Variations of intensity and anisotropy of cosmic rays measured at the geomagnetic equator

Abstract

A study of cosmic ray intensity variations has been conducted during 1956–57 at the equatorial mountain station of Kodaikanal, using a standard neutron monitor. The data have been examined to look for the relationship between the day-to-day changes of intensity, the variance of bi-hourly deviations, the occurrence of large bi-hourly deviations at different hours of the day and the associated parameters of the daily variation. The results are related to the electromagnetic state of interplanetary space as determined by streams of solar matter in the neighbourhood of the earth, carrying with them frozen magnetic fields. Comparison is made with the model elaborated by Dorman. The principal conclusions are as follows: (1) Day-to-day changes of intensity involve increases as well as decreases with respect to a base intensity for the period in question. (2) The daily variation of intensity of local neutrons, at an equatorial mountain station during 1956–57, has often a large diurnal as well as a semi-diurnal component. (3) On days of high geomagnetic disturbance, the daily variation exhibits abrupt changes indicative of the source being situated at a distance shorter than the range of the geomagnetic field. On geomagnetically quiet days, the daily variation has a form consistent with its being related to an anisotropy in interplanetary space. On days of moderate geomagnetic disturbance, the daily variation has changeable characteristics. (4) Correlated day-to-day changes of mean intensity and daily variation have been confirmed. For geomagnetically disturbed days, the semidiurnal component is greater than the diurnal component for increases of intensity, and conversely for decreases of intensity. (5) An examination of the time series of Cp for high intensity and for low intensity indicates the presence of a component of the frozen magnetic field in the direction of the solar dipole field. However, during the period of observation the solar dipole field and the sunspot field were in the same direction. Therefore, the results obtained cannot be considered either to confirm or refute the possibility of beams carrying sunspot fields with them.