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.

Anomalies to the Ideal Model

The statements above assumed that each layer was smooth and uniform. In reality the ionosphere is a lumpy, cloudy layer with irregular patches of ionization.

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Winter Anomaly

At mid-latitudes, the F2 layer daytime ion production is higher in the summer, as expected, since the sun shines more directly on the earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F2 ionization is actually lower, not higher, in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

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Equatorial Anomaly

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Within approximately ± 20 degrees of the magnetic equator, is the Equatorial Anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Related Topics:
Equator - Magnetic field - Tidal - Horizontal

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