Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle PVO ONMS Neutral Density High-Res. Data Collection Description PDS3_DATA_SET_ID = PVO-V-ONMS-3-NEUTRALDENSITY-HIRES-V1.0 START_TIME = 1978-12-07T14:29:47 STOP_TIME = 1992-10-07T19:50:36 PDS3_DATA_SET_RELEASE_DATE = 1993-03-31 PRODUCER_FULL_NAME = Dr. Wayne Kasprzak Collection Overview =================== The instrument was designed to determine the composition of the neutral thermosphere/exosphere of Venus. The term composition includes both the type of neutral gases present and their quantitative amount. The measurements begin at the orbit's periapsis altitude and extend to a limiting altitude at which the ambient signal becomes comparable to the gas background and/or detector measurement threshold. The neutral composition includes helium, atomic nitrogen, atomic oxygen, molecular nitrogen, carbon monoxide and carbon dioxide. The data reduction has been described in Niemann et al. (1980a) and Kasprzak et al. (1980). Data Description: Orbits Time 3 1978-12-07T14:29:47.668Z 640 1980-09-05T17:15:20.875Z 4961 1992-07-06T00:23:50.044Z 5055 1992-10-07T19:50:36.144Z This collection has at least preliminary composition for every data point measured but not necessarily final composition values. The source of the data and their corrections are summarized below: Species M/E Used Comments He 4 N 30 Surface recombined N and O O 32 Surface recombined O to O2; corrected for CO2 fragmentation corrected for estimated surface recombination of O to CO2(*) N2,CO 14,28 m/e 14 corrected for NO, CO and CO2 fragmentation; m/e 28 corrected for CO2 fragmentation CO2 44 Corrected for surface recombination of O to CO2(*) (*) the correction is based on matching scale height temperatures of O and CO2. The data are from the nonretarding potential mode of the instrument. Data from the retarding mode are consistent with those obtained from the nonretarding mode and have not been included. The collection does not include the factor of 1.6 increase in density needed to maintain compatibility with other data sets as discussed by Hedin et al. (1983). Two collections are provided: high resolution (high-res), every point, composition; and low resolution (low-res), 12 second sampled, composition. The low-res collection represents the best estimated composition data and is derived from the high-res data collection. The data fields are: YEAR YYYY=4 digit year DOY DDD=3 digit day of year UT Universal Time of day represented as the number of milliseconds since 00:00:00.000000 of the current day. ORBIT PVO Orbit number MASS_NUM Mass number - 4 for He 28 for N2 14 for N 29 for CO 16 for O 44 for CO2 FLAG Flag: 0 for fully corrected 1 for preliminary (not fully corrected) 2 for mass flagged (problem point; probably wrong) 3 for final density negative (only preliminary density given) PDENS Preliminary density (particles/cm**3) FDENS Final corrected density (particles/cm**3) ANGATK Angle of attack (degrees) PCERR % error in density -1 if error >127% Notes: 1) For CO2 both ODENS and FDENS are given since the final correction to this species depends on a model: (new CO2 density) = (old CO2 density) - 0.019 x (old O density) (new O density) = (old O density) + 0.019 x (old O density) Only the New O density is given. 2) Mass flagged points are usually points that fall excessively beyond range of the main body of the data. They may be wild points, points with wrong mass designations or simply wrong for other reasons. 3) The best estimate of the density is to be found in the F data. 4) Data with errors greater than about 30% should be considered unreliable. 5) The angle of attack is included to help sort out low data values due to antenna shadowing (all species) which occurs beyond 40 degrees and high value ram points seen in He at angles of attack less than 10 degrees. Some of these points have already been mass flagged. In general, it would be best to not include data in these regions. Confidence Level Overview ========================= Several criteria were invoked when inserting data for a given orbit: orbit and attitude parameters must exist (project supplied); the spacecraft format and bit rate must be appropriate for acquisition of data by the ONMS; and the command sequence for the instrument must be appropriate for useful determination of atmospheric composition. Cases where useful composition cannot be determined include special test modes (e.g., retarding potential sweeps, filament off) and 1/8 unit amu sweep modes. In addition, composition for the low-res data collection