The magnetic field vector measurement is made in the instrument XYZ coordinate system, where the Z axis was designed to be parallel to the satellite spin axis and the other two axes lie in the spin plane. It was found, however, that a correction on the order of a few tenths of a degree was necessary for the alignment of the instrument. This correction was implemented as follows:
First, the X- and Y- axis measurements were fitted for every 12-s interval with a sinusoidal wave with a shifted baseline and a linearly-varying envelop. The satellite spin period is determined from the oscillation period of these two components, whereas the cross-component phase lag gives the information about the angle between the X and Y axes. The ratio between the X/Y-component offset to the Z-component measurement provides an inclination of the Z axis from the satellite spin axis. Based on those results we rotate the measured magnetic field vector from the instrument coordinate system to the satellite coordinate system. Finally the satellite orientation is determined by rotating the satellite coordinate system so that the measured magnetic field agrees best with the model (IGRF85) magnetic field throughout a satellite pass. As a goodness function for this we adopted the sum of the squares of differences between the measured and model magnetic fields. BN and BE are the two components perpendicular to the model magnetic field, directed northward and eastward, respectively.
The Freja Summary Plots
consist of three pages per pass,
and are available as GIF files (~15kbytes each). This link will
take you to the FSP parent directory. Individual data plots are organized
in subdirectories by year and month. Descriptions of the quantities plotted
may be found in each root directory at the bottom of the listing. There is
also a forms-based FSP engine that can be used
to enter either the date or orbit number to return a list of available files.
Freja Orbit Characteristic Plots
are available to determine satellite coverage in Universal Time and approximate
location. This link will take you to the parent directory containing
all available plots in GIF format, as well as a brief description of the plot
contents. The files are in subdirectories organized
by year and month.
The Freja F2 Magnetic Field Experiment (MFE) uses a three-axis ring core fluxgate sensor and electronics unit which digitizes, filters, processes, and stores signals from the sensor. The Freja MFE records both "dc" vector magnetic fields for investigation of large scale field-aligned currents and "ac" vector magnetic fields for investigations of small-scale size structures and plasma waves. The "ac" vector magnetic field, provided by high-pass filtering the "dc" channel prior to digitization, is sampled at 128 Hz and digitized using 16 bits for a resolution of 20 pT. The "ac" channel high pass 3-pole filter has a 3 dB point set at 1.5 Hz to attenuate oscillations induced by the satellite spin in the earth's magnetic field at a rate of 0.16 revolutions per second. In order to further extend the frequency range, the "ac" signals from the z-axis were also sent to the MFE processor which performed 512 point fast Fourier transforms (FFT) of the signals with a Nyquist frequency of 256 Hz. A three-pole anti-alias filter set at 256 Hz was used for this signal. These data are referred to as the "Freja F2 On-board FFT" data. The resulting spectra are calculated at 1 Hz resolution and then are time-averaged, providing one complete spectra every 2 or 3 seconds.
We are making available "survey" data of the DC peturbation magnetic field
at 1 Hz resolution. These data are for survey purposes only, they are not
intended for publication or formal presentation. Data files, compressed
with gzip, are available for downloading and may be read with the
provided IDL routine, which may be copied
by using the "save ... source" option of your browser.
Plots of these data are also provided. The data result from the removal
of the Earth's main field and removing the spacecraft spin motion, and are
plotted in geomagnetic coordinates with the N-axis +ve toward
magnetic north and the E-axis +ve to the east.
For reference, we used the system described in:
Eccentric Dipole Coordinates For Magsat Data Presentation,
D.D.Wallis, J.R. Burrows, T.J. Hughes and M.D.Wilson
Geophysical Research Letters,
Vol. 9 No.4, pp 353-356, April 1982.
Such data are mainly useful for a qualitative assessment of the DC data and "eyeballing" interesting geophysical phenomena. The data contained in each file are:
Please contact Dr. L. J. Zanetti (zanetti@ jhuapl.edu) before using any of this data in scientific analyses or publications.
We have developed a system of near real-time notification of auroral zone expansions using Freja "AC" magnetic field data. The SWAZI (Satellite Warning of Auroral Zone Increases) reports are automatically generated and distributed via email to various and sundry interested parties. This is our contribution to Space Weather Forecasting . Further information on SWAZI reports is available in the Poster paper (Figure 5) presented under the Images heading from our large-scale Birkeland currents section, which includes links to our formal presentations.