Abstract. On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, threedimensional ion distribution of the major magnetospheric ions (H + , He + , He ++ , and O + ) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5 • ) angular resolution, and a Hot Ion AnalCorrespondence to: H. Rème (Henri.Reme@cesr.fr) yser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6 • ) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range.

We have studied in detail multi‐spacecraft observations of the exterior cusp on 04 February 2001, during a steady northward Interplanetary Magnetic Field (IMF) interval. At a radial distance of 11 Re, Cluster encountered a well‐bounded region where the magnetic field exhibited very low diamagnetic values and the ions displayed high levels of isotropisation. We refer to this region as the Stagnant Exterior Cusp (SEC). Its equatorward edge is magnetopause like, whereas on the poleward side of the SEC, high‐speed plasma jets were observed consistent with a reconnection site poleward of the cusp. The SEC/magnetosheath boundary is characterized by abrupt changes in the magnetic field and plasma parameters that satisfy the Walén test, and by an S‐shaped magnetic hodogram. The latter may suggest the presence of an intermediate/slow transition.

Abstract:The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previous authors have suggested that closed flux can become 'trapped' in the lobe. Their hypothesis holds that this plasma trapping process explains another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the 'trapped' closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.One Sentence Summary: Plasma observed in the magnetotail lobes is due to 'trapped' closed magnetic flux, and reveals the process behind the formation of transpolar arcs. Main Text:The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes (Fig 1.). The lobes consist of the regions in which the terrestrial magnetic field lines are directly connected to the interplanetary magnetic field (IMF), which is referred to as being topologically 'open' (indicated by the dashed gray lines in Fig. 1). Magnetic field lines threading the plasma sheet (solid gray lines in Fig. 1) are not connected to the IMF, and are therefore 'closed' (1, 2). Topology changes are caused by the process of magnetic ‡ This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science, volume 346 on 19 th December 2014, DOI:10.1126/science.1257377. reconnection, which drives magnetospheric dynamics when the IMF is southward (1). Different plasma populations are observed in these regions -plasma in the lobes is very cool, whereas the plasma sheet is more energetic. The key way to distinguish between open and closed magnetic field lines is that electron distributions on closed field lines may exhibit a 'double loss cone', in which the distribution peaks perpendicular to the magnetic field (e.g. 3). This requires the presence of magnetic mirrors on both sides of the observation site, therefore double loss cones are unambiguous indicators that the magnetic field lines observed by a spacecraft are closed.A major problem in magnetospheric physics is the adaptation of this picture to times when the IMF is northward. A recent study (4) has reported relatively hot plasma in the lobes, which is unexpected in standard magnetosphere model. The authors attributed the presence of the pla...