Penetration of magnetosheath plasma to low altitudes through the dayside magnetospheric cusps

Abstract

Daytime high-latitude fluxes of low energy (<1 kev) electrons and protons have been observed with the soft particle spectrometer on the polar satellite ISIS-I. These fluxes occur just above the limit Λ[subscript CL] of closed geomagnetic field lines in the dayside magnetosphere. The boundary, Λ[subscript CL], can be identified through a sharp decrease in the flux of electrons of somewhat higher energy (~10 kev). The electron energy spectrum shows a sharp peak at about 100 to 200 ev, with a total flux of the order of 10� electrons cm�² ster�¹ sec�¹ carrying an energy of a few tenths of an erg cm�² ster�¹ sec�¹. The electron and proton spectral shapes are shown to be similar to those in the magnetosheath and for protons the absolute intensities are approximately equal (no absolute intensities could be found in the literature for electrons). The proton spectrum peaks at a slightly higher energy (~300 ev) again in agreement with magnetosheath observations. The proton flux is typically >10� cm�² ster�¹ sec�¹ with typical energy fluxes in the range 0.01 to 0.1 ergs cm�² ster�¹ sec�¹. It is concluded that solar wind plasma can penetrate to low altitudes through the high-latitude cusp in the magnetopause, often referred to as the neutral point. This flux is related to a number of geophysical phenomena, including magnetospheric surface currents, day time auroras, VLF and LF emissions, ionospheric irregularities, and geomagnetic fluctuations.