28 research outputs found
Direct observation of the energy release site in a solar flare by SDO/AIA, Hinode/EIS and RHESSI
We present direct evidence for the detection of the main energy release site
in a non-eruptive solar flare, SOL2013-11-09T06:38UT. This GOES C2.7 event was
characterised by two flaring ribbons and a compact, bright coronal source
located between them, which is the focus of our study. We use imaging from
SDO/AIA, and imaging spectroscopy from RHESSI to characterise the thermal and
non-thermal emission from the coronal source, and EUV spectroscopy from the
Hinode/EIS, which scanned the coronal source during the impulsive peak, to
analyse Doppler shifts in Fe XII and Fe XXIV emission lines, and determine the
source density. The coronal source exhibited an impulsive emission lightcurve
in all AIA filters during the impulsive phase. RHESSI hard X-ray images
indicate both thermal and non-thermal emission at the coronal source, and its
plasma temperature derived from RHESSI imaging spectroscopy shows an impulsive
rise, reaching a maximum at 12-13 MK about 10 seconds prior to the hard X-ray
peak. High redshifts associated with this bright source indicate downflows of
40-250 km/s at a broad range of temperatures, interpreted as loop shrinkage
and/or outflows along the magnetic field. Outflows from the coronal source
towards each ribbon are also observed by AIA images at 171, 193, 211, 304 and
1600 A. The electron density of the source obtained from a Fe XIV line pair is
which is collisionally thick to electrons with energy up to 45-65
keV, responsible for the source's non-thermal X-ray emission. We conclude that
the bright coronal source is the location of the main release of magnetic
energy in this flare, with a geometry consistent with component reconnection
between crossing, current-carrying loops. We argue that the energy that can be
released via reconnection, based on observational estimates, can plausibly
account for the non-thermal energetics of the flare.Comment: 10 pages, 7 figure
Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares
The extreme ultraviolet portion of the solar spectrum contains a wealth of
diagnostic tools for probing the lower solar atmosphere in response to an
injection of energy, particularly during the impulsive phase of solar flares.
These include temperature and density sensitive line ratios, Doppler shifted
emission lines and nonthermal broadening, abundance measurements, differential
emission measure profiles, and continuum temperatures and energetics, among
others. In this paper I shall review some of the advances made in recent years
using these techniques, focusing primarily on studies that have utilized data
from Hinode/EIS and SDO/EVE, while also providing some historical background
and a summary of future spectroscopic instrumentation.Comment: 34 pages, 8 figures. Submitted to Solar Physics as part of the
Topical Issue on Solar and Stellar Flare
Microflares and the Statistics of X-ray Flares
This review surveys the statistics of solar X-ray flares, emphasising the new
views that RHESSI has given us of the weaker events (the microflares). The new
data reveal that these microflares strongly resemble more energetic events in
most respects; they occur solely within active regions and exhibit
high-temperature/nonthermal emissions in approximately the same proportion as
major events. We discuss the distributions of flare parameters (e.g., peak
flux) and how these parameters correlate, for instance via the Neupert effect.
We also highlight the systematic biases involved in intercomparing data
representing many decades of event magnitude. The intermittency of the
flare/microflare occurrence, both in space and in time, argues that these
discrete events do not explain general coronal heating, either in active
regions or in the quiet Sun.Comment: To be published in Space Science Reviews (2011
An Observational Overview of Solar Flares
We present an overview of solar flares and associated phenomena, drawing upon
a wide range of observational data primarily from the RHESSI era. Following an
introductory discussion and overview of the status of observational
capabilities, the article is split into topical sections which deal with
different areas of flare phenomena (footpoints and ribbons, coronal sources,
relationship to coronal mass ejections) and their interconnections. We also
discuss flare soft X-ray spectroscopy and the energetics of the process. The
emphasis is to describe the observations from multiple points of view, while
bearing in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is far from
complete, so we conclude with a brief discussion of models, and a list of
missing but important observations.Comment: This is an 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
Evidence for magnetic flux cancelation leading to an ejective solar eruption observed by
Aims. We study the onset of a solar eruption involving a filament ejection on 2007 May 20.
