The Low-Energy Cutoff to the Nonthermal Electron Spectrum Determined from RHESSI Solar Flare Observations

Details The Low-Energy Cutoff to the Nonthermal Electron Spectrum Determined from RHESSI Solar Flare Observations The Low-Energy Cutoff to the Nonthermal Electron Spectrum Determined from RHESSI Solar Flare Observations

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmsp41c..03s&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #SP41C-03

Astronomy and Astrophysics

Astrophysics

7500 Solar Physics, Astrophysics, And Astronomy, 7514 Energetic Particles (2114), 7519 Flares, 7554 X Rays, Gamma Rays, And Neutrinos

Scientific paper

Since the spectrum of electrons accelerated during a solar flare is a steeply falling function, most of the energy in these electrons resides with the lower energy particles. Thus, it is most important to determine the low energy end of the spectrum, particularly the low energy cutoff (or flattening from the steep power-law spectrum seen at higher energies) that must surely exist to keep the total energy finite. The determination of such a cutoff to the electron spectrum is decisive for the estimation of the total energy in nonthermal electrons at any given time during a flare. Unfortunately, thermal bremsstrahlung usually dominates the low-energy part of flare X-ray emission so that the flattening of the nonthermal X-ray spectrum that such a cutoff would produce at low energies has never been unequivocally identified. An M1.0 flare observed with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 2002 June 2 had a particularly favorable time history for the detection of a spectral flattening at low X-ray energies. Two impulsive X-ray peaks occurred within 1 minute of the soft X-ray start of the flare. They were noticeable in the time histories over a broad energy range down to RHESSI's 3 keV lower limit. Both peaks showed a spectral flattening at energies below 20 to 30 keV, as expected for a low-energy cutoff to the distribution of nonthermal electrons. The deduced cutoff electron energy varies from ~20 to as high as 38 keV during the 30 seconds that spectral flattening is evident. We will discuss the implications of these results on the flare energetics and on particle acceleration and transport models.

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