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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 204 tric fields are involved in producing the ionospheric irregularities, often by plasma instabilities resulting from drifts or from steep density gradients created by nonuniform convection (e.g., Fejer and Kelley, 1980). The irregularities tend to be strongest at high latitudes, where electric fields and their consequences are greatest, but the equatorial region is also subject to strong irregularities, both in the equatorial electrojet and in the nighttime upper ionosphere. Rapid magnetic fluctuations caused by changing ionospheric currents disturb geophysical surveys of magnetic anomalies in the Earth's crust. In addition, the fluctuations induce electric currents in the Earth. Analysis of the Earth currents can be useful for geophysical studies, but when they enter large man-made structures like electric transmission lines and pipelines they can cause disruptive electrical signals and corrosion. Lanzerotti and Gregori discuss these problems in more detail in Chapter 16, this volume. Finally, it should be noted that the ionospheric electric field extends down into lower atmospheric regions. In fact, the presence of the field at lower altitudes has been used for many years to measure ionospheric electric fields from stratospheric balloons (e.g., Mozer and Lucht, 1974). Roble and Tzur (Chapter 15, this volume) discuss the relation of the ionospheric electric field to the global atmospheric electric circuit. SOME OUTSTANDING PROBLEMS The entire range of subjects discussed in this chapter is undergoing scientific investigation, but there are certain topics that currently present questions of fundamental importance to the understanding of upper-atmospheric electric- field sources. Since the beginning of the artificial satellite era measurements of space plasmas, energetic particles, and electromagnetic fields have provided a broad picture of the interaction of the solar wind with the magnetosphere. Yet theory still is unable to explain quantitatively, without the use of ad hoc parameterizations, how much magnetic flux interconnects the Earth with the solar wind or what the details of the magnetic-and electric-field configurations are in the vicinity of the magnetopause and in the distant magnetospheric tail. Measurements alone cannot answer these questions because of the impossibility of measuring simultaneously the entire spatial structure of the continuously changing magnetosphere. However, some future observational programs may aid in the solution of the problem by combining continuous monitoring of electrodynamic features of the solar wind and of the entire auroral oval ionosphere with spot measurements in the poorly explored high-latitude magnetopause and distant tail regions. Because solar wind/magnetosphere dynamo processes intimately involve magnetospheric plasma energization, ionospheric conductivity alterations, and coupling of magnetospheric and ionospheric electric fields and currents, the dynamo cannot be fully understood without taking account of the magnetosphere-ionosphere interactions. Theoretical models incorporating the mutual interactions have made great strides of progress in recent years (e.g., Spiro and Wolf, 1984), but much remains to be learned. Observational programs that can measure a large number of the important phenomena involved in the interactions are a necessary adjunct to theoretical studies. The distribution of ionospheric conductivity is important in determining the distributions of both electric fields and currents in the ionosphere and magnetosphere, yet our understanding of nightside conductivities is still rudimentary. We know that the conductivities vary greatly but they are difficult to measure when the ionization density is low, and we do not yet fully understand the nature of nighttime ionization sources. In the auroral oval and polar cap the nightside conductivities can be highly structured, having features correlated with structured features of the electric fields and currents. Because the conductivity features are so variable, useful models are difficult to construct, although a number of encouraging developments in modeling auroral-oval conductivities have been made in the past few years (Reiff, 1984). Concerning the ionospheric wind dynamo, theoretical models and observations are in general agreement, except that the morphology of nighttime electric fields is not yet well explained, a problem related to the conductivity uncertainties. The nature of day-do-day variability in the electric fields and currents, as well as smallerscale variations, are only poorly understood at present. Winds in the dynamo region, especially those of a tidal nature, exhibit day-to- day variability that has not been fully explained and is currently unpredictable. It appears that day-to-day variability in the distribution of atmospheric heating as well as changes in atmospheric tidal propagation characteristics and interactions with other wave motions are largely to blame, but these conditions are not now monitored on a global scale. Advances in this area may be forthcoming in the wake of intensified study of the middle atmosphere (10-120 km altitude). In addition, further clarification of upper-atmospheric circulation changes during magnetic storms is needed in order to understand how storms affect the ionospheric wind dynamo.
