$F_L$ at low $x$ and low $Q^2$
  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. 
A.M. Stasto