peak fluxes and rise times of the associated >10 MeV
and >50 MeV protons. We perform magnetic field extrapolation with the
potential field source surface (PFSS) model to locate well-connected
field lines with respect to the source region. Once evaluated against
multiple criteria, the PFSS extrapolation would be a useful tool to
characterize the magnetic field topology in and around the active
region responsible for the intense flares and energetic CMEs. This
study is expected to partially answer the question of whether flare-
accelerated particles contribute to gradual SEP events.
Title: Coronal Temperature Properties Measured With the GOES-13 SXI
Authors: Lemen, J. R.; Nitta, N.; Boerner, P. F.; Morrison, M. D.
Bibcode: 2006AGUFMSH23B0357L
Altcode:
The Solar X-ray Imager (SXI) instrument, designed and built by the
Lockheed Martin Solar and Astrophysics Laboratory, was launched on May
24, 2006 on the National Oceanic and Atmospheric Administration (NOAA)
GOES-13 spacecraft. The SXI is operated by NOAA's Space Environment
Center (SEC) in Boulder, CO and its X-ray images of the Sun are used
by space weather forecasters to monitor solar activity. The GOES-13
SXI has better resolution than either the Yohkoh SXT or the GOES-12
SXI and its active internal jitter compensation system is performing
as expected to provide stable viewing even when the GOES-13 spacecraft
is moving. The SXI observes between 6 and 60A with broad-band filter
photometry. Full-disk images covering a 42 arcmin x 42 arcmin field
of view with 5-arcsec pixels are normally returned each minute. The
instrument is capable of viewing solar flares, active regions, and
coronal hole boundaries. We present updated computations of the response
of the SXI to thermal plasmas assuming Chianti V5.2 and present early
results from temperature and differential emission measure analyses
of solar active regions.
Title: Goals and Progress of the LWS Focused Science Topic on the
CME--ICME Connection
Authors: Mikic, Z.; Deforest, C.; Devore, R.; Georgoulis, M.; Jackson,
B.; Nitta, N.; Pizzo, V.; Odstrcil, D.
Bibcode: 2006AGUFMSH21B..05M
Altcode:
Our team addresses the NASA Living With a Star (LWS) Focused Science
Topic "to determine the solar origins of the plasma and magnetic flux
observed in an interplanetary Coronal Mass Ejection (ICME)." In short,
this team is examining the CME--ICME connection. Our team was formed
as a result of awards from the LWS Targeted Research &Technology
competition in the fall of 2004. Our team is investigating the detailed
relationship between the plasma and magnetic fields in active regions,
the source regions of CMEs, and subsequent in situ measurements in
interplanetary magnetic clouds. We plan to study this connection through
detailed numerical simulations of CME initiation and propagation,
theoretical investigations, and studies of the properties of active
regions, CMEs, and magnetic clouds. We will discuss the goals of
our team, how it fits into NASA's missions, and our progress so
far. Research supported by NASA's Living With a Star Program.
Title: Solar Sources of Impulsive Solar Energetic Particle Events
and Their Magnetic Field Connection to the Earth
Authors: Nitta, Nariaki V.; Reames, Donald V.; De Rosa, Marc L.; Liu,
Yang; Yashiro, Seiji; Gopalswamy, Natchimuthuk
Bibcode: 2006ApJ...650..438N
Altcode:
This paper investigates the solar origin of impulsive solar energetic
particle (SEP) events, often referred to as 3He-rich flares,
by attempting to locate the source regions of 117 events as observed
at ~2-3 MeV amu-1. Given large uncertainties as to when
ions at these energies were injected, we use type III radio bursts
that occur within a 5 hr time window preceding the observed ion onset,
and search in EUV and X-ray full-disk images for brightenings around
the times of the type III bursts. In this way we find the solar sources
in 69 events. High cadence EUV images often reveal a jet in the source
region shortly after the type III burst. We also study magnetic field
connections between the Earth and the solar sources of impulsive SEP
events as identified above, combining the potential field source
surface (PFSS) model for the coronal field and the Parker spiral
for the interplanetary magnetic field. We find open field lines in
and around ~80% of the source regions. But only in ~40% of the cases,
can we find field lines that are both close to the source region at the
photosphere and to the Parker spiral coordinates at the source surface,
suggesting challenges in understanding the Sun-Earth magnetic field
with observations available at present and in near future.
Title: Observations of simultaneous coronal loop shrinkage and
expansion during the decay phase of a solar flare
Authors: Khan, J. I.; Fletcher, L.; Nitta, N. V.
Bibcode: 2006A&A...453..335K
Altcode:
We report what we believe are the first direct and unambiguous
observations of simultaneous coronal magnetic flux loop shrinkage
and expansion during the decay phase of a solar flare. The retracting
and expanding loops were observed nearly face-on (i.e., with the loop
major axis approximately orthogonal to the line of sight) in emission
in imaging data from the Yohkoh Soft X-ray Telescope (SXT). The
retracting loop is observed to shrink with a speed of 118 ± 66 km
s-1. The faint outward moving loop-like feature occurred
~200´´ above the shrinking loop during the time of the shrinking
loop. We estimate the speed of the outward moving loop was ~129 ± 74
km s-1. We interpret the shrinking loop and simultaneous
outward moving loop as direct evidence for reconnected magnetic field
lines during a flare.
Title: The PFSS Model in the Context of Impulsive SEP Events
Authors: Nitta, Nariaki; De Rosa, M.
Bibcode: 2006SPD....37.2406N
Altcode: 2006BAAS...38..251N
We have located the solar sources of 67 impulsive solar energetic
particle (SEP) events, using type III bursts to narrow down the times
of particle injection in the solar corona. This information serves
as a direct means to test the model of the Sun-Earth magnetic field
connection because the particles simply trace the field lines. We
consider the standard technique to model the Sun-Earth magnetic field,
known as the potential field source surface (PFSS) model for the coronal
part and the Parker spiral for the interplanetary part. In each of
selected SEP events, we calculate the distance of the footpoint of
the well-connected field line from the observed source location. It is
found that the technique does not work as well as when it is used to
predict the solar wind speed and the polarity of the interplanetary
magnetic field. We suggest what we need to do to better understand
the Sun-Earth magnetic field connection, or, more broadly speaking,
the interplanetary magnetic field, which is an important element in
NASA's Exploration Initiative.
Title: The Great Geomagnetic Storm of 9 November 1991: Origin in a
Disappearing Solar Filament
Authors: Cliver, E. W.; Nitta, N.; Balasubramaniam, K.; Li, X.
Bibcode: 2006AGUSMSH43A..06C
Altcode:
The largest geomagnetic storms are characteristically associated with
major solar flares. The great storm of 9 November 1991 (Dst = -375 nT)
provides an exception to this rule of thumb. It is ranked tenth of the
largest Dst storms from 1932-2002, surpassing in intensity such well
known events as 14 July 1982 and 16 July 2000. The November 1991 storm
can be traced to a large disappearing solar filament from the southeast
quadrant late on 5 November. The filament was located outside of an
active region and its disappearance was well observed in both H-alpha
and soft X-rays. The associated long-duration 1-8 Angstrom event had a
C5 peak. This solar-terrestrial event indicates that neither a large
complex active region nor an intense solar flare is a requirement
for even first rank geomagnetic storms, thus providing insight to the
physics of such events while making their prediction more difficult.
Title: Comparison of Heliospheric Magnetic Field Lines from PFSS
Models with SEP Observations
Authors: Nitta, N. V.; Liu, Y.; De Rosa, M. L.
Bibcode: 2005AGUSMSH13A..12N
Altcode:
Impulsive Solar Energetic Particle (SEP) events are thought to come
locally from solar flares, in contrast with large gradual SEP events
that are attributed to extended shocks driven by fast CMEs. For several
impulsive SEP events, we identified the possible solar sources, using
the timings of type III bursts. The solar sources thus indentified
tend to be minor brightenings, sometimes not even detectable by the
GOES X-ray Spectrometer. We found whether the source active region is
open to the heliosphere, using potential field source surface (PFSS)
models. We also traced field lines from the spacecraft observing SEPs
to the source surface assuming constant solar wind speed, and then
mapped them to the photosphere using PFSS models. In a number of cases,
these traced field lines go close to the flare site. In other cases,
their foot-points are far from the flare, or the source active region
shows no open field lines. We interpret these various results in terms
of different magnetograms for PFSS modeling, and assumptions used in
the models.
Title: The Ability of Extreme Ultraviolet and Soft X-ray Imaging
Telescopes to Constrain Coronal Differential Emission Measure
Authors: Boerner, P.; Lemen, J.; Nitta, N.; Martinez-Galarce, D.
Bibcode: 2005AGUSMSP13B..05B
Altcode:
Using the Chianti database, we construct temperature response functions
for the extreme ultraviolet and soft x-ray imaging channels on a variety
of current and future solar observatories. Simulated observations in
these channels are then inverted to recover the coronal differential
emission measure. We investigate the effect of statistical error,
and systematic uncertainties in the atomic physics and instrument
calibration used to calculate the instrument temperature response
functions, on the accuracy of the DEM recovery. By selecting different
combinations of channels, we can characterize the ability of different
instrument architectures to constrain the DEM, and identify an optimal
combination of bandpasses for accurate DEM recovery.
Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images
Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser,
J.; Alexander, D.
Bibcode: 2004AGUFMSH22A..02A
Altcode:
We simulate 3D models of EUV images of flare and CME events, using
TRACE EUV movies. TRACE movies show 2D images in projection along a
particular line-of-sight. We simulate 3D models of erupting filaments,
flare loops, and postflare loops using: (1) a ``finger printing''
technique to trace linear structures in 2D images; (2) geometric 3D
models based on force-free fields and curvature radius maximization
of flare loop and flux rope structures; (3) conservation of velocity
and acceleration parameters; (4) multi-temperature plasma filling
according to hydrodyamic scaling laws; and (5) 2D projections from
secondary line-of-sights that correspond to viewpoints of the secondary
STEREO spacecraft. From such simulations we envision to illustrat
3D time-dependent models, what would be observed at the two STEREO
spacecraft positions as well as from a near-Earth spacecraft such as
SoHO. These simulations are used to test STEREO analysis software and
to investigate what physical parameters and geometric 3D reconstructions
can be retrieved from STEREO/SECCHI data.
Title: Coronal Shocks of November 1997 Revisited: The Cme Type II
Timing Problem
Authors: Cliver, E. W.; Nitta, N. V.; Thompson, B. J.; Zhang, J.
Bibcode: 2004SoPh..225..105C
Altcode:
We re-examine observations bearing on the origin of metric type
II bursts for six impulsive solar events in November 1997. Previous
analyses of these events indicated that the metric type IIs were due to
flares (either blast waves or ejecta). Our point of departure was the
study of Zhang et al. (2001) based on the Large Angle and Spectrometric
Coronagraph's C1 instrument (occulting disk at 1.1 R0) that
identified the rapid acceleration phase of coronal mass ejections (CMEs)
with the rise phase of soft X-ray light curves of associated flares. We
find that the inferred onset of rapid CME acceleration in each of the
six cases occurred 1-3 min before the onset of metric type II emission,
in contrast to the results of previous studies for certain of these
events that obtained CME launch times ∼25-45 min earlier than type
II onset. The removal of the CME-metric type II timing discrepancy in
these events and, more generally, the identification of the onset of
the rapid acceleration phase of CMEs with the flare impulsive phase
undercuts a significant argument against CMEs as metric type II shock
drivers. In general, the six events exhibited: (1) ample evidence
of dynamic behavior [soft X-ray ejecta, extreme ultra-violet imaging
telescope (EIT) dimming onsets, and wave initiation (observed variously
in Hα, EUV, and soft X-rays)] during the inferred fast acceleration
phases of the CMEs, consistent with the cataclysmic disruption of the
low solar atmosphere one would expect to be associated with a CME; and
(2) an organic relationship between EIT dimmings (generally taken to
be source regions of CMEs) and EIT waves (which are highly associated
with metric type II bursts) indicative of a CME-driver scenario. Our
analysis indicates that the broad (∼90° to halo) CMEs observed in
the outer LASCO coronagraphs for these impulsive events began life
as relatively small-scale structures, with angular spans of ∼15°
in the low corona. A review of on-going work bearing on other aspects
(than timing) of the question of the origin of metric type II bursts
(CME association; connectivity of metric and decametric-hectometric
type II shocks; spatial relationship between CMEs and metric shocks)
leads to the conclusion that CMEs remain a strong candidate to be
the principal/sole driver of metric type II shocks vis-à-vis flare
blast waves/ejecta.
Title: Comparison of X-ray Images from Yohkoh/SXT and GOES-M/SXI
Authors: Nitta, N. V.; Lemen, J. R.; Hill, S. M.
Bibcode: 2004AAS...204.5601N
Altcode: 2004BAAS...36..761N
The Yohkoh satellite completed its mission on 14 December 2001. The Soft
X-ray Telescope (SXT) on board Yohkoh observed the dynamic solar corona
nearly continuously for more than 10 years, leading to many important
discoveries in solar physics. Now similar soft X-ray images of the Sun
are taken by the Soft X-ray Imager (SXI) on the GOES-M satellite. The
primary purpose of SXI is to improve space weather forecasting, but its
data are useful for science as well. In order to understand solar cycle
effect on the solar output, stationary or transient, it is important
to understand how SXI and SXT images compare with each other. We
study the images taken during the last few months of Yohkoh's life,
when SXI took more than 100000 images as a test run. We report on how
these images are photometrically compared and how different instrument
characteristics affect the capability to observe transient phenomena.
Title: Homologous large-scale activity in solar eruptive events of
24-26 November 2000
Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V.
Bibcode: 2004JGRA..109.2112C
Altcode:
We study large-scale activity on the solar disk associated with a
24-26 November 2000 series of six recurrent major flares and "halo"
coronal mass ejections (CMEs). The analysis is based mainly on the
SOHO/EIT data, particularly by using properly rotated difference
full-disk images with 12-min intervals at 195 Å as well as with
6-hour intervals at 171, 195, 284, and 304 Å. We demonstrate that
these eruptive events were homologous not only by their flare and
CME characteristics, as [2001] showed, but also in terms of their
large-scale CME-associated manifestations in the EUV corona. These
include long and narrow channeled dimmings, some transequatorial;
anisotropic coronal waves, propagating in a restricted angular sector;
and additional quasi-stationary emitting fronts. As a whole, in all of
these six events, the homologous CME-associated disturbances covered a
considerable portion of the solar disk. The homology tendency appears
to be due to significant disturbance, partial eruption, and relatively
fast restoration of the same large-scale structures involved in the
repeating CME events. We briefly discuss the implications of the
analysis in connection with the nature of coronal equilibrium.
Title: Probing Solar Energetic Particles with SIRA
Authors: Aschwanden, M. J.; Nitta, N.; Lemaster, E.; Byler, E.; Gary,
D.; Kassim, N.; Gopalswamy, N.
Bibcode: 2003AGUFMSH42C0555A
Altcode:
The space-based SIRA (Solar Imaging Radio Array) will provide a powerful
capability to track high energy particles from solar flare and CME sites
through interplanetary/heliospheric space all the way to Earth. Together
with two other overlapping planned radio interferometers, i.e., FASR
(Frequency-Agile Solar Radiotelescope) and LOFAR (Low-Frequency Array)
the entire plasma frequency range from 30 GHz all the way down to
the plasma frequency cutoff of 30 kHz at 1 AU will be covered. These
instruments will track the magnetic trajectory of high energy particles,
beam-driven radio emission, and localize the acceleration sites in
the corona or interplanetary shocks. We simulate some CME and type III
events, as they will be mapped with these instruments, using realistic
scattering functions of radio waves on coronal and heliospheric density
inhomogeneities.
Title: Homologous large-scale activity in solar eruptive events of
November 24-26, 2000
Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V.
Bibcode: 2003AGUFMSH22A0180C
Altcode:
We study large-scale activity on the solar disk associated with a
November 24-26, 2000 series of six recurrent major flares and ``halo''
coronal mass ejections (CMEs). The analysis is based mainly on the
SOHO/EIT data, particularly by using properly rotated difference
full-disk images with 12-min intervals at 195Å~ as well as with
6-hour intervals at 171, 195, 284, and 304Å. We demonstrate that
these eruptive events were homologous not only by their flare and CME
characteristics, as Nitta and Hudson [2001] showed, but also in terms of
their large-scale CME-associated manifestations in the EUV corona. These
include long and narrow channeled dimmings, some transequatorial;
anisotropic coronal waves, propagating in a restricted angular sector;
and additional quasi-stationary emitting fronts. As a whole, in all of
these six events, the homologous CME-associated disturbances covered a
considerable portion of the solar disk. The homology tendency appears
to be due to significant disturbance, partial eruption and relatively
fast restoration of the same large-scale structures involved in the
repeating CME events. We briefly discuss the implications of the
analysis in connection with the coronal equilibrium as indicated by
recent TRACE observations of oscillating loop systems.
Title: A catalogue of white-light flares observed by Yohkoh
Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.;
Nitta, N. V.
Bibcode: 2003A&A...409.1107M
Altcode:
The aspect camera of the Soft X-ray Telescope (SXT) on Yohkoh provided
the first systematic survey of white-light flares from an observatory in
space. The observations were made in the Fraunhofer g-band at a pixel
size of 2.46 arcsec and a typical sample interval on the order of ten
seconds. A total of 28 flares with clear white-light signatures were
detected, corresponding to GOES events down to the C7.8 level in one
case. Above the X-class threshold, all 5 events observed by SXT were
observed in white light, and the maximum average contrast observed
was 30% relative to the pre-flare continuum brightness of the flare
location. We have made comprehensive comparisons of Yohkoh soft X-ray
and hard X-ray data for this list of flares. In addition we compare
the properties of the WLF sample to a sample of 31 flares that showed
no white-light emission. These comparisons show that while white-light
continuum emission has a strong association with hard X-ray emission
it is also strongly related to coronal overpressure, as determined
from the soft X-ray spectrum, indicating a component with a thermal,
rather than non-thermal origin. Appendices are only available in
electronic form at http://www.edpsciences.org
Title: Source Regions of Major Solar Energetic Particle Events
Authors: Nitta, N. V.; Cliver, E. W.; Tylka, A. J.; Smit, P.
Bibcode: 2003ICRC....6.3363N
Altcode: 2003ICRC...28.3363N
We examine the source regions of the largest prompt solar energetic
particle (SEP) events (Jproton [>10 MeV] >100 pr/cm2 /s/sr)
occurring between 1992 and 2002. We find that the 25 such events
originated in a broad spectrum of solar regions, ranging from large
complex active regions with delta sunspot groups (e.g., 30 October 1992)
to a very weak active region in which the ma jor feature was a large
filament that erupted to produce the SEP event (12 September 2000). Most
source regions are less than two rotation old. In terms of recent work
to identify two types of large SEP events on the basis of composition,
spectra, and charge state, we find that large complex active regions
can give rise to both types, whereas simple and magnetically weak
regions are preferentially linked to one type.
Title: Long-duration hectometric type III radio bursts and their
association with solar energetic particle (SEP) events
Authors: MacDowall, R. J.; Lara, A.; Manoharan, P. K.; Nitta, N. V.;
Rosas, A. M.; Bougeret, J. L.
Bibcode: 2003GeoRL..30.8018M
Altcode: 2003GeoRL..30lSEP6M
It has recently been suggested by Cane et al. [2002] that a class of
type III solar radio bursts, called type III-l, is reliably associated
with intense solar energetic particle (SEP) events. They proposed that
the causative electrons for these bursts are accelerated in regions
of reconnecting magnetic field in the wakes of coronal mass ejections
(CMEs). In this paper, we examine the durations, intensities, and other
characteristics of such radio bursts in the hectometric frequency range
and compare them to several groups of control events. We conclude
that simple criteria, based on hectometric data alone, can identify
the majority (~80%) of type III-l radio bursts, which are associated
with >20 MeV SEP proton events, while excluding almost 100% of the
control events. Detailed study of these type III-l bursts may play
a significant role in a better understanding of the acceleration of
SEPs and of the magnetic field evolution in the vicinity of CMEs.
Title: The nature of impulsive solar energetic particle events
Authors: Nitta, N. V.; Hudson, H. S.; De Rosa, M. L.
Bibcode: 2003SPD....34.1606N
Altcode: 2003BAAS...35..833N
Impulsive solar energetic particle (SEP) events, as opposed to
gradual SEP events, are usually thought to originate from flares in
the well-connected regions. In order to test this idea, we compute
the solar locations of the field lines that cross the spacecraft
encountering particles from SEP events, and compare them with
the flare locations. We combine two frequently used techniques,
i.e., the ballistic approximation for the interplanetary magnetic
field and the potential-field source-surface model for the coronal
field. Such comparisons are made for selected impulsive SEP events
during 1995-2001. We check the validity of the techniques using
Yohkoh SXT and SOHO EIT images, which often show coronal holes on
the disk. Furthermore, we study the properties of the flares that are
identified with impulsive SEP events, and compare them with those of
other flares in the well-connected areas but without impulsive SEP
events. This will give us a clue as to the importance of the flare
processes relative to the magnetic field connectivity on the detection
of impulsive SEP events.
Title: Hard X-ray Focusing Optics for Solar Physics
Authors: Ulmer, M. P.; Nitta, N.; Stern, R. A.
Bibcode: 2003SPD....34.2004U
Altcode: 2003BAAS...35..844U
Magnetic reconnecion is widely believed to play an essentrial
role in energy release in solar flares and associated coronal
mass ejections.Theory predicts particle acceleration and plasma
heating. Hard X-rays are used as an important diagnostic for these
processes. The indirect imaging hard X-ray telescopes, such as
RHESSI and Yohkoh/HXT, have been very successful in characterizing
the properties of areas of intense signals. In order to advance the
knowledge about the role of magnetic reconnection in flares and CMEs,
however, we need to measure high-energy radiation from areas where
it is not intense. This is not an obvious task for indirect imaging
telescopes, for which the dynamic range depends on the source complexity
and extension. Over-exposure (or long integration) does not essentially
contribute to enhancing the dynamic range limited by the inevitably
missing Fourier components. Therefore we have begun to explore ways
to construct a direct imaging hard X-ray telescope. We describe our
design in terms of three major requirements: (1) arc second spatial
resolution, (2) full-disk coverage, (3) short focal length and light
weight to fit in a SMEX class payload.
Title: Low Coronal Signatures of Large Solar Energetic Particle Events
Authors: Nitta, Nariaki V.; Cliver, Edward W.; Tylka, Allan J.
Bibcode: 2003ApJ...586L.103N
Altcode:
We report on the low coronal signatures of major solar energetic
particle (SEP) events. Because large SEP events are highly
associated with both flares and coronal mass ejections, we focused on
flare-associated motions in soft X-rays. In a sample of a half-dozen
well-observed flares associated with SEP events, we identified two basic
types of motions or ejections. For one class of events including those
of 2001 November 4 and 1998 April 20, the ejections occur on active
region or larger scales. They have an extended ``preeruption'' phase
in which the involved structures slowly rise or expand on timescales
of tens of minutes. For the second class of events, including those on
1997 November 6 and 2001 April 15, the large-scale preeruption phase
is absent. In these events, ejecta appear explosively at the onset of
the flare impulsive phase. The observed differences in ejections appear
to correlate with spectral/compositional/charge state characteristics
of large SEP events, suggesting that flare ejecta are diagnostic of
shock properties/environment.
Title: Soft X-ray observation of a large-scale coronal wave and
its exciter
Authors: Hudson, Hugh S.; Khan, Josef I.; Lemen, James R.; Nitta,
Nariaki V.; Uchida, Yutaka
Bibcode: 2003SoPh..212..121H
Altcode:
Recent extreme ultraviolet (EUV) observations from SOHO have shown
the common occurrence of flare-associated global coronal waves
strongly correlated with metric type II bursts, and in some cases
with chromospheric Moreton waves. Until now, however, few direct soft
X-ray detections of related global coronal waves have been reported. We
have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to
understand this apparent discrepancy, and describe the problems in this
paper. We have found good X-ray evidence for a large-scale coronal wave
associated with a major flare on 6 May 1998. The earliest direct trace
of the wave motion on 6 May consisted of an expanding volume within
20 Mm (projected) of the flare-core loops, as established by loop
motions and a dimming signature. Wavefront analyses of the soft X-ray
observations point to this region as the source of the wave, which began
at the time of an early hard X-ray spike in the impulsive phase of the
flare. The emission can be seen out to a large radial distance (some
220 Mm from the flare core) by SXT, and a similar structure at a still
greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on
SOHO. The radio dynamic spectra confirm that an associated disturbance
started at a relatively high density, consistent with the X-ray
observations, prior to the metric type II burst emission onset. The
wavefront tilted away from the vertical as expected from refraction if
the Alfvén speed increases with height in the corona. From the X-ray
observations we estimate that the electron temperature in the wave,
at a distance of 120 Mm from the flare core, was on the order of 2-4
MK, consistent with a Mach number in the range 1.1-1.3.
