FAST-TO-ALFVEN MODE CONVERSION MEDIATED BY THE HALL CURRENT. I. COLD PLASMA MODEL

The photospheric temperature minimum in the Sun and solar-like stars is very weakly ionized, with an ionization fraction f as low as 10(-4). In galactic star-forming regions, f can be 10(-10) or lower. Under these circumstances, the Hall current can couple low-frequency Alfven and magnetoacoustic waves via the dimensionless Hall parameter epsilon = omega/Omega(i)f, where omega is the wave frequency and Wi is the mean ion gyrofrequency. This is analyzed in the context of a cold (zero-beta) plasma and in less detail for a warm plasma. It is found that Hall coupling preferentially occurs where the wavevector is nearly field-aligned. In these circumstances, Hall coupling in theory produces a continual oscillation between fast and Alfven modes as the wave passes through the weakly ionized region. At low frequencies (mHz), characteristic of solar and stellar normal modes, epsilon is probably too small for more than a fraction of one oscillation to occur. On the other hand, the effect may be significant at the far higher frequencies (Hz) associated with magnetic reconnection events. In another context, characteristic parameters for star-forming gas clouds suggest that. (1) or more full oscillations may occur in one cloud crossing. This mechanism is not expected to be effective in sunspots, due to their high ion gyrofrequencies and Alfven speeds, since the net effect depends inversely on both and therefore inverse quadratically on field strength.