373 research outputs found
RHESSI and SDO/AIA observations of the chromospheric and coronal plasma parameters during a solar flare
X-ray and EUV observations are an important diagnostic of various plasma
parameters of the solar atmosphere during solar flares. Soft X-ray and EUV
observations often show coronal sources near the top of flaring loops, while
hard X-ray emission is mostly observed from chromospheric footpoints. Combining
RHESSI with simultaneous SDO/AIA observations, it is possible for the first
time to determine the density, temperature, and emission profile of the solar
atmosphere over a wide range of heights during a flare, using two independent
methods. Here we analyze a near limb event during the first of three hard X-ray
peaks. The emission measure, temperature, and density of the coronal source is
found using soft X-ray RHESSI images while the chromospheric density is
determined using RHESSI visibility analysis of the hard X-ray footpoints. A
regularized inversion technique is applied to AIA images of the flare to find
the differential emission measure (DEM). Using DEM maps we determine the
emission and temperature structure of the loop, as well as the density, and
compare it with RHESSI results. The soft X-ray and hard X-ray sources are
spatially coincident with the top and bottom of the EUV loop, but the bulk of
the EUV emission originates from a region without co-spatial RHESSI emission.
The temperature analysis along the loop indicates that the hottest plasma is
found near the coronal loop top source. The EUV observations suggest that the
density in the loop legs increases with increasing height while the temperature
remains constant within uncertainties.Comment: 23 pages, 6 figures, accepted for publication in Ap
Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare
We present observations of electron energization in magnetic reconnection
outflows during the pre-impulsive phase of solar flare SOL2012-07-19T05:58.
During a time-interval of about 20 minutes, starting 40 minutes before the
onset of the impulsive phase, two X-ray sources were observed in the corona,
one above the presumed reconnection region and one below. For both of these
sources, the mean electron distribution function as a function of time is
determined over an energy range from 0.1~keV up to several tens of keV, for the
first time. This is done by simultaneous forward fitting of X-ray and EUV data.
Imaging spectroscopy with RHESSI provides information on the high-energy tail
of the electron distribution in these sources while EUV images from SDO/AIA are
used to constrain the low specific electron energies. The measured electron
distribution spectrum in the magnetic reconnection outflows is consistent with
a time-evolving kappa-distribution with . The spectral
evolution suggests that electrons are accelerated to progressively higher
energies in the source above the reconnection region, while in the source
below, the spectral shape does not change but an overall increase of the
emission measure is observed, suggesting density increase due to evaporation.
The main mechanisms by which energy is transported away from the source regions
are conduction and free-streaming electrons. The latter dominates by more than
one order of magnitude and is comparable to typical non-thermal energies during
the hard X-ray peak of solar flares, suggesting efficient acceleration even
during this early phase of the event.Comment: 9 pages, 5 figures, accepted for publication in Ap
Observational evidence for return currents in solar flare loops
Context: The common flare scenario comprises an acceleration site in the
corona and particle transport to the chromosphere. Using satellites available
to date it has become possible to distinguish between the two processes of
acceleration and transport, and study the particle propagation in flare loops
in detail, as well as complete comparisons with theoretical predictions.
Aims: We complete a quantitative comparison between flare hard X-ray spectra
observed by RHESSI and theoretical predictions. This enables acceleration to be
distinguished from transport and the nature of transport effects to be
explored.
Methods: Data acquired by the RHESSI satellite were analyzed using full sun
spectroscopy as well as imaging spectroscopy methods. Coronal source and
footpoint spectra of well observed limb events were analyzed and quantitatively
compared to theoretical predictions. New concepts are introduced to existing
models to resolve discrepancies between observations and predictions.
Results: The standard thin-thick target solar flare model cannot explain the
observations of all events. In the events presented here, propagation effects
in the form of non-collisional energy loss are of importance to explain the
observations. We demonstrate that those energy losses can be interpreted in
terms of an electric field in the flare loop. One event seems consistent with
particle propagation or acceleration in lower than average density in the
coronal source.
Conclusions: We find observational evidence for an electric field in flare
loops caused by return currents.Comment: A&A, in pres
The Solar X-ray Limb
We describe a new technique to measure the height of the X-ray limb with
observations from occulted X-ray flare sources as observed by the RHESSI (the
Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has
model dependencies different from those present in traditional observations at
optical wavelengths, which depend upon detailed modeling involving radiative
transfer in a medium with complicated geometry and flows. It thus provides an
independent and more rigorous measurement of the "true" solar radius, meaning
that of the mass distribution. RHESSI's measurement makes use of the flare
X-ray source's spatial Fourier components (the visibilities), which are
sensitive to the presence of the sharp edge at the lower boundary of the
occulted source. We have found a suitable flare event for analysis,
SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique
here. Using a 4-minute integration over the 3-25 keV photon energy range, we
find arcsec, at 1 AU, where
the uncertainties include statistical uncertainties from the method and a
systematic error. The standard VAL-C model predicts a value of 959.94 arcsec,
about 1 below our value.Comment: 12 pages, 5 figures, accepted for publication in Ap
Location of Decimetric Pulsations in Solar Flares
This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nançay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30 - 240Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal sourc
Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?
Before the onset of a flare is observed in hard X-rays there is often a pro- longed pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduc- tion, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations
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