# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 3 Results
### Bubbles and Depletions
In this region, the perturbation is particularly interesting as the peaks occur roughly at the same positions for several passes or channels, with similar values of \(S_{4}\) in each of them. The event starts around 7:00 h UTC and ends around 10:00 h UTC, so given that the timezone is UTC-4, the local time is from 3 h to 6 h in the morning, coinciding with the hours immediately before the sunrise, that, in this place, on 24 August 2017, took place at 5:36 h LT. Focusing on this peak, the values of \(SNR\) and the derived \(S_{4}\) are plotted in Figure 12a,b versus LT. The sudden increase of \(SNR\) values is clearly visible in two of the channels of the satellite CYG06. In other satellites (not shown in these graphs), there are also similar events, that constitute the peak in \(S_{4}\) shown in Figure 11. It is important to note that the angle of the incident signal over the ocean is smoothly varying around \(28^{\circ}\) for channel 3, and from \(14^{\circ}\) to \(10^{\circ}\) in the case of channel 1, so the change in the reflection angle is not the cause of the rapid fluctuation of the signal. Figure 10: Filtered high \(S_{4}\) values overlaid with the wave height for 24 August 2017, showing the calm sea surface. Figure 9: \(S_{4}\) values over the Northern Atlantic for 24 August 2017, showing two regions of moderate scintillation. Figure 11: Peak in \(S_{4}\) value in Western part of North Atlantic on 24 August 2017. Figure 6:0 \(SNR\) and \(S_{4}\) values plotted against LT for western peak in Figure 11 during 24 August 2017, for the four channels of one of CYGNSS satellites. Note that the \(S_{4}\) rises with the transitions of SNR. For channel 3, it appears 2 peaks of \(S_{4}\), one before and one after the SNR peak, and the same happens for channel 1. In the \(S_{4}\) map for the Eastern peak in North Atlantic (Figure 13), a region of about 900 km \(\times\) 600 km of moderate scintillation is depicted. As in the previous case, different passes of CYGNSS satellites in several channels show values of \(S_{4}\) larger than 0.2. This region is around 200 km to 600 km southwest from the Azores islands, and the water surface was also in calm, as shown in previous Figure 10 for the whole day in almost all the North Atlantic ocean. The information on these and other \(S_{4}\) peaks is summarized in tables, recording for every event its date and time, duration, length, and magnitude of the \(S_{4}\) value. In the data analysis these peaks are identified and characterized using the mentioned parameters. An event to be recorded must meet that the peak of \(S_{4}\) must be larger than 0.2 during at least 5 s. Given that the integration window for computing \(S_{4}\) is 12 s width, it means that a reported peak implies at least 17 seconds of fluctuations in the \(SNR\) of the received signal. Events shorter than this duration (or dimension) cannot be detected with the current coherent and incoherent integration times of the GNSS-R payload onboard. Table 2 shows the recorded peaks for 24 August 2017, from 7 h to 10 h (UTC), in all the North Atlantic region. Table 2 shows the date and LT, the satellite and channel in which the peak was detected, the coordinates where the event started, its duration and length, the inclination of the incident signal on the sea (measured from the vertical), and the maximum value of \(S_{4}\) achieved during the peak. The length of the event is computed measuring the distance between the first and the last geolocated points belonging to the peak. Note that the highlighted row in Table 2 corresponds to the peak measured by satellite 6 in channel 3 on (37.27\({}^{\circ}\)N, 62.21\({}^{\circ}\)W) at 5:31 LT, which is the one plotted in yellow in Figure 12a,b. As it can be seen, the peak lasts for 7 seconds over the threshold of 0.2, archiving a maximum height of 0.31.