DIPOLE AND NONDIPOLE FIELDS OF THE GEOMAGNETIC FIELD ON THE BASIS OF PALAEOINTENSITIES OF THE BRUNHES VOLCANIC ROCKS.

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The experimental instruments were developed and used in the measurents of the test samples in the first year. The alternating field demagnetiser has a 120mT maximum output with a constant rate of increasing and decreasing fields. This constancy is of 100mT/60sec because the higher rate tends to result in a smaller ARM.The ARM is acquired in parallel to the NRM or TRM which is compared. Such an ARM can correct the NRM or TRM intensity for the anisotropy effects. The heating time at the top temperature is 10min. and 20min. in the first and the second heating, respectively. The samples were collected from Daisen Volcano (Japan), Andean volcanic region (Peru), and Datong monogenetic volcanoes (China). All of them were formed in the Brunhes epoch. Double heating of the Shaw method results in the success of 13 samples out of 32. The VDMs, calculated from the measured palaeointensities and the inclinations, show the splitted distribution around the mean VDMs in the previous reports. This implies the geodynamo intensities may have more complicated distribution than those previously suggested by the palaeomagnetists. The relative intensity change in the nondipople/dipole fields was investigated using the direcions of the sedimentary records of the Matuyama-Brunhes transition. The records were obtained from the Boso marine sediments in Japan. The spectral analyzes suggest that the short-period variations, probably of the order of 10^2 years, are remarkable in the transition field.