Title and Abstract
Title: On the Ribbon Front: Chromospheric Response to the Initial Beam of Electrons
Abstract: With unprecedented high resolutions, ~20 km/pix, provided by BBSO/GST, leading edges of the moving flare ribbon are identified with unique properties. In helium line at 10830 Å, enhanced absorption, also know as "negative flare", are observed within a narrow edge about 350 km wide. At the same location, blue-shfited Hɑ, broadened and central reversed Mg lines are reported. Such behaviors of these chromospheric lines differ significantly from typical spectra at emission areas, such as the peak and trailing part of the flare ribbon. It is worth mentioning that the leading edge represent the precipitation site of the initial beam of electrons.Therefore, a plausible and straightforward explanation is that the enhance helium absorption should be caused by non-thermal effects, for instance the collisional ionisation and recombination. The corresponding modeling, involving RADYN and RH codes, confirms the dominate role of collisional ionisation in overpopulating the orthohelium levels leading to stronger absorption. These results demonstrate the importance of high resolution observations as crucial diagnostic of flare energy transport.