Daily differential emission measure (DEM) distributions of the solar corona are derived from spectra obtained by the Extreme-ultraviolet Variability Experiment (EVE) over a 4-year period starting in 2010 near solar minimum and continuing through the maximum of solar cycle 24. The DEMs are calculated using six strong emission features dominated by Fe lines of charge states VIII, IX, XI, XII, XIV, and XVI that sample the non-flaring coronal temperature range 0.3-5 MK. A proxy for the non-Fe XVIII emission in the wavelength band around the 93.9Å line is demonstrated. There is little variability in the cool component of the corona (T < 1.3 MK) over the four years, suggesting that the quiet-Sun corona does not respond strongly to the solar cycle, whereas the hotter component (T > 2.0 MK) varies by more than an order of magnitude. A discontinuity in the behavior of coronal diagnostics in 2011 February-March, around the time of the first X-class flare of cycle 24, suggests fundamentally different behavior in the corona under solar minimum and maximum conditions. This global state transition occurs over a period of several months. The DEMs are used to estimate the thermal energy of the visible solar corona (of order 10 31 erg), its radiative energy loss rate (2.5-8 ×10 27 erg s −1 ), and the corresponding energy turnover timescale (about an hour). The uncertainties associated with the DEMs and these derived values are mostly due to the coronal Fe abundance and density and the CHIANTI atomic line database.
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 ± 2.1 sfu (∼ 20%) 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.
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