Monitoring solar wind in the near-earth environment with SWIM of the SARA experiment aboard the Indian lunar Mission Chandrayaan-1

Abstract

The SARA experiment aboard the Indian lunar mission Chandrayaan-1 mission consists of two instruments: Chandrayaan-1 Energetic Neutral Analyzer (CENA) and the Solar Wind Monitor (SWIM). CENA will provide measurements of low energy neutral atoms sputtered from lunar surface in the 10 eV-3.3 keV energy range by the impact of solar wind ions. SWIM will monitor the solar wind flux precipitating onto the lunar surface and in the vicinity of moon. SWIM is basically an ion-mass analyzer providing energy-per-charge and number density of solar wind ions in the energy range 10 eV-15 keV. It has sufficient mass resolution to resolve H, He, O, Na/Al-group, K/Ca-group and Mn/Fe, with energy resolution of 7%, and angular resolution 9 deg (elevation) × 22.5 deg (azimuth). The viewing angle of the instrument is 17 deg × 160 deg. Mechanically SWIM consists of a sensor and an electronic board that includes high voltage supply and sensor electronics. The sensor part consists of an electrostatic deflector to analyze the arrival angle of the ions, cylindrical electrostatic analyzer for energy analysis and the time-of-flight system for particle velocity determination. The total size of SWIM is slightly greater than a credit card and has a mass of 500 g. The moon's orbit provides a unique location to observe solar wind continuously over 27 days period around the Earth. The measurement with SWIM will provide a unique data base for near-Earth solar wind conditions over a 2 year period of the Chandrayaan-1 nominal mission life time. With solar wind measurements being made at L1 point (240 Re; Re = Earth's radius) and at around 6-10 Re, the SWIM will not only help to study lunar-solar wind interactions, but will also be of great importance to study changes in solar wind as it propagate from L1 point to Moon's orbit (60 Re) to Geostationary orbits.