Discovery of Alfven vortices in the plasma turbulence of the Earth's magnetosheath

HR: 15:40h
AN: SM43A-06 INVITED [Abstracts]
TI: Discovery of Alfven vortices in the plasma turbulence of the Earth's magnetosheath
AU: Alexandrova, O
EM: olga.alexandrova@obspm.fr
AF: LESIA, Observatoire de Paris, 5, place Jules Janssen, Meudon, 92195, France
AU: * Mangeney, A
EM: mangeney@despace.obspm.fr
AF: LESIA, Observatoire de Paris, 5, place Jules Janssen, Meudon, 92195, France
AB: The Earth's magnetosheath represents one of the natural laboratories of the collisionless plasma turbulence. The presence of the boundaries, the bow shock and the magnetopause, can play a crucial role in the turbulence nature. Here, as in the solar wind, the spectrum of magnetic fluctuations can be described by two well defined power laws separated by a spectral break, which is observed in the vicinity of the proton cyclotron frequency f_cp. In the low frequency part, the magnetosheath turbulent spectrum follows a ~ f^-1 law and it is ~ f^-3 in the high frequency part. The low frequency part of the spectrum thus does not follow the Kolmogorov's f^-5/3 law, as it is the case in the solar wind. But another important difference is that in the magnetosheath the spectral break is frequently accompanied by a large maximum. This maximum is usually interpreted by Alfvén Ion Cyclotron waves generated by the anisotropic ion distribution downstream of quasi- perpendicular bow shocks. We have established that this maximum corresponds to space-localized coherent magnetic structures in the form of Alfvén vortices [Alexandrova et al., JGR, 2006]. The Alfvén vortex is a non-linear cylindrical Alfvén wave, with an axis quasi- parallel to the mean magnetic field B₀ and which is propagating in a plane perpendicular to B₀. The frequent observation of such structures is an indication of their stability in the magnetosheath plasma. We discuss the role of the bow shock in the generation of the Alfvén vortex structures in the magnetosheath turbulence.
DE: 2159 Plasma waves and turbulence
DE: 2728 Magnetosheath
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Joint Assembly