Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle PVO ONMS Ion Max Count Rate Data Collection Description PDS3_DATA_SET_ID = PVO-V-ONMS-4-IONMAXCOUNTRATE-12SEC-V1.0 START_TIME = 1978-12-05T15:06 STOP_TIME = 1992-10-07T19:50 PDS3_DATA_SET_RELEASE_DATE = 1993-03-31 PRODUCER_FULL_NAME = Dr. Wayne Kasprzak Collection Overview =================== This data collection 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+. Parameters ========== Sampling Parameter Name : Time Sampling Parameter Resolution : 12.0 seconds Minimum Sampling Parameter : N/A Maximum Sampling Parameter : N/A Sampling Parameter Interval : 12.0 seconds Minimum Available Sampling Interval : 12.0 seconds Data Collection Parameter Name : Ion Rate Noise Level : Unknown Data Collection Parameter Unit : particles/second This data collection 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. Processing ========== 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 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. PVONMS ------ Software Release Date : 1993-03-23 Cognizant Full Name : Dr. Wayne T. Kasprzak All data were processed at NASA/Goddard Space Flight Center using custom programmed software. 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. 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. NOTE: Software is available by contacting the PPI Node Data ==== -------------------------------------------------------- Table 1. Data fields -------------------------------------------------------- Variable Comment -------------------------------------------------------- YEAR YY=2 digit year (e.g. 78 for 1978) DOY Day of Year 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 from 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) 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. 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). References ========== Colin, L., 'Pioneer Venus Overview', IEEE Transactions on Geoscience and Remote Sensing, January 1980, Vol. GE-18, No. 1, p5-10 Fimmel, R.O., Colin, L., and Burgess, E., 'Pioneering Venus: A Planet Unveiled', NASA SP-518, 1995. Guenther, Y.P., 'Pioneer Venus Neutral Mass Spectrometer,' NASA/Goddard Space Flight Center Summer Institute on Atmospheric Science, Laboratory for Planetary Atmospheres, Code 910, 1989. 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, 1980b. Niemann, H.B. and W.T. Kasprzak, Comparative Neutral Composition Instrumentation and New Results, Advances in Space Research, vol. 2, 261-270, 1983. Nothwang, G. J., 'Pioneer Venus Spacecraft Design and Operation,'IEEE Trans. on Geosci. and Remote Sensing, vol. GE-18, no. 1 pp 5-10, January, 1980.