Ground Measurements of the ATLID Laser Beam at Ultra-High-Energy Cosmic Ray Observatories

Kurzfassung

Following the activation of the Atmospheric Lidar (ATLID) laser transmitter onboard EarthCARE in summer 2024, its ultra-violet (UV) laser pulses have been consistently observed by the Pierre Auger Observatory (Auger) in Argentina and the Telescope Array experiment (TA) in the USA. These observatories are the world's largest facilities dedicated to detecting ultra-high-energy cosmic particles, whose origins remain one of the most profound mysteries in high-energy astrophysics. A key detection method involves capturing UV fluorescence light produced during air showers - cascades of secondary particles created when cosmic rays interact with the Earth's atmosphere. Auger and TA deploy arrays of telescopes to monitor fluorescence emissions over areas of approximately 3000 km² and 700 km², respectively. During its 25-day orbital repeat cycle, EarthCARE overpasses each observatory at least once, allowing sideward-scattered UV light from the ATLID laser beam to be recorded similar to air-shower fluorescence. The ground-based observations enable precise geometric reconstruction of the laser beam's trajectory through the atmosphere. By combining these observations with detailed modelling of molecular and aerosol scattering, as well as optical losses along the beam path, ATLID's laser energy at its exit point in space can be determined. The approach was successfully demonstrated during ESA's Doppler wind lidar mission Aeolus, which also utilized a UV laser transmitter, providing independent validation of the instrument performance and improving the precision of its laser beam's ground track reconstruction. Unlike Aeolus, whose visibility at Auger was limited to a few months per year, ATLID's year-round observability at both Auger and TA enables continuous monitoring of its laser signal strength and beam divergence. Additionally, the close temporal proximity of EarthCARE's overpasses of the two observatories in opposite hemispheres enables precise cross-calibration of the energy scales of Auger and TA. This calibration is essential for refining the celestial map of cosmic ray arrival directions and increasing sensitivity to the highest-energy particles in the universe.