Three dimensional, spherically polarized magnetic fields

Abstract

Turbulence in the solar wind is characterized by Alfv\'enic fluctuations that exhibit spherical polarization, a geometric condition resulting in the nearly constant magnitude of the magnetic field. This property persists even during the largest field fluctuations, sometimes leading to local polarity reversals known as switchbacks. A longstanding question is whether three-dimensional smooth magnetic fields can simultaneously satisfy the constant-$|{\bf B}|$ constraint, and how such fields can be constructed analytically or numerically. Here we propose a new numerical method that allows to construct a magnetic field that is exactly spherically polarized, reproducing key features of solar wind fluctuations. Using this framework, we find evidence that discontinuities are unavoidable for generic three-dimensional configurations. Fundamentally, this implies that field rotations cannot maintain exactly constant $|{\bf B}|$ in an arbitrarily large spatial domain. Rather, field rotations with constant magnitude can exist in limited regions of space. We argue that these finite spatial domains are separated by discontinuities where a local departure from constant $|{\bf B}|$ is expected. These results provide insights into the structure of solar wind turbulence and more generally into the nature of nonlinear magnetic fluctuations in plasmas.