Long-term correlation between solar and geomagnetic activity

Details Long-term correlation between solar and geomagnetic activity Long-term correlation between solar and geomagnetic activity

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jastp..66.1019e&link_type=abstract

Journal of Atmospheric and Solar-Terrestrial Physics, Volume 66, Issue 12, p. 1019-1025.

Physics

27

Geomagnetic Activity, Solar Activity, Solar Wind, Sunspot Number

Scientific paper

A long-term correlation study between solar and geomagnetic activity is reported in this work using annual averages of the aa index and of the sunspot number Rz in the period of 1868-2000. Dst and AE geomagnetic indices and solar wind speed data are used for more recent periods. It is shown that the geomagnetic and solar activity correlation has decreased since the end of the 19th century, and the lag between them has increased. The variations of Rz and aa were in phase in the early period (solar cycles 11-14, around 1868-1910), and became out of phase in later periods (with a lag of 2 years in solar cycle 22, with aa maximum after Rz). Nevertheless, this trend is not monotonic and superposed fluctuations are seen, which does not permit determine if this correlation decrease is part or not of a long period solar activity cycle. The probable cause of the correlation decrease seems to be related to the aa index dual peak structure. The second aa peak seems to have increased relative to the first one. This second peak is more related to the high-speed streams originated from co-rotating structures whereas the first one is related to sunspot (coronal mass ejections) activity. In recent periods, since 1964, it has been observed that aa annual values have higher correlation with the fraction of days per year with daily solar wind speed peaks larger than 500km/s(Fpk) than with Rz. The aa index also shows larger correlation with AE index than with Dst. Thus, it seems that average aa is strongly influenced by AE activity, which is influenced mainly by high speed streams from coronal holes. One can conclude that the decrease in correlation between aa and Rz occurs because the second aa peak has becoming stronger relative to the first one. The cause seems to be that open solar magnetic field structures have increased their activity relative to the closed (sunspot-related) solar magnetic field structures. This implies that the global solar magnetic field could have experienced a differential (between closed and open structures) large-scale variation in the last 130 years.

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