urn:nasa:pds:context:instrument:pvo.oims 1.0 1.7.0.0 Product_Context 2020-08-26 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.pvo::1.0 instrument_to_instrument_host Brinton, H.C., L.R. Scott, M.W. Pharo, III, and J.T.C. Coulson, The Bennett ion-mass spectrometer on Atmosphere Explorer-C and -E, Radio Sci., vol. 8, 323, 1973. reference.BRINTONETAL1973 Taylor, H.A., Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, T.M. Donahue, P.A. Cloutier, F.C. Michel, R.E. Daniell, Jr., B.H. Blackwell, Ionosphere of Venus: First observations of the dayside ion composition near dawn and dusk, Science, vol. 203, 752, 1979. reference.TAYLORETAL1979C Taylor, H.A., Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, P.A. Cloutier, F.C. Michel, R.E. Daniell, Jr., T.M. Donahue, R.C. Maehl, Ionosphere of Venus: First observations of the effects of dynamics on the dayside ion composition, Science, vol. 203, 755, 1979. reference.TAYLORETAL1979D PVO ORBITER ION MASS SPECTROMETER Spectrometer not applicable not applicable Instrument Overview =================== ---------------------------------------------------------- Copyright (c) 1980 IEEE, Reprinted, with permission, from IEEE Transactions on Geoscience and Remote Sensing, GE-18, Num 1, 36-38, 1980. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of PDS's products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or resdistribution must be obtained from the IEEE by sending a blank email message to info.pub.permission@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it. ---------------------------------------------------------- NOTE: References to figures are included in the article even though the figures aren't. This enables you to look up the actual article in the IEEE transaction and find the figures. ---------------------------------------------------------- Bennett Ion Mass Spectrometers on the Pioneer Venus Bus and Orbiter H.A. Taylor, Jr., H.C. Brinton, T.C.G. Wagner, B.H. Blackwell, and G.R. Cordier IEEE Transactions on Geoscience and Remote Sensing January 1980 --------------------------------------------------- Abstract Identical Bennett radio frequency ion mass spectrometer instruments on the Pioneer Venus Bus and Orbiter have provided the first in-situ measurements of the detailed composition of the planet's ionosphere. The sensitivity, resolution, and dynamic range are sufficient to provide measurements of the solar-wind-induced bow-shock, the ionopause, and highly structured distributions of up to 16 thermal ion species within the ionosphere. The use of adaptive scan and detection circuits and servo-controlled logic for ion mass and energy analysis permits detection of ion concentration as low as 5 ions/cm3 and ion flow velocities as large as 9 km/s for 0+. A variety of commandable modes provides ion sampling rates ranging from 0.1 to 1.6 s between measurements of a single constituent. A lightweight sensor and electronics housing are features of a compact instrument package. I. Introduction Owing to the weak intrinsic magnetic field of the planet, direct interaction between the solar wind and the Venusian ionosphere creates a more complex and variable structure than encountered in the Earth ionosphere. As a result, theoretical predictions advanced prior to the Pioneer Venus (PV) mission set the requirement for instrument capabilities exceeding those previously demonstrated in extensive flight experience with the Bennett spectrometer in the Earth ionosphere on the Orbiting Geophysical Observatory and Atmosphere Explorer missions. The primary objective of the PV Ion Mass Spectrometer (IMS) investigations has been to make global measurements of the composition of the ionosphere and, to the extent possible, measure ion drift with sufficient accuracy to contribute to the understanding of the solar-wind-induced dynamics of the ionosphere. Development of three unique measurement functions, 1) the step-dwell peak sampling technique, 2) the charge/velocity servo system, and 3) the explore/adapt ion mass sequencing system have proven to be essential for providing accurate ion concentration measurements compatible with both the data rate available from the spacecraft, and the high degree of variability encountered in the Venusian ionosphere. II. Instrument Description The Bus IMS (BIMS) and Orbiter IMS (OIMS) instruments are identical both electrically and mechanically, with the physical characteristics shown in Fig. 1. the instrument design and operational characteristics are similar to those of ion spectrometers flown on numerous rocket