In-flight Performance and Initial Results of Plasma Energy Angle and Composition Experiment (PACE) on SELENE (Kaguya)

Abstract

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its ∼1.5-year observation of low-energy charged particles around the Moon. MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measured the distribution function of low-energy electrons in the energy range 6 eV-9 keV and 9 eV-16 keV, respectively. IMA and IEA measured the distribution function of low-energy ions in the energy ranges 7 eV/q-28 keV/q and 7 eV/q-29 keV/q. All the sensors performed quite well as expected from the laboratory experiment carried out before launch. Since each sensor has a hemispherical field of view, two electron sensors and two ion sensors installed on the spacecraft panels opposite each other could cover the full 3-dimensional phase space of low-energy electrons and ions. One of the ion sensors IMA is an energy mass spectrometer. IMA measured mass-specific ion energy spectra that have never before been obtained at a 100 km altitude polar orbit around the Moon. The newly observed data show characteristic ion populations around the Moon. Besides the solar wind, MAP-PACE-IMA found four clearly distinguishable ion populations on the dayside of the Moon: (1) Solar wind protons backscattered at the lunar surface, (2) Solar wind protons reflected by magnetic anomalies on the lunar surface, (3) Reflected/backscattered protons picked-up by the solar wind, and (4) Ions originating from the lunar surface/lunar exosphere.

Solar wind electron interaction with the dayside lunar surface and crustal magnetic fields: Evidence for precursor effects

“…One expects a charge separation electric field to form in magnetic anomaly regions due to the fact that ions, with larger momentum and gyro-radius, penetrate more easily into crustal field regions than electrons, thus producing an electric field that excludes ions and pulls electrons into the interaction region. Surface experiments (Neugebauer et al, 1972;Goldstein, 1974;Clay et al, 1975) and recent spacecraft measurements (Saito et al, 2010) provide evidence for such charge separation fields. Unfortunately, this electric field should point upward, in the wrong direction to explain our observations.…”

“…More recently, measurements from Lunar Prospector, Kaguya, and Chandrayaan have shown that some of the stronger magnetic anomalies can at least partially shield the lunar surface from the solar wind flow (Lin et al, 1998;Halekas et al, 2008a;Saito et al, 2010;Wieser et al, 2010). The magnetic anomaly interaction can also compress and/or shock the incoming solar wind (Lin et al, 1998), generate plasma waves (Halekas et al, Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”

“…doi:10.5047/eps.2011.03.008 2006), and heat the incoming solar wind electron population (Halekas et al, 2008b;Saito et al, 2010).…”

Evidence for small‐scale collisionless shocks at the Moon from ARTEMIS

“…New observations show significant (up to 50% or more) reflection of solar wind protons above crustal magnetic fields [Futaana et al, 2003;Saito et al, 2010;Lue et al, 2011]. Given the sub-gyroradius and sub-inertial-length scale of the lunar fields, one would not expect to see such efficient reflection if protons experienced only magnetic forces.…”

Science Summary of Kaguya Mission

“…Interesting results on "Science on the Moon" have been obtained from PACE; these results include evidence that 0.1 to 1.0% of the solar wind (SW) protons are reflected back from the lunar surface; He+, C+, O+, Na+, K+, and probable Ar+ originating from the Moon are detected by the ion mass analyzer IMA 30) . The perpendicular entry of SW protons into the near-Moon wake is studied; SW protons coming into the deepest lunar wake have been studied; IMA has directly detected ions originating from the Moon in the Earth's magnetosphere 31) .…”