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ABSTRACT: Macroscopic gas motions are widespread throughout the solar atmosphere and
shearing motions couple to the non--ideal effects, destabilising low frequency
fluctuations in the medium. The origin of this non-ideal magnetohydrodynamic
instability lies in the collisional coupling of the neutral particles to the
magnetized plasma in the presence of a sheared background flow. Unsurprisingly,
the maximum growth rate and most unstable wavenumber depend on the flow
gradient and ambient diffusivities.
The orientation of the magnetic field, velocity shears and perturbation wave
vector play a crucial role in assisting the instability. When the magnetic
field and wave vector are both vertical, ambipolar and Ohm diffusion can be
combined as Pedersen diffusion and cause only damping; in this case only Hall
drift in tandem with shear flow drives the instability. However, for
non-vertical fields and oblique wave vectors, both ambipolar diffusion and Hall
drift are destabilizing.
We investigate the stability of magnetic elements in the network and
internetwork regions. The shear scale is not yet observationally determined,
but assuming a typical shear flow gradient $\sim 0.1 \,\mbox{s}^{-1}$ we show
that the magnetic diffusion shear instability grows on a time scale of one
minute. Thus, it is plausible that network--internetwork magnetic elements are
subject to this fast growing, diffusive shear instability, which could play an
important role in driving low frequency turbulence in the plasma in the solar
photosphere and chromosphere.
07/2012;
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ABSTRACT: Several examples of thin, Keplerian, sub-parsec megamaser disks have been
discovered in the nuclei of active galaxies and used to precisely determine the
mass of their host black holes. We show that there is an empirical linear
correlation between the disk radius and black hole mass and that such disks are
naturally formed as molecular clouds pass through the galactic nucleus and
temporarily engulf the central supermassive black hole. For initial cloud
column densities below about 10^{23.5} cm^{-2} the disk is non-self
gravitating, but for higher cloud columns the disk would fragment and produce a
compact stellar disk similar to that observed around Sgr A* at the galactic
centre.
04/2012;
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ABSTRACT: We investigate the stability of partially ionised, differentially rotating,
diffusive disc threaded by both azimuthal and vertical magnetic field. The
general stability criterion of such a disc in the presence of axisymmetric
fluctuations can be stated purely in terms of ambipolar and Hall diffusivities.
It is shown that the disc is magnetorotationally unstable if the sum of scaled
ambipolar and Hall diffusivities are larger than some numerical constant
determined by the rotation profile of the disc. This criterion suggests that
the diffusive discs are always unstable to almost radial fluctuations. The
field geometry and obliqueness of wavevector (encapsulated together in the
topological factor $g$), plays dual role of not only assisting MRI in
ambipolar--Ohm dominated disc but also making otherwise stable region in
Hall--ambipolar diffusion plane unstable.
11/2011;
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ABSTRACT: The magnetic network which consists of vertical flux tubes located in
intergranular lanes is dominated by Hall drift in the photosphere-lower
chromosphere region ($\lesssim 1 Mm$). In the internetwork regions, Hall drift
dominates above $0.25 Mm$ in the photosphere and below $2.5 Mm$ in the
chromosphere. Although Hall drift does not cause any dissipation in the ambient
plasma, it can destabilise the flux tubes and magnetic elements in the presence
of azimuthal shear flow. The physical mechanism of this instability is quite
simple: the shear flow twists the radial magnetic field and generates azimuthal
field; torsional oscillations of the azimuthal field in turn generates the
radial field completing feedback loop. The maximum growth rate of Hall
instability is proportional to the absolute value of the shear gradient and is
dependent on the ambient diffusivity. The diffusivity also determines the most
unstable wavelength which is smaller for weaker fields.
We apply the result of local stability analysis to the network and
internetwork magnetic elements and show that the maximum growth rate for
kilogauss field occurs around $0.5 Mm$ and decreases with increasing altitude.
However, for a $120 G$ field, the maximum growth rate remains almost constant
in the entire photosphere-lower chromosphere except in a small region of lower
photosphere. For shear flow gradient $\sim 0.01 s^{-1}$, the Hall growth time
is 10 minute near the footpoint. Therefore, network fields are likely to be
unstable in the photosphere, whereas internetwork fields could be unstable in
the entire photosphere-chromosphere. Thus the Hall instability can play an
important role in generating low frequency turbulence which can heat the
chromosphere.
08/2011;
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ABSTRACT: Several thin, Keplerian, sub-parsec megamaser disks have been discovered in
the nuclei of active galaxies and used to precisely determine the mass of their
host black holes. We show that there is an empirical linear correlation between
the disk radius and the black hole mass. We demonstrate that such disks are
naturally formed by the partial capture of molecular clouds passing through the
galactic nucleus and temporarily engulfing the central supermassive black hole.
Imperfect cancellation of the angular momenta of the cloud material colliding
after passing on opposite sides of the hole leads to the formation of a compact
disk. The radial extent of the disk is determined by the efficiency of this
process and the Bondi-Hoyle capture radius of the black hole, and naturally
produces the empirical linear correlation of the radial extent of the maser
distribution with black hole mass. The disk has sufficient column density to
allow X-ray irradiation from the central source to generate physical and
chemical conditions conducive to the formation of 22 GHz H2O masers. For
initial cloud column densities less than ~10^{23.5} cm^-2 the disk is non-self
gravitating, consistent with the ordered kinematics of the edge-on megamaser
disks; for higher cloud columns the disk would fragment and produce a compact
stellar disk similar to that observed around Sgr A* at the galactic centre.
