Can a 1-D mantle electrical conductivity model generate magnetic jerk differential time delays?

Abstract

Worldwide jerks occurring in 1969, 1978 and 1991 are not simultaneous at the earth's surface, showing differential delays of about 2 yr. One way to explain this intriguing temporal pattern is to consider the earth's mantle as a conductor. Consequently the geomagnetic field observed at the earth's surface will correspond to a filtered version of the original field generated at the core-mantle boundary (CMB). We developed the forward approach to this problem assuming the temporal part of the jerk as a simultaneous impulse in the third time-derivative of the magnetic field at the CMB. Two synthetic spherical harmonic models of the jerk amplitudes are built by using two different power spectra. The effect of the mantle is illustrated by a 1-D radial electrical conductivity model that acts as linear, causal and time-invariant filter. The key point is that the mantle filter is different for each harmonic degree. Therefore, because the mixing of harmonics varies with location, distinct time delays will exist at the earth's surface. By using Backus' mantle filter theory, we demonstrate that a simple 1-D mantle electrical conductivity model is able to generate differential jerk time delays that depend strongly on the jerk morphology input model. We also illustrate that the time delays will vary for each component of the magnetic field. © 2008 The Authors Journal compilation © 2008 RAS.