This paper extends a recent study of electric field penetration into the inner magnetosphere observed by the Combined Release and Radiation Effects (CRRES) satellite and the Defense Meteorological Satellite Program (DMSP) satellite F8 during the magnetic storm of June 4--6, 1991, to consider its ionospheric consequences. Effects include the development of >1 km/s subauroral ion drift (SAID) structures, the formation of midlatitude density troughs, and the vertical transport of equatorial plasma bubbles. Nearly simultaneous auroral electron and plasma drift measurements were acquired by three DMSP satellites with F8 and F9 in one hemisphere and F10 in the other. Moderate to strong SAID structures were consistently detected for ~10 hours during the early main phase of the storm. Weak SAIDs were encountered during ~8 hours of the early recovery phase. DMSP data show that SAIDs with similar characteristics developed at magnetically conjugate locations and extended for at least 3 hours in local time. Simultaneous measurements show that the SAIDs spanned temporally growing but latitudinally narrow plasma density troughs. These observations suggest that the magnetospheric sources of SAIDs act more like voltage than current generators. Energetic electron fluxes, electric fields, and plasma waves measured by CRRES indicate that during this storm the ring current shielding changes and SAID sources were located in regions of high plasma density characteristic of the plasmasphere. The sequence in which DMSP detected equatorial plasma density irregularities is consistent with model predictions that stormtime electrodynamics at low latitudes operate on distinctive fast and slow timescales [Fejer and Scherliess. 1997>. ¿ 2000 American Geophysical Union