Modified compensating functions for the incompressible Euler--Vlasov--Fokker--Planck system: Global classical solutions and pointwise-in-space decay

Abstract

We consider the Cauchy problem for the incompressible Euler-Vlasov-Fokker-Planck (Euler-VFP) system in the whole space \(\mathbb R^3\) near the global Maxwellian equilibrium. The Fokker-Planck operator and the particle-fluid drag dissipate the relative momentum but do not separately control the common particle-fluid momentum; in Fourier variables, this degeneracy occurs in the transverse momentum components. To recover the missing coercivity, we augment the classical four-moment compensator with a finite-rank skew-adjoint correction constructed from second-order Hermite modes. Combined with the cancellation between the kinetic and fluid drag terms and the incompressibility constraint, the resulting compensated Fourier energy yields a unique global classical solution for sufficiently small initial data $(u_0,f_0)\in H^N\times L_v^2(H^N)$, with $N\geq 4$. The high-order energy argument involves only spatial derivatives of the kinetic perturbation and requires no mixed \(x\)-\(v\) derivative estimates. We further construct a positive-order Lyapunov functional and establish the decay rate \((1+t)^{-1/2}\) for all positive-order spatial derivatives in the \(L^2\)-norm and for the corresponding pointwise-in-space norms, without any additional \(L^1\) integrability or low-frequency assumption on the initial data. Although no uniform algebraic decay rate is asserted for the zero-order energy of \((u,f)\), the directly dissipative variables \(u-J(f)\) and \(\{\mathbf I-\mathbf P_0\}f\) decay in the \(L^2\)-norm at the same rate, where $ J(f)=\int_{\mathbb R^3}v\sqrt M f\,{\rm d}v$ denotes the particle momentum and \(\mathbf P_0\) is the orthogonal projection onto \(\operatorname{span}\{\sqrt M,v_1\sqrt M,v_2\sqrt M,v_3\sqrt M\}\). To the best of our knowledge, these positive-order and zero-order decay estimates have not previously been established for the incompressible Euler-VFP system.