# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 2 Ionosphere Characteristics
### The Regions of the Ionosphere in Terms of Latitude  
The Earth is divided into three regions regarding the ionosphere, which present different characteristics due to their geomagnetic latitude [11]. The mid-latitude is the widest studied and described. The ionization is produced almost entirely by energetic ultra-violet and X-ray emissions from the Sun. These processes are also present in high-latitude and low-latitude, but other factors produce major changes to the performance of the ionosphere. For instance, absorption of radio signal strength varies depending on the latitude. For mid-latitude and equatorial regions, radiowaves of frequency above 70 MHz will assure penetration of the ionosphere without significant absorption; whereas, at 30 MHz the absorption of one-way vertical incidence signals can have typical values of 0.2 to 0.5 dB, which can go up to 5 dB during solar flares [12].  
Low latitude regions, around \([20,30]^{\circ}\) around both sides of the magnetic Equator, are very influenced by electromagnetic forces that appear because geomagnetic fields run horizontally over the magnetic equator [11]. The electrical conductivity is larger than usual over the Equator, forming a current named _electrojet_ and being subject to electrodynamic lifting and _fountain effect_, distorting the general form of the ionosphere. Scintillation on Earth-space paths, which is a rapid fluctuation of radio-frequency signal phase and/or amplitude, generated as a signal passes through the ionosphere, is quite severe in low latitudes as well as in high latitudes, with observed effects up to gigahertz frequencies [12]. In equatorial latitudes the scintillation peak activity occurs at vernal, as well as autumal equinox, mostly after the ionospheric sunset and events can last from 30 min to hours.  
Finally, in high latitudes the behaviour is the opposite [11; 13]. In this region, the electromagnetic field runs nearly vertically, and this leads to a more complex situation in the ionosphere than in any other of the latitudes [12]. The magnetic field-lines connect the high latitudes to the outer part of the magnetosphere, driven by solar wind. This implies four essential consequences:  
* The high-latitude ionosphere is clearly dynamic.
* The Sun energy particles access easily to this region, and produce an additional ionization, most of the times affected also by sporadic events, worsening the polar radio propagation.
* The high-latitude region is also the auroral zone. This phenomena includes also _electrojets_ and _substorms_, where the rate of ionization is greately increased due to the arrival of energetic electrons.
* A zone of lesser ionization can be formed between the auroral and the mid-latitude ionosphere, despite the mechanism of formation of the trough are not clear.  
These consequences, among others that are explained in Reference [13], cause that any propagation in high-latitudes may be affected by one or more of those deformations, and have to be studied individually and with detail. Regarding absorption at high latitudes, enhanced levels of absorption can occur due to polar cap and Auroral events, which occur at random intervals, last for different periods of time, and their effects are functions of the locations of the terminals and the elevation angle of the path [12].