Characterization of turbulent magnetic reconnection in solar flares with microwave imaging spectroscopy

Abstract

<p>Magnetic reconnection plays a central role in highly magnetized plasma, for example, in solar corona. Release of magnetic energy due to reconnection is believed to drive such transient phenomena as solar flares, eruptions, and jets. This energy release should be associated with a decrease of the coronal magnetic field. Quantitative measurements of the evolving magnetic field strength in the corona are required to find out where exactly and with what rate this decrease takes place. The only available methodology capable of providing such measurements employs microwave imaging spectroscopy of gyrosynchrotron emission from nonthermal electrons accelerated in flares. Here, we report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field at the cusp region; well below the nominal reconnection X point. The field decays at a rate of ~5 Gauss per second for 2 minutes. This fast rate of decay implies a highly enhanced, turbulent magnetic diffusivity and sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. Moreover, spatially resolved maps of the nonthermal and thermal electron densities derived from the same microwave spectroscopy data set allow us to detect the very acceleration site located within the cusp region. The nonthermal number density is extremely high, while the thermal one is undetectably low in this region indicative of a bulk acceleration process exactly where the magnetic field displays the fast decay. The decrease in stored magnetic energy is sufficient to power the solar flare, including the associated eruption, particle acceleration, and plasma heating. We discuss implications of these findings for understanding particle acceleration in solar flares and in a broader space plasma context.</p>

What aspects of solar flares can be clarified with mm/submm observations?

“…Landi et al (2021) found that within 22 min of flare onset the magnetic field measured with the Fe X MIT line underwent a strong enhancement at a location which later corresponded to the footpoints of post-flare loops; also, this magnetic field enhancement stored enough magnetic energy to power then entire flare, and was completely dissipated during the flare (see Figure 1). A similar conclusion has been made for an X8.3 limb flare with the imaging microwave spectroscopy technique (Fleishman et al, 2020). Frontiers in Astronomy and Space Sciences frontiersin.org…”

“…The GS emission in solar flares is characterized by a spectrum that typically has a spectral peak at the microwave range, while falls following roughly power-laws towards lower and higher frequencies. Supplementary videos to (Fleishman et al, 2020) display dependence of the GS spectrum on the key physical parameters including the high-energy cut-off of the nonthermal electron spectrum. This high-energy cut-off mainly affects the high-frequency end of the GS spectrum.…”

“…Microwave spectroscopy observations permit measuring magnetic field, plasma density, and nonthermal electron properties with high spatial and temporal resolution (Fleishman et al, 2020). The available diagnostics employing the EOVSA data in 1-18 GHz range provide the number density, the spectral index, and (in some cases) the low-energy cut-off; however, the high-frequency cut-off cannot reliably be determined in most cases because of the lack of the highfrequency observations.…”

Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

“…The scientific potential of ALMA observations of the Sun is promising but requires significant effort and resources to be fully unlocked due to the complexity of the instrument and the observational targets, namely, the dynamic and intermittent chromosphere, both in a quiescent and active state, the atmosphere above ARs including the often dramatically evolving coronal magnetic field (Fleishman et al, 2020), and non-thermal electrons in solar flares (Fleishman et al, 2021b). The resulting challenges for numerical modeling of the chromosphere, prominences, and the flaring corona, their appearances at the wavelengths observed using ALMA, and not least, the impact of ALMA's complex instrumental properties pose critical tests of our current understanding that will inspire future progress.…”

Rapid variations of Si IV spectra in a flare observed by interface region imaging spectrograph at a sub-second cadence

“…Figure 4E in Chen et al (2020b), see also Bastian et al (1998)]. We have therefore adopted the gyrosynchrotron (GS) forward fit method described in Fleishman et al (2020) to fit the brightness temperature spectrum T B (ν) using an isotropic non-thermal electron source with a power-law energy distribution. We restricted our spectral fit to frequencies above 2 GHz only, as the spectrum below 2 GHz contains contributions from coherent radiation (Bastian et al, 1998).…”