379 research outputs found
The Evolution of Sunspot Magnetic Fields Associated with a Solar Flare
Solar flares occur due to the sudden release of energy stored in
active-region magnetic fields. To date, the pre-cursors to flaring are still
not fully understood, although there is evidence that flaring is related to
changes in the topology or complexity of an active region's magnetic field.
Here, the evolution of the magnetic field in active region NOAA 10953 was
examined using Hinode/SOT-SP data, over a period of 12 hours leading up to and
after a GOES B1.0 flare. A number of magnetic-field properties and low-order
aspects of magnetic-field topology were extracted from two flux regions that
exhibited increased Ca II H emission during the flare. Pre-flare increases in
vertical field strength, vertical current density, and inclination angle of ~
8degrees towards the vertical were observed in flux elements surrounding the
primary sunspot. The vertical field strength and current density subsequently
decreased in the post-flare state, with the inclination becoming more
horizontal by ~7degrees. This behaviour of the field vector may provide a
physical basis for future flare forecasting efforts.Comment: Accepted for Publication in Solar Physics. 16 pages, 4 figure
A Parametric Study of Erupting Flux Rope Rotation. Modeling the "Cartwheel CME" on 9 April 2008
The rotation of erupting filaments in the solar corona is addressed through a
parametric simulation study of unstable, rotating flux ropes in bipolar
force-free initial equilibrium. The Lorentz force due to the external shear
field component and the relaxation of tension in the twisted field are the
major contributors to the rotation in this model, while reconnection with the
ambient field is of minor importance. Both major mechanisms writhe the flux
rope axis, converting part of the initial twist helicity, and produce rotation
profiles which, to a large part, are very similar in a range of shear-twist
combinations. A difference lies in the tendency of twist-driven rotation to
saturate at lower heights than shear-driven rotation. For parameters
characteristic of the source regions of erupting filaments and coronal mass
ejections, the shear field is found to be the dominant origin of rotations in
the corona and to be required if the rotation reaches angles of order 90
degrees and higher; it dominates even if the twist exceeds the threshold of the
helical kink instability. The contributions by shear and twist to the total
rotation can be disentangled in the analysis of observations if the rotation
and rise profiles are simultaneously compared with model calculations. The
resulting twist estimate allows one to judge whether the helical kink
instability occurred. This is demonstrated for the erupting prominence in the
"Cartwheel CME" on 9 April 2008, which has shown a rotation of \approx 115
degrees up to a height of 1.5 R_sun above the photosphere. Out of a range of
initial equilibria which include strongly kink-unstable (twist Phi=5pi), weakly
kink-unstable (Phi=3.5pi), and kink-stable (Phi=2.5pi) configurations, only the
evolution of the weakly kink-unstable flux rope matches the observations in
their entirety.Comment: Solar Physics, submitte
Current Helicity and Twist as Two Indicators of The Mirror Asymmetry of solar Magnetic Fields
A comparison between the two tracers of magnetic field mirror asymmetry in
solar active regions, twist and current helicity, is presented. It is shown
that for individual active regions these tracers do not possess visible
similarity while averaging by time over the solar cycle, or by latitude,
reveals similarities in their behaviour. The main property of the dataset is
anti-symmetry over the solar equator. Considering the evolution of helical
properties over the solar cycle we find signatures of a possible sign change at
the beginning of the cycle, though more systematic observational data are
required for a definite confirmation. We discuss the role of both tracers in
the context of the solar dynamo theory.Comment: 14 pages, 6 figure
High-Energy Aspects of Solar Flares: Overview of the Volume
In this introductory chapter, we provide a brief summary of the successes and
remaining challenges in understanding the solar flare phenomenon and its
attendant implications for particle acceleration mechanisms in astrophysical
plasmas. We also provide a brief overview of the contents of the other chapters
in this volume, with particular reference to the well-observed flare of 2002
July 23Comment: This is the introductory article for a monograph on the physics of
solar flares, inspired by RHESSI observations. The individual articles are to
appear in Space Science Reviews (2011
Estimation of solar prominence magnetic fields based on the reconstructed 3D trajectories of prominence knots
We present an estimation of the lower limits of local magnetic fields in
quiescent, activated, and active (surges) promineces, based on reconstructed
3-dimensional (3D) trajectories of individual prominence knots. The 3D
trajectories, velocities, tangential and centripetal accelerations of the knots
were reconstructed using observational data collected with a single
ground-based telescope equipped with a Multi-channel Subtractive Double Pass
imaging spectrograph. Lower limits of magnetic fields channeling observed
plasma flows were estimated under assumption of the equipartition principle.
