Inductive electric fields in the inner magnetosphere during geomagnetically active periods

The present study examines the characteristics of electric fields in the nightside inner magnetosphere during geomagnetically active periods. Electric field and magnetic field measurements made by the Cluster spacecraft on their perigee passes are used. The results are summarized as follows: (1) The duskward electric field component E Y tends to be larger in the premidnight sector and off the equator, presumably corresponding to the more frequent occurrence of substorms and boundary layer crossings, respectively. (2) The occurrence distribution of E Y is biased positively with an average of 0.6–0.8 mV/m, which reflects enhanced convection at active time. (3) The occurrence distribution of E Y is also characterized by extending tails with a standard deviation larger than the twice the average. Although the occurrence ratio decreases sharply with increasing magnitude of E Y , ∣ E Y ∣ occasionally exceeds 5 mV/m. (4) The sign of E Y is well organized by the change of magnetic field. When the local magnetic configuration becomes more dipolar, E Y tends to be positive (duskward), whereas it tends to be negative (dawnward) when the configuration becomes more stretched. (5) As for strong electric fields, E Y tends to be proportional to the change of the H magnetic component, and from the induction equation, the typical spatial scale of E Y is estimated at 4.2 R E . Results 4 and 5 strongly suggests that those strong electric fields are inductive. However, the corresponding process/phenomenon can be different from event to event. It is also suggested that substorm(‐like) processes inside the ring current effectively intensify the ring current.