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 on its nightside, known as the lunar wake. 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 (Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun) missions (2012–2023). We find systematic moonward deviations of ~tens of km/s in a direction perpendicular to the solar wind (moonward), at altitudes of up to 1000 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 lunar 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.