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SUMMARY We introduce a formalism to track magnetic energy transfer between spherical harmonic degrees due to the interaction of fluid flow and radial magnetic field at the top of the Earth’s core. Large‐scale synthetic single harmonic flows are characterized by a fixed difference between harmonics participating in the transfer. Large‐scale toroidal flows result in more local energy transfer than small‐scale poloidal flows. Axisymmetric poloidal flows are most efficient in producing energy transfer and dipole changes. The azimuthal phase relation between the field and the flow may play a major role in the energy transfer. Geomagnetic energy transfer induced by core flow models exhibit a striking transfer spectrum pattern of alternating extrema suggestive of energy cascade, but the detailed transfer spectrum matrix reveals rich behaviour with both local Kolmogorov‐like transfer and non‐local transfer, the latter about twice larger. The transfer spectrum reverses from even maxima and odd minima between 1840 and 1910 to odd maxima and even minima between 1955 and 1990. The transfer spectrum matrix shows geomagnetic energy cascade from low to high degrees as well as non‐local transfer from the dipole directly to higher degrees, explaining the simultaneous dipole decrease and non‐dipole increase during the historical period.

The convective state of the top of Earth's outer core is still under debate. Conflicting evidence from seismology and geomagnetism provides arguments for and against a thick stably stratified layer below the core-mantle boundary. Mineral physics and cooling scenarios of the core favor a stratified layer. However, a non-zero secular variation of the total geomagnetic energy on the core-mantle boundary is evidence for the presence of radial motions extending to the top of the core. We compare the secular variation of the total geomagnetic energy with the secular variation of the geomagnetic dipole intensity and tilt. We demonstrate that both the level of cancellations of the sources and sinks of the dipole intensity secular variation, as well as the level of cancellations of the sources and sinks of the dipole tilt secular variation, are either larger than or comparable to the level of cancellations of the sources and sinks of the total geomagnetic energy secular variation on the core-mantle boundary, indicating that the latter is numerically significant hence upwelling/downwelling reach the top of the core. Radial motions below the core-mantle boundary are either evidence for no stratified layer or to its penetration by various dynamical mechanisms, most notably lateral heterogeneity of core-mantle boundary heat flux.