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RHESSI Images and spectra of two small flares  

Ambretta Falchi   Submitted: 2005-09-12 02:27

We studied the evolution of two small flares (GOES class C2 and C1) that developed in the same active region with different morphological characteristics: one is extended and the other is compact. We analyzed the accuracy and the consistency of different algorithms implemented in RHESSI software to reconstruct the image of the emitting sources, for energies between 3 and 12 keV. We found that all tested algorithms give consistent results for the peak position while the other parameters can differ at most by a factor 2. Pixon and Forward-fit generally converge to similar results but Pixon is more reliable for reconstructing a complex source. We investigated the spectral characteristics of the two flares during their evolution in the 3~-~25 keV energy band. We found that a single thermal model of the photon spectrum is inadequate to fit the observations and we needed to add either a non-thermal model or a hot thermal one. The non-thermal and the double thermal fits are comparable. If we assume a non-thermal model, the non-thermal energy is always higher than the thermal one. Only during the very final decay phase a single thermal model fits fairly well the observed spectrum.

Authors: L. Maltagliati, A. Falchi, L. Teriaca
Projects: RHESSI

Publication Status: Solar Physics (in press)
Last Modified: 2005-09-12 02:27
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SOHO/CDS and Ground Based Observations of a Two-Ribbon Flare:Spatially Resolved Signatures of Chromospheric Evaporation  

Ambretta Falchi   Submitted: 2003-01-20 06:46

During a coordinated observing campaign (SOHO JOP 139), we obtained simultaneous spectroheliograms of a solar active region in several spectral lines, sampling levels from the chromosphere to the corona. Ground based spectroheliograms were acquired at the Dunn Solar Tower of NSO/Sacramento Peak in four chromospheric lines, while the Coronal Diagnostic Spectrograph (CDS) aboard SOHO was used to obtain rasters of the active region in transition region (TR) and coronal lines. Such a complete dataset allowed us to compare the development of intensity and velocity fields during a small two-ribbon flare in the whole atmosphere. In particular, we obtained for the first time quasi-simultaneous and spatially resolved observations of velocity fields during the impulsive phase of a flare, both in chromosphere and upper atmosphere. In this phase, strong downflows (up to 40 km s-1) following the shape of the developing ribbons are measured at chromospheric levels, while strong upward motions are instead measured in TR (up to -100 km s-1) and coronal lines (-160 km s-1). The spatial pattern of these velocities have a common area about 10 arcsec wide. This is the first time that opposite directed flows at different atmospheric levels are observed in the same spatial location during a flare. These signatures are highly suggestive of the chromospheric evaporation scenario predicted in theoretical models of flares.

Authors: Teriaca, L., Falchi, A., Cauzzi, G., Falciani, R., Smaldone, L.A. and Andretta, V.
Projects:

Publication Status: Accepted by ApJ
Last Modified: 2003-01-20 06:46
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Chromospheric Models of a Solar Flare including Velocity Fields  

Ambretta Falchi   Submitted: 2002-05-02 02:03

We study the chromospheric structure of a small flare, before, during and after the first hard X-ray spike. We construct 5 semiempirical models for different times, which will reproduce the profiles of the H_delta, Ca II K and Si I 3905 lines during the flare evolution. In order to reproduce the intensity and the main characteristics of the line asymmetry we introduce the velocity fields in the profile calculations. We find that the whole chromosphere undergoes a strong upward motion within 1 s of the maximum of the first hard X-ray spike, when the temperature and the density begin to rapidly increase. This seems the first chromospheric response to the energy deposition and/or release. Only 6 s after the hard X-ray peak, a strong downflow begins in the low chromosphere and its velocity continues to increase even during the cooling phase.

Authors: Falchi, A. and Mauas P.J.D.
Projects:

Publication Status: Accepted by A&A
Last Modified: 2002-05-02 02:03
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Abstracts by Author
RHESSI Images and spectra of two small flares
SOHO/CDS and Ground Based Observations of a Two-Ribbon Flare:Spatially Resolved Signatures of Chromospheric Evaporation
Chromospheric Models of a Solar Flare including Velocity Fields

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MSU Solar Physics.
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