First Solar energetic particles measured on the Lunar far-side

TL;DR

This study reports the first detection of solar energetic particles on the Moon's far side by Chang'E-4, revealing insights into particle release times, propagation mechanisms, and the Moon's role in space environment observations.

Contribution

It demonstrates the utility of lunar-based measurements for SEP observation and provides new data on particle release timing and propagation that challenge existing models.

Findings

Electrons released 22 minutes after flare onset.

Protons released over an hour after electrons.

SEPs propagation likely deviates from Parker spiral model.

Abstract

On 2019 May 6, the Lunar Lander Neutron & Dosimetry (LND) Experiment on board the Chang'E-4 on the far-side of the Moon detected its first small solar energetic particle (SEP) event with proton energies up to 21MeV. Combined proton energy spectra are studied based on the LND, SOHO/EPHIN and ACE/EPAM measurements which show that LND could provide a complementary dataset from a special location on the Moon, contributing to our existing observations and understanding of space environment. Velocity dispersion analysis (VDA) has been applied to the impulsive electron event and weak proton enhancement and the results demonstrate that electrons are released only 22 minutes after the flare onset and $\sim$15 minutes after type II radio burst, while protons are released more than one hour after the electron release. The impulsive enhancement of the in-situ electrons and the derived early release time indicate a good magnetic connection between the source and Earth. However, stereoscopic remote-sensing observations from Earth and STA suggest that the SEPs are associated with an active region nearly 100$^\circ$ 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 mechanism of electrons and protons are likely to differ significantly during this event.