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We present a theoretical argument suggesting that steady, adiabatic convection probably cannot occur throughout a closed‐magnetic‐field‐line region that extends into a long magnetotail. If plasma in a typical flux tube in the Earth's outer plasma sheet were compressed adiabatically as it convected into the near‐Earth part of the plasma sheet, the plasma pressure would be absurdly large there. This excess‐pressure problem is demonstrated numerically for several standard models of the magnetospheric magnetic field, for the case of isotropic pressure. We argue that the excess‐pressure problem results from the general shapes of field lines in the inner and outer plasma sheet, and not from simple inaccuracies in all the magnetic‐field models. We hypothesize that sunward convection must necessarily be time‐dependent, and that the magnetospheric substorm may be the essential time‐dependent process in which plasma is suddenly and non‐adiabatically released from plasma‐sheet flux tubes.
Erickson et al. (Sat,) studied this question.