Abstract
Abstract Rapid changes in the magnetic field characterised by an abrupt change in the secular variation have been named “secular variation impulses” or “geomagnetic jerks”. Three of these events, around 1968, 1978 and 1990, occurred during the time-span covered by the comprehensive model CM4 (Sabaka et al., 2002, 2004). This model, providing the best temporal resolution between 1960 and 2002 as well as a fine separation of the different magnetic sources, can be used to study rapid phenomena of internal origin. In order to analyse these events all over the globe, synthetic time series were obtained from the CM4 model between 1960–2002. Geomagnetic jerks are detected here as a rapid movement of the zero isoline of the second field derivative. Analysis of the area swept out by this isoline as a function of time allows us to map the spatial extent of jerks though time, and to identify an event around 1985 that is localized in the Pacific area. At the core surface, we compute the fluid flows under the frozen-flux and tangentially geostrophic assumptions. The flows do not exhibit any special pattern at jerk times, but instead show a smooth temporal evolution over the whole time period. However, the mean amplitude of the dynamical pressure associated with these flows present maxima at each jerk occurrence and helps to confirm the identification of a jerk in 1985.
Highlights
The Earth’s magnetic field occasionally shows rapid changes in its temporal variations
We concentrate on the core surface, using the comprehensive model CM4 to analyse, more accurately than was previously possible, the modifications of core surface flows linked to the jerks
A global characterisation of geomagnetic jerks is difficult to achieve when only data provided by observatories are available: they are very unevenly distributed in space, being concentrated over the continents of the northern hemisphere while being sparser over the oceans and over the southern hemisphere
Summary
The Earth’s magnetic field occasionally shows rapid changes in its temporal variations. The event; the corresponding secular variation (the first time derivative of the geomagnetic field) has a V-shaped feature, the second time derivative is step like and the third derivative is like a Dirac-delta function (Courtillot et al, 1984) This definition of a geomagnetic jerk observed at one point at the Earth’s surface is an ideal one. Some previous studies have already attempted to characterise the temporal occurrences and extents of geomagnetic jerks at the Earth’s surface using the magnetic field itself (Alexandrescu et al.,1996; Nagao et al, 2002) or its secular variation (Macmillan, 1996; Le Huy et al, 1998; De Michelis et al, 1998; Chambodut and Mandea, 2005). We concentrate on the core surface, using the comprehensive model CM4 to analyse, more accurately than was previously possible, the modifications of core surface flows linked to the jerks
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