Statistical studies of impulsive events at high latitudes

Abstract

A statistical study has been made of the high‐latitude impulsive events that were observed during the 1985–1986 South Pole Balloon Campaign. The events were selected by searching for unipolar pulses ≥ 10 nT above background in the vertical component of the magnetic field on the ground and/or pedestal or “W” shaped horizontal electric field perturbations ≥ 10 mV/m in amplitude and accompanied by perturbations in the vertical electric field at balloon altitude. A main event list comprising 112 events was compiled from the 468 hours of data available. Three aspects of the events were examined: the solar wind conditions prior to the event, local time of observation, and intrinsic properties of the events. The local time distribution was obtained from the 112 entry main event list and was found to be nearly uniform across the dayside, with no midday gap. The event rate found using our low‐amplitude selection criteria was 0.7 event/hr, comparable to expectations based on in situ studies of the magnetopause. A total of 42 events were found for which data were available from Interplanetary Monitoring Platform (IMP) 8. Of these events, 12 occurred when the ZGSM component (BZ) of the interplanetary magnetic field (IMF) was northward and 30 occurred when BZ was southward or fluctuating. Only three of the BZ northward cases and only five of the BZ southward cases were preceded by pressure pulses greater than 0.4 nPa in amplitude. Ten of the events were studied in detail by means of a model‐fitting method discussed elsewhere. This method infers values of several parameters, including the total current flowing in a coaxial or monopole system and a two‐dimensional dipole system. The intrinsic properties of the events showed that only ∼ 10% of the total current contributed to momentum transfer to the high‐latitude ionosphere, that the direction of the motion depended more on local time of observation than IMF By, and that events were usually several hundred kilometers in size. The observed Bz control found in the 42 event list and the prevalence of coaxial current dominated events are inconsistent with the predictions of the pressure pulse model.