Low-Energy-Proton Regime in the Geomagnetic Tail at Lunar Distance
The Rice University Suprathermal Ion Detector Experiments (SIDE) on the lunar surface have observed a region of low-energy protons in the geomagnetic tail. This region is similar to the ‘boundary layer’ reported by Hones et al from Vela observations and to the’plasma mantle’ reported by Rosenbauer et al from HEOS-2 in the polar magnetosphere. The plasma is characterised by a proton differential energy spectrum with its peak usually at 50–100 eV, and is clearly a phenomenon distinct from the magnetosheath and the plasma sheet. These protons are seen outside the plasma sheet, but inside the inner edge of the magnetosheath. Continuous observations have been made over 17 R E in the cross-tail direction and over 12 RR E in the direction perpendicular to the ecliptic. Integral fluxes observed are generally 106 –108 cm−2 s−1 sr−1, number densities 0.1–5 cm−3, bulk velocity~100–250 km/s, temperature ~ 0.4–5×105°K. The fraction of time these protons are seen while in this region of the tail varies widely from lunation to lunation.
Preliminary multispacecraft comparisons are being investigated, and it may be possible to follow the evolution of this region from near Earth out to lunar distance. There is reasonable expectation that one of the SIDE instruments will still be operating during the IMS.
KeywordsPlasma Sheet Lunar Surface Bulk Velocity Integral Flux Geomagnetic Tail
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