During the great magnetic storm of 15 July 2000 the ROCSAT-1 low Earth-orbiting satellite detected an unusually large region of density depletion at low latitudes (<35¿) in the ionosphere in corotation with the South Atlantic anomaly (SAA). Adjacent to the region of density dropout was a region of density enhancement with irregularities. We report characteristics of convective electric fields deduced from analyzing measurements of ROCSAT-1 drift meters and a retarding potential analyzer in the region of ion density structures during the event. Specifically, the ROCSAT-1 satellite observed penetration of large convective electric fields at apex height below 5000 km (L = 1.78) in the low-latitude nightside ionosphere. The zonal electric field component was eastward, corresponding to outward convection, and the radial electric field component on the magnetic meridian plane was outward, corresponding to westward convection. In general, both components of the observed convective electric fields were reduced and characterized with wave structures or fluctuations with a wavelength of the order of 100 km in the density enhancement region. In the density depletion region, electric fields increased steadily as density decreased. Distinctly, the observed electric field had a peak near the apex height of the magnetic field line connected to the boundary between ion density dropout and enhancement. The magnitude of the peak electric field was largest in the beginning of the storm main phase and decayed with time. We have conducted a modeling calculation to demonstrate that energetic electron precipitation in the SAA region could increase Hall conductivities in the E region. We have theoretically examined electric field patterns for a circular region of enhanced conductivity in the ionosphere in the presence of a background electric field. Theoretically, electric fields are reduced inside the high-conductance region but are enhanced immediately outside the conductive region with radial decay from the conductive boundary. It is interpreted that the localized electric field signatures observed by ROCSAT-1 are produced when ionospheric conductivities are enhanced in the density enhancement region and reduced in the density depletion region. On the basis of modeling results we speculate that energetic particle precipitation might occur in the density enhancement region near the SAA during magnetic superstorms. The obtained results suggest that ionospheric density variation in the SAA region during intense magnetic disturbances might play an important role in the low-latitude ionosphere electrodynamics.