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 Alfvénic , slow wind is generally populated by smaller amplitude and less Alfvénic 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 Alfvénic spectrum would be superposed to it .