Observations of gamma-ray lines from solar flares by SMM demonstrated that energetic protons and heavy ions are accelerated during the impulsive phase.
In order to understand the acceleration mechanism for gamma-ray producing protons and heavy ions, we have studied the characteristics of the flares from which gamma-ray lines were observed by SMM. In order to identify the characteristics unique to the gamma-ray line flares, we have also studied intense hard X-ray flares with no gamma-ray line emissions. We have found the following characteristics:
i) Most of the gamma-ray line flares produced intense radio bursts of types II and IV. 2) For most of the gamma-ray line flares, the time profiles of high-energy (2 300 keV) hard X-rays are delayed by order of several seconds with respect to those of lowenergy hard X-rays. The delay times seem to be correlated with the spatial sizes of the flares. 3) In Ha importance, the gamma-ray line flares range from sub-flares to importance-3 flares.
- The hard Xray spectra of the gamma-ray line flares are generally flatter (harder) than those of flares with no gamma-ray line emission.
From these characteristics, we conclude that the first-order Fermi acceleration operating in a flare loop is likely to be the acceleration mechanism for energetic protons and heavy ions as well as relativistic electrons.