Solar Wind Spectrometer

Description

The solar wind spectrometer, designed to measure protons and electrons at the lunar surface, was part of the Apollo 12 ALSEP package left on the Moon. It consisted of seven modulated Faraday cups opened toward different, but slightly overlapping, portions of the lunar sky. The instrument was used to observe the directional intensities of the electron (6-1330 eV) and positive ion (18-9780 eV) components of the solar wind and magnetotail plasma that strike the surface of the Moon. The scientific objectives of the Solar Wind Spectrometer Experiment were to study the existence of the solar wind plasma on the Moon, the properties of the lunar surface and interior, general solar wind properties, and the magnetospheric tail of the Earth.

Each Faraday cup had a circular opening, five circular grids, and a circular collector. The five circular grids were an aperture grid, a modulator grid, a screen grid, a screen plate, and a suppressor grid. These served to apply an AC modulating field to incoming particles and screen the modulating field from the sensitive preamplifiers. Sensor covers were over each cup to protect against dust during the EVA's and lunar module ascent. Charged particles entered the cup and a current amplifier determined the resultant current flow. Energy spectra of positively and negatively charged particles were obtained by applying fixed sequences of square-wave AC retarding potentials to a modular grid and measuring the resultant changes in current. One cup was oriented vertically and the other six cups surrounded it symmetrically facing 60 degrees off vertical. The electronics were in a temperature-controlled container below the sensor assembly attached to a radiator. The instrument had deployed dimensions of 30.5 x 28.2 x 34.5 cm, a mass of 5.7 kg, used 12.5 W total power, and had an average data rate of 66.2 bits/sec.

A sequence of plasma measurements was made every 28.1 seconds, consisting of 14 energy steps spaced a factor of square root of 2 apart for positive ions and 7 steps a factor of 2 apart for electrons. Flux rates of 2.5E6 to 2.5E11 particles cm^-2 sec^-1 could be measured. A large number of internal calibrations are provided. The instrument as deployed had the east-west axis of the instrument 2.8 degrees north of east and 2.5 degrees off level with the west edge low. The north-south axis was self-leveling. These were well within the specifications for the instrument. The ALSEP central station was located at 3.0094 S latitude, 23.4246 W longitude. The solar wind spectrometer was situated 4 meters south of the central station.

The instrument was turned on with the sensor covers in place to provide background data and the covers were removed automatically on 20 November at about 15:30 UT, approximately one hour after lunar module ascent. The instrument was turned to standby mode during the lunar night starting on 3 March 1976 to provide more power to heat the central station electronics and was turned off on 15 January 1977 to increase power for central station thermal control.