Title: Intense Flares Without Solar Energetic Particle Events
Authors: Nitta, N. V.; Cliver, E. W.; Hudson, H. S.
Bibcode: 2002AGUFMSH61A0437N
Altcode:
We study favorably located (western hemisphere) X-class flares that were
not associated with solar energetic particle (SEP) events. Three of the
four such flares that occurred during the present cycle lacked coronal
mass ejections (CMEs), consistent with the current paradigm. Soft X-ray
data for these three events show either outward moving loops above
the main flare loop or a much slower rise of the flare loop itself,
as predicted in bipolar reconnection models. However, unlike fully
eruptive events, the outward moving loops appear to stop at a certain
distance. We speculate that they are held back by overlying magnetic
field, as implied in soft X-ray images. The flare that was associated
with a CME (but not an SEP event) produced metric and decametric type
III bursts, but those without CMEs did not. Other characteristics for
the flares not associated with SEP events include relatively short
decay times of hard X-ray emission at 30--50 keV. We are extending
our study to include additional (somewhat smaller) events to try to
identify key parameters that keep intense flares from erupting and
accompanying SEP events.
Title: Five years of Yohkoh science nuggets
Authors: Hudson, H. S.; McKenzie, D. E.; Nitta, N. V.
Bibcode: 2002AGUFMSH52A0453H
Altcode:
The Yohkoh "science nuggets", weekly Web-based reports emphasizing
current Yohkoh observations, began October 24, 1997. Many writers
(the SXT "chief observers" in particular) contributed, and over the
years a characteristic style developed: these were educational pieces
rather than public-relations puffs, and they each attempted to describe
a particular item for a technically savvy non-specialist. In this poster
we summarize the nugget philosophy and point out some of our favorites,
such as the remarkable "triple jet." This and many other interesting
observations have not yet otherwise been published. Since December 2001
we have gradually broadened our science basis to include SOHO, TRACE,
and now RHESSI input. The topic index lists more than 60 categories,
and there is a general search facility. We present this poster partly
to encourage discussion of the future development of the series.
Title: Coronal blast waves detected in soft X-rays
Authors: Khan, J.; Hudson, H. S.; Nitta, N. V.
Bibcode: 2002AGUFMSH52A0487K
Altcode:
Several examples of rather clear X-ray detection of coronal blast
waves have now been reported (e.g., Khan and Aurass, A&A 383,
1018, 2002). Interestingly, most of them have been reported from
two particular active regions -- NOAA 8100 (November 1997) and 8210
(April-May 1998). The soft X-ray observations in some cases have
high time resolution and the ability to look close to the core of the
flare responsible for the wave. We summarize the observations to date,
emphasizing the distinction between ejecta (magnetic loops) and freely
running waves. Related observations now exist at metric and centimetric
wavelengths, in the EUV, and in chromospheric lines (H-alpha and He
10830), and we describe the observational relationships among these
different observations.
Title: Importance of flares in understanding the origin of CMEs
Authors: Nitta, Nariaki V.
Bibcode: 2002ESASP.506..189N
Altcode: 2002svco.conf..189N; 2002ESPM...10..189N
Although the correlation between flares and energetic coronal mass
ejections (CMEs) is generally high, there is a trend in which flares
are considered to be of secondary importance in the processes of
CME initiation. This trend may be aligned with the existence of
flares unassociated with CMEs and vice versa. The purpose of this
paper is to remind that the term "flare" as it is widely used could
confuse the issue, and to propose a new scheme to distinguish different
physical processes in flares. Furthermore, we discuss direct low coronal
observations of CMEs in their early evolution, including outward motions
detected in temperatures 1-10 MK. In order to understand the relation
of such motions with CME initiation and coronal/interplanetary shocks,
we need to characterize them more unambiguously using multi-wavelength
observations.
Title: The Structure and Evolution of a Sigmoidal Active Region
Authors: Gibson, S. E.; Fletcher, L.; Del Zanna, G.; Pike, C. D.;
Mason, H. E.; Mandrini, C. H.; Démoulin, P.; Gilbert, H.; Burkepile,
J.; Holzer, T.; Alexander, D.; Liu, Y.; Nitta, N.; Qiu, J.; Schmieder,
B.; Thompson, B. J.
Bibcode: 2002ApJ...574.1021G
Altcode:
Solar coronal sigmoidal active regions have been shown to be precursors
to some coronal mass ejections. Sigmoids, or S-shaped structures,
may be indicators of twisted or helical magnetic structures, having
an increased likelihood of eruption. We present here an analysis of a
sigmoidal region's three-dimensional structure and how it evolves in
relation to its eruptive dynamics. We use data taken during a recent
study of a sigmoidal active region passing across the solar disk
(an element of the third Whole Sun Month campaign). While S-shaped
structures are generally observed in soft X-ray (SXR) emission, the
observations that we present demonstrate their visibility at a range of
wavelengths including those showing an associated sigmoidal filament. We
examine the relationship between the S-shaped structures seen in SXR
and those seen in cooler lines in order to probe the sigmoidal region's
three-dimensional density and temperature structure. We also consider
magnetic field observations and extrapolations in relation to these
coronal structures. We present an interpretation of the disk passage
of the sigmoidal region, in terms of a twisted magnetic flux rope
that emerges into and equilibrates with overlying coronal magnetic
field structures, which explains many of the key observed aspects of
the region's structure and evolution. In particular, the evolving flux
rope interpretation provides insight into why and how the region moves
between active and quiescent phases, how the region's sigmoidicity is
maintained during its evolution, and under what circumstances sigmoidal
structures are apparent at a range of wavelengths.
Title: The Electron Number Problem Revisited with RHESSI Flare
Observations
Authors: Aschwanden, M. J.; Alexander, D.; Metcalf, T.; Nitta, N.
Bibcode: 2002AAS...200.7608A
Altcode: 2002BAAS...34..776A
The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observed
a number of solar flares since its launch on February 5, 2002. We
analyze first images and spectra from this new mission, using a
variety of image reconstruction methods, such as Clean, Pixon, Maximum
Entropy (MEM), and Forward-Fitting. We obtain context images of the
flare regions in EUV from SoHO/EIT and TRACE, and soft X-ray light
curves from GOES. From RHESSI we reconstruct hard X-ray images with
full uv-coverage in time intervals of the spin period ( 4 s). RHESSI
provides a substantially higher spatial resolution ( 2") and spectral
resolution than previous data from Yohkoh/HXT. A longstanding problem
in solar flare physics is the so-called electron number problem,
which challenges a reconciliation between (1) the rate of injected
electrons inferred from the observed hard X-ray photon spectrum in
the framework of the thick-target model and the (2) the replenishment
rate of electrons in the coronal acceleration volume. Thanks to the
high spectral resolution of RHESSI we can for the first time properly
separate the thermal and the non-thermal electron spectral components,
and this way determine the injection rate of nonthermal electrons
more accurately. In addition, imaging in many energy bands allows us
to trace propagation and energy loss between the coronal acceleration
site and the chromospheric thick-target site with higher accuracy,
taking into account the partial energy loss of trapped and precipitating
electrons. The goal of this investigation is to quantify the geometry,
density, and inflows in the coronal acceleration region, which provides
crucial constraints for the underlying magnetic reconnection process.
Title: Coronal Dimming Associated with Coronal Mass Ejections
Authors: Nitta, N. V.; Hudson, H. S.; Newmark, J. S.
Bibcode: 2002AAS...200.2905N
Altcode: 2002BAAS...34..681N
Observations from Yohkoh SXT have identified different patterns
of coronal dimming around the times of coronal mass ejections
(CMEs), but the mapping between the dimming and the CME is still
not well understood. There is always a possibility that the observed
dimming may simply reflect cooling rather than mass depletion due to
ejection. Empirically, dimming seen in SOHO EIT images appears to be
more intimately associated with CMEs, although EIT images with narrower
sensitivities should be more susceptible to changes in temperature. In
this work, we compare SXT and EIT images systematically for various
patterns of dimming, in order to understand possibly different origins
of CMEs. We use new calibrations for both sets of images that allow us
to more accurately estimate the changes of temperature/density that
account for the observed level of dimming. We concentrate on events
with good coverage to study the time dependence of coronal dimming at
different temperatures.
Title: Properties of Large Flares During Cycle 23
Authors: Nitta, N. V.
Bibcode: 2002AGUSMSH41A..05N
Altcode:
We review various properties of flares in the present cycle that are
intense in terms of the peak GOES soft X-ray flux. One of the striking
properties is the dominance of short-duration flares, which are however
well-correlated with extended CMEs as observed by LASCO. Another was
the lack of progressive hardening of hard X-ray spectra on the basis
of Yohkoh data. The hardening was seen in a handful of flares during
the declining phase of cycle 21, and considered to be an indicator of
interplanetary protons. Based on imaging observations from Yohkoh,
SOHO and TRACE, we report the frequency of characteristic pre-event
morphologies such as sigmoids and trans-equatorial loops, which
should reflect the magnetic field environment in which the flare
occurs. Observations of flare ejecta and coronal shock waves are
shown, because the distinction the two phenomena may be important for
understanding the origin of shocks that are responsible for major SEP
events. We discuss the possibility that some of these properties may
have solar-cycle dependence. We hope that this study will provide some
physics-based clarification to the long-lasting questions (confusions)
of the flare-CME relation.
Title: Fast acceleration of a CME-related X-ray structure in the
low solar corona
Authors: Alexander, David; Metcalf, Thomas R.; Nitta, Nariaki V.
Bibcode: 2002GeoRL..29.1403A
Altcode: 2002GeoRL..29j..41A
We report on a unique observation of the early and rapid acceleration of
a fast CME in the low solar corona. The coronal disturbance associated
with a LASCO CME and concurrent X1.2 flare on 1998 April 23 was
well-observed by the Yohkoh Soft X-ray Telescope. The X-ray observations
clearly show an accelerating structure, reaching 800-1100 km/s in ~500s
at a de-projected height of only 280 Mm, indicating an acceleration
well in excess of 1700 m/s2. The soft X-ray observations
are coincident with the onset of the CME, which ultimately attained
a velocity of 1390 km/s and generated a number of interplanetary
radio signatures, frequently associated with fast events. Assuming a
constant acceleration, a0, throughout the duration of the
X-ray observations, we determine a best-fit value of a0 =
1756 m/s2, while application of a power-law acceleration
model yields a better fit to the data with a peak acceleration of
~4865 m/s2 within the first 280 Mm of the solar corona.
Title: On the Relation between Flares and CMEs
Authors: Nitta, N. V.
Bibcode: 2002mwoc.conf..309N
Altcode:
On the relation between flares and CMEs N. V. Nitta, H. S. Hudson As
we reported elsewhere (GRL, in press), six recurrent `halo' coronal
mass ejections (CMEs) occurred (in November 2000) during a 60-hour
period in clear association with major flares in an active region
on the solar disk. The region was undergoing dynamic restructuring
due to flux emergence. The flares were not long-decay events (LDEs)
in terms of soft X-ray light curves and morphologies, although, in
the impulsive phase, they produced ejections in soft X-rays that are
characteristic of CMEs. In this presentation, we extend our study to
more flare/CME events, including analysis of some data taken at and
near Earth. We address (but will not fully answer) questions including
(1) Are flare-associated CMEs triggered differently from those without
flare association, (2) Can we isolate observational signatures that
are more relevant to flares or CMEs, (3) What is the relevance of solar
coronal phenomena to solar energetic particles and geomagnetic indices.
Title: Multi-Wavelength Observations of Yohkoh White-Light Flares
Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.;
Nitta, N. V.
Bibcode: 2002mwoc.conf..289M
Altcode:
The problem of accounting for the continuum emission that is observed
in solar flares is still one which is largely unresolved. These
white-light flares place severe constraints on the energy requirements
and transport mechanisms operating in the flare, raising the question
of whether partial or total in-situ heating is required to account for
this deep atmospheric heating. Since it is widely believed that the
energy release in solar flares occurs in the corona and that energy is
then transported to the low chromosphere where the optical emission is
produced, most attempts to explain the origin of white-light flares
have centred on canonical mechanisms. However, it has become clear
that the spatial and temporal correspondence between white-light and
HXR is not one-to one. In order to further our understanding of these
events we study the temporal and spatial relationships between emission
in the visible, SXR and HXR regimes in all of the white-light flares
observed by Yohkoh prior to the failure of the Soft X-ray Telescope
(SXT) Aspect Camera in 1992; a total of approximately 30 events.
Title: Initiation of Coronal Mass Ejections
Authors: Nitta, N. V.
Bibcode: 2002stma.conf..129N
Altcode:
No abstract at ADS
Title: High-Energy Aspects of CMEs Associated with X-Class Flares
in the Present Cycle
Authors: Nitta, N. V.; Hudson, H. S.
Bibcode: 2001AGUFMSH32B..04N
Altcode:
We review the properties of X-class flares as observed by Yohkoh in
the present solar cycle. More than a half of about four dozen X-class
flares were caught by Yohkoh from an early phase. Most of these flares
have durations not as long as long decay events (LDEs), and yet their
association with coronal mass ejections (CMEs) is very high. They often
correspond to extended or halo events. The flares not associated with
CMEs do not show ejecta in soft X-rays. The flare core usually shows
compact morphology. Within the sensitivity of the Yohkoh Hard X-ray
Spectrometer, the hard X-ray spectra extend to the MeV range only
in 20% of these flares, and their temporal variations are typically
soft-hard-soft. Concerning their association with interplanetary proton
events at 20 MeV, not all the proton-associated flares are associated
with CMEs or located close to the well-connected longitudes. We plan
to incorporate analysis of additional data such as metric/kilometric
radio spectra to study when and where the shocks form.
Title: Two Recent Electron-Rich Energetic Particle Events and Their
Associated CMEs
Authors: St. Cyr, O. C.; Cane, H. V.; Nitta, N. V.; Ciaravella, A.;
Raymond, J. C.; Howard, R. A.
Bibcode: 2001AGUFMSH42A0768S
Altcode:
Electron-rich energetic particle events were defined by Evenson et
al. (1984) as a class that correlated with solar flares that produced
gamma-rays. Cane et al. (1986) demonstrated that these events are
associated with impulsive (i.e. compact) flares. We have identified
two electron-rich energetic particle events in the IMP 8 data in cycle
23. The dates of the two events are 01-May-2000 and 10-March-2001. The
March 2001 event was well-observed by Yohkoh and showed a small hard
X-ray source at the flare site. The soft/hard X-ray flux ratio was
low. We have also examined SOHO LASCO and EIT images to investigate
other forms of solar activity. Both events appear to be associated with
coronal mass ejections (CMEs) that possess somewhat unusual morphology
(bright, structured material, with little evidence of leading coronal
material). Moreover, the two CME events appear similar to each other,
at least so far as morphology in the LASCO observations. Additionally,
observations of the March 2001 CME were obtained by the ultraviolet
spectrometer (UVCS) on SOHO. UVCS observed the narrow ejecta
following the leading feature at several heights (from 1.7 up to 3.1
solar radii). The CME material appeared bright in cool lines of OVI
(1032,1037 A doublet), CIII (977 A), Lyman-beta and Lyman-alpha. The
Doppler shift suggests a very small speed along the line of sight,
consistent with the CME being near the plane of the sky. We present
the SOHO observations and the Yohkoh SXT images (for the 10-March-2001
impulsive flare), and we describe our results.
Title: A Relationship Between Transition Region Brightenings,
Abundances, and Magnetic Topology
Authors: Fletcher, Lyndsay; López Fuentes, Marcelo C.; Mandrini,
Cristina H.; Schmieder, Brigitte; Démoulin, Pascal; Mason, Helen E.;
Young, Peter R.; Nitta, Nariaki
Bibcode: 2001SoPh..203..255F
Altcode:
We present multi-instrument observations of active region (AR) 8048,
made between 3 June and 5 June 1997, as part of the SOHO Joint Observing
Program 33. This AR has a sigmoid-like global shape and undergoes
transient brightenings in both soft X-rays and transition region
(TR) lines. We compute a magneto-hydrostatic model of the AR magnetic
field, using as boundary condition the photospheric observations of
SOHO/MDI. The computed large-scale magnetic field lines show that the
large-scale sigmoid is formed by two sets of coronal loops. Shorter
loops, associated with the core of the SXT emission, coincide with
the loops observed in the hotter CDS lines. These loops reveal a
gradient of temperature, from 2 MK at the top to 1 MK at the ends. The
field lines most closely matching these hot loops extend along the
quasi-separatrix layers (QSLs) of the computed coronal field. The TR
brightenings observed with SOHO/CDS can also be associated with the
magnetic field topology, both QSL intersections with the photosphere,
and places where separatrices issuing from bald patches (sites where
field lines coming from the corona are tangent to the photosphere)
intersect the photosphere. There are, furthermore, suggestions that
the element abundances measured in the TR may depend on the type
of topological structure present. Typically, the TR brightenings
associated with QSLs have coronal abundances, while those associated
with BP separatrices have abundances closer to photospheric values. We
suggest that this difference is due to the location and manner in which
magnetic reconnection occurs in two different topological structures.
Title: Hard X-Radiation from a Fast Coronal Ejection
Authors: Hudson, H. S.; Kosugi, T.; Nitta, N. V.; Shimojo, M.
Bibcode: 2001ApJ...561L.211H
Altcode:
We have observed a high-speed coronal ejection in hard X-rays,
detectable to an altitude of some 2×105 km in the Yohkoh
23-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz from the
Nobeyama radioheliograph shows complex moving features simultaneous with
the ejection, including a compact source that we identify with the rapid
X-ray source motion. The hard X-ray and microwave observations agree on
ejection velocities in the vicinity of 1000 km s-1. The hard
X-ray sources also corresponded in position angle to a bright coronal
mass ejection (CME) detected about 15 minutes later and temporally to
both fast-drift and slow-drift radio bursts in the decimeter-meter
bands. Other components of coronal hard X-ray emission were also
detected, including an extended long-duration event with a nonthermal
spectrum. We suggest that a major eruptive flare occurred in NOAA Active
Region 9415, approximately 26° beyond the west limb at the time of the
event. Estimating a source density of 4×109 cm-3
from the compact source observed at 17 GHz, we find a total electron
number (>20 keV) of approximately N20~1.3×1036
for the compact part of the source. We infer that these electrons
were trapped in expanding loops forming a part of the CME and may have
contributed substantial pressure within these loops.
Title: Recurrent flare/CME events from an emerging flux region
Authors: Nitta, Nariaki V.; Hudson, Hugh S.
Bibcode: 2001GeoRL..28.3801N
Altcode:
We report on six recurrent ‘halo’ coronal mass ejections (CMEs) that
occurred (in November 2000) during a 60-hour period in clear association
with major flares in an active region on the solar disk. The region was
undergoing dynamic restructuring due to flux emergence. The flares were
not long-decay events (LDEs) in terms of soft X-ray light curves and
morphologies, although, in the impulsive phase, they produced ejections
in soft X-rays that are characteristic of CMEs. We do not detect global
changes in EUV and X-ray full-disk images prior to these flares. We
suggest that emerging magnetic flux in the core of an active region
may be responsible for the occurrence of such repeated flare/CME events.
Title: The Physical Nature of the Loop-Top X-Ray Sources in the
Gradual Phase of Solar Flares
Authors: Nitta, Nariaki V.; Sato, Jun; Hudson, Hugh S.
Bibcode: 2001ApJ...552..821N
Altcode:
We have analyzed Yohkoh soft and hard X-ray images of 36 flares,
primarily to study the loop-top source that often prevails in these
wavelengths during and following the impulsive phase. There are
typically two patterns for the location of the low-energy (15-30 keV)
hard X-ray (HXR) source with respect to the soft X-ray (SXR) loop. In
a quarter of the flares, the HXR source lies in an extended structure
separate from the brightest SXR loop. In other flares, the HXR source
appears to be part of the same bipolar structure as the SXR loop,
but its centroid is often displaced from the SXR loop-top source. The
fact that the HXR source is not cospatial with the SXR source may
reflect the presence of a distinct hotter structure. According to
Yohkoh X-ray emission-line spectroscopy, the ~20 MK plasma accounts
for only a fraction of the HXR counts. The temperature maps obtained
from the SXR broadband photometry occasionally reveal high-temperature
areas outside the bright loop, but they also tend to be displaced from
the HXR source, indicating that they do not represent the superhot
(>~30 MK) plasma. We discuss possible distributions of plasma of
different temperatures that could be consistent with the data.
Title: Do flare-associated CMEs represent a separate class?
Authors: Nitta, N. V.
Bibcode: 2001AGUSM..SH31C06N
Altcode:
It is generally known that high-speed CMEs are associated with flares,
and that flares with longer durations have higher correlations with
CMEs. However, we also know that short-duration flares can also be
associated with CMEs (Kahler, Sheeley and Liggett 1989, and Harrison
1995). By studying the properties of major flares as observed by Yohkoh,
and their associated CME data from SOHO/LASCO, we discuss similarities
and differences of flare-associated CMEs and those that come from
quiescent regions, and study how the flare properties are inherited
in the associated CMEs. We also emphasize the 3-d nature of CMEs when
we interpret their speeds in the plane of the sky, by including those
events that originate from disk regions. For disk events, the magnetic
properties around the initiation regions are studied. We hope that
this study further clarifies the relationship between flares and CMEs.
Title: Comparison Between Hot and Cool Ejections in CME/Flare Events
Authors: Nitta, N. V.
Bibcode: 2001AGUSM..SH22A09N
Altcode:
Comparison between hot and cool ejections in CME/flare events
Nariaki Nitta (LMSAL) , Sachiko Akiyama (GUAS) We have shown that
high-temperature ejections during the impulsive phase of flares
as seen with Yohkoh/SXT are correlated with coronal mass ejections
(CMEs) as seen by SOHO/LASCO. Since then we have collected a number
of examples of ejections observed with TRACE. In this presentation,
we compare ejections in soft X-rays with those in H-alpha and EUV,
and study the sequence of processes (reconnection, mass motion,
heating, etc.) involved in CMEs so that we can put more constraints
on the models.
Title: Analysis of X-ray counterparts for Fast and Slow Halo CMEs
Authors: Alexander, D.; Metcalf, T. R.; Nitta, N.
Bibcode: 2001AGUSM..SH22B05A
Altcode:
In many cases, coronal mass ejections exhibit a strong counterpart in
the X-ray corona, particularly in flare-related events. Observations
by the Soft X-ray Telescope on Yohkoh have exhibited a number of
manifestations in association with CME eruptions. These include the
well-known dimming events, post-event arcades and expanding loop
systems. We examine the X-ray signatures of a number of fast and
slow halo CME events for evidence of two-classes of CME eruption in
accord with the observed velocity profiles determined from white-light
data. Flare-associated CMEs, which have a tendency to exhibit constant
velocity, necessarily undergo a rapid acceleration in the low corona
and should exhibit enhanced heating of the ambient corona to X-ray
emitting temperatures. Slow CMEs, on the other hand, are expected to
display a more subtle, if any, effect in the hot corona. We examine
the Yohkoh database for evidence of a dichotomy in the X-ray signatures
of halo CMEs.
Title: Flare-CME Events Associated With a Superactive Region
Authors: Wang, J.; Zhang, J.; Wang, T.; Zhang, C.; Liu, Y.; Nitta,
N.; Slater, G. L.
Bibcode: 2001IAUS..203..331W
Altcode:
Flare-CME events in a superactive region, NOAA 8100, have been studied,
using a data base that consists of photospheric vector magnetograms,
Hα and Hβ filtergrams, SOHO EIT and LASCO, and Yohkoh SXT images. The
following results are obtained. 1) There were 5 main emerging flux
regions (EFRs) and several key sites of flux cancellation. The newly
emerging flux and its driven flux cancellation were likely to result in
the expansion of the large-scale flux loops, which became sigmoidal. 2)
All flare-CME events were initiated at the narrow magnetic interface
between a major EFR and its impacted higher magnetic loops. 3) Each
event showed nearly simultaneous occurrences of the flare, dimming
and CME in widely-separated areas. This indicates an energization of
a globally connected large-scale loop system. Eruptions of giant solar
magnetic loops seem to be responsible for extended CMEs.
