M A G N E T O M E T E R T R A C D A T A S E T This data set contains TRAC-produced full magnetic field field vectors at 2.5-sec resolution (ZAP4 memory mode only), in a spacecraft-based coordinate system (Z along s/c symmetry axis, near-radial from Earth; X near direction of spacecraft motion). It also contains the non-TRAC 8-bit data and the once-every-43-sec TRAC-produced 12-bit data. In addition it contains components of the IGRF/1980 field in a coordinate system fixed by the true radial to the Earth's center and the direction of spacecraft motion (the orbital system), and a difference vector (TRAC- produced field minus the IGRF model field) as transformed to an IGRF/1980-based geomagnetic coordinate system. This difference vector ignores the difference between the spacecraft coordinate system and the orbital system which can be up to 15 deg, although ancillary information includes data needed to transform between spacecraft and orbital systems. This ASCII data set, also contains needed spacecraft position data. Time span of data set: 81-11-08 - 83-01-31 Time span for every file is given in accompany "MAGNET.TAB" file 1. MAGNETOMETER TRAC EXPERIMENT The combined Soviet-French TRAC experiment was aimed to measure magnetic field variations due to auroral field-aligned currents. It consisted of a "zoom" which was applied to the signals from the onboard magnetic field detector for attitude restitution. These signals were read and treated by the TRAC device in a way of a "zoom". Each of the three magnetic components measured, once in 43 seconds, was digitized to ~ 13 nT/bit, and these values, together with the difference analog signal (currently measured value minus the last digitized) was given to telemetry. (For a full description see Berthelier et al., Annales de Geophysique, v.38, N 5, p.635, 1982). One of the main difficulties in the treatment of the vector magnetic measurements from a satellite is the acount of the satellite attitude. The gravity-gradient oriented satellite of about 1200 kG mass slowly oscillated around the nominal ORBITAL PLANE ORIENTATION (one axis along the local vertical, another axis along the velocity vector, the third axis to form a right- hand frame). The angular amplitude from the nominal orbital plane can reach sometimes 15 degrees. Another attitude problem with this satellite is that after a crossing of the solar terminator, the satellite oscillated during several minutes with a period of about 40-45 seconds due to thermal shock on the 17 meters boom carrying a floating magnet for damping of the satellite's oscillations.Besides that, during intervals where at least one of the magnetic field components changed rather fast, the damping magnet rotated non-homogeneously inducing additional oscillations. In such cases this component could change more than 1320 nT during 43 seconds driving the TRAC signal off scale, and a constant scale limit was given to the output. After the measured vector rotation from the magnetometer axis frame to the SATELLITE, or S/C, FRAME these off scale constant values lead to false deviations in all three components ending just before the end of 43-seconds interval. These motions around the satellite's center of mass, and off-scalings, when they occur, make quite difficult the problem of automated attitude restitution, and thus the Magnetometer-TRAC experiment automated data treatment not feasible. In view of this, in the data presentation below, a simplifying assumption about the satellite attitude was adopted for the mass data treatment, as described below, which made it possible to present these data. However, the auxiliary data are provided in the data file which allow an experienced user to determine, and then use, a more precise attitude from the same magnetometer, or from more precise TRAC data. 2. SHORT DESCRIPTION OF THE INSTRUMENT The onboard standard magnetometer was mounted on a short boom, with its axes inclined to the S/C FRAME. Tts output was digitized to 8 bit words giving about 300 nT/bit. This original magnetic field vector is rotated to the S/C FRAME and its resulting components are given in the data files under the titles, respectively, MAGNX, MAGNY, MAGNZ in units nT ( in the S/C frame) These standard data were originally aimed to only technological purposes of the satellite's attitude restitution. When looked through the TRAC "zoom", this magnetometer had a noise up to about 13-26 nT, a magnetic deviation and zero shifts. These factors became noticeable when the magnetometer outputs were digitized to 12 bits by the "zoom" provided by the TRAC device. Thus these magnetic data are obviously not of a quality of a typical scientific satellite magnetometer. But still, after some averaging, they may be used for evaluation of medium-scale (~ 50 - 500 km) field-aligned current direction and approximate magnitude. 