An examination of dipole moment variations for 0-2 Ma

Large numbers of Relative Paleomagnetic field Intensity (RPI) and Absolute Paleointensity (API) data have been used to study variations in Earth's magnetic field strength on million year timescales. We examine the behavior of the geomagnetic field and its time derivative in the PADM2M global model of Paleomagnetic Axial Dipole Moment variations for the past 2 Myr, and in a suite of increasingly smoothed time-varying models that are effectively low-pass filtered versions of the original. PADM2M is constructed using a maximum likelihood (ML) estimation technique that efficiently uses almost 5000 individual API data and 89 sedimentary RPI records of variable lengths and sedimentation rates to produce a calibrated, continuous, time-varying model of PADM. Time variations in PADM2M are parametrized by cubic B-splines. The regularized model robustly resolves features at timescales longer than 10 kyr and shows the expected paleointensity lows at field reversals and excursions. Asymmetries in dipole moment growth and decay rates have been previously noted around reversals in individual records and in the SINT2000 VADM model. The spline temporal parameterization in PADM2M combined with our modeling strategy readily allows derivative calculations at various temporal resolutions, so that we can quantify these effects. For versions in which periods shorter than 40kyr are heavily damped there is a clear asymmetry in the statistical distributions for growth versus decay of the dipole field: average growth rate is about 20% larger than the decay rate, and peak growth values are approximately twice as large. These differences are not limited to times when the field is reversing, suggesting that the asymmetry may reflect fundamental physical processes underlying the paleosecular variation. The longer decay cycle might suggest episodic periods of subcritical dynamo activity, followed by transient episodes of strong convection as envisaged in Backus's (1958) early demonstration of a successful kinematic dynamo.