Multiple electron acceleration instances during a series of solar |
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Marina Battaglia Submitted: 2021-09-28 07:10
Even small solar flares can display a surprising level of complexity
regarding their morphology and temporal evolution. Many of their properties,
such as energy release and electron acceleration can be studied using highly
complementary observations at X-ray and radio wavelengths. We present X-ray
observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) and radio observations from the Karl G. Jansky Very Large Array (VLA)
of a series of GOES A3.4 to B1.6 class flares observed on 2013 April 23. The
flares, as seen in X-ray and extreme ultraviolet (EUV), originated from
multiple locations within active region NOAA 11726. A veritable zoo of
different radio emissions between 1 GHz and 2 GHz was observed co-temporally
with the X-ray flares. In addition to broad-band continuum emission, broad-band
short-lived bursts and narrow-band spikes, indicative of accelerated electrons,
were observed. However, these sources were located up to 150 arcsec away from
the flaring X-ray sources but only some of these emissions could be explained
as signatures of electrons that were accelerated near the main flare site. For
other sources, no obvious magnetic connection to the main flare site could be
found. These emissions likely originate from secondary acceleration sites
triggered by the flare, but may be due to reconnection and acceleration
completely unrelated to the co-temporally observed flare. Thanks to the
extremely high sensitivity of the VLA, not achieved with current X-ray
instrumentation, it is shown that particle acceleration happens frequently and
at multiple locations within a flaring active region.
Authors: Marina Battaglia, Rohit Sharma, Yingjie Luo, Bin Chen, Sijie Yu, Säm Krucker
Projects: RHESSI,Very Large Array (VLA)
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Publication Status: accepted for publication in ApJ
Last Modified: 2021-09-29 08:54
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Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare |
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Marina Battaglia Submitted: 2019-01-30 23:58
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 κ=3.5-5.5. 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.
Authors: Marina Battaglia, Eduard P. Kontar, Galina Motorina
Projects: RHESSI,SDO-AIA
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Publication Status: accepted for publication in ApJ
Last Modified: 2019-02-04 19:19
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The Solar X-ray Limb |
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Marina Battaglia Submitted: 2017-06-01 01:03
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 RX-ray=964.05 ? 0.15-0.29 arcsec, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 963.48 arcsec, about 2σ below our value.
Authors: M. Battaglia, H. S. Hudson, G. J. Hurford, S. Krucker, R. A. Schwartz
Projects: RHESSI
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Publication Status: ApJ, accepted
Last Modified: 2017-06-01 10:21
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Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop? |
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Marina Battaglia Submitted: 2014-05-20 01:33
Before the onset of a flare is observed in hard X-rays there is often a prolonged 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 conduction, 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.
Authors: Marina Battaglia, Lyndsay Fletcher, Paulo J. A. Simões
Projects: Nobeyama Radioheliograph,RHESSI,SDO-AIA
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Publication Status: ApJ, accepted
Last Modified: 2014-05-21 13:39
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RHESSI and SDO/AIA observations of the chromospheric and coronal plasma parameters during a solar flare |
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Marina Battaglia Submitted: 2012-10-15 01:51
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.
Authors: Marina Battaglia, Eduard P. Kontar
Projects: RHESSI,SDO-AIA
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Publication Status: ApJ, accepted
Last Modified: 2012-10-15 12:44
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Numerical simulations of chromospheric hard X-ray source sizes in solar flares |
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Marina Battaglia Submitted: 2012-04-06 04:29
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>1011
cm-3) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.
