Narrowband Gyrosynchrotron Bursts: Probing Electron Acceleration in Solar Flares

TL;DR

This study identifies and analyzes narrowband gyrosynchrotron bursts in solar flares, providing evidence that microwave emission originates directly from acceleration regions with specific magnetic and plasma conditions, distinct from thermal X-ray sources.

Contribution

It presents a statistical analysis of narrowband microwave bursts, demonstrating their origin in electron acceleration regions during solar flares, with detailed diagnostics linking radio and X-ray observations.

Findings

Microwave emission comes directly from acceleration regions with strong magnetic fields.

No significant electron trapping occurs in these bursts, indicated by steep spectra.

Thermal X-ray emission originates from separate, lower-density loops.

Abstract

Recently, in a few case studies we demonstrated that gyrosynchrotron microwave emission can be detected directly from the acceleration region when the trapped electron component is insignificant. For the statistical study reported here, we have identified events with steep (narrowband) microwave spectra that do not show a significant trapped component and at the same time show evidence of source uniformity, which simplifies the data analysis greatly. Initially, we identified a subset of more than 20 radio bursts with such narrow spectra, having low- and high-frequency spectral indices larger than 3 in absolute value. A steep low-frequency spectrum implies that the emission is nonthermal (for optically-thick thermal emission, the spectral index cannot be steeper than 2), and the source is reasonably dense and uniform. A steep high-frequency spectrum implies that no significant electron trapping occurs; otherwise a progressive spectral flattening would be observed. Roughly half of these radio bursts have RHESSI data, which allows for detailed, joint diagnostics of the source parameters and evolution. Based on an analysis of radio-to-X-ray spatial relationships, timing, and spectral fits, we conclude that the microwave emission in these narrowband bursts originates directly from the acceleration regions, which have relatively strong magnetic field, high density, and low temperature. In contrast, the thermal X-ray emission comes from a distinct loop with smaller magnetic field, lower density, but higher temperature. Therefore, these flares occurred likely due to interaction between two (or more) magnetic loops.