ionosphere and magnetosphere

▪ atmospheric science

Introduction

      regions of Earth's (Earth) atmosphere in which the number of electrically charged particles—ions (ion) and electrons (electron)—are large enough to affect the propagation of radio waves (electromagnetic radiation). The charged particles are created by the action of extraterrestrial radiation (mainly from the Sun) on neutral atoms and molecules (molecule) of air. The ionosphere begins at a height of about 50 km (30 miles) above the surface, but it is most distinct and important above 80 km (50 miles). In the upper regions of the ionosphere, beginning several hundred kilometres above Earth's surface and extending tens of thousands of kilometres into space, is the magnetosphere, a region where the behaviour of charged particles is strongly affected by the magnetic fields (magnetic field) of Earth and the Sun. It is in the magnetosphere that the spectacular displays of the aurora borealis and aurora australis take place. The magnetosphere also contains the Van Allen radiation belts (Van Allen radiation belt), where highly energized protons and electrons travel back and forth between the poles of Earth's magnetic field (geomagnetic field).

      This article describes the layers of the ionosphere and the mechanisms by which these ionized layers are created and altered. The features of the magnetosphere are also described, particularly as they are manifested in the auroras and the Van Allen belts.

Ionosphere

Discovery of the ionosphere

      Discovery of the ionosphere (ionosphere and magnetosphere) extended over nearly a century. As early as 1839, the German mathematician Carl Friedrich Gauss (Gauss, Carl Friedrich) speculated that an electrically conducting region of the atmosphere could account for observed variations of Earth's magnetic field. The notion of a conducting region was reinvoked by others, notably in 1902 by the American engineer Arthur E. Kennelly (Kennelly, Arthur Edwin) and the English physicist Oliver Heaviside (Heaviside, Oliver), to explain the transmission of radio signals around the curve of Earth's surface before definitive evidence was obtained in 1925. For some years the ion-rich region was referred to as the Kennelly-Heaviside layer.

 The name “ionosphere” was introduced first in the 1920s and was formally defined in 1950 by a committee of the Institute of Radio Engineers (electrical and electronics engineering) as “the part of the earth's upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves.” Much of the early research on the ionosphere was carried out by radio engineers and was stimulated by the need to define the factors influencing long-range radio communication. Subsequent research has focused on understanding the ionosphere as the environment for Earth-orbiting satellites (space exploration) and, in the military arena, for ballistic missile (rocket and missile system) flight. Scientific knowledge of the ionosphere has grown tremendously, fueled by a steady stream of data from spacecraft-borne instruments and enhanced by measurements of relevant atomic and molecular processes in the laboratory.

Layers of the ionosphere

      Historically, the ionosphere was thought to be composed of a number of relatively distinct layers that were identified by the letters D, E, and F. The F layer was subsequently divided into regions F₁ and F₂. It is now known that all these layers are not particularly distinct, but the original naming scheme persists.

      It appears that Edward V. Appleton (Appleton, Sir Edward Victor), a pioneer in early radio probing of the ionosphere, is responsible for the nomenclature. Appleton was accustomed to using the symbol E to describe the electric field of the wave reflected from the first layer of the ionosphere that he studied. Later he identified a second layer at higher altitude and used the symbol F for the reflected wave. Suspecting a layer at lower altitude, he adopted the additional symbol D. In time, the letters came to be associated with the layers themselves rather than with the field of the reflected waves. It is now known that electron density increases more or less uniformly with altitude from the D region, reaching a maximum in the F₂ region. Though the nomenclature used to describe the different layers of the ionosphere continues in wide use, the definitions have evolved to reflect the improved understanding of the underlying physics and chemistry.

D region

      The D region is the lowest ionospheric region, at altitudes of about 70 to 90 km (40 to 55 miles). The D region differs from the E and F regions in that its free electrons almost totally disappear during the night, because they recombine with oxygen ions to form electrically neutral oxygen molecules (molecule). At this time, radio waves pass through to the strongly reflecting E and F layers above. During the day some reflection can be obtained from the D region, but the strength of radio waves is reduced; this is the cause of the marked reduction in the range of radio transmissions in daytime. At its upper boundary the D region merges with the E region.