08/2011;
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ABSTRACT: We continue our study of weakly ionized protostellar accretion discs that are threaded by a large-scale magnetic field and power a centrifugally driven wind. It has been argued that there is already evidence in several protostellar systems that such a wind transports a significant fraction of the angular momentum from at least some part of the disc. We model this situation by considering a radially localized disc model in which the matter is everywhere well coupled to the field and the wind is the main repository of excess angular momentum. We consider stationary configurations in which magnetic diffusivity counters the shearing and advection of the magnetic field lines. In Wardle & Königl we analysed the disc structure in the hydrostatic approximation (vertical motions neglected inside the disc) and presented exact disc/wind solutions for the ambipolar diffusivity regime. In Königl, Salmeron & Wardle (Paper I) we generalized the hydrostatic analysis to the Hall and Ohm diffusivity domains and used it to identify the disc parameter sub-regimes in which viable solutions with distinct physical properties can be expected to occur. In this paper we test the results of Paper I by deriving full numerical solutions (integrated through the sonic critical surface) of the disc equations in the Hall domain. We confirm all the predictions of the hydrostatic analysis and demonstrate its usefulness for clarifying the behaviour of the derived solutions. We further show that the outflow solutions can be extended to larger scales (so that, in particular, they also cross the Alfvén critical surface) by matching the localized disc solutions to global ‘cold’ wind solutions of the type introduced by Blandford & Payne. To facilitate this matching, we construct a library of wind solutions for a wide range of wind model parameters; this library is made available to the community. The results presented in Wardle & Königl, Paper I and this work combine to form a comprehensive framework for the study of wind-driving accretion discs in protostellar and other astrophysical environments. This theoretical tool could be useful for interpreting observations and for guiding numerical simulations of such systems.
Monthly Notices of the Royal Astronomical Society 03/2011; 412(2):1162 - 1180. · 4.90 Impact Factor
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ABSTRACT: The destabilising effect of Hall diffusion in a Keplerian disc allows the MRI
to occur for much lower ionisation levels than would otherwise be possible.
However, simulations suggest that the consequences for the saturated state are
not as significant as suggested by the linear instability. Close inspection
reveals that that the simulations have not yet probed the Hall-dominated
regime. Here we revisit the effect of Hall diffusion on the MRI and the
implications for the extent of MHD turbulence in protoplanetary discs.
We conduct a local stability analysis for a vertical, weak magnetic field
subject to axisymmetric perturbations with a vertical wave vector. The
diffusivity dependence is presented using analytic expressions and contours in
the eta_H - eta_P plane for the maximum growth rate and corresponding wave
number, the upper cut-off for unstable wave numbers, and the loci that divide
the plane into regions of different characteristic behaviour. In the
highly-diffusive limit the magnetic field decouples from the fluid
perturbations and the diffusive MRI reduces to a diffusive plane-parallel shear
instability with effective shear rate 1.5 Omega. We give analytic expressions
for the growth rate and wave number of the most unstable mode.
Finally, we illustrate the critical effect of Hall diffusion on the extent of
dead zones in protoplanetary discs by applying a local stability criterion to a
simple model of the minimum-mass solar nebula at 1 au, including x-ray and
cosmic-ray ionisation and a population of 1 micron grains. Hall diffusion
increases or decreases the MRI-active column density by an order of magnitude
or more, depending on whether B is parallel or antiparallel to the rotation
axis, respectively. Existing estimates of the depth of magnetically active
layers in protoplanetary discs are likely to be wildly inaccurate.
[Abridged]
03/2011;
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ABSTRACT: We study a model of weakly ionized, protostellar accretion discs that are threaded by a large-scale, ordered magnetic field and power a centrifugally driven wind. We consider the limiting case where the wind is the main repository of the excess disc angular momentum and generalize the radially localized disc model of Wardle & Königl, which focused on the ambipolar diffusion regime, to other field diffusivity regimes, notably Hall and Ohm. We present a general formulation of the problem for nearly Keplerian, vertically isothermal discs using both the conductivity-tensor and the multifluid approaches and simplify it to a normalized system of ordinary differential equations in the vertical space coordinate. We determine the relevant parameters of the problem and investigate, using the vertical-hydrostatic-equilibrium approximation and other simplifications, the parameter constraints on physically viable solutions for discs in which the neutral particles are dynamically well coupled to the field already at the mid-plane. When the charged particles constitute a two-component ion–electron plasma, one can identify four distinct sub-regimes in the parameter domain where the Hall diffusivity dominates and three sub-regimes in the Ohm-dominated domain. Two of the Hall sub-regimes can be characterized as being ambipolar diffusion-like and two as being Ohm-like: the properties of one member of the first pair of sub-regimes are identical to those of the ambipolar diffusion regime, whereas one member of the second pair has the same characteristics as one of the Ohm sub-regimes. All the Hall sub-regimes have Brb/|Bφb| (ratio of radial-to-azimuthal magnetic field amplitudes at the disc surface) >1, whereas in two Ohm sub-regimes this ratio is <1. When the two-component plasma consists, instead, of positively and negatively charged grains of equal mