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 n T 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 B z 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. Paper number 94JA01655. 0148-0227/95/94JA-01655505.00 variations of the interaction between the solar wind and Earth's magnetic field will change the force balance between the two sides of the magnetopause, and place the magnetopause into constant movement. Magnetopause processes, such as magnetic field merging (magnetic reconnection), compression or decompression of the magnetopause by solar wind plasma, plasma injections into the low-latitude boundary layer, or the Kelvin-Helmholtz instability, and the ionospheric signatures of these processes have been discussed by many investigators [Dungey, 1961; Lanzerotti et al.son and Southwood, 1991]. The discovery of flux transfer events (FTEs) [Russell and Elphic, 1979] indicated that the dayside magnetic reconnection process was dynamic, occurring intermittently every 5-15 min when the interplanetary magnetic 7553 7554 LIN ET AL.: STATIS...

The data base from the 1985–86 Balloon Campaign at South Pole Station, Antarctica, has been searched for impulsive magnetic and electric field events that occurred when IMP 8 was in the solar wind and data were acquired. Two events are shown that appear to be similar to published examples of high latitude “twin vortex” current systems. Neither event was preceded by a dynamic pressure pulse in the solar wind. These examples disprove the hypothesis that all such high‐latitude impulses arise solely from dynamic pressure pulses produced by the solar wind. Further, on a statistical basis, a total of 42 events were found for which IMP 8 data are available. 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. Only 3 of the BZ north cases and only 5 of the BZ south cases were preceded by pressure pulses greater than 0.4 nPa in amplitude.

Trajectories of electrons with energies of 10-2000 MeV were studied using Tsyganenko's global quantitative model of the magnetosphere. Results were compared with balloon observations of atmospheric, return albedo, and cosmic electrons made at Prince Albert on August 26, 1987. Active geomagnetic conditions were associated with unusual variations in the flux of electrons on that date, which we interpret as a manifestation of an anomalously low and highly variable geomagnetic cutoff. Increasing the ring current intensity in the Tsyganenko model reduces the calculated effective cutoff rigidity somewhat, but to explain all of our observations, we must postulate that trajectories traditionally assumed to be forbidden are sometimes systematically converted into allowed trajectories. Within this model we can also identify field lines on which bursts of auroral X rays were generated. 70 MeV brc/Brc=0.77 Fraction Forbidden=0.90 -•-. .

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