Assuming approximate electron densities of the plasma n_e = 5*10^{11} cm^{-3}
in surges and n_e = 5*10^{10} cm^{-3} in quiescent/activated prominences, we
found that the magnetic fields channeling two observed surges range from 16 to
40 Gauss, while in quiescent and activated prominences they were less than 10
Gauss. Our results are consistent with previous detections of weak local
magnetic fields in the solar prominences.Comment: 14 pages, 12 figures, 1 tabl
Multiwavelength Study of M8.9/3B Solar Flare from AR NOAA 10960
We present a multi-wavelength analysis of a long duration white-light solar
flare (M8.9/3B) event that occurred on 4 June 2007 from NOAA AR 10960. The
flare was observed by several spaceborne instruments, namely SOHO/MDI,
Hinode/SOT, TRACE and STEREO/SECCHI. The flare was initiated near a small,
positive-polarity, satellite sunspot at the centre of the AR, surrounded by
opposite-polarity field regions. MDI images of the AR show considerable amount
of changes in a small positive-polarity sunspot of delta configuration during
the flare event. SOT/G-band (4305 A) images of the sunspot also suggest the
rapid evolution of the positive-polarity sunspot with highly twisted penumbral
filaments before the flare event, which were oriented in the counterclockwise
direction. It shows the change in orientation and also remarkable disappearance
of twisted penumbral filaments (~35-40%) and enhancement in umbral area
(~45-50%) during the decay phase of the flare. TRACE and SECCHI observations
reveal the successive activations of two helical twisted structures associated
with this sunspot, and the corresponding brightening in the chromosphere as
observed by the time-sequence images of SOT/Ca II H line (3968 A). The
secondary-helical twisted structure is found to be associated with the M8.9
flare event. The brightening starts 6-7 min prior to the flare maximum with the
appearance of secondary helical-twisted structure. The flare intensity
maximizes as this structure moves away from the AR. This twisted flux-tube
associated with the flare triggering, is found to be failed in eruption. The
location of the flare is found to coincide with the activation site of the
helical twisted structures. We conclude that the activations of successive
helical twists in the magnetic flux tubes/ropes plays a crucial role in the
energy build-up process and triggering of M-class solar flare without a CME.Comment: 22 pages, 12 figures, Accepted for Publication in Solar Physic
Imaging Spectroscopy of a White-Light Solar Flare
We report observations of a white-light solar flare (SOL2010-06-12T00:57,
M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics
Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI). The HMI data give us the first space-based high-resolution imaging
spectroscopy of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric FeI line at 6173.34{\AA} and its neighboring
continuum. In the impulsive phase of the flare, a bright white-light kernel
appears in each of the two magnetic footpoints. When the flare occurred, the
spectral coverage of the HMI filtergrams (six equidistant samples spanning
\pm172m{\AA} around nominal line center) encompassed the line core and the blue
continuum sufficiently far from the core to eliminate significant Doppler
crosstalk in the latter, which is otherwise a possibility for the extreme
conditions in a white-light flare. RHESSI obtained complete hard X-ray and
\Upsilon-ray spectra (this was the first \Upsilon-ray flare of Cycle 24). The
FeI line appears to be shifted to the blue during the flare but does not go
into emission; the contrast is nearly constant across the line profile. We did
not detect a seismic wave from this event. The HMI data suggest stepwise
changes of the line-of-sight magnetic field in the white-light footpoints.Comment: 14 pages, 7 figures, Accepted by Solar Physic
Radio Bursts Associated with Flare and Ejecta in the 13 July 2004 Event
We investigate coronal transients associated with a GOES M6.7 class flare and
a coronal mass ejection (CME) on 13 July 2004. During the rising phase of the
flare, a filament eruption, loop expansion, a Moreton wave, and an ejecta were
observed. An EIT wave was detected later on. The main features in the radio
dynamic spectrum were a frequency-drifting continuum and two type II bursts.
