Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
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Volume 39, Issue 11
Displaying 1-4 of 4 articles from this issue
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  • Seasonal and Solar Cycle Variations of Spread F at the Equatorial Anomaly Crest Zone
    Yinn-Nien HUANG, Kang CHENG, Wen-Ter HUANG
    1987 Volume 39 Issue 11 Pages 639-657
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.639
    JOURNAL FREE ACCESS
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    The spread F data obtained at Taipei (25.00°N; 121.53°E geographic and 14.04°N; 191.47°E geomagnetic) and Chungli (24.95°N; 121.23°E geographic and 13.98°N; 191.19°E geomagnetic) during the period from 1960 to 1982 were used to analyze the seasonal and solar cycle variations of frequency and range type spread F at the equatorial anomaly crest zone. By use of superposed epoch method, the geomagnetic activity effects on the occurrences of frequency and range type spread F were also analyzed.
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  • An Important Role of Electric Field Reversals for the Initiation of Gigahertz Scintillations at Midlatitude during Geomagnetic Storms
    T. TANAKA
    1987 Volume 39 Issue 11 Pages 659-676
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.659
    JOURNAL FREE ACCESS
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    Large-scale features of the ionospheric disturbances which lead to the occurrence of gigahertz scintillation events at midlatitudes are re-examined using h'F data at five ionosonde stations and HF Doppler measurements at Kokubunji. From these analyses, it is recognized that ionospheric dynamics during the events are generally characterized by a two-stage process, despite case-dependent quantitative differences in the appearance of each event. In this construction, an increase and subsequent decrease of virtual heights occur simultaneously at five stations in the first stage, with the rapid fluctuations of HF Doppler around the apex of h'F. On the contrary, virtual heights increase again in the second stage with time shifts that indicate a motion from north to south. It is shown that the second stage is responsible for the plasma instabilities which cause the gigahertz scintillation event at midlatitudes, whereas the first stage which corresponds to the enhancement stage of equatorial anomalies, is not unstable. From these features, important conclusions are derived that electric field reversals are expected during the events and westward electric fields seem more important for gigahertz scintillations at midlatitudes. Some other pieces of evidence are also shown which support the electric field reversals. As the cause of the two-stage process, a push-pull mechanism between ionized and neutral atmospheres is proposed.
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  • Two-Dimensional Magnetotelluric Modelling by the Boundary Element Method
    Shi-zhe XU, Sheng-kai ZHAO
    1987 Volume 39 Issue 11 Pages 677-698
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.677
    JOURNAL FREE ACCESS
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    2-D magnetotelluric modelling is usually done by the finite element method and the finite difference method. When the anomalous body is of a complicated shape, division of the region into cells and initial data preparation are rather tedious. In this paper, the boundary element method is applied to 2-D magnetotelluric models. First, we convert the boundary value problem of electromagnetic field into the boundary integral equation. Then, we solve the integral equation with the boundary element method. The boundaries of the domain considered and the anomalous body are divided into elements. The integral along all boundaries is divided into a sum of integrals along each element. Linear interpolation and Gaussian quardrature formula are used to calculate these integrals. After these steps, the integral equation is converted into a set of linear equations. Solving the equation system with the Gaussian elimination method, the electromagnetic field can be obtained and the apparent resistivity can be derived. The boundary element method is suitable for a model with one anomalous body. When there are several anomalous bodies, formula derivation and programing are complicated. This method is applied to two examples. The results agree well with those obtained by the finite element method.
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  • Secular Change in the Geomagnetic Field in West Africa for Thirty Years
    Comparison with Fourth Generation IGRF Models
    J. VASSAL
    1987 Volume 39 Issue 11 Pages 699-707
    Published: 1987
    Released on J-STAGE: September 14, 2010
    DOIhttps://doi.org/10.5636/jgg.39.699
    JOURNAL FREE ACCESS
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    Comparisons of secular variations computed from geomagnetic reference field models were made against the data obtained at Mbour and Bangui observatories and at 24 magnetic repeat stations in West Africa. The obtained results were very similar between the observatory and the repeat station data, indicating that (1) the TGRF models provide an excellent fit to secular variation data, and (2) there are no significant anomalies in African secular variation.
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