Perturbation magnetic fields and current systems associated with eastward drifting auroral structures

Abstract

Magnetospheric substorms feature the development of distorted discrete auroral forms known as surges and loops. The westward traveling surge is known to expand in discrete steps into the evening sector, the western extremity of the surge acquiring a velocity of upward of 1 km/s in the early stages of its formation. Magnetic perturbations associated with the westward traveling surge are known to reach hundreds of nanotesla in the region below the surge form. In this study we report the existence of eastward traveling regions of large magnetic perturbations, whose velocities of propagation are in the range of 0.8–2 km/s. We find that the basic structures of these propagating perturbation regions are maintained over at least 12° of longitude, although the amplitude of the disturbance may vary considerably over that longitude range. We have modeled the eastward propagating disturbance by a three‐dimensional current system of narrow longitudinal extent in which antiparallel Birkeland current sheets are linked by southward flowing current. We have used coincident riometer data to infer that the electrons responsible for E region ionospheric conductivity enhancements can also cause ionospheric absorption; this permits us to extimate the fluxes of energetic electrons responsible for the eastward propagating disturbance and thus to make a rough estimate of the equatorward electric field in the morning sector auroral oval. In particular, the longitudinal scale size of the eastward propagating structures is ∼5°–10° at 67.5°N.