Solar wind magnetic field background spectrum from fluid to kinetic scales

The solar wind is highly structured in fast and slow flows. These two dynamical regimes remarkably differ not only for the average values of magnetic field and plasma parameters but also for the type of fluctuations they transport. Fast wind is characterized by large-amplitude, incompressible fluctuations, mainly Alfvenic, and slow wind is generally populated by smaller amplitude and less Alfvenic fluctuations, mainly compressive. The typical corotating fast stream is characterized by a stream interface, a fast wind region and a slower rarefaction region formed by the trailing expansion edge of the stream. Moving between these two regions, from faster to slower wind, we observe the following behaviour: (i) The power level of magnetic fluctuations within the inertial range largely decreases, keeping the typical Kolmogorov scaling. (ii) At proton scales, for about one decade right beyond the high-frequency break, the spectral index becomes flatter and flatter towards a value around -2.7. (iii) At higher frequencies, before the electron scales, the spectral index remains around -2.7 and, based on suitable observations available for four corotating streams, the power level does not change, irrespective of the flow speed. All these spectral features, characteristic of high-speed streams, suggest the existence of a sort of magnetic field background spectrum. This spectrum would be common to both faster and slower winds, but, any time the observer would cross the inner part of a fluxtube channeling the faster wind into the interplanetary space, a turbulent and large-amplitude Alfvenic spectrum would be superposed to it.