Local well-posedness for the two-and-a-half-dimensional EMHD system with split fractional dissipation

Abstract

We study the $2\frac12$-dimensional electron magnetohydrodynamics (EMHD) system on $\mathbb T^2$ with componentwise fractional dissipation: $\partial_t a+a_yb_x-a_xb_y=-Λ^αa$ and $\partial_t b-a_yΔa_x+a_xΔa_y=-Λ^βb$, where $0<α,β<2$. This system is a $2\frac12$-dimensional reduction of the magnetic equation in Hall--MHD/EMHD under the ansatz $B=\nabla\times(ae_z)+be_z$. We prove local well-posedness for initial data $(a_0,b_0)\in H^{s+1}(\mathbb T^2)\times H^s(\mathbb T^2)$ with $s\geq 2-\varepsilon$, provided that $α+β>2$. Thus neither component is required to carry a full Laplacian dissipation; the smoothing effects of the two fractional dissipations can be combined to control the Hall nonlinearity. The proof is based on Littlewood--Paley energy estimates, commutator bounds, and cancellations between the leading low--high interactions.