Large jets from small-scale magnetic fields

TL;DR

This study uses 3D MHD simulations to explore how rotating magnetic fields can generate collimated jets in stratified media, revealing key conditions for jet formation and stability.

Contribution

It demonstrates the formation of well-collimated jets from small-scale magnetic fields through differential rotation, highlighting the importance of magnetic geometry and rotation rate.

Findings

Jets form with differential rotation creating toroidal magnetic fields

Over 75% Poynting flux conversion efficiency observed

Jets propagate stably over 100 times the source size

Abstract

We consider the conditions under which a rotating magnetic object can produce a magnetically powered outflow in an initially unmagnetized medium stratified under gravity. 3D MHD simulations are presented in which the footpoints of localized, arcade-shaped magnetic fields are put into rotation. It is shown how the effectiveness in producing a collimated magnetically powered outflow depends on the rotation rate, the strength and the geometry of the field. The flows produced by uniformly rotating, non-axisymmetric fields are found to consist mainly of buoyant plumes heated by dissipation of rotational energy. Collimated magnetically powered flows are formed if the field and the rotating surface are arranged such that a toroidal magnetic field is produced. This requires a differential rotation of the arcades' footpoints. Such jets are well-collimated; we follow their propagation through the stratified atmosphere over 100 times the source size. The magnetic field is tightly wound and its propagation is dominated by the development of non-axisymmetric instabilities. We observe a Poynting flux conversion efficiency of over 75% in the longest simulations. Applications to the collapsar model and protostellar jets are discussed.