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

Abstract

We report on a new feature of auroral substorms, namely, the sporadic though recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structures (TDIS). Our analysis builds on a combination of measurements from Interball‐Auroral, from UV imagery onboard Polar, from ground magnetometers, and also from observations on Geotail and from geostationary spacecraft. Backward tracing of ion trajectories from Interball‐Auroral orbit using realistic three‐dimensional magnetic and electric field models indicates that the injection region can extend over a wide range of radial distances, from ∼7–40 R E in the nearly equatorial magnetosphere. Both hydrogen and oxygen ions are shown to be injected toward the Earth's upper ionosphere. At Interball altitudes we find that ion injections are associated with two types of low‐frequency torsional oscillations of the magnetic field: (1) shear Alfvén 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 Alfvén waves of the PlB type, strictly restricted to the poleward boundary of the surge, with a characteristic period of ∼40 s. The systematic observation of sporadic TDIS during the auroral bulge expansion leads us to conclude that the same physical process is at work throughout the midtail. We also show that ion injections are detected well inside the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more extensively studied by Sergeev et al . [1999]. We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheric H + and O + . These ions in the hundred of eV to the keV range are heated perpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant source of ionospheric ions for the midtail magnetosphere.