Guide To The Use of Unified Abstract Data From The Open Source Neutral Mass Spectrometer (OSS) Experiment-Atmosphere Explorers C, D and E. D. C. Kayser, W. E. Potter and A. O. Nier University of Minnesota Minneapolis, Minn. 55455 Feb. 1, 1977 Introduction The Open Source Neutral Mass Spectrometers (hereafter referred to as the OSS) perform measurements of the neutral upper atmosphere with a time resolution of 16 Hz, giving approximately 105 density determinations from a single 30 minute pass. For many aeronomic studies a lower resolution data set is sufficient and even preferred. The Unified Abstract (UA) Files provide such a data base. In the case of the OSS, measurements of an ambient density are averaged over a 15 second interval that is centered on times which are multiples of 15 seconds of the day (in Universal Time). Of course, altitude variations during the 15 second averaging interval must be con- sidered, as well as the errors in the individual data points which make up the average. The resulting UA data points for the OSS are to be interpreted as the 15 second time-averaged density about the specified abstract time for the altitude crossed by the satellite at the abstract time. This document follows the topical guidelines suggested in 'Guidelines For Submitting Data to the National Space Science Data Center' and is intended to introduce non-AE science and data processing personnel to the use of OSS UA file data. It could as well serve as a reference for those with direct access to the AE computer system. A. Experiment Rationale and Motivation The Atmosphere Explorer-C, -D and -E satellites have been described in detail by Dalgarno et al. (1973). The Open Source Neutral Mass Spectrometer was designed to measure the common neutral constituent densities from the lower thermosphere to the exosphere (Nier et al. 1973). The quasi-open source design was chosen to enhance the possibility of measuring reactive constituents such as atomic oxygen, and to complement the closed-source design of the other neutral spectrometers. Specific discussions of interesting aeronomic problems which require neutral density measurements can be found in the special issue of Radio Science referenced above. Papers published by the OSS group (see references, part K), as well as the AE publication list, should be consulted for further examples. B. Instrument Description The OSS flown on Atmosphere Explorer-C, -D and -E is a double-focusing magnetic-deflection instrument with an electron-bombardment ion source. Ambient particles enter the ion source region here they are ionized by a magnetically collimated 75 ev electron beam produced by a heated 0.015 cm diameter tungsten-rhenium hairpin filament. In the 'normal' mode of operation the ions are drawn out of the source region by an electric field, pass through a pair of focusing plates and are accelerated to a narrow ground slit. After passing a collimating slit, the ions enter a variable electric field analyzer and a constant magnetic field analyzer where energy focusing and mass separation take place. The magnetic field is produced by a 4000 gauss permanent magnet. At the exit of the magnetic analyzer are two slits which allow ions of high and low mass (in an 8:1 ratio) to be collected simultaneously by two detectors. The low mass spiral electron multiplier measures ions from 1 to 6 AMU while the high mass tandem electrometer- multiplier detector detects ions from 6 to 48 AMU. The high mass detector consists of a 50% transmission electrometer grid followed by a spiral electron multiplier, thus allowing measurement of the larger mass constituents over an extended range. Preprogrammed electronics enable measurements to be made in a peak stepping, toggle or scan mode. In the peak stepping and toggle modes, voltages are changed in steps allowing data to be taken on a series predetermined peaks without spending time measuring over the 'valleys' between peaks as is done in the scan mode. Other instrument commands allow switching to an electron beam energy of 25 ev and slowing down the 16 steps/sec sample rate by a factor of 8. Redundant filaments are employed in case one burns out while variable multiplier voltage supplies are used to offset any degradation in multiplier gain that may occur. The status of the instru- ment configuration and housekeeping voltages and temperatures are monitored through subcommutated telemetry words. In the source region a reactive gas such as O recombines to form O2 after striking the surrounding instrument surfaces. In the 'normal' mode the contribution to the 32 small peak from the recombined 16 is not easily re- solved. To alleviate that problem a fly-through mode of operation was added to the OSS. In this mode there is no electric field used to draw ions out of the electron beam when they are