Field-aligned acceleration above the polar caps during prolonged periods of Northward IMF: 1- Ion outflows

The Cluster spacecraft reveal the presence of successive current sheets of opposite polarity above the polar caps during extended periods of northward IMF. At Cluster altitude (5-7 RE), the upward part of this current system consists of ion beams accelerated by quasi-static electric fields and associated with precipitating electrons. They are surrounded by low energy upflowing electron beams carrying a downward current. In this paper, we focus on Cluster observations of upflowing ion beams in the upward current region. These Polar Cap Ion Beams (PCIB) are accelerated by quasi-static electric fields. A recent statistical study shed new light on these structures. The PCIB have lifetimes that can exceed 20 minutes, they appear after northward turning of the IMF and disappear when it turns southward with a delay estimated to respectively ~2 hours and 20 minutes. Interestingly, about 40% of PCIB are detected together with isotropic plasma clouds located in the magnetospheric lobes. When present, the isotropic plasma population has a strong spatial correlation with PCIB suggesting that it is linked to the observed ion outflows. The temperature and pitch angle distribution of these isotropic ions show that they likely originate from the plasmasheet. The statistical properties of PCIB (size, orientation, IMF dependency...) are similar to those of optical polar cap arcs suggesting that both are different signatures of the same phenomenon. This is further confirmed by a case study combining Cluster in-situ data and TIMED optical observations. Using the high-altitude Cluster data as input to a magnetosphere-ionosphere coupling model we have been able to compute the UV emissions produced by the precipitating electrons associated with the PCIB. The modeled photo-emission is in good agreement with that of a polar cap arc observed by TIMED during the same time period. We'll discuss the new insight these observations give on the interaction between the magnetosphere and ionosphere during prolonged periods of northward IMF.