Direct Effects of the IMF on the Inner Magnetosphere

Several direct and well-established inner-magnetospheric effects following changes in Interplanetary Magnetic Field direction find natural explanations in terms of the Rice Convection Model. A southward turning of the IMF normally causes an increase in both cross-polar-cap potential drop and in polar-cap size. In RCM simulations, these two factors combine to produce a condition termed undershielding, characterized by increased penetration of the dawn-dusk convection electric field into the inner magnetosphere, erosion of the nightside plasmapause, and drainage of plasmaspheric plasma via plumes that stretch to the dayside magnetopause. A northward turning of the IMF causes a condition known as overshielding, characterized by dusk-to-dawn directed electric fields across the inner magnetosphere. A recent run with the coupled BATSRUS/RCM computer code suggests that the overshielding electric field peaks 12-25 minutes after the IMF direction at the dayside magnetopause turns northward, and that this time delay is about the same on both the day- and night-sides of the Earth. Changes in solar wind and IMF conditions may also influence the inner magnetosphere in more subtle ways, through their influence on the plasma-sheet plasma distribution represented by the specific entropy. Results of combining a Tsyganenko magnetic field model and Tsyganenko-Mukai representation of plasma-sheet plasma suggest that a northward turning of the IMF may reduce the specific entropy in the plasma sheet and cause interchange instability in the plasma sheet and auroral ionosphere.