E region

      The E region is also called Kennelly-Heaviside layer, named for American electrical engineer Arthur E. Kennelly (Kennelly, Arthur Edwin) and English physicist Oliver Heaviside (Heaviside, Oliver) in 1902. It extends from an altitude of 90 km (60 miles) to about 160 km (100 miles). Unlike that of the D region, the ionization of the E region remains at night, though it is considerably diminished. The E region was responsible for the reflections involved in Guglielmo Marconi (Marconi, Guglielmo)'s original transatlantic radio communication in 1902. The ionization density is typically 10⁵ electrons per cubic centimetre during the day, though intermittent patches of stronger ionization are sometimes observed.

F region

      The F region extends upward from an altitude of about 160 km (100 miles). This region has the greatest concentration of free electrons. Although its degree of ionization persists with little change through the night, there is a change in the ion distribution. During the day, two layers can be distinguished: a small layer known as F₁ and above it a more highly ionized dominant layer called F₂. At night they merge at about the level of the F (Appleton layer)₂ layer, which is also called the Appleton layer. This region reflects radio waves with frequencies (frequency) up to about 30 megahertz; the exact value depends on the peak amount of the electron concentration, typically 10⁶ electrons per cubic centimetre, though with large variations caused by the sunspot cycle.

Mechanisms of ionization

Photoionization (photo-ionization)

      Most of the electrical activity in the ionosphere is produced by photoionization (photo-ionization) ( ionization caused by light energy). Photons (photon) of short wavelength (that is, of high frequency) are absorbed by atmospheric gases. A portion of the energy is used to eject an electron, converting a neutral atom or molecule to a pair of charged species—an electron, which is negatively charged, and a companion positive ion. Ionization in the F₁ region is produced mainly by ejection of electrons from molecular oxygen (O₂), atomic oxygen (O), and molecular nitrogen (N₂). The threshold for ionization of O₂ corresponds to a wavelength of 102.7 nm (nanometres, or billionths of a metre). Thresholds for O and N₂ are at 91.1 nm and 79.6 nm, respectively.

      Positive ions in turn can react with neutral gases. There is a tendency for these reactions to favour production of more-stable ions. Thus, ionized atomic oxygen, O⁺, can react with O₂ and N₂, resulting in ionized molecular oxygen (O₂⁺) and ionized nitric oxide (NO⁺), as shown by:

O⁺ + O₂ {long right arrow} O + O₂⁺ (1)

and

O⁺ + N₂ {long right arrow} NO⁺ + N. (2)

      Similarly, ionized molecular nitrogen (N₂⁺) can react with O and O₂ to form NO⁺ and O₂⁺ as follows:

N₂⁺ + O {long right arrow} NO⁺ + N (3)

and

N₂⁺ + O₂ {long right arrow} N₂ + O₂⁺. (4)

The most stable, and consequently most abundant, ions in the E and F₁ regions are O₂⁺ and NO⁺, the latter more so than the former. At lower altitudes, O₂⁺ can react with the minor species of atomic nitrogen (N) and nitric oxide (NO) to form NO⁺, as indicated by:

O₂⁺ + N {long right arrow} O + NO⁺ (5)

and

O₂⁺ + NO {long right arrow} O₂ + NO⁺. (6)

In the D region, NO⁺ and water vapour (H₂O) can interact to form the hydronium ion, H₃O⁺, and companion species such as H₅O₂⁺ and H₇O₄⁺. Production of hydrated ions is limited by the availability of H₂O. As a consequence, they are confined to altitudes below about 85 km (53 miles).