formed. Ambient particles which have struck the ion source and have become thermally accommodated have an energy of less than 0.l eV which is not enough to overcome the negative space charge potential holding ions in the electron beam. On the other hand, ambient particles Which have not struck surfaces retain their full energy due to the relative velocity between the particle and the spacecraft. These 'energetic' particles readily escape from the beam and thus enable direct measurements of O and O2 to be made. During spinning orbits, the shape of the fly-through signal as the instrument passes through the ram direction can be used to determine the ambient temperature. Measurements of winds can also be ob- tained by using the period between the signal maxima in fly-through. The general sensitivity of the spectrometers in the normal mode is 10**-5 amp torr-1. For low altitude data taking, a small ion sputter pump is used to reduce ion-neutral collisions within the spectrometer. Neutral con- stituent densities over the range of 10**4 to 10**12 cm-3 can generally be measured with an accuracy of +/- 20% for most gases. Direct measurements of O, O2, N2, He, Ar, N and Ne have been made by OSS. Temperatures are determined to within +/- 30 degrees K while winds above 10 m/sec can be measured. A further detailed description of modes, examples of mass spectra and instrument figures are to be found in the paper by Nier et al.,(1973). A discussion of the fly-thru mode has been given by Nier et al.,(1974). D. Data Catalog It is assumed here that the UA data catalog for all experiments will be included with the data tape. Those investigators with access to the AE Sigma 9 system can generate this catalog using the DMF QUERY program. Orbit numbers,starting and ending dates and times and the presence of data from each experiment can be determined. OSS data presence is denoted by YES in the column labeled OSS. Generally, data will be found in the OSS UA words (if YES is indicated) over most of the time interval. There will be exceptions to this rule (gaps) due to many circumstances. These include: lost telemetry in transmission to GSFC, poor quality telemetry words, instrument operations were incompatible with spacecraft operations or altitudes, the instrument was in engineering modes for part or all of the pass, or the instrument was switching modes frequently. It is therefore necessary to read the file on a point-by-point basis to determine whether sufficient data is present for the investigator's needs. It should be noted that a primary goal of the data processing effort on AE is to provide simultaneous data for specific orbits, not necessarily to reduce telemetry on all orbits. The percentage of orbits reduced by OSS, while large, still reflects this overall philosophy. E. Calibration Procedures A knowledge of instrument calibration procedures is not required for the use of the OSS UA data. Absolute calibration error are estimated to be a few percent (Nier et al., 1973) and are included in the percentage error estimate which is packed with each data word. While the above reference describes the basic calibration techniques, the reader may be interested in some additional insights gained by the OSS group since that time. Consistent with the observations of Hedin et al.,(l973) atomic oxygen entering the source region of the OSS is found to recombine as O2 and can be measured as such. The laboratory calibration for molecular oxygen was straightforward and can be used in flight to give accurate indirect measure- ments of atomic oxygen, provided that a suitable correction for ambient molecular oxygen is applied (see section J). The conversion from O2 to atomic oxygen is then [O] = 2 ([O2]measured - [O2]ambient). Because the O recombination is not complete, the actual factor used on AE-C is 2.1. Further analysis of this problem was done by Nier et al. (1976) and French et al. (1975). The laboratory calibration procedure relates the instrument signal am for a given mass m to a known ambient number density, na. For a true closed source instrument in calibration, the source density ns is given by ns=na*sqrt(Ta/Ts) where Ta and Ts are respectively the ambient and source temperatures. The sensitivity Sm can be defined by Rm = Sm*ns. Conversely, in flight am is measured, from which na = ns*sqrt(Ts/Ta)* G = (am/Sm) sqrt(Ts/Ta)* G where G is a stagnation factor, also called the source function, dependent on satellite velocity and the angle of attack. For the OSS, which is a 'quasi-open' source instrument, an additional experiment must be done to determine the effect of the 'openness' and hence a correction to G. This experiment, employing an AE OSS, has been described by Nier et al. (1974) and French et al. (1975). The OSS UA data have been corrected by this factor, which is 1.05 