First Solar Energetic Particles Measured on the Lunar Far-side

On 2019 May 6 the Lunar Lander Neutron & Dosimetry (LND) Experiment on board the Chang'E-4 lander on the far-side of the Moon detected its first solar energetic particle (SEP) event with proton energies up to 21 MeV. Combined proton energy spectra are studied based on the LND, SOHO/EPHIN, and ACE/EPAM measurements, which show that LND could provide a complementary data set from a special location on the Moon, contributing to our existing observations and understanding of space environment. We applied velocity dispersion analysis to the impulsive electron event and weak proton enhancement and show that electrons are released only 22 minutes after the flare onset and similar to 15 minutes after the type II radio burst, while protons are released more than one hour after the electron release. The beam-like in situ electrons and clear velocity dispersion indicate a good magnetic connection between the source and Earth. This is remarkable because stereoscopic remote-sensing observations from Earth and STEREO-A suggest that the SEPs are associated with an active region nearly 113 degrees away from the magnetic footpoint of Earth. This suggests that these SEPs did not propagate along the nominal Parker spiral normally assumed for ballistic mapping and that the release and propagation mechanism of electrons and protons are likely to differ significantly for this event.