Recombination

      The electron density in the D, E, and F₁ regions reflects for the most part a local balance between production and loss. Electrons are removed mainly by dissociative recombination (dissociation), a process in which electrons attach to positively charged molecular ions and form highly energetic, unstable neutral molecules. These molecules decompose spontaneously, converting internal energy to kinetic energy possessed by the fragments. The most important processes in the ionosphere involve recombination of O₂⁺ and NO⁺. These reactions may be summarized by:

O₂⁺ + e {long right arrow} O + O (7)

and

NO⁺ + e {long right arrow} N + O. (8)

      A portion of the energy released in reactions (7) and (8) may appear as internal excitation of either nitrogen, oxygen, or both. The excited atoms can radiate, emitting faint visible light (electromagnetic radiation) in the green and red regions of the spectrum, contributing to the phenomenon of airglow. Airglow originates mainly from altitudes above 80 km (50 miles) and is responsible for the diffuse background light that makes it possible to distinguish objects at Earth's surface on dark, moonless nights. Airglow is produced for the most part by reactions involved in the recombination of molecular oxygen. The contribution from reactions (7) and (8) is readily detectable, however, and provides a useful technique with which to observe changes in the ionosphere from the ground. Over the years, studies of airglow have contributed significantly to scientific understanding of processes in the upper atmosphere.

      As indicated above, dissociative recombination provides an effective path for removal of molecular ions. There is no comparable means for removal of atomic ions. Direct recombination of ionized atomic oxygen (O⁺) with an electron requires that the excess energy be radiated as light. Radiative recombination is inefficient, however, compared with dissociative recombination and plays only a small role in the removal of ionospheric electrons. The situation becomes more complicated at high altitudes where atomic oxygen (O) is the major constituent of the neutral atmosphere and where electrons are produced primarily by its photoionization. The atomic oxygen ion, O⁺, may react with N₂ and O₂ to form NO⁺ and O₂⁺, but the abundances of N₂ and O₂ decline relative to O as a function of increasing altitude. In the absence of competing reactions, the concentration of O⁺ and the density of electrons would increase steadily with altitude, paralleling the rise in the relative abundance of O. This occurs to some extent but is limited eventually by vertical transport.

diffusion

      Ions (ion) and electrons (electron) produced at high altitude are free to diffuse (diffusion) downward, guided by Earth's magnetic field. The lifetime of O⁺ is long at high altitudes, where the densities of O₂ and N₂ are very small. As ions move downward, the densities of O₂ and N₂ increase. Eventually the time constant for reaction of O⁺ with O₂ and N₂ becomes comparable to the time for diffusion, and O⁺ reacts to produce either O₂⁺ or NO⁺ before it can move much farther. The O⁺ density exhibits a maximum in this region. Competition between chemistry and transport is responsible for the formation of an electron-density maximum in the F₂ layer. The dominant positive ion is O⁺.

      The density of O⁺ decreases with decreasing altitude below the peak, reflecting a balance between production of O by photoionization and its removal by reactions (1) and (2). The density of O⁺ also decreases above the peak. In this case, removal of photo-ions is regulated by downward diffusion rather than by chemistry. The distribution of O⁺ with altitude above the peak reflects a balance of forces—a pressure-gradient force that acts to support O⁺ in opposition to gravitational (gravitation) and electrostatic (electricity) forces that combine to pull O⁺ down. The electrostatic force acts to preserve electrical charge neutrality. In its absence, the concentration of ions—which are much more massive than electrons—would tend to fall off more rapidly with altitude than electrons. The abundance of electrons would quickly exceed that of ions, and the upper atmosphere would accumulate negative charge. The electric field redresses the imbalance by drawing electrons down and providing additional upward support for positively charged ions. Though O⁺ has a mass of 16 atomic units, its abundance decreases with altitude as if it had a mass of only 8 atomic units. (One atomic unit corresponds to the mass of a hydrogen atom, 1.66 10^-24 gram.) This discrepancy occurs because the electric field exerts a force that is equivalent to that exerted by the gravitational force on a body with a mass of eight atomic units. This electrostatic force is directed upward for ions and downward for electrons, in effect buoying the ions while encouraging the electrons to sink. The concentration of electrons therefore falls off with altitude at precisely the same rate as that of O⁺, preserving the balance of positive and negative charge.

