UADS Dataset Description This file gives the Ne, Te and Vs measurements derived by fitting the radial probe voltampere curves taken whenever PVO was within the ionosphere (ie., between the inbound and outbound ionopause crossings). Data from essentially every orbit in 1979 and 1980 included ionosphere transits. After the summer of 1980, however, periapsis could no longer be maintained at low altitudes, and it rose slowly. After April 1981 periapsis was above the altitude of the dayside ionopause, so the spacecraft encountered the ionosphere only in the terminator regions and on the nightside where the ionosphere extends to much higher altitudes. Dayside measurements again became available early in 1992 when periapsis returned to low enough altitudes. The PVO Entry period of between July and October 1992 provided only nightside periapsis data. During the intervening period (1981-91), only nightside UADS measurements in the high altitude (downstream) ionosphere were available. Note that High Resolution Ne data from the 1981-91 interval provide measurements in the dayside magnetosheath and in the solar wind, but these data have limited accuracy because of spacecraft photoelectron contributions. See Brace et al, 1988 (ref. 53) in the bibliography for further details on the interpretation of High Resolution measurements made outside the ionosphere. As noted earlier, the geophysical values are listed at 12 second intervals in the UADS. Each OETP entry represents a time-weighted average of those radial probe measurements taken within approximately 10 seconds of the UADS-assigned times. If no voltampere curves were recovered within that 20 second interval (this occurs at very low spacecraft telemetry rates), no UADS value is entered in that 12 second slot. The instrument actually takes voltampere curves at a rate of 120/minute, but telemetry rate limitations permit the recovery of raw voltampere curves at intervals between 4 to 32 seconds, depending upon the telemetry rate and spacecraft data format currently in use. The Ne values in the UADS file may actually be based on either the ion or electron current collected by the probe, depending upon the magnitude of the density at the time. The radial probe electron currents saturate the electrometer when Ne > 4 x 10^4 cm^-3, so it is necessary to switch over to Ni measurements at that point. Since the ion currents are about a factor of 50 smaller, the Ni measurements can be made up to densities of about 2 x 10^6 cm^-3, much greater than is present anywhere in the Venus ionosphere. We assume that Ni = Ne everywhere in the ionosphere, so either may be used to construct the UADS file. To minimize any discontinuity that may occur at the Ne/Ni switch-over point due to systematic measurement errors, Ne is normalized to Ni using a small universal correction factor. This factor is 0.7, and is based on comparisons of the overlapping Ne and Ni measurements from many individual orbits. There are good theoretical reasons to believe that the Ni measurements are inherently more accurate at densities exceeding 3 or 4 x 10^4 cm^-3, so this normalization approach improves the accuracy of the Ne measurements. The Ni measurements become less accurate at lower densities because of uncertain changes in the ion composition, ion drift velocity, and a positive ion current component produced by photoelectrons (Ipe) leaving the probe. Ipe becomes comparable to the true ion currents at Ni of approximately 1 x 10^4 cm^-3. The pe currents produce a spin modulated signal that is modeled using measurements made in the solar wind just prior to the bow shock crossing where the ambient densities are too small to produce detectable ion currents. This spin modulated Ipe waveform, whose amplitude is different from orbit to orbit because of solar EUV variations, is subtracted from the net positive current measurements made in the subsequent ionospheric passage. This subtraction gives the true ion current which is directly convertible to Ni. The spin maximum Ipe for each orbit is also used to construct the solar EUV file, as is described later. Because of the low spatial resolution of the UADS, and the fact that only ionosphere data are included, this file is not the best source of information about the ionopause. Features such as the ionopause, and plasma clouds above the ionopause, are resolved better using the High Resolution Data File which is not restricted to measurements within the ionosphere. DATA QUALITY/ACCURACY The UADS data are based on operator-assisted voltampere curve fits. The absolute accuracy of the data depends primarily upon the accuracy of the Langmuir probe theory (Krehbiel, et al., 1980) and our success in avoiding the inclusion of data from curves that were obtained in situations in which the theory does not apply (e.g., probe in wake of the telemetry antenna, very low densities, pe contamination, spacecraft potential too negative, etc.). Where these effects have been avoided, the errors in Te should not exceed 5% when Ne exceeds 500 cm^-3 in sunlight and about 30 cm^-3 in darkness. However, Te errors may be larger in regions of great spatial structure where the plasma parameters change while they are being measured, or in regions where the electron energy distribution is nonmaxwellian or appears to have two