In early research on solar flares, attention was focused on the impulsíve or flash phase, and it was assumed implicitly that virtually all the energy of a flare is released during that short phase. In recent years, however, it has been realized that the long-lived soft X-ray emission which follows the impulsive phase may require a separate energy-release process, which has been termed the “gradual phase” (Kane 1974). The fact that the impulsive phase is often preceded by soft X-ray emission has also led to the suggestion that there may be a third phase of energy release, which might be termed the “onset phase” (Sturrock 1980). It has long been realized that filament eruptions frequently precede flares, and it has been suggested (Kiepenheuer 1964) that the two should be regarded as parts of the same process. For these and other reasons, it is appropriate to question how many phases of energy release are involved in flares and what are their characteristics.
Energy Release in Solar Flares
