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1 2 INTRODUCTION Electrical Structure of the Middle Atmosphere GEORGE C. REID .. NOAA Aeronomy Laboratory Conventional usage divides the atmosphere into lay- ers on the basis of the average temperature profile. The stratosphere is the region of positive vertical tempera- ture gradient extending from the tropopause to a height of about 50 km, and the overlying region of negative temperature gradient is the mesosphere, extending to about 85 km altitude, where the lowest temperatures in the atmosphere are reached. The main heat source in both of these regions is provided by absorption of solar- ultraviolet radiation by ozone. At still greater heights lies the thermosphere, in which absorption of extreme- ultraviolet radiation causes the temperature to increase again with height. This chapter is concerned mainly with the electrical properties of the upper stratosphere and the mesosphere. The troposphere and lower strato- sphere were considered in Chapters 11 and 12 (this vol- ume) and the thermosphere is discussed in Chapter 14. Both the composition and the temperature play im- portant roles in determining the electrical structure. As noted above, the temperature in the stratosphere rises from typical tropopause values near 200 K to values of about 270 K at the stratopause (~50 km), above which the temperature decreases to mesopause values that are seasonally and latitudinally variable, occasionally drop- ping below 140 K in the high-latitude summer. ~3 The principal atmospheric constituents are molecular oxygen and nitrogen, just as in the lower atmosphere; but there are a number of minor constituents that are important from the point of view of the electrical prop- erties. Among these are nitric oxide (NO), which dif- fuses into the region from sources below and above; atomic oxygen (O) and ozone (03), which are formed locally by photodissociation of 02; and water vapor, which can be transported from the troposphere as well as being locally produced. The role of aerosols in the atmosphere at heights above 30 km is uncertain and con- troversial and is an area of active study. The occasional presence of noctilucent clouds at the high-latitude sum- mer mesopause and the more regular existence of a sum- mertime polar scattering layer seen by satellites have certainly shown that aerosols (probably ice crystals) can exist near the top of the region, but the gap between the mesopause and the well-known aerosol layers of the lower stratosphere remains relatively unexplored. In what follows, sources of ionization, the ion chemis- try that determines the steady-state ion composition, and the present status of our knowledge of aerosol distri- bution are discussed. The final two sections discuss the theory and measurement of conductivity and electric fields in the middle atmosphere.

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184 SOURCES OF IONIZATION Figure 13.1 shows typical ion-pair production rates (q) at middle latitudes during daytime. Throughout the stratosphere, galactic cosmic rays provide the principal ionization source, as in most of the lower atmosphere. The cosmic-ray ionization rate does not vary diurnally but does vary with geomagnetic latitude and with the phase of the 11-year solar cycle. Heaps (1978) provided useful relations for computing the rate of ion production at any latitude and time. Roughly, the ion-production rate above 30 km increases by a factor of 10 in going from the geomagnetic equator to the polar caps at sun- spot minimum (cosmic-ray maximum) and by a factor of 5 at sunspot maximum. The solar-cycle modulation is near zero at the equator, increasing to a factor of about 2 in the polar caps. The ionization rate above 30 km is approximately proportional to the atmospheric density. These properties are a result of (a) the shielding effect of the geomagnetic field, which allows cosmic-ray parti- cles to enter the atmosphere at successively higher lati- tudes for successively lower energies, and (b) the reduc- tion in cosmic-ray flux in the inner solar system as solar activity intensifies. Superimposed on these long-term global variations are brief reductions in cosmic-ray flux known as For- bush decreases, after their discoverer (Forbush, 1938~. Forbush decreases occur in coincidence with geomag- netic storms and are of brief (hours) duration. However, as their magnitude can be as large as some tens of per- cent, they can change global electrical parameters sig- nificantly. In the mesosphere the major daytime source of ioniza- tion in undisturbed conditions is provided by the NO 90 8C 7C 60 - ,c~ I 50 4O 30 _ toys ~I ~I a,+ UV OR ~NO + Lyman - ~ Middle - Atmosphere ionization Sources X=45 1 1~111111 1 1 1 - - 20 1 11 1 1lil1 1 10 1 10 101 1o2 ( -3 -1) SPE / 12 Me 111111 1 1 1 11111 103 104 FIGURE 13.1 Typical ion-pair production rates in the middle atmo- sphere. GEORGE C. REID molecule, whose low ionization potential of 9.25 elec- tron volts (eV) allows it to be ionized by the intense solar Lyman-alpha radiation. The concentration of NO in the mesosphere is not well known and is almost certainly variable (Solomon et al., 1982a) in response to meteoro- logical factors. The production-rate profile in Figure 13.1 is an estimate based on reasonable values for the NO concentration and the solar Lyman-alpha