and satellite missions, including those on the Atmosphere Explorer-C and - E spacecraft [1]. The instrument consists of an analyzer tube and an electronics package. Ambient atmospheric ions sampled by the spectrometer enter the instrument through the analyzer orifice which is oriented as closely as possible in the direction of spacecraft motion, to enhance the collection of ions 'scooped up' by the relatively rapid motion of the spacecraft through the thermal plasma. Both the BIMS and OIMS instruments were mounted with the analyzer axis parallel to the spacecraft spin axis; this ensures a relatively small angle of attack throughout the periapsis pass and eliminates spin modulation of the ion currents. A. Mechanical Configuration The mass analyzer, shown schematically in Fig. 2, consists of a lightweight aluminum tube enclosing a series of grids, spacers, and long drift spaces. The grids are 0.001-in diameter knitted tungsten mesh with approximately 90-percent transparency. The intergrid spacers are machined from polyimide which has been baked to drive out volatiles. The drift spaces, which must be conducting, are made of gold- plated aluminum. Vacuum sealing of the tube, which is required only for the prelaunch calibration, is achieved by means of an O-ring in the collector area and by use of a low outgassing RTV sealant where the grid tabs protrude through the aluminum shell. The electronics housing is machined from magnesium. Component mounting in the printed-circuit boards utilizes both stitch-weld and solder techniques. All boards are conformally coated. The total mass of the instrument is 3.0 kg. B. Instrument Techniques The system block diagram for the ion spectrometer is presented in Fig. 3. The instrument is powered by the +28 V spacecraft Bus and requires approximately 1.5 W of power in all modes of operation. The electronics system performs five major functions: 1) supplies required RF and dc potentials to the ion analyzer tube, 2) detects and amplifies ion current flowing to the collectors, 3) digitizes, processes, and formats data for telemetry, 4) automatically configures the sensor for subsequent measurements during a prescribed measurement cycle, and 5) decodes and implements instrument commands. 1) Ion Analyzer: Mass analysis of the spectrum of ambient thermal positive ions entering the Bennett RF spectrometer sensor (Fig. 2) is performed by 1) imparting incremental energy to those ions which are 'resonant' as they traverse the analyzer, and subsequently 2) applying a retarding potential barrier which inhibits detection of ions except those which have gained the maximum energy with the analyzer. The instrument is identified as an RF spectrometer since the incremental ion energy is imparted by an RF potential (VRF) applied to each of four RF stages within the sensor. The sequencing of mass analysis within the chosen range of ion masses is accomplished by stepping the negative voltage (Va) which accelerates the positive ions down the longitudinal axis of the analyzer, through the four RF stages, and toward the retarding potential barrier established by the positive potential (Vs). For a particular value of the accelerating voltage (Va), the 'resonant' velocity is imparted to ions of a given amu, such that these particles pass through each RF stage in synchronism with the phase of VRF. These ions receive maximum energy as they traverse the sensor, and are thus able to penetrate the retarding potential barrier and reach the collectors. The barrier field established by the positive dc voltage Vs restricts the passage of all but the resonant ions, and thus acts as both an efficiency and resolution control for the analyzer. The gridded collector intercepts a small percentage of the total detectable ion flux and presents this current as input to a low-gain preamplifier. Ion flow to the solid collector surfaces serves as input to the high- gain preamplifier. Two additional grid structures within the tube suppress secondary electron emission from the collector surfaces and also induce (upon command) simulated ion currents into the detectors for end-to-end calibration of the electronic system. 