Issue |
A&A
Volume 446, Number 3, February II 2006
|
|
---|---|---|
Page(s) | 1157 - 1163 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20053672 | |
Published online | 20 January 2006 |
Compton backscattered and primary X-rays from solar flares: angle dependent Green's function correction for photospheric albedo
1
Department of Physics and Astronomy, University of Glasgow, G12 8QQ, UK e-mail: [eduard;john]@astro.gla.ac.uk
2
DACE, University of Glasgow, G3 6NH, UK e-mail: alec@astro.gla.ac.uk
3
SSAI, Science Systems & Applications, Inc., Code 612.1, NASA/GSFC, Greenbelt, MD 20771, USA e-mail: richard.schwartz@gsfc.nasa.gov
Received:
21
June
2005
Accepted:
30
September
2005
The observed hard X-ray (HXR) flux spectrum from
solar flares is a combination of primary bremsstrahlung photons
with a spectrally modified component from
photospheric Compton backscatter of downward primary emission. The
latter can be significant, distorting or hiding the true features
of the primary spectrum which are key diagnostics for acceleration
and propagation of high energy electrons and of their energy
budget. For the first time in solar physics, we use a Green's
function approach to the backscatter spectral deconvolution
problem, constructing a Green's matrix including photoelectric
absorption. This approach allows spectrum-independent extraction
of the primary spectrum for several HXR flares observed by the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We
show that the observed and primary spectra differ very
substantially for flares with hard spectra close to the disk
centre. We show in particular that the energy dependent photon
spectral index
is
very different for
and for
and that
inferred mean source electron spectra
differ
greatly. Even for a forward fitting of a parametric
to the data, a clear low-energy cutoff required to fit
essentially disappears when the fit is to
– i.e. when albedo correction is included. The
self-consistent correction for backscattered photons is thus shown
to be crucial in determining the energy spectra of flare
accelerated electrons, and hence their total number and energy.
Key words: Sun: flares / X-rays: general / methods: data analysis / techniques: spectroscopic / scattering
© ESO, 2006
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