flux, which is itself a function of solar activity (Cook et al., 1980~. At the upper limit of the middle atmosphere, signifi- cant amounts of ionization are produced by solar x rays, forming the base of the E region of the ionosphere, and by ionization of O2 in its metastable Id\ state, which is a by-product of ozone photodissociation. While these sources are never competitive with the NO source in terms of ionization rates, they give rise to different pri- mary positive-ion species (N2 and O2 as opposed to NO + ), and hence to different chemical reaction chains. A sporadic and intense source of ionization at high latitudes is provided by solar-proton events (SPE) (Reid, 1974), and Figure 13.1 shows an ionization-rate profile calculated for the peak of a major SPE in May 1959. These events are caused by the entry into the atmo- sphere of particles accelerated during solar flares and traveling fairly directly from the Sun to the Earth. The particles are mostly protons, with much smaller fluxes of heavier nuclei and of electrons, having typical energies of 1 to 100 MeV and considerably less atmospheric pene- tration power than galactic cosmic rays. As a conse- quence, their effects are largely confined to high mag- netic latitudes ~-60) and to altitudes well above the lower stratosphere. Solar-proton events typically reach their peak intensity within a few hours of a major solar flare and then decay exponentially over the following day or two. Their occurrence is a strong function of the phase of the solar cycle, as illustrated in Figure 13.2, which shows the distribution in the 1956-1973 period of polar-cap absorption (PCA) events and of ground-level events (Pomerantz and Duggal, 1974~. Polar-cap ab- sorption is the name given to the intense radio-wave ab- sorption caused by the enhanced mesospheric ionization during an SPE, while ground-level events are the rare events with a large enough high-energy flux to cause an increase in cosmic-ray neutron monitors at the surface. The frequency of the events is related to the solar-activ- ity cycle, which peaked about 1958 and 1969, but in- tense events can occur at any time, as evidenced by those of February 1956 and August 1972. Figure 13.1 shows clearly that SPEs cause major alterations in middle- atmospheric ionization rates, and hence in the electrical parameters. Energetic electron precipitation from the radiation

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ELEC TRI CAL S TR UC TURK OF THE MIDDLE A TMOSPHERE An 20 10 - us - o JO o _ Aug. 1972 PCA EVENTS 1 Feb. 19 56 GROUND-LEVEL EVENTS 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 FIGURE 13.2 Distribution and intensity of solar energetic-particle events, 1956-1973. The peak absorption in the upper part is a measure of the intensity of the polar-cap absorption (PCA) events resulting from high-latitude ionization in the mesosphere; the lower part shows the intensity of the cosmic-ray (CR) increases recorded by neutron monitors and caused by solar-particle-induced nuclear reactions in the lower stratosphere. belts also contributes to middle-atmosphere ionization but in a manner that is highly variable in both latitude and time. During major electron precipitation events this can become the dominant source of ionization above 70 km for brief periods, and ionization rates can be as high as 105 cm~3 sect above 80 km (Reagan, 1977~. Vampola and Gorney (1983) deduced zonally av- eraged ionization rates due to energetic electron precipi- tation at several magnetic latitudes. Maximum ioniza- tion rates occur between 80 and 90 km and vary between about 0.7 cm ~ 3 see - ~ at 45 and 6 cm ~ 3 see - ~ at 65 latitude. At the higher latitudes energetic elec- trons are competitive with solar Lyman-alpha as an ion- ization source even in daytime. They are probably the dominant source above 70 km at night, when the main competitor is photoionization of NO by the weak Lyman-alpha radiation scattered from the Earth's hy- drogen geocorona (Strobe! et al., 1974~. Bremsstrahlung x rays, generated by the energetic electrons, ionize weakly at heights below 60 km (Luhmann, 1977, Vampola and Gorney, 1983) but are probably rarely competitive with cosmic rays as a global ionization source. ION CHEMISTRY IN THE MIDDLE ATMOSPHERE The principal primary positive ions produced in the middle atmosphere are N2+, O2+, and NO +, all of which IS5, participate in a wide range of ion-molecule reactions that lead to a rich spectrum of ambient ions. An equally rich spectrum of negative ions is generated by reactions that are initiated by the attachment of electrons to form the main primary species O2- and O ~ . In this section the current state of our knowledge of this ion chemistry and of the steady-state ion composition that it produces are discussed. More detailed treatments can be found in re- view articles by Ferguson et al. (1979) and Ferguson and Arnold (1981~. Positive ions The first measurements of positive-ion composition in the mesosphere were made by a rocketborne mass spec- trometer in 1963 (Narcisi and Bailey, 1965~. The domi- nant species below the mesopause were found to be pro- ton hydrates, i.e., members of the family H+(H2O)n, with a sharp transition at about the mesopause to such simple species as O2+, NO +, and several metallic species, probably of meteoric origin. Many subsequent