Cluster observations of energetic electrons and electromagnetic fields within a reconnecting thin current sheet in the Earth's magnetotail

A. Retinò (1) , R. Nakamura (1) , A. Vaivads (2) , Y. Khotyaintsev (2) , T. Hayakawa (3) , K. Tanaka (4) , S. Kasahara (4) , M. Fujimoto (4) , I. Shinohara (4) , J. P. Eastwood (5) , M. André (2) , W. Baumjohann (1) , P. W. Daly (6) , E. A. Kronberg (6) , Nicole Cornilleau-Wehrlin (7)

1 IWF - Space Research Institute of Austrian Academy of Sciences
2 IRF - Swedish Institute of Space Physics [Uppsala / Kiruna]
3 Department of Earth and Planetary Sciences [TITECH Tokyo]
4 ISAS - Institute of Space and Astronautical Science
5 SSL - Space Sciences Laboratory [Berkeley]
6 MPS - Max-Planck-Institut für Sonnensystemforschung = Max Planck Institute for Solar System Research
7 CETP - Centre d'étude des environnements terrestre et planétaires

We study the acceleration of energetic electrons during magnetotail reconnection by using Cluster simultaneous measurements of three-dimensional electron distribution functions, electric and magnetic fields, and waves in a thin current sheet. We present observations of two consecutive current sheet crossings where the flux of electrons 35-127 keV peaks within an interval of tailward flows. The first crossing shows the signatures of a tailward moving flux rope. The observed magnetic field and density indicate that the flux rope was very dynamic, and a comparison with numerical simulation suggests a crossing right after coalescence of smaller flux ropes. The second crossing occurs within the ion diffusion region. The flux of electrons is largest within the flux rope where they are mainly directed perpendicular to the magnetic field. At the magnetic separatrices, the fluxes are smaller, but the energy spectra are harder and electrons are mainly field aligned. Reconnection electric fields E_Y ∼ 7 mV/m are observed within the diffusion region, whereas in the flux rope, E_Y are much smaller. Waves around lower hybrid frequency do not show a clear correlation with energetic electrons. We interpret the field-aligned electrons at the separatrices as directly accelerated by the reconnection electric field in the diffusion region, whereas we interpret the perpendicular electrons as trapped within the flux rope and accelerated by a combination of betatron acceleration with nonadiabatic pitch-angle scattering. Our observations indicate that thin current sheets during dynamic reconnection are important for in situ production of energetic electrons and that simultaneous measurements of electrons and electromagnetic fields within thin sheets are crucial to understand the acceleration mechanisms.

Keywords: Space Plasma Physics: Plasma energization; Space Plasma Physics: Charged particle motion and acceleration; Solar Physics; Astrophysics; magnetic reconnection; energetic particles; electric fields; Earth Science

Domains: Sciences of the Universe [physics]

A. Retinò, R. Nakamura, A. Vaivads, Y. Khotyaintsev, T. Hayakawa, et al.. Cluster observations of energetic electrons and electromagnetic fields within a reconnecting thin current sheet in the Earth's magnetotail. Journal of Geophysical Research Space Physics, 2008, 113 (A12), ⟨10.1029/2008JA013511⟩. ⟨hal-04110229⟩