<i>RHESSI</i> OBSERVATIONS OF THE PROPORTIONAL ACCELERATION OF RELATIVISTIC &gt;0.3 MeV ELECTRONS AND &gt;30 MeV PROTONS IN SOLAR FLARES

Abstract

We analyze all RHESSI measurements from 2002 to 2005 (29 flare events) of the 2.223 MeV neutron-capture γ-ray line and >0.3 MeV electron bremsstrahlung continuum emissions, produced by >30 MeV accelerated protons (depending on assumptions) and >0.3 MeV accelerated electrons, respectively. We find a close proportionality between the two emissions over >3 orders of magnitude in fluence, from the largest flares down to the limits of detectability. This implies that the processes in flares that accelerate electrons above 0.3 MeV and protons above 30 MeV are closely related, and that the relative acceleration of these two populations is roughly independent of flare size. We find an overall weak correlation between the 2.223 MeV fluence and the peak GOES 1–8 Å soft X-ray (SXR) flux, but with a close proportionality for flares with 2.223 MeV fluence above a threshold of 50 ph cm−2 (equivalent to ∼2 × 1031 protons >30 MeV). Below this threshold the flares usually have large (M-class or higher) but generally uncorrelated excess SXR emission. Thus, above this threshold it appears that flares reach a maximum efficiency for >30 MeV proton and relativistic (>0.3 MeV) electron acceleration, with proportionate amounts of energy going to flare SXR thermal emission and to >50 keV electrons. Finally, we find that the electron-to-proton ratios—Je(0.5 MeV)/Jp (10 MeV)—in these flares, obtained from the γ-ray observations, are about 2 orders of magnitude larger than the ratios in gradual solar energetic particle (SEP) events, but are comparable with ratios in impulsive SEP events.