Observations of Geomagnetically Trapped Anomalous Cosmic Rays by SAMPEX

Abstract

he first detailed measurements of a belt of geomagnetically trapped heavy ions that originated as interplanetary anomalous cosmic rays (ACRs) are being made by the polar orbiting satellite SAMPEX. The singly ionized interplanetary ACRs are trapped after losing electrons in the upper atmosphere. Their subsequent lifetime against energy loss by ionization of the atmosphere allows them to reach a substantially higher intensity than in interplanetary space. The ACR composition, which includes only elements with high first ionization potentials, is reflected in the trapped ACRs with some bias due to the trapping mechanism. The elements O, N, and Ne are present, while the lower atomic number elements, He and C, are either absent or substantially depleted relative to their interplanetary abundances. The trapping mechanism also determines the location of the ACR belt, which is confined to a narrow region near L=2, and the pitch‐angle distribution of the trapped ACRs, which is nearly isotropic except for the well‐defined loss cones. The intensities of the trapped and interplanetary ACRs have been measured by SAMPEX since its July, 1992 launch. Both have been steadily increasing with the approach of the minimum of the solar sunspot cycle.