Low-energy cutoffs in electron spectra of solar flares: statistical survey

Abstract

The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base (February 2002 -- May 2006) has been searched to find solar flares with weak thermal components and flat photon spectra. Using a regularised inversion technique, we determine the mean electron flux distribution from count spectra of a selection of events with flat photon spectra in the 15--20 keV energy range. Such spectral behaviour is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in a given energy range, e.g. a low-energy cutoff in the mean electron spectra of non-themal particles. We have found 18 cases which exhibit a statistically significant local minimum (a dip) in the range of 10--20 keV. The positions and spectral indices of events with low-energy cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo correction was applied, all low-energy cutoffs in the mean electron spectrum were removed and hence the low energy cutoffs in the mean electron spectrum of solar flares above $\sim$12 keV cannot be viewed as real features in the electron spectrum. If low-energy cutoffs exist in the mean electron spectra, the energy of low energy cutoffs should be less than $\sim$12 keV.

9 pages, 5 figures, submitted to Solar Physics

References in corpus (4)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft
• Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra
• Broadband microwave burst produced by electron beams

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• Formation Of The Lyman Continuum During Solar Flares
• Growth Rates of the Upper-hybrid Waves for Power-law and Kappa Distributions with a Loss-cone Anisotropy
• HEL1OS -- A Hard X-ray Spectrometer on Board Aditya-L1