Acceleration of protons at 32 Jovian radii in the outer magnetosphere of Jupiter

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Abstract
During the inbound pass of Pioneer 10 a rapid tenfold increase of the 0.2‐ to 5‐MeV proton flux was observed at 32 RJ (Jovian radii). The total event lasted for 30 min and was made up of a number of superimposed individual events. At the time the spacecraft was in the outer magnetosphere about 7 RJ below the magnetic equator. Before and after the event the proton flux was characteristic of the low flux level normally encountered between crossings of the magnetic equator. Flux changes at different energies were coherent within 1 min, a time comparable to the time resolution of the data. Differential travel time would have led to a greater time difference if the sources had been further than 5–10 RJ from the spacecraft. The angular distributions were highly anisotropic with protons streaming toward Jupiter. A field‐aligned dumbbell distribution was observed initially, and a pancake distribution just before the flux decayed to its preevent value. The alpha particle flux changed as rapidly as the proton flux but peaked at different times. The energetic electron flux behaved differently; it increased gradually throughout the period. This acceleration event was associated with a double crossing of a current sheet, most of the acceleration being coincident with the first crossing. The average shape of the proton energy spectrum (0.3–5 MeV) remained unchanged during the event and resembled the spectrum observed in the magneto‐disk. A superposition of acceleration processes of this type is probably responsible for maintaining the energetic particle population around 30 RJ in the magnetosphere. Random fluctuations in the occurrence of acceleration events would explain the 10–50% flux changes during less than 15 min which were observed most of the time the Pioneer 10 and 11 spacecraft were in the outer Jovian magnetosphere.

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