Subscale Experiment for Investigating Lunar Magnetospheres

Lunar magnetic anomalies (LMAs) show a curious ability to reflect the high velocity ions (~400 km/s) of the solar wind, an effect of interest for manned missions. As it stands, current work in this area has focused primarily on simulation efforts supported by spacecraft data. There is a pressing need to better understand the structure of the miniature-magnetosphere system over a wide range of solar wind parameters if human missions come to rely on this shielding effect. To better target the fundamental physics of the miniature-magnetosphere, we propose an approach using a subscale experiment.

To investigate the basic physics and scaling parameters of the miniature-magnetosphere in a controlled setting, we constructed an experiment capable of recreating this plasma interaction at the laboratory scale. Specifically, we wish to investigate the magnitude, location, and thickness of the repelling electric field and how these parameters are influenced by the simulated solar wind.

A picture of the experiment in operation can be seen in [FIG. 1]. The simulated solar wind is created using an RF discharge and a DC voltage across two molybdenum grids. The resulting ion beam is neutralized by a hollow cathode mounted in the test chamber. The solar wind impacts the experiment assembly consisting of a Garolite (G-10) sheet acting as the lunar surface, a neodymium magnet beneath the surface mimicking the LMA, and a 3-axis translation stage actuating the probes. The entire platform can rotate ≤30° to simulate different solar wind incidence angles.

Emissive and Langmuir probes were chosen as diagnostics. The first measures plasma potential while operating in half-wave AC heating mode. The second measures ion density, electron temperature, and plasma potential. Initial results only report the ion saturation current which scales linearly with density and the root of the electron temperature. The  scaling is important because spacecraft data shows elevated electron temperatures produced in the mini-magnetosphere.

The experiment is supported by 3D particle in cell (PIC) simulations to bridge the gap between experimental and lunar length scales. The two work in tandem to inform one another to better isolate the driving principles of the system.

Initial results from the emissive probe [FIG. 2] show a peak plasma potential of ~200 V directly above the magnet. This value monotonically decreases with distance to the magnet which is consistent with an outward electric field being established. The map of ion saturation current [FIG. 3] is not fully complete at the time of submission but does further corroborate the formation of an ion cavity surrounded by a higher density barrier region.

Visual observations of the plasma show an asymmetry across the magnetic axis that is consistent with the 3D PIC model. This “stretching” of the magnetosphere in one direction is consistent with an  drift.

Complete 3-D maps of the density, potential, and temperature of the plasma will be ready by the conference date. A parametric investigation of various solar wind input conditions will also be conducted.