  A number of extragalactic jets show periodic structures at different scales
that can be associated with growing instabilities. The wavelengths of the
developing instability modes and their ratios depend on the flow parameters, so
the study of those structures can shed light on jet physics at the scales
involved. In this work, we use the fits to the jet ridgeline obtained from
different observations of S5 B0836$+$710 and apply stability analysis of
relativistic, sheared flows to derive an estimate of the physical parameters of
the jet. Based on the assumption that the observed structures are generated by
growing Kelvin-Helmholtz (KH) instability modes, we have run numerical
calculations of stability of a relativistic, sheared jet over a range of
different jet parameters. We have spanned several orders of magnitude in
jet-to-ambient medium density ratio, and jet internal energy, and checked
different values of the Lorentz factor and shear layer width. This represents
an independent method to obtain estimates of the physical parameters of a jet.
By comparing the fastest growing wavelengths of each relevant mode given by the
calculations with the observed wavelengths reported in the literature, we have
derived independent estimates of the jet Lorentz factor, specific internal
energy, jet-to-ambient medium density ratio and Mach number. We obtain a jet
Lorentz factor $\gamma \simeq 12$, specific internal energy of $\varepsilon
\simeq 10^{-2}\,c^2$, jet-to-ambient medium density ratio of $\eta\approx
10^{-3}$, and an internal (classical) jet Mach number of $M_\mathrm{j}\approx
12$. We also find that the wavelength ratios are better recovered by a
transversal structure with a width of $\simeq 10\,\%$ of the jet radius. This
method represents a powerful tool to derive the jet parameters in all jets
showing helical patterns with different wavelengths.

High Energy Astrophysical Phenomena