Extent of Magnetic-Field Wrapping Around the Planet

Determine to what extent the magnetic field can wrap around a hot Jupiter when the nightside atmosphere is sufficiently resistive that the field is largely decoupled from the flow, particularly when dayside regions have magnetic Reynolds numbers exceeding unity.

Background

The paper evaluates the magnetic Reynolds number to assess whether magnetic-field advection and deformation are important relative to magnetic diffusion. The authors find that the nightside atmosphere is generally characterized by low magnetic Reynolds numbers at higher pressures, suggesting that the field remains decoupled there, whereas dayside and lower-pressure regions can have magnetic Reynolds numbers near or above unity.

This creates an unresolved global-geometry problem: even if the dayside flow can advect and deform the field, it is unclear how far that deformation can extend around the planet when the nightside remains weakly coupled. Resolving this question is important for determining whether static-field magnetic-drag models adequately represent the field geometry or whether fully time-dependent MHD calculations are required.