EVIDENCE FOR ORDINARY MODE EMISSION FROM MICROWAVE-BURSTS

We analyze high-resolution, one-dimensional observations of simple mic rowave bursts, obtained at 4.9 GHz with the Westerbork Synthesis Radio Telescope in 1980, together with Halpha photographs of the associated flares from the Observatories of Athens and Meudon. In most cases the polarization structure can be interpreted in terms of extraordinary m ode emission, taking into account the polarity of the underlying magne tic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward part of the flaring loop. We fou nd evidence for ordinary mode emission in two classes of events. In on e class the o-mode comes from regions overlying strong magnetic field, which we interpret in terms of thermal gyroresonance absorption of th e extraordinary mode at the third harmonic of the gyrofrequency. In th e other class the entire burst emits in the o-mode, which may be attri buted to high gyrosynchrotron optical depth.