# Monitoring of Ionospheric Anomalies Using GNSS Observations to Detect Earthquake Precursors
## 1 Introduction
Spatial gradients and temporal variations computed starting from calibrated STEC measurements are translated in form of indices such as the Disturbance Ionosphere indeX (DIX), the Gradient Ionosphere indeX (GIXx, GIXy, and GIX), and the Sudden Ionosphere Disturbance indeX (SIDX) to obtain a clear and easily readable description of the level of perturbation of the ionosphere. The conciseness of these indices provides immediate and accessible information to GNSS users [24; 25]. The Rate Of TEC (ROT) and Rate of TEC Index (ROT and ROTI), as time series or maps for monitoring the ionosphere during severe conditions, together with the scintillation index can be used to detect the size of irregularities between on a scale of hundreds of meters to kilometers [13; 26; 27; 28]. Among the local sources of ionospheric perturbations, the most interesting and studied are certainly earthquakes. Ionospheric anomalies over earthquake preparation zones have been detected for many decades [29; 30; 31]. Anomalies consist of an increase or reduction in TEC around the epicenter starting from a few days to weeks before large earthquakes [32; 33; 34; 35; 36]. The main characteristics of an anomaly are of temporal and spatial nature and are closely related to the typical features of the precursor phenomena. The temporal parameters that characterize the anomaly are (i) the moment in which it occurs (with reference to the period in which the earthquake occurs) and (ii) its duration. In general, the manifestation occurs in the 15 days preceding the event, with a duration that can vary from a few hours to 12 hours. Spatially, the anomaly is characterized by (i) the fact that its geographical position must overlap with the earthquake area; (ii) its altitude, which is aligned with that of the F-layer (F-region, although traces may also be present at other altitudes); and (iii) its extent, the order of magnitude of which is comparable with the radius of the earthquake preparation zone. Many earth-atmosphere and Earth-ionosphere coupling mechanisms have been proposed in the literature to explain the nature of these sources (critical argumentation can be found in [PERSON] and [PERSON] [36]): acoustic coupling mechanisms, Acoustic Gravity Waves, seismogenic electric fields, and electromagnetic coupling of the ground and ionosphere driven by electric currents and fields or by currents generated by charged aerosols. The pre-earthquake anomalies produce effects on the ionospheric potential (as well as the above-cited volcanic ash emissions, huge desert storms, nuclear accidents, and tests): the mechanism acts on the conductivity of the atmosphere, inducing variations in the ionospheric potential that can be detected using different observation techniques such as (i) vertical ionospheric sounding via ground-based ionosondes [32; 37], (ii) vertical topside sounding from satellites [38], (iii) TEC monitoring via GNSS observations [19; 39; 40; 33], (iv) low-orbit ionospheric tomography [41; 42], and (v) the GNSS occultation technique [43]. Among these observations, those collected from GNSS receivers are widely used: TEC changes can be studied analyzing Global Ionosphere Maps (GIMs) produced by different centers such as CODE, ESA, and JPL [14] or computing them directly starting from the data collected by permanent GNSS stations [33; 44]. The peculiarities of ionospheric TEC variations induced by an imminent seismic event are generally as follows:  
1. Temporal variability occurs days before the main shock, usually in particular hours of day, with a certain duration;
2. Spatially speaking, the extent covers an area around the epicenter (with a symmetric manifestation in the opposite hemisphere due to the magnetically conjugated effect);
3. The seismic event induces an altitude distribution effect on the concentration of electrons.