  We derive the spin-dependent Bethe-Salpeter equation for $q\bar{q}$ bound
state at finite temperature in the ladder and cross box diagram approximations.
The imaginary time formalism is adopted in the adiabatic approximation where
the flux tube thermal excitation is separated from the thermal excitation of
the constituent quarks. The thermal excitation of the flux tube is implemented
by using a temperature dependent potential inferred from the lattice gauge
calculations. The thermal excitations of the constituent quarks are evaluated
by summing Matsubara frequencies. The resultant equation is used to study the
thermal charmonium spontaneous dissociation mechanism at temperatures below the
critical one of the phase transition to the quark gluon plasma. Our results
show that when the thermal excitation of the constituent quarks is considered
beside the flux tube thermal excitation, the spontaneous dissociation unlikely
takes place in the hadronic phase. In contrary, when the thermal excitation of
the flux tube is considered only in the calculations, the spontaneous
dissociation likely takes place just below the critical temperature.
