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Abstract A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to the acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares.
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Cold Solar Flares. I. Microwave Domain
Abstract We identify a set of ∼100 “cold” solar flares and perform a statistical analysis of them in the microwave range. Cold flares are characterized by a weak thermal response relative to nonthermal emission. This work is a follow-up of a previous statistical study of cold flares, which focused on hard X-ray emission to quantify the flare nonthermal component. Here, we focus on the microwave emission. The thermal response is evaluated by the soft X-ray emission measured by the GOES X-ray sensors. We obtain spectral parameters of the flare gyrosynchrotron emission and reveal patterns of their temporal evolution. The main results of the previous statistical study are confirmed: as compared to a “mean” flare, the cold flares have shorter durations, higher spectral peak frequencies, and harder spectral indices above the spectral peak. Nonetheless, there are some cold flares with moderate and low peak frequencies. In the majority of cold flares, we find evidence of the Razin effect in the microwave spectra, indicative of rather dense flaring loops. We discuss the results in the context of the electron acceleration efficiency.
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- Award ID(s):
- 2121632
- PAR ID:
- 10539448
- Publisher / Repository:
- AAS
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 954
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 122
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/acea20
