Spectroscopy of the hydrogen Lyman lines during solar flares using MEGS-B
Stephen Brown
University of Glasgow
The hydrogen Lyman lines provide important diagnostic information about the solar chromosphere during flares, but until recently we have had no
systematic measurements. The Multiple EUV Grating Spectrograph B (MEGS-B) detector on the Solar Dynamics Observatory observes the Sun continuously for 3 hours per day.
Operating in the 37-105nm wavelength range with a resolution of 0.1nm, MEGS-B covers the hydrogen Lyman lines (from Lyman-beta upwards) and
continuum. We use the hydrogen Lyman lines to investigate the flows during solar flares, presenting two methods for the detection of Doppler-shifts in the spectral lines during 18 flares from the current
solar cycle. The first method performs Gaussian fits to the lines to obtain their positions, and compares the quiet-Sun centroids with the flaring ones to obtain
the Doppler shifts. The second method uses cross-correlation to detect wavelength shifts between quiet-Sun and flaring spectra. Both methods allow a precision of 10kms^-1, although the cross-correlation
method allows for greater time-resolution as it does not attempt to time-average the spectra. We find
that most Doppler velocities are on order of tens of kms^-1, and that the Doppler velocities increase as the flare intensity increases.
In addition, around 70% of the flares exhibit redshift in the Lyman lines during their progress, with the remainder exhibiting blueshifts.