Enhanced Ground Magnetic Response Near the South American Magnetic Anomaly During the 10 May 2024 Sudden Commencement

In this study, we analyze 1-min resolution ground-based magnetometer observations associated with the Sudden Storm Commencement (SSC) preceding the 10 May 2024 geomagnetic superstorm. The analysis focuses on São Martinho da Serra (SMS), located within the South American Magnetic Anomaly (SAMA), and its geomagnetically conjugate counterpart, San Juan (SJG), together with four additional conjugate station pairs distributed across different longitudinal sectors. Four event-based parameters were used to characterize complementary aspects of the geomagnetic response: the SSC amplitude (ASSC), the rapid magnetic-field variation (VSSC), the Pc5 pulsation amplitude (Pc5SSC), and the accumulated perturbation (PASSC). The latter quantifies the cumulative absolute magnetic variation throughout the analysis interval. For each parameter, a dimensionless South-to-North response index was subsequently defined by comparing the values measured at the Southern Hemisphere station with those at its Northern Hemisphere counterpart, yielding RA, RV, RPc5, and RPA. The SMS/SJG pair exhibited the strongest interhemispheric asymmetry across the four response metrics, with consistently larger values at SMS relative to SJG. The other conjugate pairs showed smaller South-to-North differences. Agreement among the four complementary metrics indicates that the enhanced response at SMS was reflected in SSC amplitude, rapid magnetic-field variations, Pc5band fluctuations, and cumulative magnetic perturbation. These results provide quantitative ground-based evidence of a pronounced interhemispheric asymmetry in the SAMA sector during the initial impact of the 10 May 2024 event and support an association between the SAMA region and the enhanced geomagnetic response observed during the SSC. Plain Language Summary The South American Magnetic Anomaly is a region where Earth’s magnetic field is unusually weak. This study investigates whether this anomaly affects the magnetic disturbances measured at the ground during a sudden compression of the magnetosphere on 10 May 2024. We compared measurements from São Martinho da Serra, located within the anomaly, with those from a magnetically related station in the Northern Hemisphere and with several other station pairs around the world. The response at São Martinho da Serra was consistently stronger across different measures of the magnetic disturbance, including the initial field change, rapid variations, Pc5 fluctuations, and the accumulated magnetic variability. These results indicate that the South American Magnetic Anomaly may enhance the ground magnetic response to sudden magnetospheric disturbances.