Properties of large electric fields in the plasma sheet at 4-7 R _E measured with Polar

Measurements from the Polar satellite provide evidence for large electric field structures in the plasma sheet at geocentric distances of 4-7 R _E. These structures had amplitudes perpendicular to the ambient magnetic field that can exceed 100 mV m^-1 (6 s averaged). Two years (from May 1, 1996, to April 30, 1998) of electric field data (E_z component, approximately along GSE z) were surveyed. The distribution in invariant latitude (ILAT) and magnetic local time (MLT) of large perpendicular electric field events (defined as ≥ 20 mV m^-1 for a 6-s average) delineates the statistical auroral oval with the majority of events occurring in the nightside centered around midnight and a smaller concentration around 1500 MLT. The magnitude-versus-altitude distribution of the electric fields between 4 and 7 R_E in the nightside could be explained by models which assume either shear Alfvén waves propagating into regions of larger background magnetic fields or electrostatic structures being mapped quasi-statically along equipotential magnetic field lines. In addition, this survey yielded 24 very large amplitude events with |ET | ≥ 100 mV m^-1 (6 s averaged), all of which occurred in the nightside. In the spacecraft frame, the electric field structures occurred on timescales ranging from 10 to 60 s. About 85% of these events occurred in the vicinity of the outer boundary of the plasma sheet; the rest occurred in the central plasma sheet. The polarity of the electric fields was dominantly perpendicular to the nominal plasma sheet boundary. For a large fraction of events (≤50%) the ratios of electric and magnetic fields in the period range from 10 to 60 s were consistent with Alfvén waves. Large Poynting flux (up to 2.5 ergs cm^-2s^-1) dominantly directed downward along the background magnetic field was associated with 21 events. All 24 events occurred during geomagnetic disturbances such as magnetic substorms. A conjugate study with ground stations for 14 events (out of the 24 events) showed that these structures occurred during times of rapid changes in the H component (or X component) of magnetometer data. For most events this time corresponded to the expansion phase; two events occurred during a quick recovery of the negative H bay signature. Thus there is evidence that large electromagnetic energy transfer processes in the plasma sheet occur during the most dynamic phase of geomagnetic disturbances. From the statistical analysis it was found that Polar observed events larger than 100 mV m^-1 (50 mV m^-1) in the plasma sheet between 2100 and 0300 MLT with a 2-4% (15%) probability per crossing. These probabilities will be compared to the probability of substorm occurrence during Polar plasma sheet crossings.