What is the ionosphere composed of?

The main constituents of the Earth's atmosphere is molecular oxygen and nitrogen, but at altitudes of 100 km or higher, some of these molecules become photoionized by the solar x-ray and ultraviolet radiation. The atmosphere is so rarefied that the electrons produced in this way take a long time to recombine with positively-charged and ionized atoms, so a layer forms which is present even on the night time side of the Earth. This layer, the ionosphere or formerly the Kenneley-Heaviside Layer ( ca 1910) has an average density of about 100 electrons per cubic centimeter, together with the donor nitrogen and oxygen atoms. Currents flow in this layer, as well as vast 'plasma waves' triggered by a variety of instabilities in this electro-magnetic medium laced with the Earth's magnetic field. The currents also create their own magnetic fields which, during major geomagnetic storm events, can modify the Earth's magnetic field at ground-level. It is also a turbulent medium that causes distant radio sources in the universe to 'twinkle' when studied at the right frequencies and angular resolution.

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