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During the large magnetic storm of November 1, 1968, the Ogo 5 spacecraft encountered the apparent direct penetration of magnetosheath plasma into the dayside magnetosphere at magnetic latitudes as low as 43°. Because this region of magnetosheath plasma occurred on magnetospheric field lines immediately adjacent to the zone of trapped energetic particles, it is interpreted to be the polar cusp. The temperature of the electrons in the polar cusp was 4 times greater than the electron temperature measured simultaneously by Vela 4B in the downstream magnetosheath at local dusk, and the electron energy distribution in the cusp was similar to the distribution observed later when Ogo 5 entered the magnetosheath. During the encounters with the polar cusp the amount of depression of the magnetic field implies the presence of magnetosheath protons together with the directly measured electrons. During this period of time the polar cusp was very turbulent at both ULF (magnetic) and VLF (electric) frequencies. Energetic (≥50 kev) electrons were also observed in the cusp. The cusp moved equatorward and poleward in response to changes in the north-south component of the interplanetary magnetic field.
Russell et al. (Fri,) studied this question.