  (Abridged) The upper atmospheres of close-in gas giant exoplanets are
subjected to intense heating/tidal forces from their parent stars.
Atomic/ionized hydrogen (H) layers are sufficiently rarefied that magnetic
pressure may dominate gas pressure for expected planetary magnetic field
strength. We examine the magnetospheric structure using a 3D isothermal
magnetohydrodynamic model that includes: a static "dead zone" near the magnetic
equator containing magnetically confined gas; a "wind zone" outside the
magnetic equator in which thermal pressure gradients and the
magneto-centrifugal-tidal effect give rise to transonic outflow; and a region
near the poles where sufficiently strong tidal forces may suppress transonic
outflow. Using dipole field geometry, we estimate the size of the dead zone to
be ~1-10 planetary radii for a range of parameters. To understand appropriate
base conditions for the 3D isothermal model, we compute a 1D thermal model in
which photoelectric heating from the stellar Lyman continuum is balanced by
collisionally-excited Lyman {\alpha} cooling. This 1D model exhibits a H layer
with temperatures T=5000-10000K down to pressures of 10-100 nbar. Using the 3D
isothermal model, we compute H column densities and Lyman {\alpha} transmission
spectra for parameters appropriate to HD 209458b. Line-integrated transit
depths of 5-10% can be achieved for the above base conditions. Strong magnetic
fields increase the transit signal while decreasing the mass loss, due to
higher covering fraction and density of the dead zone. In our model, most of
the transit signal arises from magnetically confined gas, some of which may be
outside the L1 equipotential. Hence the presence of gas outside the L1
equipotential does not directly imply mass loss. Lastly, we discuss the domain
of applicability for the magnetic wind model described in this paper and in the
Roche-lobe overflow model.
