Objectives

Scarf et al.(1980) proposed to provide instrumentation to study the wave-particle interactions found near the Earth's magnetic equator in the original US OPEN program. A fundamental objective of the so-called OPEN program was to analyze the flow of mass, momentum and energy through the solar wind/magnetosphere/ionosphere system. Many of the physical processes controlling the entry, transport, storage, acceleration and loss of plasma in the Earth's magnetosphere are related directly to the underlying wave-particle interactions.
To meet the requirement for studying wave-particle interactions, design and manufacturing specifications for a Plasma Wave Instrument (PWI) on board the GEOTAIL spacecraft were solicited. In the design of the subsystem, an attempt in simultaneously capturing wave forms of two electric and three magnetic components of the waves was proposed, in addition to the highly sensitive measurement of electric and magnetic field spectra with fine frequency and time resolution.
During the period from October 1982 to September 1983, the ISEE-3 spacecraft performed the first scientific survey of the distant magnetic tail (Tsurutani and Rosenvinge,1984). Based on the observational results of the ISEE-3 studies, several observational areas of interest to be emphasized by GEOTAIL were proposed by Scarf (1985) and other PWI members (Matsumoto, 1985; Nagano, 1985; Hashimoto, 1985; Omura, 1985). These proposals addressed those aspects of the wave phenomena which remained unclear and pointed out where more details investigation was warranted. The following paragraphs contain a brief overview of their main points.

1. Intense broadband electrostatic noise (BEN) has been observed not only in the near-Earth regions of the magnetosphere (Rodriguez and Gurnett, 1975; Anderson, 1982; Gurnett and Frank, 1977, 1978; Scarf et al., 1972; Gurnett et al., 1979; Catell et al., 1986) but also in the tail regions (Gurnett et al., 1976; Scarf et al., 1974, 1984a; Nishida et al., 1985). The ISEE-3 spacecraft observed BEN in the magnetosheath, the lobe and the low-latitude boundary layer (Scarf et al., 1984a), as well as in conjunction with sheath flux ropes (Kennel et al., 1986) and slow shocks (Sibeck et al., 1984). Many theoretical studies directed towards understanding the generation mechanism of BEN have been published (Huba et al., 1978; Grabbe, 1987; Schriver and Ashour-Abdalla, 1987). Strong and persistent efforts by Schriver and Ashour-Abdalla (1987, 1988, 1990) have been undertaken in an attempt to explain the BEN generation mechanism by computer experiments. Their work provides a synopsis of the current state of understanding for the generation of BEN in the plasma sheet boundary layer (PSBL), showing that it is generated by interactions between hot ion beams accelerated in the distant tail region and cold ions flowing from the polar ionosphere. However, their theory and simulation results cannot provide a sufficient explanation for the frequencies of the noise spectra which are actually observed. The BEN does not consist of continuous waves, but impulsive ones as shown by Anderson et al.(1982), Nishida et al.(1985), and Tsutsui et al.(1991). We should investigate further other possible generation mechanisms for BEN. The most useful step for solving this problem is to analyze the actual wave forms of BEN. Since the PWI has a wide band receiver for capturing wave forms, its analysis together with computer experiments would provide us with useful insight into the generation mechanism of BEN.
2. Auroral kilometric radiation (AKR) which was first observed by Gurnett (1974) and termed terrestrial kilometric radiation (TKR) in a relation to auroral substorms was examined using data from the IMP-6 spacecraft by Voots et al .(1977) and Kaiser et al.(1977). Subsequently, Slavin et al.(1984) used data from ISEE-3 to examine strong AKR correlated with the onset of substorms. The quantitative analysis of the correlation between strong AKR and turbulent phenomena in the distant tail region is important in clarifying direct electro-dynamic relations between the geomagnetic tail and the polar magnetosphere. The AKR generation mechanism in and around the localized polar magnetosphere have been treated both theoretically (Wu and Lee, 1979; Lee et al ., 1980; Wu et al ., 1982) and observationally ones (e.g., Ungstrup et al., 1990). If the GEOTAIL observations in the distant magnetic tail could establish a one-to-one correspondence between the AKR activity and plasma turbulence in the tail, this would add greater insight into the tail-polar ionospheric coupling and its role in AKR generation.
3. Scarf et al.(1984a) found relatively weak electromagnetic continuum radiation showing specific polarization patterns which indicated multiple sources and lower cutoff at the plasma frequency in the tail lobe. Coroniti et al.(1984) examined angular distributions of the continuum radiation intensities in the equatorial plane of the tail using the ISEE-3 data. The observation method developed by Gurnettet al .(1988) and a more detailed analysis of intensity as well as polarization of waves in the tail region provide us with important information on tail structure such as the opening of the tail magnetosphere.
4. Electron whistler-mode noise bursts were found by Scarf et al .(1984b) when ISEE-3 traversed the plasma sheet. Comparing them with those observed in other regions would provide useful information on their generation mechanism and its evolution under different plasma conditions.
5. Tsurutani et al .(1974; 1977) carried out statistical studies of Chorus wave emissions from OGO-5 which has orbits with apogees of around 10 Re. In the latter paper (Tsurutani et al ., 1977), a relation between occurrence of Chorus emission and Kp indices was confirmed. The orbit of the GEOTAIL spacecraft has a perigee of around 10 Re on the dayside. Thus, Chorus emissions can be measured by GEOTAIL. Since the PWI has the capability to capture wave forms, the wave vectors, wave polarizations and Poynting flux of Chorus, Lion Roars and other electromagnetic mode waves can be estimated by analyzing the wave form data. Further, a detailed analysis of the Poynting vectors from whistler mode events observed in association with the Kp indices could provide more information on details of their generation mechanism.

Many examples of plasma waves detected in conjunction with slow shock crossings near the magnetic reconnection region (Scarf et al., 1984b), tailward passage of plasmoids (Slavin et al., 1984) and magnetotail flux ropes have been reported. However, the basic plasma mechanisms responsible for the generation of these wave emissions remain unclear. Therefore, a quantitative classification of wave characteristics and behavior using the GEOTAIL data is one of the important tasks for providing a clear understandings of the electro-dynamic behavior of the magnetosphere.
The GEOTAIL spacecraft was launched from Kennedy Space Center at 14:26 UT on July 24, 1992. The primary scientific objectives of the PWI are to characterize wave-particle interaction processes and instabilities associated with the dynamics in the different regions of the magnetosphere with emphasis placed on examining in greater detail the phenomena described above. In the following section, we will describe the PWI subsystem and its various components in greater detail, followed by an overview of some of the initial wave observations.