External forcing of the geomagnetic +eld? Implications for the cosmic ray .ux—climate variability

Data on the relation of past climate variations and changes in the geomagnetic +eld can contribute to the research on a cosmic ray–climate link. This paper presents a synthesis of geological and astrophysical results about the link of magnetic +eld variations and Earth’s climate and external forcing. On millennial timescales, geomagnetic intensity lows are apparently in phase with times of low solar magnetic activity and climate cooling. Reconstructions of quaternary continental climate systems indicate colder and dryer conditions during and after geomagnetic reversals. In the Milankovitch frequency band geomagnetic +eld reversals can occur in conjunction with minima in the carbonate content related to the 100 ka eccentricity cycle of the Earth’s orbit. A quasi 100 ka cyclicity (in phase with orbital eccentricity) is discussed in the literature for both geomagnetic intensity and solar magnetic activity. On mega-cycle periods external forcing seems to play a role in the appearance of superchrons of the geomagnetic +eld. When applying current spiral arm models superchrons developed during times when the solar system was located between spiral arms. This suggests that the galactic environment may force the geomagnetic +eld to switch from its reversing mode into superchron stages probably via modulations of the Sun. Interestingly, there is a link to the structuring of magnetic +elds in our galaxy. While the galactic magnetic +elds between spiral arms are homogenous, the passage of the solar system through turbulent magnetic +elds along the spiral arms parallels high geomagnetic reversal rates. Occurring in conjunction with a low intrinsic cosmic ray .ux the superchrons probably lowered the cosmic ray .ux at Earth even further. This may also have played a role in the evolution of life on Earth and the timing of some mass extinction events. c © 2004 Elsevier Ltd. All rights reserved.