Title: CDS UV Brightenings Explained by Quasi-separatrices and Bald
Patches in an S-shape active region
Authors: Schmieder, B.; Démoulin, P.; Fletcher, L.; López Fuentes,
M. C.; Mandrini, C. H.; Mason, H. E.; Young, P. R.; Nitta, N.
Bibcode: 2001IAUS..203..314S
Altcode:
We present multi-instrument observations of AR 8048, made between
June 3 and June5 1997 as part of SoHO JOP033. This active region
has a sigmoid-like global shape and undergoes transient erupting
phenomena which releases the stored energy. Using a force free
field approach, we defined coronal magnetic field lines which fit
with the observations. The large-scale magnetic field lines confirms
the sigmoid characteristics of the active region. The study in 3D of
the configuration explained where and how the energy is released at
different places. The Ne VI brightenings correspond to the location
of tangent to the photosphere field lines, named "bald patch", they
are localized in the low transition region and represent feet of field
lines. The Si XII brightenings at coronal temperature are at the top
of coronal loops joining quasi-separatrices.
Title: A Filament-Associated Halo Coronal Mass Ejection
Authors: Zhang, Jun; Wang, Jingxiu; Nitta, Nariaki
Bibcode: 2001ChJAA...1...85Z
Altcode:
There are only a few observations published so far that show the
initiation of a coronal mass ejection (CME) and illustrate the magnetic
changes in the surface origin of a CME. Any attempt to connect a
CME with its local solar activities is meaningful. In this paper we
present a clear instance of a halo CME initiation. A careful analysis of
magnetograms shows that the only obvious magnetic changes in the surface
region of the CME is a magnetic flux cancellation underneath a quiescent
filament. The early disturbance was seen as the slow upward motion in
segments of the quiescent filament. Four hours later, the filament was
accelerated to about 50 km s-1 and erupted. While a small
part of the material in the filament was ejected into the upper corona,
most of the mass was transported to a nearby region. About forty minutes
later, the transported mass was also ejected partially to the upper
corona. The eruption of the filament triggered a two-ribbon flare,
with post-flare loops connecting the flare ribbons. A halo CME, which
is inferred to be associated with the eruptive filament, was observed
from LASCO/C2 and C3. The halo CME contained two CME events, each event
corresponded to a partial mass ejection of the filament. We suggest
that the magnetic reconnection at the lower atmosphere is responsible
for the filament eruption and the halo CME.
Title: Solar Coronal Structure Study
Authors: Nitta, Nariaki; Bruner, Marilyn E.; Saba, Julia; Strong,
Keith; Harvey, Karen
Bibcode: 2000STIN...0085876N
Altcode:
The subject of this investigation is to study the physics of the
solar corona through the analysis of the EUV and UV data produced
by two flights (12 May 1992 and 25 April 1994) of the Lockheed
Solar Plasma Diagnostics Experiment (SPDE) sounding rocket payload,
in combination with Yohkoh and ground-based data. Each rocket flight
produced both spectral and imaging data. These joint datasets are useful
for understanding the physical state of various features in the solar
atmosphere at different heights ranging from the photosphere to the
corona at the time of the, rocket flights, which took place during the
declining phase of a solar cycle, 2-4 years before the minimum. The
investigation is narrowly focused on comparing the physics of small-
and medium-scale strong-field structures with that of large-scale, weak
fields. As we close th is investigation, we have to recall that our
present position in the understanding of basic solar physics problems
(such as coronal heating) is much different from that in 1995 (when we
proposed this investigation), due largely to the great success of SOHO
and TRACE. In other words, several topics and techniques we proposed can
now be better realized with data from these missions. For this reason,
at some point of our work, we started concentrating on the 1992 data,
which are more unique and have more supporting data. As a result, we
discontinued the investigation on small-scale structures, i.e., bright
points, since high-resolution TRACE images have addressed more important
physics than SPDE EUV images could do. In the final year, we still spent
long time calibrating the 1992 data. The work was complicated because
of the old-fashioned film, which had problems not encountered with more
modern CCD detectors. After our considerable effort on calibration, we
were able to focus on several scientific topics, relying heavily on the
SPDE UV images. They include the relation between filaments and filament
channels, the identification of hot loops, and the physical conditions
of such loops especially at their foot-points. A total of four papers
were completed from this contract which are listed in the last section.
Title: The relation between hot and cool loops
Authors: Nitta, Nariaki
Bibcode: 2000SoPh..195..123N
Altcode:
In order to study the origin of the hot (≳3 MK) corona above active
regions, we compare Yohkoh/SXT X-ray images, which represent a broad
temperature range above ∼ 2.5 MK, with TRACE EUV coronal images whose
primary sensitivities are in the 1-2 MK range. Nearly simultaneous
X-ray and EUV snapshots show that there are loops that appear similar in
these images of different temperature sensitivities, but they are not
exactly cospatial with each other. A significant difference is noted
in the active region core, where bright loops are seen in X-ray images
but not in EUV images, reflecting their high (∼5 MK) temperatures. In
SXT time-sequence images, these loops are found to undergo repetitive
minor brightenings, suggestive of their flare-like origin. This is
consistent with the absence of the EUV counterparts of the X-ray loops
in TRACE time-sequence images at earlier and later times. We need to
revisit the validity of the assumption that coronal loops are steady.
Title: Correction to “Coronal dimmings and energetic CMEs in
April-May 1998,”
Authors: Thompson, B. J.; Cliver, E. W.; Nitta, N.; Delannée, C.;
Delaboudinière, J. -P.
Bibcode: 2000GeoRL..27.1865T
Altcode:
No abstract at ADS
Title: Comparison of the 1998 April 29 M6.8 and 1998 November 5
M8.4 Flares
Authors: Wang, Haimin; Goode, Philip R.; Denker, Carsten; Yang, Guo;
Yurchishin, Vasyl; Nitta, Nariaki; Gurman, Joseph B.; St. Cyr, Chris;
Kosovichev, Alexander G.
Bibcode: 2000ApJ...536..971W
Altcode:
We combined, and analyzed in detail, the Hα and magnetograph data
from Big Bear Solar Observatory (BBSO), full-disk magnetograms from
the Michelson Doppler Imager (MDI) on board Solar and Heliospheric
Observatory (SOHO), coronagraph data from the Large Angle Spectrometric
Coronagraph (LASCO) of SOHO, Fe XII 195 Å data from the Extreme
ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray
telescope (SXT) data of the M6.8 flare of 1998 April 29 in National
Oceanic and Atmospheric Administration (NOAA) region 8375 and the
M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares
have remarkable similarities:1. Partial halo coronal mass ejections
(CMEs) were observed for both events. For the 1998 April 29 event,
even though the flare occurred in the southeast of the disk center,
the ejected material moved predominantly across the equator, and the
central part of the CME occurred in the northeast limb. The direction
in which the cusp points in the postflare SXT images determines the
dominant direction of the CMEs.2. Coronal dimming was clearly observed
in EIT Fe XII 195 Å for both but was not observed in Yohkoh SXT for
either event. Dimming started 2 hr before the onset of the flares,
indicating large-scale coronal restructuring before both flares.3. No
global or local photospheric magnetic field change was detected from
either event; in particular, no magnetic field change was found in the
dimming areas.4. Both events lasted several hours and, thus, could be
classified as long duration events (LDEs). However, they are different
in the following important aspects. For the 1998 April 29 event,
the flare and the CME are associated with an erupting filament in
which the two initial ribbons were well connected and then gradually
separated. SXT preflare images show the classical S-shape sheared
configuration (sigmoid structure). For the 1998 November 5 event, two
initial ribbons were well separated, and the SXT preflare image shows
the interaction of at least two loops. In addition, no filament eruption
was observed. We conclude that even though these two events resulted
in similar coronal consequences, they are due to two distinct physical
processes: eruption of sheared loops and interaction of two loops.
Title: Coronal dimmings and energetic CMEs in April-May 1998
Authors: Thompson, B. J.; Cliver, E. W.; Nitta, N.; Delannée, C.;
Delaboudinière, J. -P.
Bibcode: 2000GeoRL..27.1431T
Altcode:
We have analyzed the coronal dimmings for seven fast (> 600 km/s)
coronal mass ejections (CMEs) occurring between 23 April and 9 May which
were associated with flares from NOAA active region (AR) 8210. Each
of these CMEs had at least one group of interplanetary radio bursts
associated with them. These dimming regions were identified by their
strong depletion in coronal EUV emission within a half hour of the
estimated time of CME lift-off. They included areas which were as
dark as quiescent coronal holes as well as other regions with weaker
brightness depletions. While the location of the active region and
the associated flare did not correspond well with the coronagraph
observations, we found that the extended dimming areas in these events
generally mapped out the apparent “footprint” of the CME as observed
by white-light coronagraph. We briefly discuss the implications of
these results on models of CME topology.
Title: The Effect of Hydrostatic Weighting on the Vertical Temperature
Structure of the Solar Corona
Authors: Aschwanden, Markus J.; Nitta, Nariaki
Bibcode: 2000ApJ...535L..59A
Altcode: 2000astro.ph..4093A
We investigate the effect of hydrostatic scale heights λ(T)
in coronal loops on the determination of the vertical temperature
structure T(h) of the solar corona. Every method that determines an
average temperature at a particular line of sight from optically
thin emission (e.g., in EUV or soft X-ray wavelengths) of a
mutlitemperature plasma is subject to the emission measure-weighted
contributions dEM(T)/dT from different temperatures. Because most
of the coronal structures (along open or closed field lines) are
close to hydrostatic equilibrium, the hydrostatic temperature scale
height introduces a height-dependent weighting function that causes
a systematic bias in the determination of the temperature structure
T(h) as function of altitude h. The net effect is that the averaged
temperature seems to increase with altitude, dT(h)/dh>0, even if
every coronal loop (of a multitemperature ensemble) is isothermal in
itself. We simulate this effect with differential emission measure
distributions observed by SERTS for an instrument with a broadband
temperature filter such as Yohkoh/Soft X-Ray Telescope and find that
the apparent temperature increase due to hydrostatic weighting is
of order ΔT~T0h/rsolar. We suggest that this
effect largely explains the systematic temperature increase in the
upper corona reported in recent studies (e.g., by Sturrock et al.,
Wheatland et al., or Priest et al.), rather than being an intrinsic
signature of a coronal heating mechanism.
Title: Observational Signatures of the Onset of Coronal Mass Ejections
Authors: Nitta, N. V.
Bibcode: 2000SPD....31.0276N
Altcode: 2000BAAS...32..824N
Coronal mass ejections (CMEs) are observed by coronagraphs above their
occulting disks. Therefore it is in principle not straightforward to
identify CME signatures in the lower corona or below, especially at
the onset, unless they are located near the limb and observed in good
temporal resolution. For example, a bright cusp structure in soft X-rays
is often considered to result from an eruptive event, but the cusp does
not always point to the direction the associated CME emerges, suggesting
that unknown 3-D geometrical effects play an important role. In this
presentation, we select from the SoHO/LASCO database a small number
of well-observed CMEs that are known to be of disk origin, and try to
find signatures in multi-wavelengths that may represent the onset of
CMEs. We analyze data in soft X-rays (Yohkoh/SXT), EUV (SoHO/EIT and
TRACE) and H-alpha (SOON). The work is expected to further calibrate
non-coronagraph observations of ``CMEs'' and specifically to lead to
a better understanding of the role of flares in the CME initiation.
Title: Max Millennium/Whole Sun Month Observations of a Sigmoid Region
(AR 8668)
Authors: Zarro, D. M.; Canfield, R. C.; Nitta, N.; Myers, D. C.;
Gregory, S. E.; Qiu, J.; Alexander, D.; Hudson, H. S.; Thompson,
B. J.; LaBonte, B. J.
Bibcode: 2000SPD....31.0236Z
Altcode: 2000BAAS...32..817Z
We report on observations of a sigmoidal region AR 8668 obtained
during the Whole Sun Month #3 campaign and Max Millennium Coordinated
Observing Program #2. The observations pertain to the period 1999
August 16-17 during which several GOES B and C class flares occurred
in AR 8668. Near simultaneous observations were obtained by SOHO (EIT
195 Angstroms/ and MDI full-disk magnetograms), TRACE 171 Angstroms/,
Yohkoh SXT, Big Bear (Hα ), and Mees (IVM vector magnetograms). The
multi-wavelength nature of these data, combined with their overlapping
spatial and temporal coverages, provide a unique opportunity to study
the magnetic topology and flaring evolution of twisted flux structures
associated with sigmoids. An objective of this study is to co-align
images and magnetograms obtained before and during the observed flares,
and compare the results with inferences from the topological model of
Titov and Demoulin, A&A 351, 707 (1999). We will present examples
of these coalignments and identify sites of magnetic energy release
that are associated with topological features (e.g. separatrices)
predicted by this model.
Title: Small and large scale magnetic structures involved in the
development of the 1992 October 28 solar flare
Authors: Raulin, J. -P.; Vilmer, N.; Trottet, G.; Nitta, N.; Silva,
A. V. R.; Kaufmann, P.; Correia, E.; Magun, A.
Bibcode: 2000A&A...355..355R
Altcode:
We report spatially resolved observations of a solar flare,
obtained in a wide range of wavelengths including soft and hard
X-rays, decimeter-meter and millimeter radio emissions. This unique
combination of data (including the simultaneous fast localization of
emission at millimeter wavelengths) allows us to conclude that, even
if the 1992 October 28 flare is a moderate one in terms of the amount
of energy contained in the thermal plasma and in non thermal electrons,
the magnetic configuration involved in the flare development is complex
at all spatial scales. Furthermore, it is shown that stepwise temporal
evolutions of the amount of energy release in non thermal electrons
are closely related to stepwise changes of the magnetic structures of
all spatial scales illuminated either by hot plasma or by non-thermal
electrons. This provides some evidence that different episodes of
energy release are triggered by loop-loop interaction and that the site
of particle acceleration changes rapidly within the energy release
volume. These findings are discussed in the context of flare models
as well as with respect to the fragmented nature of energy release in
solar flares.
Title: Flare Loop Geometry
Authors: Nitta, N.
Bibcode: 2000ASPC..206..258N
Altcode: 2000hesp.conf..258N
No abstract at ADS
Title: Yohkoh Observations of White-Light Flares
Authors: Matthews, S.; van Driel-Gesztelyi, L.; Hudson, H.; Nitta, N.
Bibcode: 2000ASPC..206..239M
Altcode: 2000hesp.conf..239M
No abstract at ADS
Title: Small and Large Scale Magnetic Structures Involved in the
Development of the 1992 October 28 Solar Flare
Authors: Raulin, J. P.; Vilmer, N.; Trottet, G.; Nitta, N.; Silva,
A. V. R.; Kaufmann, P.; Correia, E.; Magun, A.
Bibcode: 2000ASPC..206..245R
Altcode: 2000hesp.conf..245R
No abstract at ADS
Title: Dynamics in Restructuring Active Regions Observed During
Soho/Yohkoh/Gbo Campaigns
Authors: Schmieder, B.; Deng, Y.; Mandrini, C. H.; Rudawy, P.; Nitta,
N.; Mason, H.; Fletcher, L.; Martens, P.; Brynildsen, N.
Bibcode: 2000AdSpR..25.1879S
Altcode:
JOP17 and JOP 33 are SOHO Joint Observing Programs in collaboration
with Yohkoh/SXT and ground based observatories (GBO's), dedicated to
observe dynamical events through the atmosphere. During runs of these
programs we observed in restructuring active regions (ARs), surges,
subflares, bright knots, but not large flares and jets. From these
observations we have been able to derive some of the responses of the
coronal and chromospheric plasma to the evolution of the photospheric
magnetic field. Emerging flux in an AR led to the formation of Arch
Filament Systems in the chromosphere, hot loops and knots in the
transition region, and X-ray loops. Frequent surges have been observed
in relation to parasitic or mixed polarities, but coronal jets have not
yet been found. We discuss the possible mechanisms acting during the
restructuring of the active regions (reconnection or ``sea-serpent''
geometries)
Title: The relation between hot and cool loops
Authors: Nitta, N. V.
Bibcode: 2000ssls.work..117N
Altcode:
In order to trace the origin of the hot (>~2.5 MK) corona above
active regions, we compare images from Yohkoh/SXT and TRACE. A typical
SXT snapshot of an active region shows the bright core surrounded by
diffuse emission. In time sequence SXT images, bright X-ray structures
easily recognized as loops are often found to follow minor brightenings
and to undergo structural changes on time scales that can be shorter
than the expected cooling times. These loops do not show up in TRACE
coronal images before, during or after they are seen in SXT images,
indicating their origin to be different from 1-2 MK loops. The data
suggest that heating and cooling of the diffuse loops may occur
dynamically.
Title: A Microwave Study of Coronal and Chromospheric Ejecta
Authors: Nindos, A.; Kundu, M. R.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999spro.proc..135N
Altcode:
We have studied the radio properties of 18 X-ray coronal jets (observed
by the Yohkoh SXT) using Nobeyama 17 GHz data. We also searched for
chromospheric ejecta (Hα surges) during the time intervals that the
X-ray images were available. Microwave emission was associated with the
majority of the X-ray jets. The radio emission came from the base or
the lower part of the jets. We detected radio emission from almost all
jets which showed flare-like activity at their footpoints. The 17 GHz
time profiles were gradual and unpolarized, implying that the emission
was thermal. When possible, we computed the physical properties of the
X-ray-emitting ejected plasma. In one two-sided-loop type jet and one
anemone-type jet, the observed microwave fluxes from the lower part of
the jets were well above the fluxes predicted from the computed electron
temperatures and emission measures of the soft X-ray-emitting material
on the basis of thermal free-free emission. We interpreted the large
discrepancies in terms of the presence of lower temperature material
which cannot be detected by the SXT but produces strong microwave
free-free emission. This is the first time that such material is
observed in two-sided-loop type jets. Thus our observations confirm the
theoretical prediction by Yokoyama and Shibata (1996). We detected no
cool material at the base of the jets. We also observed an Hα surge
which was not associated with an X-ray jet and showed no signatures on
the SXT images but was detected with the Nobeyama Radioheliograph. The
emission of the microwave surge-associated source was free-free from the
chromospheric plasma. Constraints for the surge density were derived.
Title: Temperature Structure of Non-Flaring Loops
Authors: Nitta, N.
Bibcode: 1999spro.proc..103N
Altcode:
The Yohkoh Soft X-ray Telescope (SXT) is a grazing incidence
telescope. Temperature and emission measure maps can be obtained
from the ratio of a pair of images in two filters that have different
transmissions, assuming that each pixel or line of sight is dominated
by plasma of a single temperature. Because of the broad temperature
sensitivity of each filter, caution must be made when interpreting
such filter ratio temperatures in the presence of plasmas of different
temperatures along the line of sight. Here, we analyze SXT data in three
filters in terms of two simple forms of differential emission measure
(DEM), namely two delta functions and a single Gaussian. We also study
how departure from the isothermal assumption affects the predicted
radio brightness temperatures at 17 GHz, the primary frequency of
the Nobeyama Radio Heliograph. Within the range of temperatures SXT
can see, the effect is found to be quite small. Therefore, we need
to assume the presence of much cooler (T << 1~MK) plasma,
if the brightness temperature calculated from the SXT filter ratio
temperature and emission measure is significantly higher than observed
(c.f., Hanaoka 1999).
Title: Flare Loop Geometry
Authors: Nitta, N.
Bibcode: 1999spro.proc..343N
Altcode:
We discuss how much we can learn about the geometry of flare loops from
the X-ray imagery presently available. The stereoscopic techniques,
which make use of different projections caused by solar rotation, are
not applicable to flare loops, because of their dynamic nature. With
an assumption that the loop is contained in a plane, a single image
provides a family of loop shapes as a function of the inclination angle
of the loop plane with respect to the plane normal to the surface. It
appears that various ways exist that may enable us to constrain the
inclination angle.
Title: Relation between Flare-associated X-Ray Ejections and Coronal
Mass Ejections
Authors: Nitta, Nariaki; Akiyama, Sachiko
Bibcode: 1999ApJ...525L..57N
Altcode:
In an attempt to identify the direct signatures of coronal mass
ejections (CMEs) in soft X-ray wavelengths, we have searched for plasma
ejections in Yohkoh soft X-ray telescope (SXT) images in a total of 17
limb flares, and compared the results with the Solar and Heliospheric
Observatory LASCO data. A general correlation exists between the
presence/absence of the X-ray ejection and the CME. Although the height
versus time relation often indicates (under the assumption of constant
speed) that the CME onset coincides with the X-ray ejection, the latter
probably does not represent the CME front, because the CME speed must
result from acceleration, which would put the estimated onset at an
earlier time. In some cases, the estimated CME onset time comes well
before the impulsive phase of the associated flare. Although the role
of the flare-associated plasma ejection in a CME is still unclear, we
propose that its occurrence depends on the presence of open field lines,
which can be due to a preceding CME. Lastly, we present a rare example
of SXT observations of what appeared to be the three-part structure
of a CME, which was seen a few minutes before a major flare started.
Title: Flare loop geometry
Authors: Nitta, N.; Van Driel-Gesztelyi, L.; Harra-Murnion, L. K.
Bibcode: 1999SoPh..189..181N
Altcode:
In selected flares that occurred in AR 7260, we have studied
the geometry of the brightest soft X-ray loop by tracing it on
an image. Even under the assumption that the loop is contained
in a plane, it is clear that a single image does not permit us to
determine the full geometry. It only provides possible loop shapes as
a function of the inclination angle of the loop plane with respect
to the vertical. However, all the loops that reproduce the observed
appearance give the same direction of increasing height as projected
on to the image plane. This direction is compared with two relevant
observations. Based on 2-D reconnection models that involve a cusp
configuration, it is expected that the soft X-ray loop top source moves
upward with time and that a higher temperature region exists above the
loop top. Several flares are found to contradict these predictions,
presumably implying the inadequacy of the models. Lastly we discuss
a possibility of constraining the inclination angle (and hence the
loop shape) with spatially-unresolved soft X-ray line spectra which
are Doppler-shifted due to plasma upflows.
Title: Small and Large Scale Magnetic Structures Involved in the
Development of the 1992 October 28 Solar Flare
Authors: Vilmer, N.; Raulin, J. P.; Trottet, G.; Nitta, N.; Silva,
A. V. R.; Kaufmann, P.; Correia, E.; Magun, A.
Bibcode: 1999ESASP.446..681V
Altcode: 1999soho....8..681V
No abstract at ADS
Title: Magnetic Field Evolution and Topology of an AR
Authors: Mandrini, C. H.; Deng, Y. Y.; Schmieder, B.; Démoulin, P.;
Rudawy, P.; Nitta, N.; Newmark, J.; de Forest, C.
Bibcode: 1999ASPC..184..276M
Altcode:
Active region 7968 was observed during runs of a coordinated SOHO,
Yohkoh and ground-based observatory program (Joint Observing Program,
JOP, 17). The region appeared and decayed in a seven day period (June 3
to 10, 1996). In this time, mainly during June 6, it presented a very
dynamical behaviour. Flux emerged in between the two main polarities
and Arch Filament Systems (AFS) were observed to be linked to this
emergence. We analyze the evolution of some over dark arches observed
during flux emergence, forming two systems of AFS. Modelling the
magnetic field, we find that these systems were associated to field
lines having dips tangent to the photosphere (the so called "bald
patches", BPs). We discuss their evolution in terms of emergence of
the dipped portion of the lines or of magnetic field reconnection.
Title: Radio and X-Ray Imaging Observations of a Continuum Burst
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Raoult, A.
Bibcode: 1999ApJ...522.1100K
Altcode:
We study a metric continuum burst observed on 1993 February 18, and
its X-ray signatures from imaging observations in radio and X-rays
using the Nançay radioheliograph and the Yohkoh Soft X-Ray Telescope
(SXT). The event in question was associated with weak type III bursts;
these were detected at only one frequency (164 MHz), except for one
burst (at 10:58:05 UT), which was observed over a broad frequency range
(164-435 MHz). We believe that the early metric continuum burst is an
extension of the microwave continuum which was observed at frequencies
as high as 5 GHz, and its onset at ~10:50 UT is associated with the
development of an X-ray-emitting diffuse loop system which appears to
advance with a speed of ~50-100 km s-1. The observed type
III bursts seem to correspond to the repeated occurrence/appearance of
a collimated jet emanating from the loop system that is responsible for
the continuum burst. A few minutes prior to the main continuum onset
there is a soft X-ray ejection from the main flare region. The main
continuum has a brightness temperature greater than 108 K;
it is unpolarized, and it shows dispersion in position with frequency
and moves with speeds of ~50 km s-1 at 236-410 MHz. The SXT
images reveal that this initially ejected soft X-ray-emitting hot plasma
seems to gradually fill up the loop system with hot material. This hot
plasma must contain enough energetic electrons of energy greater than
several tens of keV, which are responsible for producing the metric
continuum burst by plasma radiation mechanism.
