title: Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere.
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. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain 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.
authors.affiliation: Department of Physics and Astronomy, University of Leicester, Leicester, UK. r.c.fear@soton.ac.uk.
authors.forename: R C
authors.initials: RC
authors.lastname: Fear
authors.affiliation: Department of Physics and Astronomy, University of Leicester, Leicester, UK. Birkeland Centre for Space Sciences, University of Bergen, Bergen, Norway.
authors.forename: S E
authors.initials: SE
authors.lastname: Milan
authors.affiliation: Belgian Institute for Space Aeronomy, Brussels, Belgium.
authors.forename: R
authors.initials: R
authors.lastname: Maggiolo
authors.affiliation: Mullard Space Science Laboratory, University College London, Dorking, UK.
authors.forename: A N
authors.initials: AN
authors.lastname: Fazakerley
authors.affiliation: Institut de Recherche en Astrophysique et Planétologie (IRAP), UMR 5277, Université Paul Sabatier-Observatoire Midi-Pyrénées, University of Toulouse, Toulouse, France. IRAP, CNRS, Toulouse, France.
authors.forename: I
authors.initials: I
authors.lastname: Dandouras
authors.affiliation: Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: S B
authors.initials: SB
authors.lastname: Mende