## 3 Geomagnetic Activity and Ionospheric Weather
### Geomagnetic Indices and Solar Activity  
Any possibleoseismic signal in the ionospheric plasma density is expected to manifest itself as a deviation from the background level typical for a particular location and time. Among the two geographic coordinates, the most important is the latitude. It is believed that the globally quiet geomagnetic conditions, as monitored by the planetary geomagnetic indices, are a primary criterion for excluding the possible external effect (i.e., associated with space weather) on the ionospheric variability. The EQ under consideration occurred during a prolonged period of geomagnetic quietness. Figure 2 depicts the storm-time ring current Dst and the midlatitude \(\mathrm{a_{p}}\) indices for the 3-month period from November 2023 to January 2024 obtained from the NASA OMNI data portal [44]. The background level of Dst and \(\mathrm{a_{p}}\) is zero. It is commonly accepted that, when Dst \(>-50\) nT, there is no magnetic storm. The \(\mathrm{a_{p}}\) index can be graded according to the US National Oceanic and Atmospheric Administration (NOAA) criteria for the planetary Kp index, namely Kp \(<2\) (\(\mathrm{a_{p}}<20\) nT), which is the geomagnetic quietness. Only in early November did a weak magnetic storm occur, during which the ring current Dst index dropped below \(-80\) nT. Magnetic activity continued to remain somewhat elevated until the end of December, and after that, the Dst index did not fall below \(-20\) nT, indicating the absence of geomagnetic storms. The planetary index \(\mathrm{a_{p}}\) increased in accordance with the evolution of Dst. During the first half of the period, \(\mathrm{a_{p}}\) sometimes reached \(40\) nT, while in January it did not exceed \(10\) nT. In Figure 3, the solar and space weather parameters are described for a shorter period centered on the day of EQ. It appears that, despite the geomagnetic quietness, the Sun was not quiet. Solar emissions could affect mainly the density of the ionospheric plasma, while the electrical currents flowing in the magnetosphere-ionosphere circuit (these currents are responsible for the ground magnetic variations) remained relatively undisturbed. The first panel (Figure 3a) depicts solar flare activity. This is the GOES-R X-Ray Sensor (XRS) Level 2 data, which are based on soft X-ray irradiance measurements covering 0.1-0.8 nm integrated passbands [45]. According to the NOAA classification in terms of peak emission, the X-ray flux level of \(\geq\)10\({}^{-8}\), 10\({}^{-7}\), 10\({}^{-6}\), 10\({}^{-5}\), and 10\({}^{-4}\) W/m\({}^{2}\) corresponds to A, B, C, M, and X flare, respectively. Figure 3a shows that on 21 January the flare activity was relatively weak (taking into account the solar maximum phase). On 22 January, the X-ray flux reached 3 \(\times\) 10\({}^{-5}\) (15 UT) and 5 \(\times\) 10\({}^{-5}\) W/m\({}^{2}\) (23 UT), implying that the M-class flares occurred. The next day, a pair of peaks of similar amplitude was observed. Typically, the increase in X-rays is accompanied by more intense EUV emissions. The increased level of X-ray and EUV radiation results in ionization in the lower layers of the ionosphere, especially on the dayside, which leads to an increase in TEC. Figure 2: Dst and \(\mathrm{a_{p}}\) indices for the 3-month period from November 2023 to January 2024. The EQ day on 22 January is marked by vertical red line. The second panel (Figure 3b) shows the solar flux of a proton with energy \(\geq\)10 MeV. Data were taken from the SpaceWeatherLive.com portal [46], then digitized, and finally averaged over an hour. Protons started to increase on 22 January. The particle fluxes reduce the ionization altitude and increase the ionization intensity and electron density in the ionosphere. Peak altitudes of the ionization rates for solar protons, depending on their energy, are about 40 km but also affect the ionospheric layers [47]. However, compared with high-intensity solar flares, the 22 January event was rather minor.