Ionosphere

I

Introduction

Ionosphere, name given to layers of ionized air in the atmosphere extending from about 60 km (about 37 mi) above the surface of Earth to altitudes of 1,000 km (600 mi) and more. At these altitudes the air is extremely thin, having about the density of the gas in a vacuum tube. When the atmospheric particles are ionized by ultraviolet radiation from the Sun or by other radiation, they tend to remain ionized, because few collisions occur between ions. Ionized atoms and molecules have lost electrons and have become electrically charged.

The regions of the atmosphere called the mesosphere (from about 50 km to 85 km/about 31 mi to 50 mi) and the thermosphere (from about 85 km to 1,000 km/about 50 mi to 600 mi) contain much of the ionosphere. Outer space officially begins at about 100 km (60 mi), so space stations and space vehicles such as the space shuttle, as well as many satellites, orbit Earth within the ionosphere.

II

Reflection of Radio Signals

The ionosphere exerts a great influence on the propagation of radio signals. Energy that is radiated from a transmitter upward toward the ionosphere is in part absorbed by the ionized air and in part reflected downward again, toward the surface of Earth. The bending effect makes possible the reception of radio signals at distances much greater than would be possible for waves that traveled along the surface of Earth. Such reflected waves, however, reach Earth only at certain definite distances from the transmitter; the distance depends on the angle of reflections and the altitude. Hence, a radio signal may be inaudible at 100 km (60 mi) from the transmitter but audible at 500 km (300 mi). This phenomenon is known as skip.

In certain other areas the ground-wave signals and the reflected signals from the ionosphere may reach the receiver and interfere with each other, producing the phenomenon known as fading.

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The amount of reflection in the ionosphere decreases with an increase in frequency and for very high frequencies is almost nonexistent. Therefore long-distance transmission of high-frequency radio waves is limited to the line of sight. Both television and frequency-modulation (FM) radio use high-frequency waves. Long-distance transmission can be achieved only in a direct line, such as between Earth and a communications satellite; the signal then may be relayed from the satellite to a distant point on Earth.

III

Auroras

The ionosphere interacts with the magnetosphere generated by Earth’s magnetic field. At higher latitudes near the poles, charged particles from the Sun called the solar wind can strike the ionosphere, creating visible auroras. Radio communications can be disrupted by solar storms, when bursts of high-intensity radiation and charged particles from the Sun reach the ionosphere. Such events happen most often during the active periods of the Sun’s 11-year sunspot cycle.

IV

Layers in the Ionosphere

The ionosphere is usually divided into three main regions that contain ionized layers. The lowest region is called the D region and rises from about 50 or 60 km to 90 km (30 or 37 mi to 57 mi) above Earth’s surface. A higher region designated the E region (sometimes called the Heaviside layer or Kennelly-Heaviside layer) lies between about 90 km and 150 km (about 57 mi and 93 mi) above the surface and reflects radio waves of low frequency. Still higher in the ionosphere is the F region (sometimes called the Appleton layer), which reflects higher-frequency radio waves. The F region is divided into an F₁ layer, which begins at about 150 km (about 93 mi) above Earth; and an F₂ layer, which begins at about 300 km (about 186 mi) from the surface. The F layers rise during the night and therefore change reflecting characteristics.