Sensing Ionospheric Turbulence Using GNSS

Electron density in ionospheric plasma exhibits fluctuations and irregularities in time and space, at several scales. Plasma, being ionized gas, is subject to a turbulent behaviour similar to that observed in fluid dynamics, with two main distinctions: a) its dynamics are coupled with electromagnetic fields; b) collisions of particles are rare. These unique properties characterize the inertial range of ionospheric plasma turbulence, which represents the energy cascade from large-scale structures (e.g. travelling ionospheric disturbances) to small-scale ones (eddies) until energy dissipation occurs. 

Kolmogorov power law would predict a spectrum of 8/3 and equivalently a structure function with a power law of 5/3 for a phase signal crossing a 3D turbulent medium. However, the previous investigation of spatial structure characteristic of the ionosphere using LOFAR array observed a power law of around 1.9 in the spatial domain. In this study, we investigate the spatio-temporal and temporal structure of the ionosphere using structure function of GNSS phase geometry free signals from both medium earth orbit satellites and geostationary ones. We found two regimes, one compatible the 5/3 Kolmogorov theory and one obeying a 2 power law. We propose an interpretation for the two regimes, the first being a 3D turbulent flow driven by local instabilities, and the second one being driven by solar radiation-induced ionization and successive recombination. The second spectrum obeys a power law of 2, that is the power spectrum of a sinusoidal function like the local sun elevation. By using receivers at almost constant solar irradiance located in polar regions, we further observe the turbulent regimes also in spatio-temporal structure function.