Photon absorption

      Ionization at any given level depends on three factors—the availability of photons of a wavelength capable of effecting ionization, a supply of atoms and molecules necessary to intercept this radiation, and the efficiency with which the atoms and molecules are able to do so. The efficiency is relatively large for O, O₂, and N₂ from about 10 to 80 nm. This is the portion of the spectrum responsible for production of electrons and ions in the F₁ region. Photons (photon) with wavelengths between 90 and 100 nm are absorbed only by O₂. They therefore penetrate deeper and are responsible for producing about half the ionization in the E layer. The balance is derived from so-called “soft” X-rays (X-ray) (those of longer wavelengths), which are absorbed with relatively low efficiency in the F region and so are able to penetrate to altitudes of about 120 km (75 miles) when the Sun is high over the region. “Hard” X-rays (those of shorter wavelengths—that is, below about 5 nm) reach even deeper. This portion of the spectrum accounts for the bulk of the ionization in the D region, with an additional contribution from wavelengths longer than 102.6 nm—mainly from photons in the strong solar emission line at Lyman α at a wavelength of 121.7 nm. (The Lyman series (spectral line series) is a related sequence of wavelengths that describe electromagnetic energy given off by energized atoms in the ultraviolet region.) Lyman α emissions are weakly absorbed by the major components of the atmosphere—O, O₂, and N₂—but they are absorbed readily by NO and have sufficient energy to ionize this relatively unstable compound. Despite the low abundance of NO, the high flux of solar radiation at Lyman α is able to provide a significant source of ionization for the D region near 90 km (55 miles).

Ionospheric variations

      The ionosphere is variable in space and time. Some of the changes are chemical in origin and can be readily understood on the basis of the general considerations outlined above. There is a systematic variation, for example, according to the time of day. In early morning the Sun is relatively low in the sky, so that radiation (solar radiation) must penetrate a large column of air before reaching a given level of the atmosphere. As a result, ionization rates are lower, and the location of ionized layers shifts to higher altitudes. As the Sun rises, the D, E, and F₁ layers shift in altitude. The layers are lowest and densities of electrons are highest at noon. At night, on the other hand, ionization in the D, E, and F₁ regions tends to disappear as electrons and ions recombine to form neutral gases.

      The diurnal, or daily, variation of the F₂ layer is less dramatic. Ions produced at high altitudes during the day maintain a sizable density of electrons at the F₂ peak throughout the day and then diffuse downward at night. This accounts for the fact that radio reception (both in the broadcast and shortwave (shortwave radio) bands) is generally best at night. Ionization at lower altitudes—primarily those corresponding to the D region—tends to interfere with radio transmissions during the day. Interference is minimal at night because ionization in the D layer effectively disappears with the setting of the Sun.

      The density of ionization varies in response to changes in the intensity and properties of radiation from the Sun. The output of solar energy is relatively constant in the visible and near-ultraviolet (electromagnetic radiation) portions of the spectrum. It varies appreciably, however, at shorter wavelengths, reflecting changes in the temperature of the outermost regions of the solar atmosphere. The changes are particularly large, in excess of a factor of 10, at X-ray wavelengths. Variations in the D region are correspondingly large, with smaller though still significant changes in the E and F layers.

      Solar activity varies on a characteristic timescale of about 11 years. It is not entirely periodic, however; successive cycles can differ significantly, and there are indications that activity can be low for centuries. The Sun was quiet for more than 200 years from about 1600 to about 1850. Solar activity was particularly intense in 1958.

