Moonward deviation of the solar wind

The solar wind's interaction with the Moon has traditionally been understood through the Moon's absorption of solar wind particles and the formation of a plasma cavity, known as the lunar wake, on its nightside. However, this study reveals unexpected large-scale perturbations in the solar wind upstream of the Moon using 11 years of data from the OMNI and ARTEMIS missions (2012–2023). We find systematic moonward deviations of ∼tens of km/s in the direction perpendicular to the solar wind (moonward) at altitudes of up to 1,000 km, particularly when the interplanetary magnetic field (IMF) lines are oblique to the solar wind (30° < θ < 60°) and connected to the lunar dayside. The longer the duration of the interaction, the greater the moonward deviation. These perturbations can be explained by neither solar wind pickup of the reflected ions nor wake dynamics. Instead, they appear to correlate with magnetic connectivity between the ARTEMIS probes and the lunar surface, suggesting a more complex solar wind interaction than previously thought.