Auroral particle precipitation is responsible for large variations in Juicy Couture Outlet Online 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 The sources and composition of the ioas that maintain the bulk electrical properties of the upper atmosphere Juicy Couture Outlet arc 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 Juicy Couture Sale 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/ magnetospherc 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 dy-namo is on the order of 1 mV/m. A total current of about 100,000 A flows horizontally in the ionosphere bccausc 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 mid- latitudes. Clobal-scale horizontal potential differences of about 5 to 10 kV are gener-ated by the ionospheric wind dynamo. The ionospheric winds that drive the dynamo are mainly caused by upward propa-gating tides from the lower atmosphere that have large day-to-day fluctuations. Dur-ing gcomagnctic 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.