High-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal that many protoplanetary disks contain multiple rings and gaps. While isolated deep gaps may be produced by embedded planets, the widespread presence of weaker rings suggests additional mechanisms contribute to disk substructure. This study investigates whether magnetic reconnection in weakly ionized disks can generate persistent structures similar to those observed.
Using the magnetohydrodynamics code Athena++, I perform two-dimensional simulations of disks threaded by vertical magnetic fields. The models include two non-ideal magnetic effects, Ohmic resistivity and ambipolar diffusion, whose strengths are computed from chemical networks to produce realistic spatial variations in magnetic diffusivity. A parameter study spanning three orders of magnitude in field strength and four diffusion regimes explores sixteen simulations.
The simulations reveal two distinct evolutionary pathways. Ohmic-dominated cases rapidly form rings through magnetorotational turbulence within hundreds of orbits, but the structures dissipate within a few thousand orbits. In contrast, ambipolar-dominated cases suppress turbulence, producing rings later that persist for more than ten thousand orbits. This difference in longevity suggests that long-lived disk substructures may trace reconnection processes rather than turbulence.
Ring and gap formation occurs through redistribution of magnetic flux via reconnecting current sheets created by radial gas motions. Measured ring spacings are broadly consistent with scales predicted by Sweet-Parker reconnection theory within factors of two to three. The simulations predict an anticorrelation between surface density and vertical magnetic field strength, with field variations exceeding a factor of seventy between rings and gaps.
Ambipolar-dominated disks exhibit accretion rates two orders of magnitude lower than turbulent cases, with angular momentum transported primarily by magnetically driven winds. This wind-dominated regime leads to inside-out disk dissipation and may help explain transition disks with evacuated inner cavities. These results demonstrate that magnetic reconnection can produce persistent disk substructures observed by ALMA and provide observational signatures that distinguish reconnection-driven rings from planetary gaps.