Flare Ribbon Fronts: The Chromosphere’s Window on Magnetic Reconnection

The narrow leading edge of solar flare ribbons (ribbon fronts) represent the first response of the Sun’s lower atmosphere to energy release following magnetic reconnection. They offer the opportunity to study the release and transport of energy during flares, and to probe models of particle acceleration. Recent very high spatial and spectral resolution observations of flare ribbons has revealed several interesting features at the ribbon fronts, including that the He I 10830 line undergoes a period of enhanced absorption before brightening, and that the Mg II spectra show unique characteristics compared to the main section of ribbon (e.g. slight blueshifts, with deeper self-absorption features). We have previously demonstrated that the He I 10830 ‘negative’ flares demand the presence of non-thermal electrons. We have now modelled the Mg II spectra using flare radiation hydrodynamic simulations, finding that the simulations that best represented the He I 10830 dimming also are the most self-consistent with Mg II ribbon front observations. Those are simulations driven by a weaker energy flux with a harder electron energy spectrum. We have also found that in order to reproduce the timescales involved (ribbon fronts have a ~1-3 minute lifetime) it is necessary for our simulations to include an extended period of precipitation by a weak flux of non-thermal electrons with a hard spectrum, before the non-thermal particle flux increases to more traditional flare-like magnitudes. The He I 10830 dimming persists for the whole period of weaker bombardment. The implication of this is that there seems to be two phases of energy release along each reconnecting field line, which has potential consequences on particle acceleration / energy release models.