COMPRESSIVE COHERENT STRUCTURES AT ION SCALES IN THE SLOW SOLAR WIND

We present a study of magnetic field fluctuations in a slow solar wind stream, close to ion scales, where an increase of the level of magnetic compressibility is observed. Here, the nature of these compressive fluctuations is found to be characterized by coherent structures. Although previous studies have shown that current sheets can be considered the principal cause of intermittency at ion scales, here we show for the first time that, in the case of the slow solar wind, a large variety of coherent structures contributes to intermittency at proton scales, and current sheets are not the most common. Specifically, we find compressive (delta b(parallel to) >> delta b(perpendicular to)), linearly polarized structures in the form of magnetic holes, solitons, and shock waves. Examples of Alfvenic structures (delta b(perpendicular to) > delta b(parallel to).)are identified as current sheets and vortex-like structures. Some of these vortices have delta b(perpendicular to) > delta b(parallel to), as in the case of Alfven vortices, but the majority of them are characterized by delta b(perpendicular to) greater than or similar to delta b(parallel to). Thanks to multi-point measurements by the Cluster spacecraft, for about 100 structures we could determine the normal, the propagation velocity, and the spatial scale along this normal. Independently of the nature of the structures, the normal is always perpendicular to the local magnetic field, meaning that k(perpendicular to) >> k(parallel to). The spatial scales of the studied structures are found to be between two and eight times the proton gyroradius. Most of them are simply convected by the wind, but 25% propagate in the plasma frame. Possible interpretations of the observed structures and the connection with plasma heating are discussed.