Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 170 Figure 12.3 A comparison of a normal cn profile (January 15, 1981) with one obtained during a period of the 30-km cn event. Figure 12.4 A comparison of the single-mode stratospheric aerosol size distribution (solid line) typical of the quasi- steady-state period with a recently obtained distribution (dashed line) that is believed to be still under the influence of the April 1982 eruption of El Chichon. ATMOSPHERIC CONDUCTIVITY, SMALL ION CONCENTRATION, AND MOBILITY The atmospheric conductivity depends on the existence of positive and negative ions, whereas the contribution of free electrons can be neglected below about 45 km. There is normally a mixture of ions and the resulting conductivity can be expressed in terms of the number densities and mobilities of the individual species as with e the electronic charge and n and k the number densities and mobilities of the particular positive and negative ions. The ion mobility is defined by the drift velocity v of the ions in an electric field E as and depends on the mass, the collisional cross section, and charge of the individual ions (ions are believed to be singly charged in the lower atmosphere) as well as on the density and polarizability of the surrounding gas. It is inversely proportional to the air density and can be expressed by the reduced mobility k 0 as where p 0 and T0 are STP pressure and temperature (1013 mbar, 273 K) and p and T are pressure and temperature at height h. An empirical relationship between reduced mobility and ion mass in nitrogen is given by Meyerott et al., (1980) and shown later in Figure 12.13. In the troposphere and lower stratosphere the atmospheric conductivity is maintained by the so-called small ions with reduced mobilities around 1.5 cm2/V sec, whereas the mobilities of large ions (charged aerosol particles or large molecular clusters) are too small to contribute directly to the conductivity (e.g., IsraÃ«l, 1973b). As will be discussed later, large ions as well as attachment by aerosol particles can reduce the conductivity significantly. The production and annihilation of small ions is shown schematically in Figure 12.5. The molecular ion and remaining electron created by the ionizing process form charged molecular clusters (the small ions) after several reactions. These small ions are annihilated by mutual recombination and neutralization, or they become almost immobile by attachment to aerosol particles or large ions. Under steady-state conditions and assuming equal densities of positive and negative small ions, the fundamental balance equation becomes in its most simplified form
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 171 Figure 12.5 Schematic representation of the production and annihilation of atmospheric small ions. where q is the ionization rate, Î± the recombination coefficient, Î² the attachment coefficient, and Z the aerosol concentration. An expression for (r) that is relevant to stratospheric conditions has been developed by Zikmunda and Mohnen (1972) and is approximately Î²(r) = 2.35 Ã 10-4 r 1.457 , where r is the radius of the aerosol particle in micrometers. Recombination coefficients were reported by Smith and Church (1977) and experimentally derived from nearly simultaneous balloon measurements of the ionization rate, the small ion density, and the conductivity by Rosen and Hofmann (1981a, 1981b) and Gringel et al. (1983). The atmospheric conductivity within balloon altitudes is measured mostly with the Gerdien condenser technique (e.g., IsraÃ«l, 1973a). In contrast to iondensity measurements, conductivity measurements are largely independent of the airflow rate through the Gerdien tube. This seems to be the main reason for the relatively good agreement among different conductivity profiles as reported by Meyerott et al. (1980). A direct comparison of three different Gerdien- type sondes has shown in fact an agreement of the resulting conductivity profiles within 10 percent to 32 km height (Rosen et al., 1982). Figure 12.6 shows a mean profile (21 soundings) of the positive polar conductivity for quiet solar conditions and a geomagnetic latitude around 50Â° (Gringel, 1978). This profile can be analytically expressed by the relation with z the altitude (in kilometers) and Î±0 = 6.363 Ã 10-1, Î± 1 = 3.6008 Ã 10-1, Î± 2 = â 8.605 Ã 10-4, and Î± 3 = 1.0331 Ã 10-1. The scattering of the mean values as shown for kilometer intervals is considerably below 15 km, indicating a highly variable aerosol density throughout the troposphere. The noticeable increase of the conductivity around 13 km can also be attributed to a sharp decrease of aerosol particle concentrations, especially condensation nuclei, above the tropopause. For comparison, Figure 12.6 shows the mean of three conductivity measurements conducted within 1 week following solar flares. One profile was obtained on August 8, 1972, during a period of intense solar events. The other two flights were conducted 3 and 7 days, respectively, after a solar flare associated with an intense type-IV radio burst on April 11, 1978. For all three flights the Figure 12.6 Profiles of the positive polar conductivity versus altitude (Ï+ = s-) during quiet and active solar conditions. Mean values and standard deviations are shown for altitude intervals in kilometers (Gringel, 1978).