Title: SOHO EIT Observations of Extreme-Ultraviolet ``Dimming''
Associated with a Halo Coronal Mass Ejection
Authors: Zarro, Dominic M.; Sterling, Alphonse C.; Thompson, Barbara
J.; Hudson, Hugh S.; Nitta, Nariaki
Bibcode: 1999ApJ...520L.139Z
Altcode:
A solar flare was observed on 1997 April 7 with the Soft X-ray Telescope
(SXT) on Yohkoh. The flare was associated with a ``halo'' coronal
mass ejection (CME). The flaring region showed areas of reduced soft
X-ray (SXR) brightness--``dimmings''--that developed prior to the CME
observed in white light and persisted for several hours following the
CME. The most prominent dimming regions were located near the ends of
a preflare SXR S-shaped (sigmoid) feature that disappeared during the
event, leaving behind a postflare SXR arcade and cusp structure. Based
upon these and similar soft X-ray observations, it has been postulated
that SXR dimming regions are the coronal signatures (i.e., remnants)
of magnetic flux ropes ejected during CMEs. This Letter reports
new observations of coronal dimming at extreme-ultraviolet (EUV)
wavelengths obtained with the Extreme-ultraviolet Imaging Telescope
(EIT) on the Solar and Heliospheric Observatory (SOHO). A series of
EIT observations in the 195 Å Fe XII wavelength band were obtained
simultaneously with SXT during the 1997 April 7 flare/CME. The EIT
observations show that regions of reduced EUV intensity developed at
the same locations and at the same time as SXR dimming features. The
decrease in EUV intensity (averaged over each dimming region) occurred
simultaneously with an increase in EUV emission from flaring loops in
the active region. We interpret these joint observations within the
framework of flux-rope eruption as the cause of EUV and SXR coronal
dimmings, and as the source of at least part of the CME.
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999ApJ...520..391K
Altcode:
Using Nobeyama 17 GHz data, we have studied the radio properties of
19 coronal jets identified in Yohkoh soft X-ray imaging telescope
(SXT) X-ray observations. The radio data provide information on the
physical conditions in the jets, which complements the data from the
X-ray surveys. Microwave emission was associated with the majority of
the X-ray jets in our sample. The radio emission typically came from
the base or the base and lower part of the jets. We detected radio
emission from almost all jets that showed flarelike activity at their
bases. The jets that were not associated with radio emission did not
show any significant increase in X-ray emission at their bases. The
strongest radio emission came from two of the largest jets in our
sample. Our data show a general correlation between the X-ray jet
fluxes and the associated radio fluxes. The 17 GHz time profiles were
gradual and unpolarized, implying that the emission was thermal. In a
two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993
February 9 event), the observed microwave fluxes from the lower part
of the jets were well above the fluxes calculated from the computed
physical parameters of the soft X-ray-emitting material on the basis
of thermal free-free emission. We interpret the large discrepancies
in terms of the presence of lower temperature material, which cannot
be detected by the SXT (the SXT is most sensitive to hot plasma above
2×106 K), but which produces strong microwave free-free
emission. This is the first time that such material has been observed
in two-sided-loop-type jets. We also observed motion of a jet-associated
microwave source with a velocity of 55 km s-1. The microwave
motion occurred after the appearance of the X-ray jet. There is clear
evidence that the microwave emission of that source was associated
with the jet and not with the associated small flare.
Title: X-ray and radio manifestations of a solar eruptive event
Authors: Gopalswamy, N.; Nitta, N.; Manoharan, P. K.; Raoult, A.;
Pick, M.
Bibcode: 1999A&A...347..684G
Altcode:
We report on a study of the changes in the vicinity of a disappearing
solar filament (DSF) that occurred on 1993 April 30. The DSF was
associated with a long duration X-ray event (LDE) observed by the
GOES and Yohkoh spacecraft. A detailed analysis of the X-ray images
obtained by the Yohkoh Soft X-ray Telescope revealed that X-ray
manifestations of the eruption were wide-spread: (i) X-ray enhancement
over a coronal volume several times larger than that of the eruption
region, probably the X-ray counterpart of a coronal mass ejection (CME),
(ii) X-ray ejecta accelerating to 670 km s(-1) into the corona, and
(iii) quasi-stationary X-ray loops as in long decay events (LDEs) were
observed. One of the important findings of this study is the large-scale
X-ray enhancement which we identify with the frontal structure of a
CME, apart from the well-known X-ray ejecta and post-eruption arcade
formation. There is evidence for triggering of a sympathetic flare in
an adjoining active region due to the X-ray ejecta from the eruption
region. Stationary metric radio continuum observed by the Nançay
Radioheliograph was found to be associated with the brightest X-ray
loops that formed following the filament eruption. The unpolarized
continuum radio emission was found to be bremsstrahlung radiation
from the hot plasma observed in X-rays. The event was also associated
with a low frequency metric type II radio burst due to a coronal shock
wave from the eruption region. The onset time of the type II emission
precludes the possibility of a CME-driven shock causing it. Although
we do not have positional information for the type II burst, we found
that the X-ray ejecta was fast enough to drive the coronal shock. We
confirmed this by comparing the speed of the X-ray ejecta with the
shock speed obtained from the radio data which agreed within 10%.
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
Bibcode: 1999AAS...194.1704K
Altcode: 1999BAAS...31..853K
Using Nobeyama 17 GHz data, we have studied the radio properties
of 19 coronal jets identified in Yohkoh SXT X-ray observations. The
radio data provide information on the physical conditions in the jets
which complements the data from the X-ray surveys. Microwave emission
was associated with the majority of the X-ray jets in our sample. The
radio emission typically came from the base or the base and lower part
of the jets. We detected radio emission from almost all jets which
showed flare-like activity at their bases. The jets which were not
associated with radio emission did not show any significant increase
in X-ray emission at their bases. The strongest radio emission came
from two of the largest jets in our sample. Our data show a general
correlation between the X-ray jet fluxes and the associated radio
fluxes. The 17 GHz time profiles were gradual and unpolarized, implying
that the emission was thermal. In a two-sided-loop jet (July 22, 1992
event) and one anemone-type jet (February 9, 1993 event), the observed
microwave fluxes from the lower part of the jets were well above the
fluxes calculated from the computed physical parameters of the soft
X-ray-emitting material on the basis of thermal free-free emission. We
interpret the large discrepancies in terms of the presence of lower
temperature material which cannot be detected by the SXT (the SXT is
most sensitive to hot plasma above 2 x 10(6) K) but which produces
strong microwave free-free emission. This is the first time that
such material has been observed in two-sided-loop type jets. We also
observed motion of a jet-associated microwave source with a velocity
of 55 km/sec. The microwave motion occurred after the appearance of
the X-ray jet. There is clear evidence that the microwave emission of
that source was associated with the jet and not with the associated
small flare.
Title: Flare Loop Geometry
Authors: Nitta, N.
Bibcode: 1999AAS...194.3109N
Altcode: 1999BAAS...31..870N
Data from Yohkoh/SXT have shown that a single loop dominates in the main
phase of a flare, no matter how complicated its initial morphology may
be. We try to understand the geometry of this central loop. Because of
the dynamic nature of the loop, we cannot use a stereoscopic technique
that is based on different projections made by solar rotation. Instead,
we have to rely on a single image, which gives only minor constraints on
the geometry because one cannot determine the intrinsic shape and the
inclination angle at the same time. However, we can at least show that
the loop as seen in an image is often inconsistent with the commonly
assumed geometry, i.e., a symmetric loop standing vertically. Moreover,
the motion of the soft X-ray loop top source, and the offset of the
hard X-ray loop top source from it, appear to be not consistent with
the expectations from 2-d reconnection models. We discuss how soft
X-ray spectra from Yohkoh/BCS and radio polarization maps can be used
to put further constraints on the loop geometry.
Title: On the relationship between coronal mass ejections and
magnetic clouds
Authors: Gopalswamy, N.; Hanaoka, Y.; Kosugi, T.; Lepping, R. P.;
Steinberg, J. T.; Plunkett, S.; Howard, R. A.; Thompson, B. J.;
Gurman, J.; Ho, G.; Nitta, N.; Hudson, H. S.
Bibcode: 1998GeoRL..25.2485G
Altcode:
We compare the substructures of the 1997 February 07 coronal mass
ejection (CME) observed near the Sun with a corresponding event in
the interplanetary medium to determine the origin of magnetic clouds
(MCs). We find that the eruptive prominence core of the CME observed
near the Sun may not directly become a magnetic cloud as suggested by
some authors and that it might instead become the ”pressure pulse”
following the magnetic cloud. We substantiate our conclusions using time
of arrival, size and composition estimates of the CME-MC substructures
obtained from ground based, SOHO and WIND observations.
Title: Two-Sided-Loop Type X-ray Jets and Metric Radio Bursts
Authors: Kundu, M. R.; Raulin, J. -P.; Nitta, N.; Shibata, K.;
Shimojo, M.
Bibcode: 1998SoPh..178..173K
Altcode: 1998SoPh..178..611K
We have searched for nonthermal radio signatures in the form of metric
type III bursts in conjunction with two-sided-loop-type X-ray jets
observed by the Yohkoh/SXT experiment. We have found no evidence of
type III bursts in association with this particular type of X-ray jets
in contrast to the positive evidence of type III's in association
with anemone-type X-ray jets. This result is consistent with the
simulation results of Yokoyama and Shibata (1995), which show that
anemone-type jets are produced by vertical/oblique plasma flow whereas
the two-sided-loop-type jets are produced by horizontal plasma flow.
Title: Coordinated Observations with SOHO, YOHKOH and VLA
Authors: Aschwanden, Markus J.; Bastian, Tim S.; Nitta, Nariaki;
Newmark, Jeff; Thompson, Barbara J.; Harrison, Richard A.
Bibcode: 1998ASPC..155..311A
Altcode: 1998sasp.conf..311A
No abstract at ADS
Title: 3-Dimensional Models of Active Region Loops
Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson,
B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.;
Nitta, N.; Hudson, H. S.; Zucker, A.
Bibcode: 1998ASPC..155..145A
Altcode: 1998sasp.conf..145A
No abstract at ADS
Title: Filament Disparition Brusque and CME - September 25-26,
1996 Event
Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Aulanier, G.;
Demoulin, P.; Martens, P. C. H.; Zarro, D.; Deforest, C.; Thompson,
B.; St. Cyr, C.; Kucera, T.; Burkepile, J. T.; White, O. R.; Hanaoka,
Y.; Nitta, N.
Bibcode: 1998ASPC..150..366V
Altcode: 1998IAUCo.167..366V; 1998npsp.conf..366V
No abstract at ADS
Title: Surges and filaments in active regions during SOHO campaigns
Authors: Schmieder, B.; Deng, Y.; Rudawy, P.; Nitta, N.; Mandrini,
C. H.; Fletcher, L.; Martens, P.; Innes, D.; Young, P.; Mason, H.
Bibcode: 1998ESASP.421..323S
Altcode: 1998sjcp.conf..323S
No abstract at ADS
Title: Eruptive Events Observed by YOHKOH
Authors: Nitta, N.
Bibcode: 1998cee..workE..56N
Altcode:
Following Shibata et al. (1995), who documented plasmoid-like ejections
from what appear to be compact flares, we have surveyed a larger number
of events for moving structures detatched from the brightest soft X-ray
loop(s). It is found that 35-40 % of flares, either near the limb or on
the disk, show eruptive signatures as observed by Yohkoh/SXT. However,
morphology differs from flare to flare. Roughly speaking, there appear
to be three types of eruptions; expanding loops, jets and bubbles. We
suspect that these have different meterwave counterparts. We discuss
X-ray data of selected flares in terms of their eruptions and possible
magnetic field configurations. We also compare eruptions with and
without associated flares.
Title: YOHKOH Observations of Superhot Plasma in Solar Flares
Authors: Nitta, N.
Bibcode: 1998ASSL..229..107N
Altcode: 1998opaf.conf..107N
No abstract at ADS
Title: Hard X-Ray Emission from Active Region Transient Brightenings
Authors: Nitta, Nariaki
Bibcode: 1997ApJ...491..402N
Altcode:
We present the results of a search for hard X-ray counterparts to
soft X-ray transient brightenings in solar Active Region 7260 based
on data from the soft and hard X-ray telescopes on board the Yohkoh
spacecraft. A total of 72 soft X-ray events are found to accompany
hard X-ray emission. They represent strong transient brightenings. The
scatter plot between the time-integrated hard X-ray flux and the peak
soft X-ray flux indicates that these transient brightenings are not
particularly hard X-ray-rich or -deficient in comparison with more
intense flares. As in flares, hard X-ray emission is most noticeable
during the rise phase of the soft X-ray temporal variations, indicative
of its nonthermal origin. We conclude that these soft X-ray transient
brightenings are nothing but small flares and predict that hard X-ray
emission could be detected from less intense events if an instrument
with higher sensitivity were available.
Title: Detection of Microwave Emission from Coronal X-Ray Jets
Authors: Kundu, M. R.; Shibasaki, K.; Nitta, N.
Bibcode: 1997ApJ...491L.121K
Altcode:
We present evidence of the detection of microwave emission at 17 GHz in
association with coronal X-ray jets. We present two typical cases--one
on the disk (1995 March 31) and the other at the limb (1992 August
25). For the disk event we see 17 GHz emission from the upper part of
the jet base (active region loop or loops), but no emission from the
collimated X-ray jet itself, implying that it must be optically thin
at 17 GHz. For the limb event, we see the base of the jet as well as
the bottom part of the jet itself, implying that the optical depth is
higher at the bottom part (obviously because of higher electron density)
than at the top. We believe that the 17 GHz emission is thermal,
because it is gradual and unpolarized, and that the heating process
that gives rise to the jet X-ray plasma also results in the 17 GHz
emission. The calculated 17 GHz flux densities seem to agree with the
observed values within a factor of 2. We consider this disagreement
to be quite reasonable in view of the various uncertainties involved
in computing the emission in both radio and X-rays.
Title: X-Ray and Radio Studies of a Coronal Eruption: Shock Wave,
Plasmoid, and Coronal Mass Ejection
Authors: Gopalswamy, N.; Kundu, M. R.; Manoharan, P. K.; Raoult, A.;
Nitta, N.; Zarka, P.
Bibcode: 1997ApJ...486.1036G
Altcode:
On 1994 July 31, a fast (900 km s-1) eruptive structure
was observed in X-rays, followed by a slower plasmoid (180 km
s-1). They were associated with a coronal mass ejection,
prominence eruption, and a host of metric radio bursts. The X-ray
structure seems to be a part of a white light coronal mass ejections
(CME), as inferred from the white light images of July 30 and 31. A type
II burst was observed at the leading edge of the X-ray eruption, while a
type IV burst was spatially associated with the detached plasmoid. The
type III radio bursts occurred on thin overdense structures associated
with the eruption. We detected the rise of plasma levels because
of mass addition to the type III burst sources as a result of the
eruption. This event further clarifies the manifestation of a CME in
X-rays. We identify the X-ray eruption as the driver of the coronal
shock wave. This provides answer to the long-standing question regarding
the origin of coronal and interplanetary shock waves. We have also found
evidence to support the idea that herringbone bursts are produced when
the coronal shock wave crosses open magnetic field lines.
Title: Magnetic reconnection driven by emergence of sheared magnetic
field.
Authors: Schmieder, B.; Aulanier, G.; Demoulin, P.; van
Driel-Gesztelyi, L.; Roudier, T.; Nitta, N.; Cauzzi, G.
Bibcode: 1997A&A...325.1213S
Altcode:
Recurrent subflares (Class C) were observed in the NOAA 7608 active
region on 27 October 1993. From multi-wavelength observations
(white-light, magnetic field, H-alpha, X-ray), obtained during a
coordinated campaign between Pic du Midi and Yohkoh, it appears that
these flares were double ribbon flares caused by new flux emergence. As
the flare begins, the X-ray emission observed with Yohkoh/SXT is
loop-shaped with the axis almost parallel to the magnetic inversion
line, while during the flare development, X-ray loops appear at the
location of the emerging flux. The extrapolation of the photospheric
magnetic field in a linear force-free field configuration allows
identification of the magnetic configuration given by the flares. The
Hα flare ribbons are located at the intersections of the computed
quasi-separatrice layers (QSLs) with the chromosphere. We show that
the initial loop-shaped X-ray emission region is in fact formed by
several smaller loops directed in a nearly orthogonal direction with
their feet anchored close to or in the Hα ribbons. During the flare
development there are X-ray loops which represent only one foot of
open or largescale magnetic loops. For the studied flares the puzzling
soft X-rays observations could only be understood with the help of Hα
and magnetic data combined with a modeling of the coronal magnetic
field. Further, from the deduced magnetic field topology, the width
of the QSLs and our present knowledge of 3-D magnetic reconnection,
we conclude that the flare was due to magnetic reconnection driven
by emergence of sheared magnetic field impacting in the pre-existing
coronal field.
Title: A Superhot Flare Observed by Yohkoh
Authors: Nitta, Nariaki; Yaji, Kentaro
Bibcode: 1997ApJ...484..927N
Altcode:
We report the first solar flare for which direct X-ray imaging
(with focusing optics) has shown the location of superhot (T ~ 30
MK) plasma. This flare consists of two separate loop structures. The
smaller structure dominates the soft X-ray emission, but the longer
structure develops with persistently higher temperatures, which overlaps
with low-energy hard X-ray sources with very soft spectra. Using a
two-temperature model, we describe how the soft X-ray images lead
to the inference of superhot temperatures in the latter structure,
as we combine three broadband measurements at slightly different
effective wavelengths.
Title: Imaging the Chromospheric Evaporation of the 1994 June 30
Solar Flare
Authors: Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.; Nitta,
Nariaki; Zirin, Harold
Bibcode: 1997ApJ...481..978S
Altcode:
We analyze simultaneous Hα images (from the Big Bear Solar
Observatory), soft and hard X-ray images and spectra (from the soft
X-ray telescope [SXT], the Bragg Crystal Spectrometer [BCS], and the
hard X-ray telescope [HXT] on Yohkoh), and radio time profiles (from
the Owens Valley Radio Observatory) during the first 3 minutes of the
1994 June 30 flare. The strong blueshifts observed in the Ca XIX soft
X-ray line are interpreted as evidence of chromospheric evaporation,
with maximum up-flow velocities occurring 2 minutes prior to the hard
X-ray emission peak. In this study, we search for moving sources in
Hα, soft and hard X-ray images that correspond to the blueshifted
component. The chromospheric evaporation in this flare is divided
into two phases: an early phase with up-flow velocities of 350-450
km s-1, and a later phase (during the hard X-ray peak)
characterized by velocities of 100-200 km s-1. During the
first chromospheric evaporation phase, the footpoints of a loop seen
in HXT low-energy maps are seen to move toward the loop-top source. No
source displacement is observed in SXT images at this time. Images
of the later phase of chromospheric evaporation show a change in
the source morphology. The early HXT loop is no longer visible, and
HXT maps during this time display the two footpoints of a new loop
visible in SXT images. Now the HXT sources are stationary, and a SXT
footpoint source is seen to move toward the loop top. We interpret
the observed displacement of footpoint sources in HXT (early phase)
and SXT (later phase) maps to be the images of the evaporating front
projected onto the solar disk, while the up-flow velocities (inferred
from the blueshifts) are due to the movement of the same evaporating
material along the line of sight. By combining the up-flow velocities
with the proper motion of the footpoint sources seen in the maps,
we constructed a three-dimensional view of the magnetic loop for each
chromospheric evaporation phase. The early loop is almost semicircular,
with a height of 1.7 × 109 cm, whereas the later magnetic
loop is more elongated (a height of 3.2 × 109 cm), with
its apex closer to the footpoint where most of the evaporation took
place. The implications of these magnetic configurations and the
distinct evaporation phases are discussed.
Title: Tracking a CME from Cradle to Grave: A Multi-wavelength
Analysis of the February 6-7, 1997 Event
Authors: Gopalswamy, N.; Kundu, M. R.; Hanaoka, Y.; Kosugi, T.; Hudson,
H.; Nitta, N.; Thompson, B.; Gurman, J.; Plunkett, S.; Howard, R.;
Burkepile, J.
Bibcode: 1997SPD....28.0501G
Altcode: 1997BAAS...29..908G
The partially earth-directed coronal mass ejection (CME) event of 1997
February 6-7 originated from the southwest quadrant of the sun. The
CME accelerated from 170 km/s to about 830 km/s when it reached a
distance of 25 solar radii. The CME was an arcade eruption followed
by bright prominence core structures. The prominence core was tracked
continuously from the solar surface to the interplanetary medium by
combining data from the Nobeyama radioheliograph (microwaves), Mauna Loa
Solar Observatory (He 10830 { Angstroms}), SOHO/EIT (EUV) and SOHO/LASCO
(white light). The CME was accompanied by an arcade formation, fully
observed by the YOHKOH/SXT (soft X-rays) and SOHO/EIT (EUV). The X-ray
and EUV observations suggest that the reconnection proceeded from
the northwest end to the southeast end of a filament channel. In the
SOHO/EIT images, the the feet of the soft X-ray arcade were observed
as EUV ribbons. The CME event also caused a medium sized geomagnetic
storm: The hourly equatorial Dst values attained storm level during
18:00-19:00 UT on February 09. This means the disturbance took about
2.25 days to reach the Earth. The first signatures of an IP shock was
a pressure jump in the WIND data around 13:00 UT on Feb 09, 1997 which
lasted for about 14 hours, followed by flux rope signatures. This CME
event confirms a number of ideas about CMEs: The three part structure
(frontal bright arcade, dark cavity and prominence core), disappearing
filament, elongated arcade formation, and terrestrial effects. We make
use of the excellent data coverage from the solar surface to the Earth
to address a number of issues regarding the origin and propagation of
the geoeffective solar disturbances. We benefited from discussions at
the first SOHO-Yohkoh Coordinated Data Analysis Workshop, held March
3-7, 1997, at Goddard Space Flight Center.
Title: A Comparison of the MEM and Pixon Algorithms for HXT Image
Reconstruction
Authors: Metcalf, Thomas R.; Alexander, David; Nitta, Nariaki;
Kosugi, Takeo
Bibcode: 1997SPD....28.0217M
Altcode: 1997BAAS...29Q.896M
Recently a workshop was held in Palo Alto, CA to discuss image
reconstruction for the Hard X-ray Telescope (HXT) on the Yohkoh
satellite. At the workshop, the participants concluded that a
detailed comparison of the primary reconstruction algorithms should
be undertaken. We will report on the results of a comparison of the
Maximum Entropy and Pixon algorithms using pseudo data. The comparison
will check photometric accuracy, speed, and image quality using a number
of test images. The test images utilized in the comparison will examine
a broad range of reconstruction problems, including the ability of the
algorithms to accurately reconstruct single sources, multiple sources
and loop-like features, as well as the ability to reconstruct weak
sources in the presence of spatially distinct bright sources.
Title: Observations of Superhot Plasma in Solar Flares
Authors: Nitta, N.; Hudson, H. S.
Bibcode: 1997SPD....28.0162N
Altcode: 1997BAAS...29..890N
We have shown in an M1-class flare that a structure away from
the main bright loop filled with superhot ( ~ 30 MK) plasma. The
analysis incorporated Yohkoh SXT measurements in three filters and
BCS Fe XXV and Ca XIX spectra. We made use of the fact that even the
thickest filter of SXT is sensitive to plasma of ~ 10 MK, biasing
the temperature determination towards lower values. This resulted in
an estimated temperature of the imaged superhot plasma consistent
with that derived from hard X-rays (the Yohkoh HXT M1/L channel
ratio). We now apply the same technique to several other flares to
study the existence of superhot plasma, its timing and location with
respective to the impulsive component and its range of parameters. We
identify flares with superhot plasma irrespective of the results from
the BCS Fe XXVI diagnostic (which has large uncertainty), suggesting
that the latter should not be the only method to determine whether a
flare has superhot plasma. We also discuss the data in terms of the
true differential emission measure (a local property of the plasma)
rather than the spatial-composite emission measure (the average along
the line of sight).
Title: Movies of flares observed by YOHKOH/HXT
Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.;
Nitta, N.
