The observation of the solar ultraviolet radiation scattered by the terrestrial atmosphere was performed by a satellite-borne instrument. A grating spectrometer and a interference filter photometer are used for the experiment. The experiment intends to measure spectra of the scattered radiation between 2500Å and 3200Å in daylight side of the earth and to determine the density distribution of the terrestrial ozone layer. The observation covers regions between 80°N and 80°S in latitude. Preliminary data shows that the absolute intensity obtained by the instrument have good agreement with predicted values.

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Planetary plasma sounder (PPS) experiments system is installed in the Ohzora (EXOS-C) satellite which was launched on February 14, 1984, in an orbit with initial apogee and perigee of 865km and initial perigee of 354km, respectively, and initial inclination of 74.6°. Utilizing orthogonal 40m (tip to tip) antennas and swept frequency receivers in the range from 100kHz to 16MHz and using a swept frequency VLF receiver in the range from 500Hz to 20kHz, PPS is operated for two main purposes. One operation is for the reception of natural plasma waves, both in the high frequency and the VLF ranges, and the other operation is for active experiments using the high power (300W maximum) transmitter of the RF pulse where, in the normal mode, the swept frequency signal from 100kHz to 16MHz is modulated with a pulse width of 244μsec. During the first three months after the deployment of the 40m antennas which was made on May 16, 1984, data from more than 500 orbits have been obtained. From these data the following phenomena have been discovered:
1) Leaked component of auroral kilometric radiation,
2) Terrestrial hectometric radiation from the topside region of the ionosphere.
In the frequency range from 100kHz to 450kHz, there exist emissions separated from LF Hiss; the cause of the emissions is attributed to leakage of the auroral kilometric radiation. The leaked components are believed to be produced by the conversion of Z-mode waves to whistler mode waves. With cooperative operation of the stimulated plasma wave experiments (SPW), it is shown that an enhancement of terrestrial hectometric radiation is related to irregularities of plasma density. This result also suggests that the conversion of Z-mode waves to L-O mode waves is taking place near the source regions.

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The data of natural plasma waves obtained from the initial three month observations of the planetary plasma sounder (PPS) system are investigated, focusing on VLF to LF range phenomena in the polar ionosphere. It is disclosed that the dynamic spectrum of the auroral VLF hiss phenomena consists of the main portion and wings. The main portion whose lowest frequency is less than 1kHz is associated with the leaked AKR phenomena, suggesting that the main portion is generated at the acceleration region of the auroral particles. The wings that have a cut-off frequency at several kHz, higher than the main portion, are formed as the result of the cut-off at the lower hybrid resonance frequency in side area of the source region. These features suggest that the auroral VLF hiss emissions are generated through the Landau type interactions with the auroral particles in the large wave number domain of the whistler mode which is of the nature of a electrostatic plasma wave. The detailed features of the LF hiss emissions are derived for the first time from the data with high time resolution in time and frequency. The auroral LF hiss emissions in a frequency range from 100kHz to 450kHz have weak and relatively steady dynamic spectra showing a clear contrast with the leaked AKR phenomena which shows the very intense and highly variable feature. It is found that the generation of the LF hiss is sometimes taking place at the level of 500km altitude. The upper hybrid mode waves are also generated in the polar ionosphere by the electron beams streaming down from the acceleration region, and the waves are observed in the form of locally trapped Z-mode waves that are converted while propagation through the inhomogenous media.

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The data of the stimulated plasma wave experiments (SPW) obtained from the planetary plasma sounder (PPS) aboard the Ohzora (EXOS-C) satellite have been analyzed for the initial three months. From the ionograms of SPW experiments 1) vertical profiles of the topside ionosphere, 2) irregularity of the plasma distribution and 3) identification of the modes of the naturally emitted plasma waves have been obtained. The studies on these subjects lead to the following results.
(i) In the polar region ionosphere, the auroral particle precipitations generate the intense plasma waves even in the satellite level.
(ii) Associated with the generations of the intense plasma waves caused by the particle precipitation, there are irregularities of the plasma distribution in the topside ionosphere which become the origin of the scattering effects on the transmitted RF pulses.
(iii) Enhancement of the electron density takes place caused by the enhanced precipitation of the auroral particles in the auroral region. At the same time remarkable heat input which is indicated by increasing of the electron temperature is also confirmed.
(iv) Near the auroral zone in the lower latitude side than the auroral regions, there is also the region where the heat input is taking place in milder manner than the case of the auroral regions.
(v) Large energy input into the auroral zone triggers a relatively large scale disturbance propagating in the polar cap ionosphere in the form of the traveling ionospheric disturbance (TID).

