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Keywords:

  • lunar exosphere;
  • pickup ions;
  • Moon;
  • Kaguya

We present latitude and longitude distributions of Na+ and K+ fluxes from the Moon derived from Kaguya low-energy ion data. Although the latitude distribution agrees with previous ground-based telescope observations, dawn-dusk asymmetry has been determined in the longitude distribution. Our model of the lunar surface abundance and yield of Na and K demonstrates that the abundance decreases to approximately 50% at dusk compared with that at dawn due to the emission of the exospheric particles assuming the ion fluxes observed by Kaguya are proportional to the yield. It is also implied that the surface abundance of Na and K need to be supplied during the night to explain the observed lunar exosphere with dawn-dusk asymmetry. We argue that the interplanetary dust as well as grain diffusion and migration/recycling of the exospheric particles may be major suppliers.