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 set 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 DATE YYDDD YY:2 digit year (e.g. 78 for 1978) 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) 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 (LORES data set); 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 sets 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.

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.