Electron Cyclotron Maser Emission and the Brightest Solar Radio Bursts

TL;DR

This study analyzes a large catalog of solar radio bursts, revealing that a significant portion exhibit characteristics of electron cyclotron maser emission, especially at 1 GHz, with implications for terrestrial and astrophysical applications.

Contribution

It provides a revised catalog and analysis demonstrating that most bright solar radio bursts at 1 GHz are dominated by coherent ECM emission, refining understanding of solar radio burst mechanisms.

Findings

75% of bursts >100 sfu at 1 GHz show coherent emission.

Half of the 2 GHz bursts are dominated by ECM.

Most bright 1 GHz bursts are highly circularly polarized.

Abstract

This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 10 billion Jansky have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion it is found that, for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets.