  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 Alfv\'{e}nic, slow wind is generally populated by smaller
amplitude and less Alfv\'{e}nic 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 behavior: a) the power level of magnetic 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, the spectral index becomes flatter and flatter towards a value
around -2.7; c) at higher frequencies, before the electron scales, the spectral
index remains around -2.7 and, {based on suitable observations available for
$4$ 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 wind 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 Alfv\'{e}nic
spectrum would be superposed to it.
