Abstract
Abstract. The main purpose of this study is to investigate the four-peak structure observed in the low-latitude equatorial ionosphere by the FORMOSAT/COSMIC satellites. Longitudinal distributions of NmF2 (the density of the F layer peak) and hmF2 (ionospheric F2-layer peak height) averages, obtained around September equinox periods from 2007 to 2015, were submitted to a bi-spectral Fourier analysis in order to obtain the amplitudes and phases of the main waves. The four-peak structure in the equatorial and low-latitude ionosphere was present in both low and high solar activity periods. This kind of structure possibly has tropospheric origins related to the tidal waves propagating from below that modulate the E-region dynamo, mainly the eastward non-migrating diurnal tide with wavenumber 3 (DE3, E for eastward). This wave when combined with the migrating diurnal tide (DW1, W for westward) presents a wavenumber-4 (wave-4) structure under a synoptic view. Electron densities observed during 2008 and 2013 September equinoxes revealed that the wave-4 structures became more prominent around or above the F-region altitude peak (∼ 300–350 km). The four-peak structure remains up to higher ionosphere altitudes (∼ 800 km). Spectral analysis showed DE3 and SPW4 (stationary planetary wave with wavenumber 4) signatures at these altitudes. We found that a combination of DE3 and SPW4 with migrating tides is able to reproduce the wave-4 pattern in most of the ionospheric parameters. For the first time a study using wave variations in ionospheric observations for different altitude intervals and solar cycle was done. The conclusion is that the wave-4 structure observed at high altitudes in ionosphere is related to effects of the E-region dynamo combined with transport effects in the F region.
Highlights
The equatorial ionization anomaly (EIA) is an important characteristic of the low-latitude and equatorial ionosphere
It was found that DE3 and stationary planetary wave with wavenumber 4 (SPW4) relative amplitudes have their maximum and minimum in 2008 and 2010, respectively, differing from the absolute amplitudes, which were correlated with the solar flux observations, possibly caused, according to the authors, by interannual variability in the DE3 component in the mesosphere– lower thermosphere (MLT) region due to lower and middle atmospheric sources such as ENSO (El Niño–Southern Oscillation) and QBO
This study has shown that the DE3 wave is the main nonmigrating tide responsible for the wave-4 longitude variations observed in satellite data
Summary
The equatorial ionization anomaly (EIA) is an important characteristic of the low-latitude and equatorial ionosphere. It was found that DE3 and SPW4 relative amplitudes have their maximum and minimum in 2008 and 2010, respectively, differing from the absolute amplitudes, which were correlated with the solar flux observations, possibly caused, according to the authors, by interannual variability in the DE3 component in the mesosphere– lower thermosphere (MLT) region due to lower and middle atmospheric sources such as ENSO (El Niño–Southern Oscillation) and QBO (quasi-biennial oscillation). The main subject of the present study is to observe the wave-4 structures in ionospheric parameters during different solar activity periods around the September equinox and verify which waves will result using bi-spectral Fourier analysis from NmF2 (the density of the F layer peak), hmF2 and Ne observations. The fountain effect elevates ionospheric plasma in the regions near the magnetic equator to high altitudes, where it diffuses down through the geomagnetic field lines to latitudes located around 15–20◦ from the magnetic equator In this way it affects the electron density distribution.
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