## 2 Materials and Methods  
### Ionospheric Scintillation Measurement Using GNSS-R Data  
In this study, ionospheric amplitude scintillation (\(S_{4}\)) data are inferred using GNSS-R products from the NASA Cyclone Global Navigation Satellite System (CYGNSS) mission, following the approach presented in [9]. This technique computes the scintillation in the signal's intensity using the following expression:  
\[S_{4}=\sqrt{\frac{\left\langle l^{2}\right\rangle-\left\langle l\right\rangle^{2 }}{\left\langle l\right\rangle^{2}}}, \tag{1}\]  
in which \(I\) is the signal's intensity or, in our case, the magnitude of the Signal-to-Noise Ratio (SNR) of the Delay-Doppler Map (DDM) of the reflected signal over oceans in linear units, and its average (\(\left\langle l\right\rangle\)) is calculated over a 10 s moving window. This way, global \(S_{4}\) maps can be elaborated on calm oceanic regions between 40\({}^{\circ}\)S and 40\({}^{\circ}\)N latitudes.  
The signals emitted by the GNSS transmitters cross the ionosphere in the down-welling path, then coherently reflect on calm water surfaces, and reach the GNSS-R receiver, in this case, a CYGNSS satellite, which compares the signals with the direct ones. As the signals follow different paths, the reflected ones are more influenced by the ionosphere than the direct ones, therefore the ionospheric scintillation can be detected in the resulting DDM intensity.  
The points where the scintillation values are geolocated correspond to the specular reflection point over oceans, which are provided in the CYGNSS L1 dataset. This can be seen as a problem because the actual scintillation is mainly induced at the altitude of the ionosphere, approximately 350 km, near the electron density peak. However, as the reflected signal crosses this layer two times, the middle point between these two pierce points nearly matches the specular reflection point. Furthermore, in later steps, a 100 km radius is used to aggregate data when searching for earthquakes around anomalies in the \(S_{4}\) value, which helps to smooth this effect along with the uncertainty in the distance where earthquakes disturb the ionosphere.  
The GNSS-R constellation used, CYGNSS, was originally designed for wind speed and cyclone monitoring. It consists of eight micro-satellites orbiting at around 520 km altitude, with an orbital inclination of 35\({}^{\circ}\). Each satellite can track up to four GNSS signals reflected on the ground [27], allowing an effective coverage between latitudes \(-40^{\circ}\) to 40\({}^{\circ}\). The constellation was launched in December 2016, and the science data are available from March 2017, with a cadence of 1 DDM per channel per second, resulting in around \(2.76\cdot 10^{6}\) samples per day for the whole constellation. The DDM data rate was set to 2 Hz in July 2019.  
After quality filtering, the total number of samples per day per pixel of \(1^{\circ}\times 1^{\circ}\) varies from 100 in equatorial regions to 200 in \(\pm 30\) to 35\({}^{\circ}\) North and South, as shown in Figure 1.  
Figure 1: Map showing the average number of DDM samples per \(1^{\circ}\times 1^{\circ}\) pixel per day, after filtering the land cover. The average is computed during the whole month of March 2019. On the left plot the computed average number of DDMs per latitude is presented.  
CYGNSS L1 data are filtered out when the \"poor_overall_quality\" flag is true, which is the union of several flags described in the CYGNSS manual [27]. The most important flags refer to the land cover filter, which removes all the land points, and the ones that are less than 25 km from the coast. Furthermore, they remove the data when the satellite had suffer from attitude perturbations, or the DDM obtained has low confidence or a large noise figure, or when Radio-Frequency Interference (RFI) has been detected in the measurements.