Coast effect of geoelectric field: a case study based on St. Patrick's Day geomagnetic storm

Coast effect usually occurs when the conductance difference between sea and land is prominent, and the spatial geomagnetic disturbance lead to charge accumulation at the sea-land interface, with the abnormal increase in the geomagnetic vertical component from tens of seconds to several hours between different observatories. To explore the coast effect of geoelectric field, we propose an integral method for induced charges due to magnetic storms, and estimate the surface current vector intensity and surface charges caused by geomagnetic pulses at different time-scales based on the geoelectric field observations during the St. Patrick' s Day storm. The result shows that the surface current vector of stations near the sea-land boundary points inland, and its influence area is over two times larger than previous results. It indicates that the sea-land interface is the main area of charge accumulation, and the positive charge accumulates on the higher resistivity side. In the North-South sea-land boundary of the bay region, the surface current and surface charge density in the East-West direction are greater than in the North-South direction; during typical pulse, the surface charge accumulation in the bay is significantly larger than that in the peninsula. This work quantifies the coast effect of surface currents and charges and the characteristics of charge accumulation in the bay, providing a scientific basis for magnetotelluric data correction and geomagnetic induced current estimation in the offshore area.