Galactic Cosmic Ray Variations at the Moon, as Measured by the CRaTER Instrument

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) is an energetic particle telescope onboard the Lunar Reconnaissance Orbiter and is currently in orbit around the Moon. The telescope measures the linear energy transfer spectrum of particles with energies greater than about 10 MeV/nucleon. This paper will discuss the temporal and spectral variations seen in the energetic particle flux during the instrument’s first year on orbit. These events include Forbush decreases from corotating interaction regions and interplanetary coronal mass ejections, multiple small solar particle events and the beginning of the overall decrease in galactic cosmic ray (GCR) flux associated with the global reconfiguration of the interplanetary magnetic field. This decrease is associated with the beginning of solar cycle 24. In particular we will investigate two step-like persistent decreases in GCR flux that occurred in March and April of 2010. Though shorter in duration, these step-like decreases may be similar to events that occurred at the beginning of solar cycles 21 and 23.