Extragalactic jets with helical magnetic fields: relativistic MHD simulations

TL;DR

This study uses high-resolution relativistic MHD simulations to analyze how helical magnetic fields influence the morphology, stability, and energy transfer processes in extragalactic jets, revealing magnetic field transport and jet reacceleration mechanisms.

Contribution

It presents novel grid-adaptive simulations of relativistic jets with helical magnetic fields, highlighting magnetic field transport, jet stability, and reacceleration effects in a detailed morphological context.

Findings

Helical magnetic fields are transported along the jet, forming stronger toroidal regions.

High-speed jets show localized toroidal fields within backflows, unlike low-relativistic jets.

Magnetic compression aids jet reacceleration beyond 100 jet radii.

Abstract

Extragalactic jets are inferred to harbor dynamically important, organized magnetic fields which presumably aid in the collimation of the relativistic jet flows. We here explore by means of grid-adaptive, high resolution numerical simulations the morphology of AGN jets pervaded by helical field and flow topologies. We concentrate on morphological features of the bow shock and the jet beam behind the Mach disk, for various jet Lorentz factors and magnetic field helicities. We investigate the influence of helical magnetic fields on jet beam propagation in overdense external medium. We use the AMRVAC code, employing a novel hybrid block-based AMR strategy, to compute ideal plasma dynamics in special relativity. The helicity of the beam magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. In comparison with equivalent low-relativistic jets which get surrounded by cocoons with vortical backflows filled by mainly toroidal field, the high speed jets demonstrate only localized, strong toroidal field zones within the backflow vortical structures. We find evidence for a more poloidal, straight field layer, compressed between jet beam and backflows. This layer decreases the destabilizing influence of the backflow on the jet beam. In all cases, the jet beam contains rich cross-shock patterns, across which part of the kinetic energy gets transferred. For the high speed reference jet considered here, significant jet deceleration only occurs beyond distances exceeding ${\cal O}(100 R_j)$, as the axial flow can reaccelerate downstream to the internal cross-shocks. This reacceleration is magnetically aided, due to field compression across the internal shocks which pinch the flow.