  There is growing evidence that relativistic jets in active galactic nuclei
undergo extended (parsec-scale) acceleration. We argue that, contrary to some
suggestions in the literature, this acceleration cannot be purely hydrodynamic.
Using exact semianalytic solutions of the relativistic MHD equations, we
demonstrate that the parsec-scale acceleration to relativistic speeds inferred
in sources like the radio galaxy NGC 6251 and the quasar 3C 345 can be
attributed to magnetic driving. Additional observational implications of this
model will be explored in future papers in this series.