temperatures. These conditions are often found in the nightside ionosphere and at the ionopause. In these cases, the curve-fitting is done so as to measure the temperature of the lower temperature component of the plasma. The curves would have to be refitted to obtain information on the higher temperature component. The accuracy of the Ne measurements is determined by the accuracy of the Ni measurements to which they are normalized by a fixed factor that was determined by comparisons at densities in the vicinity of 4 x 10^4 cm^-3. Therefore the Ne accuracy is nominally 10%, but the error increases at low densities where pe background and/or spacecraft charging effects can be important, as described earlier. Ne is given in the UADS file for densities down to 2 cm^-3 and Te for densities down to 10 cm^-3, which are observed only in the nightside ionosphere and ionotail. The Ne error is expected to grow as the density approaches these limits, but the Te measurements are less subject to error at low densities because knowledge of Vs is not needed to obtain the temperature. In spite of the reduced accuracy, Ne measurements below 30 cm^-3 are retained in the UADS file because they do reflect real variations that may be of interest even when their absolute accuracy may be uncertain by a factor of 2 or more. Examples include the detection of weak ionospheric tail rays and plasma clouds (Brace et al., 1987). The error in Ni is not expected to exceed 10% at densities above 4 x 10^4 cm^-3. Ne is used for densities below 4 x 10^4 cm^-3. As noted earlier, Ne is normalized to Ni at their overlap point to gain the greater inherent accuracy of the ion measurements. The normalization factor is based on the overlapping Ne and Ni measurements from many orbits, and the factor does not change throughout the mission. Therefore, small discontinuities in the density measurement may sometimes be seen at the crossover point if ionospheric conditions lead to unusual spacecraft potentials, ion compositions, or other factors that are assumed constant when adopting a fixed relationship between the ion and electron currents. We assume that the normalization factor remains constant over the full range of Ne, and this may not be correct. FORMAT OF THE DATA FILE The following is a guide to reading the data in the Pioneer Venus Unified Abstract Data System tape format. The first record gives the number of parameters, followed by the 4 letter mnemonic that identifies the parameter. The parameters given are ELTE (Te in units of Deg. K), ELNE, (Ne in number/cc), and VS (Vs in volts) in following format 3 ELTE ELNE VS The second record contains the format which may be used to read all of the remaining records. (I8,I9,I5,I6,3F11.2) The third record contains values which are used to indicate that no measurement was available. 0 0 0 0 9999999.00 9999999.00 9999999.00 The fourth record to the end of the file, consists of the date (year and day of year), the UT (milliseconds), the orbit number and the time from periapsis (in seconds). The last three columns are Te, Ne, and Vs. 1978339 54454817 1 -228 9999999.00 574.00 -2.10 1978339 54550817 1 -132 5470.00 12900.00 -0.42 1978339 54682817 1 12 3960.00 11800.00 -0.57 1978339 54814817 1 144 6900.00 1090.00 0.38 1978340 51262817 2 -444 9999999.00 243.00 2.84 1978340 51322817 2 -384 8760.00 55800.00 -1.75 REFERENCES Alexander, C. J., C. T. Russell, Solar cycle dependence of the location of the Venus bow shock, Geophys. Res. Lett., 12, 369, 1985. Brace, L. H., W. T. Kasprzak, H. A. Taylor, Jr., R. F. Theis, C. T. Russell, A. Barnes, J. D. Mihalov, and D. M. Hunten, The ionotail of Venus: Its configuration and evidence for ion escape, J. Geophys. Res., 92, 15, 1987. Brace, L. H., W. R. Hoegy, and R. F. Theis, Solar EUV measurements at Venus based on photoelectron emission from the Pioneer Venus Langmuir probe, J. Geophys. Res., 93, 7282, 1988. Brace, L. H., R. F. Theis, and J. D. Mihalov, The Response of the Venus Nightside Ionosphere and Ionotail to Solar EUV and Solar Wind Dynamic Pressure, J. Geophys. Res., 95, 4075, 1990. Elphic, R.C., L. H. Brace, R. F. Theis, and C. T. Russell, Venus Dayside Ionosphere Conditions: Effects of magnetic field and solar EUV flux, Geophys. Res. Lett., 11, 124, 1984. Krehbiel, J. P., L. H. Brace, J. R. Cutler, W. H. Pinkus, and R. B. Kaplan, Pioneer Venus Orbiter Electron Temperature Probe, IEEE Transactions on Geoscience and Remote Sensing, GE-18, 49, 1980. Mahajan, K. K, W. T. Kasprzak, L. H. Brace, H. B. Niemann, and W. R. Hoegy, Response of the Venus Exospheric Temperature Measured by Neutral Mass Spectrometer to the Solar EUV Measured by Langmuir Probe on the Pioneer Venus Orbiter, J. Geophys. Res., 95, 1091, 1990. Russell, C. T., E. Chou, J. G. Luhmann, P. Gazis, L. H. Brace, and W. R. Hoegy, Solar and interplanetary control of the location of the Venus bow shock, J. Geophysic. Res., 93, 5461, 1988. Theis, R. F., L. H. Brace, K. H. Schatten, C. T. Russell, J. A. Slavin, J. A. Wolf, The Venus ionosphere as an obstacle to the solar wind, Advances in Space Research, 1, 47, 1980. Theis, R. F., L. H. Brace, R. C. Elphic, and H. G. Mayr, New empirical models of the electron temperature and density of the Venus ionosphere, with applications to transterminator flow, J. Geophys. Res., 89, 1477, 1984.