## 1 Introduction
For several decades, the GNSS ROTI has effectively been used for the monitoring and research of ionospheric irregularity occurrence at high [31; 32; 33; 34; 35; 36; 37] and equatorial [38; 39; 40; 41; 42] latitudes. The application of the ROTI approach with different mapping techniques allows us to trace plasma irregularity dynamics at different temporal resolutions, on both global and regional scales [43; 44; 45]. It is important to acknowledge that ground-based GNSS measurements have a notable limitation--the challenge in identifying the altitude range where exactly ionospheric irregularities occur. GNSS Radio Occultation (RO) is a satellite remote sensing technique that uses GNSS signal measurements received by low-Earth-orbiting (LEO) satellites to profile the Earth's ionosphere with a high vertical resolution and global coverage. LEO RO satellites typically operate at orbit altitudes between 300 and 700 km, thus providing a unique opportunity to estimate the vertical distribution of ionospheric plasma below LEO altitude at various locations around the globe. By placing a GPS receiver onboard a LEO satellite in 1995, the GPS/MET Experiment was a pioneer in demonstrating the applicability of the GPS-based RO technique to retrieve a vertical distribution of electron density in the Earth's ionosphere [46; 47]. Since that time, many satellite missions were launched into LEO orbits featuring GPS/GNSS RO receivers onboard--Oerstedt, SAC-C, CHAMP, GRACE, COSMIC, COSMIC-2, Spire, CASSIOPE, MetOp, TerraSAR, etc. [48; 49; 50; 51; 52]. Nowadays, the ionospheric profiles derived from LEO-based RO observations have become a critical data source for real-time monitoring and research of the Earth's ionosphere [53; 54; 55]. For ionospheric irregularity investigation, analysis of the LEO-based RO profiles allows for identifying altitude distributions of the steep ionospheric plasma gradients and to detect the presence of the plasma irregularities in the vertical domain. This technique was used for the climatological study of low-latitude ionospheric irregularities [56; 57; 58], as well as for case-studies of ionospheric responses to the geomagnetic storms [59; 60]. Recently, [PERSON] at al. [61] and [PERSON] et al. [62] presented interesting results of the ionosphere and plasmasphere RO soundings using the geometry of the links between geostationary Earth orbit (GEO) and GPS satellites when a GPS receiver is placedonboard a GEO satellite. In those papers, we discussed a novel retrieval of the ionospheric EDPs not from LEO but from the GEO, specifically from the Geostationary Operational Environmental Satellites (GOES) mission, which operate at \(\sim\)35,800 km altitude. They have demonstrated the applicability of GEO RO for retrieving the vertical distribution of the electron density in the Earth's ionosphere and evaluated the performance of the retrieved electron density profiles by direct comparisons with ground-based ionosondes and LEO RO products as the references. It was found that the GEO-GPS RO sounding geometry is quite repeatable from day to day due to the slow precession of the GPS satellites' orbit. This allows for assessing the ionospheric conditions over a specific location for many days in a row, which is rather similar to ground-based instruments, like ionosondes or incoherent scatter radars. Inspired by these results obtained for geomagnetically quiet conditions, our new investigation focused on GEO-GPS RO sounding results during geomagnetic storms. The aim was to examine the possibilities of identifying the signatures of ionospheric disturbances and ionospheric plasma density gradients and irregularities with this new type of observation.