Solar Wind Magnetic Field Background Spectrum from Fluid to Kinetic Scales

During solar activity minima, the solar wind is highly structured in fast and slow wind flows. These two dynamical regimes remarkably differ not only for average magnetic field and plasma values but also for the type of fluctuations they transport. Fast wind is characterized by large amplitude, incompressible fluctuations, mainly Alfvénic, slow wind is generally populated by smaller amplitude and less Alfvénic fluctuations, mainly compressive. Moving from fast to slow wind, along the speed profile of a high velocity stream, we observe the following behavior:a) the power level of magnetic field fluctuations within the inertial range largely decreases, keeping the typical Kolmogorov scaling;b) at proton scales, for about one decade, right beyond the high frequency break generally corresponding to the location of the ion-cyclotron resonance condition, the spectral index becomes flatter and flatter towards a value of -2.7, typically found in literature;c) at higher frequencies, before the electron scales, the spectral index remains around -2.7 and the power level does not change showing to be irrespective of the flow speed. This behavior is typically encountered during quiet solar activity conditions and suggests the existence of a sort of magnetic field background spectrum. Then, an Alfvènic spectrum would be added to this background any time the observer would enter and cross a fluxtube channeling the fast wind into the interplanetary space. Several example, in the limits of the available data, will be reported and the corresponding spectra from different epochs and source regions will be compared.