Polar observations of transverse magnetic pulsations initiated at substorm onset in the high‐latitude plasma sheet

Abstract

This paper presents simultaneous observations of 6‐mHz magnetic pulsations in the nightside high‐latitude plasma sheet, inner plasma sheet at the geosynchronous distance, and auroral region on the ground in association with substorm onset. We study an isolated substorm (AE ∼ 300 nT) onset on 19 October 1999 at ∼0145 UT. The Polar spacecraft was located in the plasma sheet near the plasma sheet boundary and was magnetically conjugate with the Greenland west coast magnetometer chain. Polar measured large‐amplitude transverse magnetic (10 nT; toroidal) and electric (20 mV m−1) field oscillations (∼6 mHz) during the early development of the negative bay (300 nT) in Greenland. On the ground, pulsations (50 nT) with the same frequency were superimposed on the negative bay. The geostationary GOES 8 spacecraft was located 2 hours west of Greenland. It observed compressional magnetic field oscillations (1 nT and ∼6 mHz). At Polar the pulsations were initiated ∼3 min before the first outward propagating substorm signature, an abrupt enhancement of the plasma sheet electron fluxes. Such a rich data set allows us to study in detail the spatial origin of the pulsations, their timing with respect to the negative bay onset, and their role in initiation of the substorm current wedge. It is concluded that the pulsations were initiated between the plasma sheet boundary and the tailward expanding region of the enhanced plasma sheet electron fluxes. The pulsations were then later observed on the ground and at GOES 8. It can also be argued that the pulsations were an integral part of the formation of the substorm current wedge. Finally, we suggest that the pulsations were generated by periodic variations in the rate of the current diversion from the braking region of the earthward flows generated by reconnection at the near‐Earth neutral line.