2012 ARTEMIS SCIENCE NUGGETS
ARTEMIS observations of pick-up ions in the terrestrial magnetotail lobes
by A. R. Poppe, UC Berkeley Space Sciences Laboratory
Introduction
Airless bodies throughout the solar system, including the Moon, are typically thought to be surrounded by a very thin atmosphere, typically referred to as an 'exosphere.' Exospheres are typically comprised of neutral atoms originating either from an external source, such as when micrometeoroids that bombard the object and vaporize, or from the object itself, such as when the intense solar wind sputters atoms from the lunar surface into the exosphere. Decades of observations, starting from the Apollo era, have attempted to directly measure the various species of the lunar exosphere; however, only a handful of species have been detected. The ARTEMIS probes have recently measured the ionized products (known as 'pick-up ions') of this neutral exosphere while the Moon was deep within the Earth’s magnetotail, allowing us to understand the formation and subsequent behavior of exospheric pick-up ions near the Moon.
Observations
When the Moon enters the Earth’s magnetotail lobes, the ambient plasma becomes very tenuous and the magnetic fields become very coherent, pointing almost entirely along the Earth-Sun line for more than a hundred Earth-radii downstream. In this environment, it becomes easy to identify ions that originate from the Moon itself. An example of such a measurement from ARTEMIS P2 on November 11, 2011 is shown in Figure 1. In the top panel, the energy flux of ions as a function of time shows mainly background noise (the green-blue static) with a large, extended burst of ions (shown in yellow through red) appearing in the middle of the observation. The bottom panel shows the height of the ARTEMIS spacecraft above the lunar surface in units of lunar radii. Given the proximity of the ARTEMIS spacecraft to the Moon and the lack of any other plasma in the background, we can conclude that these ions originate from the Moon, either directly from the surface (from a process such as micrometeoroid bombardment) or as ionized products of the lunar exosphere.
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| Figure 1. A time-series from ARTEMIS P2 on November 11, 2011 showing the observation of pick-up ions above the lunar surface. |
By analyzing other characteristics of the observed ions, such as the velocity and energy, we have developed a theory describing the relevant physical processes involved in producing and accelerating these ions, shown in cartoon form in Figure 2. The magnetic fields in the terrestrial magnetotail (shown in black) are known to drift (shown in red), or 'convect,' which produces an electric field. This convection electric field will accelerate these ions, which when combined with the ambient magnetic fields, causes ions to simultaneously loop around and drift with the magnetic fields. Additionally, solar ultraviolet light (orange) causes the lunar surface the charge up, producing a sheath electric field very near the lunar surface (shown in blue). Ions born above the sheath region (purple ions) feel only the convection field while ions born within the sheath region (green ions) feel both the convection electric field and the sheath electric field. The combination of both these electric fields makes these low-altitude ions disperse in a different direction than those that feel only the convection field. The fact that ARTEMIS sees the pick-up ions dispersed with a wide range of velocities implies that the ions must be produced from photo-ionization of the lunar exosphere.
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| Figure 2. A cartoon representing the various particles and fields present during ARTEMIS observation of lunar pick-up ions in the tail lobes. (Described in the text.) |
Conclusion
While these first ARTEMIS measurements have shown that pick-up ions are produced via photo-ionization of the lunar exosphere, much more can be learned about the lunar exosphere through further observation and study. This includes the development of a model of production of pick-up ions in the tail lobes. Using such a model, we can explore the effect of various physical parameters on the pick-up ion population near the Moon and compare these with current and future ARTEMIS measurements. Additionally, in late-2013, NASA will launch the Lunar Atmosphere and Dust Environment Explorer satellite, which will carry instruments capable of measuring the neutral constituents of the lunar exosphere with great precision. By comparing ARTEMIS and LADEE measurements, we can achieve a more thorough understanding of the Moon’s exosphere.
Reference
Poppe, A. R., R. Samad, J. S. Halekas, M. Sarantos, G. T. Delory, W. M. Farrell, V. Angelopoulos, and J. P. McFadden (2012), ARTEMIS observations of lunar pick-up ions in the terrestrial magnetotail lobes, Geophys. Res. Lett., 39, L17104, doi:10.1029/2012GL052909.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