Relationship between magnetospheric electric fields and the motion of auroral forms

Abstract

Twenty-two hours of balloon-measured electric fields obtained on five different nights are compared with simultaneous all-sky camera pictures of auroral forms for the purpose of interpreting the large-scale motions of visible auroras. The poleward surge characteristic of the auroral explosive phase is not associated with a large eastward electric field and therefore is not associated with an E×B/B2 drift of the source of auroral primaries. At times other than during the explosion, the meridional velocity of auroral forms is in the same direction as that of the vector EW×B/B2, where EW is the several hundred kilometer spatial average of the horizontal westward electric field measured by the balloon experiment. The magnitude of the meridional auroral drift velocity during these times was equal to |EW|/B within experimental errors in 72% of the measurements and was generally smaller than |EW|/B in the remaining examples. The occasional decrease of the drift velocity below that expected from the large-scale electric field measurement may be due to a reduction in amplitude of the small-scale electric fields inside precipitation structures because of the resistance to flow of magnetic field lines caused by the higher conductivity of the ionosphere at the ends of the magnetic field lines on which particle precipitation is occurring. Measurements of the longitudinal drift of auroral forms were sparse but were consistent with the conclusions deduced from the meridional motions.