Signature of the quiet‐time magnetospheric magnetic field and its electromagnetic induction in the rotating Earth

SUMMARY Accurate models of the magnetospheric field during magnetically quiet times are essential for high‐resolution mapping of core field dynamics, mantle and ocean induction, crustal fields and ionospheric currents. Satellite data sampled at low‐Earth orbit allow for a separate determination of the external contributions from currents in the magnetosphere. We have used Ørsted and CHAMP data from the years 1999–2004 to investigate this field component. In contrast to earlier studies, the field is decomposed here into contributions from sources in the solar‐magnetic (SM) frame and those in the geocentric‐solar‐magnetospheric (GSM) frame. For an observer on the Earth, stable fields in those frames generate different diurnal and annual variations which, in response, induce currents in the subsurface. All of these effects have been modelled here. Our primary findings are: in the GSM frame, there is a dominant constant magnetic field of about 13 nT, pointing due southward. This field component is attributed to the quiet‐time tail current system. The interplanetary magnetic field (IMF) contributes to the near‐Earth field with 10 per cent of its Bx and about 25 per cent of its By component. For the SM frame, we obtain a constant field of 7.6 nT and a variable part which can be parametrized by the D ST index. The field in SM is attributed to the combined effect of the magnetopause and ring current. A comparison of the external field variations, predicted by our satellite‐derived model, with the measurements of five latitudinally distributed ground observatories shows a remarkable agreement.