PVO-V-ONMS-4-IONMAXCOUNTRATE-12SEC-V1.0
PVO VENUS ONMS BROWSE ION MAX. COUNT RATE 12 SECOND V1.0
START_TIME = 1978-12-05T15:06:31.820Z
STOP_TIME = 1992-10-07T19:50:28.680Z
DATA_OBJECT_TYPE = "TIME SERIES"
PROCESSING_LEVEL_ID = 4
PRODUCER_FULL_NAME = "DR. WAYNE KASPRZAK"
PRODUCER_INSTITUTION_NAME = "GODDARD SPACE FLIGHT CENTER"
orbits time
1 1978-12-05T15:06:31.820Z
5055 1992-10-07T19:50:28.680Z
This data set represents the maximum count rate per second in a
12 second period beginning with the time of the first data point for
a given mass number. Although species other than O+ cannot be reduced to
to a flux and direction it is possible to estimate the approximate
flux from the maximum count rate per second occurring within one
spin period (about 12 seconds). The maximum count rate can be
approximately converted to a flux using:
[4.E7 (particles/cm**2/sec)]/[1.E4 (counts/sec)].
This sensitivity is for O+. At higher mass numbers the sensitivity is
less, being about a factor of 2.5 lower for Ar+.
Background
The instrument has 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 sensible 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.
The superthermal ion species regularly monitored include: He+, N+, O+,
(N+ + 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 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 dataset fields are:
VARIABLE COMMENT
--------------------------------------------------------
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 number
PSEC Time after periapsis (sec)
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+
PULSE Maximum count rate/sec in 12 second interval
VALT Altitude (km)
VLAT Latitude (deg N)
VLST Local solar time (hr)
VSZA Solar zenith angle (deg)
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.
CONFIDENCE LEVEL NOTE
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).
SAMPLING PARAMETER
This data set represents the maximum count rate per second in a
12 second period beginning with the time of the first data point for
a given mass number.
REFERENCES
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.
Niemann, H.B. and W.T. Kasprzak, Comparative Neutral Composition
Instrumentation and New Results, Advances in Space Research, vol. 2,
261-270, 1983.