There is improved the model for the description of the longitude-temporal variations of the F2-layer critical frequency which is based on the notion about the universal variation. This variation is characterized by the united change of foF2 at the Universal Time (UT) at all longitudes of the given latitude ring. In this paper the temporal and longitudinal dependence of the universal component amplitude is investigated. It was done by the selection of the best approximation of the observed foF2-variations by means of the model in the form of the sum of the local and universal components.

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This is a preliminary report of the first Polar Patrol Balloon (PPB) observation for auroral X-rays arose widely in the south polar region. The PPB No. 6 was launched from Syowa Station, Antarctica, on 5 January 1993, and flew along one and a half circumpolar paths, covering 6-13 g/cm² atmospheric depth and 49°-81° invariant latitude. Distinct enhancements of bremsstrahlung X-rays (E=30-120 keV) measured by an omni-directional NaI(Tl) scintillation counter were often observed during the entire flight period of 27 days with low to moderate geomagnetic activity. Ninety-three percent of the enhancements of greater than 3000 counts/30 s, being about twice the atmospheric X-ray background flux, were observed in 60°-70° invariant latitude, and 71 percent in the magnetic local dayside sector. Thirty-four percent of the dayside enhancements occurred at low latitudes below 60°, suggesting a large-scale expansion of the X-ray auroral oval to lower latitudes. Auroral X-ray enhancements observed at >65° invariant latitude exhibit that their energy spectra progressively harden in the time interval from 6 to 18 h MLT, consisting with the statistical summary of Bewersdorff et al. (1966). A double exponent spectrum in which the exponent E₀ becomes smaller in the low energy end below 50 keV is also found. Time profiles of the enhancements are well characterized by the systematic E₀-flux relation that E₀s are held within a definite range during the enhancement, otherwise they vary taking a minimum (or maximum) at the peak flux. These may suggest two different modes of energetic electron precipitations accompanying by stationary or variational spectrum from onset to recovery of the enhancement.

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We summarize the history of the Coimbra Observatory and identify the major discontinuities or gaps in the monthly mean values for the interval 1860 to 1990. Where possible, corrections are applied and gaps are filled to obtain a more complete time series. The Coimbra data must be split into two intervals (1873-1932 and 1951-1987) corresponding to different operating conditions. The external signal was removed from the geomagnetic series by the use of a deterministic model using sinusoidal terms to represent the annual and semi-annual fluctuations, and the as index to characterize the effect of long-period external disturbing fields. The values obtained for the coefficients of this model are stable for the two intervals and support the conclusion that the disturbing field affects mainly the X-component with a negative contribution and the Y- and Z-components with small but positive contributions (as expected in moderate north latitudes). The remaining signal, thought to represent the core field and its secular variation (SV), was used to analyse the occurrence of geomagnetic jerks, and their associated time scale and amplitudes. The first differences of the “internal” signal display an approximately piecewise-linear structure, where the change of slope corresponds well to some of the previously proposed jerks. A comparison of the results obtained for Coimbra with existing studies for other european observatories shows similar features concerning the 1969-70 jerk.

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The processing of the MAGSAT data have been carried out over Russian-Indian area for the singling out of the anomalous magnetic field. Using the method of the separation of the observed fields into the parts connected with their physical sources, the scalar and vector maps have been constructed over the considered area. These maps are compared with earlier MAGSAT anomalous magnetic field maps for this region and their features are discussed. The chains of the intensive magnetic anomalies correlating with the global and regional tectonic structures have been discovered on these maps. It was made the comparison of the anomalous magnetic field along the profile on 5°S latitude with other parameters of the geophysical section.

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An improved method of magnetization intensity mapping is proposed to interpret magnetic anomalies in terms of subsurface structure, where the side effect of source bodies surrounding the observational area is taken into account. The problem, which usually becomes underdetermined, can be solved iteratively by the conjugate gradient method. The asymptotic iteration gives the minimum Euclidean norm solution, to be suitable for subsurface modeling. This scheme was tested against synthetic data to evaluate its performance and applied to actual aeromagnetic data. From the analysis of aeromagnetic anomalies in the Tama area, a model of the distribution of magnetization intensity was derived, which revealed good correlation with active faults. Lower magnetization intensities appeared to the west of the southernpart and to the east of the northernpart of the Tanna fault, and in a narrow zone between the Ukihashi central and west faults. They were well correlated to highly fractured zones, and the quadrant pattern around the Tanna fault was interpreted as the effect of the compressional stress field brought about by strike-slip faulting.