for all masses except He, where the factor 1.26 is required. Direct comparisons of OSS densities with those of the NACE experiment, and with the mass densities from the MESA experiment, as well as ensemble comparisons with other satellite mass spectrometers (Trinks et al, 1976) give us additional confidence in our absolute density calibrations. F. Validity of the OSS Data Generally, when OSS data are present in the UA files, they are valid within the associated percentage error. Therefore, it is important that conclusions drawn from the data are examined in terms of the data accuracy. Our experience has indicated that usually the errors are conservative and may be nearly 2-sigma errors in some cases. Considerations of scale height and density are also important in determining practical limitations to the data. The following summarizes these limits: Helium: Data from perigee to about 800 km. Densities down to about 10**5 /cm3 can be measured. The altitude where this density is reached is related to the location of the helium bulge. Atomic oxygen: Data from perigee to about 600 km. Densities down to about 5 x 10**6/cm3 can be measured. Below this density, source background pressures can become a significant part of the density seen by the instrument. Molecular nitrogen: Data from perigee to about 550 km. Densities down to about 5 x 10**4/cm3 can be measured. Argon: Data from perigee to about 180 km. Densities down to about 5 x 10**6/cm3 can be measured. These measurements are usually done on the electrometer detector in the presence of large N2 and O signals. A small number of 'minor mode' passes have been run where Ar can be measured on the higher sensitivity multiplier detector. In this case argon is measured to about 250 km. Data which are low density measurements should always be examined by the user to verify that the above limitations are not exceeded. If high accuracy is required, the plotting and examination of altitude profiles may identify marginal points. The low altitude (perigee to 220 km) atomic oxygen data should receive special attention in applications requiring high accuracy. This data is de- rived from the mass 32 (O2) signal in the presence of an ambient O2 component. Sufficient information on global O2 densities and a practical means of correcting the mass 32 signal for the O2 contribution has evolved with the AE-C, D and E missions. OSS UA data written before Jan. 1, 1976 is being rewritten to reflect this new knowledge. Data in the UA file with revision dates before this time is not corrected. At 150 km the typical contribution of O2 will vary from 15 to 40%, increasing with increasing magnetic activity. A further discussion of O2 densities is to be found in Nier et al. (1974), Nier et al. (1976) and Kayser et al. (1976) On-line users who find low altitude data where [O] = 2 x word 6 exactly, or where the revision date is before Jan. 1, 1976, (UAQUERY-OSS can be used for this) should request a rewrite by the OSS group. Off-line users may use word 6 and the N2 density to correct the data as described in this document, part J. G. Unusual Events Relating to the Experiment Data users should be reminded that mass spectrometer sources which are initially exposed to reactive atmospheres will undergo a period of surface conditioning in the manner described by Hedin et al. (1973) and Nier et al. (1977). Because the condition is transient, we have made no special effort to eliminate these effects in the data taken in the first month after launch. As a result, OSS UA data points from this period should not be used for high precision calculations unless the OSS group is consulted. In the period near orbit 7000 on AE-C the automatic electrometer zeroing circuit reached the limit of its range, thereby inducing a periodically varying bias level on the electrometer data. For a period around orbit 7000 the effect is seen with increasing frequency until operations with the electrometer are curtailed. Data collected on AE-C since this time has been acquired on the main mass multiplier employing the minor mode mass programs. Atomic oxygen and N2 are not directly obtained in these modes. H. Known Anomalies in the Data There are no instrument-generated anomalies of any consequence in the data being delivered to the Data Center. Gain changes in the electrometer circuit occasionally introduce a spurious data point at the telemetry level. Where possible these isolated points have been excluded from the UA averaged data. On AE-C non-physical variations in the O and N2 densities will be ob- served on some orbits for altitudes between 350 and 400 km. The problem is due to automatic data processing, not the instrument. This altitude region corresponds to the overlap range of the high mass multiplier and electro- meter. When the electrometer signal reaches