Our analysis shows that if the first type II burst was formed in the low
corona, the burst heights and speed are close to the projected distances and
speed of the Moreton wave (a chromospheric shock wave signature). The
frequency-drifting radio continuum, starting above 1 GHz, was formed almost two
minutes prior to any shock features becoming visible, and a fast-expanding
piston (visible as the continuum) could have launched another shock wave. A
possible scenario is that a flare blast overtook the earlier transient, and
ignited the first type II burst. The second type II burst may have been formed
by the same shock, but only if the shock was propagating at a constant speed.
This interpretation also requires that the shock-producing regions were located
at different parts of the propagating structure, or that the shock was passing
through regions with highly different atmospheric densities. This complex
event, with a multitude of radio features and transients at other wavelengths,
presents evidence for both blast-wave-related and CME-related radio emissions.Comment: 14 pages, 6 figures; Solar Physics Topical Issue, in pres
Transient Magnetic and Doppler Features Related to the White-light Flares in NOAA 10486
Rapidly moving transient features have been detected in magnetic and Doppler
images of super-active region NOAA 10486 during the X17/4B flare of 28 October
2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely
energetic white-light events. The transient features appeared during impulsive
phases of the flares and moved with speeds ranging from 30 to 50 km s.
These features were located near the previously reported compact acoustic
\cite{Donea05} and seismic sources \cite{Zharkova07}. We examine the origin of
these features and their relationship with various aspects of the flares, {\it
viz.}, hard X-ray emission sources and flare kernels observed at different
layers - (i) photosphere (white-light continuum), (ii) chromosphere (H
6563\AA), (iii) temperature minimum region (UV 1600\AA), and (iv) transition
region (UV 284\AA).Comment: 26 pages, 13 figures, 2 tables, accepted for publication in Solar
Physic
A Statistical Study on Photospheric Magnetic Nonpotentiality of Active Regions and Its Relationship with Flares during Solar Cycles 22-23
A statistical study is carried out on the photospheric magnetic
nonpotentiality in solar active regions and its relationship with associated
flares. We select 2173 photospheric vector magnetograms from 1106 active
regions observed by the Solar Magnetic Field Telescope at Huairou Solar
Observing Station, National Astronomical Observatories of China, in the period
of 1988-2008, which covers most of the 22nd and 23rd solar cycles. We have
computed the mean planar magnetic shear angle (\bar{\Delta\phi}), mean shear
angle of the vector magnetic field (\bar{\Delta\psi}), mean absolute vertical
current density (\bar{|J_{z}|}), mean absolute current helicity density
(\bar{|h_{c}|}), absolute twist parameter (|\alpha_{av}|), mean free magnetic
energy density (\bar{\rho_{free}}), effective distance of the longitudinal
magnetic field (d_{E}), and modified effective distance (d_{Em}) of each
photospheric vector magnetogram. Parameters \bar{|h_{c}|}, \bar{\rho_{free}},
and d_{Em} show higher correlation with the evolution of the solar cycle. The
Pearson linear correlation coefficients between these three parameters and the
yearly mean sunspot number are all larger than 0.59. Parameters
\bar{\Delta\phi}, \bar{\Delta\psi}, \bar{|J_{z}|}, |\alpha_{av}|, and d_{E}
show only weak correlations with the solar cycle, though the nonpotentiality
and the complexity of active regions are greater in the activity maximum
periods than in the minimum periods. All of the eight parameters show positive
correlations with the flare productivity of active regions, and the combination
of different nonpotentiality parameters may be effective in predicting the
flaring probability of active regions.Comment: 20 pages, 5 figures, 4 tables, accepted for publication in Solar
Physic
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