Trapped ACR: Low energy Argon

Anomalous cosmic rays (ACR) are a sample of the local interstellar medium. ACRs begin as neutral atoms with high first ionization potential (e.g. He, O, Ne, Ar) that drift into the heliosphere, become singly ionized by solar UV or collisions with the solar wind, and then are accelerated to beyond 10 MeV/nucleon at the termination shock. Below about 10 MeV/nucleon most ACR are singly ionized and can become trapped within the Earth’s magnetosphere upon collision with the atmosphere. The trapped ACR above 10 MeV/nucleon have been studied in detail with SAMPEX, where the dominant species are C, N, O, Ne.

The top histogram of the figure shows the mass spectrum measured in a solar energetic particle event at 1-5 MeV/nucleon, uncorrected for instrument efficiencies. Although the LICA instrument’s mass resolution is best below 1 MeV/nucleon, in this energy range it resolves the most abundant elements.

The bottom histogram is the mass spectrum at 1-5 MeV/nucleon in the ACR trapping region (L=1.8 to L=3.0), accumulated over 6.5 years. Compared to the trapped species observed with SAMPEX above 10 MeV/nucleon, this lower energy sample also shows C, N, O, and Ne, but the outstanding difference is the higher relative abundance of Ar. Although it’s measured in the interplanetary ACR component, this is the first measurement of Ar as a trapped species below 10 MeV/nucleon. The interplanetary Ar spectrum peaks near 1 MeV/nucleon, and the lower energy trapped population correspondingly shows an enhancement. Numerical simulations of the trapping of Ar also suggest an enhancement due to a high trapping probability. We will explore this possibility further, as well as constraints the LICA measurements can place on the trapping of rare ACR species such as Mg, SI, and S recently reported in interplanetary space at 1 AU.

Contributed by Joe Mazur, Aerospace Corp.