Pioneer Venus Orbiter Plasma Analyzer (OPA) General Instrument Description The plasma analyzer aboard the Pioneer Venus Orbiter (PVO) measures the incident plasma distribution parameters for ions and electrons. This instrument consists of a 90 degree quadrispherical electrostatic analyzer with collectors at its exit aperture. Particles that pass through the entrance aperture will move on Keplerian orbits in the electrostatic field produced by applying a voltage across the plates. Most of these particles will impact one of the plates and be lost, but particles with a suitable range of energy/charge (E/Q) and arrival direction will survive to pass through the exit apperture. There they will be collected by an array of current collectors, each of which is connected to an electrometer amplifier. By varying the voltage difference, and hence the strength of the electrostatic field between the plates, a range of E/Q values can be covered for both positive ions and electrons. If this range of E/Q values is sufficiently large, the complete particle spectrum will be measured. The incident direction of the incoming particles is determined from the spacecraft azimuthal angle and the response of the five collectors. The multiple collectors resolve directions parallel to the spacecraft's spin axis while the rotation of the spacecraft provides a scan through a full 360 degrees of azimuth. In this way, the instrument can provide direct observation of the three-dimensional velocity distribution of incident ions and electrons. Bulk plasma parameters, such as velocity, density, and temperature are obtained by a least-squares fit of a convected spherical Maxwellian to this velocity distribution. The instrument has two logical modes (step and scan) and three E/Q ranges (high-energy ion, low-energy ion, and electron). These modes are discussed in detail below. Science Objectives The orbiter plasma analyzer (OPA) measures the flow velocity, density, temperature, and distribution functions of solar wind plasma in the vicinity of Venus. It also senses plasma of planetary origin. Data from the OPA have been used to study the detailed morphology of the interaction between the solar wind and the Venus ionosphere, including the planetary bow shock and magnetosheath, planetary O+ picked up by the solar wind and in the wake of the planet, escaping particles in the nightside ionosphere, O+ pickup upstream of the nose of the planet, and the acceleration of solar wind particles by the bow shock. Data taken in the upstream solar wind have been used for multiple-spacecraft studies of the global morphology of the heliosphere. Instrument Calibration The calibration was performed in two phases. First the instrument electronics transfer function was measured at several temperatures by simulating the electrometer input current over all ranges and polarities. Second, the optics was calibrated by placing the instrument in the ARC plasma chamber and measuring instrument response for all view angles. In this phase the instrument was held in a 2-axis gimbal system and rotated while illuminated with a monoenergetic plasma beam. Operational Considerations When deflection plate or suppression grid voltage is changed significantly, a spurious pulse may appear in an electrometer amplifier. These may be recognized in spectral data by a peak current near azimuthal sector 4. They should not affect the parameter data. Instrument Mounting The instrument is rigidly attached to the spacecraft. It is mounted near the outer edge of the spacecraft equipment shelf and looks out in a direction parallel to a radius vector from the spacecraft spin axis. The view angle is 140 deg in a plane parallel to the spin axis and ~3 deg in the plane normal to the spin axis. Instrument Mode Descriptions The instrument has two commandable logic modes of operation: scan mode and step mode: Scan Mode: In the scan mode the instrument first steps through its E/Q steps, one step per spacecraft rotation, to determine the maximum flux, as well as the collector number and azimuthal angle for which the maximum flux occurs for each E/Q step. Then a polar scan and an azimuthal scan are carried out at four consecutive E/Q steps, starting with the step before the one corresponding to the highest flux detected during the E/Q scan. The polar scan returns the maximum flux and corresponding azimuthal angle for all 5 collectors at a given E/Q step. The azimuthal scan returns the flux measured in 12 azimuthal sectors centered on the peak flux direction from the collector which measured the peak flux in the polar scan. The polar and azimuthal scans are accomplished consecutively at each E/Q step. Step Mode: In the step mode only the maximum flux scan is done at one energy step per second. Appropriate ground commands permit either the step or scan mode to be used in three different E/Q ranges. The high-energy ion range covers the interval 50 V to 8000 V in 32 logarithmically equal steps. The other two ranges (the low-energy ion range and its negative, the electron range) measure ions or electrons over the E/Q range 3 to 250 V in 15 logarithmically equal steps plus a "zero" step at 0.25 V. The combination of scan or step mode and the three E/Q ranges give six effective modes of operation (e.g. high-energy ion scan, high-energy ion step, low-energy ion scan, low-energy ion step, electron scan, and electron step). A complete instrument cycle in either scan or step mode is stored in a memory chip, so that the instrument sampling time for a complete spectrum is independent of the spacecraft telemetry rate. The following table gives the instrument data generation rates: _____________________________________________________________ Mode Energy Data Scan Period Data Gen Range Bits Revs Seconds Rate, bps 5 RPM 15 RPM 5 RPM 15 RPM 5 RPM 15 RPM _____________________________________________________________ Scan E1 1552 45.00 45.00 540 180 2.87 8.62 Scan E4 1384 36.00 36.00 432 144 3,20 9.61 Scan E2,E3 1216 27.00 27.00 324 108 3.75 11.3 Step E1 704 3.08 9.25 37 37 19.00 19.0 Step E4 536 2.42 7.25 29 29 18.5 18.5 Step E2,E3 368 2.17 6.50 26 26 14.2 14.2 E1 is the 32 step range for ions, E2 and E3 are the 15 step range for electrons and ions, respectively, and E4 is similar to E1 but the highest energy ranges are not covered. _____________________________________________________________ Instrument Section Descriptions The instrument consists of a detector assembly and an instrument housing which contain six electrical subassemblies. These electrical assemblies are A/D converter, high voltage control, logic and motherboard, low voltage power supply, high voltage power supply, and electrometer filament supply. The electronic housing contains the A/D converter, high voltage control, logic and motherboard, and low voltage power supply, while the electrometer housing contains the detector assembly, high voltage power supply, and electrometer filament supply. The detector section consists of a nested concentric pair of conducting quadrispherical plates, whose mean radius of curvature is 12 cm. The entrance aperture is circular, 1 cm in diameter. The plates are separated by 1 cm., attached to a target mount on wwhich rest 5 electrometer assemblies, each containing an electrometer tube. The field of view is ~3 degrees in the azimuthal direction and 140 degrees in the polar direction; spacecraft rotation results in full 360 degree azimuthal coverage. Instrument Detector Descriptions Each individual collector plate is connected to an electrometer amplifier. The input current sensitivity is 10E-14 to 10E-9 amp for ions and 10E-14 to 10E-10 amp for electrons. The three central collectors have a field of view of ~3 degrees azimuthal, 15 degrees polar; the two outer collectors have a field of view of ~3 degrees azimuthal, 47 degrees polar. Instrumental uncertainties Instruments such as the OPA are best at measuring velocity, and can return bulk velocities reliable to better than 1%. Since temperature is in some sense also a measurement of velocity (e.q. the thermal speed of the spectrum), much of this reliability extends to temperatures measurements as well. Typical thermal widths of the solar wind in the inner heliosphere are on the order of 50 km/s, which is large compared with the resolution of the OPA. This, and simulations conducted for similar instruments aboard Pioneer 10 and 11 suggest that temperatures measured by the OPA could be reliable to better than 20%. Density is more difficult to measure and there is some controversy regarding the relative calibration of measurements from different spacecraft. References Intriligator, D.S., J.H. Wolfe, and J.D. Mihalov, The Pioneer Venus Orbiter plasma analyzer experiment, IEEE Transactions on Geoscience and Remote Sensing, GE-18, 39-43, 1980. For more information, contact: Dr. Paul R. Gazis, SJSU Research Associate phone: (415) 604-5704 fax: (415) 604-6779 email: gazis@arwen.arc.nasa.gov ------------------------------------------------------------------------------- [Go to] Top of page. [Go to] ARC Pioneer Plasma Group Home Page last modified 3 May 1995.