The diagnostic temperature discrepancy as evidence for non-Maxwellian coronal electrons

TL;DR

The paper presents evidence for non-Maxwellian electron velocity distributions in the solar corona, based on persistent temperature diagnostic discrepancies and modeling with kappa distributions.

Contribution

It introduces the idea that non-Maxwellian electron distributions explain the temperature discrepancy, supported by observational data and theoretical modeling.

Findings

Radio diagnostics yield T_e ~ 0.6 MK and T_e ~ 1.5 MK, with a factor of 2.4 discrepancy.

The temperature ratio R is cycle-invariant despite turbulence variations.

Kappa distribution models with kappa ~ 2-3 match the observed ratio R.

Abstract

Two independent electron temperature diagnostics applied to the quiet solar corona yield systematically different results. Radio brightness temperatures from the Nancay Radioheliograph indicate T_e ~ 0.6 MK, while hydrostatic scale-height modeling requires T_e ~ 1.5 MK. Both probe electrons; they disagree by a factor of R = 2.4 +/- 0.3. This discrepancy persists across eight years spanning solar minimum and is confirmed by LOFAR at lower frequencies. We consider turbulent scattering, which suppresses brightness temperature, but comparison with the FORWARD/PSIMAS Maxwellian model shows the standard thermal structure predicts ~1.6 MK; scattering accounts for the reduction toward observed MWA values but not the gap to 620 kK. The ratio R is also cycle-invariant despite measured variations in turbulence. We propose the residual discrepancy reflects non-Maxwellian electron velocity distributions. Radio bremsstrahlung samples the distribution core; ionization and scale heights are dominated by the suprathermal tail. For kappa distributions, the predicted ratio kappa/(kappa - 3/2) matches R = 2.4 at kappa ~ 2-3, consistent with spectroscopic measurements in active regions but in tension with perturbative predictions of kappa ~ 10-25. We predict Active Region cores should show a collapsed ratio (R <= 1.5) as collisionality restores thermal equilibrium.