Spontaneous Geometric Structures in 2D MHD Flows

Abstract

Under suitable spatial decay assumptions on the initial data, we establish precise asymptotic expansions of the velocity and magnetic fields for two-dimensional incompressible MHD flows at each positive time. We identify in the far-field profile the hexagonal pattern previously discovered for the 2D Navier-Stokes equations by L. Brandolese \cite{Bra22}, characterized by six equally spaced directions of faster decay for each component of $u(x,t)-u_0(x)$. Surprisingly, the corresponding contribution to the magnetic field vanishes identically, owing to an exact cancellation associated with the antisymmetric structure of the magnetic nonlinearity. Thus, at order $|x|^{-3}$, the hexagonal structure appears only in the velocity field.