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OVERVIEW AND RECOMMENDATIONS 10 ultraviolet radiation associated with auroral particle precipitation are variable sources related to geomagnetic activity. Solar protons are a sporadic and intense source of ionization at high latitude following intense solar flares. These solar proton events can increase the electrical conductivity of the magnetic polar-cap mesosphere by several orders of magnitude (at altitudes down to about 50 km) during such events. In addition, the current carried by the bombarding solar protons can often exceed the local air-earth conduction current flowing in the circuit. The principal primary positive ions produced in the mesosphere are , , and NO+, but all participate in a wide range of reactions that lead to a rich spectrum of ambient positive ions. An equally rich range of negative ions is generated by reactions initiated by the attachment of electrons to form the main primary species, and O-. Rocketborne mass-spectrometer measurements have shown that below the mesopause the positive ions are proton hydrates with as many as 20 water molecules clustering to individual ions. The positive-ion chemistry of the mesosphere is better understood than is the negative-ion chemistry (see Chapter 13). The interaction of the terminal ions with aerosol particles is probably a significant sink for ions in the polar aerosol layers near the summer mesopause where noctilucent clouds are commonly observed. The electrical conductivity of the mesosphere is important because it governs the electrical properties of the equalization layer in the global circuit. Below about 60 km, the terminal small ions are the main charge carriers; but above 60 km, free electrons can exist and their high mobility is responsible for the abrupt increase in electrical conductivity observed in the mesosphere. Furthermore, above 70 km, collisions between electrons and air molecules become infrequent enough so that electrons are confined to spiral about a magnetic field line and the motion perpendicular to the field becomes more difficult than motion along the field. The electrical conductivity becomes anisotropic, and this anisotropy has a dominant influence on the electrical properties of the global circuit above 70 km. Rocketborne measurements of the upper atmosphere conductivity and electric field indicate some puzzling features. There appear to be regions in the upper stratosphere and mesosphere that have abrupt increases and decreases in vertical conductivity profiles. The decreases are probably associated with aerosol layers, but the increases are difficult to interpret. On occasion, the electric field near 50to 70-km altitude has been observed to increase enormously from what is expected if the mesosphere is a passive element in the global circuit. The mesosphere may not be electrically passive but may, in fact, contain active electrical generators that are not currently known. Ionosphere and Magnetosphere The major sources of ionization above about 85 km are extreme-ultraviolet (EUV) radiation and auroral particle precipitation (see Chapter 14). The ionizing portion of the solar spectrum (i.e., wavelengths below 102.7 nm) is absorbed in the thermosphere and creates an ionosphere that consists of positive molecular and atomic ions (e.g., , NO+, , O+) and negative electrons. The solar EUV radiation and the electron and ion densities throughout the ionosphere are highly dependent on solar activity; there are known variations with the 11-yr sunspot cycle, the 27-day rotation of the Sun, and solar flares. Auroral particle precipitation is responsible for large variations in ion and electron densities at high latitudes. The bulk of the precipitation occurs within the auroral oval that encircles the geomagnetic pole in magnetic conjugate polar caps. Observations over many years show that there is always auroral activity within the oval. The activity varies considerably over the day and even from hour to hour owing to interactions of
OVERVIEW AND RECOMMENDATIONS 11 the solar-wind plasma with the Earth's magnetic field. The total power dissipated by particles bombarding the upper atmosphere is typically 109 W, but during large geomagnetic storms it can approach 1012 W. The sources and composition of the ions that maintain the bulk electrical properties of the upper atmosphere are generally known on the dayside of the Earth, but at night there are still uncertainties with regard to the ionization sources. In the classical view of the global circuit (see Chapter 15), the ionosphere is assumed to be at a uniform potential with respect to the surface; however, the known upper-atmosphere generators are not included. The two major generators that operate in the ionosphere above about 100 km are the ionospheric wind dynamo and the solar-wind/ magnetosphere dynamo (see Chapter 14). Atmospheric winds have the effect of moving the weakly ionized ionospheric plasma through the geomagnetic field. This movement produces an electromotive force and generates electric currents and fields. This process is complicated by the variability of the ionospheric winds and the anisotropic electrical conductivity in the ionosphere. The magnitude of the horizontal electric field associated with the wind-driven dynamo is on the order of 1 m V/m. A total current of about 100,000 A flows horizontally in the ionosphere because of the combined action of the wind and electric field, mainly on the sunlit side of the Earth. This current flows in two counterrotating vortices on opposite sides of the equator, and these patterns dominate at low latitudes and midlatitudes. Global-scale horizontal potential differences of about 5 to 10 kV are generated by the ionospheric wind dynamo. The ionospheric winds that drive the dynamo are mainly caused by upward propagating tides from the lower atmosphere that have large day-to-day fluctuations. During geomagnetic storms, however, thermospheric winds increase in response to high-latitude auroral heating and cause disturbances at low latitudes to the fields and currents of the ionospheric wind dynamo. The solar-wind/magnetosphere dynamo results from the flow of the solar wind around and perhaps partly into and within the Earth's magnetosphere. The motion of this plasma through the geomagnetic field produces an electromotive force and currents at high latitudes that result in an antisunward flow of plasma over the magnetic polar cap and a sunward flow of ions in the vicinity of the dawn and dusk auroral zones. This motion is described by a two-cell counterrotating ion circulation with one cell on the dawn side and the other on the dusk side of the magnetic polar caps. The polar-cap electric field is typically 20 m V/m, with an ionospheric convection velocity of 300 m/sec. Larger fields of about 50 to 100 m V/m occur in the vicinity of the auroral ovals. The large-scale potential difference that is associated with this horizontal ion flow over the polar caps has a total dawn-to-dusk drop of about 50 kV. This potential drop and the configuration of the two-cell pattern are highly variable. The potential drop has values of 20 to 30 kV during geomagnetic quiet conditions that increase to 100 to 200 kV during geomagnetic storms. These fields are mainly confined to the polar caps because of the shielding from currents within the magnetosphere. During geomagnetic storms, however, the shielding currents can be altered and electric fields have been observed to propagate all the way from the polar caps to the equator. Currents are an integral part of the complex electrical circuit associated with the solar-wind/magnetosphere dynamo. Currents flowing along the direction of the magnetic field couple the auroral oval and high-latitude ionosphere with outer portions of the magnetosphere. Typically about a million amperes of current flow in the solar- wind/magnetosphere dynamo. The dynamo currents and fields with this high-latitude system are extremely complex and highly variable (see Chapter 14). The large-scale horizontal fields (scale sizes 100 to 1000 km) within the ionosphere can propagate or map downward in the direction of decreasing electrical conductivity. Horizontal fields of a smaller scale (1 to 10 km) on the other hand are rapidly attenu