MIMS NSSDC DOCUMENTATION A. Summary of the Rationale and Motivation for the Experiment. An Ion-mass spectrometer is an instrument which samples ions from the ionosphere in the vicinity of the spacecraft and measures the concentration of each species. The Atmosphere Explorer (AE) ion-mass spectrometer is basically a magnetic deflection-type, similar to the one flown on the ISIS-2 spacecraft, but with a mass range from 1 to 90 amu divided into three sections scanned simultaneously. Sensitivity of the instrument is better than 1 ion cm-3 for low masses and several ions per cubic centimeter for the upper end of the mass range. The main objective of the experiment is to determine the abundance of each ion species as a function of position of the satellite. This information is essential to the understanding of the chemistry, dynamics, and heat budget of the upper atmosphere. Also of interest is the behavior of the meteoritic ions near the magnetic equator where it appears that the presence of Fe+ ions is somehow related to the occurrence of spread-F. Measurement of the phase shift from the nominal ram position of the roll modulation maximum of each individual ion current yields the vertical-drift velocity for that species. This information is applicable to polar-wind studies and in the evaluation of the magnitude of interhemisphere plasma transport along magnetic field lines. B. Description of Instrument. The magnetic ion mass spectrometer (MIMS) measures the abundances of the ambient positive ions along the orbit of the AE satellite. It scans the mass range from 1 to 90 amu, or portions of that range, depending on its operational mode. The instrument consists of an entrance aperture, a 7 cm screen, flush mounted to the surface of the spacecraft, a set of collimating slits, a magnetic analyzer and a triple detector system, each detector consisting of a collector slit, an electron multiplier and a log amplifier. Positive ions are rammed through the entrance grid due to the orbital motion of the spacecraft and accelerated through the collimator forming a beam which enters the magnetic analyzer. This analyzer consists of a permanent magnet with a field strength of 3600 gauss in the gap. Three ion beam trajectories are allowed through the magnet. These split the ion beam into 3 parts in the mass ratio 1:4:16. Collector slits are placed at the exit of the magnetic lens system corresponding to the image point of the allowed ion trajectories. As the ion accelerating voltage is varied, the ion beams are scanned across the collector slits forming a series of peaks in the 103 current detector. The mass ranges covered are 1 to 4, 4 to 16 and either 16 to 90 (AE-C) or 14 to 72 (AE-D) amu at the low, mid and high mass collectors respectively. The scan times are either 4 sec (AE-C) or 2 sec (AE-D) for the analog short mode, or 8 sec for the analog long mode. When the analog short mode is used, the mass ranges of each channel are cut in half. The analog short mode is used when the satellite is spinning; the analog long for despun operation. The overall accuracy is better than 20%, except in very disturbed regions of the ionosphere, or at extremely low densities. Sensitivity of the instrument is somewhat mass dependent, varying from 1.0 ion cm-3 for H,to ~10 ions cm-3 for O+ . C. Description of Data The tapes are produced for the whole project by the GSFC Sigma 9 computer group of the AE project. The tape format of the UA data should be common to all experiments. The MIMS data format in the UA file consists of the concentration of 10 ion species as a function of UT. These occur at the 00, 15, 30 45 sec times of each minute. The data points are interpolated from the nearest measured value, which for despun data occur every 2, 4 or 9 seconds, depending on instrument mode, and for spinning data once each satellite spin period (usually l5 seconds.) The abstract file contain comprehensive data for the concentrations of the following principal ion species H+, He+ , N+ , O+, Mg+, N2+, NO+ and O2+ . Data are also avaluable for the concentrations of the minor ions, D+ and O++ , for the restricted ranges of geophysical conditions where their abundance is above the instrument threshold for reliable measurement. Particle concentrations are presented in units of ions per cm . D. A Catalog of Data See Datastat File E. Instrument Calibration An ion-mass spectrometer measure relative abundances of the ions in samples from the ionosphere but can be calibrated, as described below, to give absolute ion concentrations. Several laboratory calibrations were performed as follows: instrumental mass discrimination factors were measured by determining the ratios of gasses in standard known mixture. Conversion of output voltage (V) from the log amplifiers to input current (I) was accomplished thru the relationship I = A*e**(C*V) - B where A, B, and C are constants determined by passing known currents into the amplifiers. A special feature of the Magnetic Ion-Mass Spectrometers MIMS on the AE satellites concerns their inflight calibration. This calibration as accomplished through software interaction with data analysis procedures for the onboard Retarding Potential Aanalyzer (RPA). The sum of the measured ion currents for the molecular species, N2+, NO+ and O2+ , were compared with the corresponding unresolved molecular concentrations from the RPA over the altitude range 200-250 km. A comparable comparison was carried out for the sum of N+ and O+ near the F peak. H+ and He+ calibrations were accomplished at altitude above 500 km under those select conditions where these ions contributed distinctively features to the current-voltage output characteristics of the RPA. Calibration was thus performed for an extensive set of satellite orbits which span the full time of the AE mission. In this complementary manner, the two instruments ( i.e. the MlMS and RPA) have supplied absolute ion concentrations for which the accuracy is limited only by the