      As noted in the section on Mechanisms of ionization (ionosphere and magnetosphere), ionization above the F₂ peak is removed mainly by downward diffusion of ions and electrons. Ions are constrained, however, to move along the magnetic field. The field is oriented horizontally at the magnetic equator, which is equidistant between the magnetic North Pole and the magnetic South Pole, so vertical diffusion is inhibited at low latitudes. The density of ionized atomic oxygen (O⁺) and electrons at low latitudes is therefore controlled by chemistry to a larger extent than at high latitudes. The F₂ peak is correspondingly higher in altitude, and the density of electrons is elevated accordingly.

      Ions and electrons formed at high altitudes and low latitudes are transported to higher latitudes by thermospheric (thermosphere) winds (wind). As a result, the highest density of electrons at the F₂ peak is observed at intermediate latitudes, offset from the magnetic equator by about 10 degrees.

      Transport can also affect the distribution of ionization at lower altitudes. The diurnal pattern of heating in the troposphere and stratosphere excites a spectrum of waves, some of which are free to propagate vertically. The amplitude of the waves grows significantly as the disturbance enters regions of lower density. Passage of the waves is associated with strong alternating horizontal winds. Ionization can be driven up inclined magnetic field lines (force, line of) at one altitude, while winds blowing in an opposite direction at higher altitudes can induce simultaneous downward motion. This can lead to a bunching of ionization—a local enhancement of the electron density. The mechanism is particularly important in the E region and is responsible for the phenomenon known as sporadic E.

      The buildup of ionization is normally limited by dissociative recombination of molecular ions. At D and E region altitudes, however, the ionosphere contains a small but variable concentration of atomic ions, derived from ionization of metals ablated from meteorites. The density of metallic ions—notably those of sodium (Na⁺), magnesium (Mg⁺⁺), and potassium (K⁺)—is sometimes high enough to supply a layer of ionization with a density comparable to that of the F layer. This can result in a major temporary disruption of radio communications.

      Winds generated in the lower ionosphere by thermal forcing from below have characteristic periods expressed as submultiples of a day. Waves with a period of 24 hours dominate at low latitudes, whereas those with a characteristic period of 12 hours are more important at high latitudes. The origin of the waves is basically similar to that of oceanic tides (tide) caused by the pull of lunar gravity (gravitation). The vertical motion that generates ionospheric waves, however, is the result of the diurnal pattern of heating and cooling rather than gravity. Additional waves can arise owing to irregular forcing, associated, for example, with thunderstorms (thunderstorm), motion over mountain ranges, and other small-scale meteorological disturbances. These small-scale disturbances are referred to as gravity waves to distinguish them from the more regular planetary-scale motions excited by the diurnal cycle of heating and cooling. The regular response to thermal forcing is known as the atmospheric tide.

      Tides and gravity waves have similar effects on ionization in the E region. They both are responsible for concentrating ionization in layers. In combination with the large-scale system of winds in the lower thermosphere, they are also effective in driving an irregular current that flows in the E and lower F regions of the ionosphere. The current owes its origin to differences in the facility with which motions of ions and electrons are constrained by the magnetic field. It is associated with an electric field and results in a modulation of the magnetic field that can be readily detected at the surface. The current is particularly intense in the equatorial region, where it is known as the electrojet. The region of strong current flow is known as the dynamo (dynamo theory) region.

      Protons (proton) (H⁺) and helium ions (He⁺) are important components of the ionosphere above the F₂ peak. They increase in abundance relative to ionized atomic oxygen (O⁺) with increasing altitude. Protons are produced by photoionization of atomic hydrogen (H),

hv + H {long right arrow} H⁺ + e (9)

      and by charge transfer from O⁺ to H,

O⁺ + H {long right arrow} O + H⁺. (10)

      Helium ions are formed by photoionization of helium. The distribution of H⁺ and He⁺ with altitude reflects the influence of the polarization electric field set up to preserve charge neutrality. When O⁺ is the dominant ion, the polarization field acts to lift H⁺ and He⁺ with a force equivalent, but in opposite direction, to that exerted by the gravitational field on a particle with a mass of eight atomic units, as described in the section on Diffusion (ionosphere and magnetosphere). Protons behave as though they have an effective gravitational mass of −7 atomic units (−7 = 1 − 8). The effective mass of He⁺ is −4 atomic units (−4 = 4 − 8).

