Characteristics of geomagnetic solar flare effects from the 2024 X-class flares and the influence of local current systems

Solar flares can cause fluctuations in the Earth's magnetic field, known as the solar flare effect (SFE). However, their impact on the geomagnetic field is highly complex and exhibits regional dependence, and its characteristics have not been established. Many X-class flares occurred consecutively in May and October 2024, resulting in numerous observed SFEs. This study investigated the characteristics of SFE by conducting a detailed analysis of geomagnetic data from a global network of magnetometers, including high-cadence observation stations such as Kakioka and Chichijima, for five major X-class flares that occurred during this period. The SFE signal was extracted using a 1-h moving-average-based difference method to reduce uncertainties associated with traditional quiet-day curve subtraction. Our analysis revealed a positive correlation between the SFE magnitude and solar elevation angle for each event; however, the magnitude of SFE is primarily determined by the GOES X-ray class (solar flare magnitude). The time evolution of the SFE showed typical response, with the SFE peak lagging flare X-ray peak by approximately 3 min, reflecting the ionospheric response time to flare emission. We found anomalous response during the X9.0 class flare occurred on 3rd Oct, 2024: an enhanced SFE at the high-latitude station Lerwick (LER), attributed to the superposition of the flare effect on the active auroral current system. This fact strongly suggests that a different physical mechanism is responsible for generating the local westward electric field. These results demonstrate that the SFE response is governed not only by the flare magnitude and solar elevation angle but also by the local ionospheric current system.