Statistical analysis of geomagnetically induced current data measured around Tokyo, Japan

Abstract We started measuring geomagnetically induced current (GIC) in substations around Tokyo, Japan in 2017 to study the GIC effects on power systems. We defined a period with GIC continuously exceeding 3 A as a GIC event to analyze the long-term data. This threshold is sufficiently above the noise level of the measurement data to clearly distinguish the events. Approximately 90% of the GIC events selected using this threshold have duration of less than twenty minutes. The GIC events between February 2018 and December 2024 were analyzed for the GIC data at SFS substation because GIC amplitude observed at SFS was the largest among the four substations where GIC measurements have been conducted. Occurrence of the GIC events with large amplitude and long duration increased according to increase of solar activity towards solar maximum of cycle 25. Most of the GIC events occurred during significant variations in the geomagnetic field associated with magnetic storms regardless of their main or recovery phase although the GIC event with the largest amplitude occurred associated with a sudden commencement (SC) at the start of initial phase of the storms, such as the storm on 10 October 2024. The relationship between the peak H-component geomagnetic variation (∆H) of the magnetic storm and the absolute value of a peak GIC (|A peak |) showed a good correlation with a correlation coefficient of 0.92. The GIC event with the longest duration were recorded in the main phase of the storm on 10 May 2024, which had a minimum provisional Dst of − 406 nT. We also found an example of a long-duration GIC event related to a SC caused by a high-density solar wind region, which was not accompanied by a magnetic storm. Graphical Abstract