Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP‐PACE onboard SELENE (KAGUYA)

Saito, Y.; Yokota, Shoichiro; Tanaka, Takaaki; Asamura, Kazushi; Nishino, Masaki N.; Fujimoto, M.; Tsunakawa, Hideo; Shibuya, H.; Matsushima, Masaki; Shimizu, Hisayoshi; Takahashi, Futoshi; Mukai, T.; Terasawa, T.

doi:10.1029/2008gl036077

Cited by 192 publications

(205 citation statements)

References 24 publications

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“…• Specular reflection from the lunar surface has been observed to accelerate solar wind ions to three times that of the bulk plasma velocity [Saito et al, 2008]. This is however judged unlikely at Rhea due to high water-group photodetachment rates in Saturn's magnetosphere [Coates et al, 2010] precluding negatively charged O − existing in abundance as an ambient magnetospheric population.…”

Section: Velocity Space Analysismentioning

confidence: 99%

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Desai

Taylor

Regoli

et al. 2018

Geophysical Research Letters

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Saturn's largest icy moon, Rhea, hosts a tenuous surface‐sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO2+ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, C2H−, C2−, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics.

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Section: Velocity Space Analysismentioning

confidence: 99%

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Desai

Taylor

Regoli

et al. 2018

Geophysical Research Letters

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“…Even outside of the influence of crustal fields, we now know that part of the solar wind ion population reflects from the surface, in both ion (Saito et al, 2008) and more often in neutral form (Wieser et al, 2009;McComas et al, 2009). The dayside lunar surface also provides a source of low energy photoelectrons produced by solar photons and secondary electrons produced by both electron and ion impact, as well as reflected and backscattered primary electrons (Feuerbacher et al, 1972;Willis et al, 1973;Whipple, 1981;Horányi et al, 1998).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2012

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Electron distributions measured by Lunar Prospector above the dayside lunar surface in the solar wind often have an energy dependent loss cone, inconsistent with adiabatic magnetic reflection. Energy dependent reflection suggests the presence of downward parallel electric fields below the spacecraft, possibly indicating the presence of a standing electrostatic structure. Many electron distributions contain apparent low energy (<100 eV) upwardgoing conics (58% of the time) and beams (12% of the time), primarily in regions with non-zero crustal magnetic fields, implying the presence of parallel electric fields and/or wave-particle interactions below the spacecraft. Some, but not all, of the observed energy dependence comes from the energy gained during reflection from a moving obstacle; correctly characterizing electron reflection requires the use of the proper reference frame. Nonadiabatic reflection may also play a role, but cannot fully explain observations. In cases with upward-going beams, we observe partial isotropization of incoming solar wind electrons, possibly indicating streaming and/or whistler instabilities. The Moon may therefore influence solar wind plasma well upstream from its surface. Magnetic anomaly interactions and/or non-monotonic near surface potentials provide the most likely candidates to produce the observed precursor effects, which may help ensure quasi-neutrality upstream from the Moon.

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“…This sputtering erodes surface grains at a rate of 0.01-0.04 nm/yr [e.g., Wehner et al, 1963;Johnson and Baragiola, 1991], however, the porous nature of the regolith may trap >90% of forward-directed sputtered species [Hapke, 1986]. Ion sensors onboard the SELENE spacecraft in lunar orbit recently detected backscattered solar wind particles and found that $0.1-1% of the solar wind protons bombarding the lunar surface backscatter from the Moon as ions [Saito et al, 2008]. During the backscatter process, however, incident solar wind ions are mostly neutralized, and the vast majority should exit as ENAs with a broad distribution of energies up to the solar wind energy.…”

Section: Introductionmentioning

confidence: 99%

Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon

McComas

Allegrini

Bochsler

et al. 2009

Geophysical Research Letters

120

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[1] The solar wind continuously flows out from the Sun, filling interplanetary space and impinging directly on the lunar regolith. While most solar wind ions are implanted into the lunar dust, a significant fraction is expected to scatter back and be emitted as energetic neutral atoms (ENAs). However, this population has never been observed, let alone characterized. Here we show the first observations of backscattered neutral atoms from the Moon and determine that the efficiency for this process, the lunar ENA albedo, is $10%. This indicates that the Moon emits $150 metric tons of hydrogen per year. Our observations are important for understanding the universal processes of backscattering and neutralization from complex surfaces, which occur wherever space plasmas interact with dust and other small bodies throughout our solar system as well as in exoplanetary systems throughout the galaxy and beyond.

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Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP‐PACE onboard SELENE (KAGUYA)

Cited by 192 publications

References 24 publications

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

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

Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon

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