Abstract This study investigates the ionospheric response to the intense geomagnetic storm of 20–21 December 2015 over the equatorial and low‐latitude South American sector. Ionosonde data from São Luís (2.6°S, 44.2°W; dip angle: 3.9°S) and four additional Brazilian stations were analyzed to examine variations in the peak height of the F2 layer (hmF2) and vertical plasma drift velocities. A notable feature of this event was the occurrence of post‐midnight equatorial plasma bubbles (EPBs) during the recovery phase, an uncommon storm‐time response. In contrast, a suppression of post‐sunset EPBs was observed on 20 December, despite enhanced vertical drifts associated with the pre‐reversal enhancement. The post‐midnight EPBs are attributed to disturbance dynamo electric fields (DDEFs), which uplifted the F layer to higher altitudes and created favorable conditions for the Rayleigh‐Taylor instability (RTI) growth rate . On 20 December evening, the vertical drifts exceeded quiet‐time reference at equatorial stations, reflecting undershielding Prompt Penetration Electric Field activity. However, the absence of post‐sunset EPBs under seemingly favorable RTI conditions suggests the influence of additional suppressive mechanisms. The suppression of EPBs may be attributed to disturbance meridional winds driven by DDEFs, which generate interhemispheric asymmetries in the Equatorial Ionization Anomaly. These asymmetries can increase the integrated flux‐tube E‐region conductivity along magnetic field lines. Additionally, enhanced chemical recombination—associated with storm‐time variations in the ratio—may further reduce under geomagnetically disturbed conditions.
Vital et al. (Thu,) studied this question.