In order to measure the weak electric and magnetic fields from plasma waves
in the geomagnetic tail region, we have designed two kinds of electric
sensors and tri-axial search coils, having taken into account both the most
suitable condition for field detection and reduction of spacecraft noise.
Figure 2 shows the configuration of the GEOTAIL spacecraft along
with the definition of spacecraft coordinates and their relation to the
Two sets of long dipole antennas with a length of 100 m tip-to-tip are
dedicated to measuring the electric field.
They are wire and probe antennas
termed `WANT' and `PANT', respectively.
Two masts of 6 m in length are used for mounting the magnetic field sensors
well away from the spacecraft in an attempt to the reduce the spacecraft
One of these masts, called `MST-F', mounts two sets of the flux-gate
magnetometers which are used by the MGF team. They are dedicated primarily
to the measurement of the DC magnetic field.
Two sets of search coil and their pre-amplifiers which are used by both
the PWI and MGF teams to measure wave magnetic fields or magnetic field
fluctuations are mounted on the other mast called `MST-S'.
In Fig. 2, the spacecraft coordinates X and Y are schematically
shown by planar cross-sections in order to show the
orientations of the antennas more clearly.
The antenna elements of the WANT and PANT are radially deployed from the
spacecraft and orthogonal to each other.
Their axes, U and V, deviate by 15 ° clockwise from
the X and Y coordinate directions, respectively.
The assembly of PWI tri-axial search coils (which is hereafter called
PWI-SC) and their pre-amplifiers is mounted on the top of the MST-S.
The tri-axial search coils associated with the MGF (MGF-SC) are
mounted on the same mast but 2 m inside of the PWI-SC.
The PWI-SC is normally connected to the PWI measurement system. However,
we can select the MGF-SC by a telemetry command if necessary.
The details on the MGF-SC are provided elsewhere in this issue
(Kokubun et al ., 1993).
The direction of the MST-S (and MST--F) deviates by 45 °
clockwise from the spacecraft Y (and --Y) axis.
The three axes of the PWI search coils are defined in a cylindrical
coordinate system as follows:
\alpha is in the tangential direction, \beta measures in the
radial direction, and \gamma is parallel to the spin Z axis of the