2) Charge/Velocity Servo: The potentials Va and Vs work together to regulate the ion detection process with respect to the effects of 1) spacecraft velocity, 2) spacecraft skin charge, and 3) ion flow velocity generated by electric fields and/or solar-wind viscous interaction. For a sensor at rest relative to the plasma, equation (1) of Fig. 2 applies, and the value of VA required to produce resonance for an ion of mass M is simply determined by the fixed coefficients K, S, and F. The potential Vs is set to provide nominal analyzer efficiency and resolution. Under flight conditions (equation (2) and Fig. 2) the sum of the axial components of spacecraft and ion drift velocities (v) results in a ram energy term for each ion mass, varying as 1/2 mv2. The servo system automatically compensates for this energy shift by appropriate adjustment of the Va and Vs voltages to maintain constant instrument efficiency and mass resolution. The effect of spacecraft skin charge is to add an energy offset proportional to phi sc, independent of ion mass. This additional term in the mass analysis equation is also automatically accommodated by the instrument servo. The axial component of ion drift velocity and the value of spacecraft potential are determined by analysis of the servo coefficients included in the BIMS and OIMS data stream. 3) Adaptive Mass Scan: A second unique feature incorporated in the BIMS and OIMS instruments is the explore/adapt logic sequence for regulating the consecutive measurements of individual ion species. This accomplished in two steps: 1) periodic exploration of all 16 preselected ion species, and 2) adaptive sequencing of repetitive ion measurements according to the relative significance of ion currents detected during the exploratory cycle. Because of the significant variations in the distribution of the ions within the Venusian ionosphere, and the data rate limitations afforded by the PV mission, the explore/adapt measurement sequence was employed to insure the maximum possible repetition rate for sampling of species found to be prominent in a given altitude and/or local time range. Prompted by theoretical considerations, a total of 16 probable ion species to be identified in the Venusian ionosphere were selected and are identified in Table I. The sequence of ions listed in the table is sampled by stepping the accelerating potential Va to the appropriate value for each amu. The explore/adapt concept is shown in Fig. 4. The basic explore/adapt cycle is repeated every 6.3 s, and consists of an explore interval during which a sequential search is made for each of the 16 species, followed by a series of shorter adapt intervals during which repeated measurements of as many as eight prominent ions detected during the explore interval are performed. If eight or more ions are detected during the explore interval, adapt measurements of the eight most prominent of these will be repeated to fill out the remainder of the 6.3 s cycle as shown in the upper part of Fog. 4, thereby providing a total of six measurements of the eight prominent ions and one measurement of up to eight less prominent ions during the 6.3-s cycle. If less than eight ions are detected during the initial explore interval, the adapt intervals will contain repeated measurements of the most prominent ions detected up to the maximum number of eight. Thus, as shown in the lower part of Fig. 4, if only one ion is detected during the explore interval, forty repeat measurements of the same ion are made during the adapt interval, providing maximum temporal and spatial resolution for that single specie. The explore/adapt sequence thereby provides a spatial resolution of measurements inversely proportional to the number of ions encountered and thus automatically adjusts the measurement sequence so that information returned is optimized relative to the conditions encountered during the mission. TABLE 1 --------------------------------------------------------- BIMS/OIMS Dedicated Ions Masses --------------------------------------------------------- Measurement Sequence Ion Mass Ion Position (AMU) Species ========================================================= 0 1 H+ 1 18 H20, 18^0+ 2 12 C+ 3 32 02+ 4 4 He+ 5 28 N2+, CO+ 6 16 O+ 7 44 C02+ 8 2 H2+ 9 24 Mg+ 10 14 N+ 11 40 Ar+ 12 8 O++ 13 30 NO+ 14 17 OH+ 15 56 Fe+ ========================================================= Several commandable options extend the flexibility and reliability of the explore/adapt system. As appropriate to conditions encountered, the instrument may be commanded to 1) explore only, and 2) adapt to less than eight prominent ions. 4) Step-Dwell Ion Current Detection: As an improvement over previous designs, the BIMS/OIMS ion spectrum scan is accomplished by a step-dwell sequence of ion detection, rather than the less efficient continuous sweep used in earlier instruments. The step-dwell sequence consists of a series of dwell intervals of approximately 0.1-s duration