Statistical study of O+ transport from the cusp to the lobes with Cluster CODIF data

Ionospheric origin O+ accelerated in the cusp/cleft region convects over the polar cap and flows along open field lines to the tail lobes. Some O+ in the lobes enters the near-Earth plasma sheet where it is then convected to the inner magnetosphere, while some O+ ends up in the distant tail where it is lost. In order to understand the transport of ionospheric O+ to the plasma sheet so as to understand its contribution to the formation of geomagnetic storms, we have determined the occurrence frequency of cusp source O+ over the polar caps and in the lobes to determine where and when it is observed. The results show that the probability of observing O+ along the transport path is high even during nonstorm times, although, as expected, the highest probability is found during storm times. It was also found that when interplanetary magnetic field (IMF) By is positive, O+ from the northern cusp/cleft tends to stream toward the dawnside tail lobe while O+ from the south are observed on the duskside. The transport path for negative IMF By is more symmetric, but shows some evidence for a reversed asymmetry when IMF By is strongly negative. IMF Bz has little influence on the asymmetry. The asymmetry for positive By and lack of mirror symmetry between positive and negative By most likely result from the combination of convection driven by the solar wind and coupling with the ionosphere. Similar asymmetries have been observed in the convection patterns over the polar caps, which are attributed to a day-night ionospheric conductivity gradient adding to the IMF By effect. However, there are some disagreements between the asymmetries observed in polar cap potential patterns and the asymmetries observed in the O+ spatial distribution, indicating there may be other causes for the symmetry breaking, in addition to the day-night conductivity gradient.