DMSP/F2 and DMSP/F4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with Kp as a function of local time. This work extends a previous study of dawn and dusk boundaries to the noon and midnight regions. The boundaries are well‐ordered by Kp in the night sector but show increasingly greater scatter from dawn to postnoon. In the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity. The equatorward diffuse auroral boundary is well fit by a circle at each activity level. The center of the circle is offset from the geomagnetic pole, and the radius of the circle increases with increasing magnetic activity. The circular fits are projected to the ecliptic plane by using the Mead‐Fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with Volland‐Stern type convection electric field predictions. Previously found variations of the electric field with Kp are confirmed, including a rotation of the axis of symmetry, away from the dawn‐dusk meridian. All equatorward auroral boundaries from DMSP for 1978 were compiled. Using the equations for boundary variations with Kp, each evening sector boundary was projected to a midnight boundary. The projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large‐scale magnetospheric convection electric field. The auroral boundary index is given in monthly plots for 1978.

The data from the SSJ/4 detector on the Defense Meteorological Satellite Program (DMSP) F6 satellite are used to study the difference in the location of the equatorward boundaries of auroral ion and electron precipitation, and the variation in the difference with magnetic local time and activity. Large geometric factors of the ion sensors of the SSJ/4 detector make identification of the ion boundary unambiguous in most cases. In this study, approximately 900 boundaries each for electrons and ions were determined from all DMSP F6 auroral passes in January of 1983. The boundaries occur over local times from 0400 to 0700 on the morningside of the oval and from 1700 to 2100 on the eveningside. The ion and electron boundaries both move systematically to lower latitudes with increasing magnetic activity, as measured by Kp. Over the evening sector sampled, the ion boundary is on average 1.4° equatorward of the electron boundary, with the difference commonly ranging up to 3°. For the morning sector sampled, the ion boundary is on average 2.6° poleward of the electron boundary with a significant number of cases with differences above 5.0°. The separation between the electron and ion boundaries is not dependent on Kp but does increase with MLT from midnight toward noon on both the morningside and the eveningside of the oval. The separation in boundaries can be explained by motion in a large‐scale, quasi‐static convection electric field if the time for development of the ion boundary is explicitly taken into account and if ion pitch angle diffusion is highly energy dependent.

Results of a statistical study of the occurrence and characteristics of the polar rain (Winningham and Heikkila, 1974) are reported. Precipitating electron data in the energy range from 50 eV to 20 keV from the SSJ/3 sensor on the polar‐orbiting DMSP/F2 satellite were used for the study. Intervals of clear polar rain were identified in all orbits in the 1‐year interval September 1977 to August 1978. The spectra from the intervals were binned in a two‐dimensional spatial array in geomagnetic latitude and magnetic local time. Separate arrays were maintained for different levels of magnetic activity, values of the components of the interplanetary magnetic field (IMF), and season. The average spectra were determined in each bin and were used to calculate the integral flux and average energy. Two‐dimensional maps of average energy and integral flux show significant large‐scale spatial variations that are roughly symmetric about an axis running prenoon to premidnight. Integral flux (average energy) decreases (increases) from the dayside to the nightside along this axis by a factor of almost 15 (3). This basic variation was maintained in all separations by magnetic activity, season, intensity of the polar rain, and sector structure, although small rotations in magnetic local time of the symmetry axis could be seen in some cases. The second most prominent variation is one that has been noted previously (Yeager and Frank, 1976; Meng and Kroehl, 1977); polar rain occurs preferentially in the northern (southern) cap for away (toward) IMF sectors. In the preferred cap the precipitation is stronger in the morning. Approximately 70% of the spectra occurred for Bz negative. The overall intensity of the polar rain increases and cools with increasing magnetic activity. The occurrence of polar rain falls within a circular region whose center is offset toward premidnight and whose radius increases with magnetic activity.

The question of the mechanism producing the premidnight region 2 currents is investigated using magnetic field and precipitating particle data from the Defense Meteorological Satellite Program F7 satellite. In this study, the variations in the extent and intensity of the region 2 currents on the nightside, in the darkened hemisphere, are determined for a small, isolated substorm on January 18, 1984, and the variations related to the characteristics of the latitudinal profile in electron and ion precipitation. The region 2 currents are found to collocate with a steep slope in the ion energy density. The ion energy density decreases with decreasing latitude. The region 2 current intensity increases proportionally to the increase in the ion energy density gradient. In the premidnight sector the equatorward boundary of auroral ion precipitation generally extends to lower latitudes than that for electrons, suggesting charge imbalance in the right sense to drive region 2 currents. We find no correlation between the extent of the latitudinal separation of the two boundaries and the strength of the region 2 currents. In fact there was one pass with a clear region 2 current where this boundary behavior reversed. The observations are consistent with an azimuthal gradient in the ion energy density near the inner edge of the plasma sheet being an important factor in the generation of the premidnight region 2 currents if the increase in the azimuthal gradient is proportional to the increase in the radial gradient there.

Approximately 2500 equatorward boundaries of auroral electron precipitation were determined for times when hourly averages of the interplanetary magnetic field and solar wind velocity V were available. The equatorward boundaries were determined from data returned by the SSJ/3 electron detector on board the DMSP/F2 satellite in magnetic local sectors between 0400 and 1100 hours on the morningside of the oval and between 1500 and 2300 hours on the eveningside of the oval. The boundary data were separated into 1‐hour zones in magnetic local time. Within each zone the boundary locations were studied as functions of Bz and Bz², VBz and VBz². Significant results were obtained when the boundaries were correlated with hourly average of Bz and VBz for the hour preceding the one in which the boundary was measured and when the linear regression was performed separately for data points where Bz ≤ 1 nT and Bz > 1 nT. For points where Bz ≤ 1 nT, the correlation with Bz and VBz gave slopes generally between 0.8° and 1.2°/nT and between 1.5° and 2.5°/mV/m, respectively, with correlation coefficients of ∼0.7. The latitudes of the intercepts tend to decrease with increasing local time in the evening sector and with decreasing local time in the morning sector. In the range above 1 nT, the slope of the best fit line for correlation with Bz, and VBz changes sign. Slopes fall in the range −0.1° to −0.4°/nT and 0.21° to 1°/mV/m. Correlation coefficients are typically worse than −0.4. Correlations of the boundary with Bz² and VBz² for both ranges of Bz are uniformly worse than those for Bz and VBz. The trend in slopes and intercepts with magnetic local time for Bz ≤ 1 nT is found to be similar to that previously found for the correlation of the boundaries with Kp (Gussenhoven et al., 1981). In this range of Bz, Kp and VBz are found to be related by the equation Kp=2.01–0.91 VBz (mV/m). From the work of Ejiri et al. (1978) an equation relating the magnetospheric electric potential to VBz is derived. The results are in agreement with the general features of the half‐wave rectifier model of the magnetosphere and measurements of polar cap potential.

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