**NSSDCA ID:** PSFP-00581

**Availability:** Archived at NSSDC, accessible from elsewhere

**Time span:** 1989-08-24 to 1989-08-26

This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= The Voyager magnetometer investigation (P.I.- Norman F. Ness) makes available archival data through the National Space Science Data Center (NSSDC) located at NASA/GSFC as well as through the Planetary Data System (PDS) and other channels. The primary archive format, referred to as a 'summary tape' or 'conjoint summary tape' has been used consistently since the beginning of the Voyager mission to the outer planets (1977). This format makes available magnetometer observations, supplementary engineering and ephemeris data in one data file, and it is one product of Voyager magnetometer routine data processing. Users are referred to the summary format data for all data requirements with one exception: Neptune encounter high field observations. Neptune close approach observations are archived separately and in a different format from that with which many are familiar. This high-field archive is described here. The special demands of the Neptune encounter flyby exceeded (finally) the capabilities of the routine data processing system conceived and implemented in the mid 1970's. As a result, it was necessary to implement an additional data processing system with which the near-encounter, high field magnetometer observations were processed. The data products available with this new data processing system are not available in the same format as the standard Voyager magnetometer observations; thus the need for a separate archive and a new format description. We appreciate the desirability of a consistent archive format, but find no reasonable alternative to the present solution. We expect, however, that users interested in near encounter observations will find this new format both useful and easy to assimilate. The magnetic fields investigation on Voyager carries a total of four tri-axial ring core fluxgate magnetometers: two identical high field magnetometers mounted on the spacecraft body (HFM's) and two identical low field magnetometers (LFM's) arranged on a 13 m boom (Behannon et al., 1977). The two LFM's automatically step through a total of eight dynamic ranges (ranges 0 through 7) in response to changes in the measured field, starting with a nominal dynamic range of 8 nT, and increasing to a nominal dynamic range of 50,000 nT. The two HFM's each operate in two dynamic ranges (ranges 0 and 1) with nominal values of 50,000 nT and 200,000 nT. In the Neptune encounter mode, each magnetometer was sampled periodically with a temporal resolution of between 0.600 s and 0.060 s, and analog-to-digital converted with 12 bit resolution for subsequent telemetry. The magnitude of the maximum observed field at Neptune (approximately 10,000 nT) was sufficiently large that data from all four of the magnetometers proved useful. Parameters ========== Derived Parameters -----------------Sampling Parameter Name : time Sampling Parameter Resolution : 12.0 seconds Minimum Sampling Parameter : 19770820120000.000000 Maximum Sampling Parameter : UNK Sampling Parameter Interval : 12.0 seconds Minimum Available Sampling Interval : 0.060000 seconds Data Set Parameter Name : magnetic field vector Noise Level : 0.006000 nT Data Set Parameter Unit : nanotesla Magentic field vector: A derived parameter which combines the 3 orthogonal magnetic field component measurements. Measured Parameters ------------------Magentic field component: A measured parameter equaling the magnetic field strength (e.g. in nanoteslas) along a particular axis direction. Usually the three orthogonal axis components are measured by three different sensors. Data ==== 1) MAGNETOMETER DATA PRODUCT ctime - Decimal seconds past 1966-01-02T00:00:00 (an internal time format) SCET - Spacecraft event time PDS/ISO standard time format. B_R - Magnetic field vector radial component in units of nanotesla; planetocentric right-handed spherical coordinate system. B_THETA - Magnetic field vector theta component, units nanotesla. B_PHI - Magnetic field vector phi component, units nanotesla. The vector magnetic field is rendered in a right handed spherical coordinate system in which the angles THETA and PHI are the usual polar angles, with THETA (colatitude) measured from the axis of rotation and PHI increasing in the direction of rotation. The orientation of Neptune's pole is specified by a right ascension of 298.90 and declination of 42.84 at the time of the encounter, as given