Implications of Flat Optically Thick Microwave Spectra in Solar Flares for Source Size and Morphology

TL;DR

This study analyzes high-resolution microwave spectra of solar flares to understand source size and morphology, revealing that flat optically thick spectra indicate inhomogeneous, complex source structures that evolve during flare decay.

Contribution

It provides new insights into the spectral dynamics of microwave emissions, demonstrating that flat spectra result from inhomogeneous sources and evolve over the flare duration.

Findings

Flat spectra are linked to inhomogeneous, multi-component sources.

Source size at 2.6-3 GHz is approximately 120 arcseconds.

Flat spectra tend to grow flatter during the decay phase.

Abstract

The study aims to examine the spectral dynamics of the low-frequency, optically thick gyrosynchrotron microwave emission in solar flares to determine the characteristics of the emitting source. We present the high-resolution spectra of a set of microwave bursts observed by the Expanded Owens Valley Solar Array (EOVSA) during its commissioning phase in the $2.5-18$ GHz frequency range with $1$ second time resolution. Out of the 12 events analyzed in this study, nine bursts exhibit a direct decrease with time in the optically thick spectral index $\alpha_l$, an indicator of source morphology. Particularly, five bursts display "flat" spectrum ($\alpha_l\leq1.0$) compared to that expected for a homogeneous/uniform source ($\alpha_l\approx2.9$). These flat spectra at the low-frequencies (<$10$ GHz) can be defined as the emission from a spatially inhomogeneous source with a large area and/or with multiple emission components. In a subset of six events with partial cross-correlation data, both the events with flat spectra show a source size of $\sim120$ arcsec at $2.6-3$ GHz. Modeling based on inhomogeneity supports the conclusion that multiple discrete sources can only reproduce a flat spectrum. We report that these flat spectra appear predominantly in the decay phase and typically grow flatter over the duration in most of the bursts, which indicates the increasing inhomogeneity and complexity of the emitting volume as the flare progresses. This large volume of flare emission filled with the trapped energetic particles is often invisible in other wavelengths, like hard X-rays, presumably due to the collisionless conditions in these regions of low ambient density and magnetic field strength.