Convection in the magnetotail



It is well known that the Earth's magnetosphere is permeated by a large-scale electric field, E, which in its turn produces a global flow pattern known as magnetospheric convection, with velocity u = (E × B)/B2 . Convection is the key to understanding the global features of the magnetosphere, for instance the large-scale flow of electric currents along magnetic field lines (“Birkeland currents”) into and out of the polar ionosphere.

A successful theory of convection in the inner magnetosphere was formulated about 20 years ago. That, however, explains only part of the puzzle, because the inner magnetosphere receives its E from more distant regions, mainly the magnetotail and its boundary layers, where convection is still poorly understood. One problem has been the Erickson-Wolf effect [Erickson and Wolf, 1980; Hau et al., 1989] by which convection rapidly deforms the tail until a reconnection crisis is likely. Another problem was the observational uncertainty about the polytropic (or “adiabatic”) exponent γ of the gas law expected to hold in the plasma sheet [Zhu, 1990; Baumjohann and Paschmann, 1989; Huang et al., 1989]. MHD simulations appear to be of limited use, because convection theory in the inner magnetosphere goes beyond MHD and depends on actualparticle motion. In the weak fields of the magnetotail, these may involve not only guiding center drifts but also nonadiabatic motions [e.g., Büchner and Zelenyi, 1989] and finite-gyroradius effects [Macmahon, 1965; Stasiewicz, 1987].