Local time effects in satellite estimates of electromagnetic induction transfer functions

The current satellite magnetic missions offer new opportunities to determine the electrical conductivity of the Earth. However, satellites are nearly stationary in local time and therefore sample the inducing and induced fields quite differently than geomagnetic observatories, which rotate with the Earth. We show that estimates of induction transfer functions obtained from CHAMP magnetic data under the traditional symmetric magnetospheric ring current source (Y-1(0)) assumption depend systematically on local time, suggesting that source fields contain also a coherent non-axisymmetric component. An extended magnetospheric source model that incorporates a coherent non-axisymmetric quadrupole (Y-2(1)), and allows for Earth rotation qualitatively explains the observations.