Statistical analysis to investigate the characteristics of Sq and SFE current using the Kakioka magnetometer

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Abstract The solar flare effect (SFE) is a transient geomagnetic variation caused by a sudden increase in solar electromagnetic radiation. Although soft X-rays have traditionally been considered the primary driver, the contribution of extreme ultraviolet (EUV) radiation and its temporal evolution remain unresolved. In this study, we statistically analyzed SFE events associated with 21 X-class solar flares from 2011 to 2014 using GOES soft X-ray and SDO/EVE EUV data combined with high-time-resolution geomagnetic data from the Kakioka Magnetic Observatory. By removing the magnetospheric disturbances using the Akima-interpolated SYM-H index, we accurately extracted pure SFE components. The geomagnetic $$\:Y$$ component exhibited the strongest dependence on both the Sq amplitude and flare class. Furthermore, hodogram analyses of specific events (e.g., SOL2011-09-07 and SOL2013-05-14) demonstrated that the SFE trajectories deviated significantly from the circular Sq paths. The turning points in these hodograms were synchronized with the reversal of the normalized soft X-ray and He II (EUV) fluxes. We propose that this trajectory shift reflects a transition between Hall (soft X-ray-driven, ~ 105 km) and Pedersen (EUV-driven, ~ 130 km) conductivity dominance, highlighting the $$\:Y$$ -component as a superior and robust indicator of direct ionospheric responses to solar flares.

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