Non-collisional decay of solar type-U radio bursts

The decay time (tau) of solar type-U radio bursts in the frequency range from 470 MHz to 150 MHz are studied using high-time-resolution digital records of the Nancay Multichannel Radio Spectrograph. It is found that tau is larger for the descending branch of the type U than for the ascending branch at the same frequency, and that tau increases with decreasing frequencies for the ascending branch, while tau does not exhibit any clear statistical dependence on frequency for the descending branch. On the contrary, tau clearly increases with the elapsed time. Hence tau appears to be much more beam dependent than medium dependent. This is interpreted as a new strong indication that at high frequencies the time profile of a type-III burst is similar to that of the exciter, thus indicating that plasma waves decay no slower than the exciter. This extends to the low corona a property previously established in the solar wind. It may be accounted for by various damping mechanisms (classical collisional damping, Landau damping on the tail electrons, etc.); in low-corona conditions, a possible process is also plasma-wave refraction outside narrow overdense fibers, resulting in their fast Landau damping.