Document title: Readme file for NDADS DE datatype MAGA_GMS Project: DE NDADS Datatype: MAGA_GMS EID: DOCUMENT Super-EID: DOCUMENT There may be other documents also identified by this super-EID. NDADS filename: MAGA_DE_GMS_README.TXT TRF entry: b48485.txt in NSSDC's controlled digital document library, Mar. 1900. Document text follows: ---------------------- DYNAMICS EXPLORER 1 HIGH RESOLUTION MAGNETIC FIELD DATA SET J.A. Slavin, NASA/GSFC, Code 696, Greenbelt, MD 20771 (slavin@lepjas.gsfc.nasa.gov; 301-286-5839) DESCRIPTION OF SPACECRAFT Dynamics Explorer 1 spacecraft was one of two satellites in the Dynamics Explorer program. The DE-1 and DE-2 satellites were launched by the same vehicle so that their orbits would be coplanar, allowing two-point measurements along magnetic field lines, for the purpose of studying coupling between the magnetosphere, ionosphere, and upper atmosphere. The DE-1 orbit was highly elliptical with an apogee of 4.35 Re and a perigee of 500 km whereas the DE-2 spacecraft was placed in a much lower 300 x 1000 km altitude orbit. DE-1 was spin stabilized with its spin axis normal to the plane of the orbit. DE-2 was three axis stabilized with one face being nadir oriented. INVESTIGATION OBJECTIVES The study of field-aligned currents and MHD waves were the primary objectives of the DE-1/2 magnetometer investigation. Comparison of the magnetometer data with measurements of precipitating charged particles yielded new information on the field-aligned current carriers. In combination with the electric field measurements, it was possible to determine the vertical Poynting Flux of electromagnetic energy flowing between the magnetosphere and ionosphere and to separate small-scale field-aligned currents from MHD waves through the evaluation of the local ratio of the electric to magnetic field amplitudes in these perturbations. The field-aligned current measurements and neutral atmosphere observations also provided an opportunity for investigating atmosphere-magnetosphere coupling and assessing the total rate of energy transfer into the upper atmosphere. Finally, the DE-1/2 magnetometer investigation provided a vital service in so far as a knowledge of magnetic field direction and intensity is essential to any number of space plasma science investigations utilizing the various DE-1/2 particles and fields data sets. DESCRIPTION OF THE DATA The DE-1 magnetic field (MAG-A) high time resolution data set consists of triaxial fluxgate measurements taken every 62.5 msec (i.e., 16 vectors/second). As described in Farthing et al. (1981), the DE-1 magnetometer had a digital resolution of +1.5 nT in its low altitude, least sensitive mode. Two higher sensitivity modes were used at higher altitudes with digital resolutions of +0.25 nT and +0.02 nT, respectively. The MAG-A data set consists of the three components of the magnetic field, B-Radial (Br), B-Theta (Bth), and B-Phi (Bph), in Geomagnetic Spherical (GMS) Coordinates. This is a local Cartesian coordinate system. The R, Theta and Phi axes are oriented relative to a MAGSAT magnetic field model (Langel et al., 1980) positive in the directions of increasing radial distance from the center of the Earth (i.e., outward), increasing magnetic colatitude (i.e., southward) and increasing azimuth angle (i.e., magnetic east). The following Orbit Attitude (OA) parameters are also included in the archive data set: model magnetic field in GMS coordinates; altitude of the satellite; magnetic latitude and longitude; magnetic local time, and invariant latitude. The data set consists of daily files from 81258 to 91049. Each file contains all of the data available for a given day. If there were no magnetometer data for a given time, the time record was left out. If there were magnetometer data, but no orbit or model field data, a fill value of 9999999.0 was used for the missing values. Data Accuracy The dominant source of error in the DE-1 magnetic field measurements is the uncertainty in the attitude of the spacecraft. The DE-1 spacecraft was designed to an attitude uncertainty specification of about 0.3 degree which appears to have been met much of the time. As a "rule of thumb" each 0.1 degree in attitude uncertainty near perigee corresponds to an error of approximately 100 nT in each component of the field when the magnetic field measured at the sensors is transferred to an inertial frame of reference or a model field is transferred into the spacecraft frame and subtracted from the measured field. For this reason it is common for the residual, or "delta-B" field obtained by subtracting the model field at low altitudes (i.e., high fields) to show a gradual shift of several 100 nT from the start of a passage across the polar cap to the other side. (These slow shifts in the "baselines" of the vector field components do not affect most scientific analyses, e.g., field-aligned current measurements, but they can be effectively dealt with through modeling if need be.) At higher altitudes the ambient field intensity is less and the uncertainty due to attitude errors is correspondingly smaller. The absolute accuracy of the DE-1 total magnetic field measurement has also been evaluated through comparison with the precision vector/scalar magnetic field observatories located on the ground which are used to monitor the geomagnetic field. On the basis of such cross-comparisons utilizing DE-1 perigee data over the life of the mission, R. Langel (private communication, 1994) found excellent agreement between the MAG-A and ground-based observatory scalar data sets at the 20 to 40 nT level. On time scales comparable to or less than the DE-1 spin period, 6 sec, other artifacts are present in the data set which must be considered for some scientific investigations. Like most telemetered geophysical data, the vector components archived here suffer from occasional bad data points. These spurious data entries were caused, for the most part, by noise introduced in the satellite-receiving station