Circular Polarization Observed in Interplanetary Type III Radio Storms

We report the detection and analysis of circular polarization in solar type III radio storms at hectometric-to-kilometric wavelengths. We find that a small (usually less than 5%), but statistically significant, degree of circular polarization is present in all interplanetary type III radio storms below 1 MHz. The sense of the polarization, which is right-hand circular for some storms and left-hand circular for others, is maintained for the entire duration of the type III storm (usually many days). For a given storm, the degree of circular polarization peaks near central meridian crossing of the associated active region. At a given time, the degree of circular polarization is found to generally vary as the logarithm of the observing frequency. The radiation characteristics, including the polarization, for one interplanetary type III storm exhibits an unusual 1.6 hour oscillation. Based on the standard plasma emission theory of type III radiation, we discuss the implications of these observations for the magnitude and radial dependence of the solar magnetic field above active regions on the Sun.