Spectral Properties of Kilometer-scale Equatorial Irregularities as Seen by theSwarm Satellites

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. The spectral characteristics of these 16 Hz density estimates were analyzed to study the F-region ionospheric
irregularities at altitudes between about 440 and 510 km. The data were obtained during the period from October 2014 to October 2018. The Power
Spectral Densities (PSDs) observed followed to a very good approximation a power law. The values of spectral indices obtained showed a peak centered at around -2.5, located at the Equatorial Ionization Anomaly (EIA) belts. The spectral indices were found to be sensitive to the amplitudes of the irregular
variations. Most spectra were obtained  within the time sector 20:00 LT - 22:00 LT, and they became slightly shallower towards later local times. 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 20° and 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.