Long-term variations in the magnetic fields of the Sun and the heliosphere: Their origin, effects, and implications

Abstract

Recent studies of the variation of geomagnetic activity over the past 140 years have quantified the "coronal source" magnetic flux Fs that leaves the solar atmosphere and enters the heliosphere and have shown that it has risen, on average, by an estimated 34% since 1963 and by 140% since 1900. This variation of open solar flux has been reproduced by Solanki et al. [2000] using a model which demonstrates how the open flux accumulates and decays, depending on the rate of flux emergence in active regions and on the length of the solar cycle. We here use a new technique to evaluate solar cycle length and find that it does vary in association with the rate of change of Fs in the way predicted. The long-term variation of the rate of flux emergence is found to be very similar in form to that in Fs, which may offer a potential explanation of why Fs appears to be a useful proxy for extrapolating solar total irradiance back in time. We also find that most of the variation of cosmic ray fluxes incident on Earth is explained by the strength of the heliospheric field (quantified by Fs) and use observations of the abundance of the isotope 10Be (produced by cosmic rays and deposited in ice sheets) to study the decrease in Fs during the Maunder minimum. The interior motions at the base of the convection zone, where the solar dynamo is probably located, have recently been revealed using the helioseismology technique and found to exhibit a 1.3-year oscillation. This periodicity is here reported in observations of the interplanetary magnetic field and geomagnetic activity but is only present after 1940. When present, it shows a strong 22-year variation, peaking near the maximum of even-numbered sunspot cycles and showing minima at the peaks of odd-numbered cycles. We discuss the implications of these long-term solar and heliospheric variations for Earth's environment. Copyright 2001 by the American Geophysical Union.