Chromospheric heating by the Farley-Buneman instability

Abstract

Context. Chromospheric heating produces UV emissions that can only occur in an enhanced electron temperature medium. In the quiet Sun the radiative losses are orders of magnitude larger than those in the much hotter corona. Chromospheric heating mechanisms considered previously (e.g. shock waves and nanoflares) have failed to account for the observed persistency and uniformity of UV lines and continua. Also, resistive magnetic free-energy dissipation is not efficient enough in the highly electrically conductive solar atmosphere. Aims. In this paper we consider plasma effects in the low chromosphere and propose that the Farley-Buneman (hereafter FB) plasma-instability mechanism provides the mechanism for dissipating the energy of convectively driven motions of neutral atoms into chromospheric heating in the Sun and other cool stars that have a partially ionized chromosphere. Methods. Analysis of the ion acoustic sound speed and consideration of recent measurements of magnetic field in the quiet, internetwork, solar low chromosphere are carried out in the context of understanding the characteristics and onset of chromospheric heating. The FB instability is triggered by the cross-field motion of the partially ionized gas at velocities in excess of the ion acoustic velocity. The ions acquire their cross-field velocities through collisions with the much denser chromospheric neutral atoms. Estimates of cross-field velocities are obtained from consideration of both spectral line widths and convection numerical simulations that indicate values from a few to several km s -1 at the top of the practically radiative-equilibrium low chromosphere. Results. The FB instability is triggered by the cross-field motion of the neutral component of the partially ionized gas at velocities in excess of the ion acoustic velocity. This instability occurs in the solar chromosphere because electrons become strongly magnetized just above the photosphere, while heavy ions and protons remain unmagnetized, and only at the very top of the chromosphere do they become magnetized. Conclusions. We find that convective overshoot motions are drivers of the FB instability and provide enough energy to account for the upper chromospheric radiative losses in the quiet-Sun internetwork and network lanes.