The evolution of Alfvénic perturbations in a three‐dimensional MHD model of the inner heliospheric current sheet region

We investigate an idealized model of the evolution of magnetohydrodynamic (MHD) scale velocity and magnetic field fluctuations in the equatorial region of the inner heliosphere at solar minimum. In this model a three‐dimensional current sheet is constructed from a sheared magnetic field embedded between two velocity shear layers. Simulations of the incompressible MHD equations are used to study the evolution of a variety of perturbations to this configuration. We also perform a linear stability analysis using the solutions of the Orr‐Sommerfeld and related equations for the current sheet model. From simulations in which the model is perturbed with small amplitude Alfvén waves, we find that the perturbing velocity and magnetic fields become decorrelated in the region of velocity and magnetic shear during the linear evolution of the fluid instability. This result and similar results for large amplitude perturbations agree qualitatively with previous two‐dimensional simulations and with observations made by the Helios spacecraft at current sheet crossings between 0.3 and 1 AU and gives further evidence that shear‐driven fluid instabilities provide a mechanism for the destruction of velocity‐magnetic field correlation. The simulations also show enhancement of the power in the radial components of the perturbing velocity and magnetic fields in regions where they are sheared, in agreement with observations. We show that the linear evolution of transverse perturbations to the sheared fields gives rise to these radial power enhancements. While these simulations elucidate the three‐dimensional evolution of shear flows, they must be extended further to include compression and expansion of the wind.