telemetry link. Such bad data can usually be recognized by workers familiar with such data sets. These are for the most part single point data excursions which show no geophysical correlation between the magnetic field components and the observations of plasma phenomena by the other DE instruments. Similarly, there sometimes exist spurious data points in the ancillary orbit/attitude database. Some are obvious such as model magnetic field values for which the sign values have been corrupted. Others, such as occasional millisecond jumps in the time, produce small, unphysical discontinuities in the processed field components. Small discontinuities are also sometimes present at the point where the magnetometer changes mode due to slight imperfections in calibration parameters which are independently determined for each mode. (N.B., mode changes can be readily detected by the change in the digital resolution of the data in an expanded vertical scale plot of B versus time.) In using any unfamiliar data set, caution is advised and tests to screen out instrumental artifacts should be devised before reaching important conclusions. De-spinning high sensitivity, boom mounted vector magnetometer data in high fields (i.e., >1000 nT) frequently results in a readily observable residual signal at the spin period and its harmonics. In the case of the DE-1 magnetometer measurements, the dominant causes of residual spin tone were found to be small (0.1 to 0.01%) changes in the instrument scale factors and boom bending of up to several tenths of a degree in response to varying thermal inputs due to orbit/attitude driven changes in solar illumination (e.g., seasonal variations, eclipses, etc.). These effects were minimized through an orbit by orbit calibration procedure which analyzed the residual spin tone around apogee and perigee and adjusted the scale factors and sensor attitude accordingly. Even after these in-flight calibration activities, residual spin tone signals in the MAG-A data with amplitudes of tens of nanotesla are common in high fields around perigee. The most probable cause of these residuals is the transverse field dependence of fluxgate magnetometers in high fields which was not well-appreciated at the time that DE-1/2 magnetometers were designed and calibrated in the late 1970's. As discussed by Luhr et al. (1995) in regards to the magnetometer on the low altitude, spin stabilized Freja spacecraft, this non-linear effect can easily produce the residual spin frequency signals present in the MAG-A data set. DYNAMICS EXPLORER-1 MAG-A DATA FORMAT Logical record: Fixed length, 60 byte records for binary files, VMS binary Year integer*4 Day integer*4 Day of year, January 1 = day 1 Time integer*4 Time of measurement in milliseconds from beginning of day Spare integer*4 Spare field BR real*4 Radial-Component of Measured magnetic field in GMS coord., nT BTH real*4 Theta-Component of Measured magnetic field in GMS coord., nT BPH real*4 Phi-Component of Measured magnetic field in GMS coord., nT MR real*4 Radial-Component of Model magnetic field in GMS coord., nT MTH real*4 Theta-Component of Model magnetic field in GMS coord., nT MPH real*4 Phi-Component of Model magnetic field in GMS coordinates, nT GALT real*4 Satellite altitude, km MLONG real*4 Magnetic longitude, degrees, -90.0 to 90.0 deg. MLAT real*4 Magnetic latitude, degrees, -180.0 to 180.0 deg. MLT real*4 Magnetic local time, hours, see Note 1 ILAT real*4 Invariant latitude, degrees, see Note 1 The files are unformatted so the files may be read using the following FORTRAN statement: READ(UNIT) YEAR, DAY, TIME, SPARE, BR, BTH, BPH, MR, MTH, MPH, GALT, + MLONG, MLAT, MLT, ILAT Remember to declare year, day, time, spare as integer*4. ================================================================ Note 1: The MLT and ILAT algorithms were supplied by M. Sugiura (PI for the Magnetometer Investigation) prior to launch and used in the generation of the Orbit-Attitude database. SOFTWARE Two IDL (Interactive Data Language from Research Systems, Inc.) procedures are included with this data set. The first IDL procedure reads the data and does not average. The second program reads and averages the data. A brief explanation of the IDL procedures follows: 1. Program DEHIRES_load loads the high resolution magnetometer data for a time interval no greater than one hour. dehires_load, filename, start, finish, br, bth, bph, mod_br, mod_bth,$ mod_bph, galt, mlt, ilat, mlong, mlat, time, flag Inputs: Daily file name, start time, end time Outputs: Br, Bth, Bph, Mr, Mth, Mph, altitude, magnetic local time, invariant latitude, magnetic longitude, magnetic latitude, time, and a flag if there was no data in the requested interval. 2. Program DELOAD_AVE loads and averages the magnetometer data. deload_ave, filename, start, finish, res, abr, abth, abph, amod_br,$ amod_bth, amod_bph, mid_alt, mid_mlt, mid_ilat, mid_mlat,$ mid_mlng, mid_time, flag Inputs: Daily file name, start time, end time, averaging resolution Outputs: Averaged Br, Bth, Bph, Mr, Mth, Mph, and midpoints of altitude, magnetic local time, invariant latitude, magnetic longitude, magnetic latitude, time, and a flag if there was no data in the interval that was to be averaged. Note: For both procedures, the start and stop input times should be in milliseconds. The averaging resolution input to DELOAD_AVE should be "1/2", "1/4" or "1/8" (second). REFERENCES Farthing, W.H., M. Sugiura, B.G. Ledley, and L.J. Cahill, Jr., Magnetic field observations on DE-A and DE-B, Space Science Instr., 5, 551, 1981. Langel, R.A., R.H. Estes, G.D. Mead, E.B. Fabiano, and E.R. Lancaster, Initial geomagnetic field model from MAGSAT vector data, Geophys. Res. Lett., 7, 793, 1980. Luhr, H., F. Primdahl, and T. Risbo, The transverse field dependence of fluxgate magnetometers and its implications for low altitude satellites (abstract), Chapman conference on Measurement Techniques for Space Plasmas, Sante Fe, New Mexico 1995. J. Slavin Updated 25 August 1995