Magnetic flux expulsion from the core as a possible cause of the unusually large acceleration of the north magnetic pole during the 1990s
Résumé
The north magnetic pole (NMP) has been drifting in a north‐northwesterly direction
since the 19th century. Both local surveys and geomagnetic models derived from
observatory and satellite data show that the NMP suddenly accelerated during the 1990s.
Its speed increased from about 15 km/yr in 1989 to about 60 km/yr in 2002, after which it
started to decrease slightly. Using a Green’s function, we show that this acceleration is
mainly caused by a large, negative secular variation change in the radial magnetic field at
the core surface, under the New Siberian Islands. This change occurs in a region of the
core surface where there is a pair of secular variation patches of opposite polarities, which
we suggest could be the signature of a so‐called “polar magnetic upwelling” of the type
observed in some recent numerical dynamo simulations. Indeed, a local analysis of the
radial secular variation and magnetic field gradient suggests that the secular variation
change under the New Siberian Islands is likely to be accompanied by a significant amount
of magnetic diffusion, in agreement with such a mechanism. We thus hypothesize that
the negative secular variation change under the New Siberian Islands that produced the
NMP acceleration could result from a slowdown of the polar magnetic upwelling during
the 1990s. We finally note that the NMP drift speed is determined by such a combination
of factors that it is at present not possible to forecast its future evolution
Domaines
Planète et Univers [physics]
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