Contrasting Weak and Intense Geomagnetic Storm Effects on the Low‐Latitude and Equatorial Ionosphere Over Brazil: The 15–16 April 2025 Events

Abstract The responses of the ionospheric F‐region, as measured by Global Navigation Satellite System and ionosonde, in the equatorial and low‐latitude regions over Brazil during two geomagnetic storms, one very weak (Dst min = −33 nT) and another intense (Dst min = −138 nT), occurred on 15–17 April 2025, are investigated. Furthermore, the nighttime OI 135.6 nm emissions measured by Global‐scale Observation of the Limb and Disk (GOLD) are used to visualize equatorial plasma bubbles (EPBs) to investigate their pattern change. Several prompt penetration electric fields (PPEFs) occurred during both very weak and intense storms. The westward PPEF during the intense storm suppressed the pre‐reversal enhancement (PRE), leading to the suppression of EPBs. The storms induced equatorward thermospheric winds and eastward PPEF, which caused unusual uplifts of the ionosphere. These effects resulted in strong positive ionospheric storms during both events. The very weak geomagnetic storm caused an atypical intensification of the fountain effect that enhanced the equatorial ionization anomaly. Negative ionospheric storms occurred over the Brazilian low‐latitude stations, probably related to changes in the thermospheric N 2 /O ratio. Therefore, these findings also highlight the significant role of thermospheric composition changes in driving negative ionospheric storms at low latitudes.