The Coronal Volume of Energetic Particles in Solar Flares as Revealed by Microwave Imaging

Gregory D. Fleishman, Maria A. Loukitcheva, Varvara Yu Kopnina, Gelu M. Nita, Dale E. Gary

Research output

Abstract

The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effect on the low-frequency spectrum is often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with RHESSI during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of ≥108 K. The low-frequency spectra in the remaining events are affected to varying degrees by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.

Original languageEnglish
Article number81
JournalAstrophysical Journal
Volume867
Issue number1
DOIs
Publication statusPublished - 1 Nov 2018

Fingerprint

energetic particles
solar flares
solar arrays
energetics
low frequencies
microwaves
valley
valleys
self absorption
thermal plasmas
radio
plasma
frequency dependence
brightness temperature
retarding
plasma diagnostics
particle production
magnetic field
nonuniformity
electron

Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Fleishman, Gregory D. ; Loukitcheva, Maria A. ; Kopnina, Varvara Yu ; Nita, Gelu M. ; Gary, Dale E. / The Coronal Volume of Energetic Particles in Solar Flares as Revealed by Microwave Imaging. In: Astrophysical Journal. 2018 ; Vol. 867, No. 1.
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abstract = "The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effect on the low-frequency spectrum is often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with RHESSI during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of ≥108 K. The low-frequency spectra in the remaining events are affected to varying degrees by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.",
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The Coronal Volume of Energetic Particles in Solar Flares as Revealed by Microwave Imaging. / Fleishman, Gregory D.; Loukitcheva, Maria A.; Kopnina, Varvara Yu; Nita, Gelu M.; Gary, Dale E.

In: Astrophysical Journal, Vol. 867, No. 1, 81, 01.11.2018.

Research output

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AU - Loukitcheva, Maria A.

AU - Kopnina, Varvara Yu

AU - Nita, Gelu M.

AU - Gary, Dale E.

PY - 2018/11/1

Y1 - 2018/11/1

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AB - The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effect on the low-frequency spectrum is often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with RHESSI during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of ≥108 K. The low-frequency spectra in the remaining events are affected to varying degrees by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.

KW - acceleration of particles

KW - Sun: corona

KW - Sun: flares

KW - Sun: radio radiation

KW - Sun: X-rays, gamma rays

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