The dispersion of radio waves in the solar corona

Different arrival times of the two magnetoionic modes in solar radio bursts have been detected. The bursts are from four decimetric radio events showing narrowband millisecond spikes. They have been observed with 2 ms and 0.5 ms time resolution, respectively, by the Ikarus and Phoenix spectrometers of ETH Zurich. The four events have been selected because of their low polarization. The arrival times of the left and right circularly polarized modes have been compared by cross-correlation. In all cases the weaker mode is delayed by a fraction of a millisecond. Several tests have been carried out to ensure the significance of the delay. The delay is interpreted by the difference in group velocity of the two modes due to dispersion in the coronal plasma. Simple models show that the observed difference in travel time is consistent with this interpretation. It suggests that the radiation is polarized in the ordinary mode at the location where the polarization originates. If the polarization originates in the original source region, the possible emission processes are limited to the ones radiating in ordinary mode. More likely, the polarization seems to originate at higher altitude e.g. in a quasi-transverse region. In both cases the delay is proportional to the longitudinal component of the magnetic field in the medium of propagation.