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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 199 tant influence on the flow of electric currents and on the distribution of electric fields (e.g., Spiro and Wolf, 1984). Energetic particles drift in the Earth's magnetic fieldâelectrons toward the east and positive ions toward the westâso that a westward current flows within the hot plasma. This westward current, flowing in the geomagnetic field, essentially exerts an electromagnetic force on the plasma directed away from the Earth and thus tending to oppose the earthward convection. Charge separation associated with the current tends to create an eastward electric-field component, opposite to the nightside westward convection electric field, largely canceling the convection electric field in the inner magnetosphere. The overall pattern of magnetospheric convection tends to map along the magnetic field into the ionosphere even though this mapping is imperfect because of net electric fields that tend to develop within the non-uniform energetic plasma. In the upper ionosphere, where Eq. (14.4) is valid, the general convection pattern looks something like that shown in figure 14.4. There is antisunward flow over the polar cap and sunward flow in most of the auroral oval. The dayside magnetopause maps perhaps somewhere near the polar capâauroral oval boundary on the dayside, while the most distant closed field lines in the tail map perhaps somewhere near the polar cap boundary on the nightside (the mapping of these outer magnetospheric regions into the ionosphere is not yet well determined). The flow lines in Figure 14.4 correspond to lines of constant electrostatic potential in a steady state. There is a potential high on the dawn side of the polar cap and a low on the dusk side, with a potential difference of the order of 50,000 V. The electric- field strength in the auroral oval tends to be somewhat larger than the polar-cap electric field. Currents are an integral part of the electrical circuit associated with the solar wind/magnetosphere dynamo (e.g., Banks, 1979; Roederer, 1979; Stern, 1983; Akasofu, 1984). figure 14.5 shows schematically the current flow near the Earth. Currents flowing along the direction of the magnetic field (field-aligned currents) couple the auroral oval with outer portions of the magnetosphere. The upward and downward currents are connected by cross-field currents in the ionospheric dynamo region. The anisotropy of the dynamo-region conductivity gives rise to strong current components perpendicular to the electric field in the auroral oval, in the form of eastward and westward auroral electrojets. IONOSPHERIC WIND DYNAMO Winds in the dynamo region have the effect of moving an electric conductor (the weakly ionized plasma) through a magnetic field (the geomagnetic field), which results in the production of an electromotive force and the generation of electric currents and fields (e.g., Akasofu and Chapman, 1972; Wagner et al ., 1980). The effective electric field driving the current, E', is related to the electric field in the earth frame, E, the wind velocity u, and the geomagnetic field, B, by Figure 14.4 Schematic diagram of the magnetic north polar region showing the auroral oval and ionospheric convection (from Burch, 1977). The convection contours also represent electric potential contours, with a potential difference of order 8 kV between them. Thus we may consider that two components of current exist, one driven by the "real" (measurable) electric field E and the other driven by the "dynamo electric field" Figure 14.5 Schematic diagram of electric currents in the ionosphere and inner magnetosphere.
UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 200 u Ã B. These two components are not independent, however, because it turns out that the electric field E itself depends on the dynamo electric field u Ã B. To see how ionospheric winds cause electric fields to be set up, let us for the moment ignore the effects of the solar wind/magnetosphere dynamo. The dynamo electric field associated with the wind will drive a current. In general, this current would tend to converge in some regions of space and cause an accumulation of positive charge, while in other regions of space it would diverge and cause negative charge to accumulate. These charges would create an electric field directed from the positive toward the negative regions, which would cause current to flow tending to drain the charges. An equilibrium state would be attained when the electric-field-driven current drained charge at precisely the rate it was being accumulated by the wind-driven current. Actually, the time scale for this equilibrium to be achieved is extremely rapid, so that the electric field is effectively always in balance with the wind. The magnitude of the electric field is of order 1 mV/m. A net current flows in the entire ionosphere owing to combined action of the wind and electric field, especially on the sunlit side of the Earth (e.g., Takeda and Maeda, 1980, 1981). figure 14.5 shows a large-scale current vortex at middle-and low-latitudes flowing counterclockwise in the northern hemisphere. A corresponding clockwise vortex flows in the southern hemisphere. These vortices are known traditionally as the Sq current system because of the nature of the ground-level magnetic variations that they produce: S for solar daily variations and q for quiet levels of magnetic activity. At high latitudes the electric fields and currents produced by the ionospheric wind dynamo are relatively weak in comparison with those of the solar wind/magnetospheric dynamo. Winds in the upper atmosphere change strongly through the course of the day. They are driven in one form or another by the daily variation in absorption of solar radiation. Atmospheric heating causes expansion and the creation of horizontal pressure gradients, which drive the global-scale upper-atmospheric winds. Solarultraviolet radiation absorption at the height of the dynamo region drives a major portion of the winds. Absorption by ozone lower down (30-60 km altitude) also affects the dynamo-region winds by generating atmospheric tides that can propagate upward as global-scale atmospheric waves (e.g., Forbes, 1982a, 1982b). Figure 14.6 gives an example of three days of wind measurements at 100-130 km above Puerto Rico, showing the strong effects of propagating tides. Important contributions to the dynamo also come from altitudes above 130 km, where the conductivity is smaller (Figure 14.2) but where winds tend to be stronger and to vary less in height. Figure 14.7 shows the average global electrostatic potential generated by the ionospheric wind dynamo, expressed in magnetic coordinates. The zero potential is arbitrarily defined here such that the average ionospheric potential over the Earth is zero. The total potential difference of the average pattern (4.7 kV) is smaller than what can usually be expected on any given day and is much smaller than that associated with the solar wind/magnetospheric dynamo. This pattern was derived from observations of plasma drifts on magnetically quiet days at an altitude of about 300 km. The electric-field maps along magnetic-field lines between hemispheres, providing the symmetry about the magnetic equator even when the wind dynamo action in opposite hemispheres is asymmetric. Magnetic-field lines peaking below 300 km in the equatorial region are not represented in Figure 14.7. Electric fields in the equatorial lower ionosphere have a localized strong enhancement of the vertical component associated with the strong anisotropy of the conductivity in the dynamo re Figure 14.6 Observed eastward (unshaded) and westward (shaded) winds above Puerto Rico during three daytime periods in August 1974 (from Harper, 1977). The contour level is 20 m/sec.