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Generation mechanisms of the electric impulses in the underground chemical explosions are described in this paper. The electric impulses just after triggering were commonly observed in three explosion seismic experiments. The common characteristics of the impulses are as follows: 1) The duration time of the impulse train is always less than 10 ms and no impulse is detected after 10 ms. 2) The duration time of each impulse is approximately 1 ms, the time structure is quite different, and the occurrence is intermittent. 3) The electric charge generation process can be modeled by a vertical electric dipole. It is the first time to clearly show that the duration of the impulses is caused by the total length of the dynamite explosion. The characteristics can be explained by two closely coupled generation processes; chemical explosion and excitation of electric charges by a thermo- or mechano-electric process. The primary process is explained by means of the excitation of high-temperature and high-pressure with a dynamite explosion. The secondary process is interpreted as a theme- or mechano-electric conversion process excited either by thermal energy or by mechanical energy in surrounding media consisted of rock and water. Two thermo-electric and four mechano-electric conversion processes are discussed in terms of the time structure, the direction of expected electric dipoles, and the confinement in the vicinity of dynamite. It is concluded that the most probable source of the observed electric impulses is the fractoelectric effect.

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The summit eruption of Unzen volcano started at Jigokuato (J) and Kujukushima (K) craters on November 17, 1990, after 198 years quiescence. The volcanic activity intensified at Byobuiwa (B) crater, about 150 m west of J, on February 12, 1991, and a new dome was extruded at Jon May 20, 1991. Seven domes were extruded until early summer 1992. Preceding the major volcanic activity, a geomagnetic change of about 20 nT was detected by repeated surveys at the summit during the period of 1986 to the end of 1990. To elucidate the cause of the geomagnetic change, closely placed and continuous geomagnetic observations with proton magnetometers have been conducted near the summit of the volcano since January 1991. In association with the volcanic activity at B, geomagnetic changes amounting to about +18 and -5 nT were observed at close points in the northern and southern directions of B, respectively, from February to March 1991. During the early stages of the new dome extrusion, large geomagnetic changes of more than 80 nT were simultaneously observed at several points near J from May 12 to June 3, 1991. The changing rate of about 5 nT/day at mid-May 1991 decreased exponentially to about 0.1 nT/day by the end of 1991. The total change reached to more than 160 nT at three points closer than 400 in from J from early March 1991 to the end of June 1992, although it was only 10 nT at a point of about 500 m south of J. Efforts to clarify the causes of each geomagnetic change resulted in no systematic simple model through this eruption sequence. These magnetic changes were probably the result of many processes such as thermal demagnetization, removal effects of magma intrusion, strain/stress, rotation and displacement of magnetic materials and perhaps also electrokinetic effects.

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About 100 year variations are found in the field direction of the Matuyama-Brunhes transition. The analysed data are composed of the palaeomagnetic directions from the marine sediment in the Boso Peninsula, whichpreciselyrecordedthe Matuyama-Brunhes transition with a very high accumulation rate (3.7 mm/yr). The section is tentatively divided into two subsections from the directional change, where the main reversal and the rebound occurred in the early stage (subsection I) and subsequently the field directions show moderate changes in the late stage (subsection II). Applying the Bayesian smoothing method to the directions spanning about 4100 years, the inclination, declination and angular changes in the transition field imply short-period components of the order of 10² years. After removing long-term trends, the maximum entropy method was employed for the detection of short-period variations. The results of the spectral analyses suggest the 60-80 year and the 120-150 year periodicity in the subsections of I and II, respectively. The main reversal and the subsequent rebound seem to have been accomplished for short intervals of about 200 and 300 years, respectively. These rapid variations of the order of 10² years are suggested to be a substantial feature of the transition field.

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