Sporadic plasma sheet ion injections into the high-altitude auroral bulge:Satellite observations

We report on a new feature of auroral substorms, namely, the sporadic thoug h recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structure s (TDIS). Our analysis builds on a combination of measurements from Interba ll-Auroral, from UV imagery onboard Polar, from ground magnetometers, and a lso from observations on Geotail and from geostationary spacecraft. Backwar d tracing of ion trajectories from Interball-Auroral orbit using realistic three-dimensional magnetic and electric field models indicates that the inj ection region can extend over a wide range of radial distances, from simila r to 7-40 R-E in the nearly equatorial magnetosphere. Both hydrogen and oxy gen ions are shown to be injected toward the Earth's upper ionosphere. At I nterball altitudes we find that ion injections are associated with two type s of low-frequency torsional oscillations of the magnetic field: (1) shear Alfven waves with a period of a few minutes with the highest amplitude near the bulge front and decreasing amplitude at lower latitudes and (2) higher -frequency shear Alfven waves of the P1B type, strictly restricted to the p oleward boundary of the surge, with a characteristic period of similar to 4 0 s. The systematic observation of sporadic TDIS during the auroral bulge e xpansion leads us to conclude that the same physical process is at work thr oughout the midtail. We also show that ion injections are detected well ins ide the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more e xtensively studied by Sergeev et al. [1999]. We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheri c H+ and O+. These ions in the hundred of eV to the keV range are heated pe rpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant sour ce of ionospheric ions for the midtail magnetosphere.