Eight years of Ajisai SLR data were processed to determine the terrestrial reference frame and its time evolution. The typical precision and accuracy of the estimated geocenter position averaged over a year determined from a one-year Ajisai SLR data set are 1 cm and 1.5 cm, respectively. The baselines between SLR stations away from plate boundaries show rates of change that are in good agreement with NUVEL-1A, ITRF93 and LAGEOS results but significant deviations from geologically determined plate motion models are found for stations in plate boundary regions. Velocities of the observation stations were estimated by a weighted least squares method. The Simosato SLR station, located 100 km away from the plate boundary between the Eurasian plate and the Philippine Sea plate, moves in the direction of the subduction of the Philippine Sea plate with respect to the Eurasian plate, which infers strong coupling of the two plates at the boundary. The motion of other stations at plate boundary regions is also discussed. This study is the first attempt to use Ajisai SLR data to determine the global terrestrial reference frame and its variation, thus independent of the previous SLR studies most of which were based on LAGEOS SLR analyses.

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Accuracy of the Primary Precise Geodetic Network (PPGN) established during the period of 1975-1994 in Korea is investigated through an analysis of residual from free network adjustment. The PPGN, composed of 1155 points with mean side-length of 11 km, was surveyed by EDM trilateration to revise old triangulation network established in 1910 s. A-posteriori standard deviation of unit weight is 3.8 × 10^-6 of the observed length. Most of the estimated error ellipses of horizontal coordinates are nearly circular with a radius <5 cm except those in the marginal part of the network. Mean error of the adjusted coordinates was ±4.5 cm, a value good enough for a geodetic control network of current standard, and as the reference for future crustal deformation studies.

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The Danish satellite Ørsted is due to be launched in 1998, and should provide, for the first time since the Magsat mission (1979-1980), a dense and global coverage of the Earth's surface with vector measurements of the magnetic field. In this paper, we compare the expected error in the main field models computed for the 1970-1999 time interval using observatory data, with or without the a priori information given by the knowledge of the field at both Magsat and Ørsted epochs. This work is based on the reasonable hypothesis that the main field models derived from Ørsted data will be as accurate as the Magsat models. The a priori information given by the Magsat and Ørsted models is based on a linear behaviour of the rate-of-change of the field throughout this period, plus a noise level which can be estimated as a function of time and degree from past field changes. The expected error in the models computed for the 1980-1999 period with a priori information appears to be significantly smaller than the expected error in the models computed without this information. This result is related to the heterogeneous distribution of the observatories over the Earth surface. Consequently, when the Ørsted data is available, improved models can be computed for the 1980-1999 period particularly in regions without observatory data. This method with a priori information may allow the use of the same set of observatories throughout the entire period. Indeed, our method alleviates the requirement of a very dense data distribution.

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A new model of the stably stratified layer at the top of the core is proposed. The existence of a stably stratified layer (we name it the stratified ocean) at the top of the core makes possible the propagation of the waves akin to the Rossby waves (also named “planetary waves”), well known in oceanology and meteorology. These waves are modified and experience significant decay, due to the core's magnetic field. The “magnetic Rossby waves” are considered here, using a simple planar model, to reveal their qualitative features without going into significant mathematical complications. The core-mantle coupling, which originates from the interaction of the surface flow with the topography of the core-mantle boundary, is strongly influenced by the stably stratified layer. We consider the topographic core-mantle coupling arising due to generation of motion resembling the magnetic Rossby waves in the stably stratified layer. A simple expression is obtained for the topographic tangential stress on the core-mantle boundary.

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Long-period electromagnetic response must be estimated to derive information on the electrical conductivity distribution within the deep mantle of the Earth. For this, we estimated the electromagnetic response function for long-period geomagnetic variations, on the basis of the P₁ approximation. The data used for analyses are geomagnetic daily mean values for 10 years (1965-1974) from 59 stations distributed over the globe and for about 20 years (1960-1980) from 9 stations. It turned out that the P₁ approximation generally holds except for the periods corresponding to annual and semi-annual variations. We also found that accurate estimation is difficult for periods longer than a few years, probably because of contamination due to secular variations of core origin.

