Lyman Continuum Observations of Solar Flares Using SDO/EVE

Abstract: The Extreme ultraviolet Variability Experiment was designed to observe the Sun-as-a-star in the extreme ultraviolet; a wavelength range that has remained spectrally unresolved for many years. It has provided a wealth of data on solar flares, perhaps most uniquely, on the Lyman spectrum of hydrogen at high cadence and moderate spectral resolution. In this paper we concentrate on the analysis of Lyman continuum (LyC) observations and its temporal evolution in a sample of six major solar flares. By fitting both the pre-flare and flare excess spectra with a blackbody function we show that the color temperature derived from the slope of LyC reveals temperatures in excess of 10$^{4}$ K in the six events studied; an increase of a few thousand Kelvin above quiet-Sun values (typically $\sim$8000-9500 K). This was found to be as high as 17000 K for the 2017 September 6 X9.3 flare. Using these temperature values, and assuming a flaring area of 10$^{18}$ cm$^{2}$, estimates of the departure coefficient of hydrogen, $b_1$, were calculated. It was found that $b_1$ decreased from 10$^{2}$-10$^{3}$ in the quiet-Sun, to around unity during the flares. This implies that LyC is optically thick and formed in local thermodynamic equilibrium during flares. It also emanates from a relatively thin ($\lesssim$100 km) shell formed at deeper, denser layers than in the quiescent solar atmosphere. We show that in terms of temporal coverage and resolution, EVE gives a more comprehensive picture of the response of the chromosphere to the flare energy input with respect to those of the Skylab/Harvard College Observatory spatially resolved observations of the 1970's.