ALBERT

Description: SUMMARY Magnetic satellite data from the last decade allow to model geomagnetic secular acceleration, the second time derivative of the field, in a highly precise manner. Robust estimates of the secular acceleration (SA) are obtained by using order six B-Splines as representation of the field variability, which in turn allows us to estimate the characteristic SA timescale, τ SA . We confirm a recent finding that τ SA is of order 10 years and fairly independent of the spherical harmonic degree n . This contrasts with the characteristic timescale of geomagnetic secular variation τ SV , which is a decreasing function of n and is 100 yr for n ≤ 5. Conceivably the SA timescale might be related to short-term processes in the core, distinct from convective overturn whose timescale is reflected by τ SV . Previously it had been shown that dynamo simulations reproduce the shape of the secular variation (SV) spectrum and, provided their magnetic Reynolds number Rm has an Earth-like value of order 1000, also the absolute values of τ SV . The question arises if dynamo simulations can capture the observed timescales of geomagnetic SA. We determined τ SA ( n ) for a set of dynamo models, covering a range of values of the relevant control parameters. The selection of models was based on the morphological similarity of their magnetic fields to the geomagnetic field and not on criteria related to the time dependence of the field, or on any aspect of the spectra associated with their field variation. We find that τ SA depends only weakly on n up to degree 10, but for larger n it asymptotically approaches the 1/ n -dependence that is also found for τ SV ( n ). The acceleration timescale at low n varies with magnetic Reynolds number more strongly than τ SV and may also depend on magnetic field strength. For an Earth-like Rm ≈ 1000, τ SA is of order 10 yr for n ≃ 2–10, as found in the field models from satellite data. A simple scaling analysis based on the frozen flux assumption for magnetic variations suggests two contributions to the SA, an advective part that scales with velocity U and has a length scale dependence corresponding to n −1 , and a part that depends on the acceleration of the flow without explicit dependence on the length scale. Their combination can explain the spectral shape of τ SA ( n ) in numerical models, with the latter term dominating at n 〈 10. The characteristic timescale of acceleration of the near surface flow correlates with τ SA in the different numerical models and is of the same order as τ SA . This suggests that the observed 10 yr timescale of geomagnetic SA reflects the characteristic time of core flow acceleration. To explain the geomagnetic SV and SA timescales, we find that the rms velocity near the core surface must be 18 km yr −1 and the rms flow acceleration approximately 2 km yr −2 , although a statistical analysis of the induction equation suggests that most of the latter may occur at flow scales corresponding to harmonic degrees n 〉 12. The ability of dynamo models to match simultaneously SV and SA timescales suggests that dynamic processes in the core at the decadal timescale are not fundamentally different from those at the centennial timescale.

Description: SUMMARY We analyse the residuals between a continuous geomagnetic field model of the internal field, C 3 FM, and observed secular variation. A large part of the residual variations correlate closely with the D ST -index, suggesting an origin from unmodelled external field variations. Removal of this signal enhances the resolution of fine-scale detail in secular variation; this is useful in considering the phenomenology of geomagnetic jerks. The residual variations between different observatories show even better correlation, suggesting the possibility of the construction of a proxy for the D ST -index. Notable cross-correlation is also seen between the residuals and D ST -index with a lag of about 55 months.