Comparative study of models of earth’s magnetic field derived from Oersted, CHAMP and SAC-C magnetic satellite data

Abstract

The magnetic field near the earth contains contribution from three major sources, viz., main internal field that is due to electric currents in the outer core (97-99%), crustal field (1-2%), and external field (1-2%). The external field includes contribution due to ring currents, magnetotail, magnetopause currents and also subsurface currents induced by them. Over the past decade or so, there has been an attempt at “comprehensive modeling” that seamlessly integrates data collected over different epochs and different platforms to generate an integrated magnetic field model. It is found that the estimate of the contribution from ionospheric currents using satellite observations is sensitive to the Earth’s magnetic field models, and hence it is essential to compare various magnetic field models. In the present work, we compare CHAOS model, the most recent long term model of Earth’s magnetic field that uses Oersted, CHAMP and SAC-C satellite data with earlier epoch based models such as Oersted Initial Field Model, (OIFM) and CO2 models. CO2 model utilizes magnetic measurements from all three satellites as well as ground observatory data, whereas OIFM uses single satellite observations. While both CHAOS and CO2 models expand the static (core and crustal) field up to high order spherical harmonic (n = 50 and 49 respectively), OIFM has expansion only upto degree n = 13. The present study systematically separates and discusses the contribution from the various sources. The match between the internal field obtained from OIFM and CHAOS is found to be good in the longitudinal belt between 150°E and 250°E, indicating that in this longitude zone, the contribution due to the long wavelength crustal field is minimum. It is also observed that the difference between the internal field of OIFM and CHAOS is maximum along Indian and American sectors. Present work estimates the magnetic field variations due to the ring current, induced current, and magnetotail current, as well. It is found that the ring current contribution using OIFM is stronger compared to other two models. The external field due to ring current is discovered to be largest and that of due to the tail current is weakest. The effect of the tail current on the surface of the globe is found to be almost same everywhere, due to its far location. It is evident that in general, the ring current contributions are about five times stronger than that of due to the induced currents.