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.
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