2.0.0 spase://SMWG/Instrument/AE-C/RPA Retarding Potential Analyser/Drift Meter (RPA) RPA 2019-05-05T12:34:56Z This experiment was designed to determine vector ion drift velocities, ion concentration and temperature, and spacecraft potential. An ionospheric irregularity index was also obtained from the ion concentration sensor. The experiment consisted of a retarding potential analyzer with four planar sensor heads. The sensor head used for ion drift measurements was co-located with another head, and all were spaced nearly equally, looking outward from the satellite equator. Since the satellite spin axis was perpendicular to the orbit plane, these heads could observe along the spacecraft velocity vector in either the spin or despun mode of the spacecraft. The primary objective of this experiment was to provide accurate ion temperatures with other measurements being of secondary importance. Three of the sensor heads were similar. They had two grounded entrance grids, two retarding grids, a suppressor grid, a shield grid, and a collector. A linear sweep voltage (32 or 22 to 0 V, up or down) was normally applied to the retarding grids in 0.75 s. Interpretation of the resulting current-voltage profiles provided the ion temperature, the ion and electron concentration, some ion composition information, vehicle potential and plasma drift velocity parallel to the velocity vector. Two of the three similar sensors had an additional grid between the entrance and retarding grids in order to protect inner grids from ion bombardment during electron measurements. The other significant feature of these two sensors was that a small positive collector bias could be applied to assure adequate access of thermal electrons to the collector. With the retarding grid at constant zero volts, current changes could be observed for 3-s periods to obtain gradients of ion concentration. Electron parameters were measured in a manner similar to ions. Ions in mass ranges 1 to 4, 14 to 16, 24 to 32 and greater than 40 atomic mass units could be identified. The fourth sensor head was for the ion-drift velocity measurements, and consisted of four grounded grids, a negatively biased suppressor grid, and a four-segment collector. Differences in various collector segment currents provided ion-drift directional component information. More details of this experiment are available in W. B. Hanson et al., Radio Sci., v. 8, n. 4, p. 333, 1973. NSSDC has all the useful data that exist from this investigation. spase://SMWG/Person/William.B.Hanson PrincipalInvestigator NSSDC's Master Catalog https://nssdc.gsfc.nasa.gov/nmc/experiment/display.action?id=1973-101A-04 Information about the Retarding Potential Analyser/Drift Meter (RPA) experiment on the AE-C mission. Radio Science Journal Article https://onlinelibrary.wiley.com/doi/abs/10.1029/RS008i004p00333 Detailed information about the Retarding Potential Analyser/Drift Meter (RPA) experiment on the AE-C mission. RetardingPotentialAnalyser Retarding Potential Analyser/Drift Meter (RPA) on AE-C spase://SMWG/Observatory/AE-C