during which ion currents at each of the 16 mass positions are detected sequentially during the explore interval. At the onset of each of the 16 dwell intervals, the accelerating potential Va is stepped to the approximate value required for the resonant measurement of the specific ion. As the measurement dwell cycle proceeds, the values of Vs and Va are servoed to compensate for changes encountered in spacecraft charge and velocity, thereby ensuring mass resonance and constancy of sensor efficiency and resolution for the ion current measurement. During the dwell cycle VRF is switched an and off at a 30-Hz rate so that intervals of ion current measurements are alternated with intervals of background (zero-level) collector current. These alternating cycles of signal and noise are integrated in a manner which cancels the zero level current. At the end of the dwell cycle, the accumulated ion current value is sampled and held for A/D conversion and subsequent transfer to telemetry storage registers. In addition to providing for the servoing Vs, Va interval, the step-dwell feature of the mass scanning circuitry provides benefits for both the bandwidth requirements of the instrument and the system noise figure. In order to cover the 120-dB (106/1) dynamic range of output current from the ion analyzer, along with the desired current sensitivity, two preamplifiers are employed, each receiving its input from the appropriate collector surface within the sensor. Each preamplifier employs a low noise N- Channel field-effect transistor at its input and a high megohm feedback resistor to establish the gain. The current sensitivity provided by this system permits the measurement of ion concentrations as low as 5 ions/cm3. 5) Instrument Modes and Commandable Functions: The OIMS instrument has provision for sixteen commandable states, any one of which is selected by a serial 5-bit code, 4 bits containing the command information and one bit for initiating command. The instrument can be commanded to adapt to either the 8, 4, or 2 most prominent of the 16 ion masses. In each case, it will adapt to no more than the number commanded, but will adapt to a lesser number if there are fewer masses present than the commanded number. In addition, an EXPLORE ONLY mode overrides the adapt interval and causes the instrument to continue scanning all 16 mass positions repeatedly; (the BIMS adapt command is fixed at 8 of 16). The sensitivity of the instrument may be modified by a GUARD RING command, which applies a dc potential of either 0 or -6 V to the circular guard ring surrounding the sensor orifice. This command may be used to increase the collection efficiency for ambient positive ions, thus increasing the sensitivity. The operation of the charge/velocity servo system may be checked by use of the SERVO NORMAL/OVERRIDE command. In the OVERRIDE mode, the servo is disabled, and the Vs and Va parameters are set at nominal values predicted to be appropriate for periapsis. In this mode, the instrument is nonresponsive to changes in ion flow velocity and spacecraft charge. In addition to the foregoing, the BIMS/OIMS instruments have a POWER ON/OFF command and an internal CALIBRATION command. The CALIBRATION command couples known currents into the two preamplifiers equivalent to ion currents detectable within the dynamic range of the instrument. These simulated ion currents provide an end-to-end calibration of the electronics. III. Initial Flight Results Both the BIMS and OIMS instruments have performed accurately and reliably in flight. With repeated orbits, the OIMS has answered several basic questions which motivated the PV mission. In particular, the identity of the dominant ions O+ in the upper ionosphere and O2+ in the lower ionosphere was established immediately. In addition to the determination of the dominant ion, the OIMS also identified H+, H2+, He+, O++, C+, N+, 18O+, and/or H2O+, CO+, and/or N2+, NO+, O2+, and CO2+ in the Venusian ionosphere. Data analysis currently in progress shows positive indications that the ion energy servo system will contribute to understanding the complex dynamic nature of the ionosphere, as well as the detailed composition. In addition to the early results from OIMS and BIMS already reported [2] - [4], several examples of in-flight results are included here to illustrate the fulfillment of the instrument design goals. First, the sensitivity and temporal resolution of the ion measurements have permitted detection of numerous plasma signatures, including the bowshock region, complex ionopause