Visibility and Origin of Compact Interplanetary Radio Type IV Bursts

Abstract

We have analysed radio type IV bursts in the interplanetary (IP) space at decameter-hectometer (DH) wavelengths, to find out their source origin and a reason for the observed directivity. We used radio dynamic spectra from the instruments on three different spacecraft, STEREO-A, Wind, and STEREO-B, that were located approximately 90 degrees apart from each other in 2011-2012, and thus gave a 360 degree view to the Sun. The radio data was compared to white-light and extreme ultraviolet (EUV) observations of flares, EUV waves, and coronal mass ejections (CMEs) in five solar events. We find that the reason for observing compact and intense DH type IV burst emission from only one spacecraft at a time is due to the absorption of emission to one direction and that the emission is blocked by the solar disk and dense corona to the other direction. The geometry also makes it possible to observe metric type IV bursts in the low corona from a direction where the higher-located DH type IV emission is not detectable. In the absorbed direction we found streamers present, and these were estimated to be the locations of type II bursts, caused by shocks at the CME flanks. The high-density plasma was therefore most probably formed by shock--streamer interaction. In some cases the type II-emitting region was also capable of stopping later-accelerated electron beams, visible as type III bursts that ended near the type II burst lanes.