CHAPTER II: Descriptions of Nuclear Explosions

Scientific Aspects of Nuclear Explosion Phenomena[7]

INTRODUCTION

2.106 The events which follow the very large and extremely rapid energy release in a nuclear explosion are mainly the consequences of the interaction of the kinetic energy of the fission fragments and the thermal radiations with the medium surrounding the explosion. The exact nature of these interactions, and hence the directly observable and indirect effects they produce, that is to say, the nuclear explosion phenomena, are dependent on such properties of the medium as its temperature, pressure, density, and composition. It is the variations in these factors in the environment of the nuclear detonation that account for the different types of response associated with air, high-altitude, surface, and subsurface bursts, as described earlier in this chapter.

2.107 Immediately after the explosion time, the temperature of the weapon material is several tens of million degrees and the pressures are estimated to be many million atmospheres. As a result of numerous inelastic collisions, part of the kinetic energy of the fission fragments is converted into internal and radiation energy. Some of the electrons are removed entirely from the atoms, thus causing ionization, others are raised to higher energy (or excited) states while still remaining attached to the nuclei. Within an extremely short time, perhaps a hundredth of a microsecond or so, the weapon residues consist essentially of completely and partially stripped (ionized) atoms, many of the latter being in excited states, together with the corresponding free electrons. The system then immediately emits electromagnetic (thermal) radiation, the nature of which is determined by the temperature. Since this is of the order of several times 107 degrees, most of the energy emitted within a microsecond or so is in the soft X-ray region ( 1.77, see also 7.75).

2.108 The primary thermal radiation leaving the exploding weapon is absorbed by the atoms and molecules of the surrounding medium. The medium is thus heated and the resulting fireball re-radiates part of its energy as the secondary thermal radiation of longer wavelengths ( 2.38). The remainder of the energy contributes to the shock wave formed in the surrounding medium. Ultimately, essentially all the thermal radiation (and shock wave energy) is absorbed and appears as heat, although it may be spread over a large volume. In a dense medium such as earth or water, the degradation and absorption occur within a short distance from the explosion, but in air both the shock wave and the thermal radiation may travel considerable distances. The actual behavior depends on the air density, as will be seen later.

2.109 It is apparent that the kinetic energy of the fission fragments, constituting some 85 percent of the total energy released, will distribute itself between thermal radiation, on the one hand, and shock and blast, on the other hand, in proportions determined largely by the nature of the ambient medium. The higher the density of the latter, the greater the extent of the coupling between it and the energy from the exploding nuclear weapon. Consequently, when a burst takes place in a medium of high density, e.g., water or earth, a larger percentage of the kinetic energy of the fission fragments is converted into shock and blast energy than is the case in a less dense medium, e.g., air. At very high altitudes, on the other hand, where the air pressure is extremely low, there is no true fireball and the kinetic energy of the fission fragments is dissipated over a very large volume. In any event, the form and amount in which the thermal radiation is received at a distance from the explosion will depend on the nature of the intervening medium.

DEVELOPMENT OF THE FIREBALL IN AN AIR BURST

2.110 As seen above, most of the initial (or primary) thermal radiation from a nuclear explosion is in the soft X-ray region of the spectrum. If the burst occurs in the lower part of the atmosphere where the air density is appreciable, the X rays are absorbed in the immediate vicinity of the burst, and they heat the air to high temperatures. This sphere of hot air is sometimes referred to as the "X-ray fireball." The volume of air involved, resultant air temperatures and ensuing behavior of this fireball are all determined by the burst conditions. At moderate and low altitudes (below about 100,000 feet), the X rays are absorbed within some yards of the burst point, and the relatively small volume of air involved is heated to a very high temperature.

2.111 The energies (or wavelengths) of the X rays, as determined by the temperature of the weapon debris, cover a wide range ( 7.73 et seq.), and a small proportion of the photons ( 1.74) have energies considerably in excess of the average. These high-energy photons are not easily absorbed and so they move ahead of the fireball front. As a result of interaction with the atmospheric molecules, the X rays so alter the chemistry and radiation absorption properties of the air that, in the air burst at low and moderate altitudes, a veil of opaque air is generated that obscures the early growth of the fireball. Several microseconds elapse before the fireball front emerges from the opaque X-ray veil.

