Abstract
SUMMARY We propose a spectral transfer model for the secular variation of the geomagnetic core field to explain the simultaneous decrease in dipole field intensity and the increase in non-dipole field intensity from 1840 to the present in terms of a dynamo cascade process. The main assumption of this model is that magnetic energy is transferred between adjacent spherical harmonic degrees in the Mauersberger–Lowes spectrum of the geomagnetic field. The key parameters are a set of coefficients that indicate the rate and direction of magnetic energy transfer through the spectrum. Applying the spectral transfer model to the historical period, we find that the quadrupole family of the core field can be characterized by a persistent inverse magnetic energy cascade from higher towards lower spherical harmonics. In the dipole family of the core field, we find cascade behaviour generally from lower to higher spherical harmonics, consistent with axial dipole decrease, but with a high level of time variability that correlates with variations in the dipole family intensity. During time intervals when the dipole family intensity rapidly decreases, energy appears to cascade towards higher spherical harmonics, beyond the limit of the observable part of the core field spectrum. During time intervals when the dipole family intensity is nearly constant, a more limited forward cascade appears to trap energy at intermediate spherical harmonics. Similar fluctuations in the rate and direction of spectral transfer are also seen in the Mauersberger–Lowes spectrum of a numerical dynamo model during a dipole decrease event that led to a polarity excursion. We discuss the possibility of this scenario for the current geomagnetic dipole decrease.
Highlights
During the historical period, the shape of the low degree part of the geomagnetic field spectrum has rapidly changed with time
We argue that local spectral transfer is the dominant process that shapes the core field spectrum, because so much of the core field secular variation (SV) can be explained as frozen-flux advection by an evolving large-scale dominantly toroidal flow (e.g. Jackson 1997; Amit & Olson 2006) with the non-local energy transfer from smaller scales being of secondary importance
We interpret the changes in the low degree Mauersberger–Lowes geomagnetic spectrum at the core–mantle boundary (CMB) since 1840 using a magnetic energy cascade model
Summary
The shape of the low degree part of the geomagnetic field spectrum has rapidly changed with time. According to the historical gufm field model (Jackson et al 2000), the geomagnetic dipole moment has dropped from 8.50 × 1022 Am2 in 1840 to 7.84 × 1022 Am2 in 1990, and its dominant axial component has decreased from 8.33 × 1022 Am2 in 1840 to 7.70 × 1022 Am2 in 1990. According to the xCHAOS field model (Olsen & Mandea 2008), the 2009 dipole moment was 7.75 × 1022 Am2, its axial component was 7.63 × 1022 Am2, and its current rate of change is −4.92 × 104 Am2 yr−1, indicating that the dipole decrease has recently accelerated. One possibility is that magnetic diffusion is preferentially weakening the dipole field by decay of its higher modes (Moffatt 1978) or by expulsion of reversed dipolar field from the core (Bloxham 1986), while at the same time, dynamo processes are strengthening the non-dipole field. We apply our cascade model to estimate the rates of spectral transfer in the core during the present-day dipole decrease
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