AURORAL AND METEOR-SCATTER PROPAGATION

In the presence of unusual solar activity, the aurora often reflect radio waves at some frequencies. This is called auroral propagation. The aurorae occur in the ionosphere at altitudes of 25 km (40 mi) to 400 km (250 mi) above the surface. Theoretically, auroral propagation is possible, when the aurorae are active, between any two points on the Earth’s surface from which the same part of the aurora lies on a line of sight. Auroral propagation seldom occurs when either the transmitter or the receiver is at a latitude less than 35 degrees north or south of the equator. Auroral propagation can take place at frequencies well above 30 MHz and often is accompanied by deterioration in ionospheric propagation via the E and F layers. When a meteor from space enters the upper part of the atmosphere, an ionized trail is produced because of the heat of friction. Such an ionized region reflects EM energy at certain wavelengths. This phenomenon, known as meteor-scatter propagation, can result in over-the-horizon radio communication or reception.
A meteor produces a trail that persists for a few tenths of a second up to several seconds depending on the size of the meteor, its speed, and the angle at which it enters the atmosphere. This amount of time is not sufficient for the transmission of very much information, but during a meteor shower, ionization can be almost continuous. Meteor-scatter propagation has been observed at frequencies considerably above 30 MHz and occurs over distances ranging from just beyond the horizon up to about 2,000 km (1,200 mi) depending on the altitude of the ionized trail and on the relative positions of the trail, the transmitting station, and the receiving station.

مواضيع ذات صلة

The dipole radiator

Radiation

Electromagnetism

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