Timing of energetic proton enhancements relative to magnetospheric substorm activity and its implication for substorm theories

Abstract

Enhancements of the fluxes of protons having energies of several hundred keV are commonly observed at synchronous orbit and in the magnetotail during geomagnetically active periods. In many cases these flux enhancements have been shown to be closely associated with individual magnetospheric substorms. Using energetic proton data (Ep >0.3 MeV) from synchronous orbiting satellites and from Vela satellites in the magnetotail, we find that the timing of proton enhancements relative to substorm onsets and recoveries (derived from ground magnetic records) and relative to plasma sheet thinnings and recoveries (measured with plasma probes on the Imp 7, Imp 8, and Vela satellites) can be understood in terms of a neutral line model of substorms in which the protons are impulsively accelerated, possibly near the neutral line(s), during a brief period at substorm onset. The energized particles subsequently appear to stream both sunward and antisunward as discrete particle bunches with those flowing sunward observed in the inner magnetosphere as ‘drift echoes’ and those flowing antisunward seen in the magnetotail as single short, intense ’impulsive’ bursts. The more commonly observed rapid‐rise slow‐decay type of plasma sheet proton enhancements occur during the substorm recovery phase and are, in the present model, attributed to envelopment of the observing satellite by the recovering (i.e., expanding) plasma sheet into which have leaked protons previously injected into the outer radiation zone. We find that many of the energetic proton bursts reported by other research groups, when examined together with concurrent plasma measurements, are also consistent with this model. This paper describes events from our own records and from the published reports of others and offers a unified view of such energetic proton flux enhancements.