09-SEP-93 WTK Planetary Data System Information for Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) 2.1 General Information Spacecraft: Pioneer Venus Orbiter (Pioneer 12) Instrument Name: Orbiter Neutral Mass Spectrometer (ONMS) Instrument Type: quadrupole mass spectrometer PI: H.B. Niemann, Goddard Space Flight Center Date Built: 1977 Mass: 3.8 kg including 0.4 kg for ejected break off hat assembly Size: length 35 cm, width 15 cm, height 28 cm Power: filament on 12.6 W average, 11.2-14.8 W range for mass range 0-46 amu filament off 10.6 W average, 9.2-12.8 W range for mass range 0-46 amu Manufacturer: Goddard Space Flight Center Serial Number: flight unit Temperature range: -20 C to 50 C Spacecraft 28 V bus voltage range: 25-31 V 2.2 Instrument Description The ONMS instrument is a quadrupole mass spectrometer with an electron impact ion source for measurement of neutral gas composition in the mass range 1 to 46 amu. The sensor consists of an ion source, a quadrupole mass filter and secondary electron multiplier as an ion detector. The ion source is partially enclosed and exposed to the ambient atmosphere of neutral and ion particles through an entrance aperture. Just inside the aperture is the ion repeller grid at 36 V above spacecraft ground designed to reject ions of this energy or less. On either side of the ion repeller grid is a grid at -4.3 V designed to reject low energy electrons. Neutral gas particles are not influenced by these grid potentials. They pass through to the ionization region where a small fraction are ionized by electron impact from an electrostatically focused electron beam generated by a hot filament. The ions are then focused into the quadrupole analyzer for separation according to mass to charge ratio. The ion source grid assembly can also function as a retarding potential analyzer(RPA) for analysis of direct streaming particles that have not had any surface collisions. With the filament off and the ion repeller set to 0 V, the sensor can detect thermal ions. With the ion repeller set to 36 V, superthermal ions exceeding this energy can be detected. There are two selectable electron energies which can be used to identify neutral species based on the mass spectral cracking patterns. The ionization and dissociation cross sections are a function of electron energy. Mass spectra are simplified at lower electron energy but with a considerable loss in ionization efficiency. The ion source enclosure is such that it can function as a closed or an open source. The neutral particle density in the ionization region consists of direct streaming particles and particles which have been reflected from the surfaces. As a closed source, the neutral particle flux into the enclosure is balanced by the thermalized flux exiting the enclosure. The thermalized flux results from particles colliding many times with the ion source surfaces. This increased ram pressure results in a significant increase (about 60 for N2) over the ambient pressure of the species and lowers the detection threshold. Species such as O and N chemically react on the surfaces forming O2 and NO. The open source measures direct streaming particles that have not had surface collisions. The view cone for the open source in the ionization region has a half angle of about 38 degrees. The detection threshold for this mode is higher than that for the closed source since the ambient density is directly detected. A retarding potential field is used to discriminate between direct streaming particles at spacecraft energy and surface reflected particles at a much lower energy. A radio frequency electromagnetic field is used to select a given mass to charge ratio ion. Mass peak (mass/charge ratio) selection is accomplished through a proper combination of the AC and DC voltages applied to the hyperbolic rods. Resolution is determined by the ratio of the AC to DC amplitude. The peaks produced are flat topped and stepping from one mass peak to the other can be accomplished without requiring peak searching. A secondary electron multiplier is used as a detector operating either in a pulse counting mode or a current measuring mode. Either value can be output to the telemetry stream. Normally the pulse counter values are output as long as the electrometer current does not exceed a given value. The sensor itself is constructed of stainless steel and was sealed under vacuum prior to delivery for launch. A small ion getter pump was used to maintain the vacuum below 1.E-4 pascal. The ion source was covered by a metal-ceramic break-off cap which was removed by a pyrotechnic actuator after orbit insertion. The basic instrument data consists of a mass number being sampled, pulse counter or electrometer data, RPA mode type, and housekeeping data. Proper operation of the instrument is verified through its housekeeping data. Data accumulation is always done at equally spaced time intervals. Each 16 bit telemetry represents one reading obtained over an integration time determined by the spacecraft bit rate and format. The individual 8 bit telemetry words assigned to the ONMS in a minor frame are not equally spaced in time. Therefore, the data has to be stored internally for a period of time and then sent to the telemetry. The storage and readout are accomplished through the use of two memories. One memory is reading out unequally spaced data to the spacecraft telemetry system while the other is accumulating equally spaced data from the instrument. Both memories are first in, first out (FIFO) type and are switched simultaneously. The time interval over which the ONMS data is accumulated in memory depends on the spacecraft bit rate, format, and spin segment status. With no spin segment, the memories are