in the Jet Propulsion Laboratory's distribution of physical constants dated 11/06/89. Planetary longitudes are based on a 16.11 hour rotation period (Warwick et al., 1989) adopted by the Voyager Project shortly after the encounter. The zero longitude is defined by the requirement that the West Longitude of the spacecraft at 0356 SCET day 237 (near closest approach) be 167.7 NLS; West Longitudes of the Neptune Longitude System (NLS) are simply related to the angle PHI: WLONG = 360. - PHI (degrees) This definition of the zero longitude was adopted by the Voyager Project Steering Group in order to minimize differences in longitudes resulting from changes in the assumed rotation period. The field values are instantaneous, not averaged, samples of 'full word' magnetic field observations. These 'full word' observations are obtained from each of the Voyager low-field (inboard and outboard) magnetometers and high-field (inboard and outboard) magnetometers every 0.6 seconds. (Note that 'delta' word samples are available at much higher sample rates of 16.666 samples/second). These 'full word' samples are resampled at a time resolution of 12 seconds for present purposes (in the high-field region of the magnetosphere, these data are highly redundant at 0.6 seconds resolution). All four triaxial fluxgate magnetometers operated throughout the encounter. The comprehensive data set is constructed of inboard low-field magnetometer samples and outboard high-field magnetometer samples as follows: SCET 89 236 18 0 0 557 to 89 237 3 44 0 512: inboard low-field magnetometer SCET 89 237 3 44 12 512 to 89 237 4 2 24 511: outboard high-field magnetometer SCET 89 237 4 2 36 511 to 89 238 8 19 48 379: inboard low-field magnetometer This procedure was adopted as a result of an increased level of spacecraft interference and telemetry anomalies experienced during the Neptune encounter by MAG. The composite set of observations has good time continuity. Spacecraft zeros are assumed constant throughout the interval represented by this data set, and for that reason users of low-field data are referred to the summary tape format (standard archive format) in which greater care is exercised with the estimation of time-variable spacecraft zeros (differences of 0.05 nT or less). This data set extends from inbound MP to outbound MP for the convenience of users who do not require better weak-field accuracy than that quoted above. 2.) INTERNAL - A BINARY file of observations and supplementary ephemeris data organized specifically for analysis of the planetary magnetic field (Connerney, Acuna, and Ness, Journal of Geophysical Research, 1991). These data are resampled 'full word' observations of inboard low-field magnetometer and outboard high-field magnetometer observations according to the schedule given above. These data are a subset of the Voyager 2 vector magnetic field observations obtained within 12 Rn radial distance of Neptune (SCET day 236 hr 2340 to SCET day 237 0811). During this time the field magnitude ranged from approximately 10 nT to nearly 10,000 nT. These data were resampled (decimated in time without averaging) to produce samples every 48, 24, and 12 seconds, with the highest temporal resolution reserved for the closest approach interval (where Voyager 2's motion relative to the planet was greatest). Note that the format of this ASCII file is different from that described above; a segment of the file is reproduced here, again beneath column headings inserted here only: RADIUS THETA PHI B_COMPONENT SIGMA TYPE ----------------------------------- --------------------------------1.349 0.685 4.614 6902.570 3.500 0 1.349 0.685 4.614 3598.230 3.500 1 1.349 0.685 4.614 -2457.560 3.500 2 1.339 0.669 4.602 7153.850 3.500 0 1.339 0.669 4.602 3555.870 3.500 1 1.339 0.669 4.602 -2591.020 3.500 2 1.334 0.661 4.595 7275.760 26.00 0 1.334 0.661 4.595 3540.800 26.00 1 1.334 0.661 4.595 -2623.690 26.00 2 1.328 0.652 4.589 7401.400 26.00 0 1.328 0.652 4.589 3506.690 26.00 1 1.328 0.652 4.589 -2720.230 26.00 2 RADIUS - Spacecraft radial distance in units of Rn ( 1 Rn = 24,765 km) THETA - Theta coordinate of spacecraft in units of radians PHI - Phi coordinate of spacecraft in units of radians B_COMPONENT - Magnetic vector field component in units of nanotesla according to the value of identifier TYPE TYPE - Magnetic field value designation TYPE = 0: Radial component TYPE = 1: Theta component TYPE = 2: Phi component TYPE = 3: Magnitude of magnetic field SIGMA - Estimated standard deviation of the observation in units of nanotesla. Each