this region may perturb both the electric current and electric field patterns of the global atmospheric electricity circuit that is established mainly by thunderstorms. If these perturbations affect thunderstorm characteristics, they may alter tropospheric weather and climate. H.C. Carlson pointed out (after the meeting) that it is unlikely that changes in conductivity of the D- and E-regions of the ionosphere might have much global impact on the global electric circuit, at least as long as the altered conductivity is confined to sub-auroral latitudes. The reason for this is that only 10% of the total columnar resistance and ionospheric potential occurs above an altitude of 10 km (see, e.g., Robie and Hays, 1979; Vonnegut, 1979). 2.10 POTENTIALLY IMPORTANT PHENOMENA (Vondrak) Following the practice in PEA (1978), each item in Sections 2.10, 3.5, and 4.5 will be given a two-index rating (A,B), where A = probability of occurrence, and B = importance of potential impact. Each index will be assigned the value H (high), L (low), or U (unknown). The following issues appear to be most relevant to the terrestrial environment and to users of present operational systems: 1. Formation of high-altitude clouds (NLC and nacreous clouds). This could result from a cooling of the mesopause due to the increased humidity in the mesosphere or due to the addition of nucleation centers from ablated reentry material. Such cloud formation may have the following user impact: (a) Atmospheric albedo change that may alter the tropospheric weather and climate (see e.g., Herman and Goldberg, 1978, p. 243f). (H, U/L) (b) Reduced effectiveness of satellite-borne system for surveillance and remote sensing. (H,U) 2. Alteration of mesospheric temperature structure and dynamics. This is principally due to the increased water vapor concentration in the mesosphere, which may alter the reflectivity of planetary waves of the mesopause. Changing the mode of planetary wave propagation has been suggested (Hines, 1974) as a factor that modulates the tropospheric weather and climate. (U,U) 3. Alteration of electric conductivity in the lower ionosphere and mesosphere. The global distribution of ionospheric conductivity affects the propagation of high-latitude electric fields to low latitudes and the location and intensity of high-latitude currents (auroral electrojet). These high- latitude currents are important elements of the current system that includes the Birkeland (field-aligned) currents that couple the ionosphere to the magnetosphere. These electrojet currents also affect power transmission lines, long communication (telephone) lines, and terrestrial magnetic surveys. The global distribution of total conductivity (mesospheric and ionospheric) is an element of the global atmosphereic electricity circuit.
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