Cosmic Ray Enhancements in Lunar Radiation Environment Observed by CRaTER on the LRO

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard the Lunar Reconnaissance Orbiter (LRO) characterizes the global lunar radiation environment and its biological impacts by measuring cosmic ray (CR) radiation. By using CRaTER data, we identify the lunar CR enhancements, which are similar to the terrestrial ground level enhancements (GLEs). GLE is a sudden and short increase in CR intensity recorded by the Earth's ground neutron monitors. We examine the origins and the characteristics of CR enhancements in the lunar space environment by using the CR intensity and dose rate according to CRaTER data. In order to determine origins of CR enhancements, we also use solar proton event (SPE) data, CR data of the Advanced Composition Explorer (ACE) equipped with a Solar Isotope Spectrometer (SIS), and proton flux data of Geostationary Operational Environmental Satellite 15 (GOES15). We identified 96 CRaTER enhancements (CREs) as increases in CR events in the lunar space environment during the period June 2009-December 2017. Unlike terrestrial GLEs, CREs are much longer and more frequent. Of the 96 CREs, 43 events are associated with SPE. The values of their physical characteristics are statistically larger than those of the CREs without associated SPEs. However, such non-SPE CREs are considered to originate from solar ejections. All CREs are associated with an increase in He flux by ACE/SIS. Even though CREs are associated with a small increase in He flux, this does indicate that some materials are ejected from the Sun; rather, it indicates that all CREs are associated with solar eruptions. Because of the very weak magnetic field and extremely rare atmosphere, the lunar space environment responds to even weak solar activity. An increase in He flux observed by ACE/SIS can be useful for monitoring the eruption of energetic particles from the Sun regardless of accompanying SPEs.