II .7 . Field-Aligned Currents as `Cables'
The discovery of `inverted V events' by L . A . Frank has shown that electric currents in space often flow in ` cables ' . See Figure II . 22. From the observed inverted V events, we know that cable formation is frequent in the lower magnetosphere (up to at least one R®) . They are not so often observed higher up, but this decrease may be observational . Direct measurements of strong electric fields have been made by Mozer et al. (1977) . Besides electric fields parallel to the magnetic field, they fmd very strong fields perpendicular to the magnetic field . The shape of the equipotential surfaces is depicted in Figure II .22 . Other measurements of strongelectric fields have been made both with satellite (Maynard, 1978 ; Smiddy et al., 1977) and rocket (Marklund et al., 1979).
Accelerating regions of the type that we have described have been observed with barium cloud experiments at altitudes of about one Earth-radius (Haerendel et al., 1978; Wescott et al ., 1976 ; Shawhan et al., 1978). If the current-carrying field tube has the same voltage as the environment below the double layer, there must be a lateral voltage gradient above the layer . This produces a rotational motion of the plasma (but should not be depicted as a motion of magnetic field lines!) around the current-carrying flux tube . In this way, the filamentary current is electrically insulated from the surroundings in a way similar to a current in an electric cable located in the ocean and carrying current through a low resistance metal wire . The wire is insulated from the conducting water surrounding it by a plastic cylinder in which the electric field is similar to the radial electric field surrounding the field aligned current in the magnetosphere. In the same way as two high power transmission cables connect a generator and a `consumer', a pair of plasma cables may connect a generator and a `consumer '. The generator often consists of a plasma moving with a velocity component perpendicular to the magnetic field B and hence generating an e .m.f. where the integral is taken between the ends of the two cables.