Using OpenGGCM to Compute and Separate Magnetosphere Magnetic Perturbations Measured on Board Low Earth Orbiting Satellites

We use Open Geospace General Circulation Model (OpenGGCM) simulations to predict magnetic field perturbations at Low Earth Orbiting (LEO) satellites such as Swarm, at high latitudes. The simulations allow us to separate three different major contributions to the observed perturbations, i.e., the perturbations caused by currents in the outer magnetosphere, field-aligned currents (FACs), and the currents flowing in the ionosphere. We find that at an altitude of 500 km the strongest contribution comes from FACs, followed by the perturbations caused by the ionospheric currents, while the magnetospheric currents make only a minor contribution. The high latitude perturbations do not average out over extended quiet time periods. There are significant variations in the patterns; however, on a large scale, the basic shape of the pattern remains stable. Thus, without explicitly removing the perturbations from the data, any spherical harmonics fit is expected to incur a bias. Although the predicted OpenGGCM perturbations do not compare particularly well with Swarm data, the simulations reproduce the overall pattern. However, they may still be useful to reduce the bias of the ensemble and produce better global spherical harmonic fits, by producing an ensemble whose external field contributions average out. Since this paper only scratches the surface of the role that models of the external field can play in producing unbiased internal field models, much progress is still possible, for example by improving the external model, investigating larger ensembles, and by considering data from geomagnetically disturbed times.