DATA CENTER DCUMENTATION FOR THE ATMOSPHERE EXPLORER CYLINDRICAL ELECTROSTATIC PROBE (CEP) L. H. Brace, GSFC Principle of Operation; The Langmuir probe instruments for Atmosphere Explorers C, D and E have been described in detail in Brace et al, 1973. The technique employs two independent cylindrical collectors whose potentials are varied or held fixed as the resulting collected current is measured The ion concentration Ni is derived from the amplitude of the ion cur- rent collected when the collector is negative The electron temperature Te is derived by computer fitting of the electron retardation current waveforms Proper resolution of the volt-ampere curves is achieved by the use of an automatic ranging electrometer and an adaptive sweep voltage generator These circuits continuously adjust the electrometer sensitivity and the applied voltage amplitude and starting potential to bring about "ideal" framing of each successive volt-ampere curve. By this we mean that the instrument adapts itself to the plasma it is encountering to resolve only that portion of the volt-ampere curve that is most useful for the measurement of Ni and Te. In addition the satel- lite potential Vs is derived from each curve. Resolution of the Measurements: The one second sweep period determines the maximum spatial resolu- tion of the measurement of Te (approx. one value per 8 kilometers along the orbit). Ni is also obtained once per second. This resolution is actually attained only when the spacecraft is operating despun. In this mode, the collector never encounters the spacecraft wake and provides the full resolution of the measurements. When the spacecraft is spinning, the resolution is reduced to two measurements per spin (approx. 15 second spin period). The data in the UA file are listed at these 15 second intervals. Accuracy of the Measurements: The absolute accuracy depends upon the conditions, in particular, the spatial irregularity and the amount of plasma present. The nominal accuracy of Te in a smoothly changing plasma with Ni > 10**3/cm-3 is 5%. At lower densities, uncertainties in spacecraft shielding effects and micro- phonic noise signals increase the error in both Te and Ni measurements. (The midrophonics originate primarily in the sensor itself as it responds to vibration input from the inertial wheel of the spacecraft). Compari- son of the Te measurements with coincident measurements by incoherent radar installations indicate agreement within 5%, Benson et al., 1977. The error of the Ni values is less than 10% when O+ or molecular ions are the dominant ion constituents, typically below 1000 kilometers Where H and He are the major ions, a correction for their higher ther- mal velocity must be applied. At present, this correction is not well defined, and the error of Ni may be increased to 20% at these higher altitudes. Format of the CEP Data The results from CEP are stored in words 11, 12, and 13 of each UA Data record. Word 11 is the electron temperature, word 12 is the ion density, and word 13 is the satellite potential. The words are stored as floating point numbers. References Brace, L. H., R. F. Theis, A. Dalgarno, "The cylindrical electrostatic probes for Atmosphere Explorer C, D and E, Radio Science, 8, 341- 348, 1973. Benson, R. F., P. Bauer, L. H. Brace, H. C. Carlson, J. Hagen, W. B. Hanson, W. R. Hoegy, M. R. Torr, R. H. Want, and Y. B. Wickwar, "Electron and ion temperatures - A comparison of ground-based inco- herent scatter and AE-C satellite measurements", J. Geophys. Res. 82, 36-42, 1977.