Nonthermal electrons in the quiet solar corona: prevalence, energetics, and magnetic-field diagnostics from weak microwave transients

Joint Karl G. Jansky Very Large Array (VLA) and Expanded Owens Valley Solar Array (EOVSA) microwave imaging spectroscopy has shown that at least some weak transients from the quiet solar corona (1–2 GHz) are powered by nonthermal gyrosynchrotron emission (microwave radiation from mildly relativistic electrons spiraling in the coronal magnetic field), with nonthermal energies ($10^{26}$–$10^{29}$ erg) comparable to the thermal energy of a nanoflare. This evidence, however, rests on only a handful of events, the gyrosynchrotron spectral fits are degenerate and non-unique, the emission mechanisms of many detected transients remain unclassified, the origin of the newly detected coronal-hole microwave transients (e.g., interchange reconnection) is unestablished, and independent X-ray corroboration is blocked by particle-background contamination in GOES full-disk soft X-ray monitoring. The open problem is to determine, statistically and with quantified uncertainties, how common nonthermal electrons are in the quiet corona, what their energy content and thermal–nonthermal energy partition are, what mechanisms power them, and whether weak microwave transients can routinely constrain quiet-coronal magnetic fields.

Background

The solar corona outside active regions and coronal holes — the quiet Sun — is heated to millions of kelvin, and the coronal heating problem, one of the long-standing questions of solar physics, asks what maintains this temperature. A leading hypothesis is Parker's nanoflare picture: a multitude of tiny, impulsive magnetic-reconnection events, each releasing roughly $10^{24}$–$10^{27}$ erg, whose cumulative effect heats the plasma. Such reconnection events are expected to accelerate electrons to suprathermal energies, so a direct way to detect them is to look for nonthermal (non-equilibrium) particle populations rather than only thermal brightenings.

Radio emission is the most sensitive observational probe of nonthermal electrons in the corona. At microwave frequencies (roughly 1–20 GHz, sampling the low and middle corona), accelerated electrons radiate via gyrosynchrotron emission — mildly relativistic synchrotron radiation whose spectrum and polarization depend on the magnetic field strength $B$, the viewing angle $\theta$ between the field and the line of sight, and the energy distribution of the electrons (usually modeled as a power law between low- and high-energy cutoffs $E_{min}$ and $E_{max}$ with index $\delta$). Competing mechanisms at these frequencies are thermal free-free (bremsstrahlung) emission from hot plasma and, occasionally, coherent plasma emission excited at the local plasma frequency. Distinguishing these requires simultaneous broadband spectra and polarization (Stokes $I$ and circularly polarized Stokes $V$) measurements: gyrosynchrotron can be identified by its spectral shape and higher-than-thermal circular polarization fraction, whereas optically thin free-free polarization from propagation effects follows a predictable, weaker frequency dependence. Flux densities are measured in solar flux units (1 SFU $= 10^{4}$ Jy); the relevant transients are at the mSFU level, a few percent of the quiet-Sun background, so very high dynamic-range imaging is needed.

