资源环境科技发展态势分析平台

The tens of kiloelectron volt ions observed in the ring current region at L similar to 3-7 generally have pancake pitch angle distributions, that is, peaked at 90 degrees. However, in this study, by using the Van Allen Probe observations on the dayside, unexpectedly, we have found that about 5% time, protons with energies of similar to 30 to 50 keV show two distinct populations, having an additional field-aligned population overlapping with the original pancake population. The newly appearing field-aligned populations have higher occurrence rates at similar to 12-16 magnetic local time during geomagnetically active times. In particular, we have studied eight such events in detail and found that the source regions are located around 12 to 18 magnetic local time which coincides with our statistical result. Based on the ionospheric and geosynchronous observations, it is suggested that these energetic ions with field-aligned pitch angle distributions probably are accelerated near postnoon in association with ionospheric disturbances that are triggered by tail injections.