DATA CENTER DOCUMENTATION Atmosphere Explorer Neutral Atmosphere Temperature Experiment (NATE) Rationale for the Experiment The Neutral Atmosphere Temperature Experiment (NATE) was designed to measure the kinetic temperature of the neutral gas at the spacecraft. Its design was based upon a technique first employed on the San Marco III (a and b) satellites, and later the Aeros I and II satellites (Ref). The measurement approach is based upon the concept that the neutral particles, in this case N2, are in thermal equilibrium, and hence a measure of their velocity distribution permits a kinetic temperature calculation. The desirability of measuring the local kinetic tempera- ture seems obvious - it is a basic parameter in atmospheric studies, and it reflects local conditions of thermodynamic equilibrium and hence the local dynamic situation. Most previous temperature measurements have in fact been derived from density measurements, from the density scale height, from ion temperature, or optically by observation of line widths, Recent Aeros data analysis reveal temperature derived from density does not always faithfully reflect the true temperature of the gas particularly at times when a geomagnetic disturbance is influencing the atmosphere (Ref). The NATE also provided measurement of the composition, when commanded into the appropriate mode and, for the first time, measurement of the local wind: AE-C Vertical motions AE-D, E Vertical motions and horizontal (normal to orbit plane) winds Instrument Description The basic instrument is a quadrupole mass spectrometer with a gas sampling spherical antechamber. The sampling is accomplished through a small knife-edged orifice with an area that is approximately 1% of the antechamber spherical surface, to insure that the gas reaches equili- brium temperature (with the antechamber) before measurement; Temperature is determined by measurement of the changing density of the N2 gas in the antechamber as the spacecraft spins, or alternatively from the modulation caused by a scanning baffle when the spacecraft is despun. In the spinning mode, temperature is derived from the modulation caused by stream interception by a fixed baffle. The details of the technique are discussed in the reference (Ref.). The wind values are determined by measurement of the "stream" position relative to the satellite velocity. A null measurement permits a readily computed stream direction and, hence, by subtracting the spacecraft velocity, a wind measurement. A vertically scanning baffle leads to measurements of the vertical motion, and a horizontally scanning baffle leads to measurements of the horizontal wind normal to the orbit plane. No measurement of the component in the direction of satellite motion, in the orbit plane, is possible with the AE instru- mentation. The reference-provides details of the wind measurement technique and errors for vertical motions. Data Format A physical description of the tape is given in another part of the documentation to be provided by the Information Processing Division. The data from the NATE instrument appears in words 41-45 of the records on that tape. Word 41 is the neutral gas temperature in degrees Kelvin, The remaining words are the concentrations in number/cc of molecular Nitrogen, total Oxygen (O2 + 2O), Helium, and Argon. These values are all represented as binary floating point numbers. Whenever any of the values is not being measured, a zero has been put in the position. For AE-D the NATE words occur in position 42-46. In addition, word 47 on AE-D contains the zonal wind in meters/sec, with positive wind eastward. For AE-D word 46 contains the approximate meridional wind in meter/sec. In that case southward is positive. The actual wind component measured is normal to the vehicle direction of motion at the time of the measurement. Instrument Calibration Calibration of the instrument for temperature and wind measurements is not required since these data are computed from changes in the local density due to modulation by either spacecraft spin motion or baffle motion. Calibration of the mass spectrometer for use in the composition mode was accomplished on a conventional vacuum system. Variations in calibrations in orbit due to multiplier gain change normally experi- enced have been accounted for in the data. It should be assumed that the accuracy of the data in the composition mode is better than 25%. In many cases it is much better but at times may be worse. Time has not permitted evaluation of each data set in the file. Questions relative to particular data sets, particularly if they appear to be "unreasonable", should be addressed to the P.I. Comments on Wind and Temperature Validity It should be recognized that the wind and temperature data in the file are "new" data and reflect phenomena not generally observed previously by satellite. The variations observed reflect, it is believed, true variations of wind and kinetic temperature. However, sufficient