University of Minnesota
School of Physics & Astronomy
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John Wygant

Cluster spacecraft observations of the structure of the normal component the electric and its relation to ballistic acceleration of mono-energetic ion beams across a thin reconnecting current layer
J. R. Wygant, C. A. Cattell, R. Lysak, J. Dombeck,J. McFadden, F.S. Mozer, C.W. Carlson, I. Roth, M. Temerin, E.A. Lucek, A. Balogh, M. Andre, H. Reme , J. of Geophys. Res., 2005

Abstract

Measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region at a series of tail current sheet crossings at distances of ~18 Re near 22.4 MLT on October 1, 2001. The paper focuses on the structure of the normal component of the electric field at thin current sheets at the separatrix region and its role in the non-adiabatic acceleration of ions and the production of Poynting flux. Magnetic field measurements indicate that about ~50 % of the magnetic field change from the northern to southern tail lobes occurs in thin current sheets with widths of 200 km ( 2-4 c/wpe or ~1/4 ion gyroradius of 1keV H) bounding the separatrix region . These thin current sheets are imbedded in thicker current sheets with widths of ~3000 km which contain the remainder of the magnetic field jump. In this study, it is shown that large amplitude bipolar electric field structures coincide with these thin current layers and are directed nearly normal to the current sheet/reconnection structure. The peak to peak amplitudes of these structures may range up to +/- 60 mV/m and are directed towards the mid-plane of the cross tail current sheet. The ÚEdl between the outer boundary of the thin current sheet structure and the mid-plane magnetic field reversal due to this bipolar electric field is 3-6 kV. These electric fields correlate with the Hall magnetic field perturbation which is directed perpendicular to the upstream magnetic field and perpendicular to the boundary normal vector. The associated Poynting flux is an intrinsic feature of the reconnection processes and may provide a non-negligible contribution to the outgoing energy flux. .H+ ions from the upstream direction of the thin current sheet are largely un-magnetized in the thin current sheet and are accelerated through the 3-6 kV potential drop. Observations of 3-d velocity space distribution functions for H+ from the Cluster CIS-CODIF obtained inside the thin current layers provide evidence that protons have been nearly ballistically accelerated by the cross current layer electric potential drop. The distribution functions provide evidence for a pair of counter-streaming, mono-energetic H+ beams directed within 20 degrees of the normal direction with energies of 4-6 keV. The normal component of the electric field in the thin current sheet layer drives an ExB drift of the magnetized electrons which provides the cross tail current. The broader current structures also have normal components of the electric field on the order of 5-20 mV/m directed from the tail lobes toward the separatrix region. The potential drops associated with these electric fields are on the order of 5 kV to 30 kV Although protons are adiabatic in this broader current sheet, the observed O+ population may not be. Distribution functions show counter-streaming O+ populations accelerated along the normal direction not only in the thin current layer but also in the broader current region. These O+ distributions have characteristic energies of about 16 keV. A heuristic model is presented for the structure of the thin current sheet structure and the acceleration of ions in the structure. In this model, the normal component of the electric field spatially coincides with planar current sheets with opening angles of 10-30 degrees in an X line geometry which forms a potential well defining two dimensional "quasi-electrostatic nozzle". Ions are accelerated into this nozzle gaining a kinetic energy of ~1/2 mVA2 ( ~2 keV) and bounce several times between opposing walls. Each bounce converts ion velocity normal to the current sheet to that parallel to the x gse axis forming ion beams with velocity ~VA away from the separatrix region. This scenario is a single particle picture which should be consistent with the MHD and two fluid reconnection pictures which invoke JxB forces associated magnetic field line tension to accelerate fluid elements.