Transient Parallel Electric Fields from Electromagnetic Induction Associated with Motion of Field-Aligned Currents

presumably in response to forces associated with the solar wind and its variations. The motion of the transverse magnetic field of the field-aligned currents leads to induced electric fields parallel to the main geomagnetic field. The magnitude, direction, and spatial distribution ot' the E field depend on the velocity, magnitude, and configuration of the magnetic field of the field-aligned currents. Magnitudes of 10 -5-10 -a V/m may exist for periods extending from a few seconds to nearly I hour. Reversals of polarity ma, y be expected within the spatial distribution of the transient electric field. These E fields can exist anywhere in the volume of space influenced by the magnetic fields of the field-aligned currents, extending from the E region of the ionosphere to the outer magnetosphere. Acceleration or deceleration of charged particles may occur in different regions of the volume, effected by the magnetic field of the moving currents. Also transverse electric fields may be induced by the motion ot' the horizontal ionospheric currents that close the field-aligned currents. The totality of results at least suggests that this mechanism for producing induced electric fields may be one of major importance in studies of the transfer ot' charge and dissipation of energy in the ionosphere-magnetosphere coupled system. Electric fields in the ionospheric and magnetospheric segments of the auroral oval are important in the interpretation of the many varied geophysical phenomena that occur there. Electric fields'have recently been reviewed by Maynard [1971], Heppner [1972], Cauffman and Gurnett [1972], and Gurnett [1972]. There is agreement th:at electric fields transverse to the main magnetic field exist with magnitudes typically 3 X 10 -' to 5 X 10 -' V/m and in general accord with the large-scale convection fields proposed by Axford and Hines [1961]. The subject of electric fields parallel to the main magnetic field has been and still is one of considerable controversy. From Maynard [1971] the evidence from direct observation of E fields appears weighted toward small or nonexistent steady state parallel E fields without precluding the presence of short-duration ones. From Zmuda et al. [1970], if ambient electrons or positive ions or both constitute a stationary or quasi-stationary field-aligned auroral current, the inferred driving parallel electric field is 10-8-10 -9 V/m at 1100-km altitude, a magnitude 4 orders below that presently directly detectable. It is the principal purpose here to discuss the transient parallel electric fields produced through electromagnetic induction effects associated with the movement of the transverse magnetic fields of the field-aligned currents that form an ever present feature of the ionosphere-magnetosphere coupled system. Expected magnitudes and durations are computed. We also include brief reviews of the indirect arguments for a parall,el E field and of characteristics of field-aligned currents.