TY - JOUR
T1 - Implications of flat optically thick microwave spectra in solar flares for source size and morphology
AU - Shaik, Shaheda Begum
AU - Gary, Dale E.
N1 - Publisher Copyright:
© 2021 Institute of Physics Publishing. All rights reserved.
PY - 2021/9/20
Y1 - 2021/9/20
N2 - 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 al, an indicator of source morphology. Particularly, five bursts display a "flat" spectrum (al . 1.0) compared to that expected for a homogeneous/uniform source (al ~ 2.9). These flat spectra at 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 ~120" 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 an 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.
AB - 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 al, an indicator of source morphology. Particularly, five bursts display a "flat" spectrum (al . 1.0) compared to that expected for a homogeneous/uniform source (al ~ 2.9). These flat spectra at 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 ~120" 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 an 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.
UR - http://www.scopus.com/inward/record.url?scp=85116516015&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85116516015&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/ac0fdb
DO - 10.3847/1538-4357/ac0fdb
M3 - Article
AN - SCOPUS:85116516015
SN - 0004-637X
VL - 919
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 44
ER -