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Long-term Trends in Interplanetary Magnetic Field Strength and Solar Wind Structure during the 20th CenturyLockwood et al have recently reported an approximately 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 approximately 1954. We also consider the origin of the well-documented increase in the geomagnetic alphaalpha index that occurred primarily during the first half of the twentieth century. We surmise that the coronal mass ejection (CME) rate for recent solar cycles was approximately twice as high as that for solar cycles 100 years ago. However, this change in the CME rate and the accompanying increase in 27-day recurrent storm activity reported by others are unable to account completely for the increase in alphaalpha. Rather, the CMEs and recurrent high-speed streams at the beginning of the twentieth century must have been embedded in a background of slow solar wind that was less geoeffective (having, for example, lower IMF strength and/or flow speed) than its modern counterpart.
Document ID
20020088125
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Richardson, I. G.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Cliver, E. W.
(Air Force Research Lab. Hanscom AFB, MA United States)
Cane, H. V.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
White, Nicholas E.
Date Acquired
September 7, 2013
Publication Date
January 1, 2002
Subject Category
Solar Physics
Funding Number(s)
CONTRACT_GRANT: NCC5-609
CONTRACT_GRANT: NCC5-180
CONTRACT_GRANT: NSF ATM-9912341
Distribution Limits
Public
Copyright
Public Use Permitted.
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