A rapid increase of the polar cap potential is not effectively shielded from lower latitudes on
timescales shorter than this, and thus strong, rapidly varying disturbance electric fields can reach the equator. An interesting situation arises when strong magnetospheric convection
has been active for an extended period of time and then undergoes a rapid decrease, as can happen, for example, when the interplanetary magnetic field (IMF) suddenly turns northward. The magnetospheric hot plasma that feeds the Region-2 currents, which had been strong enough largely to counteract the directly penetrating electric field from the polar cap, is then temporarily unbalanced, creating a shielding electric field that is too strong: the equatorial electric
field is not simply shielded from the polar cap, but is ‘‘overshielded,’’ such that the net equatorial electric field associated with direct penetration can temporarily reverse direction