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.