The following are excerpts from the appendix of: Brace, L. H. and A. J. Kliore, "Structure of the Venus Ionosphere", Space Science Reviews, v55, p81-163, 1991. which has several sections which describe the intercalibration of the PVO Ion detectors. We have left the appendix section titles in place in order to help organize these excerpts. Users of any of the PVO particle detector datasets are highly encouraged to seek out the original reference as part of their research. ============================================================================= Conventions [] indicate a superscript, {} indicate a subscript RPA = Retarding Potential Analyzer IMS = Ion Mass Spectrometer NMS = Neutral Mass Spectrometer LP = Langmuir Probe RO = Radio Occultation ============================================================================ Appendix. The PVO Data Base and Sources of Measurement Error This Appendix contains a discussion of the spatial resolution and accuracy of the measurements, and some of the discrepancies in N{i} and N{e} that have been reported. A.2. MEASUREMENT CAPABILITIES The ONMS detects superthermal ions having energies greater than about 40 eV. The OIMS reports the concentration of all ion species present, including minor ion species, some instrument modes give high spatial resolution. The OETP provides high resolution in N{e}, N{i}, and T{e}. The ORPA measures the concentrations of the major ions, and the ion temperature, T{i}. The ORPA is not able to distinguish between ions of similar mass (O{2}[+] and CO[+], or O[+] and N[+]) nor can it detect minor ions that represent less than a few percent of the total number density. The ORPA has an electron mode that measures T{e} and the integral electron flux for energies up to about 50 eV. The spatial resolution of the ORPA electron measurements is the spacecraft spin period (~13 sec). The OETP and OIMS acquire continuous high resolution measurements on the order of 1 s^-1 at typical telemetry rates. The OIMS sensor is mounted parallel to the spin axis which minimizes the spin modulation which enhances the effective spatial resolution. The OETP radial sensor is mounted perpendicular to the spin axis. Shadowing from the spinning spacecraft introduces measurement errors at certain spin angles. The reductions of spin effects are valuable for resolving ionospheric structures with spatial scales <<120 km (distance s/c travels in 13 sec). A.3. DISCREPANCIES AMONG THE MEASUREMENTS The OIMS and ORPA measure N{i}. OETP measures N{i} at high densities and N{e} at low densities, with overlap between 1 x 10^3 and 1 x 10^5 cm^-3. The RO yields height profiles of N{e} down to densities of the order of 10^3 cm^-3. However, RO measurements usually represent a region quite remote from the satellite which has been averaged over a relatively long horizontal path through the ionosphere. "Systematic comparisons of the early UADS data base, conducted by Miller et al. (1984), uncovered a number of discrepancies, particularly in the ion composition and density measurements. They found that the OIMS values of N{i} were larger than those from the ORPA by a factor of between 1.5 and 2.5 in portions of the day side ionosphere, with the largest discrepancies in the afternoon at altitudes between 170 and 200 km. Miller et al. attributed this to OIMS O{2}[+] concentration that were about a factor of 3 higher in that region. The density measurements above 200 km, where O[+] dominates, are in better agreement. The ORPA measurements of N{i} were in essential agreement with the OETP measurements of N{e} except at the lowest altitudes on the nightside, where N{e} values were a bit higher. The RO density profiles agreed well statistically at these altitudes with the ORPA and OETP densities, and generally gave lower densities than those given by the OIMS. The T{e} values from the OETP were in general agreement with those from the ORPA, except on the dayside below 200 km, where the OETP values were several percent higher." A.4. POSSIBLE SOURCES OF MEASUREMENT ERROR The instruments all include onboard data processing schemes to transform their raw spectra or volt-ampere curves into physical parameters in order to reduce their bits-to-ground requirements. Some of these onboard processed measurements show that undetected errors are still present in some situation. A.4. 1. Suprathermal Electron Effects Suprathermal electrons in the nightside ionosphere cause errors in the measurements of cold ionospheric electrons by making the spacecraft potential so negative that the thermal electrons cannot reach the sensors. Electrostatic shielding is most important for the ORPA measurements (sensor is mounted on the spacecraft surface) where the cold electrons must overcome the full spacecraft potential to enter the sensor. The OETP is less affected by spacecraft potential because its sensors are mounted 40 cm (axial probe) and 100 cm (radial probe) from the spacecraft surface. A.4.2. Non-Maxwellian Electron Effects Brace et al. (1980) noted that thermal and superthermal electrons often exist together with similar densities, making it difficult to characterize the electron temperature accurately. Two component temperature distributions are common in the ionospheric holes (Brace et al., 1982b) and in the lower nightside ionosphere in the vicinity of small-scale N{e} structure (Hoegy et al., 1989). Strong spatial gradients in T{e} cause the electron energy distribution to be non-Maxwellian. A.4.3 Superthermal Ion Effects Taylor et al. ( 1980) observed superthermal ions at the ionopause and within the nightside ionosphere. The evidence for their existence is a shift in the apparent mass of O[+] from 16 to 14 amu, a shift that corresponds to ion energies in the range of 9 eV to 16 eV. Superthermal H[+] ions are not observed because H[+] at these energies fall below the mass range of the OIMS. Thus the OIMS total density may be underestimated when superthermal ions are present because the H[+] ions are not included. The N{i} measurements by the OETP radial probe are also affected by superthermal ions