2.112 The X-ray fireball grows in size as a result of the transfer of radiation from the very hot interior where the explosion has occurred to the cooler exterior. During this "radiative growth" phase, most of the energy transfer in the hot gas takes place in the following manner. First, an atom, molecule, ion, or electron absorbs a photon of radiation and is thereby converted into an excited state. The atom or other particle remains in this state for a short time and then emits a photon, usually of lower energy. The residual energy is retained by the particle either as kinetic energy or as internal energy. The emitted photon moves off in a random direction with the velocity of light, and it may then be absorbed once again to form another excited particle. The latter will then re-emit a photon, and so on. The radiation energy is thus transmitted from one point to another within the gas; at the same time, the average photon energy (and radiation frequency) decreases. The energy lost by the photons serves largely to heat the gas through which the photons travel.

2.113 If the mean free path of the radiation, i.e., the average distance a photon travels between interactions, is large in comparison with the dimensions of the gaseous volume, the transfer of energy from the hot interior to the cooler exterior of the fireball will occur more rapidly than if the mean free path is short. This is because, in their outward motion through the gas, the photons with short mean free paths will be absorbed and re-emitted several times. At each re-emission the photon moves away in a random direction, and so the effective rate of transfer of energy in the outward direction will be less than for a photon of long mean free path which undergoes little or no absorption and re-emission in the hot gas.

2.114 In the radiative growth phase, the photon mean free paths in the hot fireball are of the order of (or longer than) the fireball diameter because at the very high temperatures the photons are not readily absorbed. As a result, the energy distribution and temperature are fairly uniform throughout the volume of hot gas. The fireball at this stage is consequently referred to as the "isothermal sphere." The name is something of a misnomer, since temperature gradients do exist, particularly near the advancing radiation front.

2.115 As the fireball cools, the transfer of energy by radiation and radiative growth become less rapid because of the decreasing mean free path of the photons. When the average temperature of the isothermal sphere has dropped to about 300,0000C, the expansion velocity will have decreased to a value comparable to the local acoustic (sound) velocity. At this point, a shock wave develops at the fireball front and the subsequent growth of the fireball is dominated by the shock and associated hydrodynamic expansion. The phenomenon of shock formation is sometimes called "hydrodynamic separation." For a 20-kiloton burst it occurs at about a tenth of a millisecond after the explosion when the fireball radius is roughly 40 feet.

2.116 At very early times, beginning in less than a microsecond, an "inner" shock wave forms driven by the expanding bomb debris. This shock expands outward within the isothermal sphere at a velocity exceeding the local acoustic velocity. The inner shock overtakes and merges with the outer shock at the fireball front shortly after hydrodynamic separation. The relative importance of the debris shock wave depends on the ratio of the yield to the mass of the exploding device and on the altitude of the explosion ( 2.136). The debris shock front is a strong source of ultraviolet radiation, and for weapons of small yield-to-mass ratio it may replace the X-ray fireball as the dominant energy source for the radiative growth.

2.117 As the (combined) shock front from a normal air burst moves ahead of the isothermal sphere it causes a tremendous compression of the ambient air and the temperature is thereby increased to an extent sufficient to render the air incandescent. The luminous shell thus formed constitutes the advancing visible fireball during this "hydrodynamic phase" of fireball growth. The fireball now consists of two concentric regions. The inner (hotter) region is the isothermal sphere of uniform temperature, and it is surrounded by a layer of luminous, shock-heated air at a somewhat lower, but still high, temperature. Because hot (over 8,0000C) air is effectively opaque to visible radiation, the isothermal sphere is not visible through the outer shocked air.

