On the slow-time geomagnetic field modulation of cosmic rays

Abstract

In this work, monthly means of cosmic ray count rates from two mid latitude (Hermanus and Rome), and two higher latitude (Inuvik and Oulu) neutron monitors (NM) were employed and their variability was compared with geomagnetic stations that are in close proximity to the NMs. The data spans 1966 to 2008 and covers four solar cycles. The difference (CRdiff) between the mean count rate of all days and the mean of the five quietest days for each month was compared with the Dst-related disturbance (Hdiff) derived from the nearby geomagnetic stations. Zeroth- and First-correlation between the cosmic ray parameters and geomagnetic parameters was performed to ascertain statistical association and test for spurious association. The present results show that solar activity is generally strongly correlated (>0.75) with mean strength of GCR count rate and geomagnetic field during individual solar cycles. The correlation between mean strength of cosmic ray intensity and geomagnetic field strength is spurious and is basically moderated by the solar activity. The signature of convection driven disturbances at high latitude geomagnetic stations was evident during the declining phase of the solar cycles close to the solar minimums. The absence of this feature in the slow-time varying cosmic ray count rates in all stations and in the mid latitude geomagnetic stations suggests that the local geomagnetic disturbance do not play a significant role in modulating the cosmic ray flux. Key words: Geomagnetic field variability, solar activity, galactic cosmic rays, cosmic ray modulation.