Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle
PVO ONMS Thermal Ion 12 Second Data Collection Description
PDS3_DATA_SET_ID = PVO-V-ONMS-4-THERMALION-12SEC-V1.0
START_TIME = 1980-03-15T10:26:29
STOP_TIME = 1992-09-25T23:09:39
PDS3_DATA_SET_RELEASE_DATE = 1993-03-31
PRODUCER_FULL_NAME = Dr. Wayne Kasprzak
Collection Overview
===================
Thermal ions can be measured with the filament off and the ion repeller set at 0 V. Species
observed include He+, N+, O+, CO+ and/or N2+, NO+, O2+ and CO2+. H+ is not measurable with the
current instrument configuration. One component of the ion drift in the ecliptic plane can also
be determined. Thermal ion measurements have been taken sporadically at the end of neutral
density passes and on alternate orbits when superthermal ions are not being measured.
The ONMS instrument was not operated on all orbits and some orbits are devoted to engineering
studies. Typically neutral density passes occupied -40 min. to +30 min. relative to the time
of periapsis. Ion and superthermal ion mode passes typically are 15 to 20 minutes in duration
on either side of periapsis. Neutral density passes during entry also took about this same
amount of time.
The data values of the data colletion are sampled approximately once per 12 seconds based on
GMT times that have been supplied by the Pioneer Venus Project. Each representative data point
is constructed using an exponentially weighted average of the data over a 24 second interval
centered at sample point time.
The data have the following characteristics:
Logical Record Size: 80 bytes
Format: ASCII
Files: 1
The field names used in Record 1:
Variable Comment
YEAR YYYY=4 digit year (e.g. 1978)
DOY DDD=3 digit day of year (e.g. 053)
UT Universal Time represented as the number of
milliseconds since 00:00:00.000000 of the current day.
ORBIT Orbit number
PSEC Time after periapsis (sec)
DENS Density in particles/cm**3
WPXY Minimum ion drift m/sec
PHSE Phase shift (degrees)
MASS Mass number -
4 for He+
12 for C+
14 for N+
16 for O+
28 for N2+ and/or CO+
30 for NO+
32 for O2+
44 for CO2+
VALT Altitude above the mean surface of
Venus in km
VLAT Venus latitude in degrees
VLST Venus local solar time in hr
VSZA Venus solar zenith angle in degrees
The relationship between thermal ion density and instrument output was established by direct
comparison of the O+ signal with the O+ density determined from the Orbiter Ion Mass
Spectrometer (OIMS) instrument using O+ data from orbit number 530 at 300 seconds from
periapsis. Other species are assumed to have the same sensitivity as that of O+. In this mode
superthermal ions cannot be distinguished from thermal ions.
The data reduction process has been described in Kasprzak et al. (1992) but was similar to the
technique used for the Superthermal O+ ion data. In order to fit the superthermal O+ 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.
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. The ion species regularly monitored include: He+,
N+, O+, N2+ and/or CO+, NO+ and CO2+. As part of the reduction process the minimum ion drift in
the ecliptic plane of the apparent ion flow in spacecraft reference frame has been deduced. The
density is computed by assuming the ions are thermal energy with a speed equivalent to that of
the spacecraft An approximate correction for spacecraft potential has been applied. The data
are normalized to the OIMS instrument for O+ and all species are assumed to have the same
sensitivity.
Several parameters result from the fit: 1) the best estimate of the density for an approximate
12 second interval (low-res data collection); and 2) the phase shift of signal maximum; and 3)
the minimum ion drift speed in the ecliptic plane. The phase angle is negative if the predicted
signal maximum from the spacecraft velocity is ahead of the true signal maximum when viewed
along the -Z spacecraft axis with clockwise rotation. The drift component is derived from the
condition that the total relative velocity in the moving reference frame has no component
perpendicular to the (ONMS axis, Z axis) plane.
All data were processed at NASA/Goddard Space Flight Center using custom programmed software.
The software is available. The data represent a reduction to physical units (density, flux)
and were processed from an intermediate engineering unit file (current, count/sec etc.). Unit
and 1/8 unit amu sweeps are not contained in the processed data collection but are available
from the engineering unit data set. The engineering unit data is converted to ambient values
using spacecraft velocity and attitude, the theoretical expected system response, and the
corresponding calibration factors. Superthermal ion data for species other than O+ is available
in engineering unit form.
Kasprzak, W.T. and H.B. Niemann, Evidence for Enhanced Dynamic Flow in Ionospheric Holes from
the Pioneer Venus Neutral Mass Planetary Space Sciences, 40, 33-45,1992.
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. (1992).
Kasprzak, W.T. and H.B. Niemann, Evidence for Enhanced Dynamic Flow in Ionospheric Holes from
the Pioneer Venus Neutral Mass Planetary Space Sciences, 40, 33-45,1992.
References
==========
Brace, L.H., W.T. Kasprzak, H.A. Taylor, R.F. Theis, C.T. Russell, A. Barnes, J.D.
Mihalov and D.M. Hunten, The Ionotail of Venus: Its Configuration and Evidence for Ion
Escape, J. of Geophys. Res, vol 92, 15-26, 1987.
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. and H.B. Niemann, Evidence for Enhanced Dynamic Flow in Ionospheric Holes
from the Pioneer Venus Neutral Mass Planetary Space Sciences, 40, 33-45,1992.
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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