Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle PVO ONMS Neutral Density 12 Sec Data Collection Description PDS3_DATA_SET_ID = PVO-V-ONMS-4-NEUTRALDENSITY-12SEC-V1.0 START_TIME = 1978-12-07T14:30:47 STOP_TIME = 1992-10-07T19:49:39 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 Product Description: orbits time 3 1978-12-07T14:30:47.000Z 640 1980-09-05T17:14:34.016Z 4961 1992-07-06T00:26:53.847Z 5055 1992-10-07T19:49:39.752Z 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 data 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 data collections are provided: high resolution (high-res), every point, composition; and low resolution (low-res), 12 second sampled, composition. The low-res data collection represents the best estimated composition data and is derived from the high-res data collection. The field names described in RECORD 1: Name Description Units YEAR YYYY=4 digit year DOY DDD=3 digit day of year UT Universal Time represented as the number of milliseconds ms since 00:00:00.000000 of the current day. ORBIT PVO Orbit number TIME_TAG Nominal time tag assigned by project (usual value ranging of the day from -1800 to 1800 in increments of 12) DEN_HE Number density of He particles/cm**3 DEN_N Number density of N particles/cm**3 DEN_O Number density of O particles/cm**3 DEN_N2 Number density of N2 particles/cm**3 DEN_CO Number density of CO particles/cm**3 DEN_CO2 Number density of CO2 particles/cm**3 MASS_DEN Total mass density g/cm**3 NUM_DEN Total number density particles/cm**3 This data is a representative sample, approximately once per 12 seconds, of the high resolution data. It is constructed at designated times which have been supplied by the Project. Data with errors greater that 30% are not included nor are data with angles of attack greater than 40 degrees. An absolute altitude cutoff of 250 km was used for all species except for He for which 350 km was used. Each representative data point is constructed using an exponentially weighted average of the data over a 24 second interval centered at the sample point time. Corrections to the number densities of CO2 and O for surface reactions were made at this time based on empirical model results. A minimum of 3 data points per species and all data available for corrections are required to be present in order for a sample point to be output. The total number density and total mass density are computed if all major species (CO2, CO, N2, and O) are present. The data spacing is nominally 12 seconds except for the -12, 0, 12 time tags. Although time tags from -1800 to 1800 seconds are generated, only those data records for which at least one species has a valid value for that time tag are output. No spacecraft positional parameters have been included in the data collections. These can be obtained from the SEDR data submitted separately by the Project. 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. Fimmel, R.O., Colin, L., and Burgess, E., '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. Kasprzak. W.T., H.A. Taylor, L.H. Brace and H.B. Niemann, Observations of Energetic Ions Near the Venus Ionopause, Planetary Space Sciences, vol. 30, 1107-1115, 1982. 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.