OVSA Science Highlight No. 3: The First EOVSA "Cold" Solar Flare
Contributed by Gregory Fleishman1, 2 (1Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 323 Martin Luther King Jr Blvd., Newark, NJ 07102-1982, USA; 2Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401 A, D-79110 Freiburg, Germany); Edited by Bin Chen; Posted on August 20, 2025.
A “cold flare” refers to a solar flare event characterized by strong non-thermal emissions (in hard X-rays and microwaves) but unusually weak thermal signatures (i.e., minimal soft X-ray or heated plasma response). In other words, there appears to be substantial particle acceleration with only a modest flare heating (see Fleishman et al. 2016, who coined this term).
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Fig. 1: A subset of inferred flare parameters and their evolution. Top row: magnetic field strength. Bottom row: nonthermal electron power-law index. Left column: An example parameter map corresponding to a specific time frame derived from EOVSA microwave imaging spectroscopy data. Middle column: evolution of the fit parameters for the selected region of interest (ROI) pixel (blue symbols) along with their corresponding median values (horizontal black line). Right column: evolution of parameter distributions for all pixels of the selected ROI, corresponding to the same time frames as shown in the middle column plots. |
A paper recently published in the Astrophysical Journal reports a detailed case study of the energy budget in the 2017 September 7 “cold” solar flare observed with the Expanded Owens Valley Solar Array (EOVSA), along with a combination of other multi-wavelength instruments. In this case, the EOVSA data permitted the dynamical measurement of the coronal magnetic field and other parameters at the flare site. 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 (see Fig. 1). 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. In addition, we discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons (see Fig. 1). Using the data and a developed 3D model, we computed energy partitions and concluded that the nonthermal energy deposition is sufficient to drive the flare thermal response similarly to other cold flares (Fig. 2).
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Fig. 2: Evolution of energy components in the 2017 September 7 flare. The thermal energy WAIA computed from the ROI of the AIA DEM maps is shown in green. The thermal energy WthGBM deduced from the thermal part of the Fermi/GBM fits assuming the flux tube 2 volume in the 3D model (see the paper for details) is shown in black. The cumulative nonthermal energy deposition WnthGBM obtained from the nonthermal part of the Fermi/GBM fits is shown with the dark yellow histogram, while the red histogram shows the rate of the Fermi/GBM nonthermal energy deposition dWnthGBM/dt. The total nonthermal energy input inferred from the three loops of the 3D model is shown by a light-blue circle. The magenta curve shows the Konus-Wind 18.1–76.5 keV light curve. The scarlet, yellow, and green diamonds indicate the model values of thermal energies in Loops 1, 2, and 3, respectively; see Table 1 of the paper. |
Based on the recent paper by Fleishman, G., Motorina, G., Yu, S., & Nita, G. (2025), Energy Budget in the 2017-09-07 "Cold" Solar Flare, The Astrophysical Journal, 988, 260. DOI: 10.3847/1538-4357/ade983