Bibcode: 1997SPD....28.0169L
Altcode: 1997BAAS...29..892L
The Yohkoh Hard X-ray Telescope (HXT) has revealed new information
about solar flares by providing high spatial resolution (5 arcsec)
images in four energy bands covering 14 to 99 keV. The comparison of
images obtained in hard X-rays with those obtained with the Yohkoh Soft
X-ray Telescope (SXT) has led to important interpretations of flare
heating mechanisms (e.g., Masuda et al, 1994, Nature, 374, 495). The
HXT images are formed by reconstructing data obtained from 64 detectors
located behind a bi-grid modulation pattern. Reconstruction algorithms
using Maximum Entropy and PIXON methods have been successfully applied
to the HXT data (Alexander and Metcalf, 1997, ApJ, submitted). At the
recent HXT Coordinated Data Analysis Workshop held in January 1997, new
calibration data for the response of the grids were incorporated and an
error in the application of the reconstruction algorithms was identified
and corrected (see Kosugi et al 1997 in these proceedings). As a result,
it is now possible to reconstruct the HXT images semi-automatically to
produce quick-look movies. We are in the process of generating movies
for all flares obtained with the HXT using an MEM reconstruction. The
automated procedure selects time intervals on the basis of the count
rate statistics. We present a selection of reconstructed images and
movies from this on-going project. We expect that the ability to view
easily time sequences from many flares observed with the HXT will
provide new insights for flare studies.
Title: Microwave Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Shibasaki, K.; Nitta, N.
Bibcode: 1997SPD....28.0142K
Altcode: 1997BAAS...29..887K
We report the first detection of microwave emission from coronal
X-ray jets using simultaneously obtained imaging data at 17 GHz
(Nobeyama Radioheliograph data) and in X-rays (Yohkoh/SXT data). We
present detailed results for one jet on the disk and another at the
limb. The 17 GHz emission in the disk event originates from the base
(a loop or a system of loops) of the jet, and for the limb jet the
microwave emission comes from the base as well as from a part of the
X-ray jet. We believe that the collimated jet is optically thin at 17
GHz in most cases. We have investigated more than two dozen jet events
and found 17 GHz emission in at least 70% of the cases. The 17 GHz
emission is unpolarized. We interpret the microwave emission as thermal,
related to the heating of the plasma responsible for the X-ray jet.
Title: Shrinkage of Coronal X-Ray Loops
Authors: Wang, J.; Shibata, K.; Nitta, N.; Slater, G. L.; Savy, S. K.;
Ogawara, Y.
Bibcode: 1997ApJ...478L..41W
Altcode:
We present the first set of examples of the shrinkage of large-scale
nonflare loops in the solar corona, observed by the Yohkoh Soft X-Ray
Telescope in 1993 February. A large and isolated active region exhibited
an unusual south-north asymmetry in coronal dynamics and heating. The
northern part, referred to the main magnetic axis, showed episodic
expansion and heating. In contrast, the southern part displayed obvious
shrinking and cooling. This asymmetry was correlated with a severe
asymmetry in the surface magnetic activity revealed by Huairou vector
magnetograms. Observations suggest that this shrinkage is not an
apparent motion, but a real contraction of coronal loops that brighten
as a result of heating at footpoints followed by gradual cooling.
Title: A Multi-Wavelength Analysis of the February 6/7, 1997 Coronal
Mass Ejection
Authors: Plunkett, S. P.; Gopalswamy, N.; Kundu, M. R.; Howard, R. A.;
Thompson, B. J.; Gurman, J. B.; Lepping, R. P.; Hudson, H. S.; Nitta,
N.; Hansoka, Y.; Kosugi, T.; Burkepile, J. T.
Bibcode: 1997ESASP.404..615P
Altcode: 1997cswn.conf..615P
No abstract at ADS
Title: Solar Magnetic Field Events related to CMEs observed with SOHO
(MDI, EIT, SUMER, LASCO)
Authors: Schmieder, B.; van Driel-Gesztelyi, L.; Wiik, J. E.; Thompson,
B.; de Forest, C.; Saint Cyr, C.; Vial, J. -C.; Nitta, N.; Simnett, G.
Bibcode: 1997IAUJD..19E..42S
Altcode:
We shall present two CMEs observed by LASCO during the minimum of
activity of the Sun. These are associated with filament disparitions
brusques (DB). CME and DB definitively seem to be consequences of global
magnetic field instability, which causes reconnection of pre-existing
field lines in the corona. We shall demonstrate how cancelling flux
and converging magnetic field in photosphere may destabilize coronal
streamers overlying one or two filament channels.
Title: 3-D reconnection related to new emerging flux
Authors: Schmeider, B.; Démoulin, P.; Aulanier, G.; Malherbe, J. M.;
van Driel-Gesztelyi, L.; Mandrini, C. H.; Roudier, T.; Nitta, N.;
Harra-Murnion, L. K.
Bibcode: 1997AdSpR..19.1871S
Altcode:
We present evidences that emergence of new flux in the lower
atmosphere leads to magnetic reconnection of field lines. In a
first phase the phenomenon is observed in the chromosphere by the
formation of dark filaments (arch filament system) which are overlaid
by bright loops visible in soft X-rays. Different types of event appear
according to the magnetic field configuration and the amount of energy
involved. 3-D modelling of the photospheric magnetic field provides a
new tool for understanding reconnection in real configurations. The
observed chromospheric and coronal loops are good diagnostics for
the modelling. We document our statement by examples obtained during
coordinated campaigns with the Hα Multichannel Subtractive Double
Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh
instruments.
Title: Radio Continuum and Type III Bursts Associated with Coronal
X-Ray Structures
Authors: Raulin, J. P.; Kundu, M. R.; Nitta, N.; Raoult, A.
Bibcode: 1996ApJ...472..874R
Altcode:
In this paper, we report the detection of a metric continuum burst
source at the top of a coronal loop observed in soft X-rays. The
continuum burst was probably a flare continuum that lasted more than
1 hr. This is the first observation of such metric continuum emission
produced by energetic electrons with high-spatial resolution imaging
instruments in both X-rays and radio. The nonthermal radio emission
appears to be associated with the rupture of a part of the loop
top and the ejection of soft X-ray plasma at the top of the coronal
loop. We have also identified X-ray coronal structures in which type
III emitting electron beams propagate. The metric continuum is most
likely caused by second-harmonic plasma emission, and the electron
density in the soft X-ray structure where type Ills are observed is
close to the critical plasma density derived from radio observations.
Title: The Solar Flare of 1992 August 17 23:58 UT} %
Authors: Takahashi, Masaaki; Watanabe, Tetsuya; Sakai, Jun-Ichi;
Sakao, Taro; Kosugi, Takeo; Sakurai, Takashi; Enome, Shinzo; Hudson,
Hugh S.; Hashimoto, Shizuyo; Nitta, Nariaki
Bibcode: 1996PASJ...48..857T
Altcode:
A small flare (C4.3 in the GOES X-ray class) was well observed by
all of the instruments on board Yohkoh. The X-ray light curves have
double peaks which are about 5 min apart. Until the first peak from
flare onset, four compact areas brighten up in the soft X-ray region,
which are aligned almost on one straight line. We regard them as being
footpoints of two sets of loops, which are identifiable in soft X-ray
images, since their locations match those of hard X-ray sources. Indeed,
after the second peak, the temporal behavior of the temperature and
emission measure at each point is consistent with the existence of
two such loops. Comparing our results with recent MHD simulations,
we propose a possible scenario for this flare that is based on the
coalescence of two loops.
Title: A Filament Eruption and Accompanying Coronal Field Changes
on November 5, 1992
Authors: McAllister, A. H.; Kurokawa, H.; Shibata, K.; Nitta, N.
Bibcode: 1996SoPh..169..123M
Altcode:
An Hα filament eruption on November 5, 1992 was fully observed in Hα
with the Hida Flare Monitoring Telescope, while Yohkoh's Soft X-ray
Telescope observed the pre- and post-eruption evolution of the coronal
magnetic fields. From the Hα data, including the red and blue wings,
we have reconstructed the rise of the filament, including trajectory,
velocity, and acceleration. In combination with the Yohkoh data this
reconstruction suggests that the filament had several interactions
with other coronal magnetic fields during the eruption. The Yohkoh
data also shows pre-eruption changes in the coronal fields and several
post-eruption bright coronal structures. The pre-eruption changes are
interpreted as a partial opening of the corona, indicating that it is
not necessary to have a complete opening of the corona in order for a
filament to erupt and we discuss the several possible contributions
from emerging flux. The post-event bright coronal structures are
compared with theory and with a cleaner filament eruption event on
July 31, 1992. These comparisons suggest that, although there are many
similarities, it is hard to completely reconcile the observations with
the existing theory.
Title: Microwave and Soft X-Ray Emission from a Flare-activated
Coronal Loop
Authors: Gary, D. E.; Wang, H.; Nitta, N.; Kosugi, T.
Bibcode: 1996ApJ...464..965G
Altcode:
We study the microwave and soft X-ray emissions produced by a
flare-activated coronal loop, using total power spectral data from
the Owens Valley Radio Observatory Solar Array, as well as soft X-ray
data from the Yohkoh Soft X-ray Telescope, and the GOES soft X-ray
monitors. We show that both the microwave and soft X-ray emissions,
late in the loop development, can be well understood as thermal
bremsstrahlung (free-free emission) from a hot (∼8 MK) plasma. From
an investigation of the microwave spectrum we suggest that an overlying
or cospatial, cooler (∼4 MK) plasma may also be present. We
also study the time development of the loop, as observed with the
three instruments. The loop appears to grow in size from a lower
lying, initially invisible loop. New loops appear at greater heights,
or else the initial loop rises at a top speed of more than 150 km
s-1, but the growth of the loop or loops slows after about 3
minutes and becomes stable at its inflated size. We study the subsequent
brightening of the loop, using the SXT, GOES, and microwave data to
investigate the changing physical parameters of the loop as it evolves.
Title: Conditions for energetic flares
Authors: Nitta, Nariaki
Bibcode: 1996AIPC..374..294N
Altcode: 1996hesp.conf..294N
Hard X-ray emission is commonly observed in association with solar
flares and it is believed to signify interactions of accelerated
electrons with the ambient ions. As a first step to understand what
conditions are conducive to acceleration of electrons in solar flares,
we have analyzed Yohkoh X-ray images for a total of four-dozen events
whose hard X-ray spectra extend at least to 100 keV. The morphology of
soft X-ray emission relative to hard X-ray sources shows a wide variety,
but it is usually confined in a compact loop, which is sometimes topped
with a cusp that is much more diffuse. While eruptive behaviors are
often seen in the soft X-ray images, it is hard to decide whether they
trigger or result from the nonthermal processes. We give a case study
which indicates that double hard X-ray sources do not always come from
conjugate footpoints of a loop.
Title: Yohkoh observations of flares with superhot properties
Authors: Hudson, H. S.; Nitta, N.
Bibcode: 1996AIPC..374..285H
Altcode: 1996hesp.conf..285H
Solar flares, almost as their defining property, fill coronal magnetic
flux tubes with hot plasma. When the temperature of a significant
fraction of this plasma exceeds about 30×106 K, we call
the event ``superhot'', following the initial observation of the
hard X-ray continuum of such an event by Lin et al. (11). The Yohkoh
observations include many examples of similar events, of which three
have been published thus far. This paper reports a survey of the Yohkoh
observations, based mainly on the hard X-ray spectra obtained by the HXT
instrument. While comprehensive conclusions will not be possible until
the survey includes the Yohkoh imaging observations, we make tentative
suggestions here about the nature of flares with superhot properties.
Title: Nonthermal radio emission from coronal X-ray structures
Authors: Kundu, Mukul R.; Raulin, Jean-Pierre; Nitta, Nariaki
Bibcode: 1996AIPC..374..402K
Altcode: 1996hesp.conf..402K
We have provided evidence that certain coronal X-ray structures such as
flaring X-ray bright points and X-ray jets give rise to nonthermal radio
emission in the form of metric type III bursts. We have shown an example
of a metric type IV/flare continuum being associated with the rupture
of a flaring loop-top and the ejection of X-ray emitting material.
Title: Simultaneous UV and X-ray Observations of Coronal Bright Points
Authors: Bruner, M. E.; Nitta, N.; Wuelser, J. P.; Harvey, K.; Handy,
B.; Dame, L.
Bibcode: 1996AAS...188.8607B
Altcode: 1996BAAS...28..964B
High resolution ultraviolet filtergrams recorded during the 1992 and
1994 flights of the Solar Plasma Diagnostics Experiment (SPDE) sounding
rocket payload revealed a number of coronal bright points that were
simultaneously observed with the Yohkoh soft x-ray telescope. UV images
made at 1550 Angstroms/, which include substantial contributions from
the C IV resonance lines, reveal pairs of sources under the x-ray bright
points; consistent with the conventional interpretation of the latter
as un-resolved loops. The 1994 flight also recorded high resolution
EUV images at 171 and 195 Angstroms/, corresponding to strong lines
of Fe IX and Fe XII, respectively. Excellent correspondence was found
between coronal bright points seen in these lines, which are formed at
relatively low coronal temperatures (1 -- 2 x 10(6) k), and the 2 --
5 x 10(6) k that typifies the Yohkoh SXT images. In this study, we
use the Yohkoh database to study the temporal development of several
coronal bright points both before and after each rocket flight in order
to determine the stage of evolution of the sources at the epoch of the
flight. The relationship between the plasma properties of the sources
and their stages of evolution will be discussed.
Title: Flare Activity Associated with Large-Scale Loops in AR 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.
Bibcode: 1996AAS...188.1901N
Altcode: 1996BAAS...28..849N
Active region NOAA AR 7260 (August 1992) is marked as one of the fastest
growing regions in solar cycle 22 and has extensively been studied by
several workers. We have learned from the Yohkoh SXT data that flares in
this region were generally confined rather than eruptive. In addition
to the flares, we notice the formation of large-scale X-ray loops
connecting the large preceding spot and the emerging flux region as
the latter became complexed. One of them produced a LDE after a cusp
structure formed on top it. These loops were occasionally observed to
erupt, perhaps responsible for coronal mass ejections. Even some of
the apparently localized flares may have been due to interactions of
compact loops with such large-scale loops. It is possible that they
also caused sympathetic flares. We try to understand how such loops
formed in the context of evolution of the region, which is known to
have consisted of intrinsically twisted magnetic field.
Title: Imaging the Chromospheric Evaporation of the 1994 June 30
Solar Flare
Authors: Silva, Adriana V. R.; Wang, H.; Gary, D. E.; Zirin, H.;
Nitta, N.
Bibcode: 1996AAS...188.3310S
Altcode: 1996BAAS...28R.869S
We analyze simultaneous H _alpha images (from Big Bear Solar
Observatory), soft and hard X-ray images and spectra (from Yohkoh
during the first three minutes of the 1994 June 30 flare. The strong
blueshifts observed in the Ca XIX soft X-ray line are interpreted as
evidence of chromospheric evaporation, with maximum up--flow velocities
occurring two minutes prior to the hard X-ray emission peak. In this
paper, we search for moving sources in H_alpha , soft and hard X-ray
images that correspond to the blueshifted component. The chromospheric
evaporation in this flare is divided into two phases: an early phase
with up-flow velocities of 300-450 km s(-1) , and a later phase (during
the hard X-ray peak) characterized by velocities of 100-200 km s(-1)
. During the first chromospheric evaporation phase, the footpoints
of a loop seen in HXT maps are seen to move towards the loop top
source. No source displacement is observed in SXT images. The hard
X-ray spectra of individual sources, obtained from HXT maps, display a
very steep slope (gamma ~ 10-12). Thermal fitting of the spectra yield
temperatures of 20-50 MK. Images of the later phase of chromospheric
evaporation show the magnetic configuration to have changed. The early
HXT loop is no longer visible and HXT maps during this time display
the two footpoints of a new loop also visible in SXT images. Now
the HXT sources are stationary and a SXT footpoint source is seen to
move toward the loop top. We interpret the observed displacement of
footpoint sources in HXT (early phase) and SXT (later phase) maps to
be the images of the evaporating front projected onto the solar disk,
while the up--flow velocities (inferred from the blueshifts) are due
to the movement of the same evaporating material along the line of
sight. By combining the up--flow velocities with the proper motion
of the footpoint sources seen in the maps, we constructed a 3-D view
of the magnetic loop for each chromospheric evaporation phase. The
early loop is almost semi--circular with a height of 1.7x 10(9) cm,
whereas the later magnetic loop is more elongated (height of 2.3x 10(9)
cm) and asymmetric with its apex closer to the footpoint where most
of the evaporation took place. The implications of these magnetic
configurations and the distinct evaporation phases are discussed.
Title: Coronal X-Ray Structures and Metric Radio Type III and
IV Bursts
Authors: Kundu, M. R.; Raulin, J. -P.; Nitta, N.; Raoult, A.
Bibcode: 1996AAS...188.8609K
Altcode: 1996BAAS...28R.964K
Over the past several years Yohkoh/SXT experiments have led to
the discovery of many new and interesting dynamic phenomena. Of
particular interest are the flaring X-ray bright points (XBPs),
X-ray jets, and flare associated plasmoids and other ejecta. We have
looked for evidence of nonthermal processes occurring in these X-ray
events, using radio bursts of spectral types III and IV. We have used
positional information of metric radio bursts using the Nancay (France)
Radioheliograph in the frequency range 150-450 MHz simultaneously with
the Yohkoh experiments. We have evidence of nonthermal type III burst
emission in the meter wave range in association with flaring XBPs and
certain classes of X-ray jets. We have detected metric structures,
namely flaring loops (possibly plasmoids) and other ejecta. The
implications of these findings will be discussed in terms of our
understanding of radio bursts of different spectral types.
Title: Emerging flux and flares in NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Shibata, K.
Bibcode: 1996AdSpR..17d.201N
Altcode: 1996AdSpR..17..201N
We have studied the relation between flux emergence and flare
activity in the active region NOAA 7260, using images from the Soft
X-ray Telescope aboard the Yohkoh spacecraft and other supporting
ground-based data. It is found that microflares start around the time
of flux emergence as recorded in white-light data, which generally
precedes a major flare by several hours. We interpret the microflares as
due to fast reconnection that takes place intermittently in the slow
reconnection stage while more energy is accumulated in preparation
for a larger flare.
Title: Metric Type III bursts associated with soft X-ray jets.
Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.;
Raoult, A.
Bibcode: 1996A&A...306..299R
Altcode:
From soft X-ray and metric radio observations with high temporal and
spatial resolution, we show that electron acceleration in the form of
Type III bursts occurs in association with coronal jets observed by the
Yohkoh soft X-ray telescope. The excellent correspondence between the
positions of the radio sources observed at different frequencies and
the X-ray jets strongly suggests that electron beams propagate along
the relatively dense paths formed by the jets. Assuming a constant
temperature for the jets, one can estimate the electron density from
the soft X-ray measurements. These computed electron densities agree
well with the values derived from Type III bursts produced by the
plasma emission process. The observations are consistent with the idea
that strong particle acceleration accompanies magnetic reconnection
in these events as well as in solar flares.
Title: A Study of Major Flares Observed by YOHKOH
Authors: Nitta, Nariaki
Bibcode: 1996ASPC..111..156N
Altcode: 1997ASPC..111..156N
The author has studied hard and soft X-ray images for some 130 flares
observed by Yohkoh. The sample covers events with wide ranges of
intensity, duration and hardness (in terms of hard X-ray spectra). It
is found to be difficult to identify a simple soft X-ray loop whose
footpoints coincide with hard X-ray emission. Instead, a large fraction
of the flares studied show soft X-ray ejecta and a high-temperature
area outside the bright flare loop, as predicted by reconnection models.
Title: X-Ray Bright Point Flares Due to Magnetic Reconnection
Authors: Van Driel-Gesztelyi, L.; Schmieder, B.; Cauzzi, G.; Mein,
N.; Hofmann, A.; Nitta, N.; Kurokawa, H.; Mein, P.; Staiger, J.
Bibcode: 1996SoPh..163..145V
Altcode:
Ground-based optical observations coordinated with Yohkoh/SXT X-ray
observations of an old, disintegrating bipolar active region AR NOAA
7493 (May 1, 1993) provided a multiwavelength data base to study
a flaring `active region' X-ray bright point (XBP) of about 16 hr
lifetime, and the activity related to it in different layers of the
solar atmosphere. The XBP appeared to be related to a new minor bipole
of about 1020 Mx. Superposed on a global evolution of soft
X-ray brightness, the XBP displayed changes of brightness, lasting for
1-10 min. During the brightenings the XBP apparently had a spatial
structure, which was (tiny) loop-like rather than point-like. The
X-ray brightenings were correlated with chromospheric activity: (i)
brightenings of underlying chromospheric faculae, and (ii) appearance
of strong turbulent velocities in the arch filament system. We propose
that the XBP brightenings were due to reconnection of the magnetic
field lines (sketched in 3D) between the new bipole and a pre-existing
plage field induced by the motion of one of the new pores (v = 0.2
km s−1) towards the plage, and that the XBP itself was
a reconnected hot loop between them.
Title: Signatures of New Emerging Flux in the Solar Atmosphere
Authors: Schmieder, B.; Malherbe, J. M.; Mein, P.; Mein, N.; van
Driel-Gesztelyi, L.; Roudier, T.; Nitta, N.; Harra-Murnion, L. K.
Bibcode: 1996ASPC..111...43S
Altcode: 1997ASPC..111...43S
The emergence of new flux in the low atmosphere leads to magnetic
reconnection of field lines. In a stable phase the phenomenon is
observed in the chromosphere by the formation of dark filaments (arch
filament system, AFS). The authors show how bright loops visible in soft
X-rays are co-aligned with the AFS. Different types of events appear
as the released energy increases. With less energetic phenomena than
flares one observes surges, jets or X-ray bright points, according
to the configuration of the field lines (open/closed). A low-level
reconnection process is detectable as an X-ray bright point. If
the energy is ≡1028ergs cm-3, one observes
subflares. The authors document their statement by showing examples
observed in coordinated observations obtained with the MSDP (Pic du
Midi and Tenerife) and Yohkoh/SXT and BCS for the events occurring on
Oct 5, 1994, Oct 27, 1993, and May 1, 1993.
Title: Joint Radio and Soft X-Ray Imaging of an `Anemone' Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Nitta, N.; Aschwanden, M. J.
Bibcode: 1996SoPh..163...99V
Altcode:
The Very Large Array and the Soft X-ray Telescope (SXT) aboard the
Yohkoh satellite jointly observed the rapid growth and decay of a
so-called `anemone' active region on 3-6 April, 1992 (AR 7124). The
VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general,
discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths
because of limited brightness contrast due to optical depth effects and
wave scattering. Due to its unusual anemone-like morphology, however,
several discrete loops or loop systems are resolved by both the VLA
at 1.5 GHz and the SXT in AR 7124.
Title: OVRO, BBSO, BATSE, and YOHKOH Observations of a Twin Solar
Flare
Authors: Wang, H.; Gary, D. E.; Zirin, H.; Nitta, N.; Schwartz, R. A.;
Kosugi, T.
Bibcode: 1996ApJ...456..403W
Altcode:
We present the results of studies on two solar flares that occurred
on 1993 February 11: an M1.1 flare at 18:07 UT and an M2.7 flare at
18:31 UT. Our study was based on comprehensive observations by the
following observatories: Owens Valley Radio Observatory, which obtains
1-18 GHz microwave images; Big Bear Solar Observatory, which obtains
magnetograms, Hα and He D3 filtergrams; BATSE on board Compton Gamma
Ray Observatory, which obtains high-resolution hard X-ray spectra;
and the Japanese satellite Yohkoh, which obtains high-resolution soft
and hard X-ray images. We find the following: (1) While the optical
and hard X-ray emissions are confined to a small loop near the leading
spot of the active region for both flares, a large-scale soft X-ray
loop connects from the leading to the following spot 160" away. In
low-frequency micro-waves (<4 GHz), sources appear at each end
of the big loop, and the source near the following spot (away from
the Hα flare site) dominates at frequencies <2.8 GHz. For both
flares, as frequency increases, the source near the leading spot
becomes dominant, and the source near the following spot vanishes
gradually. (2) As frequency increases, the centroid of the leading
microwave source moves progressively downward until it reaches the
footpoint at high frequencies. (3) For the M2.7 event, in the compact
loop near the leading spot, two footpoints are seen in both soft and
hard X-rays. The dominant hard X-ray source has a softer spectrum than
the weaker one, suggesting that the weaker one may become dominant
at the higher energies (>100 keV) responsible for the microwave
emission. The high-frequency microwave emission is better associated
with this latter footpoint. (4) The large soft X-ray loop in the M2.7
flare is the postflare loop of the M1.1 flare. This flare is associated
with a different compact loop which is 40" away from the main flare. (5)
For the M2.7 flare, the microwave brightness temperature spectra in
the sources at the two ends of the big loop require very different
source parameters. The primary source near the leading spot can be
explained by nonthermal gyrosynchrotron emission from electrons with
a power-law energy index (δ) of 5.3. The same group of electrons can
explain the observed BATSE hard X-ray spectra. The low-frequency radio
source near the following spot is due to either a thermal component, or
a nonthermal component with a steep energy index (δ= 9.4). Based on the
available information, we cannot distinguish these two possibilities.
