Deriving the Undisturbed Near-surface Lunar Electric Field : Simulations of the electrostatic environment of the Umeå Lunar Venture electric field instrument

Abstract: The existence of an electric field on the Moon has been theorized since the end of the 1960s, when reports from astronauts suggested that charged particles from the lunar surface could be seen interacting with light at high altitudes. Today it is believed that an electric field drives the motion of charged particles on the Moon, and although a good number of simulations investigating the field have been made, the electric field has not been measured experimentally. Space Science Sweden and Umeå Lunar Venture have developed an electric field mill instrument that is to be attached to a lunar lander with the intention of measuring the electric field near the lunar terminators. The main problem that arises when this approach is used to measure electric fields is that the presence ofthe lunar lander, and of the instrument itself, distorts the ambient fields intended to be measured. A numerical electrostatic model treating the electric field distortions caused by conductive materials and their charge is developed in COMSOL Multiphysics, and is shown to agree well with both theoretical and experimental results. The model is then applied to a system consisting of a lunar lander with the electric field instrumen tattached, and the resulting field distortions are investigated. The system is calibrated for different locations of the field mill, as well as for different modifications of the instrument in order to find the optimal location and instrument that minimizes the errors in the calculated ambient fields. The results indicate that if the instrument consists of three field mills, and is placed near a corner of the lunar lander, it will be able to measure the ambient electric field larger than a few 100 mV/m, unless the potential of the lunar lander is around 10 V, in which case the charge induced field drowns out the ambient field. However, the model of the lunar lander that was used lacked information regarding its materials and solar panels. Consequently, the assumption that the surfaces of the lander were conductive and held at the same potential was made. Similarly, only very simplified models of the solar panels were used to estimate their importance. Because of these simplifications, the results should be seen as preliminary, and not conclusive. In order to obtain more reliable results that can be used together with actual lunar data, a few changes and additions should be made to the model. Specifically, a more detailed and accurate computer model of the lunar lander is necessary to obtain more correctly estimated field distortions. Additional information regarding the material and coating of the solar panel is required to properly model the solar panels and account for their effect on the ambient electric field.