Three Solar Cycles of Non-Increasing Magnetic Field

Details Three Solar Cycles of Non-Increasing Magnetic Field Three Solar Cycles of Non-Increasing Magnetic Field

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh51a08h&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #SH51A-08 INVITED

Physics

Geophysics

2788 Storms And Substorms, 7524 Magnetic Fields, 7537 Solar And Stellar Variability

Scientific paper

Since measurements started in the late 19th century, there has been a secular increase (with superposed ripples due to solar cycles) of the aa geomagnetic index. Starting from this observation, Lockwood, Stamper, and Wild (hereafter, LSW) conclude (Nature, 399, 1999; see also Lockwood et al., Astronomy and Geophysics, 40, 1999) that the total source's magnetic flux in the Sun's atmosphere has risen by 41% since 1964\" and by 130% in the 20th century. However, solar data over nearly three solar cycles - near-daily magnetograms from Mt Wilson, and Wilcox Solar Observatories and newly reanalyzed data from the National Solar Observatory - show no secular trend in overall photospheric flux. More importantly, the magnetic field open to interplanetary space (as calculated from photospheric measurements and assuming potential fields to a height of 2.5 Rsun) fails to show a secular increase over the last three solar cycles. Like LSW, we do not explicitly take account of transient events. Thus, both data and calculations imply that the Sun's average coronal magnetic flux has not increased over the last three solar cycles. Analysis of simulations with the potential field source surface model shows that the interplanetary magnetic flux is not simply related to the overall, photospheric, solar magnetic flux. Both results are in agreement with the findings of Wang, Lean, and Sheeley (GRL, 27, 2000). The topology, not just the strength, of the emergent solar magnetic field is a major determinant of the interplanetary magnetic field experienced at Earth. In principle, secular change in non-potentiality of the coronal field could lead to secular increase in interplanetary magnetic flux, but this seems unlikely.

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