Protons in near earth orbit

TL;DR

This paper reports measurements of proton spectra in near-Earth orbit, revealing a primary power-law spectrum above the geomagnetic cutoff and a secondary, geographically restricted spectrum below it, with implications for space radiation understanding.

Contribution

It provides the first detailed measurement of proton spectra in near-Earth orbit, including the characterization of a secondary proton population below the geomagnetic cutoff.

Findings

Primary proton spectrum follows a power law above cutoff.

Secondary proton spectrum is concentrated at equatorial latitudes.

Secondary protons originate from a restricted geographic region.

Abstract

The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at an altitude of 380 km. Above the geomagnetic cutoff the observed spectrum is parameterized by a power law. Below the geomagnetic cutoff a substantial second spectrum was observed concentrated at equatorial latitudes with a flux ~ 70 m^-2 sec^-1 sr^-1. Most of these second spectrum protons follow a complicated trajectory and originate from a restricted geographic region.