Pioneer Venus Orbiter (PVO) Neutral Mass Spectrometer (ONMS) Data Bundle PVO ONMS Superthermal Oxygen 12 Second Data Collection Description PDS3_DATA_SET_ID = PVO-V-ONMS-4-SUPERTHRMLOXYGN-12SEC-V1.0 START_TIME = 1978-12-05T15:07:22 STOP_TIME = 1992-07-20T00:22:15 PDS3_DATA_SET_RELEASE_DATE = 1993-03-31 PRODUCER_FULL_NAME = Dr. Wayne Kasprzak Collection Overview =================== 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 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+ + 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. Several parameters result from the fit: 1) the best estimate of the flux for the interval (used to generate the low resolution (low-res) data collection; 2) the phase shift of signal maximum with respect to that predicted by the position of the velocity vector and its error; 3) the fitting parameter B (Kasprzak et al., 1987); and 4) the effective angle of attack. Other items can be derived from this data: 1) the apparent direction of the ion flow projected into the ecliptic plane; and 2) one component of the ion drift perpendicular to the plane of axis of the ONMS and the spin axis. 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. The data values of the LORES data collection 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 fields are: VARIABLE COMMENT -------------------------------------------------------- YEAR YYYY=4 digit year 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 Seconds after periapsis DO+ Density of superthermal atomic oxygen in cm**-3 FO+ Flux of superthermal atomic oxygen in (cm**-2)(s**-1) FANG Apparent angle, in degrees, of the ion flow in the ecliptic plane measured with respect to the sun 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 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). 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.