Authors: M. Battaglia, E. P. Kontar, L. Fletcher, A. L. MacKinnon
Projects: RHESSI
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Publication Status: ApJ, accepted
Last Modified: 2012-04-09 08:27
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Height structure of X-ray, EUV and white-light emission in a solar flare |
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Marina Battaglia Submitted: 2011-07-21 03:56
Context. The bulk of solar flare emission originates from very compact sources located in the lower solar atmosphere and observable at a broad range of wavelengths such as near optical, UV, EUV, soft and hard X-rays, and gamma-rays. Nevertheless, very few spatially resolved imaging observations have been performed to determine the structure of these compact regions. Aims: We investigate the above-the-photosphere heights of hard X-ray (HXR), EUV, and white-light (6173 Å) continuum sources in the low atmosphere and the corresponding densities at these heights. By considering the collisional transport of solar energetic electrons, we also determine where and how much energy is deposited and compare these values with the emissions observed in HXR, EUV, and the continuum. Methods: Simultaneous EUV/continuum images from AIA/HMI on-board SDO and HXR RHESSI images are compared to study a well-observed gamma-ray limb flare. Using RHESSI X-ray visibilities, we determine the height of the HXR sources as a function of energy above the photosphere. Co-aligning AIA/SDO and HMI/SDO images with RHESSI, we infer, for the first time, the heights and characteristic densities of HXR, EUV, and continuum (white-light) sources in the flaring footpoint of the magnetic loop. Results: We find 35-100 keV HXR sources at heights of between 1.7 and 0.8 Mm above the photosphere, below the 6173 Å continuum emission that appears at heights 1.5-3 Mm and the peak of EUV emission originating near 3 Mm. Conclusions: The EUV emission locations are consistent with energy deposition from low energy electrons of ~12 keV occurring in the top layers of the fully ionized chromosphere/low corona and not by ≳ 20 keV electrons that produce HXR footpoints in the lower neutral chromosphere. The maximum of white-light continuum emission appears between the HXR and EUV emission, presumably in the transition between ionized and neutral atmospheres, implying that it consists of free-bound and free-free continuum emission. We note that the energy deposited by low energy electrons is sufficient to explain the energetics of both the optical and UV emissions. Two movies are available in electronic form at http://www.aanda.org
Authors: Marina Battaglia, Eduard P. Kontar
Projects: RHESSI,SDO-AIA,SDO-HMI
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Publication Status: Astronomy & Astrophysics Letters, accepted
Last Modified: 2011-07-21 08:42
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Observations of conduction driven evaporation in the early rise phase of solar flares |
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Marina Battaglia Submitted: 2009-03-16 07:19
The classical flare picture features a beam of electrons, which were accelerated in a site in the corona, hitting the chromosphere. The electrons are stopped in the dense chromospheric plasma, emitting bremsstrahlung in hard X-rays. The ambient material is heated by the deposited energy and expands into the magnetic flare loops, a process termed chromospheric evaporation. In this view hard X-ray emission from the chromosphere is succeeded by soft-X-ray emission from the hot plasma in the flare loop, the soft X-ray emission being a direct consequence of the impact of the non-thermal particle beam. However, observations of events exist in which a pronounced increase in soft X-ray emission is observed minutes before the onset of the hard X-ray emission. Such pre-flare emission clearly contradicts the classical flare picture.
For the first time, the pre-flare phase of such solar flares is studied in detail. The aim is to understand the early rise phase of these events. We want to explain the time evolution of the observed emission by means of alternative energy transport mechanisms such as heat conduction.
RHESSI events displaying pronounced pre-flare emission were analyzed in imaging and spectroscopy. The time evolution of images and full sun spectra was investigated and compared to the theoretical expectations from conduction driven chromospheric evaporation.
The pre-flare phase is characterized by purely thermal emission from a coronal source with increasing emission measure and density. After this earliest phase, a small non-thermal tail to higher energies appears in the spectra, becoming more and more pronounced. However, images still only display one X-ray source, implying that this non-thermal emission is coronal.
The increase of emission measure and density indicates that material is added to the coronal region. The most plausible origin is evaporated material from the chromosphere. Energy provided by a heat flux is capable of driving chromospheric evaporation. We show that the often used classical Spitzer treatment of the conductive flux is not applicable. The conductive flux is saturated. During the preflare-phase, the temperature of the coronal source remains constant or increases. Continuous heating in the corona is necessary to explain this observation.
The observations of the pre-flare phase of four solar flares are consistent with chromospheric evaporation driven by a saturated heat flux. Additionally, continuous heating in the corona is necessary to sustain the observed temperature.
Authors: Battaglia, M., Fletcher, L., Benz, A. O.
Projects: RHESSI
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Publication Status: A&A, accepted
Last Modified: 2009-03-16 11:54
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