measure- ments have verified these results and have shown that the size spectrum of the proton hydrates is very tempera- ture sensitive. At the cold high-latitude summer meso- pause, as many as 20 water molecules have been seen clustered in individual ions (Bjorn and Arnold, 1981~. The currently proposed positive-ion reaction scheme leading from the primary ions to the proton hydrates is illustrated in Figure 13.3. Since N2+ is rapidly converted into O2+ by charge exchange with O2; the two primary ions of concern are O2+ and NO +. The chain that con- verts O2+ into the proton hydrates was identified by Fehsenfeld and Ferguson (1969) and Good et al. (1970) and is fairly straightforward. Clustering of O2+ to O2 forms 04, which rapidly undergoes a switching reac- tion in which the O2 molecule forming the cluster switches with an H2O molecule to form O2+(H2O). When they are energetically allowed, such switching re- actions are usually fast, occurring at virtually every col- lision between the two species. Subsequent collisions with water molecules lead rapidly to the proton hy- drates. The failure of this mechanism above the mesopause is probably due to a combination of factors: the decreasing water-vapor concentration, the increasing electron con- centration leading to shorter ion lifetimes against re- combination, and the increasing concentration of atomic oxygen. The latter attacks the O4+ clusters through the reaction O4+ + O - O2+ + O3 (13.1) The chain of reactions leading from NO + to the pro- ton hydrates is less certain but probably involves several

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86 FIGURE 13.3 Schematic diagram of the principal positive-ion reactions in the meso- sphere. The details of the NO+ hydration scheme, enclosed by the broken lines, are not yet established. . _ NO+(N ~ r l _ NO 002' T ~ 1 NO+(H20XNA | - -a I L - . ~ I I NAHUM I NO (H,O)~No) steps of clustering and switching as shown in Figure 13.3. This mechanism, first proposed by Ferguson (1974), yields steady-state ion distributions that are rea- sonably close to those observed when appropriate reac- tion rates are used in model calculations (Reid, 1977~. Most of the critical reaction rates are still unmeasured at mesospheric temperatures, however. In the stratosphere, the picture is rather more compli- cated. Mass spectrometry has recently been developed for use at the high ambient gas pressures of the strato- sphere, and measurements of positive-ion composition have been made from rockets and balloons (Arnold et al., 1978~. These experiments showed the existence of the proton hydrates, as in the mesosphere, and also that below about 40 km the proton hydrates are replaced as the dominant species by ions with a core of mass 42 emu. This has been tentatively identified as protonated aceto- nitrile, H + (CH3CN) (Arnold et al., 1978) an identifi- cation that is reasonable in view of the high proton affin- ity of CH3CN and its recent discovery in the troposphere (Becker and Ionescu, 1982~ . It should be emphasized that our knowledge of strato- spheric ion composition is very sketchy. Almost nothing is known of the composition at heights below 30 km or at locations other than continental middle latitudes. Negative ions Our knowledge of negative-ion composition in the middle atmosphere is in an unsatisfactory state. Labora GEORGE C. REID ionization 1 1 . I _ 1 N2+ - 02+ 1 - !~ -~T ~1 ~3 ~ N~(H30)3 1 j ~ ~1 | H3O (H20) ~ - ;~1 . ~ t 3 tory measurements of the negative-ion reactions thought to be the most important ones in the atmosphere have led to the reaction scheme shown in Figure 13.4 (Ferguson et al., 1979~. In this scheme, direct attach- ment of electrons takes place only to O2 and 03; associa- tive detachment reactions occurring chiefly with atomic oxygen quickly destroy most of the resulting O2 and O~ ions in regions where O is present. The ions that escape destruction in this way, however, go on to form a wide variety of species whose electron affinity increases as we progress down the chain. In the absence of annihilation by positive ions, the dominant terminal species in the chain would be the nitrate ion, NO3, with the high elec- tron affinity of 3.9 eV (Ferguson et al., 1972~. Mass-spectrometer measurements of negative-ion composition are much more difficult to make than the corresponding positive-ion measurements, largely ow- ing to the problem of contamination by electrons. As a result, few measurements have been made in the meso- sphere, and these have given somewhat conflicting results (Narcisi et al., 1971; Arnold et al., 1971, 1982~. The predicted dominance of such species as NO3 and CO3 at heights below 80 km appears to be borne out, but many unidentified light ions have been seen in the mesosphere. Above 80 km, there appears to be a layer of heavy (> 100 emu) ions (Arnold et al., 1982) that may be a result of attachment to neutral species of meteoric origin, perhaps forming the very stable silicon species SiO3 (Viggiano et al., 1982~. In the stratosphere, the first mass-spectrometer mea

