Ionospheric electric-field disturbances at middle and low latitudes are generated both by time-varying magnetosphere-ionosphere interactions at high latitudes accompanied by direct penetration to lower latitudes [e.g., it Nishida et al., 1966; Vasyliunas, 1970, 1972; Jaggi and Wolf, 1973; Fejer et al., 1979, 1990; Gonzales et al., 1979; Kelley et al., 1979; Peymirat and Fontaine, 1994; Fejer and Scherliess, 1995; Kikuchi et al., 2000] and by secondary effects of disturbance thermospheric winds [e.g., Blanc and Richmond, 1980; Spiro et al., 1988; Forbes and Harel, 1989; Fejer et al., 1990; Fejer and Scherliess, 1995; Fuller-Rowell et al., 2002]. These disturbance electric fields can have a strong influence on the low-latitude ionosphere [e.g., Tanaka, 1981; Kelley and Maruyama, 1992; Deminova, 1995; Lakshmi et al., 1997; Abdu, 1997; Palmroth et al., 2000].
 The direct penetration of electric fields from the polar cap to low latitudes is modulated by interaction of the hot plasma in the magnetosphere with the ionosphere. This interaction drives the Region-2 currents that flow into and out of the auroral region equatorward of the polar cap. The ionospheric electric field associated with these Region-2 currents has a tendency to counteract the east-west component of the electric field coming from the polar cap, and thus to reduce the penetration of the polar cap electric field toward middle and low latitudes [e.g., Schield et al., 1969; Vasyliunas, 1972]. This effect is called “shielding” [e.g., Wolf and Jaggi, 1973]. Modeling studies show that the shielding effect is established with a characteristic timescale on the order of 3–300 min, depending on magnetospheric plasma properties and ionospheric conductivity [Vasyliunas, 1972; Jaggi and Wolf, 1973; Wolf and Jaggi, 1973; Southwood and Wolf, 1978; Wolf et al., 1982; Senior and Blanc, 1984; Spiro et al., 1988]. 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 [e.g., Kelley et al., 1979; Spiro et al., 1988; Fejer et al., 1990]. According to model studies, this temporary reversal should last only on the order of 10–60 min [e.g., Spiro et al., 1988; Peymirat et al., 2000].
 Equatorial ionospheric electric fields are also affected by thermospheric winds in the dynamo region, which lies primarily between 100 km and 200 km at day and in the lower F region at night. The winds are driven by day-night differences in solar heating, by upward propagating atmospheric tides, by collisional interaction with rapidly convecting ions in the presence of strong high-latitude electric fields, and by Joule heating associated with the strong high-latitude electric currents. The latter two sources are highly variable, and depend on the level of geomagnetic activity. They produce thermospheric disturbance winds, which affect the global ionospheric dynamo [Blanc and Richmond, 1980; Spiro et al., 1988; Fejer et al., 1990; Fuller-Rowell et al., 2002]. Because of the inertia of the neutral air, a few hours are required to set up disturbance winds. Once set up, they can persist for several hours. Thus low-latitude electric-field disturbances associated with the disturbance winds tend to be more persistent than those associated with changes in direct penetration from the polar cap and with changes in the Region-2 currents. Different aspects of the dynamo influences of disturbance winds have been given different names. Banks  and Coroniti and Kennel  pointed out that high-latitude winds accelerated by coupling with the convecting ions can have a dynamo effect that may tend to smooth out fluctuations of magnetospheric-ionospheric convection by acting as what Banks  called a type of “flywheel.” Blanc and Richmond  examined the ionospheric “disturbance dynamo” associated with midlatitude winds that are driven by high-latitude Joule heating. Spiro et al.  examined the dynamo effects of “fossil winds,” which, like winds of the “flywheel,” are accelerated by strong ion convection in the auroral regions, but which can be found lying equatorward of the Region-2 currents during the recovery phase of a magnetic storm, after the polar cap and auroral region have contracted poleward. Because the disturbance winds lie equatorward of the Region-2 currents, any electric fields they generate are not shielded from lower latitudes, and thus may affect the equatorial electric fields.
 The present study pursues the earlier fossil-wind studies by Spiro et al.  and Fejer et al. , which used the Rice Convection Model (RCM) [Harel et al., 1981] to quantify the effect of a sudden decrease of the polar cap potential drop that corresponds to a northward turning of the IMF. This decrease results in an immediate overshielding effect, which then decays away. Spiro et al.  and Fejer et al.  compared their simulations with observations of the SUNDIAL 1984 and 1986 campaigns and found general agreement of the amplitudes. However, the observed disturbances persisted for hours, while the simulated disturbances lasted only about 10–60 min. To reconcile the model with the observations, Spiro et al.  examined how fossil winds might affect global electric fields. Only when they specified an equatorward displacement of the neutral wind distribution associated with ion convection in the auroral region during the period of southward IMF, were they able to get long-lasting equatorial electric-field perturbations that agreed with the observations. This equatorward displacement of the winds relative to the Region-2 currents was intended to simulate the contraction of the polar cap, which in reality was held fixed in their simulations. Fejer et al.  pursued this study with the RCM and discussed the fossil wind idea in a more quantitative way. They reached similar conclusions as Spiro et al. , but also proposed another mechanism which should lead to mid- and low-latitude electric field perturbations similar to those of the fossil wind mechanism: the reconfiguration of the magnetospheric magnetic field that occurs when the magnetic activity quiets very abruptly, resulting in a poleward motion of the polar cap necessary to explain the mid- and low-latitude perturbations. However, they did not simulate this proposed mechanism.
 In the present study, we use the Magnetosphere-Thermosphere-Ionosphere-Electrodynamics General Circulation Model (MTIE-GCM) built by Peymirat et al. . The MTIE-GCM couples the model of inner-magnetospheric plasma convection of Peymirat and Fontaine  with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model discussed by Richmond et al. . Although the MTIE-GCM uses a simple dipole geomagnetic field and thus cannot simulate the influences of magnetic-field reconfiguration proposed by Fejer et al. , it has an advantage over the RCM used in the earlier fossil-wind studies in that the winds in the MTIE-GCM are calculated using the full three-dimensional dynamical equations with realistic forcing. Furthermore, in the present study we make an attempt to simulate realistically the expansion and contraction of the polar cap, instead of artificially moving the wind pattern equatorward with respect to a fixed polar cap boundary.