Version: 11/26/96 JFC ******************************************************************************** OVERVIEW OF VOYAGER MAGNETOMETER EXPERIMENT The magnetic field experiment carried onboard the Voyager 1 and 2 missions consists of dual low field (LFM) and high field magnetometer (HFM) systems. The dual systems provide greater reliability and, in the case of the LFMs, permit the separation of spacecraft magnetic fields from the ambient fields. Additional reliability is achieved through electronic redundancy. The wide dynamic ranges of +/- 0.5 G for the LFMs and =/- 20 G for the HFMs, low quantization uncertainty of +/- 0.002 nT in the most sensitive +/- 8 nT LFM range, low sensor RMS noise level of 0.006 nT, and use of data compaction schemes to optimize the experiment information rate al combine to permit the study of a broad spectrum of phenomena during the mission. Objectives include the study of planetary fields at Jupiter, Saturn, Uranus and Neptune; satellites of these planets; solar wind and satellite interactions with planetary fields; and the large-scale structure and microscale characteristics of the interplanetary magnetic field. The interstellar field may also be measured. Space Science Reviews, 21 (1977) 235-257. DESCRIPTION OF VOYAGER MAG DATA SUBMISSIONS TO THE NSSDC The high resolution data submitted to the NSSDC has its origins in the original MVS "Summary" data sets formally produced on the NSSC IBM MVS mainframe system. The original data sets contain a mix of engineering, electron, magnetic field and plasma data. The Voyager magnetometer (MAG) experiment now resides on a dedicated workstation where only MAG data is processed. High resolution MAG data continues to use the same format as that of archived MVS data sets. All Voyager MAG data submitted to the NSSDC consists wholly of high resolution LFM averages. These files consist of a set of averages applied across all of the mission's differing telemetry modes. In the case of the Magnetometer Experiment, the records contain both 1.92 second, 9.6 second and 48 second averages. 1.92 second averages are created from the detail detail data, 9.6 second averages are created from 1.92 second averages and 48 second averages are created from the 9.6 second averages. Thus, there are 25 1.92 second averages, 5 9.5 averages and one 48 second average per record. In producing this submission to the NSSDC, the magnetic field records have been stripped from MVS "Summary" files and converted from MVS to VAX binary representation. File content remains unchanged, except that the header records preceding and following each data record have been stripped. Since the main function of these header records is to identify the interplanetary or planetary coordinate system used to process the data, they are not needed because all data in this NSSDC data set are interplanetary and in heliographic coordinates (see below). A second, ASCII format data set containing key components of the 48 second magnetic field data and ephemeris data has been created allowing more convenient access to high resolution Voyager magnetometer data. COORDINATE SYSTEMS Interplanetary magnetic field studies make use of two important coordinate systems, the Inertial Heliographic (IHG) coordinate system and the Heliographic (HG) coordinate system. The IHG coordinate system is use to define the spacecraft's position. The IHG system is defined with its origin at the Sun. There are three orthogonal axes, X(IHG), Y(IHG), and Z(IHG). The Z(IHG) axis points northward along the Sun's spin axis. The X(IHG) - Y(IHG) plane lays in the solar equatorial plane. The intersection of the solar equatorial plane with the ecliptic plane defines a line, the longitude of the ascending node, which is taken to be the X(IHG) axis. The X(IHG) axis drifts slowly with time, approximately one