Twin Null-Point-Associated Major Eruptive Three-Ribbon Flares with Unusual Microwave Spectra

TL;DR

This study analyzes two successive three-ribbon solar flares with unusual microwave spectra, revealing complex multi-loop magnetic configurations and eruption dynamics through multi-instrument observations and modeling.

Contribution

It provides new insights into the magnetic structures and spectral properties of successive eruptive flares with three ribbons, supported by detailed multi-instrument data and modeling.

Findings

Microwave spectra were flattened at low frequencies with a lower turnover in the stronger flare.

Both flares were eruptive, emitting high-energy X-rays and gamma-rays, with a rare three-ribbon configuration.

A distributed multi-loop system in an asymmetric magnetic configuration explains the unusual microwave spectra.

Abstract

On 23 July 2016 after 05:00\,UTC, the first 48-antenna stage of the Siberian Radioheliograph detected two flares of M7.6 and M5.5 GOES importance that occurred within half an hour in the same active region. Their multi-instrument analysis reveals the following. The microwave spectra were flattened at low frequencies and the spectrum of the stronger burst had a lower turnover frequency. Each flare was eruptive, emitted hard X-rays and gamma-rays exceeding 800\,keV, and had a rare three-ribbon configuration. An extended hard X-ray source associated with a longest middle ribbon was observed in the second flare. The unusual properties of the microwave spectra are accounted for by a distributed multi-loop system in an asymmetric magnetic configuration that our modeling confirms. Microwave images did not to resolve compact configurations in these flares that may also be revealed incompletely in hard X-ray images because of their limited dynamic range. Being apparently simple and compact, non-thermal sources corresponded to the structures observed in the extreme ultraviolet. In the scenario proposed for two successive three-ribbon eruptive flares in a configuration with a coronal-null region, the first eruption causes a flare and facilitates the second eruption that also results in a flare.