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

摘要

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 algorithm using principal component analysis has been used to categorize and classify 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 time, and 65 to 90°invariant latitude. Beams dominate the outflow prenoon and postnoon outside the cusp region, while conics are responsible for most of the cusp region outflow. Generally, the highest average number flux is found in the cusp region. The outflow intensity is higher for southward than for northward IMF, especially in the cusp and postnoon sector. Also, the outflow region increases slightly for southward IMF. However, the difference in outflow intensity and active area is stronger when the data are sorted according to quiet and active geomagnetic conditions using the AE index. The highest average UFI number flux is observed during the expansion phase of substorms, when the flux increase from quiet conditions (AE < 100) is by a factor of 2, but even during growth and recovery phase the average UFI number flux is ～30% higher than for quiet conditions. The results imply that the dayside energetic ion outflow is controlled partly by nightside (driven by the release of stored energy in the magnetotail, as observed in the AE index) and partly by dayside (directly driven, as observed in the polarity in the IMF B_z component) activity.