switched on at a minor frame beginning, data is accumulated for one minor frame and then read out to the telemetry system during the next minor frame. With spin segment mode commanded, the data is accumulated at four times the rate of the no spin segment mode and at equally spaced time intervals from -45 degrees to +45 degrees roll angle relative to the ONMS velocity ram direction. The memory switch time and data accumulation start time are determined from the spacecraft RAM pulse, which is assumed to represent the maximum velocity ram for the spacecraft +X axis (0 degrees spacecraft azimuth angle), and the ONMS azimuth angle. The ONMS has telemetry words in the formats PERC (6 words), PERB (14 words), and APOB (2 words). Each word consists of 8 bits. The length of the data cycle for housekeeping data is 128 words. The unit of 64 16-bit words contains all of the information to determine the current command status, monitor values, etc., and is the basic unit of processing for the data. In addition to the internal monitors read out by the instrument electronics, other monitors are available from the spacecraft telemetry which do not require the ONMS to be operating: instrument temperature, bus voltage and instrument on/off status. A summary of the instrument parameters are as follows: Ion source: closed/open with particle retarding deployment by metal-ceramic break off cap, pyrotechnically activated electron impact ionization dual filaments, 20 ua emission electron energy 27 eV or 70 eV Analyzer: quadrupole mass filter, hyperbolic rods 7.5 cm long, field radius 0.2 cm AC frequency 5.6 mhz Detector: secondary electron multiplier, copper-beryllium, box and grid design pulse counting up to about 852000 counts/sec (0.160 ua current) current measurement from 0.160 ua to 15 ua minimum detectable signal is 1 count per integration period multiplier noise signal < 1 count/minute Total detector dynamic range: 1.3E7 Resolution/crosstalk: < 1.E-4 Mass range: 1 to 46 amu Mass measurement modes: programmed mass, 8 individual mass numbers, 1 to 46 amu unit sweep, 1 to 46 in steps of 1 amu 1/8 unit sweep, 1 to 46 in steps of 0.125 amu Sample rates: a) Normal mode equally time spaced samples for all spacecraft spin angles 6 samples/sec with nominal bit rate 1024 bits/sec and PERC format actual sample rate dependent on spacecraft telemetry format and bit rate used b) Spin segment mode equally time spaced samples for 45 degrees with respect to occurrence of velocity ram effective data rate 4 times the normal rate Number of words in telemetry format per minor frame: PERB: 14 8-bit words PERC: 6 8-bit words APOB: 2 8-bit words 1 minor frame = 64*8 bits Integration time: 0.171875 s maximum, 0.006 s minimum dependent upon telemetry bit rate and format The spacecraft orbit is nearly polar (105.6 degrees inclination) with periapsis near the equator (17 degrees north celestial latitude for the first 645 orbits; 10 degrees south celestial latitude for entry orbits 4954-5055) and has an average period of about 24 hours. The local time of periapsis increases 1.6 degrees/day (or degrees/orbit) so that it takes 224.7 days to sample one complete diurnal cycle (dayside, evening terminator, nightside, and morning terminator). For the first 600 orbits, the altitude of periapsis varied from 142 km to 250 km. After this period, the periapsis altitude was no longer controlled and increased in altitude up to about 2200 km and decreased as a result of solar gravitational perturbations. The last data taken by the spacecraft and ONMS was on orbit 5055 (07-Oct-92) at 128.8 km. The spacecraft spins with a nominal period of 12-13 seconds about an axis which points approximately toward the south ecliptic pole. 2.3 Science Objectives The prime mission of the Orbiter Neutral Mass Spectrometer (ONMS) is to perform in-situ measurements of the neutral gas composition and its variation with altitude and local solar time in the thermosphere and exosphere of Venus. Measurements of these variations are important in defining the dynamical, chemical and thermal state of the upper atmosphere. When the periapsis altitude is below about 250 km, the neutral densities of helium, atomic oxygen, atomic nitrogen, molecular nitrogen, carbon monoxide and carbon dioxide are measured. Gas kinetic temperatures can be derived from an analysis of the density scale heights. Wave-like perturbations consistent with gravity waves can be observed after removal of the altitude variation in the data. Neutral density data were taken during orbit 1-645 and during orbits 4954-5055. Superthermal ions with energy > 36 eV in the spacecraft reference have also been detected. Ions of this energy have sufficient energy to escape the planet and represent an atmospheric loss. They have been observed in the near tail region and near the dayside ionopause. The ions were first detected in orbits associated with measurements of the neutral density and are evidenced as erratic signals above the usual gas background signal at high altitudes. The ions must have an energy exceeding the voltage on the ion repeller grid (36 V) in order to be detected. The ion composition can be determined and consists of mainly O+ with traces of He+, N+, CO+ and/or N2+, NO+ and O2+. CO2+ occurs very rarely. H+ is not measurable with the current instrument configuration. After the first 935 orbits, when the altitude of periapsis was above 300 km and the neutral atmosphere could no longer be measured, the instrument was configured to perform measurements of the superthermal ions with the filament off and ion repeller at 36 V. The direction of the ion flow in the ecliptic plane can also be determined from the spin modulation of the data. Thermal ions can also be measured with the filament off and the ion repeller set at 0 V. Species observed include He+, N+, O+, CO+ and/or N2+, NO+, O2+ and CO2+. H+ is not measurable with the current instrument configuration. One component of the ion drift in the ecliptic plane can also be determined. Thermal ion measurements have been taken sporadically at the end of neutral density passes and on alternate orbits when superthermal ions are not being measured. 