What causes geomagnetic reversals?

Geomagnetic reversal is known as one of the most dramatic changes on Earth in that the north and south poles of the geomagnetic field switch their positions every 0.1–1 million years. Since Bernard Brunhes and Motonori Matuyama identified the geomagnetic reversal in the early 20th century, remarkable progress has been made on understanding the causes that drive the geomagnetic flip-flops through paleomagnetic investigations, space/ground-based observations and theoretical/numerical/laboratory experimental studies. However, based on our limited knowledge of the Earth’s deep interior, we are still unable to answer the title question clearly and definitely, but we do have found some clues that could help revealing the underlying mystery. The geomagnetic field is thought to be generated by the fluid convection in the Earth’s outer liquid core through magnetohydrodynamic process which is called the geodynamo. This hypothesis was initially proposed by Joseph Larmor in 1919 and now is widely accepted. In 1995, Gray A Glatzmaier and Paul H Roberts accomplished the first numerical self-consistent geodynamo model which maintained an Earth-like magnetic field and produced spontaneous magnetic reversals without special priori setup on fluid motion, magnetic field or boundary conditions. Later, the spontaneous geomagnetic reversals are repeated in many numerical simulations and laboratory experiments by other research groups, but the spontaneity of reversals remains sceptical because the conditions of simulations are far from those of the real Earth. Further studies have shown that heat flux variations across the core-mantle boundary induced by mantle convection or superplumes could control the frequency of geomagnetic reversals through thermal core-mantle coupling. The preference of virtual geomagnetic pole (VGP) paths during reversals for longitudinal bands associated with the subduction zones surrounding the Pacific Ocean supports the hypothesis that geomagnetic reversals are controlled by the thermal structure of the lower mantle. Moreover, the magnetic diffusion effects of the lower mantle can alter the structure of geomagnetic field and change the geodynamo process via magnetoelectric core-mantle coupling. Some studies suggested that the plate tectonics, the distribution asymmetry of the continental plates, and the growth rate of the inner core can also influence geomagnetic reversals. Even the super-rotation of the inner core or the Earth’s precession can be a candidate of driving the fluid motion in the outer core instead of thermal or compositional buoyancy force in the classical geodynamo theory and, consequently, affects geomagnetic reversals. This manuscript describes the temporal and spatial features of the geomagnetic field as well as the geodynamo theory, reviews the recent progress in the study of geomagnetic reversals. It points out that the answer to what causes geomagnetic reversals depends on our knowledge of the entire Earth and a comprehensive model that treats the Earth as a whole dynamic system with all relevant spheres taken into account.