The global ionospheric response to the St. Patrick’s Day Storm during 15–20 March 2015 is investigated using a comprehensive set of ground-based and satellite observations. Ionospheric parameters (foF2, hmF2, and h′F) from ionosonde stations distributed across different longitude sectors are analyzed in conjunction with geomagnetic field variations (H-component), thermospheric composition (O/N₂), and geomagnetic indices (Dst and Kp) to examine the coupled magnetosphere–ionosphere–thermosphere system.</p> <p>During the main phase of the storm (17 March), a pronounced uplift of the F-region is observed at equatorial and low-latitude stations in both western and eastern longitude sectors, indicating the presence of enhanced eastward prompt penetration electric fields (PPEF). The associated increase in hmF2 and h′F, followed by enhancement in foF2, confirms the occurrence of a positive ionospheric storm driven by storm-time electrodynamics. The H-component exhibits a significant decrease across all stations, reflecting the intensification of the ring current, while enhanced fluctuations at higher latitudes indicate the contribution of auroral electrojets and field-aligned currents.</p> <p>The thermospheric response, characterized by variations in the O/N₂ ratio, shows depletion during the main phase due to storm-time upwelling, followed by a marked increase during the recovery phase (18–19 March), indicating a transition toward an oxygen-rich thermosphere. This compositional change contributes to reduced recombination rates and sustained enhancement in electron density. The evolution of the equatorial electric field further reveals a transition from PPEF-dominated dynamics during the main phase to disturbance dynamo electric field (DDEF) effects during the recovery phase.</p> <p>Overall, the results demonstrate that the ionospheric response to the storm is governed by a combination of prompt electrodynamic forcing and delayed thermospheric processes, with global coherence across longitude sectors and modulation by latitude and local time.