cannot be easily determined for unit amu sweep mode. The ONMS was not operational for every orbit nor is every orbit complete due to data gaps introduced by use of telemetry formats for which the ONMS has no instrument output. Useful composition data are gathered from the lowest periapsis altitude to a maximum altitude generally around 250 km (about 300 km for He). The actual maximum altitude depends on the accumulated surface gas buildup acquired from previous orbits which creates a gas background. The gas background was estimated from high altitude averages of the data and for all species, except helium, an inbound signal/background ratio of 2 and an outbound signal/background ratio of 4 were used as cutoff values. In some cases superthermal ions (e.g., Kasprzak et al.,1982) were observed at low altitudes (e.g., below 300 km for orbit 219) and these were removed when visually detected. Some problems have been observed in the high altitude data very near cutoff, particularly for outbound N2. Several data points were never removed and appear higher than the expected extrapolation of the data to that time. Residual spin modulation which had not been completely removed is evident in the processed data. The source of the spin residuals are the gas/surface adsorption/desorption effects which were not removed from the data and a noncosine behavior for the response of the ion source density with angle of attack. Another feature observed occasionally at large angles of attack (>40 degrees) is a reduction of the data when compared to data at lower angles of attack. This has been determined to be due to antenna shadowing; that is, the ONMS geometric view cone 'sees' the spacecraft antenna at extreme angles of attack. Occasionally near minimum angle of attack (<10 degrees), enhanced data points are observed for m/e=4 (He channel) which are apparently high energy ions/neutrals traveling along the tube axis and being detected. The more extreme points in either of these two cases have been mass flagged. The data time spacing depends on the spacecraft bit rate and format, and the particular instrument commands executed. Usually programmed mass format was used but occasionally unit amu and 1/8 amu sweeps were implemented. Several orbits switched from low electron energy to high electron energy and as a result there may be a discontinuity at the transition point. The 1/8 amu sweep data have not been included. Atomic nitrogen was measured in programmed mass mode only after orbit 190. Orbits 1-19 generally do not have reliable relative composition due to the fact that gas-surface processes in the ion source had not stabilized. This affects all surface reactive species except He. Isolated (one or two points per several spin cycles) high resolution data points are occasionally observed and they should be regarded as erroneous points which are more likely wrong than right. The error associated with the points is more an indication of data quality than of absolute uncertainty. It contains the statistical error of the data determined for the principle m/e used for the species from the detector signal plus the errors coming from any other species used to correct the data. It also contains a contribution which is proportional to the background/signal ratio. The total relative error is at least an additional 5-10% above this value. References ========== Colin, L., Pioneer Venus Overview, IEEE Transactions on Geoscience and Remote Sensing, Vol GE-18, No. 1,pp. 5-10, 1980. R. O. Fimmel, L. Colin, and E. Burgess, 'Pioneering Venus: A Planet Unveiled', NASA SP-518, 1995. Hedin, A.E., H.B. Niemann, W.T. Kasprzak and A. Seiff, Global Empirical Model of the Venus Thermosphere, Journal of Geophysical Research, vol. 88, 73-83, 1983. Kasprzak, W.T., A.E. Hedin, H.B. Niemann and N.W. Spencer, Atomic Nitrogen in the Upper Atmosphere of Venus, Geophysical Research Letters, vol. 7,106-108, 1980. Niemann, H.B., J.R.Booth, J.E. Cooley, R.E. Hartle, W.T. Kasprzak, N.W.Spencer, S.H. Way, D.M. Hunten and G.R. Carignan, Pioneer Venus Orbiter Neutral Gas Mass Spectrometer, IEEE Trans. on Geoscience and Remote Sensing, vol. GE-18 (1), 60-65, 1980. Niemann, H.B., W.T. Kasprzak, A.E. Hedin, D.M. Hunten and N.W. Spencer, Mass Spectrometric Measurements of the Neutral Gas Composition of the Thermosphere and Exosphere of Venus, Journal of Geophysical Research, vol. 85, 7817-7827, 1980a. Nothwang, G.T., Pioneer Venus Spacecraft Design and Operation, IEEE Transactions on Geoscience and Remote Sensing, Vol GE-18, No. 1, pp. 5-10, January 1980.