NEUTRAL ATMOSPHERE COMPOSITION (NACE) A. The NACE experiment measures the concentrations of the neutral gas constituents in the thermosphere. This experiment provides the instantaneous and global distributions of neutral helium, atomic oxygen, nitrogen, argon, and total mass density. B. The NACE is a direct extension of the OGO-6 Neutral Composition Experiment (F0-4) and employs a quadrupole mass spectrometer. A spherical ante-chamber is coupled to the atmosphere through a knife-edged orifice. The gas in the ante-chamber is continuously sampled by the hot filament ion source which accelerates a beam of ions into the quadrupole mass filter. The ions of selected m/e ratio strike the first dynode of the off-axis electron multiplier. Amplified pulses out of the multiplier are counted to provide a measure of the density of the selected mass in the spherical ante- chamber, which is related to the ambient atmospheric density using the standard F(s) equation. The detector system provides a dynamic range of 10 degrees, and an additional factor of 40 in range is achieved by automatically desensitizing the ion source at the high densities expected to be encountered near perigee. C. The words 35 to 40 of the UA file-have the following meaning: 35. N2 density (num/cm3 ) 36. O density (num/cm3) 37. He density (num/cm3) 38. Ar density (num/cm3) 39. NO density (num/cm3 ) 40 O+2O2 density (num/cm3) The 10 least significant bits of each word provide an estimate of the fractional error which is calculated by dividing the integer value of these bits by 512. This error reflects counting statistics and background subtraction errors but not the overall instrument calibration error. E. Laboratory gas calibration was performed with helium, molecular oxygen and nitrogen, and argon using a calibration chamber pumped by Zeolite at liquid-helium temperature. Porous vycor leaks provide known conductances from pressure reference volumes. The reference standards used are Baratron capacitance manometers. Calibration accuracy for the nonreactive gases is approximately 10%. F. Atomic oxygen above 250 km, particularly in the inleg of each perigee pair, may be too low by 30 to 50% because of surface interactions. G. The NACE-C instrument failed on February 16, 1974 and data for that day (or thereafter) is suspect. H. See F above. I. Measured counts are corrected for dead time, zero level, filament emission, and warmup variation and converted to antechamber (or source) densities using the laboratory calibrations. The source density in the wake of each spin cycle is subtracted from the individual densities and the standard F(s) equation (thermal transpiration in a moving system) used to convert to ambient density. The background for despun orbits is based on nearby spinning orbits. For the UA file, the densities over a 30 second period centered on the required time are fit with a straight line as a function of time and the required density calculated. K. "A Neutral-Atmosphere Composition Experiment for the Atmosphere Explorer-C, -D, and -E" by D. T. Pelz, C. A. Reber, A. E. Hedin, and G. C. Carignan, Radio Sci, 8, 277-283, 1973.