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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE ionization 02,1~tO 1O3 O 2 ~ ~103O2 O __ I On ~ I NO NO2 CH4 I I O I NO2 ~ l OH- I 1 1 103 L NO3 ~ 1 ~1o 1 1 ~1 ~ ~ COZ CO3- ~ O NO NOR O3 CH4 FIGURE 13.4 Schematic diagram of the principal negative-ion re- actions in the mesosphere. The chain leading to the terminal species HCO3 is probably of minor importance. surements (Arnold and Henschen, 1978) showed the dominance of heavy ions. Laboratory measurements by Viggiano et al. (1980) showed that in the presence of sulfuric acid the species HSO4 would become an impor- tant core ion. Sulfuric acid is known to be the major component of the stratospheric aerosol layer and is a by- pro~luct of volcanic activity, as discussed in Chapter 12 (this volume). There thus appear to be three fairly distinct negative- ion strata in the middle atmosphere. The central region between about 55 and 80 km is formed mainly by ion- molecule reactions involving the commoner minor con- stituents, following initial attachment to O2. This cen- tral layer has layers of heavier ions both above and below the upper one probably a result of reactions in- volving meteoric species and the lower one built by clus- tering around lISO4 . At present this is a very sketchy and incomplete picture, and many more observations are needed to clarify it. ~7 got an~- ~ 6c - - Q) 50 40 an 1 1 1 1 QUIET Proton Hydrates H+(H~O)n O2 SPE 2 Intermediates , ~1 1 3 ) 20 40 60 80 100 20 40 60 80 100 Percentage Composition FIGURE 13.5 Model calculations of the steady-state positive-ion composition of the middle atmosphere, omitting the reactions leading to nonproton hydrates in the stratosphere. The left-hand panel repre- sents quiet conditions, and the right-hand panel is for the ease of an intense solar proton event. Model Calculations 1 co2 .If the rates of production of the various ion species HCO3-and the rates of the important chemical reactions are known, it is possible to calculate the steady-state ion composition. Many such calculations have been made, and examples are shown in Figures 13.5 and 13.6. Figure 13.5 illustrates the positive-ion composition calculated for an intense solar-particle event (right- hand panel) and for undisturbed daytime conditions 90 80 70 60 - C5) _ 50 40 30: 20' 1 1 ~1 <~CO3 NO3 QUIET l SPE ~ 20 40 60 80 1 00 20 40 60 80 1 00 Percentage Composition NO3 FIGURE 13.6 Model calculations of the steady-state negative-ion composition of the middle atmosphere, omitting reactions involving meteoric species at the higher altitudes and reactions involving sulfur species in the stratosphere. The left-hand panel represents quiet condi- tions, and the right-hand panel is for the case of an intense solar proton event.

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188 (left-hand panel). The stratospheric ion chemistry lead- ing to the formation of nonproton hydrates has been omitted. In both cases the proton hydrates are dominant in the lower mesosphere with a fairly abrupt transition to NO + at greater heights in the quiet case and to mostly O2+ in the SPE case. A thin layer containing mostly the intermediate clusters of O2+ or NO + separates the two regions. The tendency toward horizontal layering of the principal proton-hydrate species is caused by the shift in equilibrium toward lighter species as the temperature increases. Figure 13.6 shows the result of similar calculations of negative-ion composition, again omitting both the up- per meteoric layer above 80 km and the stratospheric region of heavy HSO4- derived ions. In the undisturbed case NO3- is dominant in most of the middle atmo- sphere, but the great enhancement in the rate of ion-ion recombination during the SPE inhibits the formation of NO3 and leads to an increase in the fraction of CO3-. The initial ions O ~ and O2- are the main components at the top of the mesosphere. The ultimate loss of ions in the middle atmosphere takes place mainly through recombination with elec- trons or with ions of the opposite charge. In the case of negative ions, photodetachment by sunlight provides another loss mechanism about which little quantitative information is available for the main atmospheric spe- cies. Loss to aerosol particles is presumably also a signifi- cant sink, especially in the vicinity of the stratospheric aerosol layer and possibly of the polar aerosol layers near the summer mesopause. Neither photodetachment nor aerosol attachment were included in the calculations represented by Figure 13.6. MESOSPHERIC AEROSOLS Stratospheric aerosols were discussed in Chapter 12 (this volume). The aerosol content of the mesosphere and the effect of these aerosols on the electrical proper- ties are much more speculative. Noctilucent clouds pro- vide direct evidence that particulate material does exist at mesospheric heights, at least on some occasions. These silvery-blue translucent clouds appear sporadi- cally at high latitudes during the summer months, when their great height allows them to be seen by scattered sunlight long after the Sun has set at the surface. Their phenomenology has been reviewed by Fogle and Haurwitz (1966~. Optical measurements suggest that the particle concentrations are 1 to 50 cm-3 in these clouds, with an individual particle radius of the order of 0.1 ,um. Satellite measurements of backscattered sun- light reveal the existence near the mesopause of a denser semipermanent particle layer over the summer polar GEORGE C. REID cap (Donahue et al., 1972; Thomas et al., 1982), which is probably related to the noctilucent cloud layer. There is general agreement that the