By using the self-consistent stability theory it is shown that during the growth phase of a magnetospheric substorm the stability of the tail configuration decreases. Taking two-dimensionality into account explicitly, we show that the free energy built up during the growth phase can be released when the plasma sheet has become sufficiently thin and/or the normal magnetic field component has been sufficiently reduced. Then the ion-tearing mode will abruptly set in and lead to changes in the magnetic field topology. It is speculated that there is a tendency for a macroscopic neutral line to form fairly close to the earth. Lack of equilibrium will lead to energy dissipation and enhanced precipitation.
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