Ionospheric electrodynamics depends, among other things, on the conductivity of the ionosphere and on the strength of thermospheric winds. Both of these depend on the flux of solar ultraviolet radiation absorbed in the upper atmosphere, which varies considerably with solar activity. The sensitivity of the equatorial electrojet to ionospheric electric fields of various sources makes its magnetic perturbations a valuable source of information about global electrodynamics. It should be noted that the derivation of ionospheric conductivities rests on the assumption that the collision frequencies are independent of the fluid velocities. Ionospheric currents have been measured by rocketborne magnetometers and by incoherent scatter radars. Perturbations in the geomagnetic field were one of the earliest phenomena of ionospheric electrodynamics to be observed, and magnetic perturbation data exist for a long period of time, from points widely spread over Earth. These data have provided with some of our most detailed information about global ionospheric electrodynamics and its variability.