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A study of ion injections at the dawn and dusk polar edges of the auroral oval

In the auroral dawn and dusk magnetosphere at altitudes of ∼2-3 R E , three distinct zones of ion and electron precipitation are commonly detected onboard Interball 2, near the polar edge of the auroral oval. From high to low latitudes the satellite encounters (1) magnetosheath/low-latitude boundary layer (LLBL) like plasma (Zone 1), (2) a mixing region with plasma characteristics between LLBL and plasma sheet (Zone 2), and (3) the auroral plasma sheet precipitation (Zone 3). Further equatorward, the satellite crosses the inner plasma sheet characterized by ion gaps in the morning sector. Inside Zones 1 and 2 impulsive ion injections are often detected. They consist of overlapping energy dispersed structures from about 10 keV down to several hundreds of eV with temperature close to that of the magnetosheath. Using trajectory computations backward in time, these dispersions are shown to be cause by time-of-flight effect from a distant source located close to the equatorial magnetopause. Whereas Zone 1 is located mainly poleward of region 1 (downward field-aligned currents at dawn and upward field-aligned currents at dusk), Zone 2 generally coincides with region 1. It is mainly located on closed field lines, as evidenced from the local detection of bouncing ion clusters. Finally, Zone 3 corresponds with region 2 of upward field-aligned currents. A statistical study of Zone 1 which is present in about 12% of the satellite passes at dawn and dusk reveals that its probability of occurrence seems to be controlled by the interplanetary magnetic field (IMF). That is, it is formed when the IMF has a northward component and tends to be radially directed. Moreover, it is more frequently encountered during periods of enhanced solar wind pressure. Detailed case studies uncover a remarkable correlation between the onsets of individual injections inside Zone 1 and those of pressure pulses in the magnetosheath. Both have a characteristic period of ∼ 200- 250 s, similar to that of Pc5 events associated with these injections and detected on board Interball 2. Altogether, these observations indicate that injections and related Alfven waves are driven by magnetosheath pressure pulses associated with a quasi-parallel bow shock. The plasma penetration mechanism remains to be understood.