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. 2.4 Instrument Calibration Description Initial testing of the retarding and ion modes was done using a low energy, 0-25 eV, ion beam. Gas calibrations to establish the closed source neutral density were performed over the pressure ranges expected in flight. This calibration established the overall relation between the thermalized particle density in the ionizing region and the electronics telemetry output. Gases used for calibration were CO2, N2, O2, Ar, He, CO and NO. The calibrations established mass spectral cracking patterns, low and high electron energy sensitivities, pulse counter dead time correction, ratio of electron multiplier count rate and current output, tuning and mass resolution characteristics. Comparison of the ONMS neutral densities with mass density measurements near 100 km deduced from the Pioneer Venus lower atmosphere probes, the bus mass spectrometer measurements and Orbiter drag indicate that the overall sensitivity is about a factor of 1.6 low (Hedin et al.,1983). Estimates of the gas flow into the ion source at satellite speeds indicate that the sensitivity may deviate from that assumed in the data reduction. Laboratory tests of the prototype instrument in a molecular beam system produced results consistent with this hypothesis but not conclusive because of the limited range of speeds that could be obtained. The relationship between the superthermal ion flux and instrument output was established in a post-flight calibration of the flight backup unit. In the energy range 40-200 V, the maximum transmission occurs 10 V above the ion repeller potential and drops to 15% of the maximum transmission value above 100 eV. The transmission decreases with increasing mass number and for Ar+ ions (m/e=40) it is about a factor of 2.5 below that for O+ at the maximum transmission point. 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. Stability of the electronics and sensor combination as a function of temperature was established in tests using a vacuum chamber with temperature control. 2.5 Operational Considerations Instrument commanding: cannot directly command filament A to B; must turn-off filament between A and B cannot use highest multiplier voltage; current levels too high cannot use spin segment mode in PERB format and bit rates exceeding 1024 bps; 4K memory overflow; cannot use it when less than one housekeeping cycle is stored in memory Maximum signal limitations: filament is shut off if detector multiplier current exceeds 15 ua 2.6 Instrument Mounting Mounting angles for the normal perpendicular to instrument orifice: Azimuth angle relative to spacecraft X-axis: 321.17 degrees Zenith angle relative to spacecraft Z-axis: 26.6 degrees 2.7 Instrument Mode Descriptions Currently there are three distinct types of data that can be acquired by the ONMS instrument: neutral density, superthermal ion flux and thermal ion density. Independent of these three data types, there are a number of modes in which the data can be taken as well as a number of modes in which the instrument can be configured and tuned. Generally speaking these different modes correspond to different command configurations of the instrument. Data measurement types: Neutral gas composition: closed source open source with particle retarding Ion composition: superthermal ions > 36 eV relative to spacecraft ground (ion repeller set to 36 V) normal ions > 0 V relative to spacecraft ground (ion repeller set to 0 V) Detection ranges(1): Neutral composition: N2 sensitivity: 5.E4 (particles/cm**3)/(count/sec) Open source density range: 3.E5 to 4.E12 particles/cm**3 for N2 Closed source density range: 5.E3 to 7.E10 particles/cm**3 for N2 Superthermal (>36 eV) ions: O+ sensitivity: 4.E3 (particles/cm**2/sec)/(count/sec) O+ flux range for 40 eV ions: 2.E4 to 3.E11 particles/cm**2/sec O+ density range for 40 eV ions: 0.01 to 1.E5 particles/cm**3 Thermal ions: O+ sensitivity: 0.02 (particles/cm**3)/(counts/sec), normalized to OIMS O+ density range: 0.02 to 2.6E5 particles/cm**3 for a spacecraft speed of 9.6 km/s (1) Based on minimum count rate, detector dynamic range and instrument sensitivity for mode used The instrument configuration modes: a) Ion source: Two different electron energies can be selected by command. Ions can also be measured with the filament off. An ion repeller grid just inside the entrance aperture at 36 V or 0 V rejects positive ions of this energy or less. Neutral gas particles are not affected by the 36 V grid potential and pass through into the ionization region. b) Retarding potential analyzer: Retarding and non-retarding modes can be commanded separately along with a mode in which they alternate. A retarding potential sweep through a range of retarding voltages is also commandable and is considered an engineering diagnostic tool. The retarding voltage is VR=O+G*M where the offset O and and the gain G each have 4 separate commandable levels, and M is the mass number in amu. The retarding sweep voltage is VR=O+0.0103*(64-S) where D is a fixed value and S is number 1...64. For reference, a spacecraft speed of 9.8 km/s corresponds to an energy of 8 eV. c) Quadrupole analyzer: Tuning on the peak and mass resolution are separately commandable with 4 different values. Mass peak desired can be programmed as 8 selectable mass numbers, a unit amu sweep, and 1/8 amu sweep. The latter is primarily an engineering tool to check tuning and resolution. The unit sweep is a species survey mode, and programmed mass mode is used to concentrate on high time resolution measurements of particular species. d) Secondary electron multiplier detector: There are 4 selectable gain values determined from four commandable high voltage levels. e) Pulse counter discriminator: There are 4 commandable discriminator levels used in counting the individual multiplier pulses. A commandable mode is available to alternately read out pulse counter and electrometer values for multiplier gain measurements. 