vector observation was weighted with the expected standard deviation of the observation in the analysis of the internal field. This practice insures that each residual (the difference between an observation and a model field) is compared with its expected error. For the standard deviation of each measurement, we adopted 1 nT or the nominal quantization stepsize of the measurement, whichever was greater. The former value approximates the variable magnetospheric magnetic field 'noise', the unmodeled temporal and spatial variations in the ambient field. The latter approximates the estimated measurement error ( 0.05 nT + 0.1% of full scale). The quantization step size of the measurement is a function of the ambient field strength (or time), since the Voyager magnetometers automatically change range in response to changes in the ambient field. In ranges 0 through 5, the quantization stepsize of the lfm measurement increased from a nominal value of 0.005 nT to 1.1 nT. In ranges 6 and 7 (the highest two ranges) each lfm was sampled with a quantization stepsize of approximately 3.5 and 26 nT, respectively. Each hfm, operating in range 0 throughout the encounter, was sampled with a quantization of approximately 26 nT. Therefore, weights of 1 nT were used for observations obtained in the lower instrument ranges, from SCET day 236/2340 to day 237/0330 and from day 237/0431 to 237/0811. Weights of 3.5 nT were used for observations obtained in lfm range 6, from SCET day 237/0330 to 237/0344 and from day 237/0402 to 237/0431. Weights of 26 nT were used for the hfm observations, from SCET day 237/0344 to 237/0402. The segment of data illustrated above includes the transition to hfm observations with an associated estimated standard deviation of 26 nanoteslas. The sampling interval was also adjusted according to instrument range, with a 48 second sampling interval for lfm range 5 observations, a 24 second sampling interval for lfm range 6 observations, and a 12 second sampling interval for hfm observations. 3.) INTERNAL FIELD MODEL COEFFICIENTS - electronic form at A Neptune spherical harmonic magnetic field model (Connerney et al., 1991) is listed here in electronic format for use in magnetic field calculations. The model is based upon a partial solution (44ev) to an 8th order expansion of the internal field, as is necessary to adequately represent the field measured by Voyager close to the surface of Neptune. However, few of the model coefficients are resolved, or uniquely determined; these few are marked with an asterisk (*). Some additional coefficients are marginally resolved; these are marked with a # sign. The remaining coefficients are not resolved, which means that they are not constrained by the data, and may assume practically any value. They are, however, necessary to represent the field along the Voyager trajectory close to the planet. USERS ARE ENCOURAGED TO READ CAREFULLY THE DISCUSSION IN THE NEPTUNE SPECIAL ISSUE OF JGR REGARDING MODEL PARAMETER RESOLUTION AND USE OF THE MODEL COEFFICIENTS. Beyond about 2.5 or 3 Rn, where higher order terms are sufficiently attenuated, the quadrupole approximation is a good approximation to the field globally. These terms are (more or less) uniquely determined. At close-in radial distances, our description of the field is necessarily incomplete. The entire expansion to eighth degree and order provided will well describe the field in the vicinity of the Voyager trajectory. Those parts of the field that the data are insensitive to are effectively zero along the Voyager trajectory. As one strays further from the Voyager trajectory in close to the planet, the real field may be expected to (increasingly) deviate from that calculated with the entire expansion. Given Voyager's close approach (1.18 Rn) to the planet, any low-order approximation to the field is expected to be inaccurate, even on the spacecraft trajectory (Figure 9 in the paper by Connerney et al. illustrates this problem). For use in modeling the field near the planet, (at radial distances of less than approximately 2 or 3 Rn), an octupole approximation to the field is required to even grossly approximate the field. Unfortunately, many of these coefficients are not well resolved. For global field modeling near the planet, we advocate use of a model field consisting of orders 1, 2 and 3 (dipole, quadrupole, and octupole) from Table 1. This model, a subset of the coefficients listed below, is referred to as the 'O8' model (the octupole (O) part of an eighth degree and order spherical harmonic