particles forming these layers are ice crystals formed in the extremely low temperatures of the summer mesopause region by con- densation from the low background concentration of water vapor (Hesstvedt, 1961~. Several model calcula- tions have shown that it is reasonable to expect ice crys- tals to form under these conditions (e.g., Charlson, 1965; Reid, 1975; Turco et al., 1982) provided that suit- able nucleation centers exist. Mass-spectrometer results suggest that nucleation does occur on positive ions (Goldberg and Witt, 1977; Bjorn and Arnold, 1981), and the particles themselves could then act as surfaces for ion capture. There is evidence for abnormally low electron concentrations in the high-latitude summer mesopause region, perhaps indicating enhanced elec- tron loss through attachment to particles (e. g., Pedersen et al., 1970~. Below the mesopause, the evidence is much less di- rect. Volz and Goody (1962) found evidence from twi- light measurements of low concentrations of dust parti- cles throughout the mesosphere. Hunten et al. (1980) calculated the flux of particles produced by condensa- tion of meteor ablation products. Depending on the model conditions, this calculation predicted meso- spheric concentrations of 102 to 103 cm ~ 3 and individual particle radii of a few nanometers. The influence of such a particle distribution on the ion and electron concen- trations would be small. Chesworth and Hale (1974) proposed the existence of mesospheric particle concen- "rations of 103 to 104 cm - 3 to explain certain discrepan- cies in the electrical parameters. The evidence for such large concentrations was indirect, and further work is needed in this area. In particular, the relationship, if any, between the meteor-ablation particles of Hunten et al. (1980) and the particles suggested by Chesworth and Hale (1974) should be studied. CONDUCTIVITY IN THE MIDDLE ATMOSPHERE The current density, j, and the electric field, E, are related by the familiar Ohm's law expression E. 1 = (J, (13.2) where (' is the conductivity. In the lower atmosphere and most of the middle atmosphere, (, is a scalar, and the electric field and the current lie in the same direction. Above about 70 km, however, collisions between elec- trons and air molecules become infrequent enough that the bending of the electron path by the Earth's magnetic field becomes appreciable between collisions, and mo

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ELECTRICAL STR UCTURE OF THE MIDDLE ATMOSPHERE lion perpendicular to the field becomes less easy than motion along the field. In these circumstances, ~ be- comes a tensor, and an applied electric field with a com- ponent perpendicular to the magnetic field drives a cur- rent in a different direction. This anisotropy of the conductivity becomes a dominant influence on the elec- trical properties of the thermosphere (see Chapter 14, this volume). Conductivity can be measured directly by rocket- borne probes, and a substantial number of such conduc- tivity values are reported in the literature, especially us- ing the so-called blunt-probe technique (Hale et al., 1968~. The problem of shock-wave effects associated with a supersonic rocket are usually avoided in these ex- periments by deploying the probe with a parachute at the top of the trajectory and making the measurements during the subsonic descent. In the presence of a mixture of ions, the conductivity can be expressed in terms of the mobilities of the individ- ual species as a= eEn:+k2+ + eEn'-k7~ + eneke, (13.3) where n and k are the concentrations and mobilities of the positive ions, negative ions, and electrons. tEqua- tion (13.3) is identical to Eq. (12.1) in Chapter 12, this volume, except that in the middle atmosphere electrons come into consideration. ~ An experimentally derived re- lationship between reduced mobility and ion mass in ni- trogen is shown in Figure 13.7 (Meyerott et al., 1980~. 1000 100 Jo _ 1 1 1 1 1 1 1 2 Reduced Mobility (10-4m2volt-1s-1) FIGURE 13.7 Reduced mobility as a function of ion mass. 189 The reduced mobility, ho, is the mobility in a standard atmosphere and is related to the actual mobility, k, by k = ko(2-73)( p )' (13.4) where T is absolute temperature and p is pressure in mil- libars. The ion mobility can be measured by the Gerdien condenser technique (e.g., Conley, 1974), and a num- ber of measurements using rockets have been reported. As discussed by Meyerott et al. (1980), there is no gen- eral agreement among the various measurements, al- though the data of Conley (1974) and Widdel et al. (1976) suggest a single reduced mobility of about 2.7 X 10-4 m2 V~~ sect for the entire middle atmosphere. This corresponds to an ion mass of about 30 emu accord- ing to Figure 13.7, which is clearly in disagreement with both the mass-spectrometer measurements and the model studies discussed above. Meyerott et al. (1980) suggested that the Gerdien condenser measurements were affected by the breakup of cluster ions by both shock-wave and instrumental electric-field effects. Much heavier ions have been seen in some flights. Rose and Widdel (1972), for example, reported a group of positive ions above 60 km whose altitude dependence gives a reduced mobility of about 3.7 X 10-5 m2 V-i sec~ i, corresponding to an ion mass of several thousand emu. The origin of these ions is unknown. Ion