Ionization equilibrium and equation of state of hydrogen plasmas in strongmagnetic fields

We study hydrogen plasmas at magnetic fields B similar to 10(12) - 10(13) G , densities rho similar to 10(-3) - 10(3) g cm(-3) and temperatures T simil ar to 10(5.5) - 10(6.5) K, typical of photospheres of middle-aged cooling n eutron stars. We construct an analytical free energy model of the partially ionized plasma, including into consideration the decentred atomic states, which arise due to the thermal motion across the strong field. We show that these states, neglected in previous studies, may contribute appreciably in to thermodynamics of the outer atmospheric layers at rho less than or simil ar to 1 g cm(-3) and typical B and T. We bake into account Coulomb non-idea lity of the ionized component of the plasma affected by intense magnetic fi eld. Ionization degree, occupancies and equation of state are calculated, a nd their dependences on the temperature, density and magnetic field are stu died.