# Effects of the 12 May 2021 Geomagnetic Storm on Georeferencing Precision
## 1 Introduction
The storms are usually classified as minor (G1) with a Kp = 5; moderate (G2) with a Kp = 6; strong (G3) with a Kp = 7; severe (G4) with Kp = 8, and extreme (G5) with Kp = 9 ([[https://www.swpc.noaa.gov/noaa-scales-explanation](https://www.swpc.noaa.gov/noaa-scales-explanation)]([https://www.swpc.noaa.gov/noaa-scales-explanation](https://www.swpc.noaa.gov/noaa-scales-explanation)), accessed on 3 December 2021) [10; 11]. The Ap-index provides a daily average level for geomagnetic activity. The Kp-value converts a linear scale called the a-index. The average from eight daily a-values gives us the Ap-index of a certain day. Thus, high levels of geomagnetic activity have a higher daily Ap-value ([[https://www.swpc.noaa.gov/noaa-scales-explanation](https://www.swpc.noaa.gov/noaa-scales-explanation)]([https://www.swpc.noaa.gov/noaa-scales-explanation](https://www.swpc.noaa.gov/noaa-scales-explanation)), accessed on 3 December 2021) [11]. The AE-index was originally introduced by [PERSON] and [PERSON] [7] as a measure of global electrojet activity in the auroral zone. The AE-index is derived from measurements of the horizontal components (H-components) of the Earth's magnetic field obtained from a series of observatories along the auroral zone in the northern hemisphere [7; 8; 12]. The AE-index is estimated as AE=AU-AL, where AU and AL indexes are obtained from the upper and lower envelope of the superposed H-components [7; 12]  
The geomagnetic storms can also be identified by various other parameters such as the symmetric disturbance of the magnetic field H (SYM-H); the interplanetary electric field (IEF); and the interplanetary magnetic field (IMF), where IMF-Bz is the most important parameter for the study of geomagnetic storms, as the energy input into the magnetosphere, depends on Bz orientation and its magnitude [13]. Although these indexes provide extra information regarding the space conditions, some of them are related to the Dst-index. For instance, a relationship has been shown between Dst and Bz [14]. The response of the ionosphere to geomagnetic induced disturbances is known as ionospheric storm [15; 16]. Ionosphere plasma density is mainly determined by the chemistry/composition, transport due to electric field, transport due to neutral wind, and transport due to ambipolar diffusion. During geomagnetic storms, the variations of chemistry or the thermospheric composition, and the interaction with the neutrals (neutral wind) [17; 18], and/or variations of electric field and ambipolar diffusion [19] are the final cause that alters the ionosphere plasma density. Nevertheless, the response of the ionosphere during a geomagnetic storm is complex and difficult to predict accurately, and the physical nature of many underlying mechanisms needs a better understanding to obtain precise forecasting of its behavior based on geomagnetic storms parameters [20]. In addition, the effects of these physical processes on the ionosphere have also been reported to vary with solar activity, storm intensity, storm duration, season, location, local time, and altitude of the observing station, which increases the forecast uncertainties [6; 21; 22; 23; 24; 25; 26; 27]. For instance, [PERSON] et al. [28] and [PERSON] et al. [29] have found that the response of the ionosphere to geomagnetic activity depends on the season of the year in which that portion of the ionosphere is located [30; 31]. The ionospheric plasma density can be estimated by estimating the ionospheric electron density. Thus, it is the determining variable for investigation of the spatial and temporal variations in the ionosphere. The total electron content (TEC), which can be estimated from double-frequency GNSS receiver data, is used to study the ionospheric response during ionospheric storms [1; 3; 9; 20; 32; 33]. The TEC is defined as the integral of the electron density from the ground height up to the ceiling height, i.e., the height of the transmitting satellite or infinity.