Electromagnetic Particle-in-Cell Simulations of the Solar Wind Interaction with Lunar Magnetic Anomalies

We present the first three-dimensional fully kinetic and electromagnetic simulations of the solar wind interaction with lunar crustal magnetic anomalies (LMAs). Using the implicit particle-in-cell code iPic3D, we confirm that LMAs may indeed be strong enough to stand off the solar wind from directly impacting the lunar surface forming a mini-magnetosphere, as suggested by spacecraft observations and theory. In contrast to earlier magnetohydrodynamics and hybrid simulations, the fully kinetic nature of iPic3D allows us to investigate the space charge effects and in particular the electron dynamics dominating the near-surface lunar plasma environment. We describe for the first time the interaction of a dipole model centered just below the lunar surface under plasma conditions such that only the electron population is magnetized. The fully kinetic treatment identifies electromagnetic modes that alter the magnetic field at scales determined by the electron physics. Driven by strong pressure anisotropies, the mini-magnetosphere is unstable over time, leading to only temporal shielding of the surface underneath. Future human exploration as well as lunar science in general therefore hinges on a better understanding of LMAs.

Figure 1

2D electron (top) and ion (bottom) charge density profiles, scaled to the initial density $n_0$ and along the dipole axis ($Y$ direction) at $z=L_z/2$, after the simulation has reached a quasisteady state. The solar wind is flowing perpendicular (in the $-X$ direction) to the lunar surface at $x=0$. Superimposed in white are magnetic field lines. Note that only the lower half of the simulation domain is shown in the $X$ direction.

Figure 2

Ion charge density profiles, scaled to the initial density $n_0$, along three different planes: the $XY$ plane (upper left) and $XZ$ plane (lower left) through the dipole center and the $YZ$ plane (right) at $x=0.01d_i$ above the lunar surface. Note that only the lower half of the simulation domain is shown in the $X$ direction.

Figure 3

Profiles along the direction parallel to the solar wind flow and through the center of the dipole. The upper panels hold the magnetic and kinetic pressure profiles for the electron (left) and ion (right) populations, in code units. The middle panel shows the $Z$ component of the electron velocity distribution (left) and the $X$ component of ion velocity distribution (right), normalized to $v_{\mathrm{sw}}$. The lower panels present the density profiles (left), normalized to the initial density $n_0$, and the mirror instability criterium (right) with the area of interest colored in magenta.