Document title: Readme.doc for NDADS DE VEFIMAGB datatype Project: DE NDADS Datatype: VEFIMAGB Super-EID: DOCUMENT There may be other documents also identified by this super-EID. NDADS filename: VEFIMAGB_AVG_README.DOC TRF entry B46574.txt in NSSDC's controlled digital document library. Feb. 1998. Document text follows: ---------------------- Dynamics Explorer (DE-2) Electric and Magnetic Field Data (VEFI-MAGB) Averaged (0.5 second) ASCII data files The ASCII data files contain half-second resolution averaged magnetic and electric field vectors from NASA's DE-2 spacecraft (309-1012 km altitude, 89.9 deg inclination, 98 min orbital period) taken between August 15, 1981 and February 16, 1983. 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 provide 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 These data have been extracted by the DE-2 magnetic and electric field teams from the original 1/16-second data which are also archived at the NSSDC. They contain the three components of the magnetic field (Bx, By, and Bz) and two components of the electric field (Ex and Ey); all in SPaCecraft(SPC) coordinate system (X-along S/C velocity, Y-up, , Y-down, [see table below], Z completes right handed system ), and the following Orbit Attitude (OA) parameters: Model magnetic field [1] in SPC coordinates; Altitude of the satellite; latitude and longitude; Magnetic local time, and Invariant latitude. The third component of the electric field, Ez, was not measured due to a boom deployment failure following launch. For this reason, care must be exercised in transforming the electric field into non-SPC coordinates because the missing Ez information may affect all three components of the electric field in the new coordinate system. NOTE: To convert the data from SPC-system to Topographic one (the X axis points to the North, the Y axis points to the East, and the Z axis points to the center of the Earth), use the DE-2 matrix files in the subdirectory MATRIX on the fourth ASCII disk. These files contain the elements of the 3 by 3 transformation matrix (see detailed description in the "vefimagb_avg_matrix.doc" file). The matrix values are provided at 5 second intervals and each file covers the existing data for an entire day. (Use interpolation to obtain 0.5sec resolution data). Every six months the spacecraft was inverted about the X-axis (Y and Z axes changed direction) in the SPC coordinates system for thermal control purposes. While the main maneuver required less than one day, the period for which the Z-axis of the spacecraft was not normal to the orbit plane extended over several days: Mode Inversion Period From To Start Primary Maneuver End -------------------------------------------------------------------------- Inverted Normal 81-08-29 (81241) 81-09-01 (81244) 81-09-06 (81249) Normal Inverted 82-02-19 (82050) 82-02-24 (82055) 82-03-05 (82064) Inverted Normal 82-08-27 (82239) 82-08-31 (82243) 82-09-06 (82246) Each daily file contains a 0.5 sec average of the high resolution data with a filename of VEFIMAGB_yyddd.DAT, for day of year ddd in year 19yy. During the merging of the data sets it was found that MAG-B and VEFI do not always cover the same time spans. In order to keep the largest amount of data possible, the merged data set includes all the available VEFI and MAG-B data. For those times when VEFI data were available but MAG-B was not (6.54%), a fill data value (9999999. for binary files and char '9' to fill all field according to data format for ASCII files) was given to the MAG-B data and for those times when MAG-B data were available, but VEFI was not (6.87%), the fill data value was assigned to the VEFI data. Times for which both VEFI and MAG-B data were fill values in the original data sets were not included in the merged data set. There were also times when certain OA parameters were fill values in the OA data base and they are therefore also fill values in this merged data set. The model magnetic field had fill values for 8.55% of the data. Statistics were not kept for the other OA parameters. DATA ACCURACY The dominant source of error in the DE-2 magnetic field measurements is the uncertainty in the attitude of the spacecraft. The DE-2 spacecraft was designed to an attitude uncertainty specification