3. TRAC DATA - TELEMETRY OUTPUT The TRAC device digitized the magnetometer detector outputs (3 components of the magnetic field) once in 43 seconds to 12 bit words (about 13 nT/bit). These data were readout through an additional telemetry channel. Then for each magnetic component the output signal (currently measured minus the last digitized) was zeroed, and the difference signal was given to the telemetry during the next 43 seconds, till the new 12 bit digitization, and so on. It produced for each magnetic component an analog saw-tooth shape output signal, and periodic 12 bit digital base values, which are the basis for the further data treatment. 4. TRAC DATA - TREATMENT First of all, the times of 12 bit digitisation/zeroing are determined for each component, spurios points removed, the base values are smoothed. The resulting base values (in nT) measured once in 43 seconds, are given in the columns with the names BXSAT, BYSAT, BZSAT in the S/C FRAME. These base values are qualified by the NOISE CODE for each component (a column in a data file, see below). The coding is: 0 is an unchanged analog measured value; 1 - a digital zeroing point (unchanged); 2 - an interpolated value; 3 - smoothe jump for more than ~ 50 - 100 nT between sequential points. Using these data (the base values and the saw-tooth TRAC signal between them in the S/C frame), a continuous sequence of the measured values. It is given in the columns in the BXSATF, BYSATF and BZSATF (S/C FRAME ). After these procedures, the magnetic field vector is considered as final (though with its components given in a not convenient frame of reference). They are given to the output data in the physical units (in nT). An indirect quality control of the measured magnetic field vector is provided by comparisons of the module of the measured magnetic field with the module of the IGRF80 model field calculated for the measurement date, time and location. The difference between the modules MEASURED - MODEL (BMAG) is given in the column BMODIGRF. (Such a check is justified because the local field-aligned current rotates the magnetic field, but nearly does not change its module, while effects of other currents are small and may be neglected. The difference BMODIGRF is often less than 500 nT, but occasionally may be larger). If the difference is smooth in time/space, this usually corresponds to a zero shift in one or more components. These zero shifts are not too important for the determination of characteristics of the field-aligned currents because the time/space derivative of a magnetic signal carries the most important information. But if the difference between measured and model modules jumps, this most probably means a spurious noise in one or more components, and such a measurement interval must be considred with care. To reduce the quantity of such jumps, a special filtering procedures were applied to the signals, so that when the difference between two sequential points was more than ~ 50 - 100 nT, an interpolation between neighbouring points was applied. If real, such jumps would imply abnormally large field-aligned current densities (which, however, cannot be absolutely excuded). Such a smoothing technique is somewhat similar to a sliding average procedure but applied locally. The large-scale signal trends are present in the these data due to large-scale geomagnetic field gradients along the satellite trajectory. To remove these trends by subtracting the model IGRF80 magnetic field, the rotation must be performed of the magnetic vector from the S/C FRAME, to the ORBITAL FRAME. It needs the known satellite attitude in respect to its ORBITAL FRAME. However, while for parts of some orbits this time-dependent matrix was well determined, its availability and precision are inadequate for the majority of the data given here. In view of this, it was supposed, for simplicity, that the S/C FRAME of reference is adopted to be coincident with the ORBITAL FRAME. (ORBITAL FRAME of reference is the frame with the Z-axis vertically upward, X-axis along the satellite's velocity vector, and Y-axis forming the right-hand frame). The IGRF80 model magnetic field vector is rotated to the ORBITAL FRAME. Its components are given in the data files in the columns BXIGRF, BYIGRF, BZIGRF in the ORBITAL FRAME. These values then were subtracted from the respective full field measured components BXSATF, BYSATF, BZSATF to get the components of the disturbance magnetic field vector, again in the ORBITAL FRAME. Now, this vector is rotated to the GEOMAGNETIC FRAME, The OUTPUT MAGNETIC DISTURBANCE VECTOR with the components DBXGM, DBYGM, and DBZGM in the GEOMAGNETIC FRAME is given in the first data columns just after the UT time. These results of the TRAC magnetic field data treatment allow to analyse short-term variations of the magnetic field due to field-aligned currents. These signal variations are superimposed with the variations due to the satellite's motions around its center of mass which were described above, to zero shifts of the magnetometer-TRAC system, and to not-precise attitude. Due