Leading modes of torsional oscillations within the Earth's core

Abstract

The fluid core motion cannot be observed directly, however, it can be deduced from magnetic observations. This motion can be approximated as a set of coaxial cylinders rotating along the Earth rotation axis. Our study shows that the actual motion in the core corresponds to a set of oscillations propagating from the core-mantle boundary to the inner core (with diminishing amplitude) at period of 85, 50, 35 and 28 years, respectively. These oscillations appear to be very robust in our study, moreover two weaker modes have also been isolated and require further study. Zatman and Bloxham (1997) isolated two modes with periods that correspond to our first two modes. Comparing the theoretically predicted modes of Mound and Buffett (2007 all three analyses (one theoretically based and two based on geomagnetic observations) agree on the periods of the isolated modes; this concurrence is exceptional and provides strong evidence of the existence of the modes. Copyright 2009 by the American Geophysical Union.