2012 ARTEMIS SCIENCE NUGGETS
A comparison of ARTEMIS measurements and plasma modeling of the lunar electrostatic surface charge
by A. R. Poppe, UC Berkeley Space Sciences Laboratory
Introduction
Airless bodies throughout the solar system, including the Moon, experience a variety of electrical currents to their surfaces. These currents come in many different forms, including, for example, collection of solar wind electrons and ions, and loss of surface electrons through photoemission induced by solar ultraviolet (UV) light. A fundamental question that has been pursued since before the Apollo era is how the lunar surface charges given the variety of currents to the surface. The twin ARTEMIS probes have allowed us to significantly expand our understanding of lunar surface charging, with implications for not only Moon, but also for other airless bodies, including Mercury, asteroids, and the moons of the giant planets.
Results
The use of plasma data to determine lunar surface charging was invented in the Apollo era and is known as 'electron reflectometry'. When a magnetic field line connects one of the ARTEMIS spacecraft to the lunar surface, incoming electrons and ions will flow along the field line past ARTEMIS, down to the Moon, and via some combination of magnetic and electric fields, will be partially reflected back up away from the Moon and towards ARTEMIS. By comparing the incident plasma flow (before it has interacted with the Moon) to plasma coming back up the field line, we can infer both the charge on the lunar surface, and the shape of the electrostatic potential above the lunar surface. Figure 1 shows a cartoon diagram of the geometry and electrical currents present in a typical electron reflectometry measurement.
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| Figure 1. A cartoon of the geometry and various currents used in a typical reflectometry measurement. Ambient ions and electrons travel down magnetic field lines towards the Moon, where they are partially reflect back towards the spacecraft. Photoelectrons generated at the lunar surface are partially trapped near the surface, while some of the population escapes and travels away from the Moon, towards the spacecraft. The region near the Moon where photoelectrons dominate is known as the "photoelectron sheath." |
To augment our understanding of the process by which electrical currents interact via electric fields with the lunar surface, we use a type of plasma model called particle-in-cell, or PIC, modeling. PIC modeling works by moving charged particles through a simulation space according to the laws of electromagnetics. A PIC model makes tiny steps in time (sometimes down to a nanosecond, or 1 billionth of a second) and moves the collection of electrons and ions that represent the plasma. At periodic times throughout the simulation, the particle positions, electric potential, and electric field can be printed out from the model to be used in comparing to the ARTEMIS data. By having the simulation run electrons and ions identical to those measured by ARTEMIS, we can then determine the electric potential above the lunar surface, as well as the electric charge on the surface itself.
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| Figure 2. Particle-in-cell model results showing the electric potential as a function of height above the lunar surface for three cases of ambient ion temperature. The model was run with input conditions similar to those measured by ARTEMIS in order to compare the model results with ARTEMIS data. |
Figure 2 shows a series of electric potentials as a function of height above the lunar surface from the model for conditions seen in the ARTEMIS data on the day we selected to study. The various lines come from running the simulation with different ambient ion temperatures in order to compare to a wide range of ARTEMIS data. The model shows that for conditions measured by ARTEMIS, the lunar surface will charge positively, although, at a negative potential with respect to the ARTEMIS spacecraft. This odd combination (positive charge / negative potential) is known as a "non-monotonic potential" and was theorized to occur in a series of papers from the early 1970’s. These measurements from ARTEMIS, as well as some earlier data from the Lunar Prospector mission, have confirmed the existence of these potentials and suggested that they most likely occur at a wide range of airless bodies (asteroids, other moons, etc.)
Conclusion
The use of ARTEMIS data in determining the both the lunar surface charge and the electric potential above the dayside surface have significantly improved our understanding of how airless bodies charge in space. This knowledge has implications for any robotic or manned exploration of the Moon, as any equipment, rovers, landers, and astronauts will be exposed to these currents and charges, which could potentially represent a hazard. Future work in this area includes studying surface charging in other environments, such as the solar wind, or on the lunar nightside, where no photoelectrons exist.
Reference
Poppe, A. R., J. S. Halekas, G. T. Delory, W. M. Farrell, V. Angelopoulos, J. P. McFadden, J. W. Bonnell, and R. E. Ergun (2012), A comparison of ARTEMIS observations and particle-in-cell modeling of the lunar photoelectrons sheath in the terrestrial magnetotail, Geophys. Res. Lett., 39, L01102, doi:10.1029/2011GL050321.Biographical Note
Andrew Poppe is a postdoctoral scholar in the Space Physics Research Group at the Space Sciences Laboratory / UC Berkeley. His research includes analysis of ARTEMIS data regarding solar wind and terrestrial magnetotail interactions with the Moon, as well as modeling and data comparison of interplanetary and planetary dust dynamics. See http://sprg.ssl.berkeley.edu/~poppe/ for more information.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu