Particle‐in‐cell simulations of plasma interactions with asteroidal and lunar surfaces

Motivated by the lack of knowledge about plasma interactions with asteroidal and lunar surfaces, we extended a legacy immersed‐finite‐element particle‐in‐cell (IFE‐PIC) simulation package to perform first‐principle‐based kinetic simulations of plasmas interacting with realistic‐shaped surfaces. ❧ A customized surface of arbitrary geometries described by an algebraic equation z = z(x, y) was introduced to the library of the IFE‐PIC package capable of modeling rugged and realistic surface topographies. A series of test cases were selected to validate all the key components of the IFE‐PIC package with the new‐added geometry, for both homogeneous and non‐homogeneous solvers. Surface charging was accurately resolved which is one of the key physical phenomenon associated with plasma‐surface interactions. ❧ The validated IFE‐PIC package was employed to simulate plasma interactions with both asteroidal and lunar surfaces, providing numerical solutions of plasma environments, electrostatic fields, and effects of the electric field on the levitation of charged dust grains. ❧ For the simulations of plasma interactions at asteroids, different sizes and structures of asteroids under different plasma environments are considered. The results imply that: • The presence of the photoemissions significantly affects the surface charging in the sunlit regions, which indicates that strong differential charging may be developed near terminator regions; • A consolidated core inside the asteroid does not significantly affect the plasma environment or surface charging. ❧ For plasma interactions with the lunar surface, simulations were performed for a local rugged surface topography in the terminator regions for a variety of plasma environments. Three different plasma conditions (solar wind average, solar wind low, and magnetosheath plasma) are considered for two Sun elevation angles (0° and 5°). The plasma environments and electrostatic fields near the surface are resolved and their effects on the charged dust grains are discussed. The results indicate that: • The plasma wake caused by the rugged local surface will lead to differential charging of the lunar surface; • A higher Sun elevation angle will make the surface potential less negative as a result of combined effects of impacting ions and photoemissions; • Different charging models for charged dust grains lead to significantly different net accelerations of those grains. The net accelerations from the layered‐dust charging model are about two orders of magnitude smaller than those of the single‐dust model. ❧ This study provides knowledge of the plasma environments and electrostatic field information for future exploration missions at asteroidal and lunar surfaces.

鉴于当前对等离子体与小行星、月球表面相互作用的相关认知仍存在不足，我们对一款经典的浸没有限元粒子网格（immersed-finite-element particle-in-cell, IFE-PIC）仿真软件包进行了扩展，以开展基于第一性原理的动理学模拟，研究等离子体与真实形状表面的相互作用过程。
我们向该IFE-PIC仿真包的软件库中新增了由代数方程z = z(x, y)描述的任意几何形状定制化表面，使其能够对粗糙且贴合真实场景的表面形貌进行建模。我们选取了一系列测试用例，针对新增的几何建模功能，分别在均匀与非均匀求解器下对IFE-PIC仿真包的所有关键组件进行了验证。该仿真包可精准解析表面充电过程——这是等离子体与表面相互作用的核心物理现象之一。
我们利用经过验证的IFE-PIC仿真包开展了等离子体与小行星、月球表面相互作用的模拟，得到了等离子体环境、静电场以及电场对带电尘埃颗粒悬浮过程影响的数值解。
针对小行星表面的等离子体相互作用模拟，我们考虑了不同等离子体环境下不同尺寸与结构的小行星。模拟结果显示：
• 光电子发射过程会显著影响向阳区域的表面充电，这表明在晨昏线区域可能会形成强烈的差异化充电；
• 小行星内部的固结核心不会对等离子体环境或表面充电产生显著影响。
针对月球表面的等离子体相互作用模拟，我们针对两种太阳高度角（0°与5°）下的三种不同等离子体条件（平均太阳风、弱太阳风以及磁鞘等离子体），对晨昏线区域的局部粗糙表面形貌开展了仿真。我们解析了表面附近的等离子体环境与静电场，并探讨了其对带电尘埃颗粒的影响。模拟结果表明：
• 局部粗糙表面引发的等离子体尾迹会导致月球表面产生差异化充电；
• 受入射离子与光电子发射的共同影响，更高的太阳高度角会使表面电位的负值程度降低；
• 针对带电尘埃颗粒的不同充电模型会导致颗粒获得差异显著的净加速度：基于分层尘埃充电模型得到的净加速度比单尘埃模型小约两个数量级。
本研究为未来小行星与月球表面的探测任务提供了等离子体环境与静电场相关的认知与数据参考。