Low‐latitude ionospheric disturbance electric field effects during the recovery phase of the 19–21 October 1998 magnetic storm

Low‐latitude ionospheric electric fields and currents are often strongly disturbed during periods of enhanced geomagnetic activity. These perturbations can last for several hours after geomagnetic quieting. We use incoherent scatter radar measurements from Jicamarca and Arecibo during 19–21 October 1998 to study, for the first time, the low‐latitude disturbance electric fields during the recovery phase of a large magnetic storm. On 19 October the Jicamarca data showed initially large and short‐lived (time scale of about 10–20 min) upward and westward drift perturbations in the early afternoon sector, due to the penetration of strong magnetospheric electric fields probably driven by an increase in the solar wind dynamic pressure. Following the decrease of auroral activity, very strong afternoon and nighttime zonal and vertical disturbance dynamo drifts were observed over Jicamarca but, surprisingly, only small and sporadic perturbation drifts were present over Arecibo. The latitudinal variation of the daytime zonal disturbance dynamo drifts during this day is in good agreement with that previously observed for thermospheric disturbance winds. The disturbance dynamo drifts also indicate significantly different local, storm time, and possibly latitudinal dependences for their vertical/perpendicular and zonal drift components, suggesting large variations in both the amplitude and direction of the disturbance dynamo electric field vector. In the next day, during moderately disturbed conditions, the daytime perturbation drifts were small, but following transient increases in geomagnetic activity, large upward/perpendicular drifts were observed near dusk over Jicamarca and Arecibo, where simultaneous westward perturbation drifts were also seen. These perturbations are consistent with the occurrence of strong prompt penetration electric fields reaching the magnetic equator. The measured prompt penetration drift patterns are generally in good agreement with predictions from global convection models. Later at night, under moderately disturbed conditions, relatively large zonal and meridional disturbance dynamo electric fields were observed by the two radars. Our results illustrate the large variability of the low‐latitude perturbation electric fields relative to their climatological values after large storms, probably due to the importance of additional disturbance processes. They also indicate that a much deeper understanding of solar‐wind/magnetospheric/ionospheric processes is required for accurate predictions of these electric fields.