2.8 Instrument Section Descriptions Not applicable 2.9 Instrument Detector Descriptions Not applicable 2.10 Instrument Filter Descriptions Not applicable 2.11 Instrument Electronics Descriptions See section 2.12 2.12 Instrument References Niemann, H.B., J.R. Booth, J.E. Cooley, R.E. Hartle, W.T. Kasprzak, N.W. Spencer, S.H. Way, D.M. Hunten, G.R.Carignan, Pioneer Venus Orbiter Neutral Mass Spectrometer Experiment, IEEE Trans. Geo. Rem. Sens., GE-18, 60-65. Niemann, H.B. and W.T. Kasprzak, Comparative Neutral Composition Instrumentation and New Results, Advances in Space Research,261-270, 1983. 3 Personnel Information PI: H.B. Niemnann Code 915 NASA/Goddard Space Flight Center Greenbelt, MD 20771 (301)-286-8706 Data set prepared by: W.T. Kasprzak Code 915.1 NASA/Goddard Space Flight Center Greenbelt, MD 20771 (301)-286-8253 SPAN ADDRESS: PACF::Kasprzak 4 Data Set Information There are data sets available from the ONMS for neutral composition, superthermal ions and thermal ions. As of this date, no known malfunction of the ONMS has been encountered. a) Neutral gas composition The instrument was designed to determine the composition of the neutral thermosphere/exosphere of Venus. The term composition includes both the type of neutral gases present and their quantitative amount. The measurements begin at the orbit's periapsis altitude and extend to a limiting altitude at which the ambient signal becomes comparable to the gas background and/or detector measurement threshold. The neutral composition includes helium, atomic nitrogen, atomic oxygen, molecular nitrogen, carbon monoxide and carbon dioxide. The data reduction has been described in Niemann et al. (1980a) and Kasprzak et al. (1980). The source of the data and their corrections are summarized below: SPECIES M/E USED COMMENTS He 4 N 30 Surface recombined N and O O 32 Surface recombined O to O2; corrected for CO+ fragmentation corrected for estimated surface recombination of O to CO2(1) N2, CO 14,28 m/e 14 corrected for NO, CO and CO2 fragmentation; m/e 28 corrected for CO2 fragmentation CO2 44 Corrected for surface recombination of O to CO2(1) (1) the correction is based on matching scale height temperatures of O and CO2. The data are from the non-retarding potential mode of the instrument. Data from the retarding mode are consistent with those obtained from the non-retarding mode and have not been included. The data set does not include the factor of 1.6 increase in density needed to maintain compatibility with other data sets as discussed by Hedin et al. (1983). Two data sets are provided: high resolution (HIRES), every point, composition; and low resolution (LORES), 12 second sampled, composition. The LORES data set represents the best estimated composition data and is derived from the HIRES data set. No spacecraft position parameters have been included in the data sets. These can be obtained from the SEDR data submitted separately by the Project. b) Superthermal ion flux data The instrument has also 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 and 4954-5055, 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+, N2+ 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. Several parameters result from the fit: 1) the best estimate of the flux for the interval (used to generate the low resolution (LORES) data set); 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. 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+. c) Relative thermal ion composition data The instrument has made measurements of thermal ions with the ion repeller set to 0 V. The data reduction process has been described in Kasprzak et al. (1992). 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 has for spacecraft potential. 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. 4.1 Data Set Physical Description The structure of the data corresponds to the structure defined by the Pioneer Venus Project for the low frequency data (UADS). The LORES data conforms to this specification. The HIRES data uses the same structure but with the absence of nominal time tags. The data are in increasing orbit number order and in increasing time order. a) Pioneer Venus Project UADS Data Format This section describes the format used for the submission of data to the National Space Sciences Data Center. The format was designed to be easily read by computers expected to be accessible to a requester. One major difference between this and the old (ca. 1978) LFD format is the use of text (ASCII) data formats, eliminating both binary and IBM floating point formats. The new submission format is self-defining in the sense that the first three records define the data parameters, value representations, and missing data (file) indicators. The first record defines the order in which the variables appear in the subsequent data records. The second record contains a FORTRAN-compatible format list describing the field sizes and representations of each data value in the order defined in record 1. This format may be used to decode all subsequent records. The third