Visibility and Origin of Compact Interplanetary Radio Type IV Bursts

Abstract

We have analyzed radio type IV bursts in the interplanetary (IP) space at decameter-hectometer (DH) wavelengths to determine 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, which were located approximately 90 degrees apart from each other in 2011-2012, and thus gave a 360 degree view of the Sun. The radio data were 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 that compact and intense DH type IV burst emission is observed from only one spacecraft at a time is the absorption of emission in one direction and that the emission is blocked by the solar disk and dense corona in 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, and they 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 shockstreamer interaction. In some cases, the type II-emitting region was also capable of stopping later-accelerated electron beams, which were visible as type III bursts that ended near the type II burst lanes.

A Quarter Century ofWindSpacecraft Discoveries

“…( 2016 ) concluded that the type IV emission cone is less than ∼60° in full width and that this narrow cone results from the small angular extent of the source region, the post eruption solar arcades. Another proposed explanation is that the shock‐compressed, high‐density plasmas in the foreground of the emission attenuate the intensity of the type IV bursts more on the shock flanks than the center (i.e., due to larger line of sight integration) (Pohjolainen & Talebpour Sheshvan, 2020 ; Talebpour Sheshvan & Pohjolainen, 2018 ). The reason for the narrow type IV emission cone continues to be an active area of research.…”

Interplanetary Type IV Solar Radio Bursts: A Comprehensive Catalog and Statistical Results

“…Based on this, the authors argued that there could be a directivity in the inherent emissivity, besides any effect due to background plasma opacity. A few case studies that followed this paper argued that occultation by dense overlying material, often a CME streamer shock formed toward the CME flank region, could be a prime reason for blocking the emission along certain viewing angles and not necessarily inherent emissivity (Melnik et al 2018;Talebpour Sheshvan & Pohjolainen 2018;Pohjolainen & Talebpour Sheshvan 2020). Thus, the earlier studies that explored directivity in the emission have either been on specific cases or on the distribution of observed source locations from one spacecraft without factoring in the variation in the dynamic spectral morphology detected from multiple vantage points.…”

“…Besides, Gopalswamy et al (2016) reported that the DH type IV emission was directed within a cone angle of ±60°, which is possibly associated with the line-of-sight occultation of the postflare loop by the solar disk. Using a few case studies with multiple-vantage-point data from the Wind and STEREO missions, Talebpour Sheshvan & Pohjolainen (2018) argued that the visibility of the DH type IV could be hampered by the presence of high-density structures along the line of sight that are formed by the interaction of the evolving CME with other preexisting coronal magnetic field structures like streamers. These high-density regions can attenuate the DH-band radio emission from the postflare loop.…”

Properties of High-Energy Solar Particle Events Associated with Solar Radio Emissions

“…We did not find any DH type IV bursts associated with our 58 events, while 32 m type IV bursts were found. The absence of DH type IVs can be explained, at least partially, with the observed directivity of type IV bursts: in events where the burst source (flare) is located near or behind the solar limb DH type IV bursts are not observed (Gopalswamy et al, 2016a;Talebpour Sheshvan and Pohjolainen, 2018). As m type IV bursts are thought to be formed by particles trapped in rising flare loops, their existence in intense and large soft X-ray flares can be considered as typical.…”