# Monitoring of Ionospheric Anomalies Using GNSS Observations to Detect Earthquake Precursors
## 4 Discussion  
In the cases studied, the proposed method allowed us to show possible anomalies correlated with geophysical events. Such a strategy is particularly interesting in ionosphere monitoring applications, considering that the results can be available in real-time. In the cases analyzed in this work, some possible tracks of ionospheric anomalies were detected in the maps of the DTEC, VTEC rate SNR, and DSNR of the VTEC rate over the earthquake zone. The anomalies have a duration of hours and started appearing between 7 and 10 days before the main shock. The radius of the preparation zone [36] in the earthquakes of the cases examined is approximately 2500 km (corresponding to approx. 20 degrees). The anomalies highlighted, in addition to being located positions corresponding to the earthquake zone, have an extent of tens of degrees, thus presenting characteristics compatible with those illustrated in Section 1. The Nepal earthquake occurred in an epoch (DOY 115) close to the relative maximum of solar activity, for which local anomalies are superimposed on those induced by the solar wind. As presented in Section 3, in the 24h VTEC series plots (Figure 3), there are DOYs in which potential anomalies are characterized by low values of VTEC throughout the day. However, this is a condition that can only be investigated using maps. It is possible to identify anomalies during DOY 111 by mapping the 15-day differential values of real-time VTEC (both the estimated ones and those from GIMs). By means of the VTEC rate SNR maps, anomalies seem detectable starting from DOY 106. The use of maps allows us to  
Figure 12: Maps of DTEC at 13 UT for DOYs 17 to 21: (**a**) from GIMs; (**b**) from the proposed model. Anomalies of DTEC are visible starting from DOY 18.  
take into account the space(-time) correlation of the studied parameters, emphasizing their local nature, that is, their own dependence on local geophysical events. This, together with real-time estimation, provides us a tool for a quick detection. The same is true for the use of the SNR: in case of a local anomaly, the idea is that a bounded area should be affected by peculiar values of this parameter. In the case of the Turkey earthquake (DOY 37), determining anomalies using a local model is slightly more difficult: the absence of permanent GNSS stations in the Africa region, especially in the central-northern area, together with the lack of stations in wide areas of Western Asia, makes it more difficult to estimate the VTEC and VTEC rate while avoiding border effects (no special constraints for the VTEC and VTEC rate are used at this level of the study). With the current GNSS CORS density and GNSS satellite constellations, the size of the anomaly we can detect is of the order of 1000 km. Whenever the density is insufficient or the distribution of the CORSs is not spatially homogeneous, the estimation can be affected by the above cited border effects, caused by the limited number of IPPs over these areas. This is particularly important in cases in which local models and real-time data processing are used for the investigation. Moreover, the earthquake occurred in 2023, a period in which the solar activity was increasing after the minimum reached in 2020. In Figure 8, we can see increasing values of 24h VTEC starting from DOY 10, with a minimum close to DOY 30, then the values finally increase again until the day of the earthquake. The solar activity may only partly explain this phenomenon. In this case, the maps of differential real-time VTEC show a single event, namely, a minimum DTEC on DOY 31 of a 2 h duration around 18 UT, whereas the GIM DTEC maps indicate a maximum during DOY 29 at 18 UT and an increasing value starting from DOY 35 over a wide area over the earthquake zone. A similar event is visible in the real-time DTEC on DOYs 36 and 37. The maps of the differential SNR of the VTEC rate in Figure 11 show a peculiar behavior over the earthquake area at 17 UT starting from DOY 32.