Dawn-dusk asymmetries in the near-Earth magnetosphere: ion observations

We study spatial asymmetries of the proton and oxygen ion intensity distributions in the near-Earth magnetosphere. The results are based on 7 years of Cluster observations of ion flux measurements from the RAPID and CIS instruments. We find that the oxygen distribution mainly follows the predictions of models: ions drift from the tail duskward, travel around the Earth and then get lost at the noon side. The distribution of ~10 keV oxygen ions shows a dawn-dusk asymmetry during quiet times and an even distribution in the whole magnetosphere during disturbed time as it is expected when particle motion is dominated by gradient drift and they promptly encircle the Earth rather than being lost. The >274 keV oxygen ion intensities during disturbed times also become smoother compare to those during quiet times as expected from the acceleration by inductive fields. However, the dawn-dusk asymmetry ist still seen. A possible reason is that the dawn-dusk asymmetry depends on the gyroradius of the ions, namely becomes stronger with a larger size. As it is expected from the Speiser acceleration model. This trend is well seen from our proton and oxygen observations at different energies. Apparently, there are several acceleration processes in the game. Therefore, it is important to model this complexity and check interplay of dawn-dusk electric field, inductive electric field under different turbulence level and acceleration on the dipolarization fronts.