2.118 Some of the phenomena described above are represented schematically in Fig. 2.118; qualitative temperature profiles are shown at the left and pressure profiles at the right of a series of photographs of the fireball at various intervals after the detonation of a 20-kiloton weapon. In the first picture, at 0.1 millisecond, the temperature is shown to be uniform within the fireball and to drop abruptly at the exterior, so that the condition is that of the isothermal sphere. Subsequently, as the shock front begins to move ahead of the isothermal sphere, the temperature is no longer uniform, as indicated by the more gradual fall near the outside of the fireball. Eventually, two separate temperature regions form. The outer region absorbs the radiation from the isothermal sphere in the center and so the latter cannot be seen. The photographs, therefore, show only the exterior surface of the fireball.

Temperature and pressure in the fireball

Figure 2.118. Variations of temperature and pressure in the fireball. (Times and dimensions apply to a 20-kiloton air burst.

2.119 From the shapes of the curves at the right of Fig. 2.118 the nature of the pressure changes in the fireball can be understood. In the isothermal stage the pressure is uniform throughout and drops sharply at the outside, but after a short time, when the shock front has separated from the isothermal sphere, the pressure near the surface is greater than in the interior of the fireball. Within less than 1 millisecond the steep-fronted shock wave has traveled some distance ahead of the isothermal region. The rise of the pressure in the fireball to a peak, which is characteristic of a shock wave, followed by a sharp drop at the external surface, implies that the latter is identical with the shock front. It will be noted, incidentally, from the photographs, that the surface of the fireball, which has hitherto been somewhat uneven, has now become sharply defined.

2.120 For some time the fireball continues to grow in size at a rate determined by the propagation of the shock front in the surrounding air. During this period the temperature of the shocked air decreases steadily so that it becomes less opaque. Eventually, it is transparent enough to permit the much hotter and still incandescent interior of the fireball, i.e., the isothermal sphere, to be seen through the faintly visible shock front (see Fig. 2.32). The onset of this condition at about 15 milliseconds (0.015 second) after the detonation of a 20-kiloton weapon, for example, is referred to as the "breakaway."

2.121. Following the breakaway, the visible fireball continues to increase in size at a slower rate than before, the maximum dimensions being attained after about a second or so. The manner in which the radius increases with time, in the period from roughly 0.1 millisecond to 1 second after the detonation of a 20-kiloton nuclear weapon, is shown in Figure 2.121. Attention should be called to the fact that both scales are logarithmic, so that the lower portion of the curve (at the left) does not represent a constant rate of growth, but rather one that falls off with time. Nevertheless, the marked decrease in the rate at which the fireball grows after breakaway is apparent from the subsequent flattening of the curve.

Fireball Radius Graph

Figure 2.121. Variation of radius of luminous fireball with time in a 20-kiloton air burst.

TEMPERATURE OF THE FIREBALL

2.122 As indicated earlier, the interior temperature of the fireball decreases steadily, but the apparent surface temperature, which influences the emission of thermal radiation, decreases to a minimum and then increases to a maximum before the final steady decline. This behavior is related to the fact that at high temperatures air both absorbs and emits thermal radiation very readily, but as the temperature falls below a few thousand degrees, the ability to absorb and radiate decreases.

2.123 From about the time the fireball temperature has fallen to 300,0000C, when the shock front begins to move ahead of the isothermal sphere, until close to the time of the first temperature minimum ( 2.38), the expansion of the fireball is governed by the laws of hydrodynamics. It is then possible to calculate the temperature of the shocked air from the measured shock velocity, i.e., the rate of growth of the fireball. The variation of the temperature of the shock front with time, obtained in this manner, is shown by the full line from l0-4 to 10-2 second in Fig. 2.123, for a 20-kiloton explosion. But photographic and spectroscopic observations of the surface brightness of the advancing shock front, made from a distance, indicate the much lower temperatures represented by the broken curve in the figure. The reason for this discrepancy is that both the nuclear and thermal radiations emitted in the earliest stages of the detonation interact in depth with the gases of the atmosphere ahead of the shock front to produce ozone, nitrogen dioxide, nitrous acid, etc. These substances are strong absorbers of radiation coming from the fireball, so that the brightness observed some distance away corresponds to a temperature considerably lower than that of the shock front.

Fireball Surface Temperature

Figure 2.123. Variations of apparent fireball surface temperature with time in a 20-kiloton air burst.

