Journal of geomagnetism and geoelectricity
Online ISSN : 2185-5765
Print ISSN : 0022-1392
ISSN-L : 0022-1392
Volume 37, Issue 11
Displaying 1-6 of 6 articles from this issue
  • Kazuo YAMAKOSHI, Akira FUJIWARA, Noriaki ASADA
    1985 Volume 37 Issue 11 Pages 987-997
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
    In order to study changes of chemical compositions of ejecta scattered from dust-dust and dust-meteorite collisions in space, two laboratory hypervelocity impact experiments by a two-stage light gas gun were performed. As a common material stainless steel, SUS 304, was used for both projectiles and targets in these experiments. The behaviors of the elements, Fe, Ni, Co, Cr and Sb were studied and enrichments of Ni and Co and depletion of Sb were observed in smaller ejected fragments in the grain to grain collision. In the grain to large body collision, the behavior of the elements were so complicated, because both enrichment and oxidization of the elements occurred in the smallest ejecta.
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  • Yoshihiro HIGUCHI
    1985 Volume 37 Issue 11 Pages 999-1015
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
    The normalized dispersion curve method taking into account the presence of helium ions is used to determine plasma densities in the magnetosphere from the dispersion characteristics of Pc 1 emissions. The determination of equatorial distances to the propagation paths of Pc 1 emissions indicates that the inferred source location of Pc 1 signals lies on the plasmapause. The phase trapping length along the geomagnetic field line from the equatorial plane is estimated as functions of wave amplitudes and proton pitch angles. It is found that an increase of helium ion concentration tends to produce a detrapping effect on the high frequency branch of ion cyclotron waves in the dispersion diagram.
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  • Leroy R. ALLDREDGE
    1985 Volume 37 Issue 11 Pages 1017-1033
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
    Several investigators have noted that the Earth's magnetic dipole has been decreasing during the last century. Some have indicated that if the trend continues the dipole would be zero within a few thousand years, indicating that a reversal was occurring. Few if any take such suggestions seriously because of the weakness of the conditional if clause. An example of such a “prediction” is taken from the recent series of definitive models and reasons are given describing why such predictions of reversals are quite meaningless. It is known that the Earth's magnetic field, at the surface of the Earth is predominantly Bipolar. From paleomagnetic studies it is also known that this predominantly dipolar field has reversed its direction many times. Some studies indicate that although, at the present time, the dipole of the Earth is decreasing quite rapidly, the total magnetic energy outside the core is remaining nearly constant. This idea is reexamined using the recent definitive models and found to be true. The main purpose of this paper is to investigate the characteristics of relatively simple core sources which can produce the observed predominantly dipolar field and then cause the field to reverse without much loss of the total energy outside the core. A simple source in the outer core cannot produce the outstanding dipolar pattern that is observed. The right combination of many simple sources, the strongest of which must be very deep in the outer core, can create the dipolar pattern as has been shown in several previous studies. A very simple source system deep in the outer core is contrived to illustrate one way in which the main characteristics of a reversal can be realized.
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  • K. P. KODAMA, E. B. EVENSON, J. M. CLINCH, J. RABASSA
    1985 Volume 37 Issue 11 Pages 1035-1050
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
    Glacial sediments and two volcanic flow units from northwestern Patagonia, Argentina were sampled for paleomagnetic measurements as part of an ongoing program to date and correlate the glacial stratigraphy. Samples taken from a large ball and pillow structure in the Anfiteatro Moraine are normal polarity and pass the fold test suggesting that the magnetization was locked in prior to the formation of the structure. Samples from Pichileufu-age glaciolacustrine sediments have anomalously low inclinations and show little movement of the magnetic vector over a 400 to 500 year period if rhythmites are interpreted to be varves. Tills in the Malleo valley, 120km north of Bariloche, that are tentatively correlated with the Pichileufu-age sediments are bracketed by normal polarity basalt flows from Pino Santo and Volcan Lanin. Anisotropy of magnetic susceptibility results from the Pichileufu and Anfiteatro Moraine sediments indicate that both units have a primary depositional fabric. A redeposition experiment of Pichileufu lake sediment suggests that it is capable of recording steep inclination geomagnetic fields. The Pichileufu sediments are interpreted to have recorded anomalous low inclination geomagnetic field behavior which is not characterized by the dramatic looping usually associated with geomagnetic excursions or polarity transitions. These observations may be explained by local dominance of a standing focus of the nondipole field for at least 400 to 500 years in the early- to mid-Brunhes normal polarity epoch.
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  • Randolph H. WARE, Malcolm J. S. JOHNSTON, Robert J. MUELLER
    1985 Volume 37 Issue 11 Pages 1051-1061
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
    A network of 27 proton magnetometers (PM's), designed to detect stress related magnetic events of crustal origin, has been operating near active faults in California for the past decade. We present here comparative magnetic difference field data from PM's used in this net with that obtained from new self-calibrating rubidium magnetometers (SCR's). The instruments were first compared over a 50-m baseline in an aseismic and magnetically quiet region in Colorado. For PM's having either a 0.25-nT or a 0.125-nT least count, the observed difference variations were 0.2-nT and 0.17-nT rms, respectively. For SCR's having a 0.001-nT least count and a 100-second averaging interval, the difference variation was 0.002-nT rms. Power spectra of these data indicate that the noise for the PM's is close to their least count limit. However, the SCR noise decreases at about 20dB per decade until it approaches its least count limit 40dB below the PM limit, for periods less than 30 minutes. A similar experiment was conducted using collocated SCR and PM pairs separated by 13km along the San Andreas fault. Power spectra indicate that both systems are equivalent and are dominated by external noise at periods greater than 4 minutes. Below 4 minutes the PM noise approaches its least count limit while the SCR noise continues to decrease at about 20dB per decade until it is 20dB below the PM limit at a period of 30 seconds. Improved discrimination of magnetic transients caused by fault activity with periods of several minutes to perhaps an hour appears to be possible with higher sensitivity magnetometers.
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  • Masahito KATO, Takasuke SAKAI
    1985 Volume 37 Issue 11 Pages 1063-1070
    Published: 1985
    Released on J-STAGE: September 14, 2010
    JOURNAL FREE ACCESS
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