315 research outputs found
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
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
Magneto--Acoustic Energetics Study of the Seismically Active Flare of 15 February 2011
Multi--wavelength studies of energetic solar flares with seismic emissions
have revealed interesting common features between them. We studied the first
GOES X--class flare of the 24th solar cycle, as detected by the Solar Dynamics
Observatory (SDO). For context, seismic activity from this flare
(SOL2011-02-15T01:55-X2.2, in NOAA AR 11158) has been reported in the
literature (Kosovichev, 2011; Zharkov et al., 2011). Based on Dopplergram data
from the Helioseismic and Magnetic Imager (HMI), we applied standard methods of
local helioseismology in order to identify the seismic sources in this event.
RHESSI hard X-ray data are used to check the correlation between the location
of the seismic sources and the particle precipitation sites in during the
flare. Using HMI magnetogram data, the temporal profile of fluctuations in the
photospheric line-of-sight magnetic field is used to estimate the magnetic
field change in the region where the seismic signal was observed. This leads to
an estimate of the work done by the Lorentz-force transient on the photosphere
of the source region. In this instance this is found to be a significant
fraction of the acoustic energy in the attendant seismic emission, suggesting
that Lorentz forces can contribute significantly to the generation of
sunquakes. However, there are regions in which the signature of the
Lorentz-force is much stronger, but from which no significant acoustic emission
emanates.Comment: Submitted to Solar Physic
A statistical correlation of sunquakes based on their seismic and white-light emission
Several mechanisms have been proposed to explain the transient seismic emission, i.e. “sunquakes,” from some solar flares. Some theories associate high-energy electrons and/or white-light emission with sunquakes. High-energy charged particles and their subsequent heating of the photosphere and/or chromosphere could induce acoustic waves in the solar interior. We carried out a correlative study of solar flares with emission in hard X-rays, enhanced continuum emission at 6173 Å, and transient seismic emission. We selected those flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) with a considerable flux above 50 keV between 1 January 2010 and 26 June 2014. We then used data from the Helioseismic and Magnetic Imager onboard the Solar Dynamic Observatory to search for excess visible-continuum emission and new sunquakes not previously reported. We found a total of 18 sunquakes out of 75 flares investigated. All of the sunquakes were associated with an enhancement of the visible continuum during the flare. Finally, we calculated a coefficient of correlation for a set of dichotomic variables related to these observations. We found a strong correlation between two of the standard helioseismic detection techniques, and between sunquakes and visible-continuum enhancements. We discuss the phenomenological connectivity between these physical quantities and the observational difficulties of detecting seismic signals and excess continuum radiation
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 spatio-temporal description of the abrupt changes in the photospheric magnetic and Lorentz-force vectors during the 2011 February 15 X2.2 flare
The active region NOAA 11158 produced the first X-class flare of Solar Cycle
24, an X2.2 flare at 01:44 UT on 2011 February 15. Here we analyze SDO/HMI
magnetograms covering a 12-hour interval centered at the time of this flare. We
describe the spatial distributions of the photospheric magnetic changes
associated with this flare, including the abrupt changes in the field vector,
vertical electric current and Lorentz force vector. We also trace these
parameters' temporal evolution. The abrupt magnetic changes were concentrated
near the neutral line and in two neighboring sunspots. Near the neutral line,
the field vectors became stronger and more horizontal during the flare and the
shear increased. This was due to an increase in strength of the horizontal
field components near the neutral line, most significant in the horizontal
component parallel to the neutral line but the perpendicular component also
increased in strength. The vertical component did not show a significant,
permanent overall change at the neutral line. The increase in total flux at the
neutral line was accompanied by a compensating flux decrease in the surrounding
volume. In the two sunspots near the neutral line the azimuthal flux abruptly
decreased during the flare but this change was permanent in only one of the
spots. There was a large, abrupt, downward vertical Lorentz force change during
the flare, consistent with results of past analyses and recent theoretical
work. The horizontal Lorentz force acted in opposite directions along each side
of neutral line, with the two sunspots at each end subject to abrupt torsional
forces. The shearing forces were consistent with field contraction and decrease
of shear near the neutral line, whereas the field itself became more sheared as
a result of the flux collapsing towards the neutral line from the surrounding
volume.Comment: DOI 10.1007/s11207-012-0071-0. Accepted for publication in Solar
Physics SDO3 Topical Issue. Some graphics missing due to 15MB limi
Density of states in d-wave superconductors disordered by extended impurities
The low-energy quasiparticle states of a disordered d-wave superconductor are
investigated theoretically. A class of such states, formed via tunneling
between the Andreev bound states that are localized around extended impurities
(and result from scattering between pair-potential lobes that differ in sign)
is identified. Its (divergent) contribution to the total density of states is
determined by taking advantage of connections with certain one-dimensional
random tight-binding models. The states under discussion should be
distinguished from those associated with nodes in the pair potential.Comment: 5 pages, 1 figur
Impurity induced resonant state in a pseudogap state of a high temperature superconductor
We predict a resonance impurity state generated by the substitution of one Cu
atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c
superconductor. The precise microscopic origin of the pseudogap is not
important for this state to be formed, in particular this resonance will be
present even in the absence of superconducting fluctuations in the normal
state. In the presence of superconducting fluctuations, we predict the
existence of a counterpart impurity peak on a symmetric bias.
The nature of impurity resonance is similar to the previously studied
resonance in the d-wave superconducting state.Comment: 4 pages, 2 figure
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
From GHz to mHz: A Multiwavelength Study of the Acoustically Active 14 August 2004 M7.4 Solar Flare
We carried out an electromagnetic acoustic analysis of the solar flare of 14
August 2004 in active region AR10656 from the radio to the hard X-ray spectrum.
The flare was a GOES soft X-ray class M7.4 and produced a detectable sun quake,
confirming earlier inferences that relatively low-energy flares may be able to
generate sun quakes. We introduce the hypothesis that the seismicity of the
active region is closely related to the heights of coronal magnetic loops that
conduct high-energy particles from the flare. In the case of relatively short
magnetic loops, chromospheric evaporation populates the loop interior with
ionized gas relatively rapidly, expediting the scattering of remaining trapped
high-energy electrons into the magnetic loss cone and their rapid precipitation
into the chromosphere. This increases both the intensity and suddenness of the
chromospheric heating, satisfying the basic conditions for an acoustic emission
that penetrates into the solar interior.Comment: Accepted in Solar Physic
- …