  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.