Title: Nonthermal Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Raoult, A.
Bibcode: 1996ASPC...93..375K
Altcode: 1996ress.conf..375K
No abstract at ADS
Title: Detection of Nonthermal Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Raoult, A.; Shibata, K.; Shimojo, M.
Bibcode: 1996mpsa.conf..445K
Altcode: 1996IAUCo.153..445K
No abstract at ADS
Title: Emerging Flux, Reconnection, and XBP
Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Demoulin, P.;
Mandrini, C.; Cauzzi, G.; Hofmann, A.; Nitta, N.; Kurokawa, H.; Mein,
N.; Mein, P.
Bibcode: 1996mpsa.conf..459V
Altcode: 1996IAUCo.153..459V
No abstract at ADS
Title: Emerging flux seen by Yohkoh.
Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Mandrini, C.;
Démoulin, P.; Cauzzi, G.; Hofmann, A.; Nitta, N.; Kurokawa, H.;
Mein, N.; Mein, P.
Bibcode: 1996joso.proc..124V
Altcode:
No abstract at ADS
Title: Active Region Evolution and Flare Activity
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Hudson, H. S.
Bibcode: 1996mpsa.conf..515N
Altcode: 1996IAUCo.153..515N
No abstract at ADS
Title: VLA and YOHKOH Observations of an M1.5 Flare
Authors: Gopalswamy, N.; Raulin, J. -P.; Kundu, M. R.; Nitta, N.;
Lemen, J. R.; Herrmann, R.; Zarro, D.; Kosugi, T.
Bibcode: 1995ApJ...455..715G
Altcode:
A major solar flare (X-ray importance M1.5 and optical importance SB)
was fully observed by the Very Large Array and the Yohkoh mission on
1993 April 22. Both thermal and nonthermal emissions were observed
in radio. In soft X-rays, the flare was confined to a compact region
in an arcade. In hard X-rays, there were two prominent footpoints,
coincident in projection with the soft X-ray footpoints and located
on either side of the magnetic neutral line inferred from photospheric
magnetograms The Yohkoh Bent Crystal Spectrometer (B CS) data provided
important context information which was helpful in cross-checking the
quantitative agreement between the radio and X-ray data. The microwave
spectrum peaked around 10 GHz and showed Razin suppression in the
beginning. Later on, the low-frequency spectral index dropped to a
value of 2, suggesting thermal emission. The VLA images of the flare at
1.5 GHz show that the flare emission started as a single source above
one footpoint; later on, the emission centroid moved toward the soft
X-ray structure to finally become cospatial with the latter. The two
locations of the 20 cm source corresponded to nonthermal (footpoint
source) and thermal (source cospatial with the soft X-ray structure)
emissions. We performed temperature and emission measure analysis of
the X-ray data (SXT, BCS, and HXT) and used them as input to determine
the expected radio emission. While there is morphological agreement
between the radio and soft X-ray structures in the thermal phase,
the 20 cm brightness temperature shows quantitative agreement with
temperature derived from the BCS data. We were able to identify
three emission mechanisms contributing to the 20 cm radio emission
at different times without any ad hoc assumption regarding emission
mechanisms. Razin-suppressed nonthermal gyroresonance emission,
plasma emission, and thermal free-free emission seem to be operating
and are found to be consistent with the plasma parameters derived
from the X-ray data. The magnetic field structure in the flaring
region showed differences before and after the flare as traced b soft
X-ray structures in the flaring region and confirmed by 20 cm radio
images. The superhot component with a temperature of 32 MK was observed
in hard X-ray images and in light curves during the impulsive phase of
the flare with possible radio signatures at 20 cm wavelength. We derived
the physical parameters of the flaring plasma, the magnetic field,
and the characteristics of nonthermal particles in the flaring region.
Title: Microwave and Hard X-Ray Observations of Footpoint Emission
from Solar Flares
Authors: Kundu, M. R.; Nitta, N.; White, S. M.; Shibasaki, K.; Enome,
S.; Sakao, T.; Kosugi, T.; Sakurai, T.
Bibcode: 1995ApJ...454..522K
Altcode:
We investigate radio and X-ray imaging data for two solar flares in
order to test the idea that asymmetric precipitation of nonthermal
electrons at the two ends of a magnetic loop is consistent with the
magnetic mirroring explanation. The events we present were observed in
1993 May by the HXT and SXT X-ray telescopes on the Yohkoh spacecraft
and by the Nobeyama 17 GHz radioheliograph. The hard X-ray images in
one case show two well-separated sources; the radio images indicate
circularly polarized, nonthermal radio emission with opposite polarities
from these two sources, indicating oppositely directed fields and
consistent with a single-loop model. In the second event there are
several sources in the HXT images which appear to be connected by
soft X-ray loops. The strongest hard X-ray source has unpolarized
radio emission, whereas the strongest radio emission lies over strong
magnetic fields and is polarized. In both events the strongest radio
emission is highly polarized and not coincident with the strongest
hard X-ray emission. This is consistent with asymmetric loops in
which the bulk of the precipitation (and hence the X-ray emission)
occurs at the weaker field footpoint.
Title: Detection of Nonthermal Radio Emission from Coronal X-Ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Shimojo, M.; Shibata, K.; Raoult, A.
Bibcode: 1995ApJ...447L.135K
Altcode:
We report the detection of a type III burst in association with a
dynamic X-ray coronal jet observed by Yohkoh/SXT. The type III burst
observed with the Nancay (France) multifrequency radioheliograph is
spatially and temporally coincident with the X-ray jet. The radio
locations at different frequencies (236.6 and 164 MHz) are aligned
along the length of the jet. The observation of the type III burst in
association with the X-ray jet implies the acceleration of electrons
to several tens of keV, along with the heating responsible for the
production of soft X-rays. This association implies the existence of
open field lines in dense coronal structures identified on the Sun's
disk. This is the first observation of dense coronal structures on the
disk, along which type III emitting nonthermal electrons propagate. We
find that this structure begins to form before the type III emission. At
the time of the type III burst we estimate a density of 6--10 x 108
cm-3 for a temperature of ~5--6 MK at an altitude of 20,000 km.
Title: Solar Microwave and Soft X-ray Observations of Thermal
Bremsstrahlung from a Post-Flare Loop
Authors: Gary, D. E.; Wang, H.; Nitta, N.; Kosugi, T.
Bibcode: 1995SPD....26.1214G
Altcode: 1995BAAS...27..986G
No abstract at ADS
Title: The Magnetic Field Over an Isolated Symmetric Sunspot
Authors: White, S. M.; Kundu, M. R.; Zlotnik, E. Ya.; Zheleznyakov,
V. V.; Nitta, N.
Bibcode: 1995SPD....26..718W
Altcode: 1995BAAS...27..970W
No abstract at ADS
Title: Aspect Angle Dependence of the Polarized Radio Emission from
AR 7123
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.; Nitta, N.
Bibcode: 1995SPD....26..701V
Altcode: 1995BAAS...27..965V
No abstract at ADS
Title: Nouthermal Radio Emission From Coronal X-Ray Jets
Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.;
Raoult, A.
Bibcode: 1995SPD....26.1318R
Altcode: 1995BAAS...27..991R
No abstract at ADS
Title: Characteristics of Two Simple Microwave Bursts
Authors: Kundu, M. R.; White, S. M.; Nitta, N.; Shibasaki, K.;
Enome, S.
Bibcode: 1995LNP...444...75K
Altcode: 1995cmer.conf...75K
We present simultaneous microwave and X-ray data for two microwave
bursts with simple impulsive time profiles. The 17 GHz images show
compact sources, and in the one case for which we have simultaneous
soft and hard X-ray images, they also show compact sources coincident
with the radio source. One of the bursts is barely detected in soft
X-rays, yet has a moderate 17 GHz flux,.
Title: Comparison of Synoptic Maps of Solar Soft X-Ray Features,
Photospheric Magnetic Fields, and Helium 1083 NM
Authors: Harvey, J.; Slater, G.; Nitta, N.; Shibata, K.; Tsuneta,
S.; Sakurai, T.; Hara, H.
Bibcode: 1994AAS...18512308H
Altcode: 1994BAAS...26Q1523H
We studied the wealth of structural features visible in Yohkoh/SXT
and NSO/KP synoptic maps of the Sun that cover Carrington rotations
1847 through 1879. In order to do this comparison, various methods to
reduce soft X-ray maps to simple structural elements were explored. In
the end, the best way of comparing the various data sets turned out to
be to filter the X-ray maps to emphasize high-spatial frequencies and
then to either simply blink the various images or to make colorized
composite maps that distinctively assign different colors to various
quantities. Among the results are: 1. Active regions exhibit normal or
"anemone" (fountain-like) X-ray loop structure tendency depending on
whether the surrounding large-scale unipolarity of the magnetic field
is small or large. 2. There is a systematic twist of the coronal loops
around magnetic concentrations in the southern hemisphere and vice-versa
in the north. The sense is the same as one would expect from the action
of differential rotation. 3. Dark lanes in the X-ray images are centered
over large-scale polarity patterns of one sign or the other. 4. The
X-ray loops at the boundaries between large-scale opposite polarity
patterns are frequently strongly sheared. The presence or absence of
a filament in these locations may be related in a complicated way to
the amount of shear. 5. At the resolution of the synoptic maps, the
footpoints of X-ray loops are almost always rooted in locally strong
magnetic concentrations and also in extra-dark 1083 nm elements.
Title: The Magnetic Evolution of the Activity Complex AR:7260 -
a Roadmap
Authors: Leka, K. D.; Canfield, R. C.; Mickey, D. L.; van
Driel-Gesztelyi, L.; Nitta, N.; Sakurai, T.; Ichimoto, K.
Bibcode: 1994SoPh..155..301L
Altcode:
The active region NOAA 7260 rotated onto the north solar hemisphere
as a mature bipole: a dominant negative-polarity sunspot with trailing
plage and scattered small spots in attendance. The dominantp spot itself
had strong magnetic fields and covered almost 400 × 10−6
of a solar hemisphere. For a period of seven days beginning 14 August,
1992 this active region displayed rapid and drastic evolution: no fewer
than 50 magnetic bipoles emerged in the area trailing the large sunspot,
increasing the region's magnetic flux by more than 1022
Mx. This new group of sunspots formed a complexβγδ configuration
with twoδ spots and a high degree of magnetic shear.
Title: Joint Radio and Soft X-ray Imaging of an ``Anemone'' Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M.; Nitta, N.
Bibcode: 1994AAS...185.8609V
Altcode: 1994BAAS...26Q1465V
The Very Large Array and the Soft X-ray Telescope aboard the Yohkoh
satellite observed the rapid growth and decay of a so-called ``anemone''
active region on 3-6 April (AR 7124). In general, discrete coronal
loop systems are rarely resolved at radio wavelengths due to optical
depth effects and scattering. In the case of AR 7124, however, several
discrete loops or loop systems are resolved by both the VLA and the SXT,
probably due to its unusual ``anemone''-like morphology. Furthermore,
the region exhibited a significant amount of variability, especially
on April 3, marked by many subflares and intensity changes as has been
reported in flare patrol observations. In the present study, physical
parameters derived from multiband radio and soft x-ray techniques for
discrete loops are compared. The temporal evolution of these parameters
is also examined.
Title: A Gigantic Coronal Jet Ejected from a Compact Active Region
in a Coronal Hole
Authors: Shibata, K.; Nitta, N.; Strong, K. T.; Matsumoto, R.;
Yokoyama, T.; Hirayama, T.; Hudson, H.; Ogawara, Y.
Bibcode: 1994ApJ...431L..51S
Altcode:
A gigantic coronal jet greater than 3 x 105 km long (nearly
half the solar radius) has been found with the soft X-ray telescope
(SXT) on board the solar X-ray satellite, Yohkoh. The jet was ejected
on 1992 January 11 from an 'anemone-type' active region (AR) appearing
in a coronal hole and is one of the largest coronal X-ray jets observed
so far by SXT. This gigantic jet is the best observed example of many
other smaller X-ray jets, because the spatial structures of both the jet
and the AR located at its base are more easily resolved. The range of
apparent translational velocities of the bulk of the jet was between
90 and 240 km s-1, with the corresponding kinetic energy
estimated to be of order of 1028 ergs. A detailed analysis
reveals that the jet was associated with a loop brightening (a small
flare) that occurred in the active region. Several features of this
observation suggest and are consistent with a magnetic reconnection
mechanism for the production of such a 'jet-loop-brightening' event.
Title: Detection of 17 GHz Radio Emission from X-Ray--bright Points
Authors: Kundu, M. R.; Shibasaki, K.; Enome, S.; Nitta, N.
Bibcode: 1994ApJ...431L.155K
Altcode:
Using observations made with the Nobeyama radio heliograph (NRH) at 17
GHz and the Yohkoh/SXT experiment, we report the first detection of 17
GHz signatures of coronal X-ray-bright points (XBPs). This is also the
first reported detection of flaring bright points in microwaves. We
have detected four BPs at 17 GHz out of eight identified in SXT data
on 1992 July 31, for which we looked for 17 GHz emission. For one
XBP located in a quiet mixed-polarity region, the peak times at 17
GHz and X-rays are very similar, and both are long-lasting-about 2
hr in duration. There is a second BP (located near an active region)
which is most likely flaring also, but the time profiles in the two
spectral domains are not similar. The other two 17 GHz BPs are quiescent
with fluctuations superposed upon them. For the quiet region XBP, the
gradual, long-lasting, and unpolarized emission suggests that the 17
GHz emission is thermal.
Title: Evolution of an Active Region and Flare Productivity
Authors: Kundu, M. R.; Shibasaki, K.; Enome, S.; Nitta, N.; Bruner,
M.; Sakao, T.; Kosugi, T.
Bibcode: 1994kofu.symp..353K
Altcode:
We have studied the evolution of an active region (AR 7515) in terms
of flare productivity. This region appears at the east limb on May 23,
1993 and then continues its onward march across the disk. We follow
its evolution until June 2. This region produces many small flares. We
study the topology, both magnetic and structural of the neighboring
regions as observed at 17 GHz by the Nobeyama Radio Heliograph (NRH)
and the Yohkoh/SXT to find their effects on the flare-producing AR. We
investigate the spatial structure of the flaring region from 17 GHz
and SXT maps during various times of the impulsive and decay phase,
to understand the difference in the flaring region spatial structure
during the preflare, impulsive and decay phases. In general, the maps
made during these phases show several loops. We try to relate these
flaring loops with the preflare active region structure.
Title: A Morphological Study of Magnetic Shear Development in a
Flare-Productive Region NOAA 7270
Authors: Kurokawa, H.; Kitai, R.; Kawai, G.; Shibata, K.; Yaji, K.;
Ichimoto, K.; Nitta, N.; Zhang, H.
Bibcode: 1994kofu.symp..283K
Altcode:
The evolutional changes of a flare-productive region NOAA 7270 were
examined in details with high resolution H_alpha images, magnetograms
and soft X-ray images to study the process of the magnetic shear
development and its relation to the strong flare activity of the
region. This study led us to the following results and a conclusion:(1)
Several new bipolar pairs simultaneously and or successively emerged in
NOAA 7270 from 5 through 7 September. (2) Magnetic shear configurations
developed at three locations, where most of flares occurred from 5
through 7 , September. (3)These magnetic shear configurations were
formed by successive emergences of twisted magnetic ropes from below
the photosphere.
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
Authors: Nitta, N.; Driel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1994kofu.symp..385N
Altcode:
Active region NOAA 7260 exhibited remarkable flare activity as an
emerging flux region appeared in the following part and evolved into
the delta configuration. While it is difficult to associate an emerging
bipole with a flare both temporally and spatially, there is an overall
correlation of the total darkness integrated over of the sunspot area,
as measured in the Yohkoh/SXT white-light images, with the soft X-ray
flux and flare occurrence. It appears that the flares in the emerging
flux region occurred preferentially at locations close to the spot of
preceding polarity that emerged in the earliest evolution of the region.
Title: Observation of 17 GHz Radio Emission from X-ray Bright Points
Authors: Kundu, M. R.; Shibasaki, K.; Enome, S.; Nitta, N.
Bibcode: 1994kofu.symp...79K
Altcode:
Using observations made with the Nobeyama radio heliograph (NRH) at 17
GHz and the Yohkoh/SXT experiment, we report the first detection of 17
GHz signatures of coronal X-ray bright points (XBP's). This is also the
first reported detection of flaring bright points in microwaves. We
have detected four BP's at 17 GHz out of eight observed by SXT on
July 31, 1992, for which we looked for 17 GHz emission. For one XBP
located in a quiet mixed-polarity-region, the peak times at 17 GHz
and X-rays are very similar, and both are long lasting -- at least
6 hours in duration. There is a second BP (located near an active
region) which is most likely flaring also, but the time profiles in
the two spectral domains are not similar. The other two 17 GHz BPs
are quiescent with fluctuations superposed upon them. For the quiet
region XBP, we believe that the 17 GHz emission is thermal.
Title: Flares on September 6, 1992
Authors: Kitai, R.; Kurokawa, H.; Funakoshi, Y.; Nakai, Y.; Shibata,
K.; Yaji, K.; Nitta, N.; YOHKOH Team; NAOJ Flare Telescope Team
Bibcode: 1994kofu.symp..147K
Altcode:
We present some preliminary results of our observational
study of typical eruptive flares in NOAA7270 on September 6,
1992. (1)Magnetic shear and flux emergence are strongly related to
flare production. (2)Observed flares showed a common temporal relation
between cool plasma dynamics seen in H_alpha and coronal energy releases
seen in YOHKOH data. (3)At the pre-heating stage of flares, there were
some indications of slow reconnection of adjacent magnetic loops.
Title: Energy Transport During a Solar Flare: VLA Observations of
the M1.9 Flare of 20 Aug 1992
Authors: Bastian, T. S.; Nitta, N.; Kiplinger, A. L.; Dulk, G. A.
Bibcode: 1994kofu.symp..199B
Altcode:
The GOES M1.9 flare of 20 August 1992 was observed by a large complement
of instruments including the VLA, Yohkoh, and a high-speed H-alpha
camera. We present a brief overview of the VLA data here. The VLA
acquired maps of the evolving microwave emission at 8.4 and 15 GHz
with a time resolution of 0.2 s. The main observational results are
as follows: i) the microwave sources consist of two, parallel, sheared
loops or loop systems; these two loop systems flared sequentially; ii)
the second microwave source is clearly associated with two magnetic
footpoints; iii) the microwave source shows a disturbance which
propagates from the initial footpoint and over the magnetic neutral
line; iv) the speed of the disturbance is roughly 3000 kms^(-1). Points
along the loop show a brightening that is delayed relative to that at
the primary footpoint, and the two footpoints spread apart over the
course of a few minutes.
Title: Two Types of Interaction Between Emerging Flux and Coronal
Magnetic Field
Authors: Shibata, K.; Nitta, N.; Matsumoto, R.; Tajima, T.; Yokoyama,
T.; Hirayama, T.; Hudson, H.
Bibcode: 1994xspy.conf...29S
Altcode:
No abstract at ADS
Title: Are X-Ray Bright Points the Signature of Magnetic Field
Reconnection?
Authors: Harvey, Karen L.; Strong, Keith S.; Nitta, Nariaki; Tsuneta,
Saku
Bibcode: 1994ASPC...68..377H
Altcode: 1994sare.conf..377H
No abstract at ADS
Title: Coalignment of the Radioheliograph and the YOHKOH Images
Authors: Nishio, M.; Nakajima, H.; Nitta, N.; Enome, S.; Shibasaki,
K.; Takano, T.; Hanaoka, Y.
Bibcode: 1994xspy.conf..187N
Altcode:
No abstract at ADS
Title: Diagnostics of Twisted Flux Emergence (noaa AR7260)
Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Anwar, B.; Canfield,
R. C.; Hudson, H. S.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.;
Kurokawa, H.
Bibcode: 1994xspy.conf...25L
Altcode:
No abstract at ADS
Title: The Relationship of X-Ray Bright Points to the Photospheric
Magnetic Fields
Authors: Harvey, K. L.; Nitta, N.; Strong, K. T.; Tsuneta, S.
Bibcode: 1994xspy.conf...21H
Altcode:
No abstract at ADS
Title: Flares in Active Region NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Sakurai,
T.; Shibata, K.; Ichimoto, K.; Canfield, R. C.; Wülser, J. -P.;
Metcalf, T. R.; Mickey, D. L.
Bibcode: 1994xspy.conf..111N
Altcode:
No abstract at ADS
Title: VLA Observations of a High Coronal Flare
Authors: Raulin, J. P.; Gopalswamy, N.; Kundu, M. R.; Nitta, N.
Bibcode: 1993AAS...183.0706R
Altcode: 1993BAAS...25.1300R
We present radio observations of a coronal flare which occurrred on 1993
April 22, in a weak magnetic field region to the west of AR 7477. The
observations were made by the Very Large Array (VLA) at 20 and 90
cm. The event consists of bright (brightness temperature of 10(10) K)
unpolarized bursts, followed by a longlasting unpolarized continuum with
moderately high brightness temperature (2-3 10(9) K) in the high corona
(90 cm observations). The low coronal counterpart of this flare is a
weak and moderatly polarized 20 cm radio emission. Full disk Yohkoh
images show that the corresponding radio emission is located in or
above magnetic loops connecting AR 7477 and its neighborhood. The
presence of permanent and non-varying noise storm associated with
AR 7477 seems to indicate that the overall magnetic field structure
of the active region is unaffected by the flare. The coronal radio
source which is indicative of acceleration of electrons to nonthermal
energies, is not associated with major Hα emissio n nor with bright
X ray emission. The absence of any detectable circular polarization,
as well as the high brightness temperature, seems to indicate that
the 90 cm emission is second harmonic plasma emission.
Title: Evolution of a Solar Active Region and Flare Productivity
Authors: Kundu, M. R.; Shibasaki, K.; Enome, S.; Nitta, N.; Bruner, M.
Bibcode: 1993AAS...183.6807K
Altcode: 1993BAAS...25.1396K
We have studied the evolution of an active region (AR 7515) in terms of
flare productivity. This region appeared on the east limb on May 23,
1993 and continued its onward march across the disk. We followed its
evolution until June 2. This region produced many small flares. We
studied the topology, both magnetic and structural of the flaring
region as observed at 17 GHz by the Nobeyama radio Heliograph (NRH)
with a spatial resolution of 10 arcsec and the Yohkoh SXT which has
a spatial resolution of 2.5 arcsec. Among other things, we find: (i)
Frequently a flare starts with the appearance of a new region/loop
which interacts with a pre-existing loop; this interaction acta as a
flare trigger. (ii) There appear to exist multiple sets of interacting
loops in the same active region. Different bursts on the same day
seem to come from diffrent sets of interacting loops. (iii) Sometimes
two or more sets of interacting loops can activate at the same time,
giving rise to different peaks in the same burst. In the decay phase
of some bursts there may appear a new or reactivated region/loop which
becomes the source of new burst emission. (iv) A simple spiky burst
in general originates from a narrow region and by implication from a
compact set of interacting loops.
Title: Lifetimes and distribution of coronal bright points observed
with Yohkoh
Authors: Harvey, K. L.; Strong, K. T.; Nitta, N.; Tsuneta, S.
Bibcode: 1993AdSpR..13i..27H
Altcode: 1993AdSpR..13...27H
X-ray imaging from Skylab and various sounding rockets has established
the existence of and begun the characterization of coronal X-ray bright
points (XBPs). With the launch of Yohkoh, the Soft X-ray Telescope (SXT)
provides a new opportunity to observe these small-scale structures
with higher temporal resolution, improved dynamic range, and greater
sensitivity. We present the results from the analysis of SXT full-disk
images showing the location and detailed evolution of XBPs. We derive
correlations of XBPs with magnetic bipoles, He I 10830-Å dark points,
and other coronal features. From the evolution of 518 XBPs presented in
this initial study, we derive a mean lifetime of about 12 hours, with
some XBPs lasting as long as 5 days and others less than 10 minutes. A
comprehensive study of the relationship between XBP lifetime, X-ray
output, size, location, and variability may well lead to a re-evaluation
of the nature and definition of XBPs.
