Decadal period external magnetic field variations determined via eigenanalysis

We perform a reanalysis of hourly mean magnetic data from ground-based observatories spanning 1997-2009 inclusive, in order to isolate (after removal of core and crustal field estimates) the spatiotemporal morphology of the external fields important to mantle induction, on (long) periods of months to a full solar cycle. Our analysis focuses on geomagnetically quiet days, and mid- to low-latitudes. We use the climatological eigenanalysis technique called Empirical Orthogonal Functions (EOFs), which allows us to identify discrete spatiotemporal patterns with no a priori specification of their geometry – the form of the decomposition is controlled by the data. We apply a spherical harmonic analysis (SHA) to the EOF outputs in a joint inversion for internal and external coefficients. The results justify our assumption that the EOF procedure responds primarily to the long-period external inducing field contributions. Though we cannot determine uniquely the contributory source regions of these inducing fields, we find that they have distinct temporal characteristics which enable some inference of sources. An identified annual-period pattern appears to stem from a north-south seasonal motion of the background mean external field distribution. Separate patterns of semi-annual and solar-cycle-length periods appear to stem from the amplitude modulations of spatially fixed background fields.