The electric field detector (EFD) on board GEOTAIL measures the electric field by two different techniques, one by the probe technique and the other by electron beam technique. The probe technique (EFD-P) gives electric field in the plane perpendicular to the satellite spin axis by measuring the voltage difference between the two sphere probes; each deployed 50 meters from the spacecraft in the opposite direction. The electron beam technique (EFD-B), measures the drift motion of the gyration center of artificially emitted electrons to obtain the electric field. The drift motion of the electrons is measured by two methods, one by measuring the drift motion itself and the other by measuring the time of the return flight of electrons to the spacecraft. To realize these measurements, EFD is equipped with two additional capabilities. One is the capability to measure the spacecraft potential relative to the ambient plasma and the other is that to control the spacecraft potential by emitting ions. The reliability of the electric field measurements can be improved greatly by employing the probe and beam techniques at the same time. The potential control of the spacecraft enables the plasma detectors onboard GEOTAIL to measure low energy ions which would otherwise be repelled by the positive potential of the spacecraft.This article describes the outline of the electric field experiments on GEOTAIL with emphasis on the principles of the measurements, the configuration of the hardware, the raw data processing, as well as the preliminary results from the initial operation with the intention of providing the basis for the studies which use the GEOTAIL electric field data.
In the cusp region, irregular fluctuations of the electric field are often observed by EXOS D at altitudes of several thousands of kilometers. Amplitudes of the fluctuations sometimes reach 100 mV/m and their spectra are broad. The electric to magnetic field ratios in the frequency range of 0.5-3 Hz agree well with the Alfv6n velocity at observation point. Hence the electric fluctuations are considered to be Alfv6n waves. The flux of precipitating ions at 500 eV to 10 keV and that of the electrons at 70--500 eV are enhanced in the cusp and they are well correlated to each other. On the other hand, correlation coefficients between the power spectral density (PSD) of the electric field at 1 Hz and the precipitating particle flux vary from case to case. When the latitude of 'the cusp is low and IMF is expected to be southward the coefficient is high. This suggests that the waves are generated in association with the injection of particles into the magnetosphere when reconnection occurs. On the other hand, when the latitude of the cusp is high and IMF is expected to be northward, the coefficient is low and the PSD of the electric field is smaller for the same flux of particles than when IMF is southward. In these cases the intensity of the electric fluctuations in the region of the particle injection is possibly not so great as that when reconnection occurs. 1. 2Faculty of Engineering, Toyarea Prefectural University, Toyama, reflection is partial the phase difference is between 0 ø and Japan. 90 ø depending on altitude and frequency. Gurnett et al. [1984] investigated the low-frequency (1.78 Hz to several tens of hertz) electric and magnetic fields observed by DE 1. They found that the E/B ratios are close to the Alfv4n velocity and compared the observation with both the static structure and the Alfv•n wave models. Chrnyrev et al. [1985] and Knudsen et al. [1990] also found that the 11,225
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