Sharp ill-posedness for the non-resistive MHD equations in Sobolev spaces

TL;DR

This paper establishes the precise threshold for ill-posedness of the non-resistive MHD equations in Sobolev and Besov spaces, demonstrating norm inflation caused by low-high frequency interactions in the nonlinear term.

Contribution

It proves sharp ill-posedness results for the non-resistive MHD equations in critical Sobolev and Besov spaces, extending previous local well-posedness results and analyzing the nonlinear mechanism.

Findings

Ill-posedness in $H^{d/2-1} imes H^{d/2}$ spaces.

Extension of ill-posedness to Besov spaces $B^{d/p-1}_{p,q} imes B^{d/p}_{p,q}$.

Construction of initial data showing norm inflation via low-high frequency interactions.

Abstract

In this paper, we prove a sharp ill-posedness result for the incompressible non-resistive MHD equations. In any dimension $d\ge 2$, we show the ill-posedness of the non-resistive MHD equations in $H^{\frac{d}{2}-1}(\mathbb{R}^d)\times H^{\frac{d}{2}}(\mathbb{R}^d)$, which is sharp in view of the results of the local well-posedness in $H^{s-1}(\mathbb{R}^d)\times H^{s}(\mathbb{R}^d)(s>\frac{d}{2})$ established by Fefferman et al.(Arch. Ration. Mech. Anal., \textbf{223} (2), 677-691, 2017). Furthermore, we generalize the ill-posedness results from $H^{\frac{d}{2}-1}(\mathbb{R}^d)\times H^{\frac{d}{2}}(\mathbb{R}^d)$ to Besov spaces $B^{\frac{d}{p}-1}_{p, q}(\mathbb{R}^d)\times B^{\frac{d}{p}}_{p, q}(\mathbb{R}^d)$ and $\dot B^{\frac{d}{p}-1}_{p, q}(\mathbb{R}^d)\times \dot B^{\frac{d}{p}}_{p, q}(\mathbb{R}^d)$ for $1\le p\le\infty, q>1$. Different from the ill-posedness mechanism of the incompressible Navier-Stokes equations in $\dot B^{-1}_{\infty, q}$ \cite{B,W}, we construct an initial data such that the paraproduct terms (low-high frequency interaction) of the nonlinear term make the main contribution to the norm inflation of the magnetic field.