Investigating the Annual and Semi-Annual Variations of Geomagnetic Field Components in the Northern Hemisphere

Abstract A study of the ever present ring current on the geomagnetic field strength is used to determine the annual and semi-annual geomagnetic activity variation and validate Russell-McPherron hypothesis. The D, H and Z geomagnetic field component mean monthly and yearly field strength in ten (10) geomagnetic observatories at different geomagnetic longitude were employed. Six parameters representing annual (Ddiff, Hdiff and Zdiff) and monthly (ddiff, hdiff and zdiff) residuals of the geomagnetic field strength were defined, and presented as indicators of ring current monthly/yearly influence on the strength of the components of the geomagnetic field. It is found that only mid latitude stations showed common pattern of Ddiff with peaks during solar maximum and troughs during solar minimum, a pattern which follows the SS cycle although out of phase. Hdiff showed annual variation corresponding to the 11year solar cycle especially in low and mid geomagnetic latitudes which was not in phase with the sunspot number (SSN). The Zdiff parameter exhibited annual variation which corresponded also with the 11 year solar cycle in the mid and high latitudes. We observed strong correlation (at 95% confidence) of Ddiff with SSN for only mid latitude stations. Strong inverse correlation between Hdiff and SSN was found at low and mid -geomagnetic latitudes and weaker correlation at high latitudes. Statistical association between Zdiff and SSN was stronger at high geomagnetic latitudes than low geomagnetic latitudes. The semiannual variation of the parameters- ddiff, hdiff revealed clear equinoctial peaks in April and Oct-Nov. The fall peak is slightly larger than the spring peak and this observation were the same irrespective of location. This semiannual global seasonal phenomenon is likely dominated by the North-South migration of the ring current.