Chromospheric plasma and the Farley-Buneman instability in solar magnetic regions

Laboratory for Atmospheric and Space Physics (LASP), University of Colorado, Boulder, CO, 80309, USA e-mail: fontenla@lasp.colorado.edu

Received: 21 June 2005
Accepted: 26 July 2005

Abstract

We study the plasma parameters in recent models of the observed magnetic features in the solar atmosphere and find that electrons are strongly magnetized in the chromosphere but protons are unmagnetized up to the transition region. Considering the magnetization and the classical Pedersen conductivity we find that magnetic diffusion is too small for effectively affecting propagating MHD waves of periods of a few minutes. However, the chromospheric-plasma parameters suggest a scenario in which upward-propagating fast-mode MHD waves of mHz frequencies would trigger the Farley-Buneman plasma instability at chromospheric layers where horizontal magnetic fields are present. We show that, because of the collisions between charged particles and neutral H atoms, the conditions in the chromosphere meet the instability criteria if the MHD wave velocity amplitude is lower but near the adiabatic sound speed. The instability growth is much faster than the wave frequency and the instability would quickly saturate. The electrostatic plasma waves resulting from the instability are expected to produce anomalous resistivity and wave energy dissipation that would heat the chromosphere as well as absorb the p-modes in magnetic regions.