Title: VLA Stereoscopy of Solar Active Region 7123
Authors: Aschwanden, M. J.; Bastian, T. S.; Nitta, N.
Bibcode: 1993BAAS...25.1224A
Altcode:
No abstract at ADS
Title: The M1.9 Flare of 20 August 1992: Joint Imaging with the VLA,
Yohkoh, and a High Speed Hα Camera
Authors: Bastian, T. S.; Nitta, N.; Kiplinger, A.
Bibcode: 1993BAAS...25.1222B
Altcode:
No abstract at ADS
Title: VLA and Yohkoh Observations of an M1.5 Flare
Authors: Gopalswamy, N.; Kundu, M. R.; Lemen, J. R.; Nitta, N.;
Strong, K. T.
Bibcode: 1993BAAS...25.1186G
Altcode:
No abstract at ADS
Title: The Relationship of X-ray Bright Points to the Photospheric
Magnetic Fields
Authors: Harvey, K. L.; Strong, K.; Nitta, N.; Tsuneta, S.
Bibcode: 1993BAAS...25.1179H
Altcode:
No abstract at ADS
Title: Evidence for Twisted Emerging Flux: NOAA AR 7260
Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Canfield, R. C.; Anwar,
B.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.
Bibcode: 1993BAAS...25R1187L
Altcode:
No abstract at ADS
Title: Classification of Active Regions Based on X-ray Images
1. Active Regions appearing in Coronal Holes
Authors: Nitta, N.; Shibata, K.; Hara, H.
Bibcode: 1993BAAS...25.1187N
Altcode:
No abstract at ADS
Title: Yohkoh-SXT Observations from the Spartan and Nixt Max91
Campaign
Authors: Morrison, M.; Bruner, M.; Freeland, S.; Lemen, J.; Linford,
G.; Nitta, N.; Slater, G.; Strong, K.; Hara, H.; Kano, R.; Shimizu,
T.; Tsuneta, S.; Hudson, H.; Ogawara, Y.; Kosugi, T.; Sakao, T.;
Watanabe, T.; Takeda, A.; Acton, L.
Bibcode: 1993BAAS...25.1213M
Altcode:
No abstract at ADS
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
and Reconnection
Authors: Nitta, N.; Drel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
Bibcode: 1993BAAS...25.1223N
Altcode:
No abstract at ADS
Title: Flares on 1992 September 6
Authors: Shibata, K.; Nitta, N.; Kitai, R.; Kurokawa, H.; Yaji, K.;
Kato, T.; Zarro, D. M.
Bibcode: 1993BAAS...25R1187S
Altcode:
No abstract at ADS
Title: Application of the Yohkoh Soft X-ray Telescope (SXT) to Solar
Terrestrial Prediction
Authors: Watari, S.; Akioka, M.; Nishikawa, J.; Nitta, N.; Strong,
K.; Tsuneta, S.
Bibcode: 1993stp2.conf..370W
Altcode:
No abstract at ADS
Title: Soft X-ray Imaging of Impulsive Evaporation
Authors: Hudson, H.; Nitta, N.; Strong, K.; Takakura, T.
Bibcode: 1992AAS...181.5501H
Altcode: 1992BAAS...24.1211H
The time development of a solar flare can often be broken into
two phases, viz. the impulsive phase and the gradual phase. For
many reasons the impulsive phase is known to result from powerful
electron acceleration, to energies of tens of keV (and higher), with
a total energy that is a large fraction of the energy subsequently
radiated in various flare emissions. A grazing-incidence soft X-ray
telescopes on board the Yohkoh spacecraft now enables us to study
the few-keV properties of the impulsive phase for the first time. In
a representative set of impulsive solar flares, we find an excellent
match between the soft X-ray time profiles at the footpoints of coronal
magnetic flux tubes and the hard X-ray impulsive emission. The hard
X-ray images directly show the sites of the particle precipitation. The
impulsive soft X-ray emission could arise directly as non-thermal
bremsstrahlung, extending to the few-keV range; or it could contain
contributions from the impulsively evaporating plasma seen during the
process of the flare explosion from the chromosphere. We discuss these
interpretations and the physics resulting from them.
Title: Correlation between X-ray Temporal Variability and Magnetic
Environment in Solar Flares
Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf,
T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M.
Bibcode: 1992AAS...181.5503N
Altcode: 1992BAAS...24.1211N
The X-ray time history of a solar flare can reflect basic processes of
heating and/or acceleration, which in turn may depend on the magnetic
environment of the site. Some flares show a simple rise and fall
temporal behavior, whereas others show more than one peak. Comparisons
of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh
spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and
Mitaka) reveal that, at least for a flare-productive active region
(NOAA 7260), flares with double-peaked and single-peaked time profiles
occurred at systematically different locations within the region. We
discuss this result in terms of theoretical models, especially those
of coalescence of two current loops.
Title: Comparison between YOHKOH Soft X-ray Images and 3D MHD
Simulations of Solar Emerging Flux Regions
Authors: Matsumoto, R.; Tajima, T.; Kaisig, M.; Shibata, K.; Ishido,
Y.; Tsuneta, S.; Shimizu, T.; Kawai, G.; Kurokawa, H.; Akioka, M.;
Acton, L.; Strong, K.; Nitta, N.
Bibcode: 1992AAS...181.8109M
Altcode: 1992BAAS...24.1253M
The soft X-ray telescope on the Yohkoh mission enabled us to observe
the evolution of emerging flux regions (EFR) in coronal X-rays with
high spatial and temporal resolution. Furthermore, we now have enough
computing capability to perform three-dimensional MHD simulation
of EFRs with sufficient spacial resolution to study details of the
flux emergence process. These new tools provide the opportunity to
investigate the physics involved in the formation of coronal loops
in much more detail. We carried out 3D MHD simulations of emerging
magnetic flux regions under various intial conditions; (1) a horizontal
magnetic flux sheet, (2) a bundle of horizontal flux tubes, and (3)
a flux sheet with sheared magnetic fields. Numerical results show that
coronal magnetic loops are formed due to the enhanced buoyancy resulting
from gas precipitating along magnetic field lines. The interchange modes
help to produce a fine fibrous structure perpendicular to the magnetic
field direction in the linear stage, while the undular modes determine
the overall loop structure. We observe in 3D simulations that during the
ascendance of loops the bundle of flux tubes, or even the flux sheet,
developes into dense filaments pinched between magnetic loops. We
also find that magnetic field lines are twisted by the vortex motion
produced by the horizontal expansion of magnetic loops. Our numerical
results may explain the observed signatures such as (1) the spacial
relation between soft X-ray loops and Hα arch filaments obtained by
coordinated observation between Yohkoh and ground-based observatories
(Kawai et al. 1992), (2) the rate of increase in size of soft X-ray
loops in EFRs (Ishido et al. 1992), (3) emergence of twisted magnetic
loops, and (4) the threshold flux for formation of chromospheric arch
filament systems (AFS).
Title: Observations of the Variability of Coronal Bright Points by
the Soft X-Ray Telescope on YOHKOH
Authors: Strong, Keith T.; Harvey, Karen; Hirayama, Tadashi; Nitta,
Nariaki; Shimizu, Toshifumi; Tsuneta, Saku
Bibcode: 1992PASJ...44L.161S
Altcode:
We present the initial results of a study of X-ray bright points (XBPs)
made with data from the Yohkoh Soft X-ray Telescope. High temporal
and spatial resolution observations of several XBPs illustrate their
intensity variability over a wide variety of time scales from a few
minutes to hours as well as rapid changes in their morphology. Several
XBPs produced flares during their lifetime. These XBP flares often
involve magnetic loops, which are considerably larger than the XBP
itself, and which brighten along their lengths at speeds of up to 1100
km s(-1) . We speculate on the origin of the XBP variability and flares.
Title: Simultaneous Observations of Coronal Bright Points in X-Ray
and Radio Wavelengths
Authors: Nitta, Nariaki; Bastian, Timothy S.; Aschwanden, Markus J.;
Harvey, Karen L.; Strong, Keith T.
Bibcode: 1992PASJ...44L.167N
Altcode:
We present a first explicit comparison of coronal bright points in
soft X-ray and radio wavelengths, using the Soft X-ray Telescope
aboard the Yohkoh spacecraft and the Very Large Array. About half of
the 33 compact sources indentified in a 20-cm full-disk map appear
as X-ray bright points in the X-ray data. The other half apparently
corresponds to unipolar regions with enhanced magnetic fields. Thus,
the identification of radio bright points alone cannot reliably serve
as a proxy for X-ray bright points. A preliminary analysis reveals that
bright points commonly observed at 20 cm and in X-rays have temperatures
of (1.4--2.9) times 10(6) K and emission measures of (0.4--2.5) times
10(45) cm(-3) . The observed brightness temperatures at 20 cm [(1--2.5)
times 10(5) K] can be explained in terms of optically thin free-free
emission from a plasma with these parameters.
Title: Comparison between Hα and YOHKOH Soft X-Ray Images of Emerging
Flux Regions
Authors: Kawai, Goro; Kurokawa, Hiroki; Tsuneta, Saku; Shimizu,
Toshifumi; Shibata, Kazunari; Acton, Loren W.; Strong, Keith T.;
Nitta, Nariaki
Bibcode: 1992PASJ...44L.193K
Altcode:
We carried out a detailed comparison between Hα and Yohkoh Soft X-ray
(SXR) images of three emerging flux regions. The main results are:
(1) In general, SXR bright features coincide well in space with Hα
arch filament systems in the emerging flux regions (EFR). (2) Some
young and active parts of EFRs are especially bright in SXR. (3)
The SXR structures related to EFR show fairly rapid changes in both
brightness and shape. These results are consistent with the model that
the emerging cool loops of EFRs evolve into hot coronal loops through
some heating processes.
Title: Study of Flare Productive Active Regions
Authors: Nitta, N.; Harvey, K. L.; Shibata, K.; Strong, K. T.
Bibcode: 1992AAS...180.1807N
Altcode: 1992BAAS...24..755N
The Soft X-ray Telescope (SXT) on the Yohkoh spacecraft has made it
possible for us for the first time to monitor the evolution of active
regions in soft X-rays with a good cadence and spatial resolution
over an extended period of time. One of our interests is what makes
an active region produce large flares. Since the SXT started its
operation, we have identified more than 40 active regions that have
produced flares whose GOES class are > M1. Although many such active
regions appear active when they first become visible on the eastern
limb, we have succeeded in observing some of them from their birth
through activation. We compare X-ray images taken from the SXT with
magnetograms and other groundbased data. We discuss similarities and
differences of such regions from other more quiet regions in terms of
morphology and physical parameters.
Title: X-ray Bright Point Flares Observed by YOHKOH
Authors: Harvey, K. L.; Strong, K.; Nitta, N.; Tsuneta, S.; Shimizu, T.
Bibcode: 1992AAS...180.1806H
Altcode: 1992BAAS...24..755H
X-ray images taken by the Soft X-Ray Telescope (SXT) on board the
Japanese Satellite Yohkoh are being used to study the characteristics
and variability of X-ray bright points and their relation to
the underlying photospheric magnetic field and chromospheric
structures. Though during this maximum phase of Cycle 22 there are few
X-ray bright points at any given time, more than a thousand have been
observed since Yohkoh began its observation in September 1991. Many of
these bright points flare; in many cases, these small-scale flares are
associated with with observed effects detected spanning substantial
distances away from the flaring bright point. This paper will report
on an investigation of the dynamics of the flares in these small-scale
coronal structures. Events will be discussed that show their complexity
and their relation to the large-scale coronal magnetic fields as
identified by coronal structures observed in X-rays.
Title: YOHKOH and Compton Observations of an LDE Event: Reconnection
and the Neupert Effect?
Authors: Hudson, H. S.; Nitta, N.; Paciesas, W.; Fishman, G.; Meegan,
C.; Wilson, R.
Bibcode: 1992AAS...180.3409H
Altcode: 1992BAAS...24..784H
We present X-ray images and time profiles of a gradual flare (GOES
magnitude M3.2) that occurred on the solar limb at 0430 UT (time of
GOES maximum), Feb. 21, 1992. The soft X-ray structure of this event
showed a remarkably clear loop development with complicated geometry,
including a prominent nearly-vertical bar extending from near the
top of the loop(s). During the presence of this bar, the large-area
BATSE hard X-ray detector on the Compton Observatory showed extended
but highly irregular hard X-ray emission, roughly in the proportion
predicted by the ``Neupert Effect'' that associates the integral of the
hard X-ray light curve with the instantaneous soft X-ray flux. The peak
hard X-ray flux (about 1 ph/(cm(2) sec) above 25 keV) was at 0321 UT,
and the emission extended throughout the rise phase of the soft X-ray
event. Coronal magnetic reconnection provides a natural interpretation
for this morphology, and the time profiles strongly suggest that we
can identify the hard X-ray emission with bremsstrahlung from electrons
accelerated in the coronal volume of the reconnection region (vertical
bar structure).
Title: A 17 GHz Radioheliograph Project: A High-Speed Radio Camera
Authors: Nishio, M.; Kai, K.; Enome, S.; Nakajima, H.; Shibasaki, K.;
Takano, T.; Nitta, N.; Torii, C.; Shiomi, Y.; Sekiguchi, H.; Sawa,
M.; Bushimata, T.; Kawashima, S.; Shinohara, N.; Irimajiri, Y.
Bibcode: 1991rst..work..161N
Altcode:
No abstract at ADS
Title: Coronal Magnetic Structures Observing Campaign. I. Simultaneous
Microwave and Soft X-Ray Observations of Active Regions at the
Solar Limb
Authors: Nitta, N.; White, S. M.; Kundu, M. R.; Gopalswamy, N.; Holman,
G. D.; Brosius, J. W.; Schmelz, J. T.; Saba, J. L. R.; Strong, K. T.
Bibcode: 1991ApJ...374..374N
Altcode:
Using simultaneous microwave and soft X-ray measurements made with
the Very Large Array (VLA) at 6 and 20 cm and the X-ray Polychromator
(XRP) aboard the Solar Maximum Mission (SMM), we have studied two
active regions near the solar limb. These observations were taken as
part of the Coronal Magnetic Structures Observing Campaign (CoMStOC),
a collaboration designed to study the magnetic field in the solar
corona. The images in soft X-rays and at 20 cm wavelength are similar:
both show peaks above the active regions and extended bridge of
emission 200,000 km long connecting the two regions. The brightness
temperature of the 20 cm emission is lower than that predicted from the
X-ray emitting material, however; it can be attributed to free-free
emission in cooler (<106 K) plasma not visible to XRP,
with an optical depth ∼1. The 6 cm emission is concentrated at lower
altitudes and in a ∼160,000 km long bundle of loops in the northern
active region. Comparison of the 6 cm map with the potential magnetic
field lines computed from photospheric magnetic fields (measured 2 days
earlier) indicates that the 6 cm emission is associated with fields
of less than ∼200 G. Such fields would be too weak to attribute the
observed 6 cm emission to gyroresonance radiation. Analysis of the
6 cm loop bundle indicates that it is strongly asymmetric, with the
magnetic field in the northern leg ∼2 times stronger than in the
southern leg; the 6 cm emission most likely arises from a combination
of hot ( ≥ 2 × 106 K) and cool plasmas, while the 20 cm
emission becomes optically thick in the cooler (∼9 × 103
K) plasma. We estimate an Alfvén speed ∼7000 km s-1
and ratio of electron gyrofrequency to plasma frequency ∼1.0 in the
northern leg of the 6 cm loop.
Title: On the Reconciliation of Simultaneous Microwave Imaging and
Hard X-Ray Observations of a Solar Flare
Authors: Nitta, N.; White, S. M.; Schmahl, E. J.; Kundu, M. R.
Bibcode: 1991SoPh..132..125N
Altcode:
We have compared microwave imaging data for a small flare with
simultaneous hard X-ray spectral observations. The X-ray data suggest
that the power-law index δ of the energy distribution of the radiating
electrons is 5.3 (thick-target) which differs significantly from the
estimate (δ = 1.4) from a homogeneous optically-thin gyrosynchrotron
model which fits the radio observations well. In order to reconcile
these results, we explore a number of options. We investigate a double
power-law energy spectrum for the energetic electrons in the flare,
as assumed by other authors: the power law is steep at low energies
and much flatter at the higher energies which produce the bulk of the
microwaves. The deduced break energy is about 230 keV if we tentatively
ignore the X-ray emission from the radio-emitting electrons: however,
the emission of soft photons by the flat tail strongly contributes to
the observed hard X-ray range and would flatten the spectrum there. A
thin-target model for the X-ray emission is also inconsistent with
radio data. An inhomogeneous gyrosynchrotron model with a number of
free parameters and containing an electron distribution given by the
thick-target X-ray model could be made to fit the radio data.
Title: X-Ray Observations of Two Short but Intense Solar Flares
Authors: Nitta, Nariaki; Dennis, Brian R.; Kiplinger, Alan L.
Bibcode: 1990ApJ...353..313N
Altcode:
This paper presents continuum X-ray spectra of impulsive emission in
two short but intense solar flares which have relatively weak soft
X-ray emissions, combining data obtained with soft X-ray and hard
X-ray spectrometers on board two satellites, the SMM and Hinotori. In
both flares, photon spectra of the impulsive component are found to
flatten toward low energies, suggesting that a low-energy cutoff of
the electron spectrum could be greater than about 50 keV and that the
total energy contained in the electrons is significantly less than
that usually quoted for a cutoff energy of about 20 keV. Different
shapes of the X-ray spectrum at energies below 50 keV in other flares
can be attributed to the variety in the relative strength of gradual
and impulsive emissions. In one of the two flares, observations with
the imager on Hinotori suggest that hard X-ray emission is likely to
be associated with loop footpoints. It is argued that contamination
by the gradual soft X-ray emission and/or the asymmetry of loops could
explain the detection of single sources in the majority of flares that
have been imaged in hard X-rays.
Title: CoMStOCI: Physical Properties of an Active Region Loop Observed
at the Solar Limb
Authors: Holman, G. D.; Brosius, J. W.; Nitta, N.; White, S. M.; Kundu,
M. R.; Gopalswamy, N.; Schmelz, J. T.; Saba, J. L. R.; Strong, K. T.
Bibcode: 1990BAAS...22..899H
Altcode:
No abstract at ADS
Title: A large and complex flare on 3 February 1982.
Authors: Mizugaki, Kazuo; Nitta, Nariaki; Hiei, Eijiro
Bibcode: 1990PNAOJ...1..297M
Altcode:
The authors present an Hα morphological study of a well-developed
2B flare that occurred on 3 Feb 1982. The flare, associated
with a long-enduring hard X-ray burst, has some interesting Hα
characteristics.
Title: The Radio Spectra of Rs-Canum Stars
Authors: White, S. M.; Kundu, M. R.; Uchida, Y.; Nitta, N.
Bibcode: 1990ASPC....9..239W
Altcode: 1990csss....6..239W
The results of observations of several systems are presented for the
0.327-90 GHz range to study the radio coronae and outbursts associated
with the RS CVn stars, as well as their visibility at millimeter
wavelengths. The radio spectra are observed with the VLA and the
Nobeyama 45-m millimeter-wavelength telescope. The three stars observed
with the Nobeyama and VLA telescopes are UX Ari, AR Lac, and HR 5110,
and the frequencies and conditions of the nonsimultaneous observations
are set forth. Observations of the RS CVn systems at mm wavelengths
is possible, and it is theorized that important information regarding
flares and energy release can be derived from these data. A simple model
is presented for the radio-coronae source related to outbursts of up
to 5 GHz, and the onset of outbursts are predicted to be accompanied
by low frequency type-II plasma emission.
Title: Interpretation of Multiwavelength Observations of Solar Active
Regions Obtained During CoMStOC
Authors: Brosius, J. W.; Holman, G. D.; Nitta, N.; White, S. M.; Kundu,
M. R.; Gopalswamy, N.; Schmelz, J. T.; Saba, J. R. L.; Willson, R.
Bibcode: 1989BAAS...21..838B
Altcode:
No abstract at ADS
Title: Simultaneous Microwave and Soft X-ray Observations of Active
Regions at the Solar Limb
Authors: Nitta, N.; White, S.; Kundu, M.; Gopalswamy, N.; Holman,
G.; Brosius, J.; Schmelz, J.; Saba, J.; Strong, K.
Bibcode: 1989BAAS...21..828N
Altcode:
No abstract at ADS
Title: The Spatial, Spectral, and Temporal Character of the Hard
X-Ray Flare of 1982 February 3
Authors: Nitta, Nariaki; Kiplinger, Alan L.; Kai, Keizo
Bibcode: 1989ApJ...337.1003N
Altcode:
This paper presents spatially resolved, hard and soft X-ray, 17 GHz
microwave and H-alpha observations of an X1.1 flare which occurred on
February 3, 1982. The bulk of the 25-50 keV hard X-ray emission during
impulsive peaks is produced by two sources of differing brightness
which are separated by about 50 arcsec. Soft X-ray images reveal a
single component that resides between the two hard sources. On the
decay of the event, X-ray spectra provide direct evidence for a hot
component that dominates the less than 50 keV flux. Hard X-ray images
obtained at these times show a single source that is nearly coincident
with the soft X-ray source. Interferometric microwave observations
obtained during impulsive peaks support the idea that microwave
emission is produced in the vicinity of high magnetic fields near the
sunspot. The influence of an asymmetric loop or system of asymmetric
loops is essential to interpreting the observations.
Title: Nonthermal and thermal emissions in solar flares.
Authors: Nitta, N.
Bibcode: 1989sasf.confP.169N
Altcode: 1988sasf.conf..169N; 1989IAUCo.104P.169N
In two major solar flares, the author has found evidence for hard X-ray
footpoint emission using the X-ray imager on Hinotori. Possible factors
are discussed as to why we only rarely detect footpoint emissions,
which may not always take the form of double sources, depending on
magnetic configurations in flares.
Title: Millimeter wavelength observations of solar flares for Max 1991
Authors: Kundu, M. R.; Gopalswamy, N.; Nitta, N.; Schmahl, E. J.;
White, S. M.; Welch, W. J.
Bibcode: 1988fnsm.work..107K
Altcode:
The Hat Creek millimeter-wave interferometer (to be known as the
Berkeley-Illinois-Maryland Array, BIMA) is being upgraded. The improved
array will become available during the coming solar maximum, and will
have guaranteed time for solar observing. The Hat Creek millimeter-wave
interferometer is described along with the improvements. The scientific
objectives are briefly discussed.
Title: A Study of Coronal Bright Points at 20-CM Wavelength
Authors: Nitta, N.; Kundu, M. R.
Bibcode: 1988SoPh..117...37N
Altcode:
We present the results of a study of coronal bright points observed
at 20 cm with the VLA on a day when the Sun was exceptionally
quiet. Microwave maps of bright points were obtained using data for
the entire observing period of 5 hours, as well as for shorter periods
of a few minutes. Most bright points, especially those appearing
in the full-period maps appear to be associated with small bipolar
structures on the photospheric magnetogram. Overlays of bright point
(B.P.) maps on the Ca+K picture, show that the brightest
part of a B.P. tends to lie on the boundary of a supergranulation
network. In general, the bright points exhibit rapid variations in
intensity. There is no systematic correlation between the size of a
B.P. and its intensity; the apparently slow variation of B.P. emission
may have rapid fluctuations superimposed on it.
Title: Millimeter Wavelength Observations of Solar Flares for Max'91
Authors: Kundu, M. R.; Gopalswamy, N.; Nitta, N.; Schmahl, E. J.;
White, S. M.
