This file gives the Ne, Te and Vs measurements derived by fitting the radialprobe voltampere curves taken whenever PVO was within the ionosphere(ie., between the inbound and outbound ionopause crossings). Data fromessentially every orbit in 1979 and 1980 included ionosphere transits.After the summer of 1980, however, periapsis could no longer bemaintained at low altitudes, and it rose slowly. After April 1981 periapsiswas above the altitude of the dayside ionopause, so the spacecraftencountered the ionosphere only in the terminator regions and on thenightside where the ionosphere extends to much higher altitudes. Daysidemeasurements again became available early in 1992 when periapsisreturned to low enough altitudes. The PVO Entry period of between Julyand October 1992 provided only nightside periapsis data. During theintervening period (1981-91), only nightside UADS measurements in thehigh altitude (downstream) ionosphere were available. Note that HighResolution Ne data from the 1981-91 interval provide measurements in thedayside magnetosheath and in the solar wind, but these data have limitedaccuracy because of spacecraft photoelectron contributions. See Brace etal, 1988 (ref. 53) in the bibliography for further details on theinterpretation of High Resolution measurements made outside theionosphere.As noted earlier, the geophysical values are listed at 12 second intervals inthe UADS. Each OETP entry represents a time-weighted average of thoseradial probe measurements taken within approximately 10 seconds of theUADS-assigned times. If no voltampere curves were recovered within that20 second interval (this occurs at very low spacecraft telemetry rates), noUADS value is entered in that 12 second slot. The instrument actuallytakes voltampere curves at a rate of 120/minute, but telemetry ratelimitations permit the recovery of raw voltampere curves at intervalsbetween 4 to 32 seconds, depending upon the telemetry rate andspacecraft data format currently in use.The Ne values in the UADS file may actually be based on either the ion orelectron current collected by the probe, depending upon the magnitude ofthe density at the time. The radial probe electron currents saturate theelectrometer when Ne > 4x104 cm-3, so it is necessary to switch over to Nimeasurements at that point. Since the ion currents are about a factor of50 smaller, the Ni measurements can be made up to densities of about 2 x106 cm-3, much greater than is present anywhere in the Venus ionosphere.We assume that Ni : Ne everywhere in the ionosphere, so either may beused to construct the UADS file. To minimize any discontinuity that mayoccur at the Ne/Ni switch-over point due to systematic measurementerrors, 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 andNi measurements from many individual orbits.There are good theoretical reasons to believe that the Ni measurementsare inherently more accurate at densities exceeding 3 or 4x104 cm-3, sothis normalization approach improves the accuracy of the Nemeasurements. The Ni measurements become less accurate at lowerdensities because of uncertain changes in the ion composition, ion driftvelocity, and a positive ion current component produced byphotoelectrons (Ipe) leaving the probe. Ipe becomes comparable to thetrue ion currents at Ni of approximately 1x104cm-3. The pe currentsproduce a spin modulated signal that is modelled using measurementsmade in the solar wind just prior to the bow shock crossing where theambient densities are too small to produce detectable ion currents. Thisspin modulated Ipe waveform, whose amplitude is different from orbit toorbit because of solar EUV variations, is subtracted from the net positivecurrent measurements made in the subsequent ionospheric passage. Thissubtraction gives the true ion current which is directly convertible to Ni.The spin maximum Ipe for each orbit is also used to construct the solarEUV file, as is described later.Because of the low spatial resolution of the UADS, and the fact that onlyionosphere data are included, this file is not the best source ofinformation about the