The series of annual mean relative sunspot numbers (Rz) for 1749-1996 is subjected to the recently developed methodology of Singular Spectrum Analysis (SSA). This technique also enables data-adaptive filtering of the individual spectral components. Low order autoregressive modelling of the components are combined to provide a basis for predicting the solar cycle 23. The R_z series is largely dominated by a doublet with periods 11.13 and 10.35 yr. close to the nominal solar cycle periodicity, a longer period variation (-110 yr.) which is the envelope of the amplitude maxima and two clusters of periodicities centred around 8 yr. and 5.5 yr. The solar magnetic cycle has no detectable component. The predicted maximum for cycle 23 will have a magnitude of -130 and the epoch of maximum is expected between late 2000 A.D. and early 2001 A.D.

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Pc5 pulsations which were excited during the great magnetic storm of March 24, 1991 are analysed using the data from three French observatories located in the southern Indian Ocean at the subauroral and middle latitudes. The unusual strong Pc5 pulsations observed display features depending on the interval of occurrence: a quasi-noisy regime in the frequency range of 2.5-3.2 mHz characterizes the 04-10 UT interval, whereas the oscillations are nearly monochromatic with spectral maximum around 1.8-1.9 mHz in the 10-14 UT interval. The sharp change occurring at 10.17 UT could be associated to a sharp change in the magnetosphere structure or dynamics.

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Absorption of radio waves in the polar ionosphere near the magnetic noon was observed on October 8, 1991, by the 30 MHz imaging riometer at Ny-Alesund, Svalbard (invariant latitude 76.1°). These observations showed that the initially widespread absorption features became localized and enhanced in the high-latitude sector of the field of view, and followed a poleward motion. This behavior occurred quasi-periodically and repeated every 10-20 min. Simultaneous observations by EISCAT “Polar” experiments showed that nine discrete plasma patches, with F-region electron density enhanced by an order of 10⁶ el/cm³, drifted poleward from the polar cusp to the cap during the same period. This coincidence suggested that the ionospheric absorption was associated with F-region plasma patches in the polar cap. Theoretical absorption values of 0.14 dB, estimated using the electron densities and the electronion collision frequencies from the EISCAT F-region plasma data, are smaller than the observed values (<0.8 dB). This discrepancy may be related to the difference between the theoretically- and experimentally-determined collision frequencies, as indicated by Wang et al. (1994). These localized, enhanced, and poleward drifting absorption features over Ny-Alesund may be explained as F-region plasma patches produced by a magnetosheathlike particle precipitation into the cusp, and as small-scale irregularities caused by density gradients of the patches drifting into the polar cap.

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A CCD based all-sky (180° field view) airglow imaging system was operated from a low latitude station, Kolhapur (16.8°N, 74.2°E; 10.6°N dip lat.) in India to map the thermospheric motion and to study the low latitude ionospheric irregularities by measuring the nightglow signatures at 630 nm. In general, all-sky imaging techniques offer broad, instantaneous coverage (2.5 million sq km area at 150° field of view at 300 km height) of the spatial and temporal characteristics of the airglow features from the selected layers of the thermosphere and ionosphere system. In addition to the all-sky camera, photometer and radio scintillation observations together with ionospheric soundings were carried out from Kolhapur to verify the signature of small scale as well as large scale size F-region irregularities. Initial results from these measurements display good examples of north-south motion of the thermosphere with apparent drift speeds of 143-200 m/s during geomagnetically quiet nights. The north-south aligned (>1000 km) bands of plasma depletions or bubbles with east-west dimension of 50-250 km moving with drift speed of 140 m/s towards east have been observed. The zonal component of plasma drift speed matches well with the eastward component of the neutral speed computed using Hedin's recent model. However, the meridional components of plasma drift speed are generally higher than the components of the neutral wind speed computed using the model. The results confirm the earlier quantitative investigations of equatorial depletion characteristics from Brazil and elsewhere.

