Simultaneous observation of the electron acceleration and ion deceleration over lunar magnetic anomalies
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At ~25 km altitude over magnetic anomalies on the Moon, the deceleration of the solar wind ions, acceleration of the solar wind electrons parallel to the magnetic field, and heating of the ions reflected by magnetic anomalies were simultaneously observed by MAP-PACE on Kaguya. Deceleration of the solar wind ions was observed for two major solar wind ion compositions: protons and alpha particles. Deceleration of the solar wind had the same ΔE /q (ΔE: deceleration energy, q: charge) for both protons and alpha particles. In addition, the acceleration energy of the electrons was almost the same as the deceleration energy of the ions. This indicates the existence of an anti-moonward electric field over the magnetic anomaly above the altitude of Kaguya. The reflected ions were observed in a much larger area than the area where magnetic field enhancement was observed. These reflected ions had a higher temperature and lower bulk velocity than the incident solar wind ions. This suggests the existence of a non-adiabatic dissipative interaction between solar wind ions and lunar magnetic anomalies below Kaguya.
Key wordsMoon plasma magnetic anomaly solar wind
It is well-known that the Moon has neither a global intrinsic magnetic field nor a thick atmosphere. Unlike the case of the Earth, where the intrinsic global magnetic field prevents the solar wind from penetrating into the magnetosphere, the solar wind directly impacts the lunar surface. Since the discovery of the lunar crustal magnetic field in the 1970s (Sharp et al., 1973; Fuller, 1974; Howe et al., 1974; Anderson et al., 1975), several studies concerning the interaction between the solar wind and the lunar magnetic anomalies have been made, including numerical simulations (Harnett and Winglee, 2000, 2002, 2003) and observations by lunar orbiters. The interaction between the solar wind and lunar magnetic anomalies was first detected by Explorer 35 as limb shocks (Ness et al., 1968; Colburn, 1971; Criswell, 1973). Using Apollo 15 and 16 subsatellite data, Russell and Lichtenstein (1975) suggested that the limb compressions are due to the deflection of the solar wind by the lunar surface field near the lunar terminators. MAG/ER on Lunar Prospector found heating of the solar wind electrons presumably due to the interaction between the solar wind and the lunar magnetic anomalies and the existence of the mini-magnetosphere was suggested (Lin et al., 1998). Using Lunar Prospector MAG data, Kurata et al. (2005) found a magnetic field signature indicating the presence of a mini-magnetosphere above the Reiner Gamma magnetic anomaly region in the solar wind. Halekas et al. (2006, 2008a) found that limb shocks are clearly associated with lunar crustal sources by analyzing Lunar Prospector data. Observation of the possible inner region of a lunar mini-magnetosphere was reported by Halekas et al. (2008b). Recently, Wieser et al. (2010) detected mini-magnetospheres on the lunar surface using energetic neutral atom imager on Chandrayaan-1. Saito et al. (2010) also found mini-magnetospheres by detecting the deficiency of backscattered solar wind protons at low altitude of ~50 km using low energy ion data obtained by Kaguya. Solar wind ion reflection by lunar magnetic anomalies was reported by Kaguya (Saito et al., 2010) and Chandrayaan-1 (Lue et al., 2011). Saito et al. (2010) found that more than 10% of the incident solar wind ions were reflected by lunar magnetic anomalies. Lue et al. (2011) found an average reflection efficiency of ~10% and a maximum reflection efficiency of ~50% for the strongest magnetic anomalies. As a reflection mechanism, Lue et al. (2011) suggested the existence of charge separation. Halekas et al. (2010) also mentioned the existence of charge separation around the magnetic anomalies. Despite all the efforts to understand the interaction between the solar wind and lunar magnetic anomalies, the details of the interaction are still unclear. In this paper, the first simultaneous observation of low energy electrons and ions over lunar magnetic anomalies are presented in order to understand the plasma structure over mini-magnetospheres.
MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment (MAP-PACE) on Kaguya (Saito et al., 2008a, 2010) completed its ©1.5 year observation of low energy charged particles around the Moon. Kaguya (SELENE) was launched on 14 September 2007 by an H2A launch vehicle from Tanegashima Space Center in Japan (Kato et al., 2010). Kaguya was inserted into a circular lunar polar orbit of 100 km altitude and continued observation for nearly 1.5 years until it impacted the Moon on 10 June 2009. During the last 5 months, the orbit was lowered to ~50 km altitude between January 2009 and April 2009, and some orbits had a lower perilune altitude of ~10 km after April 2009.
MAP-PACE consisted of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). Since each sensor had a hemispherical field of view, two electron sensors and two ion sensors which were 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, was an energy mass spectrometer that measured mass identified ion energy spectra (Tanaka et al., 2009; Yokota et al., 2009).
The Lunar MAGnetometer (MAP-LMAG) was another component that constituted MAP. MAP-LMAG was a triaxial flux gate magnetometer that was equipped at the top plate of a 12 m long mast to reduce an offset of the interference magnetic field caused by the spacecraft (Shimizu et al., 2008; Takahashi et al., 2009; Matsushima et al., 2010; Tsunakawa et al., 2010). LMAG measured the vector magnetic field with a sampling frequency of 32 Hz and a resolution of 0.1 nT.
Here one should note that the discussion here applies to the origin of the electric field above the Kaguya altitude where the bulk flow deceleration of the solar wind and the associated piling-up of the magnetic field is the dominating process. Closer to the lunar surface, the particle nature of the ions and electrons will be more important for understanding the plasma structure around the magnetic anomalies.
When the solar wind plasma collides with a lunar magnetic anomaly, the magnetic anomaly is compressed by the solar wind plasma and the solar wind will impact the lunar surface if the magnetic energy of the magnetic anomaly is lower than the energy of the solar wind plasma bulk motion. In case of the solar wind plasma shown in Fig. 6, the intensity of the magnetic field that can stand off the solar wind plasma bulk motion (with bulk velocity of 350 km/s and density of 4.0 cm−3) is calculated as 45 nT. The measured magnetic field intensity at Kaguya was 16 nT. It is possible that the magnetic field between Kaguya and the lunar surface is stronger than 45 nT and the solar wind plasmas are reflected back, though this depends on the location and structure of the source of the magnetic anomaly. In addition to the reflection by magnetic field, the anti-moonward electric field over magnetic anomalies will assist ion reflection.
Although the plasma structure over lunar magnetic anomalies has become clear due to this simultaneous observation of low energy electrons and ions, it has turned out that there exists an additional interaction region where the incident ions are significantly heated at a lower altitude than Kaguya at ~25 km altitude. In order to fully understand the MAP-PACE observations over magnetic anomalies, future hybrid or full particle simulations are indispensable.
The authors wish to express their sincere thanks to all the team members of MAP for their great support in processing and analyzing the MAP data. The authors thank the ACE MAG instrument team and the ACE Science Center for providing the ACE data. This work was supported by JSPS Grant-in-Aid for Scientific Research (B) 21340143.
Guest editor M. Yamauchi thanks E. Harnett and an anonymous reviewer in evaluating this paper.
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