The 2003 geomagnetic jerk and its relation to the core surface flows

Abstract

In this paper we examine the core surface flow obtained by an inversion of a continuous model of the geomagnetic field and its temporal variation using the diffusion-less induction equation. The continuous CHAOS model is derived from satellite data up to spherical harmonic degree 14 and covers the period 1999 to 2006. The CHAOS secular variation, when downward continued to the core surface, shows stripe-like features, which can be attributed to spherical harmonic degree 12 and higher. These contributions are removed by applying a tapering method, and the resulting tapered model is then inverted for the core surface flow. Satellite-based field models have a high spatial resolution; however, their temporal resolution is limited. In order to enhance the temporal resolution of the flow, we additionally constrain the flow to fit the secular variation from ground-based observatory data.A range of solutions, subject to different constraints, are computed, two flow hypotheses being considered: purely toroidal flow and tangentially geostrophic flow.We show that both flow types provide similar results; however, the purely toroidal flow provides a better fit to the secular variation in the equatorial region than Finally, we compare observed changes in the length-of-day and the predictions from the flow solutions.

Radial vorticity constraint in core flow modeling

“…Indeed, the decomposition (13) has been made in the same manner in previous studies [e.g., Bloxham, 1989;Wardinski et al, 2008].…”

“…k l n denotes the nth zero of the degree l − 1 spherical Bessel function, and in this particular calculation we let n = l. Recent studies have estimated flow models fitting SV models either at one standard deviation [Pais and Jault, 2008] or even more tightly [Holme and Olsen, 2006;Olsen and Mandea, 2008;Wardinski et al, 2008]. Here, we do not restrict ourselves to either of these cases, but study resulting models by varying the misfit level within one standard deviation.…”

“…Satellite magnetic field monitoring over the last decade has revealed the spatial and temporal structures of the core main field (MF) with increasing accuracy, probably up to its smallest-scale components that are unaffected by the noise from other sources. Core surface flow modeling has come to a phase, in which even temporal fluctuations of the flow are discussed in association with subdecadal changes of magnetic field and Earth rotation [Holme and Olsen, 2006;Wardinski et al, 2008]. It has been reported that such rapid changes of the flow are actually required to explain the recent fluctuations of secular variation (SV) [Olsen and Mandea, 2008].…”

“…The TG constraint alters the configuration of estimated flows significantly at the geographic equator, restricting flows to only the azimuthal component. As a result, estimated TG flow models tend to fail in explaining the local SV around the equator [Wardinski et al, 2008]. Furthermore, recent analyses have found that a pure TG flow model has difficulty in explaining the time variation of satellite SV models [Olsen and Mandea, 2008;Wardinski et al, 2008].…”

“…Likewise, a helical flow model may well satisfy the RV equation. With reference to the fit to magnetic observations alone, previous works have investigated both the assumptions of PT flow [Holme and Olsen, 2006;Wardinski et al, 2008] and helical flow [Amit and Olson, 2004]. Here, we seek for the presence of reasonable flow models compatible with the RV equation and either of these assumptions.…”