Data Set Information
DATA_SET_NAME PVO VENUS ONMS CALIBRATED SUPERTHERMAL OXYGEN HIGH RES. V1.0
DATA_SET_ID PVO-V-ONMS-3-SUPERTHRMLOXYGN-HIRES-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION The data reduction process has been described in Kasprzaket al. (1987). The method used to reduce the data assumescylindrical symmetry of the ion source. In actual fact, thesource is asymmetrical in its angular response (Guenther, 1989).This can introduce as much as a factor of 2 scatter in the data.No simple solution has been found for modeling this asymmetrysince the actual ion drift vector is unknown. The minimum energyof an ion detectable by the ONMS in this ion mode is 35.9 eV.The maximum transmission is assumed to occur about 10 V abovethis value. On the nightside of Venus the spacecraft potentialis negative and the most probable ion energy is near 40 eV.The ion species regularly monitored include: He+, N+, O+,(N+ + CO+), and CO2+. Because of the paucity of data at othermass numbers only mass 16 (atomic oxygen) has been reduced to aflux and number density. As part of the reduction process theangle in the ecliptic plane of the apparent ion flow in spacecraftreference frame has been deduced. The flux values are estimatedin the spacecraft reference frame relative to spacecraft ground.The density is computed from the flux by dividing it by a speedcorresponding to 40 eV. No correction has been applied to theangle, density or flux in order to remove the effect of spacecraftvelocity. Several parameters result from the fit: 1) the best estimateof the flux for the interval (used to generate the low resolution(LORES) data set); 2) the phase shift of signal maximum withrespect to that predicted by the position of the velocity vectorand its error; 3) the fitting parameter B (Kasprzak et al., 1987);and 4) the effective angle of attack. Other items can be derivedfrom this data: 1) the apparent direction of the ion flow projectedinto the ecliptic plane; and 2) one component of the ion driftperpendicular to the plane of axis of the ONMS and the spin axis.The phase angle is negative if the predicted signal maximum fromthe spacecraft velocity is ahead of the true signal maximum whenviewed along the -Z spacecraft axis with clockwise rotation. Thedrift component is derived from the condition that the totalrelative velocity in the moving reference frame has no componentperpendicular to the (ONMS axis, Z axis) plane.The dataset fields are:YEAR YY=2 digit year (e.g. 78 for 1978)DOY DDD=3 digit day of year (e.g. 053)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 numberMS Mass number - 16 for OF Flag - D single data point flux, density A fitted parameters for intervalDENSITY Effective number density assuming a 40 eV ion (particles/cm{3})FLUX Flux (particles/cm{2}/s)AZ ANG Azimuth angle of apparent ion flow direction projected into the ecliptic plane (deg)PHASE Phase shift of signal maximum with respect to that predicted by velocity vector (deg)ERROR Error in phase shift (deg)ANGATK Effective angle of attack (deg)WP DIR Direction of drift component perpendicular to (ONMS axis, Z axis) projected into the ecliptic plane (deg)WP XY Magnitude of drift component (m/s)B Fitting parameterKasprzak, W.T., H.B. Niemann and P. Mahaffy, Observations ofEnergetic Ions on the Nightside of Venus, Journal of GeophysicalResearch, vol. 32, 291-298, 1987.
DATA_SET_RELEASE_DATE 1993-03-31T00:00:00.000Z
START_TIME 1978-12-05T03:07:07.000Z
STOP_TIME 1992-07-20T12:22:49.000Z
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 In order to fit the data a minimum of 30 points were requiredin 36 seconds. In addition, the maximum to minimum count ratio wasrequired to be factor of 3 or greater in order to insure that therewas a definitive spin modulation. The center 12 seconds of data isdivided by the fitting function to derive the equivalent flux forthat point. The center of the new fitting interval is adjusted sothat it is centered on the expected signal maximum predicted fromthe previous interval fit. As a result of this method of fitting,discontinuities may exist near minimum angle of attack where one12 second interval adjoins the next interval.See Kasprzak et al. (1987).Kasprzak, W.T., H.B. Niemann and P. Mahaffy, Observations ofEnergetic Ions on the Nightside of Venus, Journal of GeophysicalResearch, vol. 32, 291-298, 1987.
CITATION_DESCRIPTION Kasprzak, W., PVO-V-ONMS-3-SUPERTHRMLOXYGN-HIRES-V1.0, PVO VENUS ONMS CALIBRATED SUPERTHERMAL OXYGEN HIGH RES. V1.0, NASA Planetary Data System, 1993.
ABSTRACT_TEXT This dataset has one point for each point measured, that wentinto a successful data fitting for that time interval. The individualflux and density values are computed by dividing each data value bythe value of the fitting function at the corresponding time. The instrument has detected superthermal, energeticor fast ions whose energy exceeds 36 eV in the spacecraftframe of reference. These ions were observed in early orbitsduring measurements of the neutral density near periapsis, havean erratic and unpredictable signature, and occur at too high analtitude to be due to the neutral atmosphere. When the altitudeof periapsis increased above the point where sensible neutraldensity measurements could be made, the instrument was configuredspecifically to detect superthermal ions. In general, for orbitnumbers 1 to 645, data were taken from the RPA mode. The gasbackground signal with the filament on is about a factor of 10less in this mode than in non-RPA mode, resulting in a lowerdetection threshold. For orbit numbers above 923, the instrumentwas deliberately configured with the filament off and non-RPAmode data was used. For mass 16 the RPA voltage is about +3.8volts.
PRODUCER_FULL_NAME DR. WAYNE KASPRZAK
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