## 1 Introduction  
Short-term ionospheric irregularities, especially those triggered by geomagnetic storms, can be monitored by numerous techniques and instruments, including global navigation satellite systems (GNSS) receivers, ground-based magnetometers, ionosondes, satellite instruments, etc. One of the most accurate and massive-data delivery methods uses the dual-frequency GNSS observations, which have been widely used to estimate regional and global total electron content (TEC) variability [1; 2; 3; 4; 5]. Ionospheric fluctuations and irregularities are mainly caused by the solar cycle and geomagnetic activity, which drive complex processes in the upper atmosphere. The meteorological activity also largely influences ionosphere disturbances [6]. The accurate understanding and modeling of these variations are extremely important for applications and for society, for example, for GNSS positioning, satellite communication infrastructures, and Earth observation through remote sensing techniques. There is a great deal of different models, but they still have much to be desired in the way of accuracy, especially during complex geomagnetic storms. A list of well-known models includes, e.g., the Klobuchar model [7], the SAMI model [8], the NTCM model [9], the NeQuick model [10; 11], or the International Reference Ionosphere (IRI) model [12]. However, the complex processes in the upper atmosphere during geomagnetic storms cause difficulties for accurate modeling; the existing models can only deliver monthly averages of the actual variability, especially during magnetically quiet times. During the last few decades, the understanding of ionosphere responses during geomagnetic storms has remained a complex task in the fields of solar-terrestrial interactions and magnetosphere-ionosphere-thermosphere coupling [13; 14; 15; 16]. Ionospheric behavior during geomagnetic storms is being regulated by several electrodynamic and chemical processes, and its accurate modeling for practical applications is still a major challenge. These processes are largely affected by the variable solar plasma wind via magnetospheric energy input during geomagnetic storms and are further electromagnetically coupled to the ionosphere. Major geomagnetic storms usually occur after extreme solar events, especially Earth-directed coronal mass ejections (CMEs) and co-rotating interaction regions (CIRs). CMEs can produce stronger storms than CIRs and are usually initiated by a southward interplanetary magnetic field (IMF), which enhances the night-side convection and increases the ring current. When CIRs and CMEs reach Earth, solar wind/magnetosphere coupling processes generate a rapid increase in Poynting flux; they result in an enhancement of Joule heating, which leads to disturbances in upper atmosphere composition, temperature, density, and winds (e.g., [17; 18; 19; 20]). The ionosphere itself is mainly created by processes of energy-absorption, ionization, and dissociation of molecules due to variable incoming extreme ultraviolet (EUV) and X radiation from the Sun, as well as high-latitude thermospheric Joule heating and precipitation of energetic particles due to solar wind and magnetospheric forcing, especially during geomagnetic storms. At the geomagnetic equator, the magnetic field is horizontal, and three factors mainly govern the equatorial ionosphere during magnetic quiet periods:  
1. The Equatorial Ionization Anomaly (EIA) [21; 22], where the E \(\times\) B drift moves plasma vertically as a result of the east/west electric field. At high altitudes, the pressure gradients and gravity become important and cause plasma to diffuse along the lines of Earth's magnetic field. As a result, two peaks of electron density at \(\pm 15^{\circ}\) from the geomagnetic equator and a trough of electron density at the geomagnetic equator are created. 2. Equatorial Electrojet (EEJ) [23] along the magnetic equator as a result of an ionospheric electric current at altitudes of about 110 km.