On The Hydrostatic Approximation of Navier-Stokes-Maxwell System with 2D Electronic Fields

In this paper, we prove the local well-posedness of a scaled anisotropic Navier-Stokes-Maxwell system in a two-dimensional striped domain with a transverse magnetic field around $(0,0,1)$ in Gevrey-2 class. We also justify the limit from the scaled anisotropic equations to the associated hydrostatic system and obtain the precise convergence rate. Then, we prove the global well-posedness for the system and show that small perturbations near $(0,0,1)$ decay exponentially in time. Finally, we show the optimality of the Gevrey-2 regularity by proving the solution to linearized hydrostatic system around shear flows $(V(y),0,0)=(y(1-y),0,0)$ with some initial data $(\zeta, \zeta ^1)$ grows exponentially. More precisely, for some large parameter $\lvert k \rvert>M\gg 1$ corresponding to the frequency in $x$, there exists a solution $h_k(t,x,y)$ of the system \begin{equation*} \begin{cases} \partial_{tt}h_k+\partial_th_k-\partial_{yy}h_k+V(y) \partial_x h_k =0,\\ h_k(0,x,y)=\zeta,\quad \partial_th_k(0,x,y)= \zeta ^1,\\ h_k(t, x,0)=h_k(t, x, 1)=0, \end{cases} \end{equation*} such that for any $s\in [0,\frac{1}{2})$ and $t\in [T_k,T_0)$ with $T_{k}\approx |k|^{s-\frac{1}{2}}\to 0$ as $|k|\to \infty$ and some $T_0$ small and independent of $k$, it satisfies \begin{align*} \lVert h_k(t) \rVert_{L^2 }\geq C \, e^{\sqrt{|k|}t}( \lVert \zeta \rVert_{L^2} + \lVert \zeta ^1 \rVert_{L^2}), \end{align*} for some $C > 0$ independent of $k$.