The Earth's Electrical Environment (1986)

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES 203 currents also occur, produced by corresponding changes in the ionospheric winds. Figure 14.10 Average quiet-day vertical component of the plasma drift velocity caused by east-west electric fields over Jicamarca, Peru, as a function of local time (from Fejer et al., 1979). 1968–1971 (a) are sunspot cycle maximum years, while 1975–1976 (b) are sunspot cycle minimum years. Rapid variations with time scales ranging from seconds to hours are a common feature of ionospheric electric fields and currents. The magnitude of the fluctuations is often as large as that of the regular daily variations, generally increasing with magnetic latitude up to the auroral oval. Besides storm and substorm phenomena, global-scale disturbances in the electric fields and currents also occur with the arrival of solar-wind shocks, with fluctuations of the IMF, and with rapid ionosphere conductivity changes during solar flares. Localized ionospheric electric-field fluctuations may be associated with wind or conductivity irregularities or with small-scale magnetospheric processes. For example, localized quasi-periodic oscillations in the electric fields and currents with periods ranging from a fraction of a second to minutes, called pulsations, are often observed at high latitudes (e.g., Nishida, 1978). EFFECTS OF UPPER-ATMOSPHERIC ELECTRIC FIELDS AND CURRENTS Electric fields and currents interact strongly with the upper atmosphere and help determine its behavior (e.g., Banks, 1979). The drifting ions, as they interact collisionally with neutral molecules, exert a force on the air and tend to bring it toward the ion motion. Above 200 km altitude this effect can be important: at high latitudes winds are common that approach the rapid velocity of the convecting plasma (e.g., Meriwether, 1983), while at low latitudes, where plasma drifts are much smaller, the collisional interaction tends to retard the winds driven by pressure gradients. Of even greater importance is the heating of the upper atmosphere caused by currents in the auroral region. The heating can make a significant contribution to the upper-atmospheric energy budget and can even be the dominant heat source above 120 km during magnetic storms. As the temperature increases the upper atmosphere expands, and the drag on near-Earth satellites is increased, changing their orbits (e.g., Joselyn, 1982). The ionosphere is affected in many ways by electric fields. Above 200 km, where the chemical lifetimes of ions range from several minutes to hours, rapid convection of ionization at high latitudes can bring dense dayside plasma to the nightside of the Earth in some places, cause stagnation and prolonged nighttime decay of ionization at other places, and generally produce highly complex patterns of ionization density (e.g., Sojka et al., 1983). Plasma temperatures and chemical reaction rates are also affected by the rapid ion convection through the air. Even at middle and low latitudes plasma drifts have an important influence on the upper ionosphere, primarily by raising or lowering the layer into regions of lower or higher neutral density, so that chemical decay is retarded or accelerated. During magnetic storms the plasma-drift effects on the ionosphere are not only intensified but are also supplemented by indirect effects through modification of the neutral atmosphere (e.g., Prölss, 1980). Auroral heating induces atmospheric convection that alters the molecular composition of the upper atmosphere and leads to more rapid chemical loss of the ionization, even at middle latitudes. winds generated by the magnetic storm impart motion to the ionization along the direction of the magnetic field, causing redistribution of the plasma as well as further modification of the loss rate. All these ionospheric phenomena affect radio-wave transmissions that reflect off the ionosphere. Radio waves at frequencies greater than about 30 MHz do not normally reflect off the ionosphere and are much less influenced by large-scale plasma density variations than are lower frequencies. However, these waves are affected by small-scale plasma irregularities that cause radio signals to scintillate, undergoing substantial amplitude and phase modulations (e.g., Aarons, 1982). The scintillations are bothersome for transmissions between satellites and the ground. Elec