Electric field detectors (EFD) on the EXOS-D (Akebono) satellite measure the vector electric field by both the standard double probe technique (EFD-P) and the newly developed ion beam technique (EFD-B). The EFD-P system has been proven to generate high quality data owing to its antenna alignment which is perpendicular to the sun-earth direction at any phase of the satellite spin. The EFD-B system has not yet been fully operated and is ready for a suitable orbital phase for beam experiments. This paper describes the instrumentation and preliminary observed results obtained by the EFD-P such as the Langmuir characteristics of the probe in the plasmasphere and the convection electric field in the auroral ionosphere. An intense electric field has been observed between the region 1 and 2 current on the morning side. Close to the mid-day meridian, fluctuating electric fields that are likely to be related in terms of a cusp/cleft structure were observed.

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The Akebono (EXOS-D) satellite carries triaxial fluxgate and search coil magnetometers with sensors mounted on 5- and 3-m masts, respectively. The fluxgate magnetometer has four automatically switchable ranges from ±1024 to ±65536nT (full scale), and resolutions commensurate with a 16-bit A/D converter in each range (0.031 to 2nT). The rate of sampling is 32 vectors per second. The triaxial search coil magnetometer has a frequency response up to 800Hz. Signals in the frequency range higher than 100Hz are used for VLF plasma wave experiments (KIMURA et al., 1990), while signals less than 100Hz are used for magnetic field experiments.
After the 3- and 5-m masts were extended on March 7 and 8, 1989, respectively, both the magnetometers are operating continuously. Intense small-scale field-aligned currents embedded in the large-scale field-aligned current system were always observed at 1-2 Re altitudes in all local time regions. The region 0 currents which flow in the poleward region adjacent to the region 1 currents were also frequently observed. The search coil magnetometer measured ion cyclotron waves at 1-2 Re altitudes near the equator.

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Plasma waves and sounder experiments (PWS) onboard the Akebono (EXOS-D) satellite have been successfully made after deployment of antenna and turn-on of the instruments on March 4, 1989. In natural plasma wave observations, the dynamic spectra of the plasma waves in the whole range of the orbits are observed in the PWS frequency range from 20kHz to 5MHz. The results are summarized as follows.
i) The upper hybrid emissions that cover a frequency range from the electron cyclotron frequency f_C to the upper hybrid frequency f_UHR for the case where f_C is higher than the local plasma frequency f_P, or cover from f_P to f_UHR for the case of f_P>f_C, are always observed in the altitude range from 1000km to apogee of the orbit of the satellite. From the global distribution of the upper hybrid frequency emissions, the structures of the plasmasphere are clarified.
ii) Auroral kilometric radiations are observed in the region associated with the auroral particle acceleration regions. Polarization and Poynting vectors of AKR are also precisely measured. The data show the possibility of studies on the structure of the acceleration regions in addition to the generation mechanism of AKR. The AKR data that have been obtained in the acceleration region of the auroral particles are especially important, suggesting the possible co-existence of the electrostatic plasma waves with the Z-mode waves.
iii) In the equatorial region of the plasmasphere the enhancement of the upper hybrid mode waves is discovered; the enhancement shows two possible plasma turbulences, the first is the enhancement of the microscopic instabilities that are identified only as the enhancements of the intensity of the upper hybrid emissions and called “equatorial enhancement of the wave turbulence (EPWAT)”; and the second is the enhancement of the irregular change of the upper hybrid frequency suggesting the turbulent variation of the plasma density that is called “equatorial enhancement of the density turbulence (EPDET)”. EPWAT's encircle the magnetic equator forming a disc within the altitude range of 1000km to 9000km, while EPDET's encircle the geographic equator within the altitude range of 1000km to 9000km, forming another disc.
iv) Sounder experiments of the PWS have also been carried out; the results show the principal resonances such as plasma resonances at f_P, upper hybrid resonances at f_UHR, and harmonics of the electron cyclotron frequency at nfC (n=1, 2, 3, ...). The sequence of diffuse plasma resonances f_Dn with the static plasma wave resonance f_Qn are also detected even in a high altitude range higher than 3000km. Very long ducted echoes of sounder transmitting pulses have also been detected even at the altitude of 6000km. All these results of the experiments reveal that the PWS/EXOS-D data provide a very promising key for studies of the plasma and plasma waves throughout the entire global ionosphere and plasmasphere, including the regions of auroral particles acceleration.

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The first study of the Poynting flux of the auroral kilometric radiation (AKR) based on detected amplitudes and phase angle of waveforms has been carried out by the PWS (Plasma Waves and Sounder Experiment) onboard the Akebono (EXOS-D) satellite. With the clarified information on the propagation and polarizations, the propagation mode of the AKR was determined. The first results of the Poynting vector indicated that there is a case where the medium intensity AKR shows L-O mode propagation. The other results indicated that there is a case where sudden polarization reversal from the L-O mode to the R-X mode is associated with explosive AKR bursts. The electrostatic plasma waves were found near the local upper hybrid frequency in the source region. Though detailed studies are deferred for future studies, the evidence from the observation of the upper hybrid mode waves at the very source of AKR indicates a basic necessary condition for the conversion theory.
The intensity and the direction of AKR Poynting flux vary even in a period of 10 milliseconds. The variation of this Poynting flux suggests very fast motion of the source location of AKR for their intense portion.

