A kinematic analysis of the role of velocity shear in expanding plasmas

We present a kinematic analysis of the effects on waves of the shear due to variations in radial flow and field‐aligned propagation speeds in an expanding medium. We show that although the expansion limits the extent to which wave vectors transverse to the shear flow are produced, shear can be very effective at turning wave vectors and producing off‐radial fluctuations. Any initial wave vector spectrum will become spread in wave vector space with no change in spectral index. For typical spectra of the sheared radial speeds, the small‐scale variability in the speed is more important than the large‐scale in the sense of effectively turning the wave vectors. Radial spectra will be unchanged by stream or microstream shear, and although field‐aligned Alfvén speed changes will produce some observable effects, nonlinear processes are still needed to fully explain observations. This work suggests a model for near‐Sun solar wind turbulence in which modes are “driven” nonlocally at all scales rather than the Kolmogoroff model, in which only large‐scale driving is important; this is consistent with well‐developed turbulence spectra in highly structured regions farther out in the wind.