Evidence for resonant transition radiation in decimetric continuum solar bursts

We investigate the properties of radio bursts observed in the 1-18 GHz frequency range that display two distinct spectral components peaking at decimetric (dm) and centimetric (cm) ranges. The dm emission is relatively smooth in both frequency and time, with timescales comparable to those of the cm component. The two spectral components display specific correlations in their temporal and spectral behavior. Through detailed analysis, we find the following: (1) A large ratio of plasma frequency to gyrofrequency is a characteristic of all of these events. (2) The dm component generally displays a high degree of o-mode polarization. (3) The dm component is produced by an incoherent emission mechanism. (4) The dm and cm components are generated by the same electron distribution. (5) The characteristic energy of the fast electrons producing the dm continuum is significantly lower than the energy of the electrons generating the microwave gyrosynchrotron component. (6) The spectral shape of the dm component is not well correlated with the fast electron distribution over energy. These findings cannot easily be explained with standard mechanisms for dm solar burst emission but agree well with the predictions of the theory of resonant transition radiation. We conclude that these two-component bursts represent a distinctive subclass of events, comprising about 10% of all bursts, with the dm continuum component generated most probably by resonant transition radiation produced by interaction of fast electrons with small-scale inhomogeneities of the background plasma. We discuss new possibilities for flaring plasma diagnostics using these two-component bursts.