Latitudinal characteristics of storm-time geomagnetic diurnal variations and Pc5 amplitudes in Japan: effects of subsurface conductivity

Geomagnetic storms exert substantial influence on ground-based geomagnetic measurements. This study examines geomagnetic responses during 21 strong to extreme storms spanning 2001–2024, utilizing data obtained via meridionally aligned stations operated by the Geospatial Information Authority of Japan (GSI) and the Japan Meteorological Agency (JMA). Variations in the horizontal north-south (X) and vertical (Z) components, along with Pc5 pulsation amplitudes, were examined to identify storm-related signatures and regional characteristics. The X-component shows coherent and strongly correlated diurnal variations across all stations, whereas the Z component exhibits a pronounced anti-correlation between northern and central-southern Japan. Pacific-facing stations display larger Z-component diurnal ranges than inland and Sea of Japan side stations. Moreover, Pc5 amplitudes in the X-component generally decrease toward lower latitudes, with localized enhancements in northern Honshu, while Z-component amplitudes are elevated at northern and Pacific side stations. Sites with amplified Z-component Pc5 signatures often show reduced X-component amplitudes, except in northern Honshu, where both components strengthen simultaneously. These findings indicate that geomagnetic variations in Japan are governed by the interplay between external storm-time disturbances and local subsurface conductivity, highlighting the need to incorporate site-specific geophysical conditions in interpreting and modeling ground-level geomagnetic responses.