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

Abstract

<p>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<br>irregularities at altitudes between about 440 and 510 km. The data were obtained during the period from October 2014 to October 2018. The Power<br>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<br>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.</p>