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The magnetic field strength at the fundamental frequencies 50Hz and 60Hz, of power line radiation in the topside ionospheric region has been observed by the instrument aboard the satellite “OHZORA” in the global area. The observation has started on June 1, 1984, in order to measure the magnetic field strength of the power line radiation in the topside ionospheric region over the eastern Asia. The observation instrument consists of a loop antenna and a receiving unit, which can receive the fundamental frequencies (50 and 60Hz) of power line radiation by means of three narrow-bandwidth filters. It is identified that the origin of the background noises of magnetic fields observed at these frequencies is due to the ELF hisses propagating from the outer plasmasphere. By means of the statistical procedure, the background noise field strength at 50 or 60Hz can be determined in comparison with the strength at 55Hz. If the magnetic field strength is higher than the level of ambiguity, the corresponding position can be specified as the reasonable position of power line radiation. According to the result of observation, the power line radiation at 60Hz radiated from the Japan Islands seems to spread over the wide area. The observation results also show that the power line radiation over the eastern China reveals a correlation between the statistically reasonable positions of magnetic field strength at 50Hz and the possible distribution of power lines on the ground.

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Observations of fluxes and energy spectra of electrons (0.19-3.2MeV) and protons (0.64-35MeV) by OHZORA satellite are reported. The results obtained during March 14-June 22, 1984 are discussed in this paper. The maximum fluxes of electrons and protons are observed in the South Atlantic Anomaly and the auroral zones. Narrow peaks of counting rate in “the midlatitude zone” of L=1.6 are observed over the south Pacific Ocean and Mexico, and the pitch angle distribution of the particles shows that these particles are geomagnetically trapped. Temporarily intensive proton and He ion fluxes at the north polar cap region are observed, and their particle energy spectrum shows that solar flare particles are injected into the polar cap.

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The low-energy particle experiment (ESP) onboard the EXOS-C satellite has given us new information on characteristics of low-energy electrons and ions. Our observations of auroral electrons and ions are mostly consistent with the observational results obtained by others. However, some interesting characteristics of ions precipitating into the auroral region have also been found and discussed in this paper. 1) Three types of energy dispersions of ions in the noon sector were observed at different magnetic activities. The first is the increasing energy with increasing invariant latitude, the second is the decreasing energy, and the last is no energy dispersion. 2) Ions penetrating into the evening and midnight sectors could be divided into two patterns of the energy spectra. Ions in the energy range of 1 to 10keV are populous in the region of the diffuse electron precipitation, while the ion spectra harden in the discrete precipitation. 3) Intense precipitating or trapped ions were observed in the region of the diffuse electron precipitation of the morning sector.

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The low-energy particle experiment (ESP) on the EXOS-C satellite was designed to measure the energy spectrum and the pitch-angle distribution of electrons over the energy range of 6eV to 16keV and positive ions from 200eV to 16keV. The experiment has been successful and it has revealed global characteristics of ionospheric photoelectrons and magnetospheric energetic particles over a wide range of latitude, longitude, altitude (350-850km) and local time. The precipitation of auroral particles is observed at latitudes higher than the trapping boundary of higher-energy particles. The electron precipitation pattern is diffuse in the dayside auroral region, whereas it is quite discrete on the nightside. The energy spectrum of ions precipitating into the auroral region is found to be generally harder than the corresponding electrons. Energetic electrons with energies of 10-100keV have also been detected at dusk hours near the equator. The nature of these electrons is not yet known. The energy spectrum of photoelectrons is found to have a peak in an energy range from 20 to 30eV and a sharp cutoff near 60eV. However, the spectral shape in the energy range of 20 to 30eV is found to vary significantly depending on latitude, longitude and local time. A large peak at 20-30eV is observed occasionally over the region of the South-Atlantic geomagnetic anomaly.

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Observations of the electron number density have been successfully carried out by the NEI instrument on board the Ohzora (EXOS-C) satellite after the launch on February 14, 1984. The NEI instrument was designed to measure the effective capacitance of the rod type probe whose length is 45cm with diameter of 2cm in the swept frequency range from 0.1MHz to 16MHz.
The results of the initial phase observation show complicated features of the polar ionosphere relating to the auroral particle precipitation and also the large scale movement of the ionospheric plasma. The trough region in the ionospheric level frequently observed in clearly defined form and the remarkable enhancements of the ionization are sometimes observed at the low latitude side of the boundary. Usually the ionosphere around the auroral oval region shows ionization enhancement and the small scale irregularity is predominant. The precipitating low energy particles in the auroral region, excite intense plasma waves. The electrostatic component with large amplitude and also charging of the probe clearly appear in the detected impedance values versus the swept frequency. The polar cap ionosphere which usually shows lower density level than the auroral oval region, indicate sometimes a very large electron density enhancement exceeding auroral oval ionization even in the night side.

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Overview of the electron temperature measurement which was carried out by means of the satellite “OHZORA” is presented. The principle of the measurement, the measuring circuit and several preliminary data are described. One of the findings which were obtained from one year observation is the unisotropy of the electron temperature. The electron temperatures show the difference when the sensor electrodes take the parallel and perpendicular directions with respect to the geomagnetic line of force; T_// is 1.2-1.7 times higher than T_⊥. This tendency is found all through the day in the auroral region but is limited to the daytime in the equatorial region.

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Accurate measurement of the radiation pattern of ground-based large array antennas in the VHF band has been a difficult task due to a large distance required to achieve the antenna far field in the vertical direction. A new system for a long-term monitoring of the MU radar antenna pattern has been developed using the Japanese scientific satellite OHZORA (EXOS-C).

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