Long-Term Trends in Interplanetary Magnetic Field Strength and Solar Wind Structure During the Twentieth Century

摘要

Lockwood et al (1999) have recently reported a -40% increase in the radial component of the interplanetary magnetic field (IMF) at Earth between 1964 and 1996. We argue that this increase does not constitute a secular trend but is largely the consequence of lower than average fields during solar cycle 20 (1964-1976) in comparison with surrounding cycles. For times after 1976 the average IMF strength has actually decreased slightly. Examination of the cosmic ray intensity, an indirect measure of the IMF strength over the last five solar cycles (19-23) also indicates that cycle averages of the IMF strength have been relatively constant since -1954. We also consider the origin of the well- documented increase in the geomagnetic aa index that occurred primarily during the first half of the twentieth century. We surmise that the coronal mass ejection (CMB) rate for recent solar cycles was approximately twice as high as that for solar cycles 100 years ago.