ionopause. Features such as the ionopause, andplasma clouds above the ionopause, are resolved better using the HighResolution Data File which is not restricted to measurements within theionosphere. OBJECT : DSPARMINFO SAMPLING_PARAMETER_NAME : TIME SAMPLING_PARAMETER_RESOLUTION : N/A MINIMUM_SAMPLING_PARAMETER : N/A MAXIMUM_SAMPLING_PARAMETER : N/A SAMPLING_PARAMETER_INTERVAL : 12 MINIMUM_AVAILABLE_SAMPLING_INT : N/A SAMPLING_PARAMETER_UNIT : SECOND DATA_SET_PARAMETER_NAME : ELECTRON DENSITY NOISE_LEVEL : 2 DATA_SET_PARAMETER_UNIT : CM**-3 END_OBJECT : DSPARMINFO OBJECT : DSPARMINFO SAMPLING_PARAMETER_NAME : TIME SAMPLING_PARAMETER_RESOLUTION : N/A MINIMUM_SAMPLING_PARAMETER : N/A MAXIMUM_SAMPLING_PARAMETER : N/A SAMPLING_PARAMETER_INTERVAL : 12 MINIMUM_AVAILABLE_SAMPLING_INT : N/A SAMPLING_PARAMETER_UNIT : SECOND DATA_SET_PARAMETER_NAME : ELECTRON TEMPERATURE NOISE_LEVEL : UNK DATA_SET_PARAMETER_UNIT : DEGREES KELVIN END_OBJECT : DSPARMINFO OBJECT : DSPARMINFO SAMPLING_PARAMETER_NAME : TIME SAMPLING_PARAMETER_RESOLUTION : N/A MINIMUM_SAMPLING_PARAMETER : N/A MAXIMUM_SAMPLING_PARAMETER : N/A SAMPLING_PARAMETER_INTERVAL : 12 MINIMUM_AVAILABLE_SAMPLING_INT : N/A SAMPLING_PARAMETER_UNIT : SECOND DATA_SET_PARAMETER_NAME : SPACECRAFT POTENTIAL NOISE_LEVEL : UNK DATA_SET_PARAMETER_UNIT : VOLTS END_OBJECT : DSPARMINFO

The UADS data are based on operator-assisted voltampere curve fits. Theabsolute accuracy of the data depends primarily upon the accuracy of theLangmuir probe theory (Krehbiel, et al., 1980) and our success in avoidingthe inclusion of data from curves that were obtained in situations in whichthe 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 notexceed 5% when Ne exceeds 500 cm-3 in sunlight and about 30 cm-3 indarkness. However, Te errors may be larger in regions of great spatialstructure where the plasma parameters change while they are beingmeasured, or in regions where the electron energy distribution isnonmaxwellian or appears to have two temperatures. These conditions areoften found in the nightside ionosphere and at the ionopause. In thesecases, the curve-fitting is done so as to measure the temperature of thelower temperature component of the plasma. The curves would have to berefitted to obtain information on the higher temperature component.The accuracy of the Ne measurements is determined by the accuracy of theNi measurements to which they are normalized by a fixed factor that wasdetermined by comparisons at densities in the vicinity of 4 x 104 cm-3.Therefore the Ne accuracy is nominally 10%, but the error increases at lowdensities where pe background and/or spacecraft charging effects can beimportant, as described earlier. Ne is given in the UADS file for densitiesdown to 2 cm-3 and Te for densities down to 10 cm-3, which are observedonly in the nightside ionosphere and ionotail. The Ne error is expected togrow as the density approaches these limits, but the Te measurements areless subject to error at low densities because knowledge of Vs is notneeded to obtain the temperature. In spite of the reduced accuracy, Nemeasurements below 30 cm-3 are retained in the UADS file because theydo reflect real variations that may be of interest even when their absoluteaccuracy may be uncertain by a factor of 2 or more. Examples include thedetection 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 x104cm-3. Ne is used for densities below 4 x104 cm-3. As noted earlier, Ne isnormalized to Ni at their overlap point to gain the greater inherentaccuracy of the ion measurements. The normalization factor is based onthe overlapping Ne and Ni measurements from many orbits, and the factordoes not change throughout the mission. Therefore, small discontinuitiesin the density measurement may sometimes be seen at the crossover pointif ionospheric conditions lead to unusual spacecraft potentials, ioncompositions, or other factors that are assumed constant when adopting afixed relationship between the ion and electron currents. We assume thatthe normalization factor remains constant over the full range of Ne, andthis may not be correct.