record defines a unique value associated with filler (missing) data for all variable fields. It is formatted according to the format used in record 2, and is immediately followed by the start of actual data records (record 4 and beyond). The overall specification requires that all data be coded into ASCII. The logical record length is fixed. The first three records are formatted as follows: Record 1: The format to be used is (I3,n(1X,A4)) where "n" is the number of data (*) items in each record. ---------------------------------------------------------------- | *4 ELTE ELNE MI VS (for OETP) | | ^ ^ ^ ^ ^ ^ ^ ^ | | 3 5 10 15 20 25 30 35 | ---------------------------------------------------------------- Example 1: The first record in each file. Note that new value types with new 4-character designations can be added as necessary. The date, time, orbit and time tag items (see record 2) are not included in the list, because they are common to all data records. Record 2: This record contains the format in which all succeeding records are written. The first 4 format items specify the date, time, orbit, and time tag, and appear in the same format. ---------------------------------------------------------------- | (I8,I9,I5,I6,4F9.2) | | ^ | | 1 | ---------------------------------------------------------------- Example 2: The second record in each file. (Appropriate for OETP) Record 3: This record contains zeroes for the first four fields (date, time, orbit, and time tag), and the fill value for each data value. This value is used by any program reading the data to identify fill data in subsequent input records. ---------------------------------------------------------------- | 0 0 0 0999999.99999999.99999999.99999999.99 | | ^ ^ ^ ^ ^ ^ ^ ^ | | 8 17 22 28 37 46 55 64 | ---------------------------------------------------------------- Example 3: The third record in each file. (Appropriate for OETP) Record 4 to : These records contain the date, time, orbit, and time tag for each time which has any non-fill data. ------------------------------------------------------------------ | 1981207 43527786 879 -1788 2345.67 78543.89999999.99 16.20 | |^ ^ ^ ^ ^ ^ ^ ^ ^ | | 8 17 22 28 37 46 55 64 | ------------------------------------------------------------------ Example 4: All records after the third in a file. (Appropriate for OETP). The date is coded as YEAR, DAY OF YEAR (1-366) with 19 included in the year. The time is in milliseconds of the day, orbit number is self-explanatory, and the time tag is the usual value ranging of the day from -1800 to 1800 in increments of 12. The project provided tape of SEDR information is the source of the official dates and times to be used. _______________________________________________________________________ b) Neutral Density LORES Data: The data has the following characteristics: LOGICAL RECORD SIZE: 100 bytes FORMAT: ASCII FILES: 1 Record 1 This record contains the variable field names for each record excluding the first 4 fields which are the YYYYDDD, UT(ms), orbit number and nominal time tag. It is read as: PARAMETER (NVAR=8) CHARACTER*4 NAMES(NVAR) READ(UNIT,'(I3,(1X,A4))') NV,(NAMES(I),I=1,NV) The field names described in RECORD 1: NAME DESCRIPTION UNITS YYYYDDD YYYY=4 digit year DDD=3 digit day of year UT Universal time (ms) ORBIT Orbit number TIMTAG Nominal time tag assigned by project DHE Number density of He part/cm**3 DN Number density of N part/cm**3 DO Number density of O part/cm**3 DN2 Number density of N2 part/cm**3 DCO Number density of CO part/cm**3 DCO2 Number density of CO2 part/cm**3 DRHO Total mass density g/cm**3 DTOT Total number density part/cm**3 Record 2 This record contains the FORTRAN format for the remaining records and includes the 4 leading fields and the 8 remaining fields. It is read as: PARAMETER (NBYT=100) BYTE FORMAT(NBYT) READ(UNIT,'(A1)') FORMAT Record 3 This record contains the fill values for the fields described. It is read as: INTEGER*4 F1,F2 INTEGER*2 F3,F4 REAL*4 F5,F6,F7,F8,F9,F10,F11,F12 READ(UNIT,FORMAT,END=?) F1,F2,F3,F4,F5,F6,F7,F8,F9, F10,F11,F12 Record 4... The main data records with one record per sample time point, maximum of 8 species per line: PARAMETER (NV=8) INTEGER*4 YYYYDDD,UT INTEGER*2 ORBIT,TIMTAG REAL*4 DATA(NV) READ(UNIT,FORMAT) YYYYDDD,UT,ORBIT,TIMTAG, (DATA(I),I=1,NV) _______________________________________________________________________ c) Superthermal O+ Flux LORES Data: The data have the following characteristics: LOGICAL RECORD SIZE: 78 bytes FORMAT: ASCII FILES: 1 The field names used in RECORD 1: VARIABLE COMMENT NAME DESCRIPTION UNITS YYYYDDD YYYY=4 digit year DDD=3 digit day of year UT Universal time (ms) ORBIT Orbit number PSEC Seconds after periapsis DO+ Density of superthermal atomic oxygen in particles/cm**3 FO+ Flux of superthermal atomic oxygen in particles/cm**2/sec 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 d) ONMS Neutral Density HIRES Data: The data have the following characteristics: LOGICAL RECORD SIZE: 60 bytes FORMAT: ASCII FILES: 1 Record 1: It is read as CHARACTER*60 DESC READ(UNIT,'(A)') DESC The fields used in record 1 are: YYYYDDD YYYY=4 digit year (e.g. 1979) DDD=3 digit day of year (e.g. 053) UT(ms) Universal time in ms ORBIT Orbit number MS Mass number - 4 for He 28 for N2 14 for N 29 for CO 16 for O 44 for CO2 F Flag - F for fully corrected P for preliminary (not fully corrected) M for mass flagged (problem point; probably wrong) N for final density negative (only preliminary density given) PRE DENS Preliminary density (part/cm**3) FIN DENS Final corrected density (part/cm**3) ANGATK Angle of attack (degrees) %ERR % error in density -1 if error >127% Notes: 1) For CO2 both PRE DENS and FIN DENS are given since the final correction to this species depends on a model: (new CO2 density) = (old CO2 density) - 0.019 x (old O density) (new O density) = (old O density) + 0.019 x (old O density) Only the New O density is given. 