Data Set Information
DATA_SET_NAME PVO VENUS ONMS BROWSE NEUTRAL DENSITY 12 SECOND V1.0
DATA_SET_ID PVO-V-ONMS-4-NEUTRALDENSITY-12SEC-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_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 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 field names described in RECORD 1: NAME DESCRIPTION UNITS 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 TIMTAG Nominal time tag assigned by project DHE Number density of He part/cm**3 DN Number density of N part/cm**3 DO Number density of O part/cm**3 DN2 Number density of N2 part/cm**3 DCO Number density of CO part/cm**3 DCO2 Number density of CO2 part/cm**3 DRHO Total mass density g/cm**3 DTOT Total number density part/cm**3 This data is a representative sample, approximately once per12 seconds, of the high resolution data. It is constructed atdesignated times which have been supplied by the Project. Datawith errors greater that 30% are not included nor are data withangles of attack greater than 40 degrees. An absolute altitudecutoff of 250 km was used for all species except for He forwhich 350 km was used. Each representative data point is constructed using anexponentially weighted average of the data over a 24 secondinterval centered at the sample point time. Corrections tothe number densities of CO2 and O for surface reactionswere made at this time based on empirical model results. Aminimum of 3 data points per species and all data availablefor corrections are required to be present in order for asample point to be output. The total number density andtotal mass density are computed if all major species (CO2,CO, N2, and O) are present. The data spacing is nominally 12seconds except for the -12, 0, 12 time tags. Although timetags from -1800 to 1800 seconds are generated, only those datarecords for which at least one species has a valid value forthat time tag are output. No spacecraft positional parameters have been includedin the data sets. These can be obtained from the SEDR datasubmitted separately by the Project.
DATA_SET_RELEASE_DATE 1993-03-31T00:00:00.000Z
START_TIME 1978-12-07T02:30:47.000Z
STOP_TIME 1992-10-07T07:49:39.000Z
MISSION_NAME PIONEER VENUS
MISSION_START_DATE 1968-06-01T12:00:00.000Z
MISSION_STOP_DATE 1992-10-07T12:00:00.000Z
TARGET_NAME VENUS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID PVO
INSTRUMENT_NAME ORBITER NEUTRAL MASS SPECTROMETER
INSTRUMENT_ID ONMS
INSTRUMENT_TYPE QUADRUPOLE MASS SPECTROMETER
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
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.
CITATION_DESCRIPTION TBD
ABSTRACT_TEXT 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).
PRODUCER_FULL_NAME DR. WAYNE KASPRZAK
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