ionosphere In the upper part of the atmosphere, at altitudes between 100 and 300 km the UV radiation of the sun causes ionisation in the form of free electrons, positive and negative ions. The ion density distribution reaches several peaks at various altitudes. The regions near the maxima are called layers, two of which are permanent. They are known as E1 (~ 100 km.) and F2 (at 200-250 km.). In addition to these permanent layers there are also subsidiary layers. These layers, due to the presence of free electron density, reflect the radio waves in the short wave range. The reflected radio waves are called sky waves. High frequency broadcasts for TV. are however transmitted through the ionosphere.