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Ground-based observations of dayside auroral forms and magnetic perturbations in the arctic sectors of Svalbard and Greenland, in combination with the high-resolution measurements of ionospheric ion drift and temperature by the EISCAT radar, are used to study temporal/spatial structures of cusp-type auroral forms in relation to convection. Large-scale patterns of equivalent convection in the dayside polar ionosphere are derived from the magnetic observations in Greenland and Svalbard. This information is used to estimate the ionospheric convection pattern in the vicinity of the cusp/cleft aurora. The reported observations, covering the period 0700-1130 UT, on January 11, 1993, are separated into four intervals according to the observed characteristics of the aurora and ionospheric convection. The morphology and intensity of the aurora are very different in quiet and disturbed intervals. A latitudinally narrow zone of intense and dynamical 630.0 nm emission equatorward of 75° MLAT, was observed during periods of enhanced antisunward convection in the cusp region. This (type 1 cusp aurora) is considered to be the signature of plasma entry via magnetopause reconnection at low magnetopause latitudes, i.e. the low-latitude boundary layer (LLBL). Another zone of weak 630.0 nm emission (type 2 cusp aurora) was observed to extend up to high latitudes (-79° MLAT) during relatively quiet magnetic conditions, when indications of reverse (sunward) convection was observed in the dayside polar cap. This is postulated to be a signature of merging between a northward directed IMF (B_Z > 0) and the geomagnetic field poleward of the cusp. The coexistence of type 1 and 2 auroras was observed under intermediate circumstances. The optical observations from Svalbard and Greenland were also used to determine the temporal and spatial evolution of type 1 auroral forms, i.e. poleward-moving auroral events occurring in the vicinity of a rotational convection reversal in the early post-noon sector. Each event appeared as a local brightening at the equatorward boundary of the pre-existing type 1 cusp aurora, followed by poleward and eastward expansions of luminosity. The auroral events were associated with poleward-moving surges of enhanced ionospheric convection and F-layer ion temperature as observed by the EISCAT radar in Tromsø. The EISCAT ion flow data in combination with the auroral observations show strong evidence for plasma flow across the open/closed field line boundary.

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We present an evidence of the coupling of a fast magnetospheric cavity mode to field line resonances. The power of filtered H and D components of Pi 2 pulsations during March-April in 1978 and 1979 from IGS chain in the United Kingdom (UK) shows that there are two enhancements located near L = 3.5 and 5.5 in the distribution profile. The irreversible hydromagnetic wave coupling is studied by using a box model for the magnetosphere. The cold linearized MHD equations is examined with realistic Alfven speed profile for the uniform and nonuniform ambient magnetic fields, respectively. With appropriate parameters to depict the magnetospheric environments during the aforementioned period, numerical results are acquired with the fourth order Runge-Kutta method and quite consistent with data analysis. We suggest that the coupling of a fast magnetospheric cavity mode driven by fast compressional waves owing to the impulsive source at the magnetotail to resonant field lines at low latitudes is a possible scenario of the propagation mechanism for low latitude Pi 2 pulsations.

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Wavelet analysis of the magnetic field fluctuations in the Martian foreshock is performed. Dynamical features of wave spectra are compared with plasma measurements of backstreaming ions and simulation results for all available data from the Phobos-2 spacecraft. A detailed analysis reveals the complex, fine structure of the ULF waves. It is shown that enhancement of this electromagnetic activity (mostly, at frequencies higher than the proton gyrofrequency) coincides with entry of the spacecraft into a bunch of exospheric protons reflected from the bow shock. Emissions near cyclotron frequencies of H⁺, He⁺, O⁺ are also recorded. These waves indicate pickup processes in the upstream region of Mars.

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