expansion). This decision reflects a compromise between the limited model parameter resolution afforded by the Voyager trajectory and the anticipated demands on the model. However, some of the quadrupole and octupole parameters are not well resolved and we expect these to be inaccurate. This is an inescapable consequence of the limited sinformation content of the observations. ------------------------------------------------------------------Table 1. Neptune I8E1 44ev Magnetic Field Model Schmidt-Normalized Spherical Harmonic Coefficients Neptune Radius = 24,765 km ------------------------------------------------------------------n m g(n,m) h(n,m) ------------------------------------------------------------------1 0 0.09732 * 1 1 0.03220 * -0.09889 * 2 0 0.07448 # 2 1 0.00664 # 0.11230 * 2 2 0.04499 * -0.00070 * 3 0 -0.06592 * 3 1 0.04098 -0.03669 # 3 2 -0.03581 0.01791 # 3 3 0.00484 # -0.00770 # 4 0 0.02243 4 1 0.00557 -0.01889 # 4 2 0.03099 0.02607 4 3 -0.01287 0.01204 4 4 -0.05073 -0.00456 5 0 -0.00202 5 1 -0.00229 -0.00739 5 2 0.00526 -0.01134 5 3 -0.02846 0.01067 5 4 -0.01425 -0.01551 5 5 -0.02835 -0.01090 6 0 -0.02175 6 1 -0.00466 0.04432 6 2 -0.01269 -0.01598 6 3 -0.02233 0.01721 6 4 -0.00887 0.00370 6 5 -0.00496 -0.01932 6 6 0.00755 0.01439 7 0 0.01671 7 1 0.01678 -0.03159 7 2 0.01625 0.01862 7 3 0.02157 -0.01120 7 4 -0.00483 0.00515 7 5 0.01873 0.01923 7 6 0.00584 -0.02749 7 7 0.00664 0.03344 8 0 -0.00689 8 1 0.00238 0.01446 8 2 -0.00090 -0.00079 8 3 -0.01304 0.01043 8 4 0.00311 -0.00022 8 5 -0.00367 -0.00465 8 6 -0.00249 0.01043 8 7 0.01333 -0.02138 8 8 -0.01239 0.02519 * = Coefficient WELL RESOLVED (Rxx > 0.95) # = Coefficient MARGINALLY RESOLVED (0.75 < Rxx < 0.95) ALL OTHER COEFFICIENTS are POORLY RESOLVED or UNRESOLVED Refer to text of Connerney et al., 1991, for explanation. Coordinate System ================= Neptune West Longitude System (NLS) Coordinates ----------------------------------------------COORDINATE SYSTEM CENTER : NEPTUNE COORDINATE SYSTEM REFERENCE EPOCH : UNK (1989-08-25:03:56:00.000) The orientation of Neptune's pole is specified by a right ascension of 298.90 and declination of 42.84 at the time of the encounter, as given in the Jet Propulsion Laboratory's distribution of physical constants dated 11/06/89. Planetary longitudes are based on a 16.11 hour rotation period (Warwick et al., 1989) adopted by the Voyager Project shortly after the encounter. The zero longitude is defined by the requirement that the West Longitude of the spacecraft at 0356 SCET day 237 (near closest approach) be 167.7 NLS; West Longitudes of the Neptune Longitude System (NLS) are simply related to the angle PHI: WLONG = 360. - PHI (degrees) This definition of the zero longitude was adopted by the Voyager Project Steering Group in order to minimize differences in longitudes resulting from changes in the assumed rotation period. Magnetic-field is defined in terms of the following: R - Radial along the Neptune-S/C line, positive away from Neptune Phi - east longitudinal component Theta - colatitudinal component Position is given in terms of the following: RANGE (R) - Range from the planet center to the spacecraft in units of Rn where Rn = 24,765km. LATITUDE (LAT) - Spacecraft latitude in degrees. Valid range -90.0 -> +90.0. LONGITUDE (W_LONG) - West longitude where the zero longitude is defined by the requirement that the West Longitude of Voyager 2 at 1989-08-25T03:56:00.00 was 167.7 degrees. References ========== Behannon, K. W., M. H. Acuna, L. F. Burlaga, R. P. Lepping, N. F. Ness, and F. M. Neubauer, 'Magnetic field experiment for Voyagers 1 and 2', Space Sci. Rev., 21, 235-257, 1977. Connerney, J. E. P., M. H. Acuna, and N. F. Ness, 'The magnetic field of Neptune', J. Geophys. Res., 96, Supplement, 19023-19042, 1991. Ness, N. F., M. H. Acuna, L. F. Burlaga, J. E. P. Connerney, R. P. Lepping, and F. M. Neubauer, 'Magnetic fields at Neptune', Science, 246, 1473-1478, 1989. Warwick, J. W. et al., 'Voyager planetary radio astronomy at Neptune', Science, 246, 1498-1501, 1989.

These data are available on-line from the Planetary Data System (PDS) at:

https://pds-ppi.igpp.ucla.edu/data/VG2-N-MAG-4-SUMM-NLSCOORDS-12SEC-V1.0/

- VG2-N-MAG-4-SUMM-NLSCOORDS-12SEC-V1.0

- Planetary Science: Fields and Particles

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

Name | Role | Original Affiliation | |
---|---|---|---|

Dr. Norman F. Ness | Data Provider | University of Delaware | nfness@bartol.udel.edu |

Dr. Norman F. Ness | General Contact | University of Delaware | nfness@bartol.udel.edu |