Studying the Lunar—Solar Wind Interaction with the SARA Experiment aboard the Indian Lunar Mission Chandrayaan-1

Abstract

The first Indian lunar mission Chandrayaan-1 was launched on 22 October 2008. The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard Chandrayaan-1 consists of an energetic neutral atom (ENA) imaging mass analyzer called CENA (Chandrayaan-1 Energetic Neutrals Analyzer), and an ion-mass analyzer called SWIM (Solar wind Monitor). CENA performed the first ever experiment to study the solar wind-planetary surface interaction via detection of sputtered neutral atoms and neutralized backscattered solar wind protons in the energy range ~0.01-3.0 keV. SWIM measures solar wind ions, magnetosheath and magnetotail ions, as well as ions scattered from lunar surface in the ~0.01-15 keV energy range. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analyzed via surface interaction technique. The ion mass analyzer is based on similar principle. This paper presents the SARA instrument and the first results obtained by the SWIM and CENA sensors. SARA observations suggest that about 20% of the incident solar wind protons are backscattered as neutral hydrogen and ~1% as protons from the lunar surface. These findings have important implications for other airless bodies in the solar system.

Plasma Measurements at Non‐Magnetic Solar System Bodies

“…Self-pickup provides a maximum velocity (for a proton) of up to 3 times the solar wind, and an energy 9 times that of the solar wind [Saito et al, 2008] in the spacecraft frame, due to the 'injection point' being at up to -u sw (see Figure 8, which illustrates the pickup ion geometry for both conventional pickup (inner circle) and for self-pickup (outer circle)). In addition, IBEX detected neutral lunar backscattered particles [McComas et al, 2009], while Chandrayaan-1 both confirmed the reflected protons, and found that up to 20% of the incident solar wind flux can be backscattered as neutrals [Wieser et al, 2009, Bhardwaj et al, 2010. These may then ionize and form part of the 'self-pickup' population.…”

Electron dynamics in the minimagnetosphere above a lunar magnetic anomaly

“…The lunar plasma environment has been investigated intensively by recent spacecraft, such as KAGUYA [e.g., Saito et al , , , ] and Chandrayaan‐1 [e.g., Barabash et al , ; Bhardwaj et al , ]. In addition to macroscopic plasma structures, such as the wake structure formed in the downstream region, small‐scale perturbations of plasma distribution and fields have been observed in the dayside region, mostly over the crustal magnetic anomalies found on the lunar surface.…”