Coronal Sources of the Solar F$_{10.7}$ Radio Flux

TL;DR

This study uses VLA radio imaging and EUV data to analyze the sources of the solar F$_{10.7}$ flux, quantifying the contributions of bremsstrahlung and gyroresonance emissions and examining optical depth effects.

Contribution

First full-disk F$_{10.7}$ radio image analysis linking radio flux sources to EUV emissions and quantifying the gyroresonance contribution.

Findings

8.1% of variable F$_{10.7}$ flux is from gyroresonance emission

Optical depth effects influence radio limb observations

Coronal iron abundance is about 4 times the photospheric level

Abstract

We present results from the first solar full-disk F$_{10.7}$ (the radio flux at $10.7$ cm, $2.8$ GHz) image taken with the S-band receivers on the recently upgraded Karl G. Jansky Very Large Array (VLA) in order to assess the relationship between the F$_{10.7}$ index and solar extreme ultra-violet (EUV) emission. To identify the sources of the observed $2.8$ GHz emission, we calculate differential emission measures (DEMs) from EUV images collected by the Atmospheric Imaging Assembly (AIA) and use them to predict the bremsstrahlung component of the radio emission. By comparing the bremsstrahlung prediction and radio observation we find that $8.1\pm 0.5\%$ of the variable component of the F$_{10.7}$ flux is associated with the gyroresonance emission mechanism. Additionally, we identify optical depth effects on the radio limb which may complicate the use of F$_{10.7}$ time series as an EUV proxy. Our analysis is consistent with a coronal iron abundance that is $4$ times the photospheric level.