      The abundance of H⁺ and He⁺ increases with altitude. Eventually H⁺ becomes the dominant component of the outermost ionosphere, which is sometimes referred to as the protonosphere. The more uniform composition of the atmosphere at this level causes a reduction in the polarization field to one equivalent to the gravitational force acting on a body with a mass of 0.5 atomic unit, directed upward for ions and downward for electrons. This field is sufficient to maintain equal densities of H⁺ and electrons. The effective masses of O⁺ and He⁺ shift to 15.5 atomic units (15.5 = 16 − 0.5) and 3.5 atomic units (3.5 = 4 − 0.5), respectively, and the abundance of O⁺, He⁺, and H⁺ declines with further increases in altitude.

magnetosphere

 The overall structure of the outer ionosphere—the magnetosphere—is strongly influenced by the configuration of Earth's magnetic field (geomagnetic field). Close to the planet's surface, the magnetic field has a structure similar to that of an ideal dipole (magnetic dipole). Field lines are oriented more or less vertically at high latitudes, sweep back over the Equator, where they are essentially horizontal, and connect to Earth in a symmetrical pattern at high latitudes. The field departs from this ideal dipolar configuration, however, at high altitudes. There the terrestrial field, Earth's magnetic field, is distorted to a significant extent by the solar wind, with its embedded solar magnetic field. Ultimately the terrestrial field is dominated by the interplanetary field, which is generated by the Sun.

      The solar wind compresses the magnetic field on Earth's dayside at a distance of about 10 Earth radii, or almost 65,000 km (40,000 miles) from the planet. At this distance the magnetic field is so weak that the pressure associated with particles escaping from Earth's gravity (gravitation) is comparable to the opposing pressure associated with the solar wind. This equilibrium region, with a characteristic thickness of 100 km (60 miles), is called the magnetopause and marks the outer boundary of the magnetosphere. The lower boundary of the magnetosphere is several hundred kilometres above Earth's surface.

      On the nightside, the terrestrial field is stretched out in a giant tail that reaches past the orbit of the Moon, extending perhaps to distances in excess of 1,000 Earth radii. The magnetotail can extend to such great distances because on the nightside the forces associated with the magnetic field and the solar wind are parallel.

      The outermost regions of the magnetosphere are exceedingly complex, especially at high latitudes, where terrestrial field lines are open to space. ionization from the solar wind can leak into the magnetosphere in a number of ways. It can enter by turbulent exchange at the dayside magnetopause or more directly at cusps in the magnetopause at high latitudes where closed loops of the magnetic field on the dayside meet fields connecting to the magnetotail. In addition, it can enter at large distances on the nightside, where the magnetic pressure is relatively low and where field lines can reconnect readily, providing easy access to the giant plasma sheet in the interior of Earth's magnetotail.

      The magnetosheath, a region of magnetic turbulence in which both the magnitude and the direction of Earth's magnetic field vary erratically, occurs between 10 and 13 Earth radii toward the Sun. This disturbed region is thought to be caused by the production of magnetohydrodynamic (magnetohydrodynamics) shock waves (shock wave), which in turn are caused by high-velocity solar wind particles. Ahead of this bow shock (bow wave) boundary, toward the Sun, is the undisturbed solar wind.

Auroras (aurora)

 Auroras (aurora) are perhaps the most spectacular manifestations of the complex interaction of the solar wind with the outer atmosphere. The energetic electrons (electron) and protons (proton) responsible for an aurora are directed by the solar wind along magnetic fields into Earth's magnetosphere.

