Measurements of the Longitudinal Proton Structure Function FL at HERA

The measurements of the longitudinal proton structure function F L in deep inelastic positron-proton scattering at low Bjorken x are presented. The data were taken by the HERA experiments H1 and ZEUS in a series of runs with different proton beam energies. The measured longitudinal structure function is compared to theoretical predictions in higher order QCD and using dipole model calculations. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.