2) Mass flagged points are usually points that fall excessively beyond range of the main body of the data. They may be wild points, points with wrong mass designations or simply wrong for other reasons. 3) The best estimate of the density is to be found in the F data. 4) Data with errors greater than about 30% should be considered unreliable. 5) The angle of attack is included to help sort out low data values due to antenna shadowing (all species) which occurs beyond 40 degrees and high value ram points seen in He at angles of attack less than 10 degrees. Some of these points have already been mass flagged. In general, it would be best to not include data in these regions. Record 2: It is read as follows CHARACTER*60 FMT READ(UNIT,'(A)') FMT Record 3: It is read as follows INTEGER*4 F1,F2,F3,F4 CHARACTER*1 F5 REAL F6,F7,F8,F9 READ(UNIT,FMT) F1,F2,F3,F4,F5,F6,F7,F8,F9 Record 4,5,6, etc.: The main data records with one record per point. They are read as follows: INTEGER*4 YYYYDDD,UT,ORBIT,MASS CHARACTER*1 F REAL ODENS,FDENS,ANGATK,PCERR READ(UNIT,FMT,END=?) YYYYDDD,UT,ORBIT,MASS,F, ODENS,FDENS,ANGATK,PCERR _______________________________________________________________________ d) Superthermal Ion O+ Flux HIRES Data: The data have the following characteristics: LOGICAL RECORD SIZE: 97 BYTES FORMAT: ASCII FILES: 1 The field names used in RECORD 1 of the data: YYYYDDD YYYY=4 digit year (e.g. 1978) DDD=3 digit day of year (e.g. 053) UT(ms) Universal time in ms ORBIT Orbit number MS Mass number - 16 for O F Flag - D single data point flux, density A fitted parameters for interval DENSITY Effective number density assuming a 40 eV ion (particles/cm**3) FLUX Flux (particles/cm**2/s) AZ ANG Azimuth angle of apparent ion flow direction projected into the ecliptic plane (deg) PHASE Phase shift of signal maximum with respect to that predicted by velocity vector (deg) ERROR Error in phase shift (deg) ANGATK Effective angle of attack (deg) WP DIR Direction of drift component perpendicular to (ONMS axis, Z axis) projected into the ecliptic plane (deg) WP XY Magnitude of drift component (m/s) B Fitting parameter _______________________________________________________________________ e) Superthermal Ion Maximum (MAX) Count Rate Data: The data have the following characteristics: LOGICAL RECORD SIZE: 67 BYTES FORMAT: ASCII FILES: 1 The field names used in RECORD 1 of the data: DATE YYDDD YY=2 digit year (e.g. 78 for 1978) DDD=3 digit day of year (e.g. 053) MSEC Universal time in ms 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) _______________________________________________________________________ f) Thermal Ion LORES Data: 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) MSEC Universal time in ms 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 _______________________________________________________________________ 4.2 Data Set Processing 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. DATA SET NAME TYPE ORBIT RANGE DATE RANGE PROCESSED Neutral density LORES 20 - 640 12/24/78 - 09/05/80 12/13/88 Neutral density HIRES 1 - 640 12/05/78 - 09/05/80 12/13/88 Superthermal O+ LORES 1 - 4372 12/05/78 - 11/25/90 01/06/92 Superthermal O+ HIRES 1 - 4372 12/05/78 - 11/25/90 01/06/92 Superthermal MAX 1 - 4372 12/05/78 - 11/25/90 01/06/92 Superthermal O+ LORES 4373 - 5055 11/26/90 - 10/07/92 01/06/92 Superthermal O+ HIRES 4373 - 5055 11/26/90 - 10/07/92 01/06/92 Superthermal MAX 4373 - 5055 11/26/90 - 10/07/92 01/06/92 Thermal ion LORES 1 - 5055 12/05/78 - 10/07/92 06/01/93 Neutral density LORES 4954 - 5055 06/29/92 - 10/07/92 06/01/93 Neutral density HIRES 4954 - 5055 06/29/92 - 10/07/92 06/01/93 4.3 Data Set Sampling Description a) Neutral density HIRES data This data set has at least preliminary composition for every data point measured but not necessarily final composition values. b) Neutral density LORES data This data is a representative sample, approximately once per 12 seconds, of the high resolution data. It is constructed at designated times which have been supplied by the Project. Data with errors greater that 30% are not included nor are data with angles of attack greater than 40 degrees. An absolute altitude cutoff of 250 km was used for all species except for He for which 350 km was used. Each representative data point is constructed using an exponentially weighted average of the data over a 24 second interval centered at the sample point time. Corrections to the number densities of CO2 and O for surface reactions were made at this time based on empirical model results. A minimum of 3 data points per species and all data available for corrections are required to be present in order for a sample point to be output. The total number density and total mass density are computed if all major species (CO2, CO, N2, and O) are present. The data spacing is nominally 12 seconds except for the -12, 0, 12 time tags. Although time tags from -1800 to 1800 seconds are generated, only those data records for which at least one species has a valid value for that time tag are output. c) Superthermal O+ Ion LORES Data The data values of the LORES 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. d) Superthermal O+ Ion HIRES Data The individual flux and density values are computed by dividing each data value by the value of the fitting function at the corresponding time. e) Superthermal Ion MAX Data 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. f) Thermal Ion LORES Data The data values of the LORES 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. 