Charged Particle Fluxes Associated with CALIPSO Low Laser Energy Shots

The cloud-aerosol lidar and infrared pathfinder satellite observation (CALIPSO) mission was launched on 28 April 2006 into a 705-km Sun-synchronous orbit. At launch, the lidar laser canisters were filled with dry air at 130 kPa. The redundant canister pressure has dropped during the mission, reaching 7.6 kPa by late December 2021. Starting on June 23, 2016, individual laser shots started to exhibit anomalously low energies in regions of enhanced energetic charged particle fluxes. Small at first (< 1 mJ), drops in laser energy up to 61 mJ have been observed. This study determines the energetic charged particle populations associated spatially with low laser energy shots and the degree to which time variations in the low laser energy shots can be explained by time variations in the energetic charged particle fluxes. Particle observations from the NOAA-19 medium energy proton and electron detector (MEPED) are traced down magnetic field lines to estimate the fluxes at CALIPSO altitudes that are associated with individual low-energy shots. Predictions of the stability of particle bounce and drift motions in the geomagnetic field are used to sort low-energy shots and fluxes into different radiation belt regions. While most of the low-energy shots occur in the inner belt, there are clear spatial associations with outer belt electrons and with galactic cosmic rays (GCRs). As canister pressure has decreased, GCRs and electrons have become more effective in causing larger laser energy drops.