concentrations are also measured by the Gerdien condenser technique, and here again there are puzzling differences between observations and predictions. Above about 60 km, ionization of nitric oxide by solar Lyman-alpha radiation becomes an important source, and a considerable amount of variability in ion concen- tration is expected (even in quiet conditions) owing to the variability in NO concentration (Solomon et al., 1982b). At lower altitudes, however, the only signifi- cant source is cosmic-ray ionization, and the ion-pro- duction rate due to cosmic rays can be calculated with a fair degree of. certainty. The loss rate through ion-ion recombination is also well known (Smith and Church, 1977; Smith and Adams, 1982), and the steady-state ion concentration can thus be calculated with correspond- ing accuracy. Figure 13.8 shows a typical set of results. The points are taken from experimental data reported by Widdel et al. (1976), and the solid line is the positive- ion concentration calculated from the same model used to produce the ion-composition profiles shown in Figure 13.6. The overall shape of the altitude profile shows rea- sonable agreement, but the measured values are gener- ally lower than the calculated values by about a factor of 3. A similar discrepancy was found by Meyerott et al.

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190 90 so Al I 1 1 11111| I I I Ill!| i~IT 70 60L ME 50 _ ~ 40 _ I 30 _ 20 _ 10 _ 1 1 1 1111 1o1 1o2 1: 1 1975 ~ / 1 1 1 111111 1 1 1 11111 103 104 lon Concentration (cm-3) FIGURE 13.8 Results of measurements of ion concentration (Wid- del et al., 1976) and of model calculations for quiet conditions. (1980), who suggested that it might be due to the lack of the experimental technique to measure low-mobility ions. Further measurements are clearly needed. Another puzzling feature shown in Figure 13.8 is the abrupt increase in concentration occurring at about 65 km in the 1968 data and 45 km in the 1975 data. A simi- lar abrupt increase in conductivity at about 65 km is seen in blunt-probe measurements and has been attrib- utec] to the transition from a cosmic-ray ionization source below to a Lyman-alpha source above (Mitchell and Hale, 1973~. The model results show such a transi- tion, but it is much less abrupt and smaller in magnitude than the one observed. The same explanation is cer- tainly ruled out for the 45-km transition, since solar Ly- man-alpha radiation is almost totally extinguished be- low 50 km. Figure 13.9 shows the theoretical profiles of positive- ion, negative-ion, and electron contributions to the day- time conductivity, using the same model as before. The dotted curve shows an average of several blunt-probe measurements of positive conductivity in quiet condi- tions (Mitchell and Hale, 1973) and is taken as represen- tative of the current experimental situation. Clearly the overall shape of the theoretical positive-conductivity profile matches the observations reasonably well par- ticularly below 60 km where cosmic rays are the princi- pal source of ionization. At higher levels, the expected GEORGE C. REID variability of ion concentration should lead to a corre- sponding variability in positive conductivity. The role of electrons is noteworthy. The theoretical profiles show that electrons make a negligible contribu- tion to the total conductivity below 50 km but com- pletely dominate the conductivity above 60 km, where they give rise to an extremely steep upward gradient in conductivity [the"equalizing" layer (Dolezalek, 1972~. The electron mobility is so large that the electron contri- bution to the conductivity becomes equal to the ion con- tribution at a level where the electron concentration is less than 1 cm ~ 3. In this region the model predictions of the electron-ion balance are not trustworthy. In partic- ular, the model used here does not include negative-ion photodetachment as a source of electrons, as photode- tachment cross sections of the principal atmospheric negative ions are not known. Even small amounts of photodetachment, however, will have an important ef- fect on electron concentrations in the region of the stra- topause, and hence on the model conductivity profiles. The conductivity is greatly reduced at night at all heights above 50 km. The decrease is partly due to the absence of solar ionizing radiation and partly to changes in neutral chemistry, notably the conversion of atomic oxygen to ozone in the mesosphere. Figure 13.10 shows model profiles of daytime and nighttime conductivity for quiet mid-latitude conditions, in which the night- time sources of ionization are mostly solar Lyman-alpha radiation scattered from the geocorona and the zonally averaged flux of energetic electrons at 45 magnetic lati- tude given by Vampola and Gorney (1983~. The night 90 80 7n ~ ~ I 1 1 1 11111 _ _ _ E 60 _ c) _ 50 ~ 40 _ 30 _ 1 1 1 1 ~~ Elections .-~ A! -a-. ,?Y Quiet: x = 45 / 1 1 111111 1 ~1 k Positive ions 20 I,,, ,,,1 , , ,,,, ,,1 , ,,,, ,,,1 , ,,,, 1 1,1 , ,,,,, 1 -l2 1o-ll 10-10 10_9 10-8 10-7 Conductivity (MHO M-1) FIGURE 13.9 Middle-atmosphere daytime conductivity. The dot- ted curve shows direct measurements (Mitchell and Hale, 1973) of pos- itive conductivity, and the other curves show the result of model calcu- lations of the contributions of the electrons, positive ions, and negative ions.

