Lunar precursor effects in the solar wind and terrestrial magnetosphere

Abstract

[1] The two ARTEMIS probes observe significant precursor activity upstream from the Moon, when magnetically connected to the dayside lunar surface. The most common signature consists of high levels of whistler wave activity near half of the electron cyclotron frequency. This precursor activity extends to distances of many thousands of km, in both the solar wind and terrestrial magnetosphere. In the magnetosphere, electrons reflect from a combination of magnetic and electrostatic fields above the lunar surface, forming loss cone distributions. In the solar wind they generally form conics, as a result of reflection from an obstacle moving with respect to the plasma frame (just as at a shock). The anisotropy associated with these reflected electrons provides the free energy source for the whistlers, with cyclotron resonance conditions met between the reflected source population and Moonward-propagating waves. These waves can in turn affect incoming plasma, and we observe significant perpendicular electron heating and plasma density depletions in some cases. In the magnetosphere, we also observe broadband electrostatic modes driven by beams of secondary electrons and/or photoelectrons accelerated outward from the surface. We also occasionally see waves near the ion cyclotron frequency in the magnetosphere. These lower frequency waves, which may result from the presence of ions of lunar origin, modulate the whistlers described above, as well as the electrons. Taken together, our observations suggest that the presence of the Moon leads to the formation of an upstream region analogous in many ways to the terrestrial electron foreshock.

Statistical characterization of the foremoon particle and wave morphology: ARTEMIS observations

“…The outward expansion of the magnetic field fluctuations in the region magnetically connected to the lunar wake (also referred to as the forewake in the literature) was observed first by Explorer 35 [ Ness and Schatten , ] and later by Wind [ Farrell et al , ] and Geotail [ Nakagawa et al , ]. The dayside magnetic field fluctuations have been observed by a number of low‐altitude spacecraft [ Lin et al , ; Halekas et al , , ; Nakagawa et al , , ; Tsugawa et al , , ] and also by ARTEMIS [ Halekas et al , , ]. These forewake and dayside waves have been reported and discussed separately perhaps because they were implicitly assumed to be different phenomena with different origins.…”

“…These additional particle components and modified velocity distributions can generate several classes of waves. Magnetometer measurements by lunar orbiters have revealed the presence of large‐amplitude ULF waves with peak frequencies ∼0.01–0.1 Hz [ Nakagawa et al , ; Halekas et al , ], broadband magnetic turbulence at 0.1–10 Hz [ Halekas et al , , ; Nakagawa et al , ; Tsugawa et al , ], and ∼1 Hz narrowband whistlers [ Lin et al , ; Halekas et al , , , ; Tsugawa et al , ]. These low‐frequency waves are generally enhanced above strong crustal magnetic fields, implying their association with reflected ions.…”

Dependence of lunar surface charging on solar wind plasma conditions and solar irradiation

“…We do not consider surface charging on the lunar 95 nightside much beyond the terminator, since including the effects of the plasma wake that forms downstream of the Moon is beyond the scope of this study (Halekas et al, 2005;Farrell et al, 2008a;Zimmerman et al, 2011;. Similarly, we also do not consider the effects of the complex "pre-cursor" region observed upstream of the Moon (Halekas et al, 2012), but rather focus on the near-surface sheath. The analysis presented here is also important for understanding 100 and interpreting more complicated surface charging models, such as Poppe and Horanyi (2010) and Zimmerman et al (2011Zimmerman et al ( , 2012, by providing both a useful reference for how different parameters are expected to influence surface charging, as well as insights into the underlying physical processes.…”

Plasmoid growth and expulsion revealed by two‐point ARTEMIS observations

“…Exceptions (Figures b and i) showing unidirectional flux occurred when the probes crossed flux tubes connected to the Moon, i.e., when d , the distance of each field line (assumed straight) to the lunar center, was less than one lunar radius ( d ≤ 1 R L in Figures g and n). Such blockage of field‐aligned superthermal electrons from the lunar direction has been reported previously [e.g., see Halekas et al ., ]. Outside these lunar connection periods, there was no evidence of unidirectional electron streaming but shown only near‐isotropic electron distributions.…”