Characterizing Electric Fields that Trigger the Farley-Buneman Instability in the Chromosphere

Intense heating in the quiet-Sun chromosphere raises the temperature from 4000 K to 6500 K but, despite decades of study, the underlying mechanism remains a mystery. This study explores the possibility that the Farley-Buneman instability contributes to chromospheric heating. An ambient electric field drives this instability, and it occurs in weakly ionized collisional plasmas in which electrons are magnetized but ions are not. Metal ions exist in the chromosphere and the picture is not so simple; some ions are weakly magnetized while neutral collisions demagnetize others. Convective motions of neutrals originating from the photosphere produce a neutral dynamo that can create the driving electric field. This study incorporates the effects of multiple, arbitrarily magnetized species of ions to the theory of the Farley-Buneman instability and examines the ramifications on heating in the chromosphere. The weakly magnetized protons introduce a sharp cutoff to the region in which the instability may occur. For a magnetic field of 30 G, the minimum threshold electric field is 25 V/m at the temperature minimum. A partial differential equation governs the electrostatic potential as a function of the neutral velocity vector and the conductivities. Solutions to this equation show that neutral flows are capable of producing an electric field greater than the threshold value. This electric field triggers the instability, which then heats the chromosphere.