Electron cyclotron maser emission from double footpoints in solar flares

It is now known from Yohkoh Hard X-ray Telescope observations that double ( or even multiple) hard X-ray sources in flares are a common occurrence. The se sources, which are positioned at the feet of coronal soft X-ray loops, a re synchronised to within 0.1s and have similar spectra, strongly suggestin g that they are produced by a single population of electrons accelerated/in jected at some point in the loop. As this electron population is reflected from the converging footpoint magnetic fields, it develops a loss cone and an electron-cyclotron maser instability may ensue. The frequency and intens ity of such emission depends on the relative strengths and orientations of the footpoint magnetic fields. In this paper, we investigate the case of an almost symmetric loop to assess whether observable maser emission from bot h footpoints can result. In particular, we relate this theory to existing o bservations of solar microwave spike bursts which have two distinct frequen cy bands that are of non-integer ratio and comparable intensities. We concl ude that differing footpoint magnetic field inclinations cannot explain the observations (specifically the comparable intensities), but that it is pos sible for slightly differing footpoint magnetic field strengths to explain the observations. The pros and cons of this 'geometric' model are then comp ared with a previous model of these events, which explained them in terms o f the growth and then coalescence of Bernstein waves. We conclude that both interpretations seem plausible given current observations, but present a l ist of observable features that might be used discriminate between them in flare observations of the next solar maximum.