Narrowband gyrosynchrotron bursts: probing electron acceleration in solar flares

Fleishman, G. D., Nita, G. M., Kontar, E. P. and Gary, D. E. (2016) Narrowband gyrosynchrotron bursts: probing electron acceleration in solar flares. Astrophysical Journal, 826(1), 38. (doi: 10.3847/0004-637X/826/1/38)

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 three 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 two), 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 allow 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 a relatively strong magnetic field, high density, and low temperature. In contrast, the thermal X-ray emission comes from a distinct loop with a smaller magnetic field, lower density, but higher temperature. Therefore, these flares likely occurred due to interaction between two (or more) magnetic loops.

• Item Type: Articles
• Additional Information: This work was supported in part by NSF grants AGS- 1250374 and AGS-1262772, NASA grants NNX14AC87G and NNX15AN48G at the New Jersey Institute of Technology, and the STFC Consolidated grant project “X-ray and radio diagnostics of energetic solar flare processes,” at the University of Glasgow.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Kontar, Professor Eduard
• Authors: Fleishman, G. D., Nita, G. M., Kontar, E. P., and Gary, D. E.
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Astrophysical Journal
• Publisher: IOP Publishing
• ISSN: 0004-637X
• ISSN (Online): 1538-4357
• Copyright Holders: Copyright © 2016 The American Astronomical Society
• First Published: First published in Astrophysical Journal 826(1): 38
• Publisher Policy: Reproduced in accordance with the publisher copyright policy

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