Local Simulations of Instabilities in Relativistic Jets I: Morphology
  and Energetics of the Current-Driven Instability
  We present the results of a numerical investigation of current-driven
instability in magnetized jets. Utilizing the well-tested, relativistic
magnetohydrodynamic code Athena, we construct an ensemble of local, co-moving
plasma columns in which initial radial force balance is achieved through
various combinations of magnetic, pressure, and rotational forces. We then
examine the resulting flow morphologies and energetics to determine the degree
to which these systems become disrupted, the amount of kinetic energy
amplification attained, and the non-linear saturation behaviors. Our most
significant finding is that the details of initial force balance have a
pronounced effect on the resulting flow morphology. Models in which the initial
magnetic field is force-free deform, but do not become disrupted. Systems that
achieve initial equilibrium by balancing pressure gradients and/or rotation
against magnetic forces, however, tend to shred, mix, and develop turbulence.
In all cases, the linear growth of current-driven instabilities is
well-represented by analytic models. CDI-driven kinetic energy amplification is
slower and saturates at a lower value in force-free models than in those that
feature pressure gradients and/or rotation. In rotating columns, we find that
magnetized regions undergoing rotational shear are driven toward equipartition
between kinetic and magnetic energies. We show that these results are
applicable for a large variety of physical parameters, but we caution that
algorithmic decisions (such as choice of Riemann solver) can affect the
evolution of these systems more than physically motivated parameters.
