1,798 research outputs found

### Localized magnetorotational instability and its role in the accretion disc dynamo

(Abriged) The magnetorotational instability (MRI) is believed to be an
efficient way to transport angular momentum in accretion discs. It has also
been suggested as a way to amplify magnetic fields in discs, the instability
acting as a nonlinear dynamo. Recent numerical work has shown that a
large-scale magnetic field, which is predominantly azimuthal, can be sustained
by motions driven by the MRI of this same field. Following this idea, we
present an analytical calculation of the MRI in the presence of an azimuthal
field with a non-trivial vertical structure. We find that the mean radial EMF
associated to MRI modes tends to reduce the magnetic energy, acting like a
turbulent resistivity by mixing the non-uniform azimuthal field. Meanwhile, the
azimuthal EMF generates a radial field that, in combination with the Keplerian
shear, tends to amplify the azimuthal field and can therefore assist in the
dynamo process. This effect, however, is reversed for sufficiently strong
azimuthal fields, naturally leading to a saturation of the dynamo and possibly
to a cyclic behaviour of the magnetic field, as found in previous numerical
works.Comment: 15 pages, 5 figure

### On the interaction between tides and convection

We study the interaction between tides and convection in astrophysical bodies
by analysing the effect of a homogeneous oscillatory shear on a fluid flow.
This model can be taken to represent the interaction between a large-scale
periodic tidal deformation and a smaller-scale convective motion. We first
consider analytically the limit in which the shear is of low amplitude and the
oscillation period is short compared to the timescales of the unperturbed flow.
In this limit there is a viscoelastic response and we obtain expressions for
the effective elastic modulus and viscosity coefficient. The effective
viscosity is inversely proportional to the square of the oscillation frequency,
with a coefficient that can be positive, negative or zero depending on the
properties of the unperturbed flow. We also carry out direct numerical
simulations of Boussinesq convection in an oscillatory shearing box and measure
the time-dependent Reynolds stress. The results indicate that the effective
viscosity of turbulent convection falls rapidly as the oscillation frequency is
increased, attaining small negative values in the cases we have examined,
although significant uncertainties remain because of the turbulent noise. We
discuss the implications of this analysis for astrophysical tides.Comment: 14 pages, 5 figures, to be published in MNRA

### On the angular momentum transport due to vertical convection in accretion discs

The mechanism of angular momentum transport in accretion discs has long been
debated. Although the magnetorotational instability appears to be a promising
process, poorly ionized regions of accretion discs may not undergo this
instability. In this letter, we revisit the possibility of transporting angular
momentum by turbulent thermal convection. Using high-resolution spectral
methods, we show that strongly turbulent convection can drive outward angular
momentum transport at a rate that is, under certain conditions, compatible with
observations of discs. We find however that the angular momentum transport is
always much weaker than the vertical heat transport. These results indicate
that convection might be another way to explain global disc evolution, provided
that a sufficiently unstable vertical temperature profile can be maintained.Comment: 6 pages, 5 figures, accepted in MNRA

### Magnetic self-organisation in Hall-dominated magnetorotational turbulence

The magnetorotational instability (MRI) is the most promising mechanism by
which angular momentum is efficiently transported outwards in astrophysical
discs. However, its application to protoplanetary discs remains problematic.
These discs are so poorly ionised that they may not support magnetorotational
turbulence in regions referred to as `dead zones'. It has recently been
suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect,
could revive these dead zones by enhancing the magnetically active column
density by an order of magnitude or more. We investigate this idea by
performing local, three-dimensional, resistive Hall-MHD simulations of the MRI
in situations where the Hall effect dominates over Ohmic dissipation. As
expected from linear stability analysis, we find an exponentially growing
instability in regimes otherwise linearly stable in resistive MHD. However,
instead of vigorous and sustained magnetorotational turbulence, we find that
the MRI saturates by producing large-scale, long-lived, axisymmetric structures
in the magnetic and velocity fields. We refer to these structures as zonal
fields and zonal flows, respectively. Their emergence causes a steep reduction
in turbulent transport by at least two orders of magnitude from extrapolations
based upon resistive MHD, a result that calls into question contemporary models
of layered accretion. We construct a rigorous mean-field theory to explain this
new behaviour and to predict when it should occur. Implications for
protoplanetary disc structure and evolution, as well as for theories of planet
formation, are briefly discussed.Comment: 18 pages, 16 figures, accepted for publication in MNRA

### On the Nature of Magnetic Turbulence in Rotating, Shearing Flows

The local properties of turbulence driven by the magnetorotational
instability (MRI) in rotating, shearing flows are studied in the framework of a
shearing-box model. Based on numerical simulations, we propose that the
MRI-driven turbulence comprises two components: the large-scale shear-aligned
strong magnetic field and the small-scale fluctuations resembling
magnetohydrodynamic (MHD) turbulence. The energy spectrum of the large-scale
component is close to $k^{-2}$, whereas the spectrum of the small-scale
component agrees with the spectrum of strong MHD turbulence $k^{-3/2}$. While
the spectrum of the fluctuations is universal, the outer-scale characteristics
of the turbulence are not; they depend on the parameters of the system, such as
the net magnetic flux. However, there is remarkable universality among the
allowed turbulent states -- their intensity $v_0$ and their outer scale
$\lambda_0$ satisfy the balance condition $v_0/\lambda_0\sim \mathrm
d\Omega/\mathrm d\ln r$, where $\mathrm d\Omega/\mathrm d\ln r$ is the local
orbital shearing rate of the flow. Finally, we find no sustained dynamo action
in the $\mathrm{Pm}=1$ zero net-flux case for Reynolds numbers as high as
$45\,000$, casting doubts on the existence of an MRI dynamo in the
$\mathrm{Pm}\leq 1$ regime.Comment: 5 pages, 6 figures, 1 tabl

### Spiral-driven accretion in protoplanetary discs - I. 2D models

We numerically investigate the dynamics of a 2D non-magnetised protoplanetary
disc surrounded by an inflow coming from an external envelope. We find that the
accretion shock between the disc and the inflow is unstable, leading to the
generation of large-amplitude spiral density waves. These spiral waves
propagate over long distances, down to radii at least ten times smaller than
the accretion shock radius. We measure spiral-driven outward angular momentum
transport with 1e-4 1e-8
Msun/yr. We conclude that the interaction of the disc with its envelope leads
to long-lived spiral density waves and radial angular momentum transport with
rates that cannot be neglected in young non-magnetised protostellar discs.Comment: 4 pages, 4 figures, accepted in A&A Letter

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