On the Directivity of Low-Frequency Type IV Radio Bursts

TL;DR

This study investigates the directivity of low-frequency Type IV solar radio bursts using multi-viewpoint observations, revealing that the emission is narrowly directed above the flare site and confirming this with statistical analysis.

Contribution

The paper provides new insights into the directivity pattern of low-frequency Type IV radio bursts through multi-spacecraft observations and statistical validation.

Findings

Type IV bursts are narrowly directed above the flare site.

The burst envelope reaches the lowest frequency before receding.

Type IV burst visibility depends on the observer's viewing angle.

Abstract

An intense type IV radio burst was observed by the STEREO Behind (STB) spacecraft located about 144 degres behind Earth. The burst was associated with a large solar eruption that occurred on the backside of the Sun (N05E151) close to the disk center in the STB view. The eruption was also observed by the STEREO Ahead (STA) spacecraft (located at 149 degrees ahead of Earth) as an eruption close to the west limb (N05W60) in that view. The type IV burst was complete in STB observations in that the envelope reached the lowest frequency and then receded to higher frequencies. The burst was partial viewed from STA, revealing only the edge coming down to the lowest frequency. The type IV burst was not observed at all near Earth because the source was 61 degrees behind the east limb. The eruption was associated with a low-frequency type II burst observed in all three views, although it was not very intense. Solar energetic particles were also observed at both STEREOs and at SOHO, suggesting that the shock was much extended, consistent with the very high speed of the CME (about 2048 km/s). These observations suggest that the type IV emission is directed along a narrow cone above the flare site. We confirm this result statistically using the type IV bursts of solar cycle 23.