# Effects of the October 2024 Storm over the Global Ionosphere
## 4 Results
### Spatio-Temporal Variations in the Ionospheric Characteristics
Moreover, the previously noted data gap migrated westward over time--from the Afro-European region to the Atlantic sector, and eventually to the Western Pacific by 18:00 UT on 11 October. These results underscore several key features of the ionospheric response during the storm. A persistent uplift of the F-region was observed across both Hemispheres during the main and early recovery phases, particularly on the dayside. Meanwhile, foF2 consistently decreased on the nightside, especially in the Southern Hemisphere, indicating enhanced recombination processes and plasma loss. The absence of foF2 and hnr2 measurements during nighttime hours across specific longitudes is particularly notable. From 15:00 to 03:00 UT, these data gaps were observed over the Afro-European sector, later shifting toward the Atlantic and Pacific regions during the recovery phase, before dissipating after 18:00 UT on 11 October. A rational justification for the absence of foF2 and hnr2 values is the occurrence of negative ionospheric storm effects, where significant plasma depletion causes ionization densities to drop below the level required for signal reflection. Such conditions are common during the main and early recovery phases of intense geomagnetic storms, as storm-induced changes in thermospheric composition lead to increased concentrations of molecular species like N\({}_{2}\) and O at F-region altitudes. This enhances recombination processes, resulting in reduced electron densities. Similar global ionospheric responses were also observed during the Mother's Day 2024 storm, as reported by [PERSON] et al. (2025) [5]. However, the decrease in hnr2 observed during the recovery phase of the October storm, particularly over the Southern Hemisphere's nighttime sector, appears to be a distinctive feature compared to the May 2024 event. The markings \"A-C\" in Figure 4 are spatially limited primarily due to the observational constraints of the COSMIC--2 mission. The data consist of discrete COSMIC--2 profiles that are sparsely distributed along satellite ground tracks and aggregated over a 3 h interval. Consequently, only areas with sufficient profile density and clearly observed apparent hnr2 reductions could be highlighted. While this representation may not capture the full spatial extent or finer-scale variability of the phenomenon, it still provides a meaningful approximation of localized storm-time effects in the F-region ionosphere. A plausible explanation for these apparent hnr2 drops is the development of G-conditions, wherein the foF2 drops below the F1 layer (foF1) as a result of pronounced F-region uplift combined with a reduction in Ne. A G-condition in the ionospheric F-region occurs when the critical frequency of the F2 layer becomes equal to or less than that of the F1 layer, i.e., foF2 \(\leq\) foF1, where foF2 and foF1 represent the critical frequencies of the F2 and F1 layers, respectively. Studies [6,7] have shown that G-conditions typically arise due to changes in thermospheric parameters--such as density, composition, temperature, and wind velocity--which primarily reduce foF2, while foF1 remains largely unaffected. These reductions in foF2 are characteristic of the negative phase of an ionospheric storm, and the link between this phase and thermospheric disturbances is widely recognized. This mechanism is frequently associated with intense geomagnetic disturbances and is supported here by the clear evidence of hnr2 elevation shown in Figure 4a-h. In the case of COSMIC--2 observations, the F2 peak may rise above the maximum detection altitude of the COSMIC--2 RO, making it inaccessible. As a result, the observed hmF2 values under such conditions do not represent the true F2 peak height but instead reflect the height of the underlying F1 layer, which the retrieval algorithm misidentifies as hmF2. This misinterpretation leads to an apparent reduction in the peak height. G-conditions were also observed during the September 2017 geomagnetic storm over European midlatitudes [7]. Similarly, [PERSON] et al.