The technique of complex demodulation is applied to low latitude geomagnetic field data to isolate the long term changes in the amplitude and phase of the semiannual and annual variations. The phase for semiannual variation is in close agreement with that anticipated from equinoctial hypothesis. The amplitude does not show significant dependence on the 11-yr solar cycle. Comparable time variation for all the hours of the day leads to the conclusion that the semiannual variation originates in the disturbance field. A regular oscillation with a 6-7 year periodicity and a long-term variation of the amplitude of annual oscillation corresponding to 14 LT appear as new results.

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An anomalously large value of the ratio of remanence coercive force (H_RC) to coercive force (H_C) (i. e. H_RC/H_C) which is much beyond a range of H_RC/H_C for a single phase magnetic mineral, is often found in natural rocks, particularly in extraterrestrial rocks. The observed large values of H_RC/H_C are interpreted as mostly due to the coexistence of a high coercivity component (a) and a low coercivity component (b). In a binary system of high and low coercivity components, H_RC is largely controlled by the high coercivity component and H_C largely by the low coercivity component in accordance with definitions of H_RC and H_C, respectively.
A simple model of such a magnetic binary system for purposes of estimating the magnetic coercivities of both components from the observed bulk values of H_RC, H_C and I_R/I_S is proposed, where I_R and I_S respectively denote saturation remanence and saturation magnetization. It is shown that the proposed model well approximates experimental data with an uncertainty of less than a factor of 2.

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The Mössbauer spectra of titanomagnetites, Fe_3-xTi_xO₄ (0≤x≤1), synthesized at 1200°C were redetermined to clarify the cation distribution in titanomagnetite. Nine samples from x=0 to x=0.33 were used in the experiments. The spectra were very distinct for samples between x=0 and x=0.20, and in that compositional range satisfactory fits were obtained with theoretical curves by nonlinear regression. Uncertainty in the fitting becomes a severe problem for x larger than 0.20, but the trend in the change of spectra can be traced nearly continuously from x=0 to x=0.33. The variation of the isomer shift with x suggests that the spectra can be divided into three subsets a, b, and c which are attributed to Fe³⁺ in A site, Fe^2.5+ in B site (arising from the electron hopping between the Fe³⁺ and Fe²⁺ pair), and other iron ions with locally varying energy states, respectively. The change of the fractional area (f_a) of subset a with composition shows that the cation distribution in the present samples follows the Akimoto model (AKIMOTO, 1954). This conclusion is in good agreement with the neutron diffraction study by WECHSLER et al. (1984). It was also concluded from the change of the fractional area (f_b) of subset b that the proportion of Fe³⁺-Fe²⁺ pairs which participate in electron hopping is much less than expected from the model.

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We carried out a paleomagnetic study on rocks from southern Peru to reveal more precise timing of the post-Cretaceous tectonic rotation of Peruvian block in Central Andes as suggested by the previous studies. The samples were taken from Late Miocene to Early Pliocene sedimentary rocks, the Changuillo, Pisco and Paracas formations, and from Latest Miocene volcanic rocks. The paleomagnetic pole obtained from these rocks is close to Miocene to Quaternary poles of the stable part in South America, indicating that the post-Cretaceous tectonic rotation of the Peruvian block ceased in or prior to the Late Miocene.

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Since 1985, a magnetic network of telemetered stations is settled on the Montagne Pelée volcanic massif of the Martinique island. Until December 1985, at which date the initial volcanic seisms are recorded, the simultaneous variations in the differences of Earth's magnetic field intensity between two stations located on the volcano and a reference station which is 9km distant from them remain constant to within 1nT. During the seismic crisis, which ends in May 1986, over twenty shallow earthquakes are sensed within 2km below the summit, and volcanomagnetic variations of small amplitudes never exceeding 3nT can be observed. The onset of the seismic crisis that occurred in December 1985-January 1986 is primarily expressed by a slow irreversible increase of the differences, whereas partly reversible volcanomagnetic variations are observed with the last seisms in May.

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