2.124 Provided the temperature of the air at the shock front is sufficiently high, the isothermal sphere is invisible ( 2.117). The rate at which the shock front emits (and absorbs) radiation is determined by its temperature and radius. The temperature at this time is considerably lower than that of the isothermal sphere but the radius is larger. However, as the temperature of the shocked air approaches 3,0000C (5,4000F) it absorbs (and radiates) less readily. Thus the shock front becomes increasingly transparent to the radiation from the isothermal sphere and there is a gradual unmasking of the still hot iso-thermal sphere, representing breakaway ( 2.120).

2.125 As a result of this unmasking of the isothermal sphere, the apparent surface temperature (or brightness) of the fireball increases (Fig. 2.123), after passing through the temperature mini-mum of about 3,0000C attributed to the shock front. This minimum, representing the end of the first thermal pulse, occurs at about 11 milliseconds (0.011 second) after the explosion time for a 20-kiloton weapon. Subsequently, as the brightness continues to increase from the minimum, radiation from the fireball is emitted directly from the hot interior (or isothermal sphere), largely unimpeded by the cooled air in the shock wave ahead of it; energy is then radiated more rapidly than before. The apparent surface temperature increases to a maximum of about 7,7000C (14,0000F), and this is followed by a steady decrease over a period of seconds as the fireball cools by the emission of radiation and mixing with air. It is during the second pulse that the major part of the thermal radiation is emitted in an air burst ( 2.38 et seq.). In such a burst, the rate of emission of radiation is greatest when the surface temperature is at the maximum.

2.126 The curves in Figs. 2.121 and 2.123 apply to a 20-kiloton nuclear burst, but similar results are obtained for explosions of other energy yields. The minimum temperature of the radiating surface and the subsequent temperature maximum are essentially independent of the yield of the explosion. But the times at which these temperatures occur for an air burst increase approximately as the 0.4 power of the yield (Chapter VII). The time of breakaway is generally very soon after the thermal minimum is attained.

 

SIZE OF THE FIREBALL

2.127 The size of the fireball increases with the energy yield of the explosion. Because of the complex interaction of hydrodynamic and radiation factors, the radius of the fireball at the thermal minimum is not very different for air and surface bursts of the same yield. The relationship between the average radius and the yield is then given approximately by

R (at thermal minimum) 90 W0.4

where R is the fireball radius in feet and W is the explosion yield in kilotons TNT equivalent. The breakaway phenomenon, on the other hand, is determined almost entirely by hydrodynamic considerations, so that a distinction should be made between air and surface bursts. For an air burst the radius of the fireball is given by

R (at breakaway) for air burst 110 W0.4, (2.127.1)

For a contact surface burst, i.e., in which the exploding weapon is actually on the surface,[8] blast wave energy is reflected back from the surface into the fireball ( 3.34) and W in equation (2.127.1) should probably be replaced by 2 W, where W is the actual yield. Hence, for a contact surface burst,

R (at breakaway) for contact surface burst 145 W0.4. (2.127.2)

For surface bursts in the transition range between air bursts and contact bursts, the radius of the fireball at breakaway is somewhere between the values given by equations (2.127.1) and (2.127.2). The size of the fireball is not well defined in its later stages, but as a rough approximation the maximum radius may be taken to be about twice that at the time of breakaway (cf. Fig. 2.121).

2.128 Related to the fireball size is the question of the height of burst at which early (or local) fallout ceases to be a serious problem. As a guide, it may be stated that this is very roughly related to the weapon yield by

H (maximum for local fallout) 180 W0.4 (2.128.1)

where H feet is the maximum value of the height of burst for which there will be appreciable local fallout. This expression is plotted in Fig. 2.128. For an explosion of 1,000 kilotons, i.e., 1 megaton yield, it can be found from Fig. 2.128 or equation (2.128.1) that significant local fallout is probable for heights of burst less than about 2,900 feet. It should be emphasized that the heights of burst estimated in this manner are approximations only, with probable errors of +30 percent. Furthermore, it must not be assumed that if the burst height exceeds the value given by equation (2.128.1) there will definitely be no local fallout. The amount, if any, maybe expected, however, to be small enough to be tolerable under emergency conditions.

