PVO-V-ONMS-5-SUPERTHERMALIONLOC-V1.0 PVO VENUS ONMS DERIVED SUPERTHERMAL ION LOCATION V1.0 START_TIME = 1978-12-05T15:06:25.000Z STOP_TIME = 1992-10-07T19:50:29.000Z DATA_OBJECT_TYPE = "TIME SERIES" PROCESSING_LEVEL_ID = 5 PRODUCER_FULL_NAME = "DR. WAYNE KASPRZAK" PRODUCER_INSTITUTION_NAME = "GODDARD SPACE FLIGHT CENTER" 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 DATA SET DESCRIPTION This dataset 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. 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 dataset fields are: Name Comment ------ ------------------------------------------------- ORBIT Orbit number MS Mass number of species 4 He+ 12 C+ 14 N+ 16 O+ 28 CO+ and/or N2+ 30 NO+ 32 O2+ 44 CO2+ YYDDD YY = last two digits of year -| DDD = day of year | HH:MM:SS HH = hour | MM = minutes | SS = seconds | START PER TIM Time from periapsis (sec) | ALT Altitude in (km) | SZA Solar zenith angle (deg) | LST Local solar time (hr) -| YYDDD YY = last two digits of year -| DDD = day of year | HH:MM:SS HH = hour | MM = minutes | SS = seconds | FINISH PER TIM Time from periapsis (sec) | ALT Altitude in (km) | SZA Solar zenith angle (deg) | LST Local solar time (hr) -| 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. 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.