Global well-posedness of planar MHD equations without heat conductivity

In this paper, we consider the Cauchy problem for the planar magnetohydrodynamics (MHD) system with both constant viscosity and constant resistivity but without heat conductivity. Global well-posedness of strong solutions in the presence of natural far field vacuum, due to the finiteness of the mass, is established for any large initial data of suitable smoothness. Density discontinuity and interior vacuum that are either point-like or piecewise-like are also allowed. Technically, the entropy-type energy inequality, which is commonly used as a basic tool in the existing literature on the planar MHD system, is not workable in this paper, as it is not consistent with the far field vacuum. Instead, besides making full use of advantages of the effective viscous flux, a new coupling structure, between the longitudinal velocity u and the transversal magnetic field h, is exploited to recover the dissipative estimate on h.