This paper describes the advantage of Mean Ionospheric Field Correction (MIFC) over the MAGSAT data in order to obtain crustal magnetic anomalies, the magnitude of which is in many cases comparable to or smaller than that of electric currents in the magnetosphere and ionosphere. In the past, the correction for non-crustal fields has been empirical without sound physical meaning, so that the non-crustal fields remain almost uneliminated in the geomagnetic anomaly maps, or the crustal anomalies are too reduced by some artificial corrections. Recently, it was proposed to calculate the Mean Ionospheric Field (MIF) separately for the dawn side and dusk side and subtract it, in order to obtain the magnetic anomalies of crustal origin (YANAGISAWA, 1983; YANAGISAWA and KONO, 1984). In this paper, we derive the magnetic anomaly maps of crustal origin for regions over the western Pacific and South America, taking the longitudinal inequality of MIF into consideration.

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Vector data acquired by MAGSAT were analysed to examine spatial properties of the geomagnetic field over the Japanese Islands and their surrounding seas. Three component data along satellite paths, which pass through the area surrounded by the parallels 8°N and 68°N and by the meridians 110°E and 170°E, are approximated by polynomial series of satellite position on the path. Two distinct structures have been found to exist in the regional field by examining the coefficients of the series, which give a kind of amplitude spectrum. One is the long wavelength field represented by lower degree terms up to degree 5 or 6. This type of field is characterized by sharp decrease of amplitude with increasing degree. The other is the short wavelength field represented by higher degree terms beyond degree 5 or 6. The amplitude spectrum is more flat with this type of field. From the high rate of attenuation of the spatial spectrum with degree, the magnetic field of wavelengths longer than 3000km is likely to have its origin in the core. On the other hand, from almost flat nature of the spectrum the shorter wavelength field has been regarded to originate in the crust.

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Scalar and vector geomagnetic anomaly maps over the Japanese islands region were derived from MAGSAT Investigator-B tape data. All the available data were classified into four subsets to examine the common appearance of the same anomaly pattern in the geomagnetic maps. These maps revealed that the anomaly patterns with wavelength longer than about 500km appeared commonly in all maps, so that these long-wavelength geomagnetic anomalies are certainly caused by crustal source.

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A magnetic anomaly map was complied based on the results of aeromagnetic surveys made in and around Japan, which have been in progress since 1967 under the auspices of the Geographical Survey Institute. Magnetic lineations were found in the following zones (in many cases in good correlation to the Bouguer anomaly and/or the main tectonic zones of Japan): (1) four lineation zones in the northeastern part of Japan, (2) a lineation parallel to the Kuril-Kamchatka Island Arc in the southeastern part of Hokkaido, and another one running almost perpendicularly on its northern side, (3) four lineations in the NW-SE direction over Tohoku and southwestern Hokkaido, (4) a lineation zone along the Fossa Magna tectonic line in central Honshu, (5) lineations of weak magnetic anomalies along the Median tectonic line in the northern part of Shikoku and from the region off south Shikoku to Mt. Aso, and (6) lineations in western Japan along volcanic zones such as the Hakusan and Kirishima volcanic belts.

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The Hydrographic Department of the Maritime Satety Agency conducted an airborne magnetic survey from 1979 to 1980 to derive a regional field model around Japan at the epoch 1980.0. The regional field model, consisting of 3rd-degree polynomials, was compared with the models MGST (4/81) and IGRF 1980 for the earth's main geomagnetic field over the world. The comparison of the regional field model and MGST (4/81) or IGRF 1980 revealed the following differences:
1) MGST (4/81) model approximates the actual magnetic field better than IGRF 1980, insofar as the RMS residuals for the region around Japan are concerned. The RMS residuals from MGST (4/81) are 18.1nT, 44.3nT, 47.3nT, and 42.3nT for the total intensity F and three components X, Y, Z respectively, whereas the residuals from IGRF 1980 are 23.6nT, 46.6nT, 67.6nT, and 50.4nT in the same order.
2) Large negative residuals of the geomagnetic total intensity in the area of the Japan Sea and Okhotsk Sea (amounting to -300nT from the IGRF 1965 or IGRF 1975) are thought to be attributable to the insufficient representation of the core-field in these IGRF models. If MGST (4/81) is used as a reference model field, the residual map shows a positive deviation along the outer region of Island Arc and a negative one in the marginal sea. This indicates the existence of a significant long-wavelength magnetic anomaly around Japan.

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A direct application of Ampere's theorem enables us to calculate the total amount of electric current passing through the plane enclosed by the MAGSAT orbit, through the line-integration of the observed magnetic field component tangential to the MAGSAT orbit. After eliminating the spurious effect arising from the earth's rotation under the MAGSAT orbit, it is concluded that the space current of a few million A flows anti-sunward below the MAGSAT level during magnetic storms in good correlation with the AE-index, whereas such a space current is almost absent under quiet conditions. The existence of an anti-sunward space current on disturbed days gives a strong support to the development of a partial ring current in the magnetosphere during magnetic storms.

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Among 21 geomagnetic sudden commencements (SC), each of which was reported by more than 2 ground observatories, 16 were detected by MAGSAT above the ionosphere (260-560km altitude) in the dawn-dusk meridian. The amplitude of the remaining 5SCs was small on the ground (less than 10nT). The SCs at MAGSAT appeared mainly in the H-component in low latitudes and in both H- and D-components in high latitudes. The Z-component did not show significant variations. The amplitude of the SCs was usually larger at the satellite altitude than on the ground, and the averaged ratio was 1.3 for the low latitude SCs. The larger amplitude at the satellite altitude was interpreted in terms of a westward zonal shielding current flowing in the ionosphere. A detailed analysis was made for one SC which occurred when the satellite was flying very closely above a ground station (Magadan in east Siberia, geomag. lat.=51.1°) in a duskside (16h-17h LT) meridian. The H-component at the satellite showed a positive impulse of 52nT and the D-component a negative impulse of 43nT in the beginning part of the SC. The duration of the impulse was 2-3min. The SC observed by normal-run magnetographs at ground stations near the satellite orbit was preceded by a typical preliminary impulse (PI) of opposite deflection to that of the pulse observed by MAGSAT. By the use of high time resolution data from the IMS magnetometer network Alska chain, the pulse detected by MAGSAT was identified with the PI observed at the ground stations, and it was concluded that the main source current for the PI flowed between the satellite and the ground, that is, in the ionosphere; but effects of other source currents including field aligned currents should be taken into account.

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In order to examine the relative locations of auroral particle fluxes and field-aligned currents and to identify the main charge carriers of the field-aligned currents at auroral latitudes, nearly simultaneous data from the vector magnetometers on MAGSAT and of precipitating electrons with energies between 300eV and 20keV observed by TIROS-N and NOAA-6 are compared. For more than fifty cases, MAGSAT and TIROS and/or NOAA orbits occurred within two minutes (mostly within one minute) of each other in the dawn/dusk sectors, during the time the IMS meridian chains of ground magnetometers were operating from November, 1979 through January, 1980. The latitudinal boundaries of precipitating electrons are found to line up within 1° with those of the field-aligned current region. Major portions of the upward field-aligned currents in the poleward half of the evening-sector auroral oval and in the equatorward half of the morining-sector auroral oval appear to be carried by the precipitating keV electrons.

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