Laboratory Simulation of Solar Wind Interaction With Lunar Magnetic Anomalies

Magnetic anomalies on the surface of the Moon interact with the solar wind plasma flow, resulting in both magnetic and electrostatic deflection/reflection of charged particles. Consequently, surface charging in these regions differs from regions without magnetic fields. Using the Colorado Solar Wind Experiment facility, this interaction is investigated with high-energy flowing plasmas (100-800 eV beam ions) that are incident upon a magnetic dipole embedded beneath an insulating surface. The dipole moment is perpendicular to the surface. The plasma potential distribution is measured above the surface using an emissive probe. In the dipole lobe regions the surface is charged to significantly higher positive potentials by the un-magnetized ion beam impinging on the surface while the electrons remain excluded by the magnetic field. At low ion beam energies the results agree with these expectations as the surface potential follows the ion beam energy. However, at high beam energies, the surface potentials in the electron-shielded lobe regions remain significantly lower than the expected magnitude. Surprisingly, electrons are detected in the shielded regions by a Langmuir probe. A test particle simulation indicates that secondary electrons induced by the high energy ion beams impinging on the surface can enter the shielded regions, thus lowering the surface potential.