  The Schwinger model at finite temperature is analyzed using the Thermofield
Dynamics formalism. The operator solution due to Lowenstein and Swieca is
generalized to the case of finite temperature within the thermofield
bosonization approach. The general properties of the statistical-mechanical
ensemble averages of observables in the Hilbert subspace of gauge invariant
thermal states are discussed. The bare charge and chirality of the Fermi
thermofields are screened, giving rise to an infinite number of mutually
orthogonal thermal ground states. One consequence of the bare charge and
chirality selection rule at finite temperature is that there are innumerably
many thermal vacuum states with the same total charge and chirality of the
doubled system. The fermion charge and chirality selection rules at finite
temperature turn out to imply the existence of a family of thermal theta vacua
states parametrized with the same number of parameters as in zero temperature
case. We compute the thermal theta-vacuum expectation value of the mass
operator and show that the analytic expression of the chiral condensate for any
temperature is easily obtained within this approach, as well as, the
corresponding high-temperature behavior.
