Ionosphere

The ionosphere is the part of the atmosphere that is ionized by solar radiation. It forms the inner edge of the magnetosphere and has practical importance because it influences high-frequency (HF) (3–30 MHz) radio propagation to distant places on the Earth.

The Ionospheric Layers

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Solar radiation, acting on the different compositions of the atmosphere with height, generates layers of ionization:

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D Layer

The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometre(nm) ionizing nitric oxide (NO). In addition, when the sun is active with 50 or more sunspots, Hard X-rays (wavelength < 1 nm) ionize the air (N2, O2). During the night cosmic rays produce a residual amount of ionization. Recombination is high in this layer, thus the net ionization effect is very low and as a result the high-frequency (HF) radio waves aren't reflected by the D layer. The frequency of collision between electrons and other particles in this region during the day is about 10 million collisions per second. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset, but remains due to galactic cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

Related Topics:
D layer - Lyman series - Wavelength - Nanometre - Nitric oxide - X-ray - Cosmic rays - Radio wave - Galactic cosmic ray - Broadcast band

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E Layer

The E layer is the middle layer, 90km to 120km above the surface of the Earth. Ionization is due to Soft X-Ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). This layer can only reflect radio waves having frequencies less than 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. On the night side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection.

Related Topics:
E layer - Oxygen - Solar wind

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E_S

The Es layer or sporadic E-layer. Sporadic E propagation is characterized by small clouds of intense ionization, which can support radio wave reflections from 25 – 225 MHz. Sporadic-E events may last for just a few minutes to several hours. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months with major occurrences during the summer, and minor occurrences during the winter. During the summer, this mode is popular due to its high signal levels. The skip distances are generally around 1000km (620 miles).

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F Layer

The F layer or region, also known as the Appleton layer, is 120km to 400km above the surface of the Earth. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes molecular oxygen (O2). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun.

Related Topics:
F layer - Oxygen - Skywave - Radio

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