J. Geomag. Geoelectr., 42, 367-370, 1990

Introduction to the Akebono (EXOS-D) Satellite Observations

By studies during the last decade of the 1970s, it has become clear that auroral particles are not directly injected from the tail region of the magnetosphere butthat there are instead two steps involved in the acceleration mechanism. The first step of the energization process occurs in the tail region of the magnetosphere while the second is found in the spatial region where the electric field is developed in a direction parallel to the earth's magnetic field. It is believed that the region of this electric fieldexists between a range of altitude of 3,000 km to 20,000km, centering around 6,000km, on fieldlines connected to the polar ionosphere. Although the general features of these acceleration processes have been identified based on previous satellite observations, there still are basic questions that remain unsolved about the physics of the acceleration region. i) What is the initial causative agent for the formation of the acceleration region? Is it induced by fast plasma injection, high energy electrons, or by magnetic field as original currents? ii) Are the particles accelerated by an electrostatic shock, a double layer, or growing large amplitude waves? iii) How is the development of the regions of acceleration related to the occurrence of magnetospheric disturbances, called substorms? The EXOS-D satellite was designed to seek solutions to these questions by conducting direct observations of the particle acceleration regions above the auroral region, with eight scientific instruments: a) Electric field detector (EFD), b) Magnetic field detector (MGF), c) Very low frequency plasma wave detectors (VLF), d) Plasma wave detectors in high frequency range and sounder (PWS), e) Low energy particle spectra analyzer (LEP), f) Suprathermal mass spectrometer (SMS),

g) Temperature and energy distribution of plasma (TED), h) Auroral television camera (ATV). With all these eight scientific instruments onboard, the EXOS-D satellite was successfully launched on February 22, 1989 into a semi-polar orbit with an initial apogee, perigee and inclination of, respectively, 10,500 km, 274 km and 75•‹and with an evolution period of 212 min. As common facilities to support the operation of these scientific instruments, the spacecraft was equipped with two sets of 60 m tip-to-tip wire antennas, one three-axial loop antenna with a 60cm•~60cm rectangular shaped winding, 5m and 3m extensible masts and a despun-mirror system. The deployment of all these antennas and masts was completely carried out on March 6 and 8; in Figs. 1 and 2, the in-orbit configuration of the EXOS-D (Akebono) satellite with its deployed antennas and masts and a photograph of the flight model after the final assembly test are shown. The spacecraft was spin-stabilized by making the direction of the spin axis pointing towards the sun in order to collect the maximum solar power. The designed spin-rate of

367 368 Introduction

(a) Front view

(b) Back view

Fig. 1. The Akebono (EXOS-D) satellite, in-flight configuration. In panels (a) and (b), the front and back views are respectively indicated. The satellite is equipped with four solar paddles, two sets of 60m tip-to-tip wire antennas, 3m and 5mlength masts and three axial-loop antennas. Telemetry antennas are also installed both on top and bottom panels of the satellite. Introduction 369

Fig. 2. Photograph of the Akebono (EXOS-D) satellite at the phase of final pre-launching test. Within its body of a 126 cm diameter and 95 cm height and of a weight of 295.4 kg, eight scientific instruments are installed with all common facilities.

7.5 rpm is controlled by magnetic torqueing which can also control the satellite's attitude. From March 27 to April I, 1989, the high tensions of power supply system of each instrument was switched on so that all the scientific items have been on the regular schedule of operation since April 2, 1989. Observed data are transmitted through a S-band system with a bit rate of 64 kbps on the high bit rate mode, 16 kbps on the medium bit rate mode and 4 kbps on the low bit rate mode. PCM telemetry data are received at four tracking ground stations, i.e. Kagoshima Space Center (Japan), Prince Albert (Canada), Esrange (Sweden) and Syowa Station (Antarctica). Owing to the 64 Mbits bubble memory installed on the satellite, we can obtain a worldwide data coverage. It is possible to send long series of commands, called OP's, to the onboard computer, which consist of a wide variety of lists for the control of the operation of the scientific instruments. Through this OP system, we can then automatically control the satellite for sufficiently long periods, up to a maximum period of one week. Data sent by the EXOS-D are of a very high quality with a large variety of types of observations for each field of science and with a high S/N ratio and time of resolution. Correlative studies from each instrument provide essential data on the microscopic and macroscopic aspects of the plasma, the field and particle phenomena, especially that relating to wave-particle interactions, on the effect of plasma heating and on the acceleration of the auroral charge particle beams. Owing to the measurement of the magnetic field, new findings on the acceleration processes and on the state of the current carrier have been clarified both from the microscopic and macroscopic aspect of the problem. In the equatorial plasmasphere, disturbances forming a ring shape in an altitude range of 1000 km to 10,000 km or above and surrounding the magnetic equator have been discovered. All these results are reported in detail in this special issue. After launching the satellite, very large sun spot activities induced an unusually large earth magnetic storm. Even though only wave and field detectors were operated during 370 Introduction

the storm time, because of the preparation phase for particle detectors, observations were carried out during the climax of the largest storm which started on March 13, 1989. The

plasmapause moved down to the invariant latitude of 48•‹ with a large oscillatory and turbulent shape. The magnetic field detectors also identified an unusually large current injection from the magnetosphere. Even though detailed studies will continue on several succeeding years, we can soon expect extremely important results on the energy transport of the solar wind into the earth's upper atmosphere through the magnetosphere during the operation of the EXOS-D. The EXOS-D (Akebono) satellite was launched as the first spacecraft associated with STEP (Solar Terrestrial Energy Program) which was extensively organized for the

period of 1990 to 1995 as the most significant SCOSTEP (Science Committee on Solar Terrestrial Physics) program endorsed by the ICSU (International Council of Scientific Union). We hope that the EXOS-D (Akebono) satellite will make a most significant contribution to this current of international collaboration in the various phases of the STEP studies. As a direct outcome from an international collaboration on the EXOS-D (Akebono) satellite, SMS science team has been invited with their instrumentation from Canada, and data acquisition is progressing from Prince-Albert, also in Canada, as well as from oversea stations at Syowa and Kirna in Sweden. Therefore, data can be acquired with a rate of almost 80%. To conclude this introduction, we as well as all the authors of this special issue, would like to express our hearty thanks to Prof. J. Nishimura who acted as the representative of the ISAS for the management of the ISAS which was essential to achieve these EXOS-D projects. The launch and the operation of the satellite were conducted by the ISAS engineering team to whom we, as well as all the authors of this issue, are very grateful to, especially to Prof. T. Hayashi, the deputy director of the ISAS who arranged for the best collaboration conditions to achieve this mission.

March 10, 1990

Hiroshi Oya Geophysical Institute Tohoku University Sendai 980, Japan

Koichiro Tsuruda Institute for Space and Astronautical Science Sagamihara, Kanagawa, Japan