Using a program for non-LTE model computations, we have computed the Lc intensities for different flare circumstances, namely, the temperature rise in the chromosphere, the downward shift of the transition region, and the nonthermal effect of a precipitating beam of electrons. All the three factors can enhance the Lc intensity by various amounts. However, the third case produces a Lc spectrum with a significantly lower color temperature than the former two cases. It thus provides a diagnostic tool to discriminate whether the nonthermal effect plays an obvious role during solar flares. It is also suggested that the relative magnitudes of Lc and Bc enhancements can be used to infer the energy distribution type, e.g., the power index of the beam electrons. Space programs carried out recently or in the future may provide relevant observations on this topic.
Finally, we like to point out that the computational results are quite model-dependent, thus they cannot be used to explain any specific event. Our purpose is only to show the potential diagnostic role of the Lc for flare nonthermal processes. It is preferably used together with line profile diagnostics.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998