We made a survey for Jovian hectometric and kilometric radiation by GEOTAIL spacecraft before, during, and after the impacts of Comet Shoemaker-Levy 9 (SL-9). We attempted to identify the radiation from Jupiter based on a k vector determination. However, we did not find clear enhancement of the Jovian radiation activity in the whole period nor around each impact time. With regard to the Jovian magnetospheric activity, the enhancement of synchrotron and X-ray radiation in association with the SL-9 event has been already reported. Synchrotron radiation increased during the week of the impacts, and X-rays was clearly detected just around K impact time at the magnetic conjugate footprints of the impact sites. This suggests that a strong perturbation at the impact sites has given non negligible change in the inner magnetosphere directly or indirectly. Our results suggest that such activities were limited in the inner magnetosphere only on low L shells, and there were few amount of direct coupling between the cometary fragments and the Jovian outer magnetosphere. This is consistent with the ground-based observation of the decametric radiation that no significant change was detected during the SL-9 event.

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In order to clarify the properties of substorm occurrence, we statistically examine the conditions for the occurrence of Pit pulsations at Memambetsu (geomagnetic latitude 34.6°) using solar wind hourly value data for the period 1965-1990. The main results are: The Pit occurrence probability increases roughly linearly with decreasing Bz component of the interplanetary magnetic field (IMF) for the range Bz < 4 nT. It also increases with increasing solar wind velocity (V); the probability is approximately proportional to V3.15, and the exponent for V becomes larger as the dipole tilt angle (χ) approaches zero. The probability is higher for positive By of the IMF than for negative By when χ is negative, and vice versa when χ is positive. The solar wind density (D) increases the probability; the probability is approximately proportional to D^0.814 The solar wind parameter dependence of the Pi2 occurrence probability shows a close similarity to that of the AL and am indices, which are considered to represent the substorm intensity, except for the density dependence. In order to understand the cause of the similarity, we suggest that the substorm occurrence probability is controlled by the rate of solar wind energy input into the magnetosphere and is enhanced as the rate increases.

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Results of measurements of the negative ion density and its ratio to the positive ion density in the lower ionosphere (D-layer) of the polar region (Andøya) are presented. A comparison is made with previous data in mid-latitude region (KSC), which were obtained by a same instrument. It has been found that the negative ion-dominant region in the D-layer is compressed toward lower altitudes in the polar region than that in the mid-latitude by about 16 km. Some mechanisms for this and its relation to the ozone distribution are discussed.

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Variations of volcanic aerosol layers originated from eruptions of Mt. El Chichón and Mt. Pinatubo observed in Fukuoka are analyzed. The arrival of the bulk of the El Chichón aerosol layer at Japan was sooner after the eruption, about two weeks after the major eruption, than the arrival of Pinatubo volcanic aerosols which was about three months after the major eruption. The difference is mainly because of the difference in the seasons of the major eruptions between the two events. Although the altitude of the layer of aerosols formed in the case of El Chichón was higher than that of Pinatubo, the speeds of the downward motions were comparable in both cases. It is suggesting that mean radius of the El Chichón aerosols was smaller than that of the Pinatubo aerosols.

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The depolarization ratio of atmospheric molecules was estimated to be 0.021 on the basis of regression analysis of total depolarization ratio and scattering ratio of atmospheric aerosols measured by a lidar in spring, 1994.

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The magnetic and gravity anomaly maps covering the Izu-Ogasawara arc are compiled using the MGD77 data measured by the Hydrographic Department of Japan (JHD). A magnetic basement map is also obtained by applying the three-dimensional inversion method. Characteristic features of the crust of the Izu-Ogasawara arc are made apparent from the view point of the magnetic, gravity and topographic features. Especially, the distribution of the magnetic bodies with low ρ/J ratio, which belong to the granitic rock of magnetite series, becomes apparent in the forearc area of the northern part of the Izu-Ogasawara arc. The magnetic anomalies of the Nisi-Sitito ridge are well explained by the normal magnetization of topographic highs. This feature suggests the N-S arrangement of the arc when it was formed. The bulk magnetization per unit area (km²) along the Quaternary volcanic front of the southern arc is about 2.4 times as large as that of the northern arc. These features can be explained by the narrow and stable location of the volcanic front in the southern arc, which is related with the steep gradient of the subducting slab. The magnetic edge effect of the Moho boundary can be recognized in the western margin of the northern arc. However, this feature becomes vague in the southern arc. A tectonic model to explain the origin of the different characteristics between the northern and southern arcs is presented by assuming the long and stable state of the steep dip angle of the subducting slab in the southern arc under the N-S arrangement of the arc. The plausible origin of NE- and NNE-trending en-echelon ridge and trough morphology is also given in consistent with the tectonic framework mentioned above.

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We present for the first time sea surface echoes observed at ranges of 650-900 km with the 46.5-MHz middle and upper atmosphere (MU) radar at Shigaraki, Japan. Ionospheric data support that the radar wave propagated toward the target by reflection due to intense sporadic E layer. Although the Doppler spectra obtained are modified more or less due to temporal and spatial changes of the sporadic E layer, fundamental characteristics of the spectra are well explained by a first-order theory of Bragg scatter from sea surface.

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