Observations of cosmic ray intensity in the Brazilian geomagnetic anomaly region

The geomagnetic field functions as a barrier against the incident of cosmic rays to the atmosphere. According to the recent studies, however, the geomagnetic field has been monotonically decreasing for past one thousand years. If the recent decreasing rate continues, the geomagnetic field will disappear within 1500 years. It should be noted that the geomagnetic field in the southern part of Brazil is already extremely weak. In order to investigate the change of radiation environment due to the decreasing geomagnetic field, we started a continuous observation of cosmic ray intensity in the Brazilian geomagnetic anomaly region from January 1997. To estimate the present cosmic ray intensity in Brazil, a wide area survey has been also conducted in August 1997. The cosmic ray intensity, absorbed dose rate in air due to ionizing component of cosmic rays, was measured with a multichannel spectrometer with a 3'' diameter x 3'' cylindrical NaI(Tl) scintillation detector. Based on the continuous observation, the cosmic ray intensity has fluctuated by some 20 percent from 1997 to 1998 which is almost the same as that measured in the north America located at the high geomagnetic latitude compared to Brazil. The wide area survey also indicated that the recent cosmic ray intensity in Brazil is relatively 5 percent large compared to the other area situated in almost the same geomagnetic latitude as Brazil. These results suggest that the cosmic ray intensity in the atmosphere will be increasing due to the decreasing geomagnetic field and the area of such phenomena will expand into global scale in the near future if the decrease of the geomagnetic field continues. (author)