New Measurements of Inner Belt Proton Flux Gradients From the Van Allen Probes Mission
Abstract
Prior studies of 10's of MeV inner belt protons in low Earth orbit have established that the atmospheric density gradient produces a proton flux gradient because of losses to the atmosphere and the comparable sizes of the proton qyroradius and atmosphere scale height. The observable is an east-west asymmetry in the proton flux that has been reported using many low-Earth orbit missions going back to the first nuclear emulsion flights in 1963. We will revisit this low-altitude east-west effect as well as higher-altitude gradients with new measurements from the Relativistic Proton Spectrometer (RPS) on the Van Allen Probes spacecraft. RPS is a particle spectrometer designed to measure the flux, angular distribution, and energy spectrum of protons from ~60 MeV to ~2000 MeV with good rejection of penetrating backgrounds by requiring a 10-fold coincidence in its stack of silicon detectors. The Van Allen Probes orbit allows for a survey of proton gradients not only at low altitudes but also as high as the outer trapping limit at McIlwain L shell L~3 corresponding to ~13,000 km altitude. The 60 MeV proton gyroradius varies from ~50 to 700 km in this altitude range. The 1-second sampling of RPS and the nominal 5 rpm rotation rate of the Van Allen Probes yields a sensitive measure of proton gradients. This is the first time that a single mission can address the gradients and trapping of high-energy protons throughout the inner belt. We will report on preliminary flux gradients of >61 MeV protons observed during the first year of the mission using RPS and ancillary geophysical data.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMSM34A..01M
- Keywords:
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- 2774 MAGNETOSPHERIC PHYSICS Radiation belts;
- 2730 MAGNETOSPHERIC PHYSICS Magnetosphere: inner;
- 7984 SPACE WEATHER Space radiation environment