12 research outputs found
Onset of Fast Magnetic Reconnection in Partially Ionized Gases
We consider quasi-stationary two-dimensional magnetic reconnection in a
partially ionized incompressible plasma. We find that when the plasma is weakly
ionized and the collisions between the ions and the neutral particles are
significant, the transition to fast collisionless reconnection due to the Hall
effect in the generalized Ohm's law is expected to occur at much lower values
of the Lundquist number, as compared to a fully ionized plasma case. We
estimate that these conditions for fast reconnection are satisfied in molecular
clouds and in protostellar disks.Comment: 19 pages, 1 figure, 1 tabl
Model of two-fluid reconnection
A theoretical model of quasi-stationary, two-dimensional magnetic
reconnection is presented in the framework of incompressible two-fluid
magnetohydrodynamics (MHD). The results are compared with recent numerical
simulations and experiment.Comment: 4 pages, 1 figure, accepted to Physical Review Letter
Amplification of magnetic fields by dynamo action in Gaussian-correlated helical turbulence
We investigate the growth and structure of magnetic fields amplified by
kinematic dynamo action in turbulence with non-zero kinetic helicity. We assume
a simple Gaussian velocity correlation tensor, which allows us to consider very
large magnetic Reynolds numbers, up to one trillion. We use the kinematic
Kazantsev-Kraichnan model of dynamo and find a complete numerical solution for
the correlation functions of growing magnetic fields.Comment: 7 pages, 3 figure
Magnetic reconnection with anomalous resistivity in two-and-a-half dimensions I: Quasi-stationary case
In this paper quasi-stationary, two-and-a-half-dimensional magnetic
reconnection is studied in the framework of incompressible resistive
magnetohydrodynamics (MHD). A new theoretical approach for calculation of the
reconnection rate is presented. This approach is based on local analytical
derivations in a thin reconnection layer, and it is applicable to the case when
resistivity is anomalous and is an arbitrary function of the electric current
and the spatial coordinates. It is found that a quasi-stationary reconnection
rate is fully determined by a particular functional form of the anomalous
resistivity and by the local configuration of the magnetic field just outside
the reconnection layer. It is also found that in the special case of constant
resistivity reconnection is Sweet-Parker and not Petschek.Comment: 15 pages, 4 figures, minor changes as compared to the 1st versio
A model of Hall reconnection
The rate of quasi-stationary, two-dimensional magnetic reconnection is
calculated in the framework of incompressible Hall magnetohydrodynamics (MHD).
The calculation is based on the solution of Hall-MHD equations that include
Hall and electron pressure terms for electric current. These equations are
solved in a local region across the reconnection electron layer, including only
the upstream region and the layer center. In the case when the ion inertial
length d_i is larger than the Sweet-Parker reconnection layer thickness, the
dimensionless reconnection rate is found to be independent of the electrical
resistivity and equal to d_i/L, where L is the scale length of the external
magnetic field in the upstream region outside the electron layer.Comment: 4 pages, 1 figur
Fast and slow two-fluid magnetic reconnection
We present a two-fluid magnetohydrodynamics (MHD) model of quasi-stationary,
two-dimensional magnetic reconnection in an incompressible plasma composed of
electrons and ions. We find two distinct regimes of slow and fast reconnection.
The presence of these two regimes can provide a possible explanation for the
initial slow build up and subsequent rapid release of magnetic energy
frequently observed in cosmic and laboratory plasmas.Comment: 16 pages, 2 figures, 1 tabl
On the two-dimensional magnetic reconnection with nonuniform resistivity
In this paper two theoretical approaches for the calculation of the rate of
quasi-stationary, two-dimensional magnetic reconnection with nonuniform
anomalous resistivity are considered in the framework of incompressible
magnetohydrodynamics (MHD). In the first, ``global'' equations approach the MHD
equations are approximately solved for a whole reconnection layer, including
the upstream and downstream regions and the layer center. In the second,
``local'' equations approach the equations are solved across the reconnection
layer, including only the upstream region and the layer center. Both approaches
give the same approximate answer for the reconnection rate. Our theoretical
model is in agreement with the results of recent simulations of reconnection
with spatially nonuniform resistivity by Baty, Priest and Forbes (2006),
contrary to their conclusions.Comment: 7 pages, 1 figur
Magnetized Turbulent Dynamo in Protogalaxies
The prevailing theory for the origin of cosmic magnetic fields is that they
have been amplified to their present values by the turbulent dynamo inductive
action in the protogalactic and galactic medium. Up to now, in calculation of
the turbulent dynamo, it has been customary to assume that there is no back
reaction of the magnetic field on the turbulence, as long as the magnetic
energy is less than the turbulent kinetic energy. This assumption leads to the
kinematic dynamo theory. However, the applicability of this theory to
protogalaxies is rather limited. The reason is that in protogalaxies the
temperature is very high, and the viscosity is dominated by magnetized ions. As
the magnetic field strength grows in time, the ion cyclotron time becomes
shorter than the ion collision time, and the plasma becomes strongly
magnetized. As a result, the ion viscosity becomes the Braginskii viscosity.
Thus, in protogalaxies the back reaction sets in much earlier, at field
strengths much lower than those which correspond to field-turbulence energy
equipartition, and the turbulent dynamo becomes what we call the magnetized
turbulent dynamo. In this paper we lay the theoretical groundwork for the
magnetized turbulent dynamo. In particular, we predict that the magnetic energy
growth rate in the magnetized dynamo theory is up to ten time larger than that
in the kinematic dynamo theory. We also briefly discuss how the Braginskii
viscosity can aid the development of the inverse cascade of magnetic energy
after the energy equipartition is reached.Comment: accepted to ApJ, 35 pages, 3 figure