New Measurements of Trapped Anomalous Cosmic Rays and Other Heavy Ions in the Inner Magnetosphere

Anomalous cosmic rays (ACR) are a sample of the local interstellar medium that originate as neutral atoms with first ionization potential greater than that of hydrogen. Once the atoms become singly ionized by solar ultraviolet radiation or charge exchange with the solar wind, they convect to the outer heliosphere, become accelerated to greater than 10 MeV/nucleon at the termination shock, and propagate back in to the inner heliosphere. The Earth's magnetosphere becomes a reservoir for ACR ions after the ions become further stripped in collisions with the upper atmosphere and are stably trapped near L=2 with long lifetimes. During the last solar cycle, observations from low-Earth orbit with SAMPEX and other satellites mapped the abundances and time dependence of the trapped ACR. Those observations revealed insights into the ACR acceleration process, the trapping process, as well as other elements (Mg-S and Fe) that might have originated as pickup ions from dust grains in the inner heliosphere. Radial diffusion of solar energetic particles into low L is also a potential source for trapped heavies near L=2. This paper reports new measurements of the trapped heavy ion abundances, energy spectra, and pitch angle distributions in the inner (L<3) magnetosphere made with the High Linear Energy Transfer (HiLET) sensor in highly elliptical Earth orbit. HiLET uses arrays of silicon detectors in multiple coincidence with high thresholds to eliminate backgrounds from penetrating protons and electrons. The energy range is ~3 to 30 MeV/nucleon (Z=8). Our measurement database begins in the spring of 2008, and therefore we are observing solar minimum conditions when the ACR and trapped ACR intensities are at their highest. Compared to the LEO measurements from the last solar cycle, HiLET has the advantage of sampling the heavy-ion environment in the inner magnetosphere near the magnetic equator where the trapped heavy ions have maximum intensity. We will report on the new findings and preliminary comparisons with previous LEO measurements and a model of the ACR trapping process.