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Swedish Institute of Space Physics
Uppsala Division

The Wave and Plasma Density Instrument (F4) on Freja

[Freja picture]


Principal investigator for the wave and plasma density instrument (F4) is Bengt Holback at the Uppsala Division of the Swedish Institute Space Physics, where most of the instrument has been designed and built. However, several co-investigators has contributed substantially with hardware and data analysis (see text below).


For the measurements of plasma density and elctric wave fields, we use six spherical probes of radius 3 cm. The spheres are mounted on wire booms (length 5.6 or 10.6 meters) deployed in the satellite spin plane (rotation rate 10 r.p.m.). You can see one of these probes at left center in the figure above, and one half of another spherical probe in the lower left corner. The wire booms to the other probes are also visible in the figure.

To measure wave magnetic fields, there is a tri-axial search coil magnetometer provided by CETP Velizy, France. This magnetometer is mounted on a stiff boom, which in the figure is seen to extend from the "rear" side of the satellite (top center). On the same stiff boom, one can barely discern the short HF antenna (Cornell University, USA) for the detection of high-frequency electric wave fields. Furthermore, a cylindrical Langmuir probe gives additional data on the plasma characteristics. The cylinder probe is seen as a thin rod from the stiff boom which extends from the satellite towards us in the figure (lower center).

Data handling

All signals are transmitted to ground as time series (wave forms); there is no on-board spectral analysis. We have several different channels with different sampling frequencies, varying from 128 samples/s to 8,000,000 samples/s! It is not possible to transmit data sampled at so high sampling frequencies to ground continuously. Instead, there is a burst memory, provided by the University of Oslo, in which data are intermediately stored before transmission to ground. Other groups who have contributed to the F4 instrument and now are working with the data analysis are found in Umeå, Sweden, and Helsinki, Finland. We are also in close cooperation with the groups responsible for the other instruments on Freja.


The wave instrument worked faithfully during all the main data acquisition period of Freja, from October 1992 to June 1995. In the very last days of this period, the instrument finally gave up, after having collected data from more than 10,000 passes over the ground stations Esrange (Sweden) and Prince Albert (Canada). During the operations period, we tried to vary our use of the instruments as much as possible, and to run it in several (around 40) different modes. The planning for the operations of F4, and to some extent also of some of the other instruments, is documented here (rather technical and boring information unless you are familiar with the instrument).

Results and data

Summaries of the Freja scientific data, including F4 data, are provided by the Freja summary plots, available at APL. At your next visit to this page, we hope to have higher resolution summaries of F4 data available here. Welcome back!
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