Suprathermal Ion Detector Experiment (SIDE)

Description

The Suprathermal Ion Detector Experiment (SIDE), part of the ALSEP package, measured positive ions reaching the lunar surface, including magnetospheric ions and those generated from ultraviolet ionization of the lunar atmosphere and from the free-streaming solar wind/lunar surface interaction. Flux, number density, velocity, and energy/unit charge were determined for these ions. The scientific objectives of the experiment were: to provide information on the energy and mass spectra of the positive ions close to the lunar surface; measure the flux and energy spectrum of positive ions in the Earth's magnetotail and magnetosheath during those periods when the Moon passes through the magnetic tail of the Earth; provide data on the plasma interaction between the solar wind and the Moon; and determine a preliminary value for the electric potential of the lunar surface. Similar instruments, differing only in look direction and mass range, were also flown on Apollo 12 and 15.

The experiment was housed in a rectangular box which was deployed on the surface of the Moon by the astronauts during their first EVA. A bubble level on top of the box was used to ensure proper leveling. The box stands on a tripod and is connected to the ALSEP central station by a ribbon cable. A wire screen is spread out on the surface under the tripod to compensate for a possibly large (tens of volts) lunar surface electric potential. The screen is connected to one side of a stepped voltage supply, the other side of which is connected to the internal ground of the detector and to a grounded grid mounted immediately above the instrument and in front of the ion entrance apertures. The top of the instrument is roughly 50 cm above the surface.

The SIDE consisted of two positive ion detectors. The first, the Mass Analyzer (MA), consisted of a velocity filter of crossed E and B fields (a Wien filter) in tandem with a curved-plate electrostatic energy-per-unit-charge filter and a channel electron multiplier behind both filters. The multiplier was operated as an ion counter that yielded saturated pulses for each input ion. The MA determined the ion flux in 20 mass channels from 6 to 750 amu for 6 energies: 0.2, 0.6, 1.8, 5.4, 16.2, and 48.6 eV. The other analyzer, the Total Ion Detector (TID), did not have a velocity filter and used a channel electron multplier to detect higher energy ions in 20 steps over the range 10 to 3500 eV. Both multipliers were biased with the input ends at -3.5 kV to boost the positive ion energies to improve detection efficiency. A mass spectrum (from MA) and an energy spectrum (from TID) were obtained each 24 s in normal mode. The potential of the entrance apertures relative to the grid deployed on the lunar surface was normally varied through 24 steps (of the following voltages: 0, 0.6, 1.2, 1.8, 2.4, 3.6, 5.4, 7.8, 10.2, 16.2, 19.8, 27.6, 0, -0.6, -1.2, -1.8, -2.4, -3.6, -5.4, -7.8, -10.2, -16.2, -19.8, and -27.6) at 2.58 min/step, in order to monitor the lunar surface potential. The detectors looked upward, 15 deg from local vertical, in a plane parallel to the lunar equator. The sensors looked approximately 2.4 deg to the right of Earth, so solar wind ions were not directly observable while the moon was outside the magnetosphere. Streaming ions in the downstream dawn-side magnetosheath were observed, as were ions upstream from the bow shock.

The Cold Cathode Ion Gauge (CCIG, also called Cold Cathode Gauge Experiment - CCGE) was carried in a compartment of the SIDE instrument and was removed and set up on the lunar surface by the deploying astronaut. The gauge unit was connected by a wire cable to the SIDE package, because the CCIG and SIDE electronics comprise an integrated system. On Apollo 12 the stiffness and spring action of the cold wrapped wire cable made it difficult to deploy and orient the gauge head properly, so the wrapping was removed for Apollo 14, but the spring effect still made deployment a problem.

The ALSEP central station was located at 3.6440 S latitude, 17.4775 W longitude. The SIDE was deployed approximately 16 meters south-southeast of the central station. The SIDE was turned on about 18:00 UT on 5 February 1971. On 5 April 1971 an anomaly in the analog-to-digital converter resulted in loss of ability to measure the input grid voltages, but the voltage steppers apparently continued to generate their nominal voltages. Hence the data were not affected. Electric arcing at high temperatures resulted in the SIDE being systematically cycled on-off for several days on each side of lunar noon, throughout the mission, to keep the internal temperatures below ~55 degrees C. Starting on 29 November 1974 the instrument became difficult to command on from the ground, possibly due to a short-circuit in the high voltage supply. (More than 1700 unsuccessful ON cammands were transmitted following 29 November). On 19 February 1976 the SIDE was commanded off permanently. An instrument description and preliminary report is given by Hills and Freeman as section 8 of the Apollo 14 Preliminary Science Report, NASA SP-272, 1971.

The results of the SIDE experiments included observations of: lunar ions accelerated by the solar wind induced field; 1-3 keV protons during lunar night, considered to be protons from the bow shock of the Earth; the energetic ion characteristics in the Earth's magnetosheath and at its boundaries, and correlation with geomagnetic activity; apparent motions of ion "clouds" related to lunar impact events, and mass spectra during the events; energetic ions during lunar night, when site is shielded from the solar wind direction; ion events near terminators suggesting a turbulent region of solar wind plasma interaction with the solid Moon; positive ion fluxes while in the geomagnetic tail, and correlation with geomagnetic storm activity; mass spectra of ions from the ambient atmosphere; the electric potential of the lunar surface in the magnetosheath or solar wind and near the terminators; solar wind during interplanetary storms; penetrating ions from solar flares (especially from the major flare event in August 1972); the effects of the LM ascent engine exhaust on magnetosheath ion fluxes, ion mass spectra due to the LM exhaust gas, and the intensity decay rate.