Turbulence Properties of Kilometer-scale Equatorial Irregularities as Deduced from Swarm Satellites LP Observations

The ESA Swarm satellites have since year 2014 provided measurements of electron density at a frequency of 2 Hz and at times also 16 Hz corresponding to about 500 m along the satellite paths. Spectral indices, structure functions and scaling exponents of these 16 Hz density estimates were analyzed to study the F-region ionospheric irregularities at altitudes between about 425 and 510 km. The data were obtained during the period from October 2014 to October 2022.The Power Spectral Densities (PSDs) observed followed to a very good approximation a power law. The values of spectral indices p obtained showed a peak centered at around -2.5, located at the Equatorial Ionization Anomaly (EIA) belts. The largest contribution to the spectra came from in the South American-Antlantic-African longitudes and it was generally low in the Asian-Pacific region. The angle between the Swarm satellite orbital path and the magnetic field (∠(B, v)) was examined. The highest percentage of occurrence of ionospheric irregularities and the peak in spectral index was obtained for ∠(B, v) between 0° and about 40°. Over this range of angles PSD spectra steepened with increasing ∠(B, v) (p becomes increasingly negative), consistent with local anisotropic turbulence at scales of a few km. The probability distributions of density differences (structure functions) are non-Gaussian at all orders, similar to many other observations in space plasma. The scaling exponent function is non-linearly concave, which is usually taken as a sign of intermittency.