ISTerre seminar

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Scaling of the geomagnetic secular variation timescales

Tuesday 25 March 2025 - 11h00

Yue-Kin Tsang - Newcastle University
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From the geomagnetic power spectrum and the secular variation spectrum, a time-scale spectrum tau_sv(l) can be constructed as a function of the spherical harmonic degree l. Although tau_sv(l) is only defined above the outer core, it is often taken as a representative time scale of the dynamo inside the outer core. To assess the validity of this surmise, we introduce a magnetic time-scale spectrum tau(l,r) that is valid for all radius r above the inner core and study its scaling in a numerical geodynamo model. We find that the scaling of tau(l,r) with l undergoes a sharp transition just under the core-mantle boundary, suggesting tau_sv(l) is not reliable in estimating time scales inside the outer core. A second question concerning tau_sv(l) is whether a frozen-flux argument can explain its scaling. We analyse the induction equation in the spectral space and find that magnetic diffusion and the radial derivatives in the induction term, which are both ignored in the argument, are important in determining the scaling of tau_sv(l) and tau(l,r). Finally, we propose that if the location of the peak of the secular variation spectrum becomes available in the future, there might be a way to estimate the time scales inside the outer core using tau_sv(l).

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Organizing team : Géodynamo

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères

Informations de visio :

https://univ-grenoble-alpes-fr.zoom.us/j/91690538748?pwd=buNDS0rAZw9rG4vS1rWxX53zHYdQaw.1