of about 0.3 degrees which it appears to have met much of the time. (N.B., sinusoidal errors in spacecraft attitude of up to several tenths of a degree due to nutation with period about 30 sec and other vehicle "dynamics" do occur and must be taken into account for some types of scientific analyses!) As a "rule of thumb" each 0.1 degree in attitude uncertainty 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 to show a gradual shift of several 100 nT from the start of a passage across the polar cap to the end on the other side. For an ideal magnetometer on a spacecraft whose attitude and location were perfectly known the difference in the measured magnetic fields at mid-latitudes on opposite sides of the polar cap due to remote ionospheric and magnetospheric electrical currents would be on the order of a few tens of nT. It is therefore common practice to use the measured magnetic field to make "baseline corrections" for these drifts in spacecraft attitude (e.g., see Gary et al., JGR, 99, 11,417, 1994). The absolute accuracy of the DE-2 magnetometers themselves can be evaluated after launch only through comparison with the precision vector/scalar magnetometer observatories located on the ground used to monitor the geomagnetic field. On the basis of such cross-comparisons, R. Langel (Geological Soc. of India Mem., 24, 3-23 1992) has determined 1-sigma errors of +/- 28 nT for the total field measured by the DE-2 magnetometer over the course of mission. The electric field measurements have normal accuracy of +/- 4 mV/meter. This accuracy has been reached after making a contact potential error correction on an orbit to orbit basis. 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 spurious data can usually be recognized by workers familiar with such data sets. These are for most part single point data excursions which show no geophysical correlation between the electric field and the magnetic field vector components. ACKNOWLEDGMENT: We thank Dr J. A. Slavin, T. L. Aggson, and C. Liebrecht (Extraterrestrial Physics Laboratory, GSFC/NASA) for their support in providing the 0.5-s data and for the help with documentation. DATA FORMAT Logical record: 109 bytes + CR/LF (2 bytes) for ASCII files. ============================================================================ Item Description Binary ASCII Units Notes ============================================================================ 1 Tenth of I*4 I9 1/10 of Time of VEFI Milliseconds of day millisec average 2 EX of electric field R*4 F8.2 mV/m in SPC coordinates 3 EY of electric field R*4 F8.2 mV/m in SPC coordinates 4 BX component of magnetic R*4 F8.1 nT field in SPC coordinates 5 BY component of magnetic R*4 F8.1 nT field in SPC coordinates 6 BZ component of magnetic R*4 F8.1 nT field in SPC coordinates 7 BX component of the model R*4 F8.1 nT magnetic field in SPC coordinates 8 BY component of the model R*4 F8.1 nT magnetic field in SPC coordinates 9 BZ component of the model R*4 F8.1 nT magnetic field in SPC coordinates 10 Altitude of the satellite R*4 F8.3 km 309 - 1022 from the surface of the Earth 11 LAMBDA Geographical R*4 F8.3 degree -90.000 - 90.000 latitude of spacecraft 12 PHI Geographical R*4 F8.3 degree -180.000 - 180.000 longitude of spacecraft 13 Magnetic Local Time (MLT) R*4 F6.2 hours See Note 1 14 INVariant LATitude R*4 F8.3 degree See Note 1 (INVLAT) ========================================================================== Note 1: The MLT and INVLAT algorithms were supplied by M. Sugiura (PI for the Magnetometer Investigation) prior to launch and used in the generation of the Orbit-Attitude database. REFERENCES 1 R.A. Langel at al., Initial Geomagnetic Field Model From MAGSAT vector data, GRL, 7,793-796, 1980. 2. G. Gustafsson, N. E. Papitashvili, and V. O. Papitashvili, A Revised Corrected Geomagnetic Coordinate System for Epochs 1985 and 1990, J. Atmos. Terr. Phys.,54, 1609-1631, 1992. 3. N. Papitashvili and J. King, Corrected geomagnetic coordinate software available, NSSDC News, 9, No. 3, 12, 1993. Project definition and guidance: Dr. Joseph H. King Director, National Space Science Data Center Data analysis, and software: Dr. Natalia E. Papitashvili Visiting Scientist, NSSDC Project and data documentation: J.H. King, N.E. Papitashvili