Impulsive (Flash) Phase of Solar Flares: Hard X-Ray, Microwave, EUV and Optical Observations

Abstract

Recent observations of impulsive hard X-ray, microwave, EUV and optical emissions during solar flares are briefly reviewed in order to deduce the characteristics of the impulsive (flash) phase phenomenon in small solar flares particularly from the point of view of the acceleration of electrons and their role in producing the various impulsive phase emissions. Observed and deduced characteristics of the various electromagnetic emission sources are summarized (Table II). The deduced characteristics of the electron acceleration process (Table III) indicate a process with high acceleration efficiency. The observations are found to be consistent with a model in which electrons are accelerated in a series of short pulses each lasting for ≲ 1 s and the accelerated electrons provide the energy necessary for all the observed electromagnetic emissions produced during the flash phase of small solar flares. Models of the impulsive phase emissions in which energetic electrons play a prominant role are examined and crucial tests to check the accuracy of these models are indicated (Table IV).