Visibility and Origin of Compact Interplanetary Radio Type IV Bursts

TL;DR

This study investigates the origin and directivity of interplanetary radio type IV bursts using multi-spacecraft observations, revealing that absorption and solar disk blockage cause observed emission patterns and linking shock-streamer interactions to burst generation.

Contribution

It provides new insights into the source regions and directivity of interplanetary type IV bursts by analyzing multi-point observations and identifying absorption effects and shock-streamer interactions.

Findings

Absorption causes emission to be observed only from certain directions.

Shock-streamer interactions likely produce high-density plasma regions.

Type II bursts are associated with shock interactions at streamer locations.

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.