Equatorial Electrodynamic Disturbances During the May and October 2024 Extreme Geomagnetic Storms: Implications for HF Communication and Geomagnetically Induced Current Risks in Indonesia

Abstract

The effects of the two biggest storms of Solar Cycle 25, the intense geomagnetic storms of May 10–11 and October 10–11, 2024, on the Indonesian equatorial region are examined in this study. For the 500-kV Jawa-Madura-Bali (JAMALI) transmission network, geomagnetically induced current (GIC) modeling was integrated with magnetometer and ionosonde observations from Pontianak (PTN), close to the geomagnetic equator, and Kupang (KPG), close to the southern crest of the Equatorial Ionization Anomaly (EIA). During both storms, significant equatorial amplification was noted. While the October storm caused lower local perturbations, the horizontal magnetic disturbance at PTN during the May event reached -471 nT, around 16% bigger than the minimum global Dst index (-406 nT). In the JAMALI network, rapid changes in the geomagnetic field produced geoelectric fields that caused quasi-DC currents of 0.6–1.6 A. Such currents may contribute to long-term transformer stress even when they are below essential operational criteria. Significant ionospheric disruptions were caused by both storms, with foF2 reductions of more than 50% and matching drops in maximum usable frequency (MUF) of about 4 MHz, suggesting increased dangers to HF communication equipment. Ionospheric variability over Indonesia is regulated by geomagnetic latitude, local time, storm-time electrodynamic forcing, and thermospheric dynamics, as demonstrated by the differences between PTN and KPG responses.