Methods. We observe the filament in Hα images from Hinode/SOT and in EUV with TRACE and
STEREO/SECCHI/EUVI. Hinode/XRT images are used to study the eruption in soft X-rays. From spectroscopic data taken with Hinode/EIS we obtain bulk-flow velocities, line profiles, and plasma densities in the onset region. The magnetic field evolution was observed in SoHO/MDI magnetograms.
Results. We observed a converging motion between two opposite polarity sunspots that form the primary magnetic polarity inversion line (PIL), along which resides filament material before eruption. Positive-flux magnetic elements, perhaps moving magnetic features (MMFs) flowing from the spot region, appear north of the spots, and the eruption onset occurs where these features cancel repeatedly in a negative-polarity region north of the sunspots. An ejection of material observed in Hα and EUV marks the start of the filament eruption (its “fast-rise”). The start of the ejection is accompanied by a sudden brightening across the PIL at the jet's base, observed in both broad-band images and in EIS. Small-scale transient brightenings covering a wide temperature range (Log Te = 4.8-6.3) are also observed in the onset region prior to eruption. The preflare transient brightenings are characterized by sudden, localized density enhancements
(to above Log ne [ cm-3] = 9.75, in \ion{Fe}{xiii}) that appear along the PIL during a time when pre-flare brightenings were occurring. The measured densities in the eruption onset region outside the times of those enhancements decrease with temperature. Persistent downflows (red-shifts)
and line-broadening (\ion{Fe}{xii}) are present along the PIL.
Conclusions. The array of observations is consistent with the pre-eruption sheared-core magnetic field being gradually destabilized by evolutionary tether-cutting flux cancelation that was driven by converging photospheric flows, and the main filament ejection being triggered by flux cancelation between the positive flux elements and the surrounding negative field. A definitive statement however on the eruption's ultimate cause would require comparison with simulations, or additional detailed
observations of other eruptions occurring in similar magnetic circumstances
Nonthermal and thermal diagnostics of a solar flare observed with RESIK and RHESSI
Aims. We aim to prove and diagnose the occurrence of nonthermal electron distributions in solar flare plasma using X-ray spectral observations.
Methods. An M4.9 flare on 2003 January 7/8 was observed with the RESIK instrument in the 3-6 Å wavelength range (2-4 keV) and with RHESSI at energies above 6 keV. The temporal
behavior of RESIK flare spectra has been analyzed for two different
types of velocity distributions – a thermal (Maxwellian) distribution and a nonthermal plasma distribution of free electrons. The \ion{Si}{xiv}, \ion{Si}{xiii}, and \ion{Si}{xii}d satellite
lines observed with RESIK in the 5-6 Å range were used to determine the degree of deviation from Maxwellian, and the equivalent non-Maxwellian pseudo-temperature, τ. The
diagnostics presented are sensitive to the shape of the distribution in the energy range where the maximum of the electron distribution occurs (where the bulk of electrons reside) and does not include the influence of the shape of the high-energy tail of the distribution.
Under the assumption of a Maxwellian distribution of electron velocities, the plasma temperature was determined from an emission measure (EM) loci analysis and a differential emission measure (DEM)
analysis of RESIK spectra. The high-energy end of the flare radiative emission was investigated through RHESSI spectral analysis.
Results. The nonthermal analysis of RESIK spectra has shown that the largest deviations of the plasma electron distribution from Maxwellian appeared during the impulsive phase of the flare. The
decay phase spectra had an almost isothermal character. The pseudo-temperature, τ, reached its maximum around the peak time of the soft and hard X-ray fluxes. The temporal behavior of the
temperatures derived from the thermal analysis was similar to the
behavior of the nonthermal pseudo-temperature. The values of the pseudo-temperature were consistent with the temperatures obtained in both thermal analyses, but lower than the temperatures derived from the slope of the RHESSI continua. In comparison with the synthetic isothermal or multithermal spectra, the nonthermal synthetic spectra fitted the observed \ion{Si}{xii}d satellite lines much more closely (the error is less than 10%). The fluxes in the Si XIId satellite lines in isothermal or multithermal spectra have been underestimated by a factor of three or more in comparison to the observed fluxes. The value of this factor varies with time and
it is different for the different satellite lines.
Conclusions. Evidence was found for considerable deviations of the distribution of free
electrons from Maxwellian in the plasma during a solar flare. These occurred mainly during the flare impulsive phase and can be diagnosed using existing X-ray spectral observations