Solar wind rotation rate and shear at coronal hole boundaries

Abstract

Context. In situ measurements by several spacecraft have revealed that the solar wind is frequently perturbed by transient structures that have been interpreted as magnetic folds, jets, waves, and flux ropes that propagate rapidly away from the Sun over a large range of heliocentric distances. Parker Solar Probe (PSP), in particular, has detected very frequent rotations of the magnetic field vector at small heliocentric radial distances, accompanied by surprisingly large solar wind rotation rates. The physical origin of such magnetic field bends and switchbacks, the conditions for their survival across the interplanetary space, and their relation to solar wind rotation are yet to be clearly understood. Aims. We aim to characterise the global properties of the solar wind flows crossed by PSP, to relate those flows to the rotational state of the low solar corona, and to identify regions of the solar surface and corona that are likely to be sources of switchbacks and bends. Methods. We traced measured solar wind flows from the spacecraft position down to the surface of the Sun to identify their potential source regions, and used a global magneto-hydrodynamic model of the corona and solar wind to analyse the dynamical properties of those regions. We identify regions of the solar corona for which solar wind speed and rotational shear are important and long-lived that can be favourable to the development of magnetic deflections and to their propagation across extended heights in the solar wind. Results. We show that coronal rotation is highly structured, and that enhanced flow shear and magnetic field gradients develop near the boundaries between coronal holes and streamers, and around and above pseudo-streamers, even when such boundaries are aligned with the direction of solar rotation. The exact properties and amplitudes of the shears are a combined effect of the forces exerted by the rotation of the corona and of its magnetic topology. A large fraction of the switchbacks identified by PSP map back to these regions, both in terms of instantaneous magnetic field connectivity and of the trajectories of wind streams that reach the spacecraft. Conclusions. We conclude that these regions of strong shears are likely to leave an imprint on the solar wind over large distances and to increase the transverse speed variability in the slow solar wind. The simulations and connectivity analysis suggest they could be a source of the switchbacks and spikes observed by PSP.