# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 4 Discussion
### Comparison with Plasma Density Data from Swarm  
ESA Swarm mission is a constellation of 3 satellites launched in 2013 that globally provides data on the geomagnetic field and plasma density. Satellites named as Swarm A and C fly together in a polar orbit of around 460 km altitude and 87.4\({}^{\circ}\) inclination, so they can sense the plasma density on the ionosphere. Swarm B is orbiting at 530 km, within another polar orbit plane, so it can sense a bit higher layer of the ionosphere. Swarm instruments can measure geomagnetic field intensity and direction, and also, local electron plasma density. In order to compare with previous results from CYGNSS, Swarm measurements during 24 August 2017 have been analyzed. In Figure 22, the plasma density during some hours of this day is plot along with an index that the authors have defined to indicate the intensity of its fluctuations, the Normalized N\({}_{\rm e}\) Fluctuation Index (NNeFI). This index is obtained from the plasma density given by Swarm (N\({}_{\rm e}\)), once per second. Ne measures the number of electrons per cubic centimeter with values that are in the order of 10\({}^{5}\) e\({}^{-}\)/cm\({}^{3}\), the typical electron density in the ionosphere at this height. Using N\({}_{\rm e}\), the NNeFI is computed as:  
Figure 21: Histogram of bubble length during 25 equally spaced days in 2017 representing the probability in bins of 5 km with a total number of samples of around 4700. \[NNeFI=\sqrt{\frac{\left\langle N_{e}^{2}\right\rangle-\left\langle N_{e}\right\rangle^ {2}}{\left\langle N_{e}\right\rangle^{2}}}, \tag{3}\]  
where Ne is the electron density measured by Swarm and the average is computed using a 12-samples moving window. This index is plotted in a map in Figure 23, for the whole day and the three Swarm satellites, and it is underlaid the mean occurrence of scintillation events, using the WBMOD model predictions of the \(90 th\) percentile \(S_{4}\) index. Note that only values of NNEFI above 0.05 are shown. It can be observed a correlation between the appearance of plasma density fluctuations and the predicted \(S_{4}\) map. In Figure 24 it is plotted the same data from Swarm NNEFI along with the peaks of \(S_{4}\) reported by CYGNSS during the same day that was studied in previous sections of this work. \(S_{4}\) values plotted are filtered to be higher than 0.12. Swarm computed NNEFI exhibits high occurrence in high latitudes, above \(60^{\circ}\), both Northern and Southern, but they are out of the region covered by CYGNSS and they will be not mentioned here. Figure 23: Normalized Ne Fluctuation Index for Swarm A, B and C during 24 August 2017 with the predicted mean occurrence of \(S_{4}\) scintillation using WBMOD model. Background image adapted from Reference [20] and overlaid with Swarm data. Figure 22: Plasma density (Ne) and Normalized Ne Fluctuation Index for Swarm A during some hours on day 24 August 2017. Note the values of NNEFI when there is fluctuations in the plasma density. Around meridian 0\({}^{\circ}\) there is a large region of fluctuations observed by Swarm A and C, over Sahara and Ghana, which, in the part that enters to the Atlantic ocean, there is also some \(S_{4}\) peaks detectable by CYGNSS. Another example is found in the line of \(S_{4}\) peaks that goes from The Guianas to Cabo Verde that seems to be completed with some peaks in the NNeFI index from Swarm B. There is also some cases along the Pacific Ocean that are marked with circles, where there is a cluster of \(S_{4}\) peaks at the same time as NNeFI peaks. There are also other regions in which there is mostly \(S_{4}\) peaks but few or none Swarm indicators, for example in the South of Mexico, in the region between Bermudas and Cuba or in the Middle South Atlantic  
It is important to note that the availability of data is much larger in the CYGNSS constellation than the Swarm one. For one day, CYGNSS has 8 satellites with 4 channels each in a orbit with less inclination; Swarm is only three satellites in polar orbit.