structure, and pronounced irregularity in the ionosphere, as shown in Fig. 5. In Fig. 6, the capability is shown for simultaneous detection of axial ion drift velocities of the order of km/sec along with associated extreme structural variations in the ion concentration. Together these measurement tools provide a mean for detailed exploration of both the composition of the Venusian ionosphere and the complexities of its dynamic interaction with the solar wind. Acknowledgement The performance of the countless engineering tasks contributing to the successful operation of the Pioneer Venus Ion Spectrometers deserves special acknowledgement. Particular among the many contributors are J.S. Burcham, B.D. Gagnon, and M.W. Pharo of GSFC, D.E. Simons, R.C. Maehl, J.T.C. Coulson, D.E. Tallon, L.T. Fry, R. Madaris, P. Lepanto, and W. Heflin of Norlin Communications, Inc., and A.A. Stern of CSTA, Inc. References H.C. Brinton, L.R. Scott, M.W. Pharo, III, and J.T.C. Coulson, 'The Bennett ion-mass spectrometer on Atmosphere Explorer-C and -E', Radio Sci., vol. 8, p. 323, 1973. H.A. Taylor, Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, T.M. Donahue, P.A. Cloutier, F.C. Michel, R.E. Daniell, Jr., B.H. Blackwell, 'Ionosphere of Venus: First observations of the dayside ion composition near dawn and dusk', Science, vol. 203, p. 752, 1979. H.A. Taylor, Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, P.A. Cloutier, F.C. Michel, R.E. Daniell, Jr., T.M. Donahue, R.C. Maehl, 'Ionosphere of Venus: First observations of the effects of dynamics on the dayside ion composition', Science, vol. 203, p. 755, 1979. H.A. Taylor, Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, P.A. Cloutier, F.C. Michel, R.E. Daniell, Jr., T.M. Donahue, 'Ionosphere of Venus: First observations of day-night variations of the ion composition', Science, vol. 205, p. 96, 1979. - End of IEEE copyrighted article - - Begin Appendix from JGR article by Grebowsky et al. - Copyright (c) 1993 American Geophysical Union. Reprinted, with permission, from Journal of Geophysical Research, Vol. 98 No. E5, 1993. This material is protected by copyright, and should not be republished, redistributed, or posted on the Internet. By choosing to view this document, you agree to all provisions of the copyright laws protecting it. *********************************************************************** Appendix: Response of OIMS to Superthermal lons The Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) [Taylor et al., 1981] was designed to measure cold ionospheric plasma at Venus. When ambient ions exist with energies comparable to or exceeding those of cold ions in the frame of reference of the spacecraft, which travels at ~10 km/s, the spectrometer can often detect their presence. These ''superthermal'' ions are detected as ion currents at instrument mass settings for cold ion species that are clearly not present in the environment. The energies of the superthermal ions collected could be due to thermal plasma motions and/or high bulk flow speeds (perhaps just a segment of the tail of the ion velocity distribution). The basic mode of operation of the instrument (a Bennett RF ion mass spectrometer) is well understood, leading from its original conception by Bennett [1950]. Figure A1 is a schematic diagram of the sensor tube. The spacing of the grids and the frequency of the RF voltage signal that is applied simultaneously across each grid set establishes a ''resonant velocity'' that an incoming ion must have to traverse the tube, from entrance to collector, and to acquire the maximum kinetic energy from the RF E field accelerations. An ion in traversing one RF grid set is accelerated to a velocity which sets its time of flight through the E-field free drift space region to the next grid set. Those ions with the resonant velocity in the drift space reach the entrance to the next RF grid set at precisely the right phase of the RF potential for further acceleration. Nonresonant ions arrive at the RF grids at non optimal phases and do not receive the maximal acceleration. In front of the current collector plate a retarding potential VS is applied to allow only the resonantly accelerated ions to pass to the collector. The RF frequency is the parameter which sets the precise speed which an incoming ion must have at the entrance to the first grid set to be in resonance with the subsequent RF field accelerations. This frequency and hence the resonant ion speed is held fixed in the OIMS. The acceleration voltage drop, VA, applied at the entrance to the