# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 4 Discussion
### Bubbles Study  
Obtained results match with previous studies and, in some cases, they can be clearly interpreted as the EPBs described, for example, in Reference [19]. In this work, a method for detecting and measure equatorial plasma depletions is explained. These depletions in the electron content of the ionosphere highly affect the radio wave signals crossing them, and they can rapidly appear, travel some distance, and then vanish. The way to study them in Reference [19] is by the use of data provided by ground-based GNSS receivers belonging to the International GNSS Service (IGS), available online since, at least, 2002, including different solar cycles.  
Figure 18: Histogram count of specular reflection points over all oceans between 40\({}^{\circ}\)N and 40\({}^{\circ}\)S for each \(S_{4}\) value and LT hour, from 0 h to 24 h (UTC) during selected days in 2017.  
GNSS satellites orbit at 20,000 km altitude (GPS), with periods of 12 h, which means that from a ground station they are in view during long times, making it possible to study the region of the ionosphere that crosses the vector from the satellite to the receiver. As this vector moves slowly compared to the estimated drift velocity of the plasma bubbles, they can cross the signal path during the tracking of a single GNSS satellite. This method is applied systematically to measure the duration and depth of these bubbles. The results show that these bubbles could have very different duration, lasting from 10 to almost 100 min during a solar maximum, or shorter periods during a solar minimum, as shown in Figure 19.  
In order to make a comparison between both studies, the equivalent length of the bubbles has been computed. Regarding the measurement from ground stations and neglecting the movement of the GNSS satellites during the transition of the bubble, the length of the bubbles can be estimated using the mean drift velocity of the ionospheric plasma, \(\sim\)100 m/s. That transformation linearly translates the horizontal scale of Figure 19, from 10-100 min to 60-600 km, respectively.  
From the recorded CYGNSS data, using equally spaced days every 15 days from March 2017 to March 2018 -25 days in total-, all the peaks lasting more than 5 s with an \(S_{4}\) value above 0.2 have been computed to display the histogram shown in Figure 20. The extension of the bubble, in this case, is computed from the coordinates of the first and last specular reflection considered part of the peak and it is shown in Figure 21. The total number of peaks analyzed in these histograms is around 4700 and they are distributed along the 25 days in 2017 using the data shown in the previous section represented in Figure 15.  
Figure 19: Probability to detect equatorial plasma bubbles (EPBs) with a particular effective time duration, for both years studied in Reference [19]: 2009 (**left**) during solar minimum and 2014 (**right**) during the last solar maximum (adapted from Reference [19]).  
Figure 20: Histogram of peak duration during 25 equally spaced days in 2017 representing the probability in bins of 1 s with a total number of samples of around 4700.  
Comparing CYGNSS data with the one studied in Reference [19], many similarities can be extracted. The shape of the distribution fits very well in terms of bubble extension, ranging from around 40 km to 200 km. In both plots, an exponential-like shape can be observed.