A Flare-related Decimetric Type-IV Radio Burst Induced by the X2 Radiation of Electron Cyclotron Maser Emission

TL;DR

This study analyzes a solar flare-related decimetric type-IV radio burst, providing evidence that it is caused by electron cyclotron maser emission, and models electron transport explaining spectral features and polarization.

Contribution

It demonstrates that the burst is induced by ECME in the harmonic X mode and reveals a shell-like electron velocity distribution, expanding understanding of solar radio emissions.

Findings

The burst has polarization levels up to medium-strong and exceeds 10^11 K in brightness temperature.

Sources are located above sunspots and align with the second harmonic of the local electron gyrofrequency.

Electrons exhibit a shell-like velocity distribution instead of the typical loss-cone distribution.

Abstract

The radiation mechanism of decimetric wideband and pulsating radio bursts from the Sun (in terms of decimetric type-IV (t-IVdm) burst) and other flaring stars is a long-standing problem. Early investigations were based on the leading-spot hypothesis for the sun and yielded contradictory results. Here, we analyzed the flare-associated t-IVdm burst on 20110924 with medium-strong levels of polarization and from sources near a sunspot. We found that the emission is intermittent and the maximum $T_B$ exceeds 10$^{11}$ K, with well-defined upper and lower frequency cutoffs. The radio sources are left-handed polarized, located above the sunspot with a negative polarity. The sources align well with the sites of the second harmonic of the local electron gyrofrequency. These findings provide essential evidence that the burst is induced by the electron cyclotron maser emission (ECME) in the harmonic X mode. We further modeled the transport of downward-streaming energetic electrons along a coronal loop and found most electrons get mirrored within the specific altitude range of 20-100 Mm. This explains why such bursts tend to have well-defined spectral ranges. We also found the ECME-radiating energetic electrons exhibit a shell-like VDF instead of the generally-presumed loss-cone distribution. The study greatly expands the application of ECME in solar radio astronomy and provides solar samples for similar bursts from other flaring stars.