High‐latitude energetic particle boundaries and the polar cap: A statistical study

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Instruments aboard SAMPEX have been used to monitor high latitude energetic particle boundaries. These boundaries are related to the polar cap areas threaded by open magnetic flux tubes. Variations of the area poleward of the energetic particle boundary and the boundary location itself are examined statistically for possible association with the interplanetary magnetic field (IMF) Bz, VBz, and Kp. We find the average location of the energetic particle boundary under weak IMF coupling conditions (i.e, |Bz| < 2.0 nT) to be ≈ 69°, (≈ 71° at noon and ≈ 67° at midnight) with the data showing considerable scatter. The boundary varies with IMF |Bz| for |Bz|> 2 nT and shifts equatorward at a rate of ≈ 0.4°/nT(≈ 0.1°/nT) for negative (positive) values of Bz. Varying the time offset of the hourly Bz values from 1 to 3 hours prior to the crossing had no significant effect on the correlation. The area poleward of the boundary is well represented by an offset circle fitted to the data. A weak dependence is also seen with VBz for VBz < −1.0 mV/m with the boundary location shifting equatorward at a rate of ≈ 0.9°/(mV/m). The equatorward motion of the boundary with increasing Kp is most pronounced about midnight and least pronounced at noon. In comparison with plasma boundaries the energetic particle boundaries show a lower degree of correlation with Bz, VBz, and Kp. This finding suggests a temporal and spatial decoupling between plasma and energetic particle populations.