Rapid core field variations over the past decade (Invited)

It has long been observed that the geomagnetic secular acceleration (defined as the second order time derivative of the field) at ground observatories undergoes some sudden changes of polarity, referred to as geomagnetic jerks. The better space-time data coverage and overall data precision provided by the Oersted and CHAMP satellites recently made it possible to model the secular acceleration on the global scale. Unlike measurements at isolated observatories, secular acceleration spherical harmonic models can be downward continued to the core-mantle boundary, thus providing new information on rapid core dynamics. We calculated secular acceleration models from CHAMP data every 30 days from 2002 to 2009.5 on a three-year sliding window. We found that the secular acceleration underwent two large "pulses" at the core-mantle boundary, one centered in 2006 (and previously pointed out by Chulliat et al., GRL, 2010) and the other centered in 2009. These two pulses are separated by a geomagnetic jerk near 2007, and delimited by two other jerks near 2003 and 2010.5. Their spatial structures at the core-mantle boundary are highly anti-correlated and suggest the existence of a stationary magnetohydrodynamic wave at the core surface, of period 5 to 6 years, in the low-latitude Atlantic sector. Possible interpretations of this wave in terms of core processes will be discussed, as well as relationships with other rapid core field variations observed in geomagnetic data.