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" Long-term evolution of 27 days signal in solar activity and terrestrial magnetic indices common features and differences ".

M.G. Shnirman (MITP RAN)



Solar activity is known to be the origin of the short-term oscillation of The Earths magnetic field. The 27-day signal created by the solar rotation is reflected by the 27-day geomagnetic signal. We study the long-term evolution of the 27 days signal and compare its properties for solar and geomagnetic activity. We suggest the running averaging of 27 days range energy packet as a principal tool of this study.


The 27-days signal exhibits the 22-year Hale cycle whose properties are different for sunspot numbers (Wolf number, sunspot group numbers, etc) and geomagnetic indices (aa-index, Dst-index, etc). The Hale cycle is manifested in the normalized aa-index through the whole 130-years time span covered by geomagnetic indices reflecting the non-symmetric part of the solar wind. We obtain that short and long period in the 27-days range are differently reflected in the solar and geomagnetic activity. Shorter periods energy has rather similar evolution when longer periods (more than 28 days) exhibit different behavior. The 22-years cycle in aa-index is clearly generated by relatively long periods (more than 28 days), which is not true for sunspot numbers. We discuss possible origin of this observation.


The 150 years evolution of the 27-days signal and its Hale cycle demonstrates quasi-stationary behavior at long time intervals with a few critical moments. One of them happens around 1915 and precedes the increase of solar activity. Another one is observed around 1940 in the aa-index and precedes the end of the growing phase. We discuss a possibility for the 27-days signal to be a sensitive indicator of interior solar processes, further development and problems of its study.