Rotationaily driven interchange instability: Reply to André and Ferrière

Abstract

We derive the dispersion relation for the rotationaily driven interchange instability, using the same basic equations as in our previous work (Vasyliunas, 1994; Pontius, 1997) but taking care to keep the terms we had overlooked (as pointed out by André and Ferrière (2007)), and show that inclusion of the Coriolis force does modify the dispersion relation, reducing the growth rate of the instability and adding an oscillating component. Its effect becomes negligible only in the limit of azimuthal wavelength very short in comparison to radial distance, in the limit of plasma acceleration time very short in comparison to rotation period, and in the singular case of flux tube content proportional to equatorial field strength. André and Ferière (2007) find no effect of the Coriolis force because they assume all wavelengths to be short. For plasma acceleration time longer than the rotation period, the effect of the Coriolis force can restrict the range of radial gradients that are unstable. Although not a generally valid result, neglect of the Coriolis force effects is in practice applicable as long as the fastest-growing (hence presumably the most significant) instabilities are those with very short wavelengths. Copyright 2007 by the American Geophysical Union.