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The VLF wave instruments on board Akebono (EXOS-D) involve a wide band analyzer (WBA), multi-channel analyzers (MCA), Poynting flux analyzers (PFX), ELF frequency analyzers and a vector impedance probe (VIP) and cover a frequency range from a few Hz to 17.8kHz for electric and magnetic field components. The main objectives of the wave observations are to investigate the wave phenomena closely associated with energetic particle precipitation in the auroral zone and the physics of wave-wave and wave-particle interaction phenomena. In order to clarify these wave characteristics, the determination of the wave normal direction and the direction of the Poynting vectors are essential. Our VLF instruments, especially PFX and ELF, are designed to satisfy such requirements. The present paper introduces all the subsystems of the VLF instruments and some preliminary results of observations to show the characteristic features of each subsystem.

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The Japanese Akebono (EXOS-D) satellite was successfully put into a semipolar, elliptical orbit in order to study particle acceleration processes in the auroral magnetosphere. The onboard LEP (low energy particle) instrument makes three distinct types of charged particle observations: 1) energy and pitch-angle distributions of auroral electrons and ions, 2) mass per charge analysis of positive ions, and 3) onboard detection of particle flux modulations in the HF and VLF ranges. In this report we describe the LEP instrumentation, and present some preliminary results from item 1 above. Due to the initial orbital configuration, emphasis is placed on dayside phenomena, including the polar cusp. Unique examples of high particle activity in the polar cap and the hot plasma distribution along the transpolar arc are also presented. The observations in the cusp region reveal intense but patchy electron fluxes accompanied by keV-range ions with a characteristic energy versus time/space dispersion. The data are interpreted as signatures of longitudinal and/or temporal fluctuations of the magnetosheath plasma injection into the polar cusp. The dayside is characterized by the frequent appearance of low-energy ion conics and beams accompanied by intense low-energy electron precipitation. Finally it is noted that low-energy accelerated ions upgoing along the magnetic field line are observed in a very limited region around the dayside edge of the transpolar arc.

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The ion mass spectrometer (IMS) installed on the Akebono satellite has been newly developed to achieve a simultaneous observation of velocity distributions of both compositions and pitch angles of ions with a very high time resolution. The IMS instrument is working perfectly, though further fine adjustment is needed to measure low energy ions. Even in the early phase of the observation, the IMS detected upflowing ions with both field-aligned (UFI) and conical (Conics) pitch angle distributions. Helium ion fluxes in the observed upflowing ions were larger than expected and nearly equal to oxygen ion fluxes. A mass dispersion phenomenon in which ions with higher mass upflow at lower latitudes were observed in the conics event. Upstreaming electrons associated with the upflowing ions were found at the edge of the inverted-V events. Electrons otherwise bouncing at the mirror point decreased in flux at the appearance of upward flowing ions. This suggests the existence of an electric field below the satellite altitude.

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The Suprathermal Ion Mass Spectrometer (SMS) was developed to study the thermal (0-25eV) and suprathermal (25eV-several keV) ion distributions in the low altitude magnetosphere. The instrument has a mass (1-70amu/e) and plasma density (10^-3 to 10⁵cm^-3) dynamic range sufficient to measure, on a regular basis, the major and the minor ion distribution functions at apogee (≈10⁴km) as well as at perigee (≈300km). The instrument is a radio frequency type mass spectrometer and has a programmable mass resolution (Δm/m from 0.06 to 0.20) which is independent of energy and mass selected.
We present here measurements of the thermal ion mass composition near apogee which show that the composition varies from one dominated by H⁺ and O⁺ to one in which the density of “minor” ions (O⁺⁺, He⁺, N⁺) is comparable to or greater than that of H⁺ and O⁺. We also present high time resolution mass/energy/angle observations in the topside ionospheric plasma in or near the cusp region where rapid heating of the bulk of the plasma and the formation of conic distributions is clearly evident. These unique observations will be compared with other plasma diagnostics on the spacecraft to increase our understanding of ionospheric ion production, convection and loss processes as well as low altitude ion energization phenomena.

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An instrument for measuring the velocity distribution of thermal electrons (TED) is installed in the Akebono (EXOS-D) satellite which was launched on February 22, 1989. The instrument was basically designed to reveal global characteristics of the thermal electrons in the energy range of 0 to a few eV. The present paper gives an outline of the experimental setup, and describes preliminary results of the TED measurements concerning both the velocity distribution and the temperature.

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An aurora-TV camera for studies of aurora dynamics and global monitoring of magnetospheric activities with simultaneous in-situ experiments is installed on the satellite Akebono (EXOS-D). The camera is a two-channel, visible and VUV, snapshot imager with the maximum rating of one shot every 8 seconds. Initial results of VUV imaging are providing promising data for imposed missions.

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A ground system for the operation and data handling of the Akebono (EXOS-D) satellite is briefly reported in this paper. The satellite is remote-controlled from the Sagamihara Operation Center (SOC). Data from the satellite are sent to the Sagamihara main computer system where real time analysis can be carried out. Scientific data can be displayed as “QL plots” of which hard copies are used to find out interesting phenomena. Ephemeris data of the satellite are also stored in the ISAS (Institute of Space and Astronautical Science) data base. ISAS is linked to remote data handling subcenters through electronic networks and gives access to all Akebono data.

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