Maximum height of burst for fallout

Figure 2.128. Approximate maximum height of burst for appreciable local fallout.

2.129 Other aspects of fireball size are determined by the conditions under which the fireball rises. If the fireball is small compared with an atmospheric scale height, which is about 4.3 miles at altitudes of interest ( 10.123), the late fireball rise is caused by buoyant forces similar to those acting on a bubble rising in shallow water. This is called "buoyant" rise. The fireball is then essentially in pressure equilibrium with the surrounding air as it rises. If the initial fireball radius is comparable to or greater than a scale height, the atmospheric pressure on the bottom of the fireball is much larger than the pressure on the top. This causes a very rapid acceleration of the fireball, referred to as "ballistic" rise. The rise velocity becomes so great compared to the expansion rate that the fireball ascends almost like a solid projectile. "Overshoot" then occurs, in which a parcel of dense air is carried to high altitudes where the ambient air has a lower density. The dense "bubble" will subsequently expand, thereby decreasing its density, and will fall back until it is in a region of comparable density.

HIGH-ALTITUDE BURSTS

2.130 For nuclear detonations at heights up to about 100,000 feet (19 miles), the distribution of explosion energy between thermal radiation and blast varies only to a small extent with yield and detonation altitude ( 1.24). But at burst altitudes above 100,000 feet, the distribution begins to change more noticeably with increasing height of burst (see Chapter VII). It is for this reason that the level of 100,000 feet has been chosen for distinguishing between air bursts and high-altitude bursts. There is, of course, no sharp change in behavior at this elevation, and so the definition of a high-altitude burst as being at a height above 100,000 feet is somewhat arbitrary. There is a progressive decline in the blast energy with increasing height of burst above 100,000 feet, but the proportion of the explosion energy received as effective thermal radiation on the ground at first increases only slightly with altitude. Subsequently, as the burst altitude increases, the effective thermal radiation received on the ground decreases and becomes less than at an equal distance from an air burst of the same total yield ( 7.102).

2.131 For nuclear explosions at altitudes between 100,000 and about 270,000 feet (51 miles) the fireball phenomena are affected by the low density of the air. The probability of interaction of the primary thermal radiation, i.e., the thermal X rays, with atoms and molecules in the air is markedly decreased, so that the photons have long mean free paths and travel greater distances, on the average, before they are absorbed or degraded into heat and into radiations of longer wavelength (smaller photon energy). The volume of the atmosphere in which the energy of the radiation is deposited, over a period of a millisecond or so, may extend for several miles, the dimensions increasing with the burst altitude. The interaction of the air molecules with the prompt gamma rays, neutrons, and high-energy component of the X rays produces a strong flash of fluorescence radiation ( 2.140), but there is less tendency for the X-ray veil to form than in an air burst ( 2.111).

2.132 Because the primary thermal radiation energy in a high-altitude burst is deposited in a much larger volume of air, the energy per unit volume available for the development of the shock front is less than in an air burst. The outer shock wave ( 2.116) is slow to form and radiative expansion predominates in the growth of the fireball. The air at the shock front does not become hot enough to be opaque at times sufficiently early to mask the radiation front and the fireball radiates most of its energy very rapidly. There is no apparent temperature minimum as is the case for an air burst. Thus, with increasing height, a series of changes take place in the thermal pulse phenomena; the surface temperature minimum becomes less pronounced and eventually disappears, so that the thermal radiation is emitted in a single pulse of fairly short duration. In the absence of the obscuring opaque shock front, the fireball surface is visible throughout the period of radiative growth and the temperature is higher than for a low-altitude fireball. Both of these effects contribute to the increase in the thermal radiation emission.