Historically, weak microwave transients were studied over narrow bandwidths (e.g., variable emission associated with coronal bright points — small, compact magnetic-loop systems visible in extreme-ultraviolet images — detected since the 1980s), which could not discriminate emission mechanisms. At metric wavelengths (below ~200 MHz), the Murchison Widefield Array revealed ubiquitous Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs) down to mSFU levels, with sub-second durations and bandwidths under 1 MHz, hypothesized to be coherent plasma-emission signatures of nanoflare-like reconnection; frequency-drift measurements support plasma emission for at least some of them. The microwave (GHz) regime remained essentially unexplored at these sensitivities until Mondal, Chen & Yu (2023) combined Karl G. Jansky Very Large Array (VLA) and Expanded Owens Valley Solar Array (EOVSA) data — the latter a dedicated solar radio interferometer delivering imaging spectroscopy — to detect transient quiet-Sun sources at 1–2 GHz. Modeling the broadband spectra and polarization, they showed that at least three transients are inconsistent with thermal free-free emission and well fit by nonthermal gyrosynchrotron, with nonthermal energies of $10^{26}$–$10^{29}$ erg; for the two events where both could be computed, thermal and nonthermal energies were comparable. They also reported the first radio transients associated with coronal holes (regions of open magnetic field and the source of the fast solar wind), tentatively attributing one to interchange reconnection between open and closed field. The same study, however, established the current limits: the gyrosynchrotron fits are under-constrained and non-unique because of sparse frequency sampling; sidelobe levels of the synthesized beam (the interferometer's effective point-spread function, whose secondary response lobes can mimic or corrupt weak sources) of roughly 40–70%, together with limited sensitivity and dynamic range, restrict image fidelity and polarization measurements; only a handful of events were analyzed; and soft X-ray light curves from GOES (the Geostationary Operational Environmental Satellites, which provide full-disk soft X-ray monitoring) are too contaminated by particle background at these flux levels to provide independent corroboration, so sensitive hard/soft X-ray instruments with focusing optics, such as the Nuclear Spectroscopic Telescope Array (NuSTAR), are needed for cross-wavelength confirmation. A 2026 review of the field (Mondal et al. 2026, in the SKA science book) confirms that these limitations still define the state of the art, with the Square Kilometre Array (SKA), expected around 2030, identified as the facility that could overcome them.

Problem Statement

Determine the prevalence, energetics, and physical origin of nonthermal electrons associated with weak transient microwave emission from the quiet solar corona, and establish whether such transients can serve as routine diagnostics of quiet-coronal magnetic fields. Throughout, "quiet Sun" carries the Background definition above — the corona outside active regions and coronal holes — so the microwave transients detected in coronal-hole regions are treated as a separate class (item 4) and are not counted in the quiet-Sun prevalence and occurrence-rate statistics of items 1–3.

1. Prevalence and statistics. Establish how commonly weak quiet-Sun microwave transients are powered by nonthermal gyrosynchrotron emission rather than thermal free-free or coherent plasma emission, via detection and mechanism classification of a statistically well-defined sample, replacing the handful of individually analyzed events of Mondal, Chen & Yu (2023). A "statistically well-defined sample" is one whose selection function is stated explicitly — observing period, detection threshold, and any exposure- or area-weighting; "quantified uncertainties" throughout means explicit confidence or posterior intervals on the per-event fitted $B$, $\delta$, $E_{min}$, and nonthermal electron column density and on the aggregate occurrence rate and energy distribution.
2. Energetics and energy partition. Determine the distribution of nonthermal energies and the thermal–nonthermal energy partition across the sample, establishing whether the near-comparable partition found for the two events of the source study is typical, and hence whether the nonthermal share must be included in the quiet-coronal energy budget relevant to nanoflare-based coronal heating.
3. Spectral degeneracy. Determine to what extent broadband spectropolarimetric data break the gyrosynchrotron fit degeneracy among magnetic field strength $B$, electron power-law index $\delta$, low-energy cutoff $E_{min}$, nonthermal electron column density, viewing angle, and source inhomogeneity, so that electron parameters and coronal magnetic fields are recovered with quantified, defensible uncertainties rather than representative non-unique values.
4. Coronal-hole transients. Identify the physical mechanism (e.g., interchange reconnection between open and closed field) that produces the microwave transients detected in coronal-hole regions, and quantify their contribution to solar-wind mass and energy supply.
5. Magnetographic utility and corroboration. Beyond the per-event recovery of $B$ and the electron parameters in item 3, assess whether modeling weak-transient gyrosynchrotron emission can routinely constrain quiet-coronal magnetic fields outside active regions — that is, whether that recovery succeeds across the population at the sensitivity and dynamic range of currently available broadband microwave imaging spectroscopy without event-specific tuning of the fit parameters — supported by independent corroboration of the nonthermal signatures at EUV and X-ray wavelengths (with sensitivity beyond GOES, which is particle-background-limited at these flux levels).

Items 4–5 build on the same sample and the same fits as items 1–3, adding the coronal-hole mechanism, the routine-recovery assessment, and the EUV/X-ray corroboration.

References

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