A new compact deep-towed proton magnetometer system was developed and tested in 11 cruises around the Japanese Islands. The pressure case towed by a steel wire rope on the winch carries a portable proton magnetometer, a depth meter and their power supplies. The magnetometer sensor is connected to the pressure case with an armored cable 15 m long. The system automatically measures the total geomagnetic field intensity and the towing depth. It also stores the observed data on the internal digital memories within the instruments. This self-contained system enables us to make deep-towed observations up to a depth of 6000 m without an expensive armored towing cable. Furthermore, the simple system facilitated the operation and the production, and it reduced the costs. In our experiments, we successfully collected a maximum of 13 hours data and recorded a maximum towing depth of 3667 m. Some observed deep-towed data showed detailed information about magnetic anomalies, which could not be detected by surface-towed magnetometers. We will here explain the system with the results of two experiments and show that it is very useful for deep-towed magnetic observations.

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Magnetic field in and around a magnetically permeable shell of cubic shape placed in a uniform external field is studied. The Laplace equation satisfied by magnetic potential is solved by use of relaxation technique in three-dimensional space. In contrast to the case of a spherical shell, rarefaction of magnetic equi-potential lines is not quite remarkable indicating considerable weakening of shielding effect in the shell interior. This is probably caused by leakage of magnetic flux from the shell wall near the sharp corners. It is therefore apparent that a shell of which the shape is close to a sphere should be used in order to have an efficient shielding. Magnetic shielding of a cubic shell having a defect on one of the shell wall is also studied. It turns out that the extent of shielding power is weakened only slightly even for a large defect. In one of the Appendixes, shielding by a flat square shell is studied reaching a conclusion that the shielding power of the shell is fairly small. It is not practicable to use such a shell for protecting a floppy disk from a magnetic disturbance arising from outside.

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Break-up of Comet Shoemaker-Levy 9 entering Jovian atmosphere was studied with highly accurate hydrocode ExCIPHER by changing the equation of state (EOS) of the comet. The comet of radius 1.5 km was completely destroyed around the atmospheric pressure of 10 bar which is shallower than the result by Mac Low and Zahnle. Furthermore, the behavior of break-up is quite different from them. This behavior depends on the type of EOS. For hard material like water and ice, the impact of Jovian atmosphere caused incompressible surface wave propagating along the comet surface thus enhancing ablation loss in low pressure atmosphere. For ideal EOS, however, the surface wave did not appear since the comet was compressed upon impact and hence the ablation loss around 5 bar was quite low. Although the comet was broken into moderate pieces at 10 bar, most of the energy had already been lost well before the break-up. Surprisingly, however, the final disintegration occurred around 10 bar for tested EOS and even for planar geometry.

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Neural network is applied to the knowledge-based prediction rule system of solar flares based on the attributes of sunspot characteristics. The inputs are the magnetic class, modified Zürich class, area and number of sunspots, and the output gives the occurrence probability of flares in the following day. Simple 3-layer network model has been trained by using the if-then rule based patterns, and almost 99% of correct answer rate is obtained which can substitute for collating data base with present sunspot characteristics. Also, some tests for the specification of input units, the necessary number of hidden units and generalization capability have been made. Further sophistication of the network is to be expected for the practical prediction of our sun-earth environment.

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The low energy particle (LEP) instrument onboard GEOTAIL is designed to make comprehensive observations of plasma and energetic electrons and ions with fine temporal resolution in the terrestrial magnetosphere (mainly magnetotail) and in the interplanetary medium. It consists of three units of sensors (LEP-EA, LEP-SW and LEP-MS) and a common electronics (LEP-E). The Energy-per-charge Analyzers (EA) measure three-dimensional velocity distributions of electrons (with EA-e) and ions (with EA-i), simultaneously and separately, over the. energy-per-charge range of several eV/q to 43 keV/q. Emphasis in the EA design is laid on the large geometrical factor to measure tenuous plasma in the magnetotail with sufficient counting statistics in the high-time-resolution measurement. On the other hand, the Solar Wind ion analyzer (SW) has smaller geometrical factor, but fine angular and energy resolutions, to measure energy-per-charge spectra of the solar wind ions. In both EA and SW sensors, the complete three-dimensional velocity distributions can only be obtained in a period of four spins, while the velocity moments up to the third order are calculated onboard every spin period (nominally, 3 sec). The energetic-ion Mass Spectrometer (MS) can provide three-dimensional determinations of the ion composition. In this paper, we describe the instrumentation and present some examples of the inflight measurements.

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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.

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The instruments for the HEP (High Energy Particle) experiments of the GEOTAIL mission, launched in July 1992, consist of 5 spectrometers (LD, BD, MI-1, MI-2 and HI). The LD (Low energy particle Detector) and the BD (Burst Detector) are mainly used to measure electrons, protons, helium and oxygen ions which reflect plasma dynamics in the magnetotail region. On the other hand, MI-1, MI-2 (Medium energy Isotope telescope-1, -2) and HI (High energy Isotope telescope) are used to measure the isotope abundance of solar flare particles and cosmic ray particles which reflect physical conditions of interplanetary space and origin of these particles. In this paper, the objectives of these experiments, the details of the instruments and preliminary results from the observation in the magnetotail are given. The preliminary results obtained by the LD and BD show that a highly collimated beam of energetic particles appear in the plasma sheet just after the substorm onset and that directional distribution of particles can be radically different for different energies. In the MI and HI telescopes, details a remarkable enhancement of anomalous cosmic ray components (N, O) in the quiet time of solar activity is reported, as well as the element composition of galactic cosmic rays.

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