# Effects of the 12 May 2021 Geomagnetic Storm on Georeferencing Precision
## 2 Materials and Methods
### Geophysical and Geomagnetic Conditions  
The geomagnetic data were downloaded from the World Data Center (WDC) for Geomagnetism, Kyoto ([[http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.htm](http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.htm)]([http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.htm](http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.htm)), accessed on 3 December 2021) [75]; OMNIWeb Plus Data Documentation ([[https://omniweb.gsfc.nasa.gov/form/dxl.html](https://omniweb.gsfc.nasa.gov/form/dxl.html)]([https://omniweb.gsfc.nasa.gov/form/dxl.html](https://omniweb.gsfc.nasa.gov/form/dxl.html)), accessed on 3 December 2021); and GFZ Helmholtz Centre Potsdam ([[https://www.gfz-potsdam.de/en/kp-index/](https://www.gfz-potsdam.de/en/kp-index/)]([https://www.gfz-potsdam.de/en/kp-index/](https://www.gfz-potsdam.de/en/kp-index/)), accessed on 3 December 2021) [76].  
On 12 May 2021 (DoY 132), a moderate geomagnetic storm (G3) took place with a Kp-index \(\geqslant\) 4 from 6 UT. The maximum Kp-index of 7 was sustained between 12 and 15 UT. The Kp-index went below 3 after the 18 UT.  
The geomagnetic storm started on the DoY 132. Its initial phase started at \(\sim\)6 UT and lasted \(\sim\)6 h. During this period, the Dst-index slowly decreased. The main phase of the storm started at \(\sim\)12.4 UT and lasted for \(\sim\)one hour. In the storm main phase, the Dst-peak of \(-\)61 nT was reached at 14 UT. At that time, the geomagnetic storm recovery phase began. The Dst-index went over \(-\)30 nT at \(\sim\)23 UT on the DoY 132 (see Figure 2b). We were unable to obtain the raw data of the AE-index, but it is possible to observe the behavior of this index in the graphical display of the data on the website of the WDC for Geomagnetism, Kyoto([[http://wdc.kugi.kyoto-u.ac.jp/ae_realtime/202105/index_20210512.html](http://wdc.kugi.kyoto-u.ac.jp/ae_realtime/202105/index_20210512.html)]([http://wdc.kugi.kyoto-u.ac.jp/ae_realtime/202105/index_20210512.html](http://wdc.kugi.kyoto-u.ac.jp/ae_realtime/202105/index_20210512.html)), accessed on 3 December 2021). In that image, it is possible to see an intensification of the AE-index with two peaks \(\sim\)1500 nT between 12 and 16 UT.  
The IMF Bz was below \(-\)10 nT between 12 and 15 UT on the day of the storm (DoY 132). The minimum Bz of \(-\)18.3 nT was reached at 13 UT. According to the Gonzalez and  
Figure 2: Panels (**a**) (Kp-index), (**b**) (Dst-index), and (**c**) (Z component of interplanetary magnetic field, IMF Bz) show the different indexes that characterize the geomagnetic conditions. Panels (**g-i**) present the different TEC variations related indexes (VTEC, DVTEC, %DVTEC, and ROTI). The panels (**d–f**) display the positioning error of each component in the vita station, Vegas de Itala, Chile (36.42\({}^{\circ}\)S, 72.86\({}^{\circ}\)W). The orange line represents \(\overline{VTEC}\) calculated with the reference days (DoYs 129, 130, and 131). (**g**) We represent the \(\overline{VTEC}\) in all the studied DoYs to compare it with the VTEC of each of the days (blue line).  
Tsurutani criteria [2], this event can be classified as a geomagnetic storm since it was caused by an interplanetary magnetic field \(\mathrm{Bz}\leq-10\) nT that lasted more than 3 h (see Figure 2c).  
The geomagnetic conditions were generally quiet between 9 and 11 of May 2021 (DoYs 129 and 131), where the Dst-index was predominantly positive, and the maximum Kp-index was 2 (see Figure 2a).