Abstract
Watari et al. (Space Weather, 2009, 7) found that the geomagnetically induced current (GIC) in Hokkaido, Japan (35.7° geomagnetic latitude (GML)), is well correlated with the y-component magnetic field (By) (correlation coefficients >0.8) and poorly correlated with Bx,z and dBx,y,z/dt. The linear correlation with By would help predict the GIC, if we have capabilities of reproducing the magnetosphere–ionosphere currents during space weather disturbances. To validate the linear correlation with By for any periods (T) of disturbances, we made correlation analyses for the geomagnetic sudden commencements and pulsations (T = 1–10 min), quasi-periodic DP2 fluctuations (30 min), substorm positive bays (60 min), geomagnetic storms (1–20 h), and quiet-time diurnal variations (8 h). The linear correlation is found to be valid for short periods (cc > 0.8 for T < 1 h) but not for long periods (cc < 0.3 for T > 6 h). To reproduce the GIC with any periods, we constructed one-layer model with uniform conductor and calculated the electric field (IEF) induced by By using the convolution of dBy/dt and the step response of the conductor. The IEF is found to be correlated with the GIC for long periods (cc > 0.9), while the GIC-By correlation remains better for short periods. To improve the model, we constructed a two-layer model with highly conductive upper and less conductive lower layers. The IEF is shown to reproduce the GIC with cc > 0.9 for periods ranging from 1 min to 24 h. The model is applied to the GIC measured at lower latitudes in Japan (25.3° GML) with strong By dependence. The mechanism of the strong By dependence of the GIC remains an issue, but a possible mechanism for the daytime GIC is due to the zeroth-order transverse magnetic (TM0) mode in the Earth-ionosphere waveguide, by which the ionospheric currents are transmitted from the polar to equatorial ionosphere.
Highlights
Geomagnetic disturbances have been known to induce electric fields on the surface of the Earth, which create a potential difference between transformers in the power transmission line system
As shown we found that the geomagnetically induced currents (GICs)-By correlation depends on the period of disturbances such that cc > 0.8 for short periods (6 h)
1) We have shown that the GIC at Memambetsu in Hokkaido (35.7° GML) is linearly correlated with the y-component geomagnetic field, By, for the short-period disturbances such as the geomagnetic sudden commencements and Pi2 pulsations, while the correlation was found to become worse as the period of disturbances increases, such that cc 0.67 for the substorm and cc 0.27 for the solar quiet diurnal variations
Summary
1. The geomagnetically induced currents in Hokkaido, Japan (35.7° GML), are correlated with By (cc > 0.8) for short periods (T < 1 h), while the correlation is poor (cc < 0.3) for long periods (T > 6 h). 2. The GICs with periods of 1 min to 24 h are well correlated with the electric field, Ex, induced by By in the semi-infinite onelayer conductivity model (cc > 0.85) and two-layer model (cc > 0.95) composed of highly conductive upper layer. 3. The strong By dependence of the GIC is observed at lower latitude in Japan (25.3° GML) with cc > 0.85. 4. The By dependence of the midlatitude GIC may be associated with the ionosphere-ground currents transmitted by the TM0 mode waves in the earth-ionosphere waveguide from high latitude to the equator
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