The dynamic aspects of electron acceleration and transport in the solar atmosphere can be sensitively probed by the radiative signatures, notably hard X-ray bremsstrahlung produced by collisional energy loss and coherent radio emission generated by kinetic plasma instabilities. In this chapter, we focus on dynamic aspects of nonthermal particles in solar flares that have been studied mainly by using hard X-ray observations obtained with the Hard X-ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) (Orwig et al. 1980) and simultaneous radio observations. We concentrate on impulsive flares or type B flares in the classification scheme of Tanaka (1983, 1987). They are characterized by double footpoint emission in hard X-rays and soft-hard-soft spectral evolution. Type B flares are by far the most common observed by SMM. Since we are mainly interested in impulsive signatures of nonthermal particles, thermal flares (type A), and gradual flares (type C) are not discussed. Accordingly we also neglect radio emission associated with extended, gradual flare phases (type IV, type II).
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