Band-splitting of coronal and interplanetary type II bursts. I. Basic properties

Abstract

Patterns analogous to the band-splitting of metric type II bursts are found in a number of type II bursts observed in the dekameter-kilometer wavelength range. A similarity of morphological and frequency-time characteristics of two emission components are indicative of a common source. Relative frequency splits span in the range Δf/f = 0.05-0.6. At radial distances between 2 and 4 R⊙ only small splits around 0.1 can be found. In the interplanetary space the relative split on average increases with the radial distance, whereas the inferred shock velocity decreases. In three events extrapolations of the split components point to the base and the peak of the jump in the local plasma frequency caused by the associated shock passage at 1 AU. This is suggestive of the plasma radiation from the regions upstream and downstream of the shock. Adopting this interpretation, one finds that the drop of Δf/f at 2-4 R⊙ is congruent with the Alfvén velocity maximum expected there. The split increase and the velocity decrease at larger distances can be explained as a consequence of declining Alfvén speed in the interplanetary space.