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 172 ionization rate was appreciably reduced by the flarerelated Forbush decrease, resulting in a remarkable conductivity reduction throughout the lower stratosphere. For the August 8, 1972, flight the conductivity reduction reached deep into the troposphere (see also Figure 12.8 below). The ratio between the values of positive and negative polar conductivity exhibit a noticeable altitude dependence as shown in Figure 12.7 (Gringel, 1978). In the troposphere the negative conductivity values exceed the positive ones by about 12 percent. Between 15 and 20 km both polarities are about equal, whereas above 20 to 25 km the negative conductivity becomes 5 to 10 percent smaller than the positive on the average. Assuming equal densities of positive and negative small ions, the same can be concluded for the corresponding average small-ion mobilities. As shown by Roble and Tzur (Chapter 15, this volume) the vertical columnar resistance is an important parameter within the scope of the global atmospheric electrical circuit. This columnar resistance R c is related to the atmospheric conductivity by and denotes the resistance of a vertical air column with a 1-m2 base between the ground and the equalization layer. From Eq. (12.6) R c is determined mainly by the low conductivity values near the ground. Figure 12.8 shows the columnar resistance Rc (h1) between the lower height h 1 and 60 km as function of h1 under quiet solar conditions as calculated from the conductivity profile in Figure 12.6 and using the ratio between positive and negative conductivity shown in Figure 12.7. The mean value from the ground to 60 km was found to be 1.3 Î§ 1017 â¦ m2 at Weissenau (South Germany, 450 m above sea level) during fair-weather conditions. The variations are on the average about 30 percent and are caused by a changing ionization from radioactive materials near the ground and by varying aerosol concentrations in the lower troposphere. The first 2 km of the atmosphere contribute about 50 percent and the first 13 km about 95 percent to the total columnar resistance. Figure 12.7 The ratio of positive to negative polar conductivity as deduced from 10 balloon flights (Gringel, 1978). Figure 12.8 Mean columnar resistance [R c (h1 )] between the lower-altitude h 1 and 60 km. The profiles are calculated from positive polar conductivity profiles only for quiet solar conditions and after solar flares measured on August 8, 1972 (A-691) and on April 14, 1978 (A-782). The right side of Figure 12.8 shows three profiles of the positive polar columnar resistance as a function of the lower height h1 for quiet solar conditions and two profiles following solar flares. These three profiles were calculated using only positive conductivity values and therefore show about twice the value of the actual R c obtained from both polarities. Whereas the Forbush decrease observed on April 14, 1978, influenced mainly the part of the columnar resistance above the troposphere, the enhancement reached deep into the troposphere on August 8, 1972, at a geomagnetic latitude of 48Â°. The part of R c above 13 km was found to be higher by 13 and 28 percent, respectively, when compared with Rc obtained under quiet solar conditions. As proposed by Markson (1978) this could cause a reduction of the current flowing from the top of the thunderclouds to
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 173 the ionosphere, resulting in a lower value for the ionospheric potential reported for periods of high solar activity by Fischer and Muhleisen (1972). Layers of an increased aerosol density can significantly reduce the atmospheric conductivity owing to attachment of small ions on these aerosol particles. Figure 12.9, as an extreme, shows a conductivity profile through a Sahara dust layer between 1.7 and 3.7 km height and 2200 km west of the West African coast (Gringel and MÃ¼hleisen, 1978). The dust concentration Z* responsible for the conductivity decrease is also shown. The authors report a mean mass concentration of 1200 g-3 throughout the layer. Owing to the low altitude of the layer the total columnar resistance was increased about 30 to 50 percent within these large-scale areas of Sahara dust transport across the North Atlantic. It is estimated that about the same increase of R c occurs Figure 12.9 Polar conductivity as a function of altitude and the concentration of mineral dust particles Z* derived from the conductivity decrease D1 in the main Sahara dust transport layer at 2200 km distance from West Africa (Gringel and Muhleisen, 1978). Figure 12.10 Small-ion-density profiles by different authors taken from Riekert (1971), who measured the profile A-480. under stratus clouds extending from 2 to 4 km height. The influence of altostratus and cirrus clouds on R c is negligible compared with the above. Up to several hundred percent increase of the columnar resistance can occur in polluted areas where high aerosol concentrations (in excess of 104 cm-3) or even smog reduce the conductivity drastically within the first few kilometers of the troposphere. The concentration of small ions is one of the more fundamental electrical properties of the atmosphere. The closely related parameter conductivity, although highly important for the electrical structure of the atmosphere, depends on the product of the ion concentration and ion mobility and is therefore of a somewhat less basic nature. Small-ion concentrations are also measured with the Gerdien condenser technique. In contrast to conductivity measurements, the measured ion current is proportional to the airflow rate through the Gerdien chamber, and the latter must be well known in evaluating the appropriate ion density values. Figure 12.10 shows some ion-density profiles measured by different
