The directivity and polarisation of thick target X-ray bremsstrahlung from solar flares

The directivity and polarisation of solar hard X-ray bursts is discussed in terms of two bremsstrahlung source models. These involve continuous and impulsive injection of electrons respectively, as described widely in the literature.A detailed analysis is made of the continuous injection model in which electrons are accelerated downward into the dense chromosphere where their velocity distribution is greatly modified by collisions. This thick target scattering is described by a simple analytical model. Directivity and polarisation of the bremsstrahlung emission are calculated in detail for a thick target model in which the guiding magnetic field is vertical.It is predicted for this model that X-ray sources should brighten from the centre to the limb of the solar disk, while the degree of polarisation should rise from zero to around 30 % near the limb. The plane of maximum intensity for any source is that containing the source and the disk centre. Both the directivity and polarisation should increase with increasing photon energy.Very recent observations agree with these predictions though they suggest that the field is significantly non-vertical. If scattering is not included in the model, or if an impulsive injection model is taken, agreement with observations is not obtained.Temporarily at the Lehrstuhl für Theoretische Astrophysik der Universität Tübingen, West Germany.