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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 90 80 70 - Cal ._ a) I 50 40 30 1 1 1 1 1 1 1 // ' sight /Day / 1 1 1 1 1 1 1 10 - 12 10 - 10 10-8 10ff Conductivity (MHO mat) 10-4 10-2 FIGURE 13.10 Model calculations of the daytime and nighttime conductivity of the middle atmosphere during quiet conditions. time equalizing layer lies nearly 10 km higher than the daytime layer, and the transition between the two should take place rapidly during twilight. During a solar-proton event, the conductivity profile of the middle atmosphere is greatly changed. Individual events are so variable in ionization rate, however, that a representative SPE-conductivity profile would not be meaningful. Figure 13.11 is simply intended as an illus- tration of the conductivity profile calculated from the above mode! for a fairly intense SPE during daytime. The largest fractional increase from the quiet-time con- ductivity occurs between 50 and 60 km, and the greatly enhanced ionization in this region tends to eliminate, or 90 70 60 50 t - F I i,lll trill 1l,ll ',lil ilill l,,l, ,~ ,: 40 _ 30 V _ 20 1 ~1, ,,,~1' ',~1' t''1~ ''t1' t''1' ''lit '''1' 1: 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 Conductivity (MHO M-1) FIGURE 13.11 Model calculations of the daytime conductivity of the middle atmosphere during quiet conditions and during an intense solar-proton event. 191 at least substantially reduce, the steep gradient immedi- ately above. To summarize, neither direct measurements nor modeling studies of the bulk electrical parameters of the middle atmosphere are in a satisfactory condition. The measurements are difficult to make and tend to be beset by problems of interpretation. Models are based on defi- cient knowledge of several of the important mechanisms and cannot yet give more than estimates of the altitude profiles of such important parameters as the ion concen- tration and the conductivity. Much remains to be done to place our knowledge on a secure footing. ELECTRIC FIELDS IN THE MIDDLE ATMOSPHERE The normal electric field in the middle atmosphere is a superposition of fields mapped upward from thunder- storm generators in the lower atmosphere and fields mapped downward from magnetospheric and ionos- pheric dynamo generators. The possible existence of lo- cal electric-field generators within the middle atmo- sphere is a controversial topic and will be discussed briefly later. The mapping of the fair-weather field in the vertical direction has been studied by a number of authors (e. g., Mozer and Serlin, 1969; Park and Dejnakarintra, 1973), and a full three-dimensional mode} that includes a real- istic thunderstorm distribution and surface topography has been constructed by Hays and Roble (1979~. The principal component of the middIe-atmosphere electric field provided by this tropospheric source is vertically directed and arises from the necessity for continuity of the vertical current. The vertical electric field is thus roughly inversely proportional to the conductivity, and its order of magnitude varies from 10- ~ V/m at balloon altitudes to 10-6 V/m at the base of the thermosphere. The horizontal component of the electric field arises from the nonuniform distribution of thunderstorm gen- erators over the Earth and is largely removed by the short-circuiting effect of the equalizing layer in the mesosphere, where the conductivity increases sharply (see Figure 13.10~. The attenuation is not complete, however, and the model of Hays and Roble (1979) pre- dicts horizontal electric fields of magnitude up to a few tenths of a millivolt per meter in the lower thermosphere arising from the fair-weather source. The corresponding problem of mapping the iono- spheric and magnetospheric electric fields downward through the middle atmosphere to the ground has also been examined by several authors (e.g., Mozer and Serlin, 1969; Volland, 1972; Chin, 1974; Park, 1976) using simple one-dimensional models and by Roble and

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192 Hays (1979) with a full three-dimensional model. The generation of these fields and their mapping into the global electrical circuit will be discussed in Chapters 14 and 15 (this volume). The dramatic increase in middle-atmosphere conduc- tivity during a major solar-proton event causes large changes in the local electric fields at high latitudes. Holzworth and Mozer (1979) carried out balloon mea- surements of the stratospheric electric field over north- ern Canada during the intense event of August 1972 and reported a decrease in the vertical field by more than an order of magnitude. The decrease closely paralleled the increase in solar-proton flux and could be explained qualitatively by conservation of the fair-weather cur- rent in the presence of the greatly enhanced