DATA DOCUMENTATION PACKAGE for the NATIONAL SPACE SCIENCE DATA CENTER Instrument: Bennett Ion Mass Spectrometer Spacecraft: Atmosphere Explorer-C Principal Investigator: Henry C Brinton Laboratory for Planetary Atmosphere Goddard Space Flight Center Greenbelt, Maryland 20771 I Rationale for Measurement The overall science objective of the Atmosphere Explorer missions, to study the response of the earth's atmosphere to solar radiation, requires the simultaneous measurement of a number of fundamental atmospheric parameters One of those parameters is ion composition, since the ionized component of the atmosphere is a direct result of solar irradiation. The Bennett ion mass spectrometer on AE-C measures the individual concentrations of thermal positive ions in the mass range 1 to 72 amu and number density range 8xl0 to 5x10**6 ions/cm**3 II Instrument Description The spectrometer is mounted on the lower baseplate of the AE-C spacecraft and looks forward (in the direction of motion) along the +X axis in the despun spacecraft mode. The instrument consists of an ion analyzer of the Bennett type and associated electronics. Mass analysis is performed by DC and RF potentials applied to sixteen parallel tungsten-mesh grids. Ambient ions entering the instrument are accelerated down the axis of the spectrometer by a DC sweep potential. For each ion mass there is value of this potential which accelerates the ion to the instrument's resonant velocity. Resonant ions which transit the first analyzer stage in the phase with the applied RF gain sufficient energy from the RF field in a11 three stages to penetrate a retarding DC field and reach the collector, where a current proportional to the ambient concentration of the resonant ion is detected by an amplifier system covering the range 5*10**-14 to 5*10**-8 amp MEASURED PARAMETERS: Primary instrument output data produced by a peak detector circuit, consist of two digital words: one indicates the measured current and the other the sweep potential for each ion detected. The ambient concentration and identity of each ion in the mass range 1-72 amu are derived from these data. In addition the ion current spectrum appearing at the output of the amplifier chain is telemetered in analog form. TEMPORAL RESOLUTION: The instrument employ three RF frequencies each corresponding to a range of ion mass: 1-4 amu 2-18 amu and 8-72 amu. Any combination of one or more ranges may be selected by command; each is swept in 1.7 sec. The resolution of the ion measurements depends on the selected mass scan mote and on whether the spacecraft is spinning or despun. For the despun case, when a single mass range has been selected, the concentration of a given ion is measured every 1.7 seconds, corresponding to about 12 km along the orbit. When the full mass range (1-72 amu) is scanned, the measurement is repeated every 5.1 seconds. In the spacecraft spinning mode, ram measurements are spaced 15 seconds apart. ADDITIONAL INFORMATION: A more completed description of the instrument may be found in Radio Science, 8, 323-332, April 1973. III. Description of Data The Bennett ion mass spectrometer (BIMS) data are included in the AE-C Unified Abstract File database; a copy of this database, in magnetic tape form, is being submitted to the NSSDC. The BIMS data in the UA file are derived by time-smoothing the Geophysical Unit File data produced by the instrument. While the GU data have a variable time resolution (depending upon spacecraft spin and BIMS instrument mode) the UA data are on 15-second centers. The BIMS words in the UA file are Words 1 thru 10; they contain the following data: Word 1 H+ concentration (ions/cm**3) Word 2 He+ concentration (ions/cm**3) Word 3 N+ concentration (ions/cm**3) Word 4 O+ concentration ions/cm**3) Word S N2+ concentration (ions/cm**3) Word 6 NO+ concentration ions/cm**3) Word 7 O2+ concentration (ions/cm**3) Word 8 Total ion concontration (ions/cm**3) Word 9 Spare Word 10 Spare IV. CATALOG OF DATA The initial AE-C UA File submission to the NSSDC includes data taken during the month of February 1974 when the satellite was in its eccentric orbit phase. Data were acquired on both spinning and despun passes, generally below 1000 km altitude; perigee was ~150 km. BIMS data in this February 1974 dataset includes ion composition measurements made on 36 despun orbits and 35 spinning orbits. V. INSTRUMENT CALIBRATION The reduction of ion mass spectrometer data requires the conversion of measured ion currents to ambient ion concentrations. This process, for the BIMS instrument, consists of converting directly from current to number density via a mathmetical expression involving instrument geometry, laboratory and in-flight calibration factors, and spacecraft orbit and attitude data. For conditions of zero or slightly