Rotating parametric instability in early Earth

摘要

The existence of Earth's magnetic field both before and during the growth of the solid inner core is consistent with a geodynamo driven by a rotating parametric instability (RPI) in the fluid outer core. Here we confirm by laboratory experiment the existence of an RPI in both a rotating fluid sphere and spherical shell through excitation by a travelling perturbation. An RPI is generated in each case by elliptically deforming the flexible, spherical boundary containing the fluid. This externally imposed strain, corresponding to Earth's Luni-solar strain of the semi-diurnal tide, couples low order rotational modes of the fluid to produce an RPI. The RPI continues to grow and decay as long as the boundary is perturbed. Our current laboratory work confirms that the RPI that appears to exist with the present inner core could have existed when the core was much smaller, or even when there may have been no inner core present. Thus an RPI could have driven the geodynamo throughout Earth's history.