ON THE CORRELATION BETWEEN SPECTRA OF SOLAR MICROWAVE BURSTS

AND SPECTRA OF PROTON FLUXES NEAR EARTH

I. M. Chertok, V. V. Grechnev and N. S. Meshalkina

Studies of an extreme solar event of 20 January 2005 strengthened the contest over of a long-standing question whether solar energetic particles (SEPs) arriving at Earth were accelerated in flares or in shocks ahead of coronal mass ejections (CMEs). Herewith, a question is important about a probable relation between the energy spectrum of SEP and parameters characterizing the frequency spectrum of flare microwave bursts. Several papers, which considered previous solar cycles, showed such a relation to exist, in particular, for protons of tens of MeV energy. Our paper presents results of analysis of this relation using data covering the period of 1987–2008. We have found a significant correlation between an index δ, which is equivalent to the power-law index of the integral energy spectrum of protons registered near the Earth orbit in a range of 10–100 MeV, on the one hand, and parameters of microwave bursts such as a ratio of peak fluxes at two frequencies (e.g., 9 and 15 GHz) and a microwave peak frequency fm as well. Flares with hard frequency spectra (S₉/S₁₅£1, fm ³ 15 GHz) correspond to proton fluxes with hard (flat) energy spectra (δ£1.5). Accordingly, flares with soft radio spectra (S₉/S₁₅³1.5, fm £ 5 GHz) correspond to proton fluxes with soft (steep) energy spectra (δ ³ 1.5-2). We have also shown that powerful high-frequency bursts with hardest radio spectra (fm » 30 GHz) can point at acceleration of significant proton fluxes in flares occurring in strong magnetic fields. These results provide an important argument in favor of acceleration of solar energetic particles (at least, during the initial pulse of a SEP event) mainly in impulsive and post-eruptive energy release in flares rather than in CME-driven shocks.