 Auroras occur in both hemispheres, confined for the most part to high latitudes in oval-shaped regions that maintain a more or less fixed orientation with respect to the Sun. The centre of the auroral oval is displaced a few degrees to the nightside with respect to the geomagnetic pole (geomagnetic field). The midnight portion of the oval is, on average, at a geomagnetic latitude of 67°; the midday portion is at about 76°. An observer between 67° and 74° magnetic latitudes generally encounters auroras twice a day—once in evening and once in morning.

Auroral zones

      The portion of Earth that traverses the midnight portion of the auroral oval is known as the auroral zone. In the Northern Hemisphere this zone lies along a curve extending from the northern regions of Scandinavia through Iceland, the southern tip of Greenland, the southern region of Hudson Bay, central Alaska, and on to the coast of Siberia. This is the prime region from which to view an aurora in the Northern Hemisphere. The phenomenon is by no means static, however. The auroral zone shifts poleward at times of low solar activity, while during periods of high solar activity it has been known to move as far south as 40° (geographic latitude). At low latitudes, an aurora assumes a characteristic red colour. In ancient times this colour was often interpreted as evidence of impending disaster. More recently it has been taken as a sign of approaching fires. Auroras assume a variety of forms, depending on the vantage point from which they are observed. The luminosity (luminous intensity) of an aurora is generally aligned with the magnetic field. Field lines are close to vertical in polar regions, and so an aurora occurring there appears to stand on end, hanging from the sky in great luminous drapes. It is a spectacular sight indeed, especially if viewed from a distance either from the north or south. At lower latitudes, the magnetic field lines are inclined with respect to the vertical. There an aurora appears as streamers radiating from the zenith. Such is the majesty of the aurora that no two displays are totally alike. light can move rapidly across the sky on some occasions, and at other times it can appear to stand in place, flickering on and off.

Causes of auroral displays

      The most common type of aurora is associated with bombardment of the atmosphere by electrons (electron) with energies of up to 10,000 electron volts (electron volt). The energy source for these electrons originates ultimately from the Sun. It is propagated through space by the solar wind along bundled, ropelike magnetic fields that form temporarily between the Sun and Earth's magnetosphere, most probably to the plasma sheet. Energetic electrons enter the atmosphere along magnetic field lines. They produce a shower of secondary and tertiary electrons, approximately one for every 35 electron volts of energy in the primary stream. Primaries can propagate to altitudes as low as 100 km (60 miles). Most of the luminosity is produced, however, by low-energy secondary and tertiary electrons. Prominent emissions in the spectrum of this luminosity are associated with the red line of atomic oxygen at 633 nm, the green line of atomic oxygen at 558 nm, the first negative bands of ionized molecular nitrogen at 391 nm and 428 nm, and a host of emissions from atomic oxygen, molecular oxygen, ionized molecular oxygen, and molecular nitrogen. Many of these features are present also in the day and night airglow. They are most notable in auroras because of their intensity and the rapidity with which they switch on and off in response to changes in the flux and energy of incoming primaries. An aurora has a characteristic red colour if the energy of primaries is relatively low. Emission in this case is dominated by atomic oxygen and is confined for the most part to altitudes above 250 km (150 miles). If the energy of the primaries is high, an aurora has a greenish blue colour and extends downward to altitudes as low as 90 km (55 miles).

      Auroral displays are also produced by bombardment of the atmosphere by energetic protons (proton). Protons with energies of up to 200,000 electron volts are responsible for auroral activity in a diffuse belt that is equatorward of the main auroral zone. These protons can be detected from the ground by observation of Doppler (Doppler effect)-shifted radiation emitted by fast hydrogen atoms formed by charge transfer from atmospheric atoms (atom) and molecules (molecule). Protons also play a role at higher latitudes, especially at times following major solar flares (solar flare). It is thought that the protons responsible for auroras at the polar caps are solar in origin. Associated energies may reach as high as one million electron volts, and particles may penetrate as deep as 80 km (50 miles). Polar cap auroras can provide a significant transient source of mesospheric and stratospheric nitric oxide (NO). They can be responsible for small but detectable short-term fluctuations in the abundance of stratospheric ozone.

