Electromagnetic induction modelling based on satellite magnetic vector data

Abstract

SUMMARY The first results on the modelling of magnetic signals induced by transient equatorial ring currents in the magnetosphere with a timescale of the order of days and recorded at satellite altitudes are presented. The input of modelling consists of the X -component of the time-series recorded by the CHAMP vector magnetometer along individual night-time, mid-latitude satellite tracks. We have not considered the magnetic signals measured along day-time tracks and above polar regions because they are disturbed by signals with sources different from magnetospheric ring currents. The modelling procedure is divided into two parts. First, the X -components of satellite magnetic signals are processed by a two-step least-squares analysis. As a result, the X -data are represented in terms of series of Legendre polynomial derivatives. Four examples of the two-step analysis of the signals recorded by the CHAMP vector magnetometer are presented. Second, forward modelling of the electromagnetic induction response of a 2-D heterogeneous conducting sphere to a transient external current excitation is carried out in the time domain using a recently developed spectral finite-element technique (Martinec et al. 2003), which has been modified to include satellite magnetic data. The modified approach has been verified against a semi-analytical solution for a 2-D geometry: a spherical inclusion eccentrically nested in a homogeneous sphere. The output of the forward modelling of electromagnetic induction, that is, the predicted Z-component at satellite altitude, can then be compared with the satellite observations to obtain data to be minimized when adjusting electrical conductivity models in future inversion modelling. To outline such a procedure, we use a 5-layer spherically symmetric model of electrical conductivity and compare the predicted and observed Z-data for the magnetic storm recorded by the CHAMP satellite between 2001 September 25 and 2001 October 7. The agreement between observations and predictions is quantitatively satisfactory. The main advantage of the present method is its ability to use the satellite magnetic data directly, without the need to continue them from the satellite altitude to the ground or to decompose them into the inducing and induced parts by spherical harmonic analysis.