2.133 A qualitative comparison of the rate of arrival of thermal radiation energy at a distance from the burst point as a function of time for a megaton-range explosion at high altitude and in a sea-level atmosphere is shown in Fig. 2.133. In a low (or moderately low) air burst, the thermal radiation is emitted in two pulses, but in a high-altitude burst there is only a single pulse in which most of the radiation is emitted in a relatively short time. Furthermore, the thermal pulse from a high-altitude explosion is richer in ultraviolet radiation than is the main (second) pulse from an air burst. The reason is that formation of ozone, oxides of nitrogen, and nitrous acid ( 2.123), which absorb strongly in this spectral region, is decreased.

Thermal Radiation Graph

Figure 2.133. Qualitative comparison of rates of arrival of thermal radiation at a given distance from high-altitude and sea-level bursts.

2.134 For burst altitudes above about 270,000 feet, there is virtually no absorption of the X rays emitted in upward directions. The downward directed X rays are mostly absorbed in a layer of air, called the "X-ray pancake," which becomes incandescent as a result of energy deposition. The so-called pancake is more like the frustum of a cone, pointing upward, with a thickness of roughly 30,000 feet (or more) and a mean altitude of around 270,000 feet; the radius at this altitude is approximately equal to the height of burst minus 270,000 feet. The height and dimensions of the pancake are determined largely by the emission temperature for the primary X rays, which depends on the weapon yield and design, but the values given here are regarded as being reasonable averages. Because of the very large volume and mass of air in the X-ray pancake, the temperatures reached in the layer are much lower than those in the fireballs from bursts in the normal atmosphere. Various excited atoms and ions are formed and the radiations of lower energy (longer wavelength) re-emitted by these species represent the thermal radiation observed at a distance.

2.135 For heights of burst up to about 270,000 feet, the early fireball is approximately spherical, although at the higher altitudes it begins to elongate vertically. The weapon debris and the incandescent air heated by the X rays roughly coincide. Above 270,000 feet, however, the debris tends to be separate from the X-ray pancake. The debris can rise to great altitudes, depending on the explosion yield and the burst height; its behavior and ionization effects are described in detail in Chapter X. The incandescent (X-ray pancake) region, on the other hand, remains at an essentially constant altitude regardless of the height of burst. From this region the thermal radiation is emitted as a single pulse containing a substantially smaller proportion of the total explosion energy but of somewhat longer duration than for detonations below roughly 270,000 feet (see 7.89 et seq.).

2.136 Although the energy density in the atmosphere as the result of a high-altitude burst is small compared with that from an air burst of the same yield, a shock wave is ultimately produced by the weapon debris ( 2.116), at least for bursts up to about 400,000 feet (75 miles) altitude. For example, disturbance of the ionosphere in the vicinity of Hawaii after the TEAK shot (at 252,000 feet altitude) indicated that a shock wave was being propagated at that time at an average speed of about 4,200 feet per second. The formation of the large red, luminous sphere, several hundred miles in diameter, surrounding the fireball, has been attributed to the electronic excitation of oxygen atoms by the energy of the shock wave. Soon after excitation, the excess energy was emitted as visible radiation toward the red end of the spectrum (6,300 and 6,364 A).

2.137 For bursts above about 400,000 feet, the earth's magnetic field plays an increasingly important role in controlling weapon debris motion, and it becomes the dominant factor for explosions above 200 miles or so (Chapter X). At these altitudes, the shock waves are probably magnetohydrodynamic (rather than purely hydrodynamic) in character. The amount of primary thermal radiation produced by these shock waves is quite small.

AIR FLUORESCENCE PHENOMENA

2.138 Various transient fluorescent effects, that is, the emission of visible and ultraviolet radiations for very short periods of time, accompany nuclear explosions in the atmosphere and at high altitudes. These effects arise from electronic excitation (and ionization) of atoms and molecules in the air resulting from interactions with high-energy X rays from the fireball, or with gamma rays, neutrons, beta particles, or other charged particles of sufficient energy. The excess energy of the excited atoms, molecules, and ions is then rapidly emitted as fluorescence radiation.

2.139 In a conventional air burst, i.e., at an altitude below about 100,000 feet, the first brief fluorescence that can be detected, within a microsecond or so of the explosion time, is called the "Teller light." The excited particles are produced initially by the prompt (or instantaneous) gamma rays that accompany the fission process and in the later stages by the interaction of fast neutrons with nuclei in the air ( 8.53).

