Forbush decrease in lunar space environment observed by CRaTER on the LRO

Abstract

The Lunar Reconnaissance Orbiter (LRO) launched on 16 June 2009 has seven instruments, including the Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The CRaTER instrument characterizes the global lunar radiation environment and its biological impact by measuring galactic and solar cosmic rays. A Forbush decrease (FD) is a sudden decrease in the intensity of galactic cosmic rays (GCRs) recorded at the ground neutron monitor. Using the cosmic ray (CR) data from CRaTER, we identified the lunar CR events with a similar FD profile to that on Earth. Thirty-two FDs were found in the lunar space environment between June, 2009 and December, 2017, by a one-to-one correspondence with the FDs recorded at the terrestrial South Pole neutron monitor and associated interplanetary coronal mass ejections (ICMEs). The minimum GCR intensity of the main phase in the lunar space environment showed a correlation with the maximum interplanetary magnetic field strength of the associated ICME and was double the terrestrial FD magnitude. Because the FDs recorded by CRaTER were identified in space not on the ground, ICME can form an effective magnetic barrier by surrounding the Moon without interactions. The lunar space environment shows the FDs unaffected by the Earth’s magnetosphere. In addition, the background intensity of GCR in the lunar space environment is anti-correlated with the solar activity.