  Three energy mechanisms invoking large-scale magnetic fields are incorporated
in a model to interpret jet production in black hole (BH) systems, i.e., the
Blandford-Znajek (BZ), the magnetic coupling (MC) and Blandford-Payne (BP)
processes. These energy mechanisms can coexist in BH accretion disc based on
the magnetic field configurations constrained by the screw instability,
provided that the BH spin and the power-law index indicating the variation of
the magnetic field at an accretion disc are greater than some critical values.
In this model the jets are driven by the BZ process in the Poynting flux regime
and by the BP process in the hydromagnetic regime, being consistent with the
spine/sheath jet structure observed in BH sources of stellar and supermassive
size.
