Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle
PVO ONMS Superthermal Ion Location Data Collection Description
PDS3_DATA_SET_ID = PVO-V-ONMS-5-SUPERTHERMALIONLOC-V1.0
START_TIME = 1978-12-05T15:06:25
STOP_TIME = 1992-10-07T19:50:29
PDS3_ DATA_SET_RELEASE_DATE = 1996-09-01
PRODUCER_FULL_NAME = Dr. Wayne Kasprzak
Collection Overview
===================
This data collection contains start and stop times/locations where superthermal ions were
detected by the Neutral Mass Spectrometer instrument aboard the Pioneer Venus Orbiter. The
instrument detected superthermal, energetic or fast ions whose energy exceeds 36 eV in the
spacecraft frame of reference. These ions were observed in early orbits during measurements of
the neutral density near periapsis, have an erratic and unpredictable signature, and occur at
too high an altitude to be due to the neutral atmosphere. When the altitude of periapsis
increased above the point where neutral density measurements could be made, the instrument was
configured specifically to detect superthermal ions. In general, for orbit numbers 1 to 645,
data were taken from the RPA mode. The gas background signal with the filament on is about a
factor of 10 less in this mode than in non-RPA mode, resulting in a lower detection threshold.
For orbit numbers above 923, the instrument was deliberately configured with the filament off
and non-RPA mode data was used. For mass 16 the RPA voltage is about +3.8 volts.
Data Collection Description =
orbits time
1 1978-12-05T15:06:25.000Z
645 1980-09-10T17:30:20.000Z
4710 1991-12-29T00:40:49.000Z
5055 1992-10-07T19:50:29.000Z
The data reduction process has been described in Kasprzak et al. (1987). The method used to
reduce the data assumes cylindrical symmetry of the ion source. In actual fact, the source 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 asymmetry since
the actual ion drift vector is unknown. The minimum energy of an ion detectable by the ONMS in
this ion mode is 35.9 eV. The maximum transmission is assumed to occur about 10 V above this
value. On the nightside of Venus the spacecraft potential is negative and the most probable
ion energy is near 40 eV.
The ion species regularly monitored include: He+, N+, O+, N+ and/or CO+ and CO2+. Because of
the paucity of data at other mass numbers only mass 16 (atomic oxygen) has been reduced to a
flux and number density. As part of the reduction process the angle in the ecliptic plane of
the apparent ion flow in spacecraft reference frame has been deduced. The flux values are
estimated in the spacecraft reference frame relative to spacecraft ground. The density is
computed from the flux by dividing it by a speed corresponding to 40 eV. No correction has been
applied to the angle, density or flux in order to remove the effect of spacecraft velocity.
The data fields are:
VARIABLE COMMENT
--------------------------------------------------------
ORBIT PVO Orbit number
MASS Mass number of species
4 He+
12 C+
14 N+
16 O+
28 CO+ and/or N2+
30 NO+
32 O2+
44 CO2+
BDATE Beginning Date - YY = last two digits of year DDD = day of year -|
BTIME Beginning Time - HH:MM:SS - HH = hour |
MM = minutes |
SS = seconds. |
BTPER Beginning time from periapsis (sec) |
BALTIT Beginning altitude in (km) |-- START
BSZA Beginning Solar zenith angle (deg) |
BLST Beginning Local solar time (hr) |
EDATE Ending Date - YY = last two digits of year DDD = day of year. -|
ETIME Ending Time - HH:MM:SS - HH = hour -|
MM = minutes |
SS = seconds. |
ETPER Ending time from periapsis (sec) |
EALTIT Ending altitude in (km) |-- FINISH
ESZA Ending solar zenith angle (deg) |
ELST Ending local solar time (hr) -|
Confidence Level Overview
=========================
In order to fit the data, a minimum of 30 points were required in 36 seconds. In addition, the
maximum to minimum count ratio was required to be factor of 3 or greater in order to insure
that there was a definitive spin modulation. The center 12 seconds of data is divided by the
fitting function to derive the equivalent flux for that point. The center of the new fitting
interval is adjusted so that it is centered on the expected signal maximum predicted from the
previous interval fit. As a result of this method of fitting, discontinuities may exist near
minimum angle of attack where one 12 second interval adjoins the next interval.
See Kasprzak et al. (1987).
Kasprzak, W.T., H.B. Niemann and P. Mahaffy, Observations of Energetic Ions on the Nightside
of Venus, Journal of Geophysical Research, vol. 32, 291-298, 1987.
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., L. Colin, and E. Burgess, '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., H.B. Niemann and P. Mahaffy, Observations of Energetic Ions on the
Nightside of Venus, Journal of Geophysical Research, vol. 32, 291-298, 1987.
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.
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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