# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 1 Introduction  
### Physics of the Ionosphere  
The ionosphere is a layer of the atmosphere that plays a very important role in satellite communications and Earth Observation. This layer, which ranges from around 60 km to more than 500 km altitude, contains free electrons and ions, making a sort of \"electric conductor\" that interacts with the electromagnetic waves crossing it.  
The shape and density in electron density profile, are highly affected by many factors in a balance between production and destruction processes driven, mainly, by solar irradiation. Also, its dynamics is influenced by the movements of the inner atmospheric layers and the outer magnetosphere. Because of all these factors, the ionospheric fluctuations take place in a wide range of amplitudes and duration, from kilometers to several meters and from a few seconds or minutes to days.  
It has been observed that there is an increase in the activity, related to solar irradiation, in the transitions (sunrise and sunset), with a rearrangement of the layers. Seasonal dependence is also noticed, with more activity during the periods around the Spring and Autumn Equinoxes. And, on top of that, there is a positive correlation with solar activity following the 11-year solar cycle, with a minimum happening this year 2020.  
Electromagnetic waves crossing the ionosphere may suffer from those perturbations, making the signal to change its direction of propagation, polarization, or creating fluctuations in intensity and phase, the so-called, ionospheric scintillation. This phenomenon is known since the first satellites put in orbit and it has been a subject of study until today. Some models have been developed to try to describe and predict its behavior. During the past years, some ionospheric scintillation models have appeared, for example:  
* Radiocommunication Sector) for the ionospheric communications [3].
* The WideBand MODel (WBMOD) [4] is also based on electron-density irregularities and uses the MPS theory to compute the scintillation effects in an statistical sense. In this model it is possible to set-up a communication scenario (time, location, and other geophysical conditions).
* The Wernik-Alfonsi-Materassi Model (WAM) [5] also uses the MPS theory, but generates its statistics from in situ measurements on ionization fluctuations measured by the Dynamics Explorer 2 satellite. It can also predict the ionospheric \(S_{4}\) index along a defined path given some environmental conditions.  
More recently, in the context of a European Space Agency (ESA) project, the GISM functionalities were extended in the UPC/OE/RDA (Universitat Politecnica de Catalunya/Observatori de l'Ebre/Research and Development in Aerospace) SCIONAV model [6], to reproduce more realistically the behavior of ionospheric scintillation at both low and high latitudes, taking into account the different physical phenomena that create them, such as the bubbles and depletions in equatorial regions and it is based on lookup tables resulting from extensive analyses.  
However, all these models require further improvements:  
* In polar and auroral regions the models still need a better description in terms of velocity, distribution, duration and intensity of the ionospheric effects.
* A better modelling (3D) of the equatorial plasma bubbles (EPBs) can be incorporated in the UPC/OE/RDA SCIONAV model, relating its 3D properties to the altitude, and the amount of plasma depletion with the \(S_{4}\) produced [7].
* In GNSS-R applications, some anomalous fluctuations have been reported in equatorial regions in regions over calm ocean, which are supposed to come from ionospheric scintillation [8].