The correct answer is option 3: Thermosphere. The thermosphere is the layer of the atmosphere that lies above the mesosphere, ranging from about 80 kilometers to 600 kilometers above the Earth`s surface. It is known for having a high concentration of ions.
The ionization of molecules and atoms in the thermosphere occurs due to the intense solar radiation in this part of the atmosphere. The absorption of solar ultraviolet and x-ray radiation leads to the ionization of gas molecules, creating a high concentration of ions.
The presence of ions in the thermosphere has several important effects. It can cause the scattering and absorption of radio waves, for example, which affects the propagation of satellite signals and radio communications. The ionization in the thermosphere also plays a significant role in the formation of the auroras, as charged particles from the Sun interact with the Earth`s magnetic field.
In summary, the thermosphere is the layer of the atmosphere that has a high concentration of ions due to the ionization of gas molecules by solar radiation.