Data Set Information
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| DATA_SET_NAME |
PVO VENUS ONMS CALIBRATED NEUTRAL DENSITY HIGH RES. V1.0
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| DATA_SET_ID |
PVO-V-ONMS-3-NEUTRALDENSITY-HIRES-V1.0
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| NSSDC_DATA_SET_ID |
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| DATA_SET_TERSE_DESCRIPTION |
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| DATA_SET_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 data set has at least preliminary composition for every datapoint measured but not necessarily final composition values. The source of the data and their corrections aresummarized 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 theinstrument. Data from the retarding mode are consistent withthose obtained from the nonretarding mode and have not beenincluded. The data set does not include the factor of 1.6increase in density needed to maintain compatibility withother data sets as discussed by Hedin et al. (1983). Twodata sets are provided: high resolution (HIRES), every point,composition; and low resolution (LORES), 12 second sampled,composition. The LORES data set represents the bestestimated composition data and is derived from the HIRES dataset.The dataset fields are: YYYY YYYY=4 digit year DDD DDD=3 digit day of year UT Universal Time represented as the number of milliseconds since 1966-01-01T00:00:00Z stored as a double precision floating point number. ORBIT Orbit number MS Mass number - 4 for He 28 for N2 14 for N 29 for CO 16 for O 44 for CO2 F Flag - F for fully corrected P for preliminary (not fully corrected) M for mass flagged (problem point; probably wrong) N for final density negative (only preliminary density given) ODENS Preliminary density (part/cm**3) FDENS Final corrected density (part/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.
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| DATA_SET_RELEASE_DATE |
1993-03-31T00:00:00.000Z
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| START_TIME |
1978-12-07T02:29:47.000Z
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| STOP_TIME |
1992-10-07T07:50:36.000Z
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| MISSION_NAME |
PIONEER VENUS
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| MISSION_START_DATE |
1968-06-01T12:00:00.000Z
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| MISSION_STOP_DATE |
1992-10-07T12:00:00.000Z
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| TARGET_NAME |
VENUS
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| TARGET_TYPE |
PLANET
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| INSTRUMENT_HOST_ID |
PVO
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| INSTRUMENT_NAME |
ORBITER NEUTRAL MASS SPECTROMETER
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| INSTRUMENT_ID |
ONMS
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| INSTRUMENT_TYPE |
QUADRUPOLE MASS SPECTROMETER
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| NODE_NAME |
planetary plasma interactions
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| ARCHIVE_STATUS |
ARCHIVED
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| CONFIDENCE_LEVEL_NOTE |
Several criteria were invoked when inserting data for a givenorbit: orbit and attitude parameters must exist (project supplied);the spacecraft format and bit rate must be appropriate foracquisition of data by the ONMS; and the command sequence for theinstrument must be appropriate for useful determination ofatmospheric composition. Cases where useful composition cannot bedetermined include special test modes (e.g., retarding potentialsweeps, filament off) and 1/8 unit amu sweep modes. In addition,composition for the LORES data set cannot be easily determined forunit amu sweep mode. The ONMS was not operational for every orbitnor is every orbit complete due to data gaps introduced by use oftelemetry formats for which the ONMS has no instrument output. Useful composition data are gathered from the lowest periapsisaltitude to a maximum altitude generally around 250 km (about 300km for He). The actual maximum altitude depends on the accumulatedsurface gas buildup acquired from previous orbits which creates agas background. The gas background was estimated from high altitudeaverages of the data and for all species, except helium, an inboundsignal/background ratio of 2 and an outbound signal/background ratioof 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 visuallydetected. Some problems have been observed in the high altitudedata very near cutoff, particularly for outbound N2. Several datapoints were never removed and appear higher than the expectedextrapolation of the data to that time. Residual spin modulation which had not been completely removedis evident in the processed data. The source of the spin residualsare the gas/surface adsorption/desorption effects which were notremoved from the data and a noncosine behavior for the response ofthe ion source density with angle of attack. Another featureobserved occasionally at large angles of attack (>40 degrees) is areduction of the data when compared to data at lower angles ofattack. This has been determined to be due to antenna shadowing;that is, the ONMS geometric view cone 'sees' the spacecraft antennaat extreme angles of attack. Occasionally near minimum angle ofattack (<10 degrees), enhanced data points are observed for m/e=4(He channel) which are apparently high energy ions/neutralstraveling along the tube axis and being detected. The more extremepoints in either of these two cases have been mass flagged. The data time spacing depends on the spacecraft bit rate andformat, and the particular instrument commands executed. Usuallyprogrammed mass format was used but occasionally unit amu and 1/8amu sweeps were implemented. Several orbits switched from lowelectron energy to high electron energy and as a result there maybe a discontinuity at the transition point. The 1/8 amu sweep datahave not been included. Atomic nitrogen was measured in programmed mass mode onlyafter orbit 190. Orbits 1-19 generally do not have reliable relative compositiondue to the fact that gas-surface processes in the ion source had notstabilized. This affects all surface reactive species except He. Isolated (one or two points per several spin cycles) highresolution data points are occasionally observed and they should beregarded as erroneous points which are more likely wrong than right. The error associated with the points is more an indication of dataquality than of absolute uncertainty. It contains the statisticalerror of the data determined for the principle m/e used for the speciesfrom the detector signal plus the errors coming from any other speciesused to correct the data. It also contains a contribution which isproportional to the background/signal ratio. The total relative erroris at least an additional 5-10% above this value.
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| CITATION_DESCRIPTION |
TBD
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| ABSTRACT_TEXT |
The instrument was designed to determine the compositionof the neutral thermosphere/exosphere of Venus. The termcomposition includes both the type of neutral gases presentand their quantitative amount. The measurements begin at theorbit's periapsis altitude and extend to a limiting altitudeat which the ambient signal becomes comparable to the gasbackground and/or detector measurement threshold. The neutralcomposition includes helium, atomic nitrogen, atomic oxygen,molecular nitrogen, carbon monoxide and carbon dioxide. Thedata reduction has been described in Niemann et al. (1980a) andKasprzak et al. (1980).
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| PRODUCER_FULL_NAME |
DR. WAYNE KASPRZAK
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| SEARCH/ACCESS DATA |
Planetary Plasma Interactions Website
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