Long-Term Changes in the Total Cosmic Ray Flux Over the Earth During the Past 400 Years and Possible Relation to Climate Changes

The possibility of a connection between galactic cosmic radiation and climate has intrigued scientists for the past decade. The initial studies of Friis-Christensen and Svensmark found a variation in the global cloud cover that appeared to correlate with the change in galactic cosmic radiation flux over a solar cycle. Further work has indicated that the correlation appears to be between the high-energy cosmic ray flux and the low cloud cover. The amount of cosmic radiation impacting the earth is a function of the geomagnetic cutoff rigidity - a quantity that changes with the evolution of the geomagnetic field. We show that the change in geomagnetic cutoff rigidity over the last 400 years results in a change in the high-energy cosmic radiation flux impacting the earth that is approximately equal to the relative change in galactic cosmic ray flux over a solar cycle. However, these changes in the geomagnetic cutoff rigidity are non-uniform over the globe with both significant increases and decreases at mid-latitude and equatorial locations. These are the locations where the higher energy portion of the cosmic ray spectrum generates ionization throughout the atmosphere. We suggest that both the solar cycle modulation of cosmic radiation and the effect of the long term evolution of the geomagnetic field on the flux impinging at the top of the atmosphere be included in studies of possible cosmic radiation correlations with climate.