A Decadal-Scale Eigen-Analysis of terrestrial magnetic data

The Swarm satellite constellation, due for launch in 2013, will provide new measurements of the Earth’s magnetic field of unparalleled precision. Induction studies to estimate the distribution of mantle conductivity are a key goal of the scientific mission of the constellation, so it is important that the spatial geometry of the long-period inducing fields be properly resolved. The annual and semi-annual period fields originating from magnetospheric and ionospheric currents required to estimate mantle conductivity in the depth range 1,200 to 2,000 km are subject to large uncertainty since they overlap with the periods on which the core field also changes significantly. Currently, the spatial structure of the long-period external field is poorly resolved and is commonly assumed to be the P_1^0 solenoidal field term associated with the symmetric magnetospheric ring current. We use a dataset, developed for the Swarm mission, of ground-based magnetic observatory hourly means in combination with a method called Empirical Orthogonal Functions (EOFs) in order to decompose the external magnetic field over a full 11-year solar cycle. EOFs can be used to infer patterns of maximum variance in a dataset, allowing us to assess the spatial and magnitude changes of dominant spatio-temporal patterns in the external magnetic field. Specifically, our focus is on isolating the spatial pattern associated with the long-period external field oscillations. We find that the annual periodicity of the external magnetic fields is dominated by a P_2^0 term with additional spatial amplitude peaks at local noon, and between local dusk and midnight. The dominant pattern on shorter periods is a P_1^0 term. The temporal oscillations of both the P_2^0 and P_1^0 patterns exhibit a modulation with a period of the length of the solar cycle. The results of this study should be useful in fulfilling the planned mantle induction objectives of the Swarm mission.