# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 3 Results  
### Bubbles and Depletions  
Using the data available from the year 2017 until present, after applying the processing method described above, several interesting results found are shown. The research is mainly focused on the detection of Equatorial Plasma Bubbles (EPBs) or depletions. Given the characteristics of this experiment, these bubbles are expected to take the shape of transient peaks in \(S_{4}\) curves as the reflected signal path crosses the bubble, while the GNSS receiver is moving along its orbit. A first visual inspection of some days, like the one depicted before, shows that there are many peaks no matter which day is analyzed. Some of them are isolated, but others occur along with other peaks in slightly different times and other satellites or channels but in a relatively small region. One of these peaks in \(S_{4}\) is located in mid the Atlantic during 21 November 2017, shown in Figure 7, and it may serve as an example to study its origin. This isolated event in the Atlantic is the consequence of the rapid fluctuation of the \(SNR\) of the signal arriving to channel 1 of CYGNSS satellite 2, at around 4:26 UTC. The approximate length of the peak is 1400 km, and it lasts for about 5 minutes. As it can be seen in the \(SNR\) plot (Figure 8a), channel 1 oscillates rapidly from 1 dB to 7 dB, starting at 2:13 h to almost 2:18 h Local Time (LT), according to the longitude of the region. During this event, the ocean is calm, with wave heights of at most 2 m, as seen in Figure 6.  
Figure 6: \(S_{4}\) values above 0.05 over Atlantic ocean for 21 November 2017. It can be see that most of the \(S_{4}\) points appear only when the ocean is relatively calm. Also, these plots help to explain that, given the way the \(S_{4}\) is computed, only the \(SNR\) fluctuations occurring in a timescale of 12 s can appear as high/moderate \(S_{4}\) values. In channel 3 of the same satellite, a slow drift of the \(SNR\) is observed at the same time as channel 1 is fluctuating quickly (Figure 7(a)). Despite this, the \(S_{4}\) value in channel 3 keeps being very small, less than 0.05 (Figure 7(b)). On the other hand, the singular parabolic shape that it exhibits is due to the change in the incident angle of the reflected signal as the receiver satellite moves in the orbit capturing the signal of the much slower and higher GPS transmitter. In this case, the angle of incidence is almost vertical (\(\sim\)4\({}^{\circ}\)), when the signal is maximum (\(\sim\)7 dB), and increases up to around 25\({}^{\circ}\) at the end of the plot. In the case of channel 1, the angle of incidence varies less than 1\({}^{\circ}\) around 70\({}^{\circ}\) during all the fluctuation period. The previous example was an isolated peak in just one of the 4 channels of one satellite, but in other days analyzed, there are concentrations of these events in a small region, occurring during a finite time period. Figure 9 shows data from 24 August 2017. The points in red mark moderate scintillation measurements around 0.2-0.3, and they appear in some concentrated regions for different channels and satellites crossing above those regions. During this day, almost all the North Atlantic was very calm, with wave heights less than 2 m, as seen in Figure 10.  
Figure 8: \(SNR\) fluctuations and \(S_{4}\) value for 21 November 2017 peak vs LT (Local Time) corresponding to previous Figure 7.  
Figure 7: \(S_{4}\) peak in middle Atlantic. To have a better view of these events, two detail plots are shown. In the first one in the Western region, (Figure 11), located about 300 km North of Bermuda island, a large number of moderate \(S_{4}\) points are closely spaced in a zone that extends approximately 250 km \(\times\) 320 km across different CYGNSS satellites and channels. All the measurements occur during the period between 6 h and 10 h UTC, even lasting during different passes of CYGNSS satellites over this zone.