4.4 Coordinates Not applicable 4.5 Data Set Confidence a) Neutral density Several criteria were invoked when inserting data for a given orbit: orbit and attitude parameters must exist (project supplied); the spacecraft format and bit rate must be appropriate for acquisition of data by the ONMS; and the command sequence for the instrument must be appropriate for useful determination of atmospheric composition. Cases where useful composition cannot be determined include special test modes (e.g., retarding potential sweeps, filament off) and 1/8 unit amu sweep modes. In addition, composition for the LORES data set cannot be easily determined for unit amu sweep mode. The ONMS was not operational for every orbit nor is every orbit complete due to data gaps introduced by use of telemetry formats for which the ONMS has no instrument output. Useful composition data are gathered from the lowest periapsis altitude to a maximum altitude generally around 250 km (about 300 km for He). The actual maximum altitude depends on the accumulated surface gas buildup acquired from previous orbits which creates a gas background. The gas background was estimated from high altitude averages of the data and for all species, except helium, an inbound signal/background ratio of 2 and an outbound signal/background ratio of 4 were used as cutoff values. In some cases superthermal ions (e.g., Kasprzak et al.,1982) were observed at low altitudes (e.g., below 300 km for orbit 219) and these were removed when visually detected. Some problems have been observed in the high altitude data very near cutoff, particularly for outbound N2. Several data points were never removed and appear higher than the expected extrapolation of the data to that time. Residual spin modulation which had not been completely removed is evident in the processed data. The source of the spin residuals are the gas/surface adsorption/desorption effects which were not removed from the data and a non-cosine behavior for the response of the ion source density with angle of attack. Another feature observed occasionally at large angles of attack (>40 degrees) is a reduction of the data when compared to data at lower angles of attack. This has been determined to be due to antenna shadowing; that is, the ONMS geometric view cone "sees" the spacecraft antenna at extreme angles of attack. Occasionally near minimum angle of attack (<10 degrees), enhanced data points are observed for m/e=4 (He channel) which are apparently high energy ions/neutrals traveling along the tube axis and being detected. The more extreme points in either of these two cases have been mass flagged. The data time spacing depends on the spacecraft bit rate and format, and the particular instrument commands executed. Usually programmed mass format was used, but occasionally unit amu and 1/8 amu sweeps were implemented. Several orbits switched from low electron energy to high electron energy and as a result there may be a discontinuity at the transition point. The 1/8 amu sweep data have not been included. Atomic nitrogen was measured in programmed mass mode only after orbit 190. Orbits 1-19 generally do not have reliable relative composition due to the fact that gas-surface processes in the ion source had not stabilized. This affects all surface reactive species except He. Isolated (one or two points per several spin cycles) high resolution data points are occasionally observed and they should be regarded as erroneous points which are more likely wrong than right. The error associated with the points is more an indication of data quality than of absolute uncertainty. It contains the statistical error of the data determined for the principle m/e used for the species from the detector signal plus the errors coming from any other species used to correct the data. It also contains a contribution which is proportional to the background/signal ratio. The total relative error is at least an additional 5-10% above this value. For the entry period data below 135 km for N, O, N2, CO and CO2 and for He below 135 km has not been included in the data set. This is due to unresolved non-linearities that occur due to the ion source response that cannot be removed from the data. Due to electronic changes in the instrument over the intervening 10 years the instrument retuning. Tuning checks were made using 1/8 amu sweeps in several different resolutions and tuning: orbits 4855-4864 and 4978-4984 were tune=[normal], resolution=[2]; orbits 4865-4977 were tune=[tune up 1], resolution=[2]; orbits 4987-5031 were tune=[tune up 1] and resolution=[0]; and orbits 5034-5055 were in tune=[tune down 1], resolution=[0]. Mass 45 was used for measuring CO2 for these orbits and beginning with orbit 5034 mass 29 was used in place of 28, mass 31 in place of 30 and mass 33 in place of 32.Below about 140 km mass 12 was used as a proxy for the CO2 density since the parent peak would have saturated the instrument detector. There is possibility that the individual mass peak sensitivities may have changed as a result and this effect has not been fully evaluated. A gross check on the sensitivity was done by comparing the total mass density at periapsis with the spacecraft speed change at periapsis, delta-V, as a result of atmospheric drag. The relationship between the mass density and delta-V in 1992 is about 10% lower than in 1978-80 assuming that the spacecraft aspect parameters have not changed substantially between the two comparisons. b) Superthermal O+ ion data See Kasprzak et al. (1987). 