Large amplitude traveling waves for the non-resistive MHD system

We prove the existence of large amplitude bi-periodic traveling waves (stationary in a moving frame) of the two-dimensional non-resistive Magnetohydrodynamics (MHD) system with a traveling wave external force with large velocity speed lambda(omega 1,omega 2) and of amplitude of order O(lambda 1+) where lambda >> 1 is a large parameter. For most values of omega = (omega 1,omega 2) and for lambda >> 1 large enough, we construct bi-periodic traveling wave solutions of arbitrarily large amplitude as lambda -> +infinity. More precisely, we show that the velocity field is of order O(lambda 0+), whereas the magnetic field is close to a constant vector as lambda -> +infinity. Due to the presence of small divisors, the proof is based on a nonlinear Nash-Moser scheme adapted to construct nonlinear waves of large amplitude. The main difficulty is that the linearized equation at any approximate solution is an unbounded perturbation of large size of a diagonal operator and hence the problem is not perturbative. The invertibility of the linearized operator is then performed by using tools from micro-local analysis and normal forms together with a sharp analysis of high- and low-frequency regimes with respect to the large parameter lambda >> 1. To the best of our knowledge, this is the first result in which global in time, large amplitude solutions are constructed for the 2D non-resistive MHD system with periodic boundary conditions and also the first existence result of large amplitude quasi-periodic solutions for a nonlinear PDE in higher space dimension.