Solar Wind Turbulence and Phase Coherence at Ion Kinetic Scales

Abstract:

It is now well established that in the free solar wind the magnetic spectral slope is close to $-5/3$ in the inertial range and becomes steeper at ion scales for frequencies higher than the so-called break frequency $f_b$. However, physical mechanisms at work at ion scales are still under debates. In particular, it is not clear whether we have to deal with a random phase fluctuations, or with a superposition of coherent structures, or with a mixture of both kind of fluctuations. In this paper, using in-situ measurements of the Wind spacecraft in a fast stream on January 30, 1995, we investigate for the first time the phase coupling between different components of the magnetic fluctuations, and in a large range of scales around $f_b$. For the fluctuations in the plane perpendicular to the mean magnetic field ${\bf B_0}$, the analysis of the phases reveals a sporadic narrow frequency band of Alfv\'en Ion Cyclotron waves. The phase coupling of the fluctuations, in the plane parallel to ${\bf B_0}$, reveals presence of coherent structures such as current sheets and Alfv\'en vortex like structures. Furthermore both waves and coherent structures have characteristic frequencies and scales close to $f_b$ which makes them suitable candidates for a clear break and a slope steepening at ion scales. We detail the spectral contribution of waves, structures and of the rest of the signal to the observed spectrum. This analysis allows us to conclude that a particular combination of these phenomena defines the spectral shape around ion scales. We discuss the necessity to consider waves and coherent structures in order to build a model to describes the solar wind turbulence at ion scales.