On the Reflection of Oxygen Ion at the Dipolarization Fronts: Cluster Observations

Abstract:

Dipolarization fronts (DFs) are known as earthward propagating leading edge of earthward high-speed flow with sharp increasing of the northward magnetic field B_z. By analyzing a DF event observed by Cluster spacecraft around the neutral sheet on 5 August 2001, we find that the variations of O⁺ parameters are obviously different with protons (H⁺). In the front of the DF, the O⁺ density enhancement starts earlier and is more pronounced. Then during the DF crossing, O⁺ density decreases more gradually. Considering that the structure has a planar shape and propagates in a constant velocity, we can estimate the distance between the density minimum position and the front. The distance of O⁺ is roughly 4 times larger than protons, which is comparable to the ratio of their gyroradius. Besides, a contour of flux cut-off is found in the O⁺ spectrogram, which is modulated by energy magnitude and pitch angles, but it agrees well with O⁺ gyroradius. These correlations indicate that, when particles in the ambient plasma sheet hit the front, they will be reflected by performing half a gyration. So the distance between front and density minimum position can be simply understood as the gyroradius of particles with the highest energy. Similarly, the difference of precursor signatures is mainly due to that O+ have larger gyroradius and can be reflected and accelerated more effectively. Furthermore, using the Cluster data from 2001 to 2004, a superposed epoch analysis is performed and shows a similar result. It is clear that all these analyses provide a more direct proof to the ions reflection scenario at the DF.