Revisiting Breakthrough Studies of Anomalous Cosmic Rays by SAMPEX

As SAMPEX celebrates its 20th anniversary, this paper will review some of the unique contributions that SAMPEX made to revealing the fascinating story of Anomalous Cosmic Rays (ACRs). ACRs begin as neutral interstellar atoms that drift into the solar system, get ionized, are picked up by the solar wind and convected to the outer heliosphere where they are accelerated to 10's of MeV/nuc, and then propagate in to 1 AU. Some ACRs become trapped in the magnetosphere to form a radiation belt composed of interstellar material. Following the US-Russian IMP8-Cosmos study, which discovered that ACRs are trapped in Earth's magnetosphere, but not their location, SAMPEX quickly pinpointed the location of the trapped ACR radiation belt at L=2 and also confirmed that the elemental composition (O, N, Ne Ar) agreed with that expected for an interstellar neutral origin. As the solar cycle progressed, the trapped ACR belt changed intensity on a time scale of ~months, revealing a relatively short trapping lifetime. SAMPEX was also able to take advantage of the Earth's field as a magnetic filter to measure ACR energy-spectra to high energies, and provide elemental and isotopic studies of ACR material while shielded from contamination by galactic cosmic rays (GCRs). Measurements of both interplanetary and trapped ACRs showed that the ACR isotopic composition differed significantly from that of GCRs, another available sample of interstellar matter. This paper will review these and related SAMPEX advances.