The Temporal Behaviour of Lyman-alpha Emission During Solar Flares From SDO/EVE

Ryan O. Milligan, Phillip C. Chamberlin

Published 2015-06-08Version 1

Despite being the most prominent emission line in the solar spectrum, there has been a notable lack of studies devoted to variations in Lyman-alpha (Ly$\alpha$) emission during solar flares in recent years. The few examples that do exist, however, have shown Ly$\alpha$ emission to be a substantial radiator of the total energy budget of solar flares (on the order of 10%). It is also a known driver of fluctuations in earth's ionosphere. The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory now provides broadband, photometric Ly$\alpha$ data at 10 s cadence, and has observed scores of solar flares in the 5 years since it was launched. However, the time profiles appear to display a rise time of tens of minutes around the time of the flare onset. This is in stark contrast to the rapid, impulsive increase observed in other intrinsically chromospheric features (H$\alpha$, Ly$\beta$, LyC, C III, etc.). Furthermore, the Ly$\alpha$ emission peaks around the time of the peak of thermal soft X-ray emission, rather than during the impulsive phase when energy deposition in the chromosphere - often assumed to be in the form of nonthermal electrons - is greatest. The time derivative of Ly$\alpha$ lightcurves also closely resembles that of the time derivative of soft X-rays, rather reminiscent of the Neupert Effect. To establish whether this atypical behaviour is a characteristic of flare heating in the lower solar atmosphere during explosive events, or a manifestation of the broadband nature of the EVE observations, comparisons have been made with spectrally-resolved Ly$\alpha$ measurements during flares from SORCE/SOLSTICE, and other broadband photometers such as PROBA2/LYRA and GOES/EUVS-E.