to the simplification adopted concering the satellite's attitude (see above), a given X- and/or Y-component's value can reach 10 000 nT or sometimes even more. Besides that, the effects of the neglected satellite's motions around the center of mass are still present in the output signals as parasites. It may be noted that only the time/space derivative of a component value is significant for evaluation of the field-aligned current direction. Differentiation of the signal with respect to time/space removes the rest of the slow trend and zero shifts from the data. Thus even a large but slowly varying shift of a component due to a not-precise satellite attitude and magnetometer zero shift is not very important for the scientific aims of the TRAC experiment. However, if an experienced user would like to determine the satellite attitude in respect to the ORBITAL FRAME more precisely, the auxiliary information given in the data files allows to do that. Indeed, the same magnetometer signals, or more precise TRAC signals in the satellite frame, are to be used together with the information on the IGRF80 field components given in other columns of the data files. The following types of spurious/erroneous data are noted within the files: 1. Oscillations after a terminator crossing with a period about 40 -45 seconds - a systematic error, may be removed by fitting of a damped oscillation and its subtraction from the signal. 2. Oscillations during perigee when the magnetic field rapidly changes in the S/C FRAME, and the damping magnet at the top of the 17-meter boom rotates not smoothly. 3. Erroneous values during the initial (incomplete) 43-second period, and/or during the final one (incomplete), in each ZAP interval due to inadequacy of the data treatment programs for such cases. To be neglected. 4. Off-scaling of the TRAC scale of +-1320 nT within a 43-second period. Due to the frame transformations involved, a truncated saw-tooth signal appears sometimes during several 43-second periods. The zeroing occurs at a proper place, but the last part of a 43-second period is false, and to be neglected. 5. Individual spurious points ("jumps") due to telemetry noise. Most of them are removed by the data treatment procedures, but some may be left in the signal, especially when it is not clear wheather a particular point is real or not. 5. DESCRIPTION OF COORDINATE SYSTEMS FOR THE TRAC EXPERIMENT. The SATELLITE FRAME is: Z-axis along the upward vertical of the Earth's oriented satellite (within +/- 15 degrees from local vertical in flight), X-axis along the forward direction ( within +/- 15 degrees from the velocity vector in flight), Y-axis forming the right-hand frame (to the left side from the satellite's velocity vector). The ORBITAL FRAME of reference is the frame with the Z-axis vertically upward, X-axis along the satellite's velocity vector, and Y-axis forming the xright-hand frame. The GEOMAGNETIC FRAME has the Z-axis along the IGRF80 model magnetic field vector (Northward in both hemispheres), Y-axis Eastward, perpendicular to the IGRF80 model local meridian plane, and X-axis forming the right-hand frame. 6. DESCRIPTION OF FILES AND FORMAT 6.1. Name of files A filename of a particular data file contains the number of the memory readout telemetry seance, and the instrument indication: the letters "trX" for the magnetic (TRAC) data. In these filenames the last digit (after "tr" instead of X) indicate the number of the ZAP4 recording time interval within the memory readout seance, for which the data are presented. Thus 00642tr2.DAT means that in this file the TRAC data are recorded during the second interval. A particular interval can be identified by its UT time as written in the header "PASSPORT..." of the file. The file extension is .DAT. Each file has a header and number of data records. Logical record -212 bytes +Cr/LF 6.2. Record Format description WORD ASCII MEANING UNITS/COMMENTS 1 I4 Hour (0-23) 2 I3 Minute (0-59) 3 I3 Second (0-59) 4 I4 Millisecond 5 F8.0 DBXGM, X-component of the Disturbance Magnetic Field in the nT (From the TRAC) Geomagnetic Frame 6 F8.0 DBYGM, Y-component of the Disturbance Magnetic Field in the nT (From the TRAC) Geomagnetic Frame 7 F8.0 DBZGM, Z-component of the Disturbance Magnetic Field in the nT (From the TRAC) Geomagnetic Frame 8 F8.0 BXSAT-Time, and Values of Zeroing Points (Digitalization, Full Field) nT (From the TRAC) X-axis, Satellite Frame 9 F8.0 BYSAT- Time, and Values of Zeroing Points (Digitalization, Full Field) nT (From the TRAC) Y-axis, Satellite Frame 10 F8.0 BZSAT-Time, and Values of Zeroing Points (Digitalization, Full Field) nT (From the TRAC) Z-axis, Satellite Frame 11 I2 X- Noise Code (0,1,2,3) for the 0- unchanged measured DBXGM component value; 1- a zeroing point (unchanged); 2- an interpolated, or modified value; 3- smoothed jump by more than ~50-100nT between the points. 