First Solar Energetic Particles Measured On The Lunar Far-side

Abstract:

On May 6, 2019 the Lunar Lander Neutron & Dosimetry (LND) Experiment on board the Chang'E-4 lander on the far side of the Moon detected its first small solar energetic particle (SEP) event with proton energies up to 21MeV. In this work we present combined proton energy spectra based on LND, SOHO/EPHIN and ACE/EPAM measurements which show that LND provides a complementary dataset from its special location on the Moon, and contributes to our existing observation and understanding of the space environment. We applied a velocity dispersion analysis (VDA) to the impulsive electron event and accompanying weak proton enhancement. We found that electrons were released 22 minutes after the flare onset and ~15 minutes after type II radio burst, while protons were released more than one hour after the electron release. The impulsive enhancement of the in-situ electrons and the derived early release time indicate a good magnetic connection between the source and Earth. However, stereoscopic remote-sensing observations from Earth and STA suggest that the SEPs are associated with an active region nearly 100 degrees away from the magnetic footpoint of Earth. This suggests that the propagation of these SEPs could not follow a nominal Parker spiral under the ballistic mapping model and the release and propagation mechanisms of electrons and protons are likely to differ significantly during this event.