A COMPARISON OF A MODEL FOR THE THETA-AURORA WITH OBSERVATIONS FROM POLAR, WIND, AND SUPERDARN

A model is presented according to which theta auroral arcs form after southward turnings of interplanetary magnetic field (IMF) and/or large variations in IMF B-y, following prolonged periods of northward IMF o r very small B-z, with \B-y\ greater than or similar to \B-z\. The arc s start on the dawnside (duskside) of the auroral oval and drift duskw ard (dawnward) across the polar cap for positive (negative) B-y in the northern hemisphere and conversely in the southern hemisphere. After the theta aurora has formed, changes in IMF B-y or B-z readjust the me rging configuration and continue the auroral pattern. The transpolar a rcs are on closed magnetic field lines that bifurcate two open section s of the polar cap and map to the outer plasma sheet. Four theta auror al events were studied using data from the ISTP/GGS Polar and Wind spa cecraft and the ground-based SuperDARN radars. Observations that are c orrectly predicted by our model include the following: (1) The formati on and evolution of theta auroras observed by the visible imaging syst em are closely related to the IMF patterns measured by the Wind magnet ic field investigation. (2) Both electrons and ions in the transpolar are and poleward part of the nightside auroral oval exhibit similar sp ectral characteristics, identified from the data acquired with Hydra a nd the comprehensive energetic particle and pitch angle distribution e xperiment. The low-energy electrons show counterstreaming distribution s, consistent with their being on closed field lines that magnetically connect to the boundary plasma sheet in the magnetotail. (3) Ion comp osition measurements obtained from the toroidal imaging mass-angle spe ctrograph show cold plasma outflows from the ionosphere and hot, isotr opic magnetospheric ions in the two regions, also indicating transpola r arcs are on closed field lines. (4) Large scale polar cap convection inferred by SuperDARN observations is well correlated with IMF patter ns. (5) Plasma convection in the transpolar arcs, inferred from the el ectric field instrument and the magnetic field investigation measureme nts, is sunward.