if they represent a significant fraction of the total density. The calculation of N{i} assumes that the ion flux to the collector is produced by the velocity of the collector through the ionosphere (10 km s^-1). If the thermal velocity of the ions is comparable to the spacecraft velocity, additional ion current is collected and N{i} is overestimated. A.4.4. Ion Drift Effects Knudsen et al. (1980b) and Taylor et al. (1980) have shown that very high ion drift velocities are present, particularly at high altitudes near the terminator. Ion drift produces changes in the ion velocity into the sensor and changes in the angle of approach, both of which affect the ion fluxes that reach the collector of the instrument. ORPA and OIMS are mounted to provide small angles of attack near periapsis, but the minimum angle of attack tends to increase with altitude. ORPA measures the ion drift component normal to the sensor, so its N{i} measurements are affected by ion drift only to the extent that the correction for the assumed angle of arrival may be incorrect. The OETP measurements of N{i} depend linearly upon knowledge of the ion drift velocity, which is assumed in the data processing to be the satellite velocity. The angle of arrival is unimportant. N{i} is measured at low altitude where drift effects are small and N{e} is used at the higher altitudes. Off-axis ion drift velocities in the OIMS reduce the transmission efficiency of the analyzer, an effect that is particularly important for the heavier ions. Ion drift leads to an underestimate of the density and an apparent change in the relative ion composition at high altitudes where the ion drift velocities may be a significant fraction of the spacecraft velocity. Both high ion drift velocities and large angles of attack tend to be important to the measurement accuracy at higher altitudes but the discrepancies among the PVO density measurements are greater at lower altitudes. A.4.5. Spacecraft Photoelectron Effects At densities greater than a few hundred cm^-3, the instruments operate in an electron environment that is dominated by the ionospheric electrons. Spacecraft photoelectrons dominate when the ambient density is lower, but the lower limit for reliable measurements depends on the mounting location of the particular sensor and its sensitivity to the photo-electron background. The OETP radial sensor is least affected because of its greater distance from the spacecraft, and because one can select the measurements taken only when the collector is on the dark side of the spacecraft where the spacecraft photoelectron background is lower (Brace et al., 1988b). The ORPA is mounted on the spacecraft surface. This limits the density range over which cold ionospheric electrons can be measured when the spacecraft is sunlit. A.4.6. Periapsis Effects Perhaps the most well established periapsis effect arises from impact ionization (Hanson et al., 1981; Whipple et al., 1983; Curtis et al., 1985). At the PVO periapsis velocity of 10 km s^-1, the impact energy (1/2mv^2) for CO, is 23 eV. This is enough energy to ionize a small fraction of the CO{2} that the spacecraft encounters and produce a measurable cloud of 1 to 2 eV secondary electrons above the leading surface of the spacecraft. Lighter molecules (O{2} and CO) contribute less impact ionization because their impact energies are only slightly greater than their ionization potentials. The OETP axial sensor is mounted on the ram end of the spacecraft. Measurable fluxes of impact electrons are seen when the spacecraft is below about 165 km on the dayside and 150 km on the nightside. The impact electron density at periapsis may be as high as 10% of the ambient N{e} at the nightside peak (order of 10^4 cm^-3). Their density has been shown proportional to the thermospheric CO2 concentration measured by the ONMS (Whipple et al. 1983). The radial OETP sensor observes no secondary electrons at its location. Miller et al. (1984) suggested that impact electrons may be responsible for the anomalous increase in T{e} in the ORPA measurements below about 167 km in the daytime ionosphere. The ORPA is mounted on the forward looking surface of the spacecraft where this effect is greatest. Impact ionization may indirectly produce errors in the ion measurements as well. A byproduct of the impact ionization process is the creation of a dc electric field upstream of the spacecraft (Parker and Holeman, 1980). This electric field is produced by the difference in mobility of the sputtered ions and electrons. The resulting charge separation creates a region of positive space charge near the surface and a region of negative space charge farther ahead. Ambient ions must pass through these electric fields to reach the ion sensors, so the composition and energy of the measured ions may be perturbed. No analysis of this effect on the PVO ion measurement techniques has been reported. In conclusion, the accuracy of the PVO measurements is difficult to assess. While the periapsis effects can be expected to cause errors at the lowest altitudes, the largest disagreements occur in the afternoon near 175 km, well above periapsis. The choice of PVO data for a particular investigation will fall to the user. References Brace. L. H., Theis. R. F.. Hoegy, W. R., Wolfe, J. H., Mihalov, J. D., Russell. C. T., Elphic, R. C., and Nagy A. F.: 1980, J. Geophys. Res. 85. 7663. Brace, L. H., Theis, R. F., Curtis, S. A., and Parker, L. W.: 1988b, J. Geophys. Res. 93, 12735. Curtis, S. A., Brace, L. H., Niemann, H. B., and Scarf, F. L.: 1985, J. Geophys. Res. 90, 6631. Hanson, W. B., Sanatani, S., and Hoffman, I. H.: 1981, J. Geophys. Res. 86, 11, 350. Knudsen. W C., Spenner, K., Miller, K. L., and Novak, V.: 1980b, J. Geophys. Res. 85, 7803. Miller, K. L.. Knudsen, W. C., and Spenner, K.: 1984, Icarus 57, 386. Taylor. H. A., Brinton, H. C.. Bauer, S. J., Hartle, R. E.. Cloutier, P. A., and Daniell. R. E.: 1980, J. Geophys. Res. 85, 7765. Whipple, E. C., Brace, L. H., Parker, L. W.: 1983, Proc. 17th ESLAB Symp. Spacecraft Interactions, 127, ESA Report SP-198.