Reflected Protons in the Lunar Wake and Their Effects on Wake Potentials

Area of Science:

• Space Physics
• Plasma Physics
• Lunar Science

Background:

• The lunar wake is refilled by ambipolar electric potential driven by electron pressure gradients.
• Solar wind protons can be reflected by lunar magnetic fields and the surface, entering the wake due to large gyroradii.

Purpose of the Study:

• Investigate how reflected protons (RPs) affect lunar wake potential structure.
• Analyze the impact of RPs on electrostatic potentials and electron beam formation in the lunar wake.

Main Methods:

• Utilized over 7 years of ARTEMIS mission data.
• Systematically analyzed reflected proton occurrence rates, densities, and their correlation with lunar wake potential.

Main Results:

• RPs can reduce lunar wake potential by hundreds of volts.
• RPs exhibit peak occurrence rates at specific downstream distances and prefer directions aligned with the solar wind's motional electric field.
• The presence of RPs leads to field-aligned accelerated electron beams and nonmonotonic potential structures.

Conclusions:

• Reflected protons significantly modify lunar wake electrostatic potentials.
• Wake potential scales logarithmically with reflected proton density when exceeding ~30% of local solar wind proton density, consistent with the Boltzmann relation.