degree per 72 years. Magnetic field orientation is defined in relation to the spacecraft. Drawing a line from the Sun's center (IHG origin) to the spacecraft defines the X axis of the HG coordinate system. The HG coordinate system is defined with its origin centered at the spacecraft. Three orthogonal axes are defined, X(HG), Y(HG), and Z(HG). The X(HG) axis points radially away from the Sun and the Y(HG) axis is parallel to the solar equatorial plane and therefore parallel to the X(IHG)-Y(IHG) plane too. The Z(HG) axis is chosen to complete the orthonormal triad. An excellent reference guide with diagrams explaining the IHG and HG systems may be found in Space and Science Reviews, Volume 39 (1984), pages 255-316, MHD Processes in the Outer Heliosphere, L. F. Burlaga. DATA FORMATS Binary MAG data files usually contain two types of records. The first and last records are header records containing a description of the parameters used to process the Experimental Data Records (EDRs). The remaining records contain the field averages from the low field magnetometers (LFM). Only the LFM records are included in the NSSDC data set. -------------------------------------------------------------------------------- Each binary LFM record contains the following format: VOYAGER LOW FIELD MAGNETOMETER (LFM) SUMMARY RECORD FORMAT LFM Record Components 1. Header block - 32 words 2. MAG data block - 496 words 3. SEDR data block - 40 words --- 568 words total (32 bit words) Header Block Description word type description ---- ---- ----------- 1 A*4 Data Identifier ("HDR1" or "LFM ") 2 A*4 Telemetry Format ("ENG ","CR-1","CR-2","CR-3","CR-4","CR-5", "CR-6","CR-7","CR5A","GS-2","GS-3","GS-4", "GS-6","GS-7","GS-8","GS10","G10A","OC-1", "OC-2","UV5A","****") 3 A*4 S/C Identification ("FLT1","FLT2","PTM ","SIM1","SIM2","UKWN") 4 I*2 Year of Data 4.5 I*2 Day of Year (Jan. 1 = 1) 5 I*2 Hour of Day 5.5 I*2 Minute of Hour 6 I*2 Second of Minute 6.5 I*2 Millisecond of Second 7 R*8 Decimal Day of Year (Jan. 1 = 0) 9 R*8 Decimal Day Count since August 20, 1977 11 A*4 Time Type (SCET = spacecraft event time, ERTS = start earth receive time) 12 R*4 Time period of data block (48 sec) 13 I*2 48 Minute Counter (2**16 sequence counter of data at time of telemetry readout.) 13.5 I*2 48 Second Counter (Modulo 60 sequence counter of data at time of telemetry readout.) 14 I*2 60 Millisecond Counter (Line counter of data at time of telemetry readout.) 14.5 I*2 Spare 15 Z*4 MAG Status Word 16 Z*4 MAG Command Word 17 I*2 Data Identifier (LFM=2,HDR1=8) 17.5 I*2 Data Rate (GS-X=0,CR-1=1,CR-2=2,CR-3=3,CR-4=4,CR-5=5, CR-6=6,CR-7=9,ENG=8,CR5A=13,UV5A=13) * CR5A and UV5A are both VIM-5 modes 18 I*4 Spare 19 A*8 Data Confidence Flag (indicates reduced data confidence due to software/hardware intervention) 21 I*4 Spare 22 I*4 Spare 23 I*4 Spare 24 I*4 Spare 25 I*4 Spare 26 I*4 Spare 27 I*4 Spare 28 I*4 Spare 29 I*4 Spare 30 I*2 Number of primary data words 30.5 I*2 Number of secondary data words 31 I*2 Number of primary full words 31.5 I*2 Number of secondary full words 32 I*2 Record number in file 32.5 I*2 Number of words remaining in logical record MAG Data Block Description 48 second averages ------------------ 33 R*4 F1: 48 second average of 9.6 second norms (nT) 34 R*4 F2: Norm of 48 second component averages (nT) F2 = sqrt(B1**2+B2**2+B3**2) 35 R*4 Latitudinal field angle = arcsin(B3/F2) (degrees) 36 R*4 Longitudinal field angle = 180 - atan(B2,-B1) (degrees) 37 R*4 B1: 48 second average of first 9.6 second averaged component (nT) 38 R*4 B2: 48 second average of second 9.6 second averaged component (nT) 39 R*4 B3: 48 second average of third 9.6 second averaged component (nT) (B1,B2,B3) = (BR,BT,BN) (HG coordinates) 40 R*4 RMS of B1 (nT) 41 R*4 RMS of B2 (nT) 42 R*4 RMS of B3 (nT) 43 I*4 Number of 9.6 averages per 48 second average (0