analyses have not yet been accomplished nor has there been sufficient correlation with related phenomena to fully establish the validity of the data. Thus there is no known significant discrepancy in the data, however, no proof of the validity has yet been identified. Nevertheless, there has been sufficient study and comparison with related phenomena to convince the investigator that the data are valid. Error analysis conducted to date confirm that the values are valid to a few percent (Ref.). Random variations in density have been shown to have an insignificant effect upon the temperature and wind data. As discussed in the reference, uncertainty in the average value of the spacecraft roll position precludes confidence in the average value of the vertical winds, however, because of the much greater magnitude of the orbit normal component usually observed, one can have confidence in the absolute value of the horizontal component. Data Reduction The data reduction procedure involves several distinct processes. The procedure used to derive the composition measurements is the same as used in the NACE experiment which is discussed in the documentation for that instrument. First, the data are read using subroutines provided on the Sigma 9 computer. The instrument mode readout is then checked to see if the instrument is being operated in a mode in which the gas temperature may be derived. The following operating modes lead to useful temperature measurements. 1. Satellite despun, baffle scanning. 2. Satellite spinning, baffle fixed in front of orifice. 3. Satellite spinning, baffle stowed. After the mode determination the data is checked for the presence of drop outs and a dead time correction is applied. Further data reduction is abandoned if excessive data drop outs occur. Next a density gradient correction is applied to the data this is done by fitting a quadratic to the portions of the data which are not dependent on the gas temperature. In the spinning mode the data are independent of the gas temperature from approximately 20 to 8O degrees angle of attack. In the despun mode the data are independent of gas tempera- ture whenever the baffle is at one of the extreme positions in its scan. To perform the analysis when the baffle is being used we establish a series of sums which we call moments. These sums are formed from teta(i) and Wi. The quantity teta(i) is just the time of each data point measured from an arbitrary reference position. Wi is the difference between each data point and the value of the quadratic used for gradient correction at that point. The first moment R1 is just the sum of Wi over the data. This moment is nearly independent of temperature or wind and can be used to normalize the remaining moments. R2 is the sum of the squares of Wi divided by Rl squared. This moment is highly dependent upon the gas temperature. teta(1) is the sum of the product of teta(i) x Wi divided by R1. This moment defines the position of the baffle wake with regard to the arbitrary reference mentioned above and leads to a measurement of the local wind. teta(2) is the product of Wi times a teta(1) dependent weight function divided by R1. The constants in the weight function are chosen to enhance the dependence of this moment on the gas temperature. Different constants are used for the different baffle configurations. To convert these moments to the gas temperature and wind we have systematically calculated a set of Wi encompassing all conditions expected to be encountered in orbit. The moments were then formed ant fitted to multi- dimensional Taylor's series. Finally, orbit and attitude data are read in to enable a conversion to winds in an earth oriented reference frame. When the baffle is not in use the gas temperature may be determined when spinning using the region near 90 degrees angle of attack. At 90 deg. particles enter the orifice because of their transverse thermal motions. These thermal motions are only important in a small angular region approx- imately 15 degrees wide centered on 90 degrees. The technique used to determine the temperature depends on the fact that the sum of the data points over this interval minus the sum of a calculation of the value those points would have if thermal motions were set to zero, is a simple function of the gas temperature. References l. Spencer, N. W., H, U. Niemann, and G. R. Carignan, "The Neutral-Atmosphere Temperature Instrument," Radio Science, 8, 287, 1973. 2. Spencer, N. W., D. T. Pelz, H. B. Niemann, G. R. Carignan, and J. R. Caldwell, "The Neutral Atmosphere Temperature Experiment," J. Geophys., 40, 613, 1974. 3. Spencer, N. W., R. F. Theis, L. E. Wharton, and G. R. Carignan, "Local Vertical Motions and Kinetic Temperature from AE-C as Evidence for Aurora-Induced Gravity Waves," Geophys. Res. Ltrs., 3, 313, 1976. 4. Chandra, S., N. W. Spencer, D. Krankowsky, and P. Lammerzahl, "A Comparison of Measured and Inferred Temperatures from Aeros-B," Geophys. Res. Ltrs., 3, 718, 1976.