# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 5 Conclusions  
This work has presented the first experimental evidence that GNSS-R can be used as a global ionospheric scintillation monitor, and in particular, of bubble and depletions. Future work on the search of ionospheric activity at all latitudes and during different solar activity periods could help in the improvement of existing ionospheric models, providing them with more complementary data than ground stations are currently providing and with a denser spatial coverage than Swarm mission. The study shows the result of scintillation events during the years 2017 and 2019, including tables with peaks that correspond to EPBs that are consistent with previous studies in terms of duration, LT, and also latitudes. The duration of the events shown here is in the order of seconds (from 5 s to 30 s), which means bubbles of around 25 km to 150 km length. The depth of these bubbles in terms of maximum \(S_{4}\) is in the order of 0.2 to 0.5. This way to study the ionospheric scintillation could be very beneficial because it can cover large areas all over the Earth, including remote regions in the middle of oceans, where no ground stations could be placed. However, there is also has some limitations and they should be carefully identified  
Figure 24: NNeFI for Swarm A, B and C above 0.05 during day 24\(th\) August, along with the \(S_{4}\) peaks above 0.12 detected by CYGNSS during the same day. Note the regions marked with a yellow circle where it can be observed a spatial correlation between both sources of data. Note also, the color scale is shared for \(S_{4}\) and NNeFI. and refined. One of them is that only calm ocean surfaces ensure coherent specular reflections. If the water surface has ripples, the signal would reflect from a larger glistening zone, arriving at the receiver with different phases, hiding the possible fluctuations in amplitude and phase. In further studies continuing the analysis of CYGNSS and other GNSS-R missions, an automated data filtering algorithm must be implemented to eliminate data affected by sea roughness. On the other hand, it is important to take into account that the results presented in the study come from years 2017 to 2019, and solar activity cycle should reach its minimum in 2020. That implies that less activity is expected, and consequently, fewer scintillation events. It would be very interesting to study GNSS-R data from future missions when approaching the solar maximum. A correlation study between these results and others obtained from different sources, for example, the Swarm geomagnetic and plasma density products is being performed to find possible confirmations of this new proposed technique and also to check what new possibilities could GNSS-R technique bring to the ionospheric monitoring field. The results presented in this study are a small sample of the total amount of information that can be obtained using this technique. CYGNSS generates around 10.4 GB per day, and further exhaustive analyses of it could bring new information or improve current models of the ionosphere, which is especially interesting regarding oceanic regions. Conceptualization, C.M. and A.C.; methodology, C.M. and A.C.; software, C.M. ; validation, C.M. and A.C.; formal analysis, C.M. and A.C.; investigation, C.M. ; resources, A.C.; data curation, C.M. ; writing-original draft preparation, C.M. ; writing-review and editing, C.M. and A.C.; visualization, C.M. ; supervision, A.C.; project administration, A.C.; funding acquisition, A.C. All authors have read and agreed to the published version of the manuscript. This work has been partially funded by the Spanish MCIU and EU ERDF project (RTI2018-099008-B-C21/AEI/10.13039/501100011033) \"Pioneering Opportunistic Techniques\", grant to \"CommSensLab-UPC\" Excellence Research Unit Maria de Maeztu (MINECO grant MDM-2016-600), and ESA project AO 1-8862/17/NL/AF/eg \"RADIO CLIMATOLOGY MODELS OF THE IONOSPHERE: STATUS AND WAY FORWARD\". Thanks to the CommsSensLab administrative and research personnel and to the NanoSatLab members for all the support.