Even if a steady convection state could exist in principle, the magnetosphere will be rarely in it, since the interplanetary magnetic field is hardly ever stationary over the 2-4 hour convection cycle (Rostoker et al., 1988). Indeed, the hourly average north-south component of the interplanetary field retained the same sign for two consecutive hours only 12.2% of the time during solar cycles 20 and 21 (Hapgood et al., 1991). If only for this reason, we cannot avoid dealing with time-dependent convection. In this section, we take up one method of coping with the issue. Correlation studies take advantage of solar wind variability without ever needing to consider the precise nature of the time-dependent response of the magnetosphere. Though laborious, they are a procedurally straightforward way to test the viscous and reconnection models of convection. Geomagnetic activity, the response of geomagnetic field to currents flowing in the ionosphere and in space, has been monitored in an increasingly systematic way since the beginning of the eighteenth century. Today, a worldwide network of ground stations provides continuous records of the magnetic field at many different locations on the earth’s surface. Before computational data displays enabled large quantities of data to be summarized at a glance, the complex multi-station records were combined into single parameters called geomagnetic indices, which were designed to characterize one aspect or another of geomagnetic activity on a global scale. We will refer frequently to the auroral electrojet (AE) index, which was designed by Davis and Sugiura (1966) as a measure of electrojet activity in the auroral zone. The index is derived from the horizontal, northern component of the geomagnetic perturbation field measured at a number of observatories in the northern hemisphere. The number of observing stations contributing to the index is occasionally indicated in parentheses as AE(12) or AE(32), and so on. The maximum and minimum perturbations recorded at any given time at the stations in the AE network are called the AU and AL indices, respectively, for “upper” and “lower.” These provide a measure of the westward and eastward electrojet strengths. The difference between AU and AL is the AE index.
Scintillations of Radio Sources and Geomagnetic Activity
