High‐energy magnetospheric protons and their dependence on geomagnetic and interplanetary conditions

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High‐energy (E > 0.3 MeV) proton increases within the magnetosphere both at synchronous orbit (6.6 RE) and in the plasma sheet (∼18 RE) have been studied. Measurements at 6.6 RE reveal that most (>80%) substorms have no associated injections of >0.3‐MeV protons (above the typical ambient level of 5 × 10³ (cm² s sr MeV)−1). Those relatively rare substorms which produce large fluxes of high‐energy particles show behavior ranging from drift echo to nondrift echo types of enhancements. The drift echo events are characterized by brief, well‐defined pulses of protons which are observed to drift azimuthally about the earth several times before dispersing. The nondrift echo events exhibit clear flux enhancements, but they do not show very evident pulsed behavior. The relative occurrence probability of high‐energy proton enhancements at 6.6 RE shows a strong positive correlation both with solar wind speed and with Kp. A substantial correlation with southward interplanetary magnetic field (IMF) is also found. Very similar Kp, solar wind, and IMF dependences are found for plasma sheet proton (Ep > 0.5 MeV) enhancements above a threshold of ∼10 (cm² s sr MeV)−1 as measured at 18 RE. The close similarity of differential energy spectra, Kp, solar wind, and IMF dependences at synchronous orbit and in the plasma sheet suggests a common acceleration source. Bursts of energetic protons (Ep ∼ 1 MeV) are also often observed simultaneously in data obtained in the interplanetary medium when proton enhancements are seen at 6.6 RE. A model depicting the relationship between proton events in various magnetospheric regions is presented. It is suggested that large transient (<1 min) induced electric fields exist within the plasma sheet at the time of those substorm onsets that occur during periods of high solar wind speed and southward IMF. Such electric fields then produce both the proton pulses seen at synchronous orbit and the previously reported plasma sheet ‘impulsive’ tailward flowing proton bursts. We relate these initial proton populations directly to the substorm onset acceleration mechanism. The more frequently observed type of plasma sheet proton enhancements which is observed in the expanding plasma sheet is, in the present model, attributed to previously injected protons from the outer radiation zone filling the plasma sheet volume during the recovery phase of substorms.