# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 2 Ionosphere Characteristics
### Ionosphere Layers: Day-Night and Latitude Dependencies  
The ionosphere is the ionized part of the Earth's atmosphere. It is formed by a group of electrons and electrically-charged particles, starting at an approximate altitude of 60 km. It is a part of the atmosphere with intense activity because it is affected by several natural phenomena, as winter and equatorial anomalies [14; 15], and also the equatorial _electrojet_[16]. Another interesting issue is the periodic day-night variance, which exists in the ionosphere due to the main source of ionization,that is, the Sun [8]. It is detailed that during daytime the gases in the atmosphere are ionized by means of the absorption of photons of short wavelength coming from the Sun. In this sense, most of the electron density loss occurs during the night, when electrons disappear due to the influence of gas dynamics and electromagnetic interactions. The phenomena of diffusion and proton absorption provides a path for the removal of the molecular ions [17].  
Several travelling ionospheric disturbances, anomalies as sporadic-E, spread-E or spread-F affect the ionosphere specially at certain seasons and latitudes, and it makes its evolution variable between days, seasons and also geographic fluctuations [18]. Furthermore, solar flares and geomagnetic storms are also present in the ionospheric channel and are unpredictable [8]. The sunspot number has a pseudo-periodic cycle of between 9 and 14 years length [19]. Solar radiation is the main influence on ionization of the heterogeneous plasma in the ionosphere. It mainly depends on the solar flux and the position and angle of the Sun in relation to the Earth's ionosphere. Therefore the ionosphere layers definition is highly correlated with the day-night cycle.  
The D layer (found in \(\sim\)60-80 km) contains free electrons that recombine with oxygen ions during the night, forming uncharged oxygen molecules. During daytime, the D layer attenuates low-frequency radio waves of the HF band and during the night disappears [20]. The E layer (found between \(\sim\)80-160 km) does not disappear at night despite its low ionization. Ionization in this layer appears mainly for soft X-ray and residual nighttime ionization is due to cosmic rays among other sources. This residual ionization is usually unimportant for HF links except in high latitudes, where it varies rapidly due to energetic particles precipitating [21; 22].  
The \(\mathbf{E_{s}}\) layer (found between \(\sim\)80-120 km), also known as sporadic E, is described as thin layers of intense ionization, that appear rarely and for a short period of time [23]. It is more frequent in summer and during the day [8]. Finally, the F layer (found between \(\sim\)160-400 km), comprises two peaks of ionization during daytime and only one layer during nighttime. It is the layer with the higher density of free electrons and its ion distribution varies from day to night. During the day, it contains a small peak of ionization called F1 and the dominant F2 is also observed, carrying the main electron density. The F layer and the E layer are the only ones that remain during the 24-h cycle. Although the F layer contains the main ionization peak, which is dependent on available frequencies, link length and ionization level, the E layer also plays an important role in the possibilities of electromagnetic signal transmission.