  In the present series of papers we propose a consistent description of the
mass loss process. To study the effects of intrinsic magnetic field of a
close-orbit giant exoplanet (so-called Hot Jupiter) on the atmospheric material
escape and formation of planetary inner magnetosphere in a comprehensive way,
we start with a hydrodynamic model of an upper atmosphere expansion presented
in this paper. While considering a simple hydrogen atmosphere model, we focus
on selfconsistent inclusion of the effects of radiative heating and ionization
of the atmospheric gas with its consequent expansion in the outer space.
Primary attention is paid to investigation of the role of specific conditions
at the inner and outer boundaries of the simulation domain, under which
different regimes of material escape (free- and restricted- flow) are formed.
Comparative study of different processes, such as XUV heating, material
ionization and recombination, H3+ cooling, adiabatic and Lyman-alpha cooling,
Lyman-alpha reabsorption is performed. We confirm basic consistence of the
outcomes of our modeling with the results of other hydrodynamic models of
expanding planetary atmospheres. In particular, we obtain that under the
typical conditions of an orbital distance 0.05 AU around a Sun-type star a Hot
Jupiter plasma envelope may reach maximum temperatures up to ~9000K with a
hydrodynamic escape speed ~9 km/s resulting in the mass loss rates ~(4-7)*10^10
g*s . In the range of considered stellar-planetary parameters and XUV fluxes
that is close to mass loss in the energy limited case. The inclusion of
planetary intrinsic magnetic fields in the model is a subject of the following
up paper (Paper II).