12 I2 Y- Noise Code (0,1,2,3) for the 0- unchanged measured DBYGM component value; 1- a zeroing point (unchanged); 2- an interpolated, or modified value; 3- smoothed jump by more than ~50-100nT between the points. 13 I2 Z- Noise Code (0,1,2,3) for the 0- unchanged measured DBZGM component value; 1- a zeroing point (unchanged); 2- an interpolated, or modified value; 3- smoothed jump by more than ~50-100nT between the points. 14 F8.0 BXSATF Full magnetic Field Component in the Satellite nT, (From the TRAC) Frame 15 F8.0 BYSATF Full magnetic Field Component in the Satellite nT, (From the TRAC) Frame 16 F8.0 BZSATF Full magnetic Field Component in the Satellite nT, (From the TRAC) Frame 17 F8.0 MAGNX Full magnetic Field Component in the Satellite nT, (Raw data from the Frame Magnetometer) 18 F8.0 MAGNY Full magnetic Field Component in the Satellite nT, (Raw data from the Frame Magnetometer) 19 F8.0 MAGNZ Full magnetic Field Component in the Satellite nT, (Raw data from the Frame Magnetometer ) 20 F8.0 BXIGRF - IGRF80 Model nT Magnetic Field Along the X-axis of the Orbital Frame 21 F8.0 BYIGRF - IGRF80 Model nT Magnetic Field Along the Y-axis of the Orbital Frame 22 F8.0 BZIGRF - IGRF80 Model nT Magnetic Field Along the Z-axis of the Orbital Frame 23 I7 BMODIGRF - a quality control nT (Difference Between the Module of the Measured Field and that of the IGRF80 Model Field 24 F10.0 Altitude km 25 F8.2 Geographic Latitude of the Degrees, +/-90 Spacecraft 26 F8.2 Geographic Longitude of the Degrees, 0-360 Spacecraft 27 F8.2 L, McIlwain's Invariant Magnetic Earth's Radii Drift Shell Parameter 28 F8.2 L0, Invariant Latitude Degrees, +/-90 L0=arc cos (sqrt(1/L)) 29 F7.2 MLT, Magnetic Local Time Hours.decimal hours (for centered dipole field) 30 F8.2 BMAG, Module of IGRF80 Magnetic milliGauss Field 31 F8.2 ZSUN, Solar Zenith Angle Degrees, 0-180 -------------------------------------------------------------------------- 6.3. STRUCTURE OF A HEADER The header starts with a title: "PASSPORT FOR THE SEANCE S-XXXX, ARCAD-3". It gives (in place of XXXX) the number of the memory readout telemetry seance and the time intervals of the data recording. The orbit numbers within a seance thus are lower, or equal, to the seance number. The first header text paragraph starts with a row "THE NUMBER OF THE TIME INTERVALS - YY". It gives (instead of YY) the number of uninterrupted memory recordings, each one in a particular memory mode ZAP (described in GENERAL.TXT). Each such recording is described as a line in the "PASSPORT...". Within a data file it corresponds to a number of lines each describing a "measurement point" by a line of the data. The number of measurement "points" (from the start to the end of each recording) is shown in two first columns of each line, the next columns give the date (dd.mm.yy) and the UT time (hh.mm.ss.mss) of the start of the interval, then the same for the end of an interval. The before last column of a passport row gives the DT - a data time step in milliseconds (mss), and the last column gives the memory mode ZAP (1-4). In the second heading paragraph, a reminder is given that the first rows (up to 7) with the data in each time/ZAP interval may be false due to telemetry switches, etc., AND THUS MUST BE DISCARDED. The third text paragraph in a header gives some relevant explanations, or a reminder, concerning the data of an instrument presented further. It describes the GEOMAGNETIC FRAME, in which the output data DBXGM, DBYGM, and DBZGM are given. The headings of each data column within a data file are provided, which contain the meaning of the data in the column (see Format description above). As the date of measurements in a data file for each time interval is given in the "PASSPORT OF THE SEANCE", it is not repeated in the data rows, and only the UT is given. The words 14-31 in format description are provided for a qualified user who would like to improve the accuracy of the satellite attitude in respect to the ORBITAL FRAME using these same magnetic field measurements and geomagnetic model calculations, to check additionally the data by the variations of their module. Note that an FTPHBrowswr interface to this dataset, providing data subsetting and and graphical browsing, is available at "http://ftpbrowser.gsfc.nasa.gov/" ------------------------------------------------------------------ Related data and directories: SPDF Data and Orbits Services ------------------------------------------------------------------- SPDF Contact: Natalia Papitashvili ------------------------------------------------------------- Please acknowledge NASA's Space Physics Data Facility and and relevant scientists identified above for data usage. -------------------------------------------------------- Authorizing NASA Official: Dr. R.E. McGuire, Head, SPDF, NASA Goddard Space Flight Center 301-286-7794, e-mail: Robert.E.McGuire@nasa.gov ------------------------------------------------------------------------- ----------------------------------------------------------------------------------