From January 1988 to December 1990, ionospheric total electron content depletions associated with amplitude scintillations observed simultaneously at two equatorial anomaly stations (Kaohsiung and Chihpen) have been used to investigate the eastward drift velocities of the equatorial plasma bubbles in East Asian region. It is found that there are considerable large variability of eastward drift velocities. The nocturnal variation pattern shows a tendency for the eastward drift velocities to decrease from early evening to presunrise hours. Seasonal variations show large eastward drift velocities in summer than in equinox. During midnight and postmidnight periods, eastward drift velocities decrease slightly with the increasing of magnetic activity.

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Using geomagnetic data from a global network equipped with fluxgate magnetometers and data loggers by Kyushu University, two moderate storm sudden commencement (SSC) events which occurred on Jun. 10 and Aug. 13, 1992 were analyzed. The network consists of about 30 stations, the main portion of which are arrayed nearly along three magnetic meridians (Japanese, Brazilian and African meridians) in low-latitude and equatorial regions. As for low-latitude and equatorial SSC-associated geomagnetic pulsations (Psc's), the following results were obtained. (1) The shape of the waveform is consistently ordered by local time for all low-latitude and equatorial stations; it is steeper near noon (or near the source) and has a more moderate slope near midnight (far from the source). This indicates that the Psc's are not cavity-mode oscillations but compressional hydromagnetic (HM) waves propagating from dayside to nightside two-dimensionally along the equatorial plane. (2) The Psc's have globally almost identical dominant frequencies which agree well with those of Pi2's. This implies that the frequencies of low-latitude and equatorial pulsations may be determined by the global structure of the magnetosphere or the plasmasphere independently of source mechanisms. During its passage through the plasmapause the compressional HM wave generated in the outer magnetosphere might be regulated in such a waveform as could be expressed as a superposition of exponentially damped sinusoids at ground stations, whose frequencies are globally almost identical.

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The self-consistent ionospheric boundary conditions, which describe the interaction between hydromagnetic (HM) waves and the anisotropically conducting ionosphere, have been derived in the form convenient for the eigenmode analysis of coupled HM waves in the magnetosphere-ionosphere system. These conditions consist of (a) the continuity of the horizontal perturbation electric field across the ionosphere, (b) the divergence of a vector formula relating a jump of the horizontal perturbation magnetic field across the ionosphere to the sheet current flowing therein and (c) the normal component of the rotation of the vector formula to the ionosphere. In order to emphasize a significant role of the derived self-consistent conditions, under the assumption that there exists no parallel inhomogeneity but a radial one of the Alfvén velocity V_A and the ionospheric conductivities in both hemispheres are uniform and symmetric, the coupling between the axisymmetric fast and Alfvén waves via the ionospheric Hall current has been investigated in case of no energy dissipation (or zero Pedersen conductivity). It is suggested that for a given fundamental eigenfrequency, the Alfvén mode perturbation is sharply enhanced with a finite amplitude at a field line position which is deviated from the position predicted by the classical field line resonance theory that the resonant frequency is determined only from the distribution of V_A along the field line and its length l_|| between the northern and southern ionospheres. Such a deviation is due to the divergent Hall current and occurs toward the direction in which V_A increases. Further, the Alfvén mode perturbation has no phase shift across the enhancement position. It is also suggested that the parallel wavelength of the fast mode perturbation is slightly greater than 2l_|| in the region with smaller V_A and 2l_||/3 in the region with larger V_A, respectively, and it changes rapidly from ∼2l_|| to r 2l_||/3 through l_|| as V_A grows large, where the wavelength l_|| appears just at the enhancement position.

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The magnetic field from Earth's core (the main field) is a global phenomena with measurable temporal variations with periods ranging from one year to millennia. Geomagnetic studies are thus heavily dependent on the availability of data well distributed over the globe and acquired over long periods of time. Satellite data provide the best geographic coverage, but are unlikely to be available except possibly at intervals of 10 to 30 years. Accurate mapping of the main field over long periods of time is mostly dependent upon a network of geomagnetic observatories, each of which contributes continuous, three-component, data of high accuracy. The overall accuracy of knowledge of the main field depends both upon the adequacy of the geographic distribution of those observatories and on the existence of periodic surveys by satellite. Analysis of models based on the existing observatory distribution reveals large geographic regions in which their accuracy is degraded such that studies of the field, its source dynamo, etc. are seriously limited. Model accuracy is studied for three distributions of 92, 162, and 252 equally spaced observatory sites and for degradation of those distributions by a large area with no data. The 92-site distribution is the most economically realistic. Expansion of the existing network so that a subset of observatories approximates this 92-site distribution can be accomplished by a phased program of collocating magnetometers at 20 sites already established, or now planned, for other geophysical networks such as FLINN, GEOSCOPE, IDA, and IRIS, at 10 additional land or island sites, and at 8 sea bottom sites. Specific locations for these sites are proposed. While not meeting all of the needs for study of current problems in geomagnetism, if implemented, this extension of the current observatory network would form a firm foundation for most such studies. Such implementation will only be accomplished if the burden for doing so is partially shouldered by most or all of the national agencies and organizations representing data users and if such users unit in expressing their own need.

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