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ATMOSPHERIC ELECTRICITY IN THE PLANETARY BOUNDARY LAYER 162 ers, where the turbulent transport time across the entire PBL can be comparable with the electrical relaxation time. The controlling meteorological variables are the surface fluxes of momentum and buoyancy and the mean conductivity and thickness of the mixed layer. It appears that the convection of electrode-effect space charge can have a major impact on the electrical structure of the boundary layer. It can even reduce the magnitude of the total downward current density locally on the order of 50 percent owing to a mechanically generated electromotive force (EMF) of more than 100 kV in extreme cases. Although these theoretical predictions are not inconsistent with existing data, they remain to be tested thoroughly in the field. Effect of the Planetary Boundary Layer on the Fair-Weather Electrical Circuit Two boundary-layer processes can have a substantial impact on the fields and currents appearing throughout the entire atmospheric column from the Earth to the ionosphere. These are variations in the columnar resistance and convection currents. To appreciate the importance of these effects, consider a steady convection-current density J c( z) below an inversion at height H when a steady ionospheric potential V â is applied from above. Since the total current density J t must be independent of height, it is easy to show that where R â is the total columnar resistance. The second term on the right may be considered an EMF generated by boundary-layer convection. This steady-state analysis is valid as long as Î», J c, and V â change slowly compared to the electrical relaxation time near the ground. If we assume V â to be constant, Eq. (11.8) shows that the magnitude of Jt is inversely proportional to R â and decreases linearly as the boundary-layer EMF increases. An aerosol-related increase in columnar resistance of 40 percent can therefore produce a similar decrease in the total current density. A simultaneous 100-kV increase in the PBL EMF can cause a further 30 percent decrease, for a total reduction in J t of 52 percent. This makes J t alone a relatively poor indicator of global processes. NEEDED RESEARCH AND POTENTIAL APPLICATIONS Measurement of Global-Scale Phenomena As discussed in detail in other chapters of this volume, there are ample reasons for interest in global-scale atmospheric-electrical phenomena. For example, valuable information about the distribution and temporal variability of horizontal potential differences in the ionosphere could be provided by monitoring the ionospheric potential simultaneously in different locations. Furthermore, the widely accepted relationship between global thunderstorm activity and ionospheric potential has yet to be verified on any but the crudest statistical basis. From the present perspective, finally, a detailed knowledge of the forcing from the global circuit would be useful in evaluating the electrical response of the PBL. Unfortunately, the measurement of global-circuit parameters is complicated by the action of boundary-layer processes. Although local PBL structure cannot appreciably affect the total current in the global circuit, or even the local ionospheric potential, it can cause a redistribution of that current and alter the vertical profile of electric field. Therefore, the proper interpretation of local measurements in terms of global parameters requires a thorough understanding of noise sources in the PBL. The electrostatic potential of the upper atmosphere with respect to Earth is the single parameter most indicative of the electrical state of the global circuit. Yet the temporal variability of this ionospheric potential is largely unknown outside of its average diurnal variation. Methods of measuring the ionospheric potential (such as aircraft and balloon soundings) have for the most part systematically excluded the detection of any shorter-term variations. Fluctuations in electric-field and current-density measurements in the PBL with periods shorter than a few hours are usually attributed entirely to local sources, primarily turbulence and pollution. One way to separate local and global sources is to correlate measurements at widely separated stations or to make instantaneous measurements averaged over large horizontal areas. A preliminary attempt (Ruhnke et al., 1983) to detect global variations with periods of seconds to minutes in the total Maxwell current (conduction, convection, and displacement) measured simultaneously in the United States and the Soviet Union revealed an apparent correlation that is difficult to attribute solely to chance. This approach deserves further attention as a relatively simple prospective method of monitoring short-period variations in the global circuit. If it can be demonstrated that short-period and day-to-day global variations do indeed exist, then not only is the source of these variations of importance but also the usual interpretation of local variations in terms of turbulence must be re-evaluated. In light of the importance of the ionospheric potential as an indicator of the electrical state of the global circuit and the need to separate its variation from local fluctuations in the PBL, the iono