Van Allen radiation belts (Van Allen radiation belt)

 The magnetosphere includes two doughnut-shaped radiation belts (Van Allen radiation belt), or zones, centred on the Equator that are occupied by appreciable numbers of energetic protons and electrons trapped in the outermost reaches of the atmosphere. No real gap exists between the two zones; they actually merge gradually, with the flux of charged particles showing two regions of maximum density. The inner belt extends from roughly 1,000 to 5,000 km (600 to 3,000 miles) above the terrestrial surface and the outer belt from some 15,000 to 25,000 km (9,300 to 15,500 miles). The belts were named in honour of James A. Van Allen (Van Allen, James A.), the American physicist who discovered them in 1958. His was a triumph of serendipity—he detected the presence of the trapped particles with a Geiger counter designed to measure the flux of cosmic rays (cosmic ray) in space. It was the first great discovery of the space age and was achieved by combining data obtained with instruments carried by three of the earliest United States scientific satellites (Earth satellite)—Explorer 1 (Explorer), Explorer 4, and Pioneer 3 (Pioneer).

      The flux of protons crossing a square centimetre of surface in the inner Van Allen belt can be as large as 20,000 per second, higher than the flux of cosmic radiation in space by a factor of 10,000. Protons in the inner belt have energies in excess of 7 × 10⁸ electron volts, enough to enable them to penetrate about 10 cm (4 inches) of lead. Spacecraft flying through the belts must be protected; otherwise, their electronic components would be subjected to irreparable damages.

      The high-energy protons in the inner Van Allen belt are thought to originate from the decay of neutrons (neutron) that are produced by the interaction of the atmosphere with energetic cosmic rays of galactic origin. Some of these short-lived neutrons—they have a lifetime of 12 minutes—are ejected upward. A fraction of them decay into energetic protons and electrons as they pass through the region occupied by the Van Allen belts. These protons and electrons become trapped and travel in spiral paths along the flux lines of Earth's magnetic field. The particles reverse their direction at intermediate altitudes (about 500 km [300 miles]) and low latitudes because, as the particles approach either of the magnetic poles, the increase in the strength of the field causes them to be reflected back toward the other pole. Collisions with atoms in the thin atmosphere eventually remove the particles from the belts, but they generally survive for about 10 years. This relatively long lifetime allows particles to accumulate in the radiation belts, providing high fluxes despite the small magnitude of the intrinsic source.

      The inner belt merges gradually with the outer belt, which extends from about two to eight Earth radii. A portion of the ionization in the outer belt is derived from the solar wind, as demonstrated by the presence of helium ions in addition to protons. Unlike the outer zone, the inner belt contains no helium ions, while it has been established that helium ions account for about 10 percent of solar wind. The flux of electrons in the outer belt can vary by orders of magnitude over intervals as short as a few days. These changes appear to correlate with times of strong magnetic disturbances. They are not, however, as yet well understood.

Michael B. McElroy

Additional Reading

Ionosphere

A general review of the underlying physics of the upper atmosphere may be found in J.A. Ratcliffe (ed.), Physics of the Upper Atmosphere (1960); and C.O. Hines et al. (eds.), Physics of the Earth's Upper Atmosphere (1965). A modern account of aeronomy is given by P.M. Banks and G. Kockarts, Aeronomy, 2 vol. (1973).

Magnetosphere and aurora

A classic treatment of the aurora and airglow is presented by Joseph W. Chamberlain, Physics of the Aurora and Airglow (1961). A specialized account of magnetospheric processes may be found in Wilmot N. Hess, The Radiation Belt and Magnetosphere (1968). J.G. Ruederer presents a readable summary article, “The Particle and Field Environment of the Earth,” Astronautics and Aeronautics vol. 7, pp. 22–28 (Jan. 1969).Michael B. McElroy

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