Photospheric flare line diagnostics and the associated velocity fields

Discernable differences in the induced photospheric velocity field were found, dependent on the combination of electron beam parameters used, ranging from near-zero induced line-of-sight velocities and up to 1 km/s. The physical characteristics of these models were then used as inputs to the RH radiative transfer code to synthesise flare line profiles of the 617.3 nm, 630.1 nm, and 630.2 nm Fe I lines. <br><br>The inferred velocities from the Doppler shifts and velocity bisectors of the flare line profiles were compared against the true velocity field of the lower photosphere and evaluated if they accurately represented the lower photosphere line-of-sight velocities.

The primary finding of this study was that changes to the contribution functions of the three spectral lines induced by flare heating resulted in increased emission from non-photospheric regions of the solar atmosphere, namely the lower chromosphere and in several cases chromospheric condensations. The influence of these higher regions' emission was the misleading Doppler shift of the flaring Fe I line profiles to apparent redshifts (downflows) or blueshifts (upflows) that were not representative of the true photospheric velocities.

In the most extreme cases, for over 40% of several model's evolution the "observed" Doppler shifts directly contradicted the nature of the photospheric velocity field, e.g. indicating a downflow while the photosphere is purely upflowing. These periods of misleading Doppler shifts in observables were generally, but not exclusively, during the period of beam heating of a model. After the electron beam heating ended and the solar atmosphere was allowed to evolve further the observables generally showed greater agreement with the photospheric velocity field.<br><br>This analysis was expanded to investigate two more extreme cases of a high-energy flux electron beam heated solar plage initial atmosphere, and the flare from an M dwarf star.

A breakdown of the total emergent intensity profiles into the three dominant components that define the line profile shapes through the use of the contribution functions revealed that photospheric velocity field information was generally irretrievable in these models. In both cases, the emission from regions higher in the atmosphere was so significant to the total profile shape that the traditional assumption that these three lines are solely photosphere-formed could provide drastically incorrect interpretations of the photospheric conditions.

In particular, the stellar flare case line profiles were so dominated by emission from non-photosphere regions that the total profile appears in emission while the photospheric emitted absorption profile containing the desired line-of-sight velocity information is completely lost.