Contribution of Field-Aligned Currents to the Variations of Mid-Latitude Magnetic Field on the Ground: Dayside and Nightside are Compared.

Abstract:

Field-aligned currents (FACs) are believed to be the main contributors to the asymmetric variation of the magnetic field on the ground at mid-latitudes during geomagnetic storms. However, the contribution from the ionospheric currents can interfere with that from FACs on the dayside where ionospheric conductivity is higher and not concentrated along the auroral oval. We present the results of the comparison of the contribution from the large-scale FAC system with the observations at the mid-latitude observatories during 12 geomagnetic storms. The contribution from the FAC system is estimated using the 10 min resolution data of AMPERE system which provides 2D map of the FAC flowing in and out of the ionosphere reconstructed from the measurements onboard of ~70 Irridium satellites . To estimate the magnetic effect of FAC system, we performed Biot-Savart integration of these currents along IGRF field from the equator to the earth center. Although in reality the FACs close via ionosphere, we generally obtain a good quantitative agreement with observations at the ground observatories on the nighside. On the other hand the agreement is much worse on the dayside. The correlation coefficient between the D-component of the magnetic field measured on the ground and that computed using Biot-Savart integration varies between ~0.65 (21-03h MLT) and < 0.1 (12-18h MLT). In addition, we discuss the closure paths of the large scale FACs during storm periods and the sources of the north-south asymmetry of the ground magnetic field at mid-latitudes.