An inversion method is proposed to estimate the wave energy distribution of magnetospheric VLF radio waves (VLF/ELF emissions and whistlers) at the ionospheric base by using the simultaneously observed two horizontal magnetic and a vertical electric field components. The simulation experiments are extensively carried out to examine the effectiveness of the method. Two models for the wave source (a single source and two simultaneous sources) have been used, in which the observed signal is simulated to be composed of a number of elementary plane waves whose arriving directions are distributed around a specific direction in the case of a single source (two specific directions for the case of two simultaneous sources) and whose polarizations are distributed around right-handed circular. The distributions how the wave energy density is distributed at the ionospheric base have been obtained as an inversion problem in which the entropy of the wave distribution function is made maximum for the spectral matrix composed of the auto-and cross-power spectra of the three field components. It is then found that the present new method is very promising especially for the situation when a few wave sources (ducts) are present or when the ionospheric exit region is widely spread for which the previous direction finding systems are ineffective. Finally, the present method is applied to the actual data of whistlers observed at a low latitude, and it is found that the method is very effective in locating the wave source distribution, being in good agreement with the results by the previous direction finding methods. Further useful information on the mechanism of magnetospheric duct propagation and the ionospheric transmission is obtainable from the present method.

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The spatial and temporal features of the peak proton flux detected near the equator in the integral energy channel of 0.64-35MeV by S-1 telescope on board the EXOS-C mission were studied for a period of three years (1984-86). The equatorial zone (±30°geomagnetic latitude) proton data obtained for the angle between the telescope axis and the magnetic field direction in the range of 50°-130°and at the peak efficiency of the instrument, were sorted out from a study of the orientation of the telescope axis and calculation of the response function of the telescope. The data, after freed from the major influence of the South Atlantic Anomaly, and subsequently analyzed, revealed some new results contrary to the earlier observation (MORITZ, 1972) in the altitude range of 350 to 850km. The spatial distribution shows that the global profile of the peak proton flux is aligned with the profile of the minimum magnetic field, and, the peak proton population straddles the minimum magnetic field equator with a full-width-at-half-maximum of about 19°. Also, the pitch angle distribution of the equatorial proton population is found to have the anisotropy index in the range 7 to 10. Further, the proton flux shows altitude, L, and longitude dependences. The flux increases as h^1.43 and L^4.25-7.50 (corresponding to different equatorial pitch angles). Besides, in the longitude range 180°<φ<300°, the flux is significantly larger than that in the range 100°<φ<180°. The fact of L independence of low energy (<5MeV) protons, and the strong L dependence (L⁸¹) of high energy (>5MeV) protons, has been used to infer the presence of a major low energy component and a minor high energy component. In the study of temporal features, comparison of absolute fluxes during two solar epochs shows that the proton flux during the solar minimum in 1984-86 was, at least, 40 times less than that during the solar maximum condition in 1982, possibly, due to lesser amount of hydrogen escape to the exosphere during the minimum condition, and thereby causing less precipitation of low energy particles in the equatorial atmosphere.

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A sounding rocket was launched in January 1988 to locate heat sources which may cause the anomalous elevation of the E-region electron temperature over Kagoshima, Japan. Fluctuations of the electron density and the electric field were measured using a fixed-biased Langmuir probe (PWN) and a pair of double probes (AEF) respectively, placed onboard the rocket. The PWN measured an abrupt decrease in DC current and an increase in AC current (5-160Hz component) at the altitude of 94-130km along the rocket's ascending path. The AEF also measured an increase in AC electric field (5-160Hz component) in the same altitude. Comparative analysis of the PWN and AEF data reveals that the potential of the rocket dropped a few volts accompanied by low-frequency fluctuations in that altitude range. An other research group of the same rocket measured an elevated electron temperature in the same altitude range. Coincident occurrence of these phenomena suggests common cause. The suprathermal electrons are considered to be a promising candidate.

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Electrojet field in Peru is analysed by making use of Hermite function technique developed by RIKITAKE (1952, 1989) that is especially useful for analysing an isolated anomaly. External and internal parts of H and Z fields thus obtained are in good agreement with those obtained by FORBUSH and CASAVERDE (1961). It should be noted, however, that the present analysis is free from such an assumption that the distribution periodically repeats outside the space range in question when a Fourier series analysis is used.
The external field is distributed fairly regularly suggesting an overhead flow of electric currents in the east-west direction. On the contrary, the internal field is distributed somewhat anomalously probably reflecting a complicated distribution of electrical conductivity in the interior of the earth beneath the Peruvian west coast.

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Electromagnetic shielding by a spherical shell having a hole is studied. Solving the differential equation satisfied by magnetic potential, distributions of potential are obtained for various hole sizes. First of all, perfectly-conducting shells of finite and infinitesimally thin thicknesses are studied in detail. When the shells are finitely-conducting, only thin shells are studied because of mathematical difficulty.
Relaxation technique is used for solving the problem numerically. The shell is placed in a uniform external field, which is time-varying, parallel to the direction connecting the hole center to that of the shell.
As expected, it turns out that the smaller the hole size, the higher the shielding power. It is clearly demonstrated that the magnetic potential lines are largely expelled from the shell interior when the shell conductivity is high. It is interesting and important that the magnetic field at the shell center is weakened one-thousandth times for a perfectly conducting shell even the hole aperture radius is as large as 30°.
Similar study of electromagnetic shielding for thin cylindrical shells having a defect is made in Appendix. The distribution of magnetic lines of force is presented in this case.

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Three scalar AMT campaigns were conducted in the mid-summer, late-fall, and late-winter seasons of 1990 in two nearby localities of southern Rio Grande do Sul State, Brazil. Comparisons of the results obtained at each season highlighted the problem of seasonal variation in the apparent resistivity measurements in the area. From 1-D inversions, the most striking fact in this temporal variation is the presence, in the last two campaigns, of a new deeper conductive layer (conductance of 6 to 7S) at depths of the order of 500m, which was not observed in the midsummer campaign. Careful observation of the regional rainfall data indicated that the seasonal conductive layer could be generated by the occurrence of a transitory deep aquifer that would grow or wane in time in accordance to the climatological conditions. Another likely explanation could be in terms of a misinterpretation of the geoelectrical section due to the distortion of the MT curves by induction effects generated by a saturated surficial layer.

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A stalagmite sample (ISA) was collected from an unnamed cave located on the West Akiyoshi Plateau in western Japan. The paleomagnetic results from four core samples covered the same growth period were of high quality and in agreement with each other, satisfying internal consistency. No depositional error was recognized in the results. The geomagnetic reversal episode was recorded in the portion between the whitish center part and the innermost of three clearer growth layers. The AF-cleaned direction variation of outer portion than the innermost clearer growth layer corresponds to other records from West Japan over period of 2.5 to 6.5ka. The estimated growth rate in this portion, therefore, is 1.6 to 1.7cm/kyr. We conclude that the reversal occurred between 350ka (²³⁴U/²³⁰Th age of the center part) and 15ka (by the comparison of the direction variation with other paleomagnetic records from Japan). The duration of the reversal is estimated to be about 2500 years, assuming a growth rate for the outer layer to the inner portion. During the period of the reversal, the virtual geomagnetic pole moved roughly along the longitude of the sampling site (131°E), and the geomagnetic paleointensity was estimated to be relatively low.

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