2.140 For bursts above 100,000 feet, the gamma rays and neutrons tend to be absorbed, with an emission of fluorescence, in a region at an altitude of about 15 miles (80,000 feet), since at higher altitudes the mean free paths in the low-density air are too long for appreciable local absorption ( 10.29). The fluorescence is emitted over a relatively long period of time because of time-of-flight delays resulting from the distances traveled by the photons and neutrons before they are absorbed. An appreciable fraction of the high-energy X rays escaping from the explosion region are deposited outside the fireball and also produce fluorescence. The relative importance of the X-ray fluorescence increases with the altitude of the burst point.

2.141 High-energy beta particles associated with bursts at sufficiently high altitudes can also cause air fluorescence. For explosions above about 40 miles, the beta particles emitted by the weapon residues in the downward direction are absorbed in the air roughly at this altitude, their outward spread being restricted by the geomagnetic field lines ( 10.63 et seq.). A region of air fluorescence, called a "beta patch," may then be formed. If the burst is at a sufficiently high altitude, the weapon debris ions can themselves produce fluorescence. A fraction of these ions can be channeled by the geomagnetic field to an altitude of about 70 miles where they are stopped by the atmosphere ( 10.29) and cause the air to fluoresce. Under suitable conditions, as will be explained below, fluorescence due to beta particles and debris ions can also appear in the atmosphere in the opposite hemisphere of earth to the one in which the nuclear explosion occurred.

 

AURORAL PHENOMENA

2.142 The auroral phenomena associated with high-altitude explosions ( 2.62) are caused by the beta particles emitted by the radioactive weapon residues and, to a varying extent, by the debris ions. Interaction of these charged particles with the atmosphere produces excited molecules, atoms, and ions which emit their excess energy in the form of visible radiations characteristic of natural auroras. In this respect, there is a resemblance to the production of the air fluorescence described above. However, auroras are produced by charged particles of lower energy and they persist for a much longer time, namely, several minutes compared with fractions of a second for air fluorescence. Furthermore, the radiations have somewhat different wavelength characteristics since they are emitted, as a general rule, by a different distribution of excited species.

2.143 The geomagnetic field exerts forces on charged particles, i.e., beta particles (electrons) and debris ions, so that these particles are constrained to travel in helical (spiral) paths along the field lines. Since the earth behaves like a magnetic dipole, and has north and south poles, the field lines reach the earth at two points, called "conjugate points," one north of the magnetic equator and the other south of it. Hence, the charged particles spiraling about the geomagnetic field lines will enter the atmosphere in corresponding conjugate regions. It is in these regions that the auroras may be expected to form (Fig. 2.143).

Phenomena associated with high altitude
explosions

Figure 2.143. Phenomena associated with high altitude explosions.

2.144 For the high-altitude tests conducted in 1958 and 1962 in the vicinity of Johnston Island ( 2.52), the charged particles entered the atmosphere in the northern hemisphere between Johnston Island and the main Hawaiian Islands, whereas the conjugate region in the southern hemisphere region was in the vicinity of the Samoan, Fiji, and Tonga Islands. It is in these areas that auroras were actually observed, in addition to those in the areas of the nuclear explosions.

2.145 Because the beta particles have high velocities, the beta auroras in the remote (southern) hemisphere appeared within a fraction of a second of those in the hemisphere where the bursts had occurred. The debris ions, however, travel more slowly and so the debris aurora in the remote hemisphere, if it is formed, appears at a somewhat later time. The beta auroras are generally most intense at an altitude of 30 to 60 miles, whereas the intensity of the debris auroras is greatest in the 60 to 125 miles range. Remote conjugate beta auroras can occur if the detonation is above 25 miles, whereas debris auroras appear only if the detonation altitude is in excess of some 200 miles.

