On the temporal evolution of midlatitude F region disturbance drifts

Superposed Epoch Analysis (SEA) is used to examine a 5-year (1999 to 2003) database of Digisonde drift measurements made at Bundoora (145.1A?E, 37.7A?S geographic, 49A?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 (49A? 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.