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MAG-L Format 1.1 Description


Please start with reading the introduction about the format and CDF.

The dataset containing high precision vector magnetic field data (MAG-L) is designed to meet the science needs of the majority of the investigations directed towards studies of the internal magnetic field - core and crustal fields. Furthermore, it serves as a basic dataset that all investigators may want in order to browse Ørsted data for magnetic activity, data acquisition/fall out, conjugacies etc. Data are fully calibrated and time-synchronous, and are distributed to all groups that formally have joined the Ørsted science investigation and who request this data. Each file in the standard distribution dataset on CD-rom will contain 24 hours of observation time whereas specifically requested datasets distributed via the Web may contain smaller or larger periods of observation time. The data files include the following data items:

  1. Calibrated vector and scalar magnetic field values in ECEF polar coordinates at approximately 1 second time resolution with an absolute timing accuracy of approximately 0.2 milliseconds. Error estimates for the vector values based on the actual field measurements which are sampled at 25 or 100 Hz rate.
  2. Signal intensity of magnetic field oscillations in the bands 5-10/30 Hz approximately (TBC), at 1 second time resolution synchronized with the data in item 1.
  3. Particle indices describing the energetic particle flux-levels. These are given at 1, 2, or 10 second time resolution depending on the actual sampling frequency of the particle detectors. They are synchronized as closely as possible to the data in item 1, but may be up to 0.5 seconds off.
  4. Satellite position in ECEF polar coordinates at 1 second time resolution fully synchronized with the vector data of item 1.
  5. Magnetic field disturbance indices, among others, one proposed by Berthelier and Menvielle. Indices are given at appropriate time resolutions and according to the actual satellite position (orbit).

Data will be distributed every 3 months. Since the calibration parameters are subject to modifications as calibration efforts proceed during the mission, revisions of earlier data releases are anticipated. Therefore, each release will contain data for the complete mission up to the release date. This strategy also allows new groups joining the Ørsted investigation easily to be brought up to date. If, however, the data volume (the number of CD-roms) grows significantly late into the mission, and earlier data are not changed, only new data may be distributed.

2.1 File Content

A 24 hour uncompressed data file from the standard distribution will contain approximately 6 Mbytes (TBC). These files will be written on CD-rom(s) in the Common Data Format (CDF). If a large community of the Ørsted investigators express the wish for data as simple columns of ASCII characters, this will be considered as an alternative distribution format. The complete mission (14 months) can be contained in three CD-roms (five to six if the plain ASCII format is chosen).
2.1.1 Position
The position information originates from the TANS or Turbo-Rogue GPS receiver, whose position data goes into an advanced orbit model to obtain maximum accuracy and provide position information at any point in time. If the GPS data for some reason is unavailable causing a decrease in position accuracy, this is indicated in the position quality flag, QPos.
2.1.2 Scalar Magnetic Field Data, F
The scalar magnetic field is the output of the OVH scalar magnetometer processed as follows:
2.1.3 Vector Magnetic Field Data, B
The vector magnetic field values are those of a not yet final extraction algorithm performing a sub-sampling like filtering of the calibrated 25 or 100 Hz data stream from the CSC instrument. Calibration of the data is described in "IN-FLIGHT CALIBRATION METHODS USED FOR THE OERSTED MISSION" by Nils Olsen et al. This document can be found at http://www.dsri.dk/Oersted/Calibration/ESA-SP.pdf

The error estimates of the three magnetic vector components derived from the filtering above are given in BX,sigma for X = r, theta, and phi.

Bac is calculated from the raw 25 or 100 samples within the 1 second time frame centered at the time of the data point. It is calculated as Bac = rms(Bpert) where Bpert is the length of the magnetic field perturbation vector.

2.1.4 Particle Index, P
Intensities Pi's are calculated from the count rates of detectors viewing parallel to the satellite boom axis, which is typically radial outwards. P1 is from energy channel 2 of detector E1 (50 keV electrons), P2 is from channel 7 of E1 (500 keV electrons), P3 is from detector P1 channel 2 (300 keV protons), and P4 is from detector P1 channel 7 (3 MeV protons). The count rates are multiplied with the geometrical factor of the sensors determined by preflight calibrations to convert to flux. Note, that no effort has been made to clean up the data for spurious counts, solar illumination, etc. Note also that the flux is really dependent on the shape of the particle distribution functions. No ``best fit'' calculation has been performed.
2.1.5 Magnetic Disturbance Indices, I
 The basic idea is: ``as many as possible''. In practice, a subset of a large set of magnetic disturbance indices is selected for each distribution. Of this subset not all indices may be available throughout - or even not at all in - the time period of the distribution, but are present as part of a minimum required set of indices. The sparseness and fill-value facilities of CDF are exploited to make this feasible using only limited file-space. Both the global set and the selected subset of indices are TBD.

One index is likely always to be included, the sector magnetic disturbance index of Berthelier and Menvielle (B&N). This index can be calculated in almost real time in contrast to conventional indices such as Dst (1-2 years). The B&N index is calculated for 3 hour intervals and it is a sector index, where the earth is divided into 9 longitudinal zones, 5 for the northern hemisphere, and 4 for the southern.

The values are given in the following sequence: North sectors in increasing longitude starting at Greenwich, then southern sectors. Details subject to change without notice.
HemisphereSequence no.IAGALongitude
Northern1HAD355
NGK12
2POD90
SVD61
3MGD151
PET158
MMB144
4VIC236
TUC249
5OTT284
FRD282
Southern6PAF70
CZT51
HER19
7GHA116
AMS77
8EYR172
CNB149
10AIA295
TRW294
PST303

Magnetic disturbance indices will also be made available on-line at the Data Centre when they become available.

2.1.6 Quality and Information Flags, Q*
The quality and information flags and the conditions they flag are yet provisional. Some conditions may yet be identified.
2.1.7 SIM boresight unit vector, n
This is a unit vector, giving the direction of the SIM boresight direction in the ECEF local coordinate system. It is given to facilitate adjusting for the non-uniform error distribution in the SIM instruments.
2.1.8 Magnetic coordinates, M
The magnetic coordinates (Quasi-dipole latitude, Quasi-dipole longitude, Modified Apex Latitude and Magnetic Local Time) are calculated using the algorithms described in: A. D. Richmond, Ionospheric Electrodynamics Using Magnetic Apex Coordinates, J. Geomagn. Geoelectr., 47, p. 191-212, 1995 using IGRF 2000 as main field model (secular variation is not accounted for) and a reference height h_R = 110 km.

More information on Quasi-dipole- and Apex-coordinates can be found at ftp://ftp.hao.ucar.edu:122/archive/cedar/apex/README.

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Last modified $Date: 2003/05/19 09:09:32 $ by $Author: kib $.