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.

Geophysics

The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer. At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.

Related Topics:
Earth's atmosphere - Troposphere - Km - Stratosphere - Mesosphere - Ozone layer - Thermosphere - Electrons - Ion - Radio propagation - Plasma

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Solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons of energy at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together. The point of balance between these two processes determines the degree of ionization present at any given time.

Related Topics:
Solar - Radiation - Ultraviolet - X-Ray - Wavelength - Photon - Electron - Atom - Molecule - Recombination

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The ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the sun. Thus there is a diurnal (time of day) effect time and a seasonal effect. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the sun is associated with the sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.

Related Topics:
Sun - Activity - Diurnal - Hemisphere - Sunspot cycle - Solar flare - Solar wind - Geomagnetic

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