Particle-In-Cell and Experimental Study of Lunar Mini-Magnetospheres for Power Extraction

Lunar magnetic anomalies (LMAs) are small regions (on the order of 100 km) of crustal magnetic fields on the lunar surface with field strengths of about 100 nT [1, 2]. Spacecraft measurements and numerical modeling of the interaction between the solar wind and the LMAs predict the formation of mini-magnetospheres [3], where the field strength magnetizes electrons but not ions. The separation between the electrons tied to the field lines and the less restrained ions produces strong electric fields (≈ 0.150 V m⁻¹) near the lunar surface [1]. At SPACE Laboratory, we are studying if this polarization electric field and the solar wind particle flux can be used for power extraction on the lunar surface using 3D particle-in-cell (PIC) modeling [4] and a subscale experiment. The figure shows key aspects employed to simulate mini-magnetosphere physics in the PIC code (left) and the experiment (right): (1) a plasma representing the solar wind, (2) a magnetic dipole field representing the LMA, (3) the lunar surface plane, (4) a current emitting cathode that enhances and allows power draw into an external load, and (5) an anode where electron precipitation balances the load current. The simulation imposes sheath electric field conditions at the electrodes [5, 6] to model the interaction with the plasma.

This work presents results from a study of mini-magnetosphere structure under various solar wind conditions, such as varying incidence angle, density, and speed, and discusses how the changing plasma dynamics would affect power extraction. Results show that net positive power in the sub-kilowatt range can be extracted from the mini-magnetosphere under favorable conditions with the injection of an electron current from the cathode and the collection of sufficient charged particles at the anode. PIC simulations show that the stability of the power generation scheme depends on the stability of the mini-magnetosphere structure, which is sensitive to the cathode electron injection. Moreover, the solar wind incidence angle is found to be a major factor in determining the power that could be generated with fixed electrodes since the mini-magnetosphere structure stretches in the direction of the wind. The subscale experiment corroborates many of the physical phenomena predicted by the simulations, lending credence to the findings. Based on the physical insights, we propose engineering solutions that could enable this technology to provide power for lunar exploration missions.

References: [1] Deca J. et al. In: Journal of Geophysical Research: Space Physics 120.8 (2015), pp. 6443–6463. [2] Bamford R. A. et al. In: The Astrophysical Journal 830.2 (2016), p. 146. [3] Deca J. et al. In: Physical Review Letters 112.15 (2014), p. 151102. [4] Markidis S. et al. In: Mathematics and Computers in Simulation 80.7 (2010), pp. 1509–1519. [5] Skolar C. R. et al. In: Physics of Plasmas 30.1 (2023), p. 012504. [6] Baalrud S. D. et al. In: Plasma Sources Science and Technology 29.5 (2020), p. 053001.