Characteristics of gamma-ray line flares

Observations of solar ..gamma..-rays by the Solar Maximum Mission (SMM) have demonstrated that energetic protons and ions are rapidly accelerated during the impulsive phase. To understand the acceleration mechanisms for these particles, we have studied the characteristics of the ..gamma..-ray line (GRL) flares observed by SMM. Because we discovered that the GRL flares emit very intense hard X-rays, we have also studied very intense hard X-ray flares without detectable nuclear ..gamma..-rays. The main characteristics of GRL flares are as follows: (1) delay of high-energy hard X-rays; (2) flat hard X-ray spectra (average power-law index 3.4); (3) emission of type II and/or type IV radio bursts; (4) intense hard X-ray and microwave emission. The majority of the GRL flares exhibit all these characteristics, and the remainder exhibit at least three of them. The first two characteristics are consistently explained by the proposition that in GRL flares an additional mechanism further accelerates high-energy electrons. Among the GRL flares we have found positive correlations between the following pairs of quantities: (1) duration of spike bursts and spatial size of flare loops and (2) delay time and duration of spike bursts.