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Variations of rock-magnetic properties in a late Pleistocene (approx. 18-130 ka) Loess-Paleosol sequence of Haro River area, Attock basin, Pakistan were studied. Isothermal remanent magnetization (IRM) acquisition and thermomagnetic analyses of the samples indicate that magnetite and maghemite are the major magnetic minerals in these samples. Saturation isothermal remanent magnetization (SIRM), anhysteretic remanent magnetization (ARM), and the ratio ARM/SIRM disclose relative enrichment of ultra-fine magnetite/maghemite grains in the paleosols as compared to loess deposits, which suggests that the magnetite/maghemite is of pedogenic origin. Despite this enrichment, the low-field magnetic susceptibility is not always enhanced in paleosols in this sequence. In the Chinese Loess it has been believed that the observed lows and highs of magnetic susceptibility values respectively correspond to loess and paleosol sequences which indicates paleoclimatic change during the formation of the sequences. However, our observations in Pakistan's Loess-Paleosol sequence demonstrate that, although the ultrafine grained magnetite/maghemite may be produced by the process of pedogenesis, the low-field magnetic susceptibility record does not directly correlate with the alternation of loess and paleosol, and therefore the magnetic susceptibility can not simply be interpreted as a proxy for paleoclimate in the area.

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The equatorial GDV fields have been widely measured in North-South magnetometer chains and the data so obtained interpreted by assuming that these variations are bi-dimensional. This hypothesis limits its application to the inference of the noon amplitude of the ionospheric current and to the case on which the suspected local Earth's structure does not have discontinuities running along the direction normal to that of the electrojet current vector. In this work we develop a method to interpret three-dimensional GDV fields, based on the Riesz and in the Fourier integral transforms. This method consists in a numerical code that allows to separate, in real time, any threedimensional low frequency field, and a system equations to infer the ionospheric current system at daylight times and to predict the field induced from the external GDV field in a layered Earth's model. We discuss the application of the method to data obtained in North-South magnetometer chains, and, by analyzing a particular case-data from the Indian geomagnetic observatories-we illustrate how the method increases the amount of information that may be obtained from these data.

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Magnetic analyses have been conducted in and around Akita-Yakeyama volcano at the northwestern edge of the Sengan Geothermal Area, northeast Japan to reveal the regional and local subsurface structures of the area. First, a magnetization intensity mapping method has been applied to analyze aeromagnetic anomalies of the area. Generally, magnetization highs and lows lie on volcanic rocks which are normally and reversely magnetized, respectively. Magnetization lows with small amplitudes are distributed on hydrothermally altered areas. These results imply the usefulness of the method to estimate the young volcanic activities of Quaternary volcanic areas. Detailed magnetic modeling reveals the subsurface structure of Akita-Yakeyama volcano itself. Rock magnetic data from volcanic rocks, both from the surface and cores in the geothermal exploration wells, have been employed for the modeling. The resultant magnetic structure indicates the following: the surface volcanic rocks are underlain by granitic intrusions which have minimum thicknesses of about 2, 000 m below the northern flank of volcano; in the southern flank, the surface volcanic rocks are underlain widely by the Old-Tamagawa Welded Tuffs which are reversely magnetized. These results show a good agreement with a geologic interpretation in and around the volcano, especially with a hypothesis of the existence of buried calderas below the present volcano.

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We find out if the ratio of cross-strait voltage to water transport for the Bering Strait (BS) is constant. For this purpose, we have developed a technique to construct BS water velocity maps. We have built up a three-dimensional conductivity model for the BS region. Using this model, we have simulated coast-to-coast voltage for the various velocity maps constructed. We have found that the voltage/transport ratio remains constant for electromagnetic field periods exceeding 2 days. We have estimated the ratio as 239 ± 11 mV/ (km² · m/s) assuming bottom sediment conductance to be 600 S. We conclude that measuring cross-strait voltage allows the monitoring of BS water transport.

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