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 antenna axes. 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 noise. 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 spacecraft.