spectrometer's ability to measure the concentration ratios and the analyzers ability to measure the concentrations of the sums of these ions. Overall accuracy is expected to be better than lO% for the major ions species Calibrations for minor constituents were ton by interpolation of the data related to the major peaks with an expected accuracy of +/- 12%. F. Scientific Areas where Data are Known to be Excellent or invalid. The MIMS experiment has supplied ion composition data which has been used quite extensively in models to explain, for example, the N+ distributions in the ionosphere, to determine ratio coefficients of reactions like N2+ + O and the recombination of NO+ in the ionosphere, and the formation of O2+ by charge exchange with metastable O+ . Also the diurnal variation of NO+ ,O2+, and O+ in the lower thermosphere and the daytime chemistry of NO+ have been studied using the MIMS data. Studies have also been carried out on the distributions of hydrogen and deuterium determined through the charge exchange reactions of atomic oxygen with H+ and D+ respectively. Measurements of the H+ concentrations below 200 km, combined with data from other AE instruments, were used to evaluate the vertical hydrogen flux. The results indicate upward daytime fluxes well in excess of that needed to offset the normal planetary losses due to evaporative escape, and suggest that some other loss or global redistribution process controls the thermospheric distribution of hydrogen. H+ data from circular orbits have been used to derive, in a direct manner, the diurnal and seasonal variabilities of hydrogen. Winter enhancements and summer depletions are observed, along with a diurnal variations which ranges from 2 to 3 between the winter and summer hemispheres. A special mode of increased sensitivity has proven useful in studies of minor atmospheric ions. D+ concentration data have yielded information on the D/H ratio and the daytime abundance of deuterium to altitudes below 300 km. A relative enhancement of deuterium has been observed in the thermosphere. Low altitude data for O++ have established reaction with atomic oxygen as the principal chemical loss process; data analysis also has provided an estimate of the reaction rate. In addition, evidence has been obtained for a low altitude X-ray source of O++ in the thermosphere. The ion composition signatures of equatorial "bubbles" has given in- formation on the plasma dynamics of the bubble phenomenon. The composition in bubbles is in general typical of that found several hundred km below the bubble altitude. Simple calculations show that recombination alone cannot produce the observed signatures; relatively rapid vertical plasma convection is required. The ion drift meter shows that upward velocities of 100- to 200 m/sec-1 exist, and the MIMS data indicate that velocities of this magnitude persist for times of the order of 20 min. to one hour in bubbles. The existence of meteoritic ions, Mg+ and Fe+ , in the equatorial region has been confirmed by the MIMS experiment. In the polar regions, areas have also been found where Mg+ is the dominant ion species near 150 km at 2200 hrs. local time. G. Unusual Events From orbits 2700 to 2760 and 3244 to 3491 the MIMS instrument was in a sensitivity mode. The resulting order of magnitude increase in sensitivity has resulted in high quality data for many of the minor species e.g. D+ and O++,to very low altitudes. The data set includes several despun orbits where the MIMS instrument was directed either backward toward the wake, or at an extremely large angle relative to ram. The light H+ ions are readily observed in such a configuration; although the concentrations so deduced from the data are not indicative of the true ambient H+ concentrations, these data may be of scientific value in investigation of the spacecraft plasma sheath. The following orbits are known to fall into this category: 527, 549, 563, 567, 571, 782, 785, 786, 787, 788, 791, 804, 807, 808, 809, 810, 813 and 826. However, any such orbits are readily identifiable, since H+ is the only observable ion below 1000 km Occasional "flyers" or "spikes" sometimes appear in the data; in such instances the concentration of a species at an isolated point could be several orders of magnitude greater than at a neighboring point. Examples exist on orbits 585, 628, 650 and 741. As variation of this amount could sometimes be real (e.g. in the polar regions or during spread-F) no attempt has been made to systematically eliminate all of it from the data base. however, the identification of possible spurious signals should be apparent to even the least informed user of the data. H. Known Anomalies. See Section. G. I. Data Reduction Procedure. The data presented in the UA file are reduced to ion concentrations with all known corrections applied. They may be considered the absolute ion concentrations at the times indicated at the position of the satellite. J. Discussion of Procedures. See Section I. K. References. 1) The Magnetic Ion-mass spectrometer on Atmosphere Explorer, J. H. Hoffman, W. B. Hanson, C. R. Lippincott and E. E. Ferguson, Radio Sci., 8, 315-322, 1973. 2) In Situ Measurements of hydrogen Concentratlon and flux Between 160 and 300 km in the Thermosphere, E. L. Breig, W. B. Hanson, J. H. Hoffman and D. C. Kayser, J. Geophys. Res.,81, 2677-2685, 1976. 3) Doubly-Charged Atomic Oxygen Ions in the Thermosphere, I. Photochemistry, E. L. Breig, Marsha R. Torr, D. G. Torr, W. B. Hanson, J. H. Hoffman, J.C.G. Walker, and A. O. Neir, J Geophys. Res. 1977 in press. 4) Deuterium in the Daytime Thermosphere, in preparation. 5) Minor Species and Other Aeronomic Studies, E. L. Breig, presented at Atmosphere Explorer Symposium, Bryce Mountain, Va., October, 1976. 6) Sample Ion Composition Results from the AE Magnetic Ion mass Spectrometer, J. H. Hoffman, presented at Atmosphere Explorer Symposium, Bryce Mountain, Va., October, 1976. L. Other Known Conditions in Data. NONE