Abstract

Quiet-time mean monthly values from the INTERMAGNET observatory at Hermanus (HER) in South Africa were used to study the changes in secular variation during the period between 2005 and 2009. After removing an annual variation resulting from magnetospheric and ionospheric currents by means of a 12-month running means applied to the respective observatory first differences of the X, Y, and Z components, clear evidence was revealed of a strong geomagnetic jerk that occurred during 2007 in this area. The GRIMM-2X model also provided evidence of the occurrence of this jerk in 2007. Of particular interest is that GRIMM-2X predicts the turning points in all the secular variation trends to occur much earlier than revealed by the observatory data. We also observed that the power of this jerk, determined as the difference in slope of the secular variation before and after the jerk, is several times stronger than the global jerk of 1982/3.

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