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The critical frequency changes continuously and the F layer of the ionosphere is mostly responsible for the reflection of radio waves back to Earth. The other layers(D) interact in other ways - absorption of frequency and during the day, the D Layers forms, and the F layer splits into F1 and F2 layers.
The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown ...
For oblique incidence, the critical frequency becomes larger. Very low frequencies (VLF: 3–30 kHz), and extremely low frequencies (ELF: <3 kHz) are reflected at the ionospheric D- and lower E-layer.
Typical ionogram indicating an F2 layer critical frequency (foF2) of approximately 5.45 MHz. An example of an ionosonde system displaying an ionogram. An ionosonde, or chirpsounder, is a special radar for the examination of the ionosphere.
In radio transmission, maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points on Earth by reflection from the ionosphere (skywave or skip) at a specified time, independent of transmitter power.
The ionosphere is a region of the upper atmosphere, from about 80 km (50 miles) to 1000 km (600 miles) in altitude, where neutral air is ionized by solar photons, solar particles, and cosmic rays. When high-frequency signals enter the ionosphere at a low angle they are bent back towards the Earth by the ionized layer. [1]
Vertical incident sounding uses a collocated transmitter and receiver and involves directing a range of frequencies vertically to the ionosphere and measuring the values of the reflected returned signals to determine the effective ionosphere layer altitude. This technique is also used to determine the critical frequency.
The F region of the ionosphere is home to the F layer of ionization, also called the Appleton–Barnett layer, after the English physicist Edward Appleton and New Zealand physicist and meteorologist Miles Barnett. As with other ionospheric sectors, 'layer' implies a concentration of plasma, while 'region' is the volume that contains the said layer.