Apollo 12 Solar Wind Spectrometer


Instrument Overview
===================
The Solar Wind Spectrometer (SWS), designed to measure protons and
electrons at the lunar surface, was part of the Apollo 12 Lunar Surface
Experiments Package (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 SWS 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
extravehicular activities and the 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 (number 7) was oriented
vertically and the other six cups (numbered 1-6 clockwise) surrounded
it symmetrically facing 60 degrees off vertical.  The number 1 cup
faced west on Apollo 12.  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 Watts total power, and had an
average data rate of 66.2 bits/second.

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 3.00942
degrees South latitude and 23.42458 degrees West longitude on the lunar
surface.  The solar wind spectrometer was situated 4 meters south of
the central station and was emplaced so that cup number 4 faced east
and the point between cups 2 and 3 faced north.

The instrument was turned on with the sensor covers in place to provide
background data and the covers were removed automatically on 20
November 1969 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.
     
																										 
References
==========
Apollo 12 Preliminary Science Report, NASA SP-235, published by NASA,
Washington D.C., 1970.

Apollo Scientific Experiments Data Handbook, NASA Technical Memorandum
X-58131, JSC-09166, published by NASA Johnson Space Center, Houston,
Texas, Aug. 1974 (revised Apr. 1976).

Bates, J.R., W.W. Lauderdale, and H. Kernaghan, ALSEP Termination Report,
NASA Reference Publication Series, NASA RF-1036, published by NASA Johnson
Space Center, Houston, Texas, Apr. 1979.

Clay, D.R., B.E. Goldstein, M. Neugebauer and C.W. Snyder, Lunar surface
solar wind observations at the Apollo 12 and Apollo 15 sites, Journal of
Geophysical Research, 80, 1751-1760, 1975.

Goldstein, B.E., D.R. Clay, C.W. Snyder and M. Neugebauer, ALSEP solar
wind spectrometer plasma data as observed at the Apollo 12 and 15 landing
sites, Jet Propulsion Laboratory, Contract NAS7-100, 1973.

Neugebauer, M., C.W. Snyder, D.R. Clay and B.E. Goldstein, Solar wind
observations on the lunar surface with the Apollo-12 ALSEP, Planetary and
Space Science, 20, 1577-1591, 1972.

Snyder, C.W., D.R. Clay and M. Neugebauer, The solar-wind spectrometer
experiment, Apollo 12 Preliminary Science Report, NASA SP-235, Chapter 5, 
pp. 75-81, published by NASA, Washington D.C., 1970.


Source
======
The NASA Space Science Data Coordinated Archive (NSSDCA, formerly NSSDC) 
provided this description.