THE ARGUS EFFECT

2.146 For bursts at sufficiently high altitudes, the debris ions, moving along the earth's magnetic field lines, are mostly brought to rest at altitudes of about 70 miles near the conjugate points. There they continue to decay and so act as a stationary source of beta particles which spiral about the geomagnetic lines of force. When the particles enter a region where the strength of the earth's magnetic field increases significantly, as it does in the vicinity of the conjugate points, some of the beta particles are turned back (or reflected). Consequently, they may travel back and forth, from one conjugate region to the other, a number of times before they are eventually captured in the atmosphere. (More will be said in Chapter X about the interactions of the geomagnetic field with the charged particles and radiations produced by a nuclear explosion.)

2.147 In addition to the motion of the charged particles along the field lines, there is a tendency for them to move across the lines wherever the magnetic field strength is not uniform. This results in an eastward (longitudinal) drift around the earth superimposed on the back-and-forth spiral motion between regions near the conjugate points. Within a few hours after a high-altitude nuclear detonation, the beta particles form a shell completely around the earth. In the ARGUS experiment ( 2.53), in which the bursts occurred at altitudes of 125 to 300 miles, well-defined shells of about 60 miles thickness, with measurable electron densities, were established and remained for several days. This has become known as the "ARGUS effect." Similar phenomena were observed after the STARFISH PRIME ( 2.52) and other high-altitude nuclear explosions.

EFFECT ON THE OZONE LAYER

2.148 Ozone (O3) is formed in the upper atmosphere, mainly in the stratosphere (see Fig. 9.126) in the altitude range of approximately 50,000 to 100,000 feet (roughly 10 to 20 miles), by the action of solar radiation on molecular oxygen (O2). The accumulation of ozone is limited by its decomposition, partly by the absorption of solar ultraviolet radiation in the wavelength range from about 2,100 to 3,000 A and partly by chemical reaction with traces of nitrogen oxides (and other chemical species) present in the atmosphere. The chemical decomposition occurs by way of a complex series of chain reactions whereby small quantities of nitrogen oxides can cause considerable breakdown of the ozone. The equilibrium (or steady-state) concentration of ozone at any time represents a balance between the rates of formation and decomposition; hence, it is significantly dependent on the amount of nitrogen oxides present. Solar radiation is, of course, another determining factor; the normal concentration of ozone varies, consequently, with the latitude, season of the year, time of day, the stage in the solar (sunspot) cycle, and perhaps with other factors not yet defined.

2.149 Although the equilibrium amount in the atmosphere is small, rarely exceeding 10 parts by weight per million parts of air, ozone has an important bearing on life on earth. If it were not for the absorption of much of the solar ultraviolet radiation by the ozone, life as currently known could not exist except possibly in the ocean. A significant reduction in the ozone concentration, e.g., as a result of an increase in the amount of nitrogen oxides, would be expected to cause an increased incidence of skin cancer and to have adverse effects on plant and animal life..

2.150 As seen in 2.08 and 2.123, nuclear explosions are accompanied by the formation of oxides of nitrogen. An air burst, for example, is estimated to produce about 1032 molecules of nitrogen oxides per megaton TNT equivalent. For nuclear explosions of intermediate and moderately high yield in the air or near the surface, the cloud reaches into the altitude range of 50,000 to 100,000 feet (Fig. 2.16); hence, the nitrogen oxides from such explosions would be expected to enhance mechanisms which tend to decrease the ozone concentration. Routine monitoring of the atmosphere during and following periods of major nuclear testing have shown no significant change in the ozone concentration in the sense of marked, long-lasting perturbations. However, the large natural variations in the ozone layer and uncertainties in the measurements do not allow an unambiguous conclusion to be reached. Theoretical calculations indicate that extensive use of nuclear weapons in warfare could cause a substantial decrease in the atmospheric ozone concentration, accompanied by an increase in adverse biological effects due to ultraviolet radiation. The ozone layer should eventually recover, but this might take up to 25 years.

 

FOOTNOTES

  1. The remaining (more technical ) sections of this chapter may be ommitted without loss of continuity.
  2. For most purposes, a contact surface burst may he defined as one for which the burst point is not more than 5 W0.3 feet above or below the surface.

 

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