To the Origin of the Fast X-ray Precursors of Solar Flares

We study the origin of the soft X-ray precursors that arise before the impulsive phase of a flare and testify to the rapid heating of the footpoints of flaring magnetic loops to temperatures of 10–15 MK. We show that the precursor heating rate at an observed duration $${\sim}$$ 10 s must exceed the quasi-steady-state coronal heating rate at comparable electric currents by three orders of magnitude. We suggest that the preflare heating is associated with a sharp increase in the longitudinal electric current as the Rayleigh-Taylor instability develops at the chromospheric footpoints of the flaring loops. We show that if the pulse current exceeds $$10^{11}{-}10^{12}$$ A, then the plasma Joule heating leads the ionization. In this case, a relatively large fraction of neutrals, $$n_{a}/n=10^{-5}$$ , which significantly exceeds the fraction of neutrals in a quasi-steady-state corona, is retained in the precursor plasma during the heating process. This ensures the rapid heating of the precursor region through an increase in the current dissipation rate under the Cowling resistivity associated with ion-atom collisions.