conductiv- ity. The upward mapping of the thunderstorm-gener- ated fields of the lower atmosphere is sensitive to changes in middle-atmosphere conductivity, since the middle atmosphere represents a low-resistance load to the generator even in quiet conditions. The downward mapping of electric fields Generated in the ionosphere ~. . GEORGE C. REID must involve either a local mesospheric generation mechanism or a dramatic local decrease in conductiv ity, in which case the strong electric field would be needed to maintain current continuity. The latter possi bility appears to be ruled out by simultaneous conduc tivitv measurements made on the same flight (Maynard et al., 1981~. These measurements show that the con ductivity is indeed low in the region of the strong electric fields but is still large enough to provide a vertical cur rent density about 200 times larger than the fair weather value. The fact that the strong fields are usually seen near the 60- to 65-km height region, where the equalizing layer exists, is a possible clue. In this region the domi nant negatively charged current carriers change from the slow-moving negative ions below to the highly mo bile electrons above, and a fairly sharp change in elec tric field must result from the need to conserve vertical current alone. However, as mentioned above, the fields measured are much larger than those associated with this upward mapping process, and there is no obvious anct magnetosphere, however, is much less sensitive, reason why strong fields should be generated in this since the conductivity of the middle atmosphere re- neighborhood. The observations challenge our picture mains much less than that of the lower ionosphere even during a major solar-proton event (see Figure 13.11~. The changes in the global electric circuit arising from the August 1972 SPE have been examined in detail by Reagan et al. (1983) and Tzur and Roble (1983), all of whom pointed out the importance in estimating the changes in middle-atmosphere electrical parameters of including the current carried by the precipitating pro tons themselves in the polar-cap region. Changes in the global circuit, however, arose mainly from the Forbush decrease in galactic cosmic-ray flux that accompanied the event rather than from the solar-proton flux. Measurements of the electric field in the mesosphere have been carried out with rocketborne techniques and have yielded conflicting and unexpected results. Most startling of these is the measurement of strong electric fields in the lower mesosphere (Tyutin, 1976; Hale and Croskey, 1979; Maynard et al., 1981) with intensities that can be orders of magnitude larger than those re quired to maintain continuity of the fair-weather cur rent. The reality of these fields has been questioned (Kelley et al., 1983), and the possibility that they are instrumental artifacts has not been entirely laid to rest. No satisfactory explanation of their existence, either as a genuine atmospheric phenomenon or as an instrumental effect, has yet been proposed. However, they remain an intriguing feature of the middle atmosphere. The anom alous fields, if they are real, cannot be mapped from above or below, since they are present only in relatively well-defined height ranges. Any plausible explanation lo ~ O of the middle atmosphere as a passive element in the global electrical circuit and suggest that there may be field-generating mechanisms that we do not yet under- stand. Even if the electric fields do turn out to be instru- mental artifacts, their explanation will contribute to our understanding of the limitations of in situ electric-field measurements in the terrestrial environment. CONCLUSION In this brief review we have summarized our present understanding of the electrical structure of the atmo- sphere in the 30- to 100-km height range. The sources of ionization in this region are reasonably well known, and their variations in time and space are at least qualita- tively understood. The complexities of the ion chemistry that connects the ionization sources to the ambient ion composition still require a great deal of unraveling. We are still quite ignorant of many aspects, including pho- todetachment of negative ions and the role of reactive neutral species with extremely low concentrations. These uncertainties lead to corresponding uncertainties in ion concentration and mobility and in such bulk elec- trical parameters as the conductivity. Direct experimen- tal measurements have led to considerable progress, but they are beset by difficulties of interpretation and by inconsistencies among themselves. Finally, the recent observations of large mesospheric electric fields have raised first-order questions about our understanding of atmospheric field-generating mechanisms or of the in

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