negative spacecraft and spectrometer orifice potential, small angle of attack, small Debye length, and ion thermal velocity less than or comparable to the satellite velocity, the relationship between measured current, I, and ambient concentration, N, for each ion detected is given by I=N*e*v*A*a where e is the electronic charge, v is the satellite velocity, A is the spectrometer orifice area and a is the spectrometer efficiency. Thus, the number density of a particular ion can be derived directly from its measured ion current, assuming that spacecraft orbit/attitude information is available and that the instrument factor a has been determined by laboratory and in-flight calibration. Spectrometer Efficiency Factor a: Spectrometer efficiency is defined as the ratio of resonant ion current reaching the collector to the total current of resonant mass ions entering the instrument. As described in the Radio Science (8, 323, 1973) paper, a is a function of the retarding potential Vs; Vs regulates a by controlling the duty cycle, or on-time, of the ion analyzer. As part of the laboratory calibration of the BIMS instrument an ion flux of known magnitude is directed into the spectrometer orifice, Vs is scanned, and the variation of collected ion current is observed. Both the absolute value of a and its variation with Vs are thereby determined. After the instrument is in orbit, in-flight Vs calibrations are performed under conditions of near-constant input ion flux, to verify the form of the a versus Vs curve. This test also verifies that the effective Vs is reduced by the incoming energy (due to spacecraft velocity) of the ion being measured; a simple correction for this effect is made in the interpretation of flight data. Interpretation of Spin-Mode Data: At the nominal spacecraft spin rate of 4 revolutions per minute the angle between the spectrometer analyzer and the satellite-velocity vector changes at a rate of 24 deg sec-1. The scan of a single mass range (1.7 sec.) occurs, then, within +/-20deg. of the velocity vector; the scan of multiple mass ranges results in a proportionately larger range of attack angle. As part of the data reduction process, ion currents measured at nonzero angles of attack are corrected to ram values. The correction factors have been empirically determined from AE-C flight data. VI. Accuracy of Ion Concentrations The BIMS data in the AE-C UA file have been derived using the laboratory and in-flight calibration techniques described briefly in Section V. However, extensive comparisons have been made between BIMS ion concentrations and those measured simultaneously by the CEP and MIMS/RPA instruments. Based on these comparisons, it appears that for despun orbits, the BIMS measurements of H+, O+, NO+, O2+, and N2+ are, in general, accurate to +/-20%, The ions He+ and N+ are usually minor constituents, and hence comparison among the three instruments for these ions has been less conclusive. Because of (a) the inherently poorer time resolution of spin- mode data, and (b) the angle-of-attack correction made to spin-mode measurements, the BIMS ion concentrations in the UA file for spacecraft spinning orbits are subject to greater error than for despun orbits. Also, because of instrument mode changes and phasing of the BIMS angle-of-attack, there are gaps in the UA data for certain ions on some spin-mode orbits. VII. Discussion of Anomalies The principal anomaly which occurred in the operation of the AE-C BIMS instrument was an amplifier baseline offset which limited amplifier sensitivity at altitudes below ~220 km. This limitation, which was successfully removed from the AE-E instrument by electronic modification, was apparently caused by impact ionization of the dense neutral atmosphere entering the BIMS instrument at high velocity (~8 km/sec.). The effect of the baseline offset was to prevent the measurement, below ~220 km altitude, of ion constituents whose density was lower than ~10**3 ions/cm**3 VIII. Data Reduction Procedure The BIMS data processing system is designed to convert the digital outputs of the instrument into mass and concentration of positive ions, calculate experiment monitors, compute and merge orbit/attitude data with the experiment data, create a Geophysical Unit (GU) file, and produce Unified Abstract (UA) data for each orbit. The system is composed of three data analysis programs (F4 program, Despun Cleanup program, Spin Mode Cleanup program), a program which produces Unified Abstract data (F8 program), and two printer plot modules (GU Printer Plot program, UA Printer Plot program). They are written in FORTRAN IV and run on the Xerox Sigma 9 AE central computer system at the Goddard Space Flight Center. The processing flow begins with the F4 program. This program accesses the raw telemetry data (one major frame at a time) and orbit/attitude data for the orbit to be processed, and performs the following major functions: * Organize raw telemetry data into sweeps * Calculate ion mass and current for each peak in the sweep * Calculate orbital parameters and associate these data with ion peaks. * Compute experiment monitors ant identify these with each sweep and peak. * Produce a temporary Geophysical Unit (GU) file with a record for each sweep, each record containing peak information, orbital parameters, and experiment parameters for the sweep. The next step in the system is the processing of the temporary GU file by either the Despun Cleanup program or the Spin Mode Cleanup program, depending on whether the orbit being processed is despun or spinning. These programs do the following tasks: * Apply a series of tests which account for special instrument and spacecraft conditions, making appropriate modifications to the current associated with each peak in the temporary GU file. * Calculate ion concentration for every ion mass in each sweep. * Produce final GU file After the final GU file has been created, the temporary GU file (previously produced by the F4 program) is deleted from the system. The final GU file is now processed by the F8 program, which does the following: * Interpolate ion concentrations at 15 second intervals for each of the seven major ion constituents (H+ ,He+, N+, O+, N2+, NO+, O2+), and for as many as three specially selected ions (presently BIMS total concentration) . * Write the interpolated concentrations onto the Unified Abstract (UA) file, where it may be accessed by all experimenters. At this point in the processing, two plot programs are executed in sequence. The GU Printer Plot program reads the GU file and products plots of ion concentration versus universal time. The UA Printer Plot program accesses the UA data produced by the F8 program, and produces a concentration versus universal time plot showing each of the BIMS ion masses in the UA file. The final step in the BIMS data processing flow is the "demotion" of the GU file onto magnetic tape for permanent storage. The entire system, from F4 program to demotion of the GU file, typically requires 3 to 4 minutes of CPU time to process a 35 minute pass. IX. Particularly Useful References 1. "The Bennett Ion Mass Spectrometer on Atmosphere Explorer-C and -E", H. C. Brinton, L. R. Scott, M. W. Pharo, III, and J. T. Coulson, Radio Science, 8, 323, 1973. 2. "In-Situ Measurements of Plasma Drift Velocity and Enhanced NO+ in the Auroral Electrojet by the Bennett Spectrometer on AE-C", H. C. Brinton, Geophys. Res. Letters, 2, 243-246, 1975. 3. "Winter Bulge and Diurnal Variations in Hydrogen Inferred from AE-C Composition Measurements", H. C. Brinton, H. G. Mayr, and W E. Potter, Geophys. Res. Letters, 2, 389-392, 1975. X. Additional Bibliography 1. "Atomic Nitrogen Densities in the Thermosphere", D. G. Torr, 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, Geophys. Res. Letters, 3, 1-4, 1976. . 2. "Diurnal and Seasonal Variations in Atomic and Molecular Oxygen Inferred from Atmosphere Explorer-C", H. G. Mayr, P. Bauer, H. C. Brinton. L. H. Brace, and W. E. Potter, Geophys. Res. Letters, 3, 77-80, 1976. 3. "Recombination of NO+ in the Ionosphere", D. G. Torr, 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, Geophys. Res. Letters, 3, 209-212, 1976. 4. "Ion Chemistry of N2+ and the Solar Ultraviolet Flux in the Thermosphere", M. Oppenheimer, A. Dalgarno, and H. C. Brinton, J. Geophys. Res., 81, 3762, 1976. 5. "The OI (5577A) Airglow: Observations and Excitation Mechanism", J E. Frederick, D. W. Rusch, G. A. Victor, J.W. E. Sharp, P. B. Hays, and H. C. Brinton, J. Geophys. Res., 81, 3923, 1976. 6. "Molecular Oxygen Abundances in the Thermosphere from Atmosphere Explorer-C Ion Composition Measurements", M. Oppenheimer, A. Dalgarno, and H. C. Brinton, J. Geophys. Res., 81, 4678, 1976. 7. "Recombination of O2+ in the Ionosphere", D. G. Torr, M. R. Torr, J.C.G. Walker, A. O. Nier, L. H. Brace, and H. C. Brinton, J. Geophys. Res., 81, 5578, 1976. 8. "Quenching of Metastable 2D Oxygen Ions in the Thermosphere by Atomic Oxygen", N. Orsini, D. G. Torr, M. R. Torr, H. C. Brinton, L. H. Brace, A. O. Nier, and J.C.G. Walker, submitted for publication in Geophys. Res. Letters, 1976. 9. "A Global Thermospheric Model Based on Mass Spectrometer and Incoherent Scatter Data: MSIS Part 2-Composition", A. E. Hedin, C. A. Reber, G. P. Newton, N. W. Spencer, H. C. Brinton, H. G. Mayr, and D. C. Kayser, accepted for publication in J. Geophys. Res., 82, 2139,1977. 10."Daytime Chemistry of NO from Atmosphere Explorer-C Measurements", M. Oppenheimer, A. Dalgarno, P. P. Trebino, L. H. Brace, H. C. Brinton, and J. H. Hoffman, submitted for publication in J. Geophys. Res., 1976.