spectrometer is varied to accelerate different incoming ion mass species to the resonance velocity; this provides the ion mass discrimination. Ions which are accelerated by the potential drop VA to speeds near multiples of the RF resonant velocity will receive partial acceleration in the RF sections. This ''harmonic'' ion acceleration in the spectrometer can result in an anomalous collection current signal which could not be distinguished from that collected from the desired resonance mass. For typical ionospheric ions which have thermal energies less than l eV and enter the spectrometer at the PVO 10 km/s velocity, the OIMS nominal voltages were designed to prevent the harmonic ions from being collected. This was only effective, however, for relatively cold (<1 eV) ion species and not superthermal ions. To minimize the telemetry data rate, the OIMS was designed to sample 16 discrete amu's rather than to sweep continuously through all masses. The accelerating VA potential drop at the spectrometer entrance was stepped through 16 discrete values for the nominal collection of thermal ionospheric species that were likely to be present at Venus. The retarding potential VS on the grid before the collector plate was similarly stepped to maintain a set current collection efficiency for each amu. Since the instrument selects an ion species by using the VA to bring it to the resonant speed of the tube and because the transmission through the retarding potential grid depends on incoming particle energy, the instrument's amu response depends upon the net energy of the ions entering the spectrometer. That is, the analyzer section does not know whether the drift energy is from the VA acceleration or from ambient plasma flow. Spacecraft electrical potential and ambient plasma drift energy have the same impact on the OIMS resonance response as do VA and VS changes. To compensate for variations in the incoming ambient plasma energies, the VA and VS potentials were servoed in tandem [Taylor et al., 1980a] starting from their expected cold plasma values for each amu channel. The servo logic was to seek and lock the instrument potentials for each amu sample at those values for a prefixed collection efficiency (i.e., the percentage of incoming ion flux that reaches the collector). The response time of the servo was designed to be rapid enough that it would fully adjust to anticipated variations in plasma ram energies along the spacecraft orbit. The rate of adjustment was engineered to vary proportionally to the magnitude of the collected ion current, so that in low plasma density regimes sudden changes in incoming ion energy would require a longer time for the instrument to adjust to its proper efficiency operating point than in high density regions. This time scale was of the order of minutes for the lowest measurable ion concentration regions. Within the main body of Venus' ionosphere the servo compensation led to reliable ambient ion concentration measurements, but near the boundary of the ionosphere two features of the instrument's operation led to anomalous and yet useful responses to ambient plasma components that it wasn't (unable to reproduce figures.) Fig. A1. Schematic diagram of the OIMS analyzer structure. directly designed to study. First, in order to maximize the sensitivity of the instrument in low density plasma regimes, the instrument voltages were automatically set more than 10 V away from the fixed servo point in regions when no, or just trace amounts of, detectable ions were present. This ensured the highest collection efficiency possible when ion concentrations increased to just measurable levels. As a result, at the inbound ionopause crossing the spectrometer was ''offtune.'' The servo voltages converged toward the desired values as the ionospheric density increased; but as the response time was of the order of minutes, they did not settle at the designed operating point until the spacecraft entered into the ionosphere. Once it attained its designed operating point it remained'' tuned'' to these voltages until the spacecraft left the ionosphere and encountered only trace ion concentrations. Second, the servo response was not designed to compensate for coexistent ion species with differing flow velocities. In a region with one species of cold ambient ions and a minor ion population of superthermal ions with the same atomic mass, the instrument voltages lock onto the desired operating point for the cold species most rapidly. As the instrument voltages step for amu scans, the voltages for each mass