Plasma Drifts in the Auroral Ionosphere Derived from Barium Releases

Abstract

Barium cloud data on plasma drifts in the auroral ionosphere are reviewed. The convection is directed essentially westward before 2200 MLT and eastward at later night hours; this is opposite to the convection on the polar cap. The west and southward components of the electric field are found to be positively correlated with each other. The correlation is more pronounced in a nonrotating frame of reference. This result and the observed ratios of both field components indicate a dominantly magnetospheric origin of the correlation. Ordering of drift velocities according to typical geomagnetic situations shows the extension of corotation up to the auroral zone during quiet periods. During mildly disturbed periods the typical auroral zone convection pattern shows up with small magnitudes of E⋱(5–l 5 mV m^-1). The growth phase of substorms is characterized by fast westward flows in the evening sector and the region of the westward electrojet by south-easterly motions with speeds of typically 1 km s^-1. Close to the evening bulge of the plasmasphere northwestward directed motions were found suggesting a deformation process at work. During a poleward expansion of the auroral oval the plasma drifted southeastward, essentially opposite to the movements of the auroral arcs. This may be understood in terms of the reconnection of magnetic field lines in the tail. Plasma drifts in the westward traveling surge of a substorm are slow and toward the south, but speed up and turn westward after leaving the region of strong auroral precipitation. A model is proposed according to which’ the westward traveling surge is a result of a reconnection of tail field lines upon which a southeastward directed flow on the polar cap is reversed to a westward one. The existence of strong upward flowing magnetic field aligned currents from the surge is implied.