2011 ARTEMIS SCIENCE NUGGETS
First Near Lunar Wake Flyby by ARTEMIS
by J.S. Halekas, UC Berkeley Space Sciences Laboratory
Introduction
On Feb 13, 2010, ARTEMIS P1 passed through the near lunar wake for the first time, encountering a region of space seething with plasma activity. As the solar wind flows past the Moon, most of it is lost to impact with the sunlit surface, creating a void downstream. The solar wind plasma gradually refills this void, creating a teardrop-shaped wake extending downstream. The refilling process, however, is not simply a smooth expansion into the void. Beams of charged particles (protons, alpha particles, and electrons) refill the wake along magnetic field lines from the two sides. Where these beams meet in the center of the wake, they stimulate significant electromagnetic and electrostatic disturbances.
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| Figure 1. Geometry of the Feb 13, 2010 ARTEMIS wake flyby in wake coordinates. Colored spacecraft trajectory indicates ion density, and arrows show magnetic field vectors. |
Results
ARTEMIS passed through the wake at a distance of ~5000 km downstream from the Moon. Two projections of the orbit are shown in wake coordinates in Figure 1, with colors representing ion density and arrows showing the magnetic field direction. Fortuitously, the spacecraft passed very near the center of the wake, and it traveled nearly along the magnetic field direction as it crossed the wake. Figure 2 shows the gross features of the wake, including ion and electron energy flux spectra, spacecraft potential, ion density, ion velocity, magnetic field, and electrostatic wave spectra. These demonstrate the remarkable capability of the ARTEMIS instrumentation, and clearly show the remarkable structure of the wake.
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| Figure 2. ARTEMIS observations of the lunar plasma wake on Feb 13, 2010. Panels show ion and electron energy-time spectrograms, spacecraft potential, ion density and velocity, magnetic field, and an electric field frequency-time spectrogram (black curve shows electron plasma frequency). Electron spectra are corrected for the effects of spacecraft charging. Data gaps (white strips) indicate times when the spacecraft potential changed too rapidly to apply corrections. |
ARTEMIS's field-aligned trajectory allowed it to observe interpenetrating proton, alpha particle, and electron populations refilling the wake along magnetic field lines from both flanks. The characteristics of these distributions match expectations from self-similar models of plasma expansion into vacuum, with an asymmetric character likely produced by a combination of a tilted interplanetary magnetic field and an anisotropic incident solar wind electron population. On this flyby, ARTEMIS provided unprecedented measurements of the interpenetrating beams of both electrons and ions naturally produced by the filtration and acceleration effects of electric fields set up during the refilling process. ARTEMIS also measured electrostatic oscillations closely correlated with counter-streaming electron beams in the wake, as previously hypothesized but never before directly measured.
Conclusion
This early flyby demonstrated the capability of ARTEMIS, promising great science from the thousands of wake crossings to come in the next few years. Even in this first orbit, we saw some phenomena never seen before or even predicted; most notably, a highly asymmetric wake potential structure, producing counter-streaming electron beams that trigger strong electrostatic wave activity.
Reference
J.S. Halekas, V. Angelopoulos, D.G. Sibeck, K.K. Khurana, C.T. Russell, G.T. Delory, W.M. Farrell, J.P. McFadden, J.W. Bonnell, D. Larson, R.E. Ergun, F. Plaschke, K.H. Glassmeier, First results from ARTEMIS, a new two-spacecraft lunar mission: Counter-streaming plasma populations in the lunar wake, Space Sci. Rev., doi:10.1007/s11214-010-9738-8, 2011.Biographical Note
Jasper Halekas is a Research Scientist at the U.C. Berkeley Space Sciences Laboratory. He has been studying the Moon since he was a graduate student, working primarily with Lunar Prospector data. He is currently working on hardware for the MAVEN and Solar Probe Plus missions, and spending as much time on ARTEMIS data analsis as he can.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu