Swedish Institute of Space Physics
The Wave and Plasma Density Instrument (F4) on Freja
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
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).
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
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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