The Slowly Varying Corona. II. The Components of F $_{10.7}$ and Their Use in EUV Proxies
Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017), we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F$_{10.7}$, the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F$_{10.7}$ is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F$_{10.7}$ variability and that coronal bremsstrahlung emission accounts for 14.2 $\pm$ 2.1 sfu ($\sim20\%$) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F$_{10.7}$ at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F$_{10.7}$. We find that the bremsstrahlung component fit, along with the Mg II index, correlates better with the observed 5-37 nm spectrum than the common 81 day averaged F$_{10.7}$ proxy. The bremsstrahlung component of F$_{10.7}$ is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models.