  The current paradigm foresees that relativistic jets are launched as
magnetically dominated flows, whose magnetic power is progressively converted
to kinetic power of of the matter of the jet, until equipartition is reached.
Therefore, at the end of the acceleration phase, the jet should still carry a
substantial fraction ($\approx$ half) of its power in the form of a Poynting
flux. It has been also argued that, in these conditions, the best candidate
particle acceleration mechanism is efficient reconnection of magnetic field
lines, for which it is predicted that magnetic field and accelerated
relativistic electron energy densities are in equipartition.Through the
modeling of the jet non--thermal emission, we explore if equipartition is
indeed possible in BL Lac objects, i.e. low-power blazars with weak or absent
broad emission lines. We find that one-zone models (for which only one region
is involved in the production of the radiation we observe) the particle energy
density is largely dominating (by 1-2 orders of magnitude) over the magnetic
one. As a consequence, the jet kinetic power largely exceeds the magnetic
power. Instead, if the jet is structured (i.e. made by a fast spine surrounded
by a slower layer), the amplification of the IC emission due to the radiative
interplay between the two components allows us to reproduce the emission in
equipartition conditions.
