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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 201 gion. This enhanced electric field drives an eastward daytime current along the magnetic equator called the equatorial electrojet, as seen in Figure 14.5 (e.g., Forbes, 1981). Figure 14.7 Average quiet-day ionospheric electrostatic potential at 300 km altitude as a function of magnetic local time (from Richmond et al., 1980). The contour level is 500 V, and extrema relative to the global average are labeled in kilovolts. Figure 14.8 Average electric potential patterns above 60Â°N magnetic latitude as a function of magnetic local time deduced from magnetic variations at the ground, for four directions of the IMF (see text). The contour level is 4 kV (from Friis-Christensen et al., 1985). VARIABILITY The preceding sections discussed the average patterns of ionospheric electric fields. Substantial deviations from these patterns occur, on a global scale as well as on a localized scale, and with a wide range of time scales. Because the solar wind and particularly the IMF undergo large variations (e.g., Hundhausen, 1979), it is not surprising that the electric fields associated with solar wind/magnetosphere dynamo action similarly show large variations (e.g., Cowley, 1983; Rostoker, 1983). The interconnection of the interplanetary and magnetospheric magnetic fields maximizes when the IMF is southward, as in Figure 14.3. As the IMF direction rotates out of the plane of Figure 14.3 the amount of interconnected magnetic flux appears to lessen. For a northward-directed IMF the pattern of magnetic-field interconnection must be quite different from that shown in Figure 14.3, and it is possible that interconnection becomes insignificant, so that the magnetosphere is closed. The east-west component of the IMF affects the interconnection morphology and consequently also the pattern of high-latitude magnetosphere convection. The third IMF component, in the plane of Figure 14.3 but directed toward or away from the Sun, seems to have a less important role in the solar wind/magnetosphere dynamo than the other two components. However, the toward/away component correlates strongly with the west/east IMF component because of the tendency for the IMF to assume a spiral pattern around the Sun. Some discussion of IMF effects has therefore taken place in reference to the toward/ away component rather than the west/east component. Figure 14.8 shows the average patterns of electric potential above 60Â° magnetic latitude deduced from ground magnetic variations for four different directions of the IMF. B z is the northward component of the IMF; B y is the eastward component (toward the east for an observer on the sunward side of the Earth). The electric fields are stronger for a southward IMF (B z < 0) than for northward IMF (B z > 0), in accord with the concept that magnetic field interconnection is greater and magnetospheric convection is stronger for a southward IMF. There are also clear differences in the patterns between the westward (B y < 0) and eastward (B y > 0) IMF cases, especially on the sunward side of the Earth. Magnetic storms are dramatic disturbances of the entire magnetosphere lasting a time on the order of 1 day, usually produced by a strong enhancement of the solarwind velocity, density, and/or southward IMF component (e.g., Akasofu and Chapman, 1972). The enhancements often come from explosive eruptions of plasma
UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 202 near the Sun's surface but sometimes are a long-lasting, relatively localized feature of the solar wind that sweeps past the Earth as the Sun rotates. During a magnetic storm magnetospheric convection varies strongly but is generally enhanced; plasma energization and precipitation into the ionosphere are greatly increased; and electric current flow is much stronger, also varying rapidly in time. Figure 14.9 gives an example of ground-level magnetic fluctuations caused by magnetospheric and ionospheric currents during a large storm. Quiet-day variations in the declination (D), vertical component (Z), and horizontal intensity (H) are seen up until 8:30 UT, when a shock wave in the solar wind hit the magnetosphere to produce a storm sudden commencement. Perturbations up to a few percent of the total geomagnetic-field strength occurred during the subsequent hours. Storms are composed of a succession of impulsive disturbances lasting 1-3 hours, called substorms (e.g., Akasofu, 1977; Nishida, 1978; McPherron, 1979). Impulsive disturbances with similar characteristics also occur on the average a few times a day even when no storm is in progress, and these are also called substorms. The characteristics of substorms can vary quite considerably from one to another but are often associated with what appears to be a large-scale plasma instability in the magnetospheric tail. The disturbed electric fields extend beyond the auroral oval and can even be seen at the magnetic equator (e.g., Fejer, 1985). Magnetic storms are predominantly a phenomenon of the solar wind/magnetosphere dynamo, but they affect the ionospheric wind dynamo as well. In addition to strong auroral conductivity enhancements, conductivities can also be altered at lower latitudes at night by over an order of magnitude (Rowe and Mathews, 1973), though they still remain well below daytime values. The nighttime ionospheric layer above 200 km altitude can be raised or lowered in response to stormtime electric fields and winds, which changes its conductive properties. During major storms the entire wind system in the dynamo region can be altered by the energy input to the upper atmosphere, so that the pattern of electric-field generation is modified (Blanc and Richmond, 1980). This effect can raise the potential at the equator by several thousand volts with respect to high latitudes. Regular changes in the electric fields and currents occur over the course of the 11-year solar cycle and with the changing seasons. Ionospheric conductivities change by up to a factor of 2 as the ionizing solar radiation waxes and wanes along with the trend of sunspots. Ionospheric winds also change as the intensity of solar extreme-ultraviolet radiation increases and decreases (e.g., Forbes and Garrett, 1979). We know that ionospheric dynamo currents change by a factor of 2 to 3 with the solar cycle (e.g., Matsushita, 1967), but the variation of electric fields is not yet so extensively documented. At Jicamarca, Peru, however, ionospheric electric fields have been measured on an occasional basis for well over a solar cycle, and the average behavior of the east-west equatorial electric-field component at 300 km altitude is shown in Figure 14.10, as represented by the vertical plasma drift that it produces. The presence of a strong upward drift after sunset at solar sunspot maximum (1968-1971) is not usually present at solar minimum (1975-1976). Clear seasonal variations appear at Jicamarca as well as at higher latitudes. Day-to-day changes in middle-and low-latitude electric fields and Figure 14.9 Magnetogram from Fredericksburg, Virginia, on March 22, 1979, showing variations of the magnetic declination (D), vertical magnetic field (Z), and horizontal magnetic intensity (H) during a magnetic storm. Scale values are not shown.