species are automatically preset prior to servoing at values consistent with the energy of that amu flowing into the instrument with spacecraft speed and these settings do not change in the short time between the cyclic scanning of all masses. Ions with energies exceeding the spacecraft speed may be detected with misidentified masses since their incoming energies effectively add to the electrical potential energies set in the spectrometer with the internal voltages alone used to identify the resonant mass. The response of the OIMS to offsets in the operating voltages and/or the presence of superthermal O+ ions in the midst of a cold thermal O+ component is demonstrated in Figure A2. The top shows the computed response to superthermal O+ ions when the spectrometer is completely outside the ionosphere. Superthermal oxygen ions could appear in the 24 and 40 amu windows even in the absence of thermal ambient ions with these masses. They would also contribute to the current collected at the 16 amu setting, but for most energies the efficiency of collection would be less than that for the collection of cold incoming O+ ions to which the plotted efficiencies are normalized. The bottom part of the figure shows a similar calculation for superthermal protons which also are detectable by the appearance of anomalous mass signatures. Superthermal O+ ions with ambient energies exceeding 30 and 46 eV can, if their fluxes are large enough, produce current contributions in the 24 and 40 amu windows respectively. The protons are detectable in the same mass windows with higher energy thresholds. The upper limit of the instrument's capability to resolve any amu is 100 eV which includes the spacecraft ram energy. The responses plotted in Figure A2 were computed assuming the internal voltages were set for a measurement outside of the ionosphere, where instrument voltages were intentionally offset from their desired ionosphere operating values by the order of 10-20 v depending on the mass setting. Once the spacecraft enters into the ionosphere, this voltage offset is eliminated, although somewhat slowly, by the servo mechanism. The instrument thereafter remains tuned to such optimal voltages until it exits the ionosphere and encounters only trace concentrations. The difference in response in the bulk of the ionosphere transit and outside the ionosphere is depicted in figure A3, where the response of the 14 amu signal to superthermal O+ is shown. This calculation shows that even cold 16 amu ions collected at the spacecraft speed could produce a signal in the 14 amu window in the low density regime outside of the ionosphere. Inside the ionosphere the threshold for the detection of superthermals as 24 or 40 amu signals would be raised by ~20 eV, a slightly higher change than is the case for the 14 amu signature. (Unable to reproduce figures.) Fig. A2 Effects of superthermal O+ (top) and H+ (bottom) ions on the OIMS response. The figures reflect the efficiency of collection in the mass positions of 16, 24, and 40 amu, as was numerically modeled by calculating O+ motions through the DC and RF grid voltage layout of the instrument. Effectively, normalized efficiencies which drop below the 0.001 level are undetectable. The efficiencies are normalized to the instrument efficiency of collection for cold O+ ions flowing at PV's ionosphere speed, which is ~1%. Fig. A3. Response of the 14 amu signal computed as a function of O+ energy into the spectrometer. Inside the ionosphere and on the outbound transit through the ionopause 16 amu ions require speeds exceeding the spacecraft speed entrance velocity to appear as 14 amu signals. Outside the ionosphere due to the intentional offset of the instrument voltages, and in the inbound crossing of the ionopause where the instrument slowly servos to optimum operating voltages, even cold ionospheric O+ ions would cause such a spurious signal. References cited in appendix. Bennett, W.H., Radiofrequency mass spectrometer, J. Appl. Phys., 21, 143, 1950. Taylor, H.A., Jr., H.C. Brinton, T.C.G. Wagner, B.H. Blackwell and G.R. Cordier, Bennett ion mass spectrometers on the Pioneer Venus bus and orbiter, IEEE Trans. Geosci. Remote Sensing, GE-18. 44, 1980a. Taylor, H.A., Jr., R.E. Daniell, R.E. Hartle, H.C. Brinton S.J. Bauer, and F.L. Scarf, Dynamic variations observed in thermal and superthermal ion distributions in the dayside ionosphere of Venus, Adv. Space Res., 1, 247, 1981. (Received September 28,1992; revised January 25, 1993; accepted February 2, 1993.) - End of Grebowsky et al. appendix. -