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. c) Thermal ion data See Kasprzak and Niemann (1992). Technique is similar to that used for processing superthermal O+ ion data. 4.6 References 4.6.1 Instrument References 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, GE-18 (1), 60-65, 1980b. Niemann, H.B. and W.T. Kasprzak, Comparative Neutral Composition Instrumentation and New Results, Advances in Space Research, 261-270, 1983. 4.6.2 Neutral Density References Brinton, H.C., H.A. Taylor, H.B. Niemann, H.G. Mayr, A.F. Nagy,T.E.Cravens, and D.F. Strobel, Venus Nighttime Hydrogen Bulge, Geophysical research Letters, 7, 865-868, 1980. Hedin, A.E., H.B. Niemann, W.T. Kasprzak and A. Seiff, Global Empirical Model of the Venus Thermosphere, Journal of Geophysical Research, 88, 73-83, 1983. Hoegy, W.R., L.H. Brace, W.T. Kasprzak and C.T. Russell, Small- Scale Plasma, Magnetic, and Neutral Density Fluctuations in the Nightside of Venus Ionosphere, Journal of Geophysical Research, 95, 4085-4102,1990. Kar, J., R. Paul, R. Kohli, K.K. Mahajan, W.T. Kasprzak and H.B. Niemann, On the Response of Exospheric Temperature on Venus to Solar Wind Conditions, Journal Geophysical Research, 96,7901- 7904,1991. Kasprzak, W.T., H.B. Niemann, A.E. Hedin, S.W. Bougher and D.M. Hunten, Neutral Compostion Measurements by the Pioneer Venus Neutral Mass Spectrometer During Orbiter Entry, Submitted to GRL, 1993. Kasprzak, W.T., H.B. Niemann, A.E. Hedin and S.W. Bougher, Wave- like Perturbations Observed at Low Altitudes by the Pioneer Venus Orbiter Neutral Mass Spectrometer During Entry, Submitted to GRL, 1993. Kasprzak, W.T., A.E. Hedin, H.B. Niemann and N.W. Spencer, Atomic Nitrogen in the Upper Atmosphere of Venus, Geophysical Research Letters, 7,106-108, 1980. Kasprzak, W.T., A.E. Hedin, H.G. Mayr and H.B. Niemann, Wavelike Perturbations Observed in the Neutral Thermosphere of Venus, Journal of Geophysical Research, 93, 11237-11245, 1988. Keating, G.M., J.L. Bertaux, S.W. Bougher, T.E. Cravens, R.E. Dickenson, A.E. Hedin, V.A. Krasnopolsky, A.F. Nagy, J.Y. Nicholson,L.J. Paxton and U. von Zahn, "Models of Venus Neutral Upper Atmosphere:Structure and Composition," in Venus International Reference Atmosphere, ed. A. V. Kloire, V.I. Moroz and G.M. Keating, Advances in Space Research, 5, 117-171, 1985. Mahajan, K.K., W.T. Kasprzak, L.H. Brace, H.B. Niemann and W.R. Hoegy, Response of Venus Exospheric Temperature Measured by Neutral Mass Spectrometer to Solar Flux Measured by Langmuir Probe on the Pioneer Venus Orbiter, Journal of Geophysical Research, 95, 1091-1095, 1990. Mayr, H.G., I. Harris, W.T. Kasprzak, M. Dube, and F. Variosi, Gravity Waves in the Upper Atmosphere of Venus, Journal of Geophysical Research, 93, 11247-11262, 1988. Niemann, H.B., R.E. Hartle, W.T. Kasprzak, N.W. Spencer, D.M. Hunten, and G.R. Carignan, Venus Upper Atmosphere Neutral Composition: Preliminary Results from the Pioneer Orbiter, Science, 203, 770-772, 1979. Niemann, H.B., R.E. Hartle, A.E. Hedin, W.T. Kasprzak, N.W. Spencer, D.M. Hunten and G.R. Carignan, Venus Upper Atmosphere Neutral Gas Composition: First Observations of the Diurnal Variations, Science, 205,54-56, 1979. Niemann, H.B., W.T. Kasprzak, A.E. Hedin, D.M. Hunten and N.W. Spencer, Mass Spectrometric Measurements of the Neutral Gas Composition of the Thermosphere and Exosphere of Venus, Journal of Geophysical Research, 85, 7817-7827, 1980a. Taylor, H.A., H. Mayr, H. Brinton, H. Niemann, and R.E. Hartle, Variations in Ion and Neutral Composition at Venus: Evidence of Solar Control of the Formation of the Predawn Bulges in H+ and He, ICARUS, 52, 211, 1982. Taylor, H.A., H. Brinton, H. Niemann, H. Mayr, R. Hartle, A. Barnes and J. Larson, In-Situ Results on the Variation of Neutral Atmospheric Hydrogen at Venus, Adv. Sp. Res., 5, 125- 128, 1985. von Zahn, U., S. Kumar, H. Niemann, and R. Prinn, "Composition of the Venus Atmosphere," in VENUS, 288-430, University of Arizona Press, Tucson, Ariz.,1983. 4.6.3 Superthermal Ion references Brace, L.H., W.T. Kasprzak, H.A. Taylor, R.F. Theis, C.T. Russell, A. Barnes, J.D. Mihalov and D.M. Hunten, The Ionotail of Venus: Its Configuration and Evidence for Ion Escape, Journal of Geophysical Research, 92, 15-26, 1987. Grebowsky, J.M.,W.T. Kasprzak, R.E. Hartle, K.K. Mahajan and T.J. Wagner, Superthermal Ions Detected in Venus' Dayside Ionosheath, Ionopause, and Magnetic Barrier Regions, Journal Geophysical Research, 98, 9055-9064,1993. Kasprzak, W.T., H.A. Taylor, L.H. Brace and H.B. Niemann, Observations of Energetic Ions Near the Venus Ionopause, Planetary Space Sciences, 30, 1107-1115, 1982. Kasprzak, W.T., H.B. Niemann and P. Mahaffy, Observations of Energetic Ions on the Nightside of Venus, Journal of Geophysical Research, 92, 291-298, 1987. Kasprzak, W.T. and H.B. Niemann, "Fast O+ Ion Flow Observed Around Venus at Low Altitudes," NASA Technical Memorandum 100717, December 1988. Kasprzak, W.T., J.M. Grebowsky, H.B. Niemann and L.H. Brace, Superthermal > 36 eV Ions Observed in the Near Tail Region of Venus by the Pioneer Venus Orbiter Neutral Mass Spectrometer, Journal of Geophysical Research, 96, 11175-11187,1991. Szego, K., V.S. Shapiro, V.I. Shevchenko, R.Z. Sagdeev, W.T. Kasprzak, and A.F. Nagy, Physical Processes in the Plasma Mantle of Venus, Geophysical Research Letters, 18, 2305-2308, 1991. 4.6.4 Thermal Ion References 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. 4.6.5 Miscellaneous References 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. Kasprzak, W.T., "The Pioneer Venus Orbiter: 11 Years of Data," NASA Technical Memorandum 100761, May 1990. 5 Data Events Superthermal Ion Location File, orbits 1 - 706, 4710 - 5055. A summary of the location of superthermal ions detected by the ONMS instrument is given in this file. The file contains the start and stop times for each superthermal ion segment. The file header description: 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) -|