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R.J. Murphy, G.H. Share (NRL)
The {\em Compton Gamma Ray Observatory}/OSSE experiment observed the impulsive X1.8, 2B flare from AR 8910 (N16W82). The fluxes observed at energies from 0.05 MeV to >16 MeV rose sharply to their maxima within ~35 s; the times of maxima coincided to \leq8 s over the full range of energies. The widths of the peaks became narrower with increasing energy; e.g. for energies >16 MeV most of the emission occurred within 8 s. During that short interval OSSE detected photons up to ~40 MeV. The spectrum accumulated over the two minute period from 07:45:06 to 07:47:26 UT was featureless and could be fitted with a continuum spectrum that hardened with energy. Specifically, we found that a broken power-law that hardened from a spectral index of ~-3.65 to ~-2.9 at ~0.255 MeV fit the data <1 MeV; an additional harder power law with index of ~-1.6 was needed to fit the data >1 MeV. The power law index below the break energy hardened from ~-4.0 to -3.2 over the first 100s of the flare covering the peak. Kiplinger (1995 ApJ, 453, 973) suggested that such progressive hardening predicts the occurrence of solar energetic proton events. We find no evidence for energetic particles following this flare in the GOES and ACE data. We also find no evidence for the emission of nuclear lines that are the signature of accelerated protons impacting the solar atmosphere. Thus this flare appears to fit into the 'electron-dominated' category (Rieger, {\em et al.} 1998, Solar Physics, 183, 123). We set upper limits on fluxes in the 2.223 MeV neutron capture and 4.44 MeV 12C lines from the flare. We then use these limits and the flux in the continuum, assumed to be due to electron bremsstrahlung, to determine the relative energies in accelerated electrons and ions. This work was supported by NASA DPR W19,343.
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The author(s) of this abstract have provided an email address for comments about the abstract: murphy@gamma.nrl.navy.mil