The Negative Ions at the Lunar Surface (NILS): first dedicated negative ion instrument on the Chang’E-6 mission to the Moon.

• Introduction

When solar wind hits the lunar surface, a fraction of it is reflected back from the Moon instead of being absorbed by its surface. Evidence of neutral hydrogen atoms [1] and solar wind protons [2] being reflected from the surface are provided by numerous studies; however, no measurements of negative ions have been done so far. It is unknown whether or not a significant negative ion population exists near the lunar surface. The Negative Ions at the Lunar Surface (NILS) instrument, the first-ever dedicated negative ion instrument flown beyond the Earth, will address this question. NILS is being developed at the Swedish Institute for Space Physics for the Chinese Chang’E-6 sample return mission. Chang’E-6 is expected to launch in 2024 and will soft-land on the lunar far-side at approximately 41°S and 180°E. NILS will operate at least 30 minutes after the landing to achieve its main science objective. NILS operations will end with the lift-off of the sample return module.

• Main science objective

Negative ions are a not yet observed plasma component at the Moon. NILS’s main objective is to detect negative ions emitted from the lunar surface as a result of the interaction with solar wind and establish the upper limits for their fluxes. That requires NILS to distinguish and resolve the energy distributions of scattered H- and sputtered lunar negative ions, as well as to coarsely resolve different mass groups. The overarching scientific question is to estimate the importance of negative ions for space-surface interactions and environments of planetary bodies with surface-bound exospheres.

• Instrument design

NILS is the 9th generation of the SWIM family [3], a series of compact, adaptive, and capable mass spectrometers analysing ions, electrons or energetic neutral atoms at the sub-keV energy range. NILS combines a compact electrostatic analyser and a time-of-flight cell to measure the energy per charge and the mass per charge of incident ions. An additional deflection system consisting of two cylindrical electrodes allows for a one-dimensional 160° scanning of the viewing direction. NILS instantaneously records negative ions and electrons from a single direction and requires electrostatic scanning to change the viewing direction. Negative ions and electrons can be separated by an electron suppression system placed between the deflection system and the first-order focusing 127° long cylindrical electrostatic analyser. The latter is equipped with micro serrations on the outer electrode to maximize suppression of UV photons and scattered particles.

Prior to entering the time-of-flight cell, ions are post-accelerated by a -400V potential improving the sensor detection efficiency. The time-of-flight start signal is generated by a channel electron multiplier, which detects the secondary electrons generated when that particle is interacting at grazing incidence with a tungsten single crystal start surface. After the reflection on the tungsten surface, the particle travels to a stop surface, where a second secondary electron is generated that then subsequently is detected by a second channel electron multiplier, providing the time-of-flight stop signal. The time difference between the start and stop signals combined with the energy setting of the electrostatic analyser allows determining the mass per charge of the particle.

• References

[1] M. Wieser, S. Barabash, Y. Futaana, M. Holmström, A. Bhardwaj, R. Sridharan, M. B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura. First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms. Geophys. Res. Lett., 37(5), 03 2010.

[2] Y. Saito, S. Yokota, T. Tanaka, K. Asamura, M. N. Nishino, M. Fujimoto, H. Tsunakawa, H. Shibuya, M. Matsushima, H. Shimizu, F. Takahashi, T. Mukai, and T. Terasawa. Solar wind proton reflection at the lunar surface: Low energy ion measurement by map-pace onboard selene (kaguya). Geophys. Res. Lett., 35, 12 2008.

[3] M. Wieser and S. Barabash. A family for miniature, easily reconfigurable particle sensors for space plasma measurements. Journal of Geophysical Research: Space Physics, 2016