## 2 Materials and Methods  
The results obtained in this study are based on the GPS observations from two GOES satellites, GOES-R (known also as GOES-16) and GOES-S (known also as GOES-17). The GOES mission operates at \(\sim\)35,800 km altitude and its observations serve for real-time weather forecasting. The GPS receivers onboard these GOES satellites were designed for navigation purposes but can track GPS signals propagated through the atmosphere and ionosphere of the Earth. Within the framework of the NASA Exploring GNSS Remote Sensing at GEO Using GOES GPS Receivers project, we obtained navigation and tracking datasets from the GOES-16 and GOES-17 GPS receivers. The GPS signals, tracked by the receiver, are coming from the GPS transmitter located on the opposite side of the Earth. Figure 1 shows the sketch view on how the GPS receivers onboard the GOES mission observe GPS satellites during a GEO-based RO experiment, which is quite different from the traditional LEO-based RO. For a LEO RO experiment, a LEO satellite is typically orbiting within the ionosphere (300-800 km in altitude) and GPS satellites are at \(\sim\)20,200 km orbit altitude. GPS and LEO satellites form a configuration such that the GPS satellite is considered almost stationary during an occultation session and the LEO satellite is moving quickly, thus forming multiple slices of the Earth's ionosphere with those GPS-LEO links. In the case of the GEO-based RO experiment, the GEO satellite is stationary and the GPS satellite is moving much faster, creating the occultation links. It is important to note that the signal transmissions from the GPS satellites are intentionally directed toward the Earth's surface for the advantage of terrestrial users (not away from the Earth or toward the geostationary orbit slots). Therefore, the only signals available to the receivers mounted on the GOES satellites are those transmitted by GPS satellites from the relative opposite side of the Earth or acquired from the side or back lobes of the GPS antennas. Thus, in the GOES-GPS RO experiment, the distance between satellites along the ray path is significantly larger than in any LEO RO. When these GPS signals propagate along this \(\sim\)60,000 km distance link, they pass through the Earth's ionosphere where they experience phase delays. These signal phase delays can be processed into estimates of the total electron content along the observation path and also can retrieve electron density profiles by the approach developed by [PERSON] et al. (2018). The general algorithm for the retrieval of electron density profiles using measurements from the GOES GPS receivers was described in [62]. The dataset we analyzed consists of ionospheric observations from July 2017 to May 2022 for GOES-16 and from November 2018 to May 2022 for GOES-17. Because the GPS receivers onboard GOES satellites were not designed and configured for RO, the number of successful RO retrievals is quite limited. Finally, we have retrieved about 13,000 GEO RO ionospheric soundings covering an altitudinal range of 80-2000 km [62]. The time period from July 2017 to May 2022 marked a transition between the 24 th to 25 th solar cycle, characterized by a low-to-moderate solar activity level. Two major geomagnetic storms took place during the specified period, one on 7-8 September 2017 (with a minimum SYM-H of around -145 nT) and another on 26 August 2018 (with a minimum SYM-H of -206 nT). The severe ionospheric responses, including the development of strong ionospheric plasma density gradients and ionospheric irregularities, were reported for both cases of these geomagnetic storms [64; 65; 66]. Our analysis focused on the GOES GEO RO ionospheric soundings dataset to identify the ionospheric plasma irregularities resulting from the geomagnetic storms on 7-8 September 2017 and 25-26 August 2018. For these two major geomagnetic storms, only data from GOES-16 were available as GOES-17 was not in operational mode yet; so, this study will utilize GOES-16 data only.