The longitudinal structure function of the proton for small x

A comparison of the H1 data on the longitudinal structure function, $F_L$, at small $x$ with the predictions from the generalized vector dominance / color dipole picture (GVD/CDP) is presented. Using the set of parameters previously determined in the fits to the total cross section, $\sigma_{\gamma^* p}$, we find good agreement with the data for $F_L$. Scaling in $\eta = (Q^2 + m^2_0) / \Lambda^2 (W^2)$ is discussed in detail for the longitudinal and transverse photoabsorption cross sections.