a certain value, the multiplier is shut off and a bit is set in the 4-sec instrument subcom. Alternatively, the multiplier will shut itself off at high count rates. Especially on spinning mode passes, the 15 second spin period of the satellite and hence the signal modulation period) may frustrate the 15 second off constraint in the multiplier control circuitry. The multiplier on-off transients in the data are not always recognized by the data reduction system and abnormal densities are written on occasion. I. Overall Data Reduction Procedures Because the OSS UA file data words contain ambient neutral number densities the user does not have to perform additional data reduction. A complete de- scription of the OSS data reduction procedure is beyond the scope of this docu- ment. Below is a brief discussion of the critical steps involved. Bytes of OSS data are retrieved from the spacecraft telemetry records and combined to give detector signals, housekeeping voltages and command words. Up to three detector signals are obtained for each 1/16 second of operation. Multiplier detector signals are corrected for dead times as obtained in labor- atory calibrations. At low altitudes (high source pressures) all detector signals must be corrected for gas scattering effects. At high altitudes some portion of the signal may be due to surface-desorbed gases and approximate corrections are applied: When the satellite is spinning, residual signals are available every 15 seconds and, at least in the case of the OSS geometry, it appears appropriate to subtract the residual signals from the forward facing signals. When the satellite is despun, the residual signals from very high altitudes are subtracted; downleg high altitude points on the downleg, upleg high altitude points on the upleg. Fortunately, the quasi-open' source has a high pumping speed and such corrections are usually small on AE-C up to 450 km. Ambient number densities are obtained from the corrected signals as described in part E. The densities are written to a Geophysical Unit File and stored on tape. A separate program reads these files and writes averaged data values into the OSS UA data words. J. Some Data Analysis Procedures Initially, because it was anticipated that word 6 (total O2) of the OSS UA data words would not be used directly by other groups, no correction was made to it for incomplete atomic oxygen recombination. This correction was applied in the computation of word 4 (atomic oxygen). Considerable use has been made of the word 6, however, and some confusion as to its meaning has resulted. We are now rewriting this data with the proper correction, but until this work is completed, the user who needs the highest accuracy must include this step in his calculations, as follows: If the revision date of the orbit is before Jan. 1, 1976, multiply word 6 by 1.05. A similar step may be required for helium in some orbits written to UA before Nov. 2, 1974. These densities must be multiplied by part of the 'openness' correction (see part E) = 1.20. Before Jan. 1, 1976, the task of correcting the atomic oxygen in word 4 for the presence of ambient molecular oxygen was left to the user. Again, some confusion resulted. We are now rewriting these data with a correction based on our experience on AE-C, D and E ( see part F). Following Nier et al. (1976) and Kayser et al. (1976), where N2 densities are measured along with [O] = 2*([total O2] - 1.51E-3*[N2]*(N2*T)**1/7). The temperature T used is not critical in this expression, and if no better estimate is available, T = 800 degress K is used. The above expression is elaborated somewhat for AE-D and E. K. References to Papers Relating to the OSS (as of October 13,1976) I. Publications of OSS Group Alone Already published: 1. Nier, A. O., W. E. Potter, D. R. Hickman, and K. Mauersberger, "The Open-Source Neutral-Mass Spectrometer on Atmosphere Explorer-C, -D, and-E," Radio Science, 8, 271, 1973. 2. Nier, A. O., W. E. Potter, D. C. Kayser, and R. G. Finstad, "The Measurement of Chemically Reactive Atmospheric Constituents by Mass Spectro- meters Carried on High-Speed Spacecraft," Geo. Res. Ltrs., 1, 197, 1974. 3. Mauersberger, K., M. J. Engebretson, W. E. Potter, D. C. Kayser, A. O. Nier, "Atomic Nitrogen Measurements in the Upper Atmosphere," Geo. Res.Ltrs., 2, 337, 1975. 4. Mauersberger, K., D. C. Kayser, W. E. Potter, and A. O. Nier, "Seasonal Variation of Neutral Thermospheric Constituents in the Northern Hemisphere," J. Geophys. Res., 81, 7, 1976. 5. Nier, A. O., W. E. Potter, and D. C. Kayser, "Atomic and Molecular Oxygen Densities in the Lower Thermosphere," J. Geophys 81, 17, 1976. 6. Mauersberger, K., M. J. Enqebretson, D. C. Kayser, and W. E. Potter, "Diurnal Variation of Atomic Nitrogen," J. Geophys. Res., 81, 2413, 1976 7. Mauersberger, K., W. E. Potter, and D. C. Kayser, "A Direct Measurement of the Winter Helium Bulge," Geo. Res. Ltrs., 3, 269, 1976. 