On the temporal evolution of midlatitude F region disturbance drifts

Abstract

Superposed Epoch Analysis (SEA) is used to examine a 5‐year (1999 to 2003) database of Digisonde drift measurements made at Bundoora (145.1°E, 37.7°S geographic, 49°S magnetic), Australia, to determine the temporal evolution of midlatitude F region electric fields associated with the magnetospheric (lifetimes of about an hour) and ionospheric disturbance (lifetimes of a few to several hours) dynamos. The magnetospheric effects are qualified using AE “step‐up” and “step‐down” temporal filters and the SEA results reveal features fairly consistent with under‐ and over‐shielding conditions described by the Rice Convention Model (RCM). The disturbance dynamo effects are qualified using onset times of AE‐ and Dst‐defined storms. These onset times are further subdivided into three categories: short‐, medium‐ and long‐duration storms. We find there are no noticeable changes in ionospheric electric fields near Bundoora during short‐duration AE‐defined storms. In contrast, the SEA responses for medium‐duration AE‐defined storms and short‐ and medium‐duration Dst‐defined storms are in good agreement with the ionospheric disturbance dynamo predictions. The SEA results associated with long‐duration AE or Dst‐defined storms indicate that the electric field perturbations agree with the effects of the high‐latitude two‐cell convection pattern expanding to the latitude of the station (49° magnetic) for up to 20 h after t = 0 h. Overall, the perturbation drifts are predominantly westward with largest amplitudes in the dusk to midnight sector and continued for nearly 50 h in storm time. These enhancements are also consistent with the influence of the sub‐auroral polarization stream (SAP) extending to the latitude of the station.