Magnetic fields in gaps surrounding giant protoplanets

TL;DR

This study investigates how non-ideal magnetohydrodynamic effects influence magnetic field structure and turbulence in gaps around giant protoplanets, impacting accretion and satellite formation.

Contribution

It provides a detailed assessment of non-ideal MHD effects on magnetic fields in protoplanetary gaps using an a posteriori approach, including Hall drift, resistivity, and ambipolar diffusion.

Findings

Hall drift dominates the magnetic field behavior in the gap.

Field alignment affects MRI stability and toroidal field strength.

Large-scale magnetic forces are weak in the circumplanetary disc.

Abstract

Giant protoplanets evacuate a gap in their host protoplanetary disc, which gas must cross before it can be accreted. A magnetic field is likely carried into the gap, potentially influencing the flow. Gap crossing has been simulated with varying degrees of attention to field evolution (pure hydrodynamical, ideal, and resistive MHD), but as yet there has been no detailed assessment of the role of the field accounting for all three key non-ideal MHD effects: Ohmic resistivity, ambipolar diffusion, and Hall drift. We present a detailed investigation of gap magnetic field structure as determined by non-ideal effects. We assess susceptibility to turbulence induced by the magnetorotational instability, and angular momentum loss from large-scale fields. As full non-ideal simulations are computationally expensive, we take an a posteriori approach, estimating MHD quantities from the pure hydrodynamical gap crossing simulation by Tanigawa et al. (2012). We calculate the ionisation fraction and estimate field strength and geometry to determine the strength of non-ideal effects. We find that the protoplanetary disc field would be easily drawn into the gap and circumplanetary disc. Hall drift dominates, so that much of the gap is conditionally MRI unstable depending on the alignment of the field and disc rotation axes. Field alignment also influences the strong toroidal field component permeating the gap. Large-scale magnetic forces are small in the circumplanetary disc, indicating they cannot drive accretion there. However, turbulence will be key during satellite growth as it affects critical disc features, such as the location of the ice line.