Magnetic and electric field variations during geomagnetically active days over Turkey

Currents in the magnetosphere flow into the ionosphere during geomagnetic disturbances and are detected at the ground magnetic stations as Geomagnetically Induced Currents (GICs). In this paper, magnetic and electric field characteristics of the GICs at midlatitudes were studied using electric field and magnetic field observations in Turkey during the geomagnetically active intervals. A magnetotelluric station consisting of an electrometer and a magnetometer were set up in Bozcaada, canakkale (37.5 degrees N, 106 degrees E). Several cases that showed large electric and magnetic field fluctuations during geomagnetic disturbances were selected and the effects of geomagnetic activity were studied using the time derivatives of horizontal component of the magnetic field and the deviations in the magnetic and electric field components from the quiet background. In magnetic field data, quiet day Sq variations were removed using cubic spline fits. Similarly, the magnitude of the deviations in the electric field were determined by subtracting the background electric field determined by using cubic spline. Corresponding to the strong geomagnetic activity identified using Kp and Dst indices, high frequency, strong fluctuations in the magnetic field, its derivatives, and electric field were observed. These fluctuations in horizontal magnetic and electric field were compared with those seen during a magnetically quiet day. The close association between the fluctuations of the time derivatives of the horizontal magnetic field and electric field components were demonstrated. Two types of variations in the electric and magnetic fields corresponding to the different phases of the geomagnetic activity were identified: those corresponding to the initial phase including the sudden commencement and those to the main phase of the geomagnetic storm. The fluctuations in both magnetic field and electric field corresponding to the sudden commencement and the initial phase indicate the effects of magnetopause currents driven by the large solar wind dynamic pressure as associated with the coronal mass ejection (CME) events. Each event shows increased levels of ground level magnetic and electric field fluctuations corresponding to the CME compression at the subsolar magnetopause. High frequency, large fluctuations continue subsequently during the main phase in the presence of the geomagnetic storms. The fluctuations during the main phase were found to be different than those corresponding to the sudden commencements. GIC occurrences in our latitudes were shown to be associated with the sudden commencement and main phase of the geomagnetic storm activity. The time rate of change in horizontal component of the magnetic field showed perturbations on the order of 0.5 nT/s in our region. The sources of the GICs based on these observations were discussed. This study presents our preliminary results on the characteristics of the GICs over Turkey based on the simultaneous measurements of electric and magnetic field during the geomagnetic storms. It is the first study of the GICs in Eurasia region and the results contribute to the worldwide understanding and modelling efforts on the GICs. (C) 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.