The Advanced Small Analyzer for Neutrals (ASAN) instrument on Chang’E-4: A review of five years on the lunar surface

Abstract

The Advanced Small Analyzer for Neutrals (ASAN) instrument [1] is the first instrument to measure energetic neutral atoms directly on the lunar surface. Installed onboard of the Yutu-2 rover of the Chinese Chang’E-4, mission it started operations in January 2019. The landing site in the Karman crater on lunar far side is at the edge of the large magnetic anomaly in the South Pole-Aitken basin. The ASAN instrument has been operated periodically since, making use of the mobility of the rover to get different observation geometries.When the solar wind interacts with the lunar surface, a portion of it is reflected back as energetic netural atoms. Previous studies have provided evidence of neutral hydrogen atoms [2] and solar wind protons being reflected from the surface. However, prior to ASAN, all these measurements were performed form orbit and no direct measurements on the surface had been conducted. ASAN’s observation location directly at the surface allows to investigate the microphysics of the solar wind-surface interaction process and to study the effects of local magnetic anomalies on the emitted energetic neutral particle flux.The ASAN instrument is an 8th generation instrument of the SWIM family [3]. ASAN uses a charge conversion surface to convert energetic neutral atoms to positive ions and analyzed them with s surface-interaction based time-of-flight system combined with an electrostatic analyzer and an electrostatic elevation scanning system. This enables ASAN to to measure the direction-, energy-, and mass-resolved energetic neutral atom flux on the surface.We report on the instrument status and present a review of energetic neutral atoms observations done by ASAN on the lunar surface and discuss possible implications for the near-surface plasma environment. [1] M. Wieser, S. Barabash, X.-D. Wang, A. Grigoriev, A. Zhang, C. Wang, and W. Wang. The Advanced Small Analyzer for Neutrals (ASAN) on the Chang’E-4 Rover Yutu-2. Space Science Reviews, 216(4):73, 2020.[2] M. Wieser, S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M. B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura. First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms. Geophys. Res. Lett., 37(5), 03 2010.[3] M. Wieser and S. Barabash. A family for miniature, easily reconfigurable particle sensors for space plasma measurements. Journal of Geophysical Research: Space Physics, 2016