Asymmetries in Geomagnetic Field Structure and Variability

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The main source of the geomagnetic field is a self-sustaining dynamo produced by fluid motions in Earth's liquid outer core. This magnetic field provides protection from the solar wind, helps retain volatile parts of the atmosphere, and protects Earth's surface from the worst impacts of space weather. Direct observations of the geomagnetic field back to 1590 AD show significant geographic variability in its time variations. The southern hemisphere is more active than the north, and since the mid nineteenth century strong variability has been particularly focused around the region known as the South Atlantic Magnetic Anomaly prompting questions about its longevity, relation to long term geomagnetic field strength, and consequences for space weather and radiation exposure for low-earth orbiting satellites. This proposal will study spatial and temporal changes in the internal magnetic field by mapping the time-varying geomagnetic field over the past 100 thousand years. This will be accomplished using globally distributed paleomagnetic records drawn from high accumulation rate sediments and from volcanic rocks. The proposed work is of importance in a broad range of geophysical disciplines, addressing fundamental assumptions used in paleomagnetic work, providing results of specific relevance to geodynamo modeling, and to general seismological, geochemical, and mineral physics studies of the core-mantle boundary regions and deep interior. The PI has a track record of training and mentoring junior scientists, and plans to continue significant service in the area of developing community databases, as well as making available geomagnetic field models, animations, and related products. Existing field studies extending back 10 thousand years show greater geomagnetic variability in the southern hemisphere than in the north, and lower average field strength. The persistence of this asymmetry on longer time scales has not been confirmed, although there are hints that it occurs in lava flow records back to 2 Ma. Persistent asymmetry would have important consequences for the geocentric axial dipole hypothesis widely used in paleomagnetism and plate tectonic reconstructions. Current time-varying global field models extending beyond 10~ka are limited to axial dipole field variations or to short snippets surrounding the Laschamp excursion (at 40ka) and the most recent reversal (780 ka), and cannot be used to study potential long term spatial asymmetries. However, dipole variations show an interesting temporal asymmetry in growth and decay rates for the geomagnetic dipole at periods longer than 25 ky, suggesting the possibility of episodic periods of subcritical dynamo activity where the field is dominated by diffusive processes, followed by transient periods of strong growth of the axial dipole. A time-varying geomagnetic field model for the period 0-100 ka will be constructed. Data and modeling results will be used to (1) test whether hemispheric asymmetry in secular variation and the time-averaged field persists on this time scale; (2) conduct a detailed study of differences in growth versus decay rates for the axial dipole at better temporal resolution than currently available from PADM2M; (3) improve knowledge of the geomagnetic power spectrum at 1-50~ky periods.