  A model for the longitudinal structure function $F_L$ in the region of low
$x$ and low $Q^2$ is discussed. It is constructed using the $k_T$ factorization
theorem and a photon-gluon fusion mechanism suitably extrapolated to the region
of low $Q^2$. A phenomenological model for higher twist is presented, which is
based on the assumption that the contribution of quarks having limited
transverse momenta is dominated by the soft pomeron exchange mechanism.
