The properties of oxygen ions in low latitude boundary layer on dayside magnetopause

Abstract:

Energetic (>3KeV) oxygen ions (O⁺) escaping from magnetosphere into magnetosheath through magnetic reconnection has already been reported. The property of escaped O⁺ in the boundary layer with different dominated IMF Bz has not been studied. Now, two cases of O⁺ escape observed by Magnetospheric Multiscale (MMS) spacecraft on magnetopause crossings are studied.

These two events are observed on the duskside of equatorial magnetopause crossing. One occurs during initial phase of substorm with southward IMF dominated, the other occurs during expansion phase of substorm with northward IMF dominated. In both of the events, O⁺ density enhances apparently in the boundary layer when remarkable negative Vz proton flows appear, but the O⁺ velocity nearly remains unchanged. The walén analysis is used and suggests that the observed negative Vz flows are reconnection jets and the O⁺ appearing in the magnetosheath boundary layer may mainly escape from magnetosphere through the magnetic reconnection. The O⁺ pitch angle distribution in the boundary layer is also checked, which shows that the tendency of the change is the same: When IMF Bz is northward, the O⁺ pitch angle distribution is close to isotropic. When IMF Bz is southward, the O⁺ pitch angle distribution is close to parallel. The Larmor radius effect on O⁺ dynamics in the boundary layer is also studied.

Generally, the O⁺ ionspheric outflow is tailward during substorm so as to the O⁺ number density in the magnetosphere is larger than that in the low latitude boundary layer, which is consistent with what the southward IMF dominated event shows. However, we found what the northward IMF dominated event shows is opposite to that. That the ionspheric outflow is dayside when IMF Bz is northward even though during the substorm is suggested. Some differences of the O⁺ dynamic behavior in the boundary layer with different dominated IMF Bz are also studied, and the reasons for these differences will be discussed in the future work.