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OVERVIEW AND RECOMMENDATIONS 9 Mid-Troposphere-Stratosphere The main source of ionization in the mid-troposphere-stratosphere (Ë 2-50-km) region is cosmic radiation; the ionization rate depends on magnetic latitude and on solar activity. At about 50Â° geomagnetic latitude and at 20-km altitude, the ion-production rate during sunspot maximum is about 30 percent smaller than during sunspot minimum; at 30-km altitude (same latitude) it is about 50 percent smaller. Following solar flares that produce energetic charged particles (solar proton events), the ion-production rate in the stratosphere may increase by orders of magnitude for periods of hours to days, but deeper in the atmosphere the effect is much smaller. Solar flares are usually followed by a reduction (Forbush decreases) in the ion-production rate for periods of hours to weeks that is caused by a temporary reduction of the incoming cosmic-ray flux. The composition and chemistry of the ions that establish the bulk electrical properties in the mid-troposphere- stratosphere are relatively unknown. The ion concentrations are also affected by aerosols whose distributions are quite variable in both space and time. Aerosols tend to accumulate at temperature inversion boundaries and can cause a general loss of visibility that can be seen by airline passengers as they pass through such layers. Such a buildup of aerosols causes a general decrease in the smallion concentration, and, thus, the electrical conductivity is also reducedâ resulting in an increase in the local electric field. The concentration of particles with a radius greater than about 0.1 micrometer decreases with altitude above the PBL, and a relative minimum occurs in the upper troposphere. The particle concentrations increase within the lower stratosphere, peak near 20 km, and then decrease again with altitude. This persistent structure is frequently referred to as the 20-km sulfate layer; the character of this layer is controlled largely by gases emitted during volcanic eruptions, as discussed in Chapter 12. Aerosol particles that have radii on the order of 0.01 micrometer are referred to as condensation nuclei (CN) and are uniformly mixed throughout the troposphere above the PBL. Near the surface, the CN concentration may be large owing to local sources; above the tropopause the concentration decreases with altitude. In recent years, a CN layer has frequently been observed near 30 km. As a result of the El Chichon volcanic eruption in 1982, the normal CN concentration at 30-km altitude increased by at least two orders of magnitude and measurably affected the ion concentration and electrical conductivity. Under steady-state conditions, the air-earth current density is constant with altitude if there is large-scale horizontal homogeneity and if no thunderstorms or other localized electrical disturbances are in the vicinity. The air- earth current varies with magnetic latitude because of the magnetic variations in cosmic-ray fluxes. The current is generally enhanced over orographic features such as mountain ranges because of the decreased columnar resistance (mountains are closer to the ionosphere than the near-sea-level surface). Estimates have been made that indicate that as much as 20 to 30 percent of the total global current flows into the high mountain peaks. Mesosphere In the mesosphere (50-85 km altitude), the major daytime source of ionization is solar Lyman-alpha photoionization of nitric oxide (NO). The major source of NO for this region is the thermosphere, where NO is produced by extreme ultraviolet (EUV) radiation (wavelengths less than 100 nm) and auroral particle precipitation. Meteorological processes in the upper atmosphere transport NO from the thermosphere to the mesosphere, where its distribution is variable. Somewhat smaller sources of ionization in the upper mesosphere include solar x-ray ionization and the photoionization of oxygen in a metastable state. At high latitudes, energetic electrons, protons, and bremsstrahlung