8. Mauersberger, K., A. O. Nier, D. C. Kayser, W. E. Potter, and M. J. Engebretson," Determination of Exospheric Neutral Gas Temperatures," Geo. Res. Ltrs., 3, 273, 1976. 9. Kayser, D. C., and W. E. Potter, " Molecular Oxygen Measurements at 200 km from AE-D Near Winter Solstice, 1975," Geo. Res. Ltrs., 3, 455, 1976. In press or submitted for publication: 10. Potter, W. E., D. C. Kayser, and K. Mauersberger, "Direct Measurements of Neutral Wave Characteristics in the Thermosphere," accepted for publication in J. Geophys. Res. 11. Engebretson, M. J., K. Mauersberger, D. C. Kayser, W. E. Potter, and A. O. Nier, "Empirical Model of Atomic Nitrogen in the Upper Thermosphere," accepted for publication in J. Geophys. Res. 12. Potter, W. E.,and D. C. Kayser, "In Situ Measurements of Neon in the Thermosphere," accepted for publication in Geo Res. Ltrs. 13. Nier, A. O., "The Study of Planetary Atmospheres with Mass Spectrometers Carried on High Speed Probes or Satellites," accepted for publication in Proc. of Rarefied Gas Conf.,1977. 14. Mauersberger, K., M. J. Engebretson, W. E. Potter, D. C. Kayser, and A. O. Nier, "Atomic Nitrogen Measurements in the Upper Thermosphere," accepted for publication in Space Res. II. Publications in Collaboration with Other Groups Already published: 1. Torr, M. R., D. G. Torr, J. C. G. Walker, P. B. Hays, W. B. Hanson, J. H. Hoffman, and D. C. Kayser, "Effects of Atomic Nitrogen on the Nocturnal Ionosphere," Geo. Res. Ltrs., 2, 385, 1975. 2. Brinton, H. C., H. G. Mayr, and W. E. Potter, "Winter Bulge and Diurnal Variations in Hydrogen Inferred from AE-C Composition Measurements," Geo. Res. Ltrs., 2, 389, 1975. 3. Reber, C. A., A. E. Hedin, D. T. Pelz, W. E. Potter, and L. H. Brace, "Phase and Amplitude Relationships of Wave Structure Observed in the Lower Thermosphere," J. Geophys. Res., 80, 4576, 1975. 4. Torr, D. G., M. R. Torr, D. W. Rusch, P. B. Hays, K. Mauersberger, J. C. G. Walker, N. W. Spencer, A. E. Hedin, H. C. Brinton, and R. F. Theis, "Atomic Nitrogen Densities in the Thermosphere," Geo. Res. Ltrs., 3, 1, 1976. 5. Mayr, H. G., P. Bauer, H. C. Brinton, L. H. Brace, and W. E. Potter, "Diurnal and Seasonal Variations in Atomic and Molecular Oxygen Inferred from Atmosphere Explorer-C," Geophys. Res. Ltrs., 3, 77, 1976. 6. Torr, D. G., M. R. Torr, J. C. G. Walker, L. H. Brace, H. C. Brinton, W. B. Hanson, J. H. Hoffman, A. O. Nier, and M. Oppenheimer, "Recombination of NO+ in the Ionosphere," Geophys. Res. Ltrs., 3, 209, 1976. In press or submitted for publication: 7. Breig, E. L., W. B. Hanson, J. H. Hoffman, and D. C. Kayser, "In-situ Measurements of Hydrogen Concentration and Flux between 160 and 300 km in the Thermosphere," accepted for publication in J. Geophys. Res. 8. Torr, D. G., M. R. Torr, J. C. G. Wal ker, A. O. Nier, L. H. Brace, and H. C. Brinton. "Recombination of O2+ in the Ionosphere," submitted to J. Geophys. Res. 9. Trinks, H., H. G. Mayr, D. C. Kayser, and W. E. Potter, "Auroral Energy Deposition and Neutral Composition Changes Observed Simultaneously by ESRO 4 and AE-C at Different Altitudes," accepted for publication in J. Atmos. Terr. Phys. 10. Marcos, F. A., K. S. W. Champion, W. E. Potter, and D. C. Kayser, "Density and Composition of the Neutral Atmosphere at 140 km from the Atmosphere Explorer-C Satellite Data," submitted to Space Res. 11. Hedin, A. E., J. E. Salah, J. V. Evans, C. A. Reber, G. P. Newton, N. W. Spencer, D. C. Kayser, D. Alcayde, P. Bauer, L. Cogger, and J. P. McClure, "A Global Thermospheric Model Based on Mass Spectrometer and Incoherent Scatter Data: MSIS Part 1 - N2 Density and Temperature," submitted to J. Geophys. Res. 12. Hedin, A. E., C. A. Reber, G. P. Newton, N. W. Spencer, H. C. Brinton, H. G. Mayr, and W. E. Potter, "A Global Thermospheric Model Based on Mass Spectrometer and Incoherent Scatter Data: MSIS Part 2 - Composition," submitted to J. Geophys. Res. 13. Trinks, H., C. A. Reber, A. E. Hedin, N. W. Spencer, D. Krankowsky, P. Lammerzahl, D. C. Kayser, A. O. Nier, and U.von Zahn, "Intercomparison of Neutral Composition Measurements from the Satellites ESRO 4, AEROS-A, AEROS-B and AE-C, submitted to J. Geophys. Res. References For This Document Not Found in Part K 1. Daigarno, A., W. B. Hanson, N. W. Spencer, and E. R. Schmerling, "The Atmosphere Explorer mission," Radio Science, 8, 263-266, 1973. 2. French, J. B., N. M. Reid, A. 0. Nier, and J. L. Hayden, "Rarefied gas dynamic effects on mass spectrometric studies of upper planetary atmospheres," AIM Journal, 13, 1641, 1975. 3. Hedin, A. E., B. B. Hinton, and G. A. Schmitt, "Role of gas-surface interactions in the reduction of' 090-6 neutral particle mass spectrometer data," J. Geophys. Res., 78, 4651, 1973.