Magnetic Reversals
Superchron cycles driven by variable core heat flow (Driscoll & Olson, 2011a)
The seafloor magnetic record reveals that the geodynamo has undergone hundreds of polarity reversals over the last 180 million years (Myr). Moreover there is a trend going back in time from frequently reversing behavior (5 reversals per Myr) over the last few Myr to non-reversing behavior during the Cretaceous Normal Superchron (80-120 Ma) when the field did not reverse over a 40 Myr span. Driscoll & Olson (2011a) presented evolving numerical dynamo models that display a similar behavior by slowly changing the core cooling rate.
(Below) The top line is the Geomagnetic Polarity Time Series (GPTS) obtained from the seafloor magnetic record. White (black) denotes reversed (normal) polarity. The Cretaceous Normal Superchron is from 84-120 Ma where there are no recorded reversals. The second line is the reversal record from a single continuous 200 Myr long numerical dynamo model with time variable core cooling rate. The model begins and ends in two 20 Myr long superchrons, so the record has been cut and reordered to align with the GPTS. To produce these trends in reversal frequency the core heat flow must be high during periods of frequent reversals, and low during superchrons. Such a 200 Myr oscillation in the core cooling rate may be driven by fluctuations in the mode of mantle convection (e.g. supercontinent breakup).
Magnetic reversals in numerical dynamos (Olson, Driscoll, & Amit, 2009)
Left: magnetic field contours in a numerical dynamo during a reversing and weakly dipolar state. Positive (negative) magnetic field lines are yellow (blue). Right: magnetic field contours in the same model during a quiescent and strongly dipolar state. Reversals in these models are convective events where magnetic field is mixed, temporarily disrupting the axial dipole, before recovering with a strong axial dipole in the reversed magnetic orientation.
Influence of rotation and buoyancy on reversal frequency in numerical dynamos (Driscoll & Olson, 2009b)
The influence of buoyancy is described by the Rayleigh number Ra (vertical axis), which is the ratio of buoyancy to diffusion. The influence of rotation is described by the Ekman number E (horizontal axis), which is the ratio of viscous to Coriolis forces. The regime boundary between non-reversing, and reversing numerical dynamos is linear in log space, meaning that an increase in E (associated with the slowing rotation of the Earth) and a decrease in Ra (associated with decreasing core cooling rate) may lead to dynamically similar behavior. The geodynamo is known to have been in a predominantly dipolar and occasionally reversing state for hundreds of millions of years. The question remains why the geodynamo would traverse this regime boundary over such a long period of time.
Polarity reversals in dynamo models with core evolution (Driscoll & Olson, 2009a)
Time series of dipole tilt during 4 numerical dynamos. Panel (d) shows a model with increasing Rayleigh number over time, demonstrating the increase in reversal frequency with core cooling rate.
