# Effects of the October 2024 Storm over the Global Ionosphere
## 3 Solar and Geomagnetic Activity  
The October 2024 geomagnetic storm was triggered by two highly active solar regions identified by NOAA as AR3848 and AR3842. These regions generated a series of powerful solar flares, including two classified as X-class--the most intense category, known for their strong X-ray emissions and frequent association with fast and wide CMEs. On 9 October, AR3848 and AR3842 collectively produced two M-class, three C-class, and two X-class flares (Figure 1a-c). The first of these, an X1.8 flare from AR3848, began at 01:25 UT, peaked around 01:56 UT, and ended by 02:43 UT. Later that day, an X1.4 flare from AR3842 erupted, starting at 15:44 UT, peaking at 15:47 UT, and fading by 15:53 UT ([[https://www.spaceweatherlive.com/en/archive/2024/10/09/xray.html](https://www.spaceweatherlive.com/en/archive/2024/10/09/xray.html)]([https://www.spaceweatherlive.com/en/archive/2024/10/09/xray.html](https://www.spaceweatherlive.com/en/archive/2024/10/09/xray.html)) accessed on 23 April 2025). The first CME associated with the X1.8 flare was observed by SOHO/LASCO to be ejected from AR3848 at 02:12 UT on 9 October ([[https://kauai.ccm.gsfc.nasa.gov/CMEScoreboard/prediction/detail/3670](https://kauai.ccm.gsfc.nasa.gov/CMEScoreboard/prediction/detail/3670)]([https://kauai.ccm.gsfc.nasa.gov/CMEScoreboard/prediction/detail/3670](https://kauai.ccm.gsfc.nasa.gov/CMEScoreboard/prediction/detail/3670)) accessed on 23 April 2025), and it reached Earth by 15:00 UT on 10 October, travelling at an average speed of \(\sim\)1200 km/s. Multiple eruptions throughout 9 October led to successive CMEs, which merged into a complex interplanetary structure, producing a strong interplanetary (IP) shock detected near Earth around 15:00 UT. The arrival of this IP shock at Earth's magnetosphere triggered a Sudden Storm Commencement (SSC) at 15:16 UT on 10 October, as shown in Figure 2a-g as an orange line. At the time of SSC, the 1 min SYM--H index (equivalent to the hourly Dst index) spiked from 11 nT to 78 nT (Figure 2f), a signature of intensified magnetopause current. Immediately afterwards, the SYM--H index began to fall sharply, indicating the onset of the storm's main phase. This phase continued until the index reached a minimum of \(-\)346 nT at 01:46 UT on 11 October, as depicted by the grey line in Figure 2a-g. A gradual recovery followed, with the SYM--H returning to near pre-storm levels by 00:26 UT on 12 October as shown by a brown line. A notable intermediate recovery point occurred around 19:38 UT on 11 October, when the index rose above \(-\)100 nT (pink line). Between SSC and 23:14 UT on 10 October, the SYM--H index exhibited notable oscillations, linked to fluctuations in the interplanetary magnetic field Bz component (IMF--Bz). Peaks in SYM--H corresponded to northward (positive) IMF--Bz, indicating a more closed magnetospheric configuration, while decreases aligned with sustained southward (negative) IMF--Bz conditions, favourable for magnetic reconnection and energy input into the magnetosphere. Figure 1: Solar flare effects recorded on 9 October: (**a**) the locations of AR3848, (**b**) AR3842 (red circle) over the solar disc, and (**c**) solar flares recorded on 9 October. Figure 2: Geomagnetic conditions during the October storm event depicted in (**a**) total magnitude B of IMF, (**b**) Bz component of IMF, (**c**) solar wind speed, (**d**) solar wind pressure, (**e**) interplanetary electric field (IEF) Ey, (f) SYM-H and Kp index and (**g**) Polar Cap (PCN) Index. The orange vertical lines represent the SSC, the grey vertical line denotes the time of the SYM-H index minima during the storm, the purple vertical line represents when SYM–H ¡\(-\)100 nT, and the brown vertical line represents the end of the recovery phase of the ionosphere. To further assess geomagnetic activity, we examined the global Kp index (Figure 2f), which peaked at 6+ between 15:00 UT on 10 October to 14:00 UT on 11 October, confirming the severity of the event.