Bibcode: 1988BAAS...20..746K
Altcode:
No abstract at ADS
Title: Comparison of hard X-ray spectra obtained by spectrometers
on Hinotori and SMM and detection of ``superhot'' component
Authors: Nitta, Nariaki
Bibcode: 1988AdSpR...8k.259N
Altcode: 1988AdSpR...8..259N
We report the results of comparison between hard X-ray spectra in
solar flares obtained by broad-band spectrometers aboard Hinotori
and SMM satellites. The energy ranges of the Hinotori spectrometer
were known only with growing uncertainty because of background
X-rays from particle-induced radioactivity in the NaI crystal, which
gradually outweighed X-rays from the radioactive calibration source
(109Cd). Within uncertainty brought about by assuming the
typical energy of the background X-rays, calibrations are consistent
with constant energy ranges, i.e. same as prelaunch values. Using
these energy ranges, we find that spectra by the Hinotori spectrometer
are usually consistent with those by the SMM spectrometer for flares
in 1981. On the contrary, flares in 1982 persistently show 20-50 %
higher flux by Hinotori than by SMM. If this discrepancy is entirely
attributable to errors in the calibration of energy ranges, the
errors would be ~10 %. There are other factors which may increase the
uncertainty in determining a spectrum. Despite such a discrepancy in
absolute flux, in the decay phase of one flare, spectra revealed a
hard X-ray component (probably a ``superhot'' component) that could
be explained neither by emission from a plasma of ~2×107 K
nor by a nonthermal power-law component. Indeed, imaging observations
during this period show nearly cospatial hard X-ray emission with
soft X-ray emission, in contrast with earlier times at which hard and
soft X-rays come from different places. Contrary to the temperature
diagnosis using FeXXVI lines, hard X-ray spectral data give temperatures
~7×107 K for the superhot component.
Title: Coronal bright points in mircrowaves.
Authors: Kundu, M. R.; Nitta, N.
Bibcode: 1988sscd.conf...89K
Altcode:
An excellent map of the quiet Sun showing coronal bright points at
20 cm wavelength was produced using the VLA on February 13, 1987. The
authors studied the locations of bright points relative to features on
the photospheric magnetogram and Ca+K spectroheliogram. They
find that most bright points appearing in the full 5-hour synthesized
map are associated with small bipolar structures on the photospheric
magnetogram; and the brightest part of a B.P. tends to lie on the
boundary of a supergranulation network. The bright points exhibit rapid
variations in intensity superposed on an apparently slow variation.
Title: Coronal Bright Points in Microwaves: Their Optical Associations
and Emission Measures
Authors: Nitta, N.; Kundu, M. R.
Bibcode: 1987BAAS...19.1122N
Altcode:
No abstract at ADS
Title: Simultaneous imaging observations of the sun at microwaves
and hard X-rays
Authors: Nakajima, H.; Takakura, T.; Nitta, N.; Ohki, K.
Bibcode: 1987SoPh..113...63N
Altcode: 1982SoPh..113...63N
No abstract at ADS
Title: Energy of Microwave-Emitting Electrons and Hard X-Ray /
Microwave Source Model in Solar Flares
Authors: Nitta, N.; Kosugi, T.
Bibcode: 1986SoPh..105...73N
Altcode:
We present a new method of estimating the energy of microwave-emitting
electrons from the observed rate of increase of the microwave flux
relative to the hard X-ray flux measured at various energies during
the rising phase of solar flares. A total of 22 flares observed
simultaneously in hard X-rays (20-400 keV) and in microwaves (17
GHz) were analyzed in this way and the results are as follows: The
observed energy of X-rays which vary in proportion to the 17 GHz
emission concentrates mostly below 100 keV with a median energy of
70 keV. Since the mean energy of electrons emitting 70 keV X-rays is
≲130 keV or ≲180 keV, depending on the assumed hard X-ray emission
model (thin-target and thick-target, respectively), this photon energy
strongly suggests that the 17 GHz emission comes mostly from electrons
with an energy of less than a few hundred keV.
Title: X-ray imaging of a solar limb flare on 1982 January 22.
Authors: Takakura, T.; Tanaka, K.; Nitta, N.; Kai, K.; Ohki, K.
Bibcode: 1986SoPh..107..109T
Altcode: 1987SoPh..107..109T
Simultaneous X-ray images in hard (20-40 keV) and softer (6.5-15 keV)
energy ranges were obtained with the hard X-ray telescope aboard the
Hinotori spacecraft of an impulsive solar X-ray burst associated with
a flare near the solar west limb.
Title: Observations of solar flare photon spectra from 20 keV to
7 MeV.
Authors: Yoshimori, M.; Watanabe, H.; Nitta, N.
Bibcode: 1985JPSJ...54.4462Y
Altcode: 1985PSJaJ..54.4462Y
Solar flare photon energy spectra in the 20 keV to 7 MeV range are
derived from the Apr. 1, Apr. 4, Apr. 27 and May 13, 1981 flares. The
flares were observed with hard X-ray and gamma-ray spectrometers on
board the Hinotori satellite. The results reveal that the shape of
hard X-ray spectrum for the gamma-ray flares does not much vary from
flare to flare, and the spectra harden apparently at energy about
400 keV. Effects of nuclear line emission on the continuum and of
higher energy electron bremsstrahlung are considered to explain the
spectral hardening.
Title: Observations of Solar Flare Photon Energy Spectra from 20
KEV to 7 Mev
Authors: Yoshimori, M.; Watanabe, H.; Nitta, N.
Bibcode: 1985ICRC....4...54Y
Altcode: 1985ICRC...19d..54Y
Solar flare photon energy spectra in the 20 keV to 7 MeV range are
derived from the Apr. 1, Apr. 4, Apr. 27 and May 13, 1981 flares. The
flares were observed with a hard X-ray and a gamma-ray spectrometer
on board the Hinotori satellite. The results show that the spectral
shape varies from flare to flare and the spectra harden in energies
above about 400 keV. Effects of nuclear line emission on the continuum
and of higher energy electron bremsstrahlung are considered to explain
the spectral hardening.
Title: Coronal Propagation of Solar Flare Particles Observed by
Geostationary Satellite
Authors: Kohno, T.; Nitta, N.; Wada, M.; Suda, T.
Bibcode: 1985ICRC....4..301K
Altcode: 1985ICRC...19d.301K
Propagation of solar flare particles in corona was studied using the
satellite data at the geostationary orbit. by selecting very fast
rise time events only, the interplanetary propagation were assumed
to be scatter free arrival. The results show that the propagation in
corona does not depend on particle energy in 4 to 500 MeV protons,
and the time delays from optical flare do not depend on the distance
between the flare site and the base of the interplanetary magnetic
field which connects to the Earth.
Title: Model of hard X-ray and microwave emission solar flares -
from the analysis flux time variations.
Authors: Nitta, N.
Bibcode: 1985AstHe..78..308N
Altcode:
No abstract at ADS
Title: Flux relations between hard X-rays and microwaves for both
impulsive and extended solar flares
Authors: Kai, K.; Kosugi, T.; Nitta, N.
Bibcode: 1985PASJ...37..155K
Altcode:
The correlation of peak fluxes between hard X-rays and microwaves from
solar flares was reexamined separately for impulsive and extended bursts
using 61 events recorded with both the hard X-ray spectrometer aboard
the Hinotori satellite and the 17-GHz polarimeter at Nobeyama. (1)
For impulsive bursts FR = 37.2 FX exp 0.77 with
a small scatter of 0.3 orders of magnitude (rms) where FR
is the 17-GHz peak flux in sfu and FX is the hard X-ray
peak flux integrated over 67-152 keV in photons/s sq cm. Extended
bursts deviate systematically above the regression line derived for
impulsive bursts by a factor of 2-10 (excess of radio emission). A
strong constraint is derived on the magnetic field intensity in
the radio emitting region of impulsive bursts: the magnetic field
intensity must be constant from flare to flare (approximately 1000
G with a scatter of less than a factor of 2). The radio excess of
extended bursts can be explained by a relatively abundant population
of relativistic electrons trapped in large magnetic loops.
Title: Hard X-ray imaging observations of solar hot thermal flares
with the HINOTORI spacecraft
Authors: Tsuneta, S.; Nitta, N.; Takakura, T.; Makishima, K.; Murakami,
T.; Oda, M.; Ogawara, Y.; Ohki, K.; Tanaka, K.
Bibcode: 1984ApJ...284..827T
Altcode:
Two solar hard X-ray bursts of a new type (hot thermal flare) were
observed with hard X-ray imaging telescopes and other instruments on
Japanese spacecraft Hinotori. The flares have no clear impulsive phase
below 40 keV and emit intense hard X-rays (10-50 keV) with extremely
steep spectra from a small region with size (FWHM) of 10-20 arcsec. This
source contains a hot thermal plasma of (3-3.5) x 10 to the 7th K with
an emission measure of the order of 10 to the 49th/cu cm. One of the
flares occurred just on the limb, and the centroid of the hard X-ray
(14-38 keV) source was located at (6 + or - 3) x 10 to the 3rd km
above the photosphere. It is concluded that the energy continuously
released goes into heating rather than acceleration almost throughout
the flare. Typical impulsive flares may usually have a similar nature
in the later phase (gradual phase) of the flare evolution.
Title: Solar flare iron K-alpha emission associated with a hard
X-ray burst
Authors: Tanaka, K.; Watanabe, T.; Nitta, N.
Bibcode: 1984ApJ...282..793T
Altcode:
High-resolution observations of the solar flare event of 20:09:30
UT on July 28, 1981, obtained with the Bragg crystal spectrometers
(line spectra of soft X-rays at 175-195 pm) and the soft-X-ray and
hard-X-ray spectrometers (continuum spectra at 1.5-12.5 and 18-400 keV,
respectively) of the Japanese solar-maximum satellite Hinotori are
reported. The data are presented in graphs, sample spectra, and tables
and analyzed. An intense K-alpha Fe emission is found to be associated
with a hard X-ray burst with a power-law photon distribution from below
10 to above 100 keV during the first part of the flare. The K-alpha
emission during the X-ray burst and after its decay are attributed to
photospheric neutral-Fe fluorescence induced by the burst X-rays and
thermal X-rays, respectively.
Title: Hard X-ray imaging of a solar two-ribbon flare on 1981
August 21
Authors: Takakura, T.; Nitta, N.; Ohki, K.; Wang, J. L.
Bibcode: 1984ApJ...281L..51T
Altcode:
X-ray images (20-35 keV) were observed with the hard X-ray telescope
aboard the Hinotori spacecraft for the solar X-ray burst associated
with a typical two-ribbon H-alpha flare with sufficient separation
for resolving in X-ray imaging whether or not the image is double over
the ribbons. The X-ray images were, however, a single source elongated
along one ribbon of the flare at the early phase of the burst. It is
likely that this spatial coincidence is due to an apparent projection
effect of the coronal X-ray source along a long ridge of the magnetic
arcade connecting the two ribbons about 30 arc sec apart or along a
long, twisted magnetic loop parallel to the two ribbons. In the later
phase of the burst, the X-ray image became a compact single source
with a spatial shift of the core by 10 arc sec-20 arc sec.
Title: Hard X-ray imaging of the solar flare on 1981 May 13 with
the HINOTORI spacecraft
Authors: Tsuneta, S.; Takakura, T.; Nitta, N.; Makishima, K.; Murakami,
T.; Oda, M.; Ogawara, Y.; Kondo, I.; Ohki, K.; Tanaka, K.
Bibcode: 1984ApJ...280..887T
Altcode:
Hard X-ray images and X-ray spectra of an intense solar flare that
occurred at E58N09 on May 13, 1981 are presented and discussed. The
observation was made with Japanese solar X-ray spacecraft Hinotori. An
unusual hard X-ray source, observed at 14-38 keV, had a steady
spatial displacement of approximately 1 arcmin toward the limb from
the two-ribbon H-alpha flare during the 16 minutes of hard X-ray
observation, including the time of maximum flux. This suggests that the
source was located near the top of a coronal loop structure connecting
the two ribbons, at an estimated altitude of approximately 40,000 km
above the photosphere. The soft X-ray (5-10 keV) source nearly coincided
in position and size with the hard X-ray source. Near the peak of hard
X-ray time profile, approximately 40 percent of the total count rate of
the hard X-ray image is estimated to come from a power-law component,
as observed with the hard X-ray spectrometer. The parameters of the
thermal plasma near the loop top were determined to be n = 3 x 10
to the 10th per cu cm, T = 2 x 10 to the 7th K, and beta = 16(pi)
n k T/(B squared) approximately 1.0. Intense heat conduction from the
thermal plasma near the loop top to the transition region appears to
be in equilibrium with the continuous energy release near the loop top.
Title: Hard X-ray dynamic spectrum of flares observed by Hinotori
Authors: Nitta, N.; Takakura, T.; Ohki, K.; Yoshimori, M.
Bibcode: 1983SoPh...86..241N
Altcode:
Time variations of the hard X-ray spectrum in solar flares are observed
by the hard X-ray spectrometer (HXM) aboard the Hinotori satellite. With
a new presentation of the dynamic spectrum we have studied the
differences between impulsive and gradual hard X-ray bursts. In the
impulsive events a "bent" spectrum up to some hundred keV persists at
least until the main peak. In the gradual events, on the other hand,
a power-law spectrum augmented by a low-energy excess is dominant.
Title: Interpretation of the Soft X-Ray Spectra from HINOTORI
Authors: Tanaka, K.; Nitta, N.; Akita, K.; Watanabe, T.
Bibcode: 1983SoPh...86...91T
Altcode:
We present analyses of the soft X-ray iron line spectra of flares
obtained from the Bragg Spectrometer on Hinotori. We first present a
case of strong Kα emission at the impulsive phase of the hard X-ray
burst, and assess net Kα emission due to the electron impact by
eliminating the fluorescence contribution. Second we discuss on the
differences in the electron temperatures and emission measures derived
respectively from FeXXVI and FeXXV spectra. A pilot two-temperatures
model which can explain the two spectra is presented. Finally,
we compare the temporal relations between the soft X-ray and
hard X-ray intensities and show two extreme classes of flares, one
characterized by the efficient formation of a hot thermal plasma above
30 million degree, and the other characterized by the spiky hard X-ray
component. Energetical relation of the thermal plasma to the electron
beam is discussed for the two classes.
Title: Hard X-ray imaging of a solar limb flare with the X-ray
telescope aboard the HINOTORI satellite
Authors: Takakura, T.; Tsuneta, S.; Nitta, N.; Makishima, K.; Murakami,
T.; Ogawara, Y.; Oda, M.; Ohki, K.; Miyamoto, S.
Bibcode: 1983ApJ...270L..83T
Altcode:
X-ray images of the intense solar X-ray burst of Apr. 27, 1981 which
were made aboard the satellite Hinotori are discussed. The hard
X-ray telescope aboard the satellite is described, and the mapping
spectrometric functions are considered. The hard X-ray images are shown
along with time profiles of the flare. The images reveal the height
structure of a strong hard X-ray source located in the corona. The
source contains two sources, a steady main one of about 20 arcsec and
a subordinate one exhibiting variable intensity relative to the main
source. Both sources were located in the corona without any evidence
of an appreciable chromospheric source throughout the observation. The
obtained photon spectra fit better to power laws than to isothermal
spectra.
Title: General aspects of hard X-ray flares observed by Hinotori:
Gradual burst and impulsive burst
Authors: Ohki, K.; Takakura, T.; Tsuneta, S.; Nitta, N.
Bibcode: 1983SoPh...86..301O
Altcode:
We survey here the observational results on five gradual and four
impulsive type events from the hard X-ray imaging (SXT) and spectrometer
(HXM) instruments on the Hinotori satellite. A set of differences are
clearly recognized between the gradual and impulsive type bursts. These
are: (1) Hard X-ray images show the existence of a large coronal
source for each gradual burst and a wide variety of source structures
for impulsive bursts. (2) The source heights of the impulsive bursts
appear to be low. (3) All gradual bursts show power-law spectra while
impulsive bursts show exponential thermal spectra at least before the
maximum phase. (4) Energy-dependent peak delays are observed only in
gradual bursts. From these differences we suggest that two different
acceleration and emission mechanisms are involved with these two kinds
of hard X-ray bursts.
Title: Thermal Evolution of Flare Plasmas
Authors: Watanabe, T.; Tanaka, K.; Akita, K.; Nitta, N.
Bibcode: 1983SoPh...86..107W
Altcode:
The evolution of hot thermal plasma in solar flares is analyzed by a
single-temperature model applied to continuum emission in the 5 keV
< E ≲ 13 keV spectral range. The general trend that the thermal
plasma observed in soft X-rays is heated by the non-thermal electrons
that emit as the hard X-ray bursts is confirmed by the observation of
an electron temperature increase at the time interval of hard X-ray
spikes and a quantitative comparison between thermal energy content and
hard X-ray energy input. Non-thermal electrons of 10 keV < E <
30 keV energy may play an important role in pre- and post-burst phases.
Title: Vertical Structure of Hard X-Ray Flares
Authors: Tsuneta, S.; Takakura, T.; Nitta, N.; Ohki, K.; Makishima,
K.; Murakami, T.; Oda, M.; Ogawara, Y.
Bibcode: 1983SoPh...86..313T
Altcode:
This paper presents studies of the vertical structure of hard X-ray
flares for two contrasting examples. The 1981 May 13 flare contained
a coronal hard X-ray source which was located above 50000 km above
the photosphere. On the other hand, the 1981 July 20 flare had a
chromospheric double source structure in the initial phase. Electrons
in this case were able to stream freely from the corona to the
chromosphere.
Title: Hard X-Ray Images of Impulsive Bursts
Authors: Takakura, T.; Ohki, K.; Tsuneta, S.; Nitta, N.
Bibcode: 1983SoPh...86..323T
Altcode:
A morphological study is made for the hard X-ray images (25-50 keV)
of nine impulsive bursts observed by Hinotori. Most of them revealed
single sources, either extended or compact, during the whole duration
of the bursts. The sources of all of four spike bursts in the present
sample are compact. After the main phase of the impulsive bursts,
generally the source size becomes smaller accompanying a shift of
position. The X-ray source size is much greater than that of the Hα
kernel in two events out of three. Four possible explanations for the
X-ray source to be single are suggested. One of these is the strong
electric field along the magnetic field as demonstrated to be produced
at the decay of force-free current.
Title: Short Period Pulsations Observed Simultaneously by X-Ray and
Radio Waves
Authors: Degaonkar, S. S.; Takakura, T.; Kaufmann, P.; Costa, J. E. R.;
Ohki, K.; Nitta, N.
Bibcode: 1983SoPh...86..237D
Altcode:
From simultaneous high-time-resolution observations of solar X-rays from
Hinotori and the millimeter waves at Itapetinga Radio Observatory in
Brazil during a solar flare on November 4, 1981 at 1827 UT, short period
(∼ 300 ms) pulsations have been detected in five time intervals of 2
s each. Both a cross-correlation analysis between X-rays and microwaves
and a Fourier analysis were made to verify the significance of the
quasi-periodic pulsations. The cross-correlation is significant but
the pulsations could not be periodic oscillation.
Title: Sub-second pulsations simultaneously observed at microwaves
and hard X rays in a solar burst
Authors: Takakura, T.; Kaufmann, P.; Costa, J. E. R.; Degaonkar,
S. S.; Ohki, K.; Nitta, N.
Bibcode: 1983Natur.302..317T
Altcode:
Sub-second time structures have been found in millimetre
waves1,2 and, independently, in hard X rays3,4
emitted during solar bursts. However, because simultaneous observations
of such fast time structures in millimetre radio and X-ray ranges
had not been available, we planned coordinated observations of the
solar burst in November 1981 with a high time resolution of a few
milliseconds. The hard X rays (30-40 keV) were observed by the hard
X-ray monitor, HXM5, aboard the Hinotori Satellite with a
time resolution of 7.81 ms and the radio emissions were observed on the
ground by the 45-ft dish at Itapetinga Radio Observatory with a high
time resolution (1 ms) and high sensitivities at 22 GHz and 44 GHz
(ref. 6), supplemented by patrol observations at 7 GHz with a time
resolution of 100 ms. Absolute timing at Itapetinga and Hinotori is
better than 10 ms. Correlated sub-second time structures at hard X-ray
and at millimetre wavelengths were found for the first time.
Title: Sub-second pulsations simultaneously observed at microwaves
and hard X-rays in a solar burst
Authors: Takakura, T.; Kaufmann, P.; Costa, J. E. R.; Degaonkar,
S. S.; Ohki, N.; Nitta, N.
Bibcode: 1982STIN...8325644T
Altcode:
Coordinated observations of solar bursts in November 1981 with a
high time resolution of a few milliseconds were planned. The hard
X-rays (30 to 40 keV) were observed with hard X-ray monitor (HXM)
aboard the Hinotori Satellite with a time resolution of 7.81 ms and
the radio emissions were observed on the ground with 45 ft dish at
Itapetinga Radio Observatory with a high time resolution (1 ms) and high
sensitivities at 22 GHz and 44 GHz, supplemented by a patrol observation
at 7 GHz with time resolution of 100 ms. The pulsations repeated with a
period of about 300 ms. The physical implication of the good correlation
is not clear at this stage, but it may give a clue to the understanding
of the high energy phenomena occurring during the solar flares.
Title: Sub-second Pulsations Simultaneously Observed at Microwaves
and Hard X-rays in a Solar Burst
Authors: Takakura, T.; Kaufmann, P.; Degaonkar, S. S.; Costa, J. E. R.;
Ohki, K.; Nitta, N.; Tsuneta, S.
Bibcode: 1982BAAS...14..879T
Altcode:
No abstract at ADS
Title: Hard X-Ray Imaging by SXT - Compact Sources
Authors: Tsuneta, S.; Ohki, K.; Takakura, T.; Nitta, N.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.; Kondo, I.
Bibcode: 1982sofl.symp..130T
Altcode:
No abstract at ADS
Title: Hard X-Ray Imaging by SXT - Extended Sources
Authors: Takakura, T.; Ohki, K.; Tsuneta, S.; Nitta, N.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.
Bibcode: 1982sofl.symp..142T
Altcode:
No abstract at ADS
Title: The Energetics of the Elementary Bursts
Authors: Tanaka, K.; Nitta, N.; Watanabe, T.
Bibcode: 1982sofl.symp...20T
Altcode:
No abstract at ADS
Title: Results from the Hard X-Ray Spectrometer - Hxm
Authors: Ohki, K.; Nitta, N.; Tsuneta, S.; Takakura, T.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.
Bibcode: 1982sofl.symp...69O
Altcode:
No abstract at ADS
Title: Hard X-ray imaging of solar flares with HINOTORI SXT
Authors: Takakura, T.; Nitta, N.; Tsuneta, S.; Makishima, K.; Murakami,
T.; Oda, M.; Ogawara, Y.; Ohki, K.; Shibuya, N.; Miyamoto, S.
Bibcode: 1982spte.symp.1597T
Altcode:
Japanese solar X-ray satellite Hinotori was launched on February 21,
1981. The hard X-ray telescope installed in this satellite is designed
to observe the images of hard solar X-ray bursts in a band typically 20
- 40 keV with a high angular resolution, a moderate time resolution and
a wide field of view. More than 100 solar X-ray bursts with sufficient
fluxes for imaging have been detected so far. The instrumentation of
the telescope is presented in this report.
Title: Solar hard X-ray images observed by Astro-A
Authors: Ohki, K.; Tsuneta, S.; Nitta, N.; Takakura, T.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.
Bibcode: 1982AIPC...77..395O
Altcode: 1982grtr.work..395O
Observations with the solar X-ray telescope on board the Astro A
satellite are reviewed and analyzed. The instrument has a sensitivity
range in the 17-60 keV interval. A total of 20 large X-ray events were
observed between February and August 1981. Analyses are presented
of one limb and one disk event. The April 27 limb flare in region
3049 produced a count increase to 60,000/sec on the lowest energy
channel. An April 2 event showed a size less than 10 arcsec on the
solar surface. An electron density of 6.2 x 10 to the 46/cu cm was
calculated, with field particles exceeding 20 billion/cu cm. Details
of the satellite scanning capabilities are described.
Title: Hard X-Ray Imaging by SXT - Comparisons with Hα Data
Authors: Ohki, K.; Tsuneta, S.; Takakura, T.; Nitta, N.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.; Miyamoto, S.
Bibcode: 1982sofl.symp..102O
Altcode:
No abstract at ADS
Title: Solar Hard X-Ray Images Observed by Astro-A
Authors: Ohki, K.; Tsuneta, S.; Nitta, N.; Takakura, T.; Makishima,
K.; Murakami, T.; Ogawara, Y.; Oda, M.
Bibcode: 1981AIPC...77..395O
Altcode:
The Solar X-ray Telescope (SXT) on board the Astro-A has observed many
events since its launch on February 21, 1981. Several of the largest
events with counting rates <104 c/s, have been analyzed to reveal
very compact sources for the large hard X-ray bursts. Although a few
limb events show some extended features up to about one arcmin, most
events have linear dimensions less than the FWHM of the SXT triangular
response, which is about 30 arcseconds. This compactness of the largest
events may conflict with traditional models of hard X-ray sources,
including thin and thick target models. In this paper, two typical
large events are presented. A disk event on April 2, 1981, shows a
single source with a very small diameter, while a April 27, 1981,
limb event shows a double source structure with unbalanced intensities.