We use continuous field models to derive the secular variation of Ḃφ at the core-mantle boundary (CMB). Here we assume the mantle to be weakly conducting and we apply a non-harmonic downward continuation procedure in order to derive this quantity at the CMB. In general, the results of the non-harmonic downward continuation has implications to the determination of torque due to electro-magnetic core-mantle coupling. In fact the radial magnetic field component is determined by the spherical harmonic coefficients directly, whereas the magnitude of Bφ and Ḃφ are given by the much larger radial derivatives of the non-harmonically continued spherical harmonic coefficients. In particular, we are interested in the properties of the geomagnetic jerk occurred around 1991 derived from the non-harmonic downward continuation. We discuss a comparison of Ḃφ, derived from different field models, among each other in the time interval 1980-2000. Further, from this quantity we derive the occurrence times of the 1991 jerk at the Earth's surface by applying a simple optimization procedure. A comparison between these times at the Earth's surfaces and the occurrence times at the CMB reveals that the occurrence at the CMB predates the corresponding Earth surface event by about one year. The optimization procedure provides a clearer picture of the jerk shape and occurrence properties than it was in recent investigations, where mantle conductivity has been neglected.

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Ballani, L.; Greiner-Mai, H.; Stromeyer, D.; Chambodut, A.; Wardinski, I. (2006): The secular variation of the geomagnetic Y-component in the core-mantle boundary region. AGU 2006 Fall Meeting (San Francisco, USA 2006).