Apollo 15 Mission
Science Experiments - Solar Wind Spectrometer
The Solar Wind Spectrometer measured the composition of the solar wind.
The Sun continually emits a flux of electrically charged particles into space. This is termed solar wind. Earth's magnetic field prevents these charged particles from reaching Earth's surface, although in the polar regions, these particles can reach the upper part of the atmosphere, causing auroras. The Moon is outside the Earth's magnetic field for most of each month and has a negligible atmosphere, so solar-wind particles can reach the Moon's surface. Two different experiments, the Solar Wind Spectrometer and the Solar Wind Composition Experiment, were deployed on the Moon to study the solar wind.
The Solar Wind Spectrometer was deployed on Apollo 12 and 15. Although the solar wind contains ions of most chemical elements (including the noble gases measured by the Solar Wind Composition Experiment), over 95% of the particles in the solar wind are electrons and protons, in roughly equal numbers. The Solar Wind Spectrometer measured the flux of protons and electrons as a function of particle velocity. The measurements were made in seven detector cups with different orientations in order to determine the direction of particle motion. Most of the measured flux was in the detector that was oriented most directly toward the Sun.
Because the Solar Wind Spectrometer made continuous measurements, it was possible to measure how Earth's magnetic field affects arriving solar-wind particles. For about two-thirds of each orbit, the Moon is outside Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar-wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath, where the Earth's magnetic field affects the solar wind but does not completely exclude it. In this region, the particle flux was reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar-wind particles were measured.