2004ApJ...608..378F

We present a detailed numerical study of the dynamics and evolution of ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under extreme magnetization conditions. We find that Lorentz factors of up to 3000 are achieved and derived a modified Michel scaling (Γ~σ) that allows for a wide variation in the flow Lorentz factor. Pending contamination induced by mass entrainment, the linear Michel scaling links modulations in the ultrarelativistic wind to variations in mass accretion in the disk for a given magnetization. The jet is asymptotically dominated by the toroidal magnetic field allowing for efficient collimation. We discuss our solutions (jets) in the context of gamma-ray bursts and describe the relevant features such as the high variability in the Lorentz factor and how high collimation angles (∼0°-5°), or cylindrical jets, can be achieved. We isolate a jet instability mechanism we refer to as the ``bottleneck'' instability, which essentially relies on a high magnetization and a recollimation of the magnetic flux surfaces. The instability occurs at large radii where any dissipation of the magnetic energy into radiation would in principle result in an optically thin emission.