Observations of solar wind fluctuations: Anisotropy properties

DASSO S.; MATTHAEUS W.H.; SMITH C.W.; MILANO L. J.

Prague, Czech Republic

Conferencia; Cosmic Particle Acceleration, 26th meeting of the IAU, Joint Discussion 1, 16-17 August, 2006, Prague, Czech Republic; 2006

IAU

The propagation of galactic and solar cosmic rays in the solar wind (SW) can be strongly influenced by the SW fluctuations properties. Magnetohydrodynamic (MHD) scale fluctuations in the solar wind are usually highly anisotropic, and have also been found to exhibit different properties in regions of high and low solar wind speed. Previous studies show that magnetic correlations present distinct lobes aligned with the parallel and perpendicular (to the mean magnetic field) axes; this 'Maltese cross' pattern motivates the identification of two idealized populations: a 'slab' population with wave-vectors aligned with the main magnetic field and a 'quasi-2D' population with almost perpendicular wave-vectors. We present here an analysis of the magnetic, velocity, and cross helicity correlations at 1AU, from one-point two-times single spacecraft correlations, employing 5 years of ACE plasma and magnetic field data, as a function of solar wind speed and as a function of the angle between the wave number and the mean magnetic field. We analyse the relative 'slab to quasi-2D' population considering fast/slow solar wind. We conclude that the fast wind (>500Km/s) contains practically only slab-like magnetic fluctuations, while the slow (<400Km/s) wind is practically only 2D-like. The anisotropy of the plasma bulk velocity and cross helicity are similar to that of the magnetic energy, implying strong coupling of fluctuations throughout the spectrum.