Long-term trends in interplanetary magnetic field strength and solar wind structure during the twentieth century

Details Long-term trends in interplanetary magnetic field strength and solar wind structure during the twentieth century Long-term trends in interplanetary magnetic field strength and solar wind structure during the twentieth century

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002jgra..107.1304r&link_type=abstract

Journal of Geophysical Research (Space Physics), Volume 107, Issue A10, pp. SSH 12-1, CiteID 1304, DOI 10.1029/2001JA000507

Astronomy and Astrophysics

Astrophysics

12

Global Change: Solar Variability, Interplanetary Physics: Cosmic Rays, Interplanetary Physics: Interplanetary Magnetic Fields, Interplanetary Physics: Solar Cycle Variations (7536), Solar Physics, Astrophysics, And Astronomy: Magnetic Fields

Scientific paper

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 (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 aa. 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.

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