  We present a method to extract, in the leading and next-to-leading order
approximations, the longitudinal deep-inelastic scattering structure function
FL(x,Q2) from the experimental data by relying on a Froissart-bounded
parametrization of the transversal structure function F2(x,Q2) and, partially,
on the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations. Particular
attention is paid on kinematics of low and ultra low values of the Bjorken
variable x. Analytical expressions for FL(x,Q2) in terms of the effective
parameters of the parametrization of F2(x,Q2) are presented explicitly. We
argue that the obtained structure functions FL(x,Q2) within both, the leading
and next-to-leading order approximations, manifestly obey the Froissart
boundary conditions. Numerical calculations and comparison with available data
from ZEUS and H1-Collaborations at HERA demonstrate that the suggested method
provides reliable structure functions FL(x,Q2) at low x in a wide range of the
momentum transfer (1 GeV2 < Q2 < 3000 GeV2) and can be applied as well in
analyses of ultra-high energy processes with cosmic neutrinos.
