The semiannual variation of geomagnetic activity: phases and profiles for 130 years of /aa data

Details The semiannual variation of geomagnetic activity: phases and profiles for 130 years of /aa data The semiannual variation of geomagnetic activity: phases and profiles for 130 years of /aa data

Jan 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002jastp..64...47c&link_type=abstract

Journal of Atmospheric and Solar-Terrestrial Physics, Volume 64, Issue 1, p. 47-53.

Physics

13

Scientific paper

We determined the phases of the maxima (spring, fall) and minima (summer, winter) in the curve of smoothed daily averages of the /aa geomagnetic index, available from 1868 to 1998. The dates we obtained are consistent with the equinoctial hypothesis which has aberration-adjusted theoretical maxima, for a ~440kms-1 (modern epoch) average solar wind speed, on 25 March (experimentally determined to be 27 March, with an uncertainty of /+/-2 days) and 27 September (27 September) and minima on 25 June (26 June) and 26 December (27 December). We also show that the overall shape of the 30-day smoothed modulation curve throughout the year (broad minima, narrow peaks) bears greater fidelity (/|r|=0.96) to the aberration-shifted solar declination /δ (the controlling angle, on average, for the seasonal variation under the equinoctial hypothesis) than to the solar B0 angle (/r=0.76; axial hypothesis) or the solar /P angle (/r=0.86; Russell-McPherron effect). Lastly, a three-parameter fit of the smoothed annual variation of the /aa data with a function consisting of the sum of the smoothed yearly curves for the /δ, B0, and /P angles yielded an amplitude of /0.58+/-0.07 for the /δ component vs. /0.16+/-0.03 for B0 and /0.20+/-0.04 for /P. Thus, the phases and profiles of the 6-month wave in the long-running mid-latitude /aa range index are consistent with control by a dominant equinoctial mechanism.

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