Magnetic fields in protoplanetary disks

By Mark Wardle. 2007

218 citations recorded by the source corpus.

Abstract Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard population of 0.1 μm grains the active surface layers have a combined column Σactive≈2 g cm−2 at 1 AU; by the time grains have aggregated to 3 μm, Σactive≈80 g cm−2. Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling to 10% of the disk material at 1 AU (i.e. Σactive≈150 g cm−2). At 5 AU the Σactive≈Σtotal once grains have aggregated to 1 μm in size.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/e53dcf05965b7d2b30cd1c7aa605f861dd7359c6

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Instrumentation

In Statistical and Nonlinear Physics

Citations in the corpus

Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics (2017, methodology): “We note that, given similar field strength, vertical transport is more efficient than radial transport by a factor of ~R H (Wardle 2007; Bai & Goodman 2009).”
Global simulations of protoplanetary disks with net magnetic flux: I. Non-ideal MHD case (2016, background): “The three non-ideal MHD effects, namely ohmic diffusion, ambipolar diffusion and the Hall effet, can be characterized by their effective diffusivities ηO, ηA and ηH respectively (Wardle 2007).”
On the diversity and statistical properties of protostellar discs (2018, background): “…ambipolar diffusion and Ohmic resistivity allow for diffusion of the magnetic field relative to the matter, and the Hall effect can cause material to either spin up or spin down depending on the relative orientation of the magnetic field and the rotation axis (e.g. Wardle & Ng 1999; Wardle 2007).”
Thanatology in protoplanetary discs - The combined influence of Ohmic, Hall, and ambipolar diffusion on dead zones (2014, background): “…is well understood (Wardle 1999; Balbus & Terquem 2001), its impact on the non-linear evolution, and in particular on the dynamics of putative dead zones, has remained mostly speculative (Sano & Stone 2002; Salmeron & Wardle 2003, 2005; Wardle 2007; Salmeron & Wardle 2008; Wardle & Salmeron 2012).”
ELECTRIC CHARGING OF DUST AGGREGATES AND ITS EFFECT ON DUST COAGULATION IN PROTOPLANETARY DISKS (2009, background): “This conclusion is in contrast to that of previous studies (e.g., Sano et al. 2000; Wardle 2007) which claimed that the ionization degree increases as the aggregates grow.”
Hall magnetohydrodynamics of partially ionized plasmas (2007, methodology): “Table 3 gives the collision frequencies, plasma Hall parameters and Hall length and time scales in a protostellar disc, for a minimum-solar-mass nebula model at 5 AU from the central star in the presence of a 10−2 G field, using the ionization fraction calculated by Wardle (2007).”
Dead Zone Accretion Flows in Protostellar Disks (2008, background): “The Ohmic resistivity is greater than the Hall resis-4H tivity near the midplane at 1 AU while the two are comparable at 5 AU (Wardle 2007).”
EFFECT OF AMBIPOLAR DIFFUSION ON THE NONLINEAR EVOLUTION OF MAGNETOROTATIONAL INSTABILITY IN WEAKLY IONIZED DISKS (2011, background): “The quantity 〈β〉 is also very useful for studying nonideal MHD effects because the value it controls the relative importance of various non-ideal MHD effects (Ohmic, Hall and AD, e.g., see Wardle (2007); Bai (2011)).”
Dawes Review 6: The Impact of Companions on Stellar Evolution (2016, background): “The nature of the viscosity that allows gas to accrete is not clear, but it might be provided at least in part by the very same magnetic field that is responsible for the jet collimation (Wardle, 2007).”
Gaps, Rings, and Non-Axisymmetric Structures in Protoplanetary Disks - From Simulations to ALMA Observations (2014, methodology): “We chose the monomer size by adopting the values from Wardle (2007).”
WIND-DRIVEN ACCRETION IN PROTOPLANETARY DISKS. I. SUPPRESSION OF THE MAGNETOROTATIONAL INSTABILITY AND LAUNCHING OF THE MAGNETOCENTRIFUGAL WIND (2013, background): “Being weakly ionized, PPDs are not perfectly conducting, which is reflected in the non-ideal MHD terms in the induction equation (e.g., Wardle 2007; Bai 2011a) ∂ B ∂t = ∇ × (u × B) − ∇ × [ηO J + ηH ( J × B̂) + ηA J⊥], (5) where J = ∇ × B is the current density, B̂ denotes unit vector along B, the…”