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 174 authors as reported by Riekert (1971). The profiles show large variations among each other and differ by almost a factor of 5 at the ion density maximum near 15 km. The small-scale fluctuations shown in the profiles of Kroening (1960) and Paltridge (1965) have been attributed to the attachment of small ions on particles. However, it is not clear that the known stratospheric aerosol size and concentration can quantitatively account for the required amount of ion depletion, except under rare circumstances. If, on the other hand, the airflow through the Gerdien chamber is maintained by the rising balloon, it becomes difficult to estimate the actual airflow rate from the balloon rise rate as shown by Riekert (1971) and Morita et al. (1971). The potential seriousness of these uncertainties has led Rosen and Hofmann (1981a, 1981b) to develop a new Gerdien-type ion-density sonde, which is force ventilated by a lobe pump producing a steady and well-defined airflow. A typical iondensity profile measured by these authors is shown in Figure 12.11. Over a 2-yr period their measurements show only small variations from flight to flight, and the fluctuations in the individual profiles are considerably less than those obtained with Gerdien tubes that are not force ventilated. The ion-mobility values among different researchers show also, similarilty to the ion density, large variations as discussed in detail by Meyerott et al. (1980). The reduced positive mobility values up to 30 km range from 1 cm2/V sec (Riekert, 1971) to 2.7 cm2/V sec (Widdel et al., 1976), corresponding to average ion masses of approximately 400 and 30 amu, respectively (see the ion mass/mobility relation shown in Figure 12.12). Ion mobilities obtained by Mitchell et al. (1977) are even larger, around 5 cm2/V sec at 30 km and above, which would indicate ion masses less than 10 amu if the theoretical curve in Figure 12.12 could be extrapolated. Meyerott et al. (1980) point out that these small ion masses might be attributed to rather large field strengths of E/p 20-30 V/cm Torr used in some of the instruments, which can cause an ion breakup during the sampling procedure. If complementary measurements of conductivity and ion concentration are used to determine the ion mobility, a poorly known and variable airflow rate through the ion-density chamber might explain some of the above discrepancies. Figure 12.11 Small-ion-density profile measured with a force ventilated Gerdien condenser (from Rosen and Hofmann, 1981a). Figure 12.12 Reduced mobility spectrum (histogram) for positive ions around 26 km (Gringel, in preparation). The relationship between the mass of molecular ions and their reduced mobility in nitrogen as reported by Meyerott et al. (1980) is also shown. Three simultaneous balloonborne measurements of ion density [force-ventilated Gerdien tube by Rosen and Hofmann (1981a)] and conductivity [Gerdien tube ventilated by the rising balloon (Gringel, 1978)] yielded a reduced positive ion mobility of 1.3 Â± 0.15 cm2/V sec between 4 and 34 km (Gringel et al., 1983). Figure 12.13 shows the result from flight W-204 conducted on May 15, 1979, at Laramie, Wyoming. The reduced positive mobility from this particular flight indicates an ''average" ions mass of about 180 amu throughout the
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 175 entire altitude range. Following these measurements a balloonborne ion-mobility spectrometer was developed and for the first time was flown successfully at Laramie, Wyoming, on June 6, 1983 (Gringel, in preparation). The spectrometer consists of a voltage-stepped Gerdien condenser with divided collector electrode, force ventilated by a lobe pump. The maximum electric-field strength is kept well below 1 V/cm Torr in order to avoid a breakup of the weakly bonded small ions. As preliminary results, the fractional abundances of positive and negative small ions with respect to the reduced mobility are shown in Figures 12.13 and 12.14 for altitudes of 26 and 24.5 km, respectively. These results indicate that most of the positive (~ 78 percent) and negative ( ~ 90 percent) ions have reduced mobilities between ~ 1 and ~ 2.2 cm2/V sec. The mass-mobility relation by Meyerott et al. (1980) indicates that most of the ions of both polarities have masses between 55 and 400 amu. Whereas only 20 percent of the negative ions show reduced mobilities larger than ~ 1.7 cm2/V sec, approximately 43 percent of the positive ions exceed this value and have masses smaller than about 100 amu. The reduced average mobility values are 1.5 cm2/V sec for the negative ions and 1.8 cm2/V sec for the positive ions with an accuracy range of + 10 to â 20 percent. It cannot be decided at this time whether the reduced mobilities measured in 1979 (see Figure 12.13) and 1983 are really different from present values. Figure 12.13 Reduced mobilities of positive small ions versus altitude calculated from simultaneous measurements of small-ion density and conductivity (Gringel et al., 1983). Figure 12.14 Reduced mobility spectrum of negative ions around 24.5 km (Gringel, in preparation). The negative ion-mobility spectrum shown in Figure 12.14 is quite consistent with ion mass spectrometer measurements reported recently by Viggiano et al. (1983). These authors found that the main mass peaks of negative ions between 125 and 489 amu belong to the main ion families (HNO3) n and . The heavy core ion family was found during their flights (September/October 1981) mainly at altitudes above 30 km. However the major volcanic eruption of E1 Chichon in early April 1982 has changed the H2SO4 content of the stratosphere dramatically, as reported among others by Hofmann and Rosen (1983a, 1983b). Their aerosol measurements as well as the negative-ion mobility spectra indicate the presence of the heavy core ion family well below 30 km. In contrast to negative-ion mass spectrometer measurements, as of this writing the presence of positive ions having masses greater than 140 amu in the lower strato