Energetic ion outflow from the dayside ionosphere and its relationship to the interplanetary magnetic field and substorm activity

The energetic ion outflow from the dayside ionosphere has been investigated for different AE and IMF conditions, Viking ion data, 1-min values of the AE index, and 5-min averaged IMF values have been used. An automated algori thm using principal component analysis has been used to categorize and clas sify the upflowing ion events (UFIs) into beams and conics. The Viking data cover altitudes between 6000 and 13,500 km, 0600 to 1800 magnetic local ti me, and 65 to 90 degrees invariant latitude. Beams dominate the outflow pre noon and postnoon outside the cusp region, while conics are responsible for most of the cusp region outflow. Generally, the highest average number flu x is found in the cusp region. The outflow intensity is higher for southwar d than for northward IMF, especially in the cusp and postnoon sector. Also, the outflow region increases slightly for southward IMF. However, the diff erence in outflow intensity and active area is stronger when the data are s orted according to quiet and active geomagnetic conditions using the AE ind ex. The highest average UFI number flux is observed during the expansion ph ase of substorms, when the flux increase from quiet conditions (AE < 100) i s by a factor of 2, but even during growth and recovery phase the average U FI number flux is similar to 30% higher than for quiet conditions. The resu lts imply that the dayside energetic ion outflow is controlled partly by ni ghtside (driven by the release of stored energy in the magnetotail, as obse rved in the AE index) and partly by dayside (directly driven, as observed i n the polarity in the IMF B-z component) activity. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.