1	Ionosphere

The ionosphere is the part of the atmosphere that contains ionized gaz, called plasma, and that affects radio propagation. It corresponds to altitudes from 50 to 500 miles at which the atmosphere is so thin that free electrons can exist.

The ionization process originates from UV solar radiation and X-Ray wavelengths. These photons are energetic enough to dislodge electrons from gaz atoms.
Simulatenously, those free electrons can be captured by positive ions. This is called recombination.
The ratio between ionization and recombination determines on overall electron density. It depends on gaz density (at lower altitudes, the recombination process accelerates) and on the amount of radiation received from space (Sun mainly, but also GRBs).

Thus, the ionosphere exhibits is a diurnal effect (day/night), a seasonal effect (summer/winter) and strong relationship with solar activity (11-years sunspot level and solar flares).

The ionosphere has been divided into layers:

• D Layer
  The D layer is the closest to Earth surface. Its altitude ranges from 30 to 60 miles (50 km to 90 km). Due to the high density, recombination is important. The overall electron density is very low. This layer is mainly present in the daytime, but during the night, cosmic rays produce a residual amount of ionization. It does not reflects HF radio waves but is mainly responsible for their absorption, particularly at lower frequencies. Consequently, the absorption is smaller at nightime than at midday. One can notice that the AM radio stations are best received during the daytime.
• E Layer
  The E layer is higher in altitude, from 60 to 90 miles (90 km to 120 km). It only reflects radio waves with a frequency lower than 10 MHz and partially absorbs higher frequencies.
• F Layer
  The F layer ranges from 90 to 250 miles (120 km to 400km) above the Earth surface. It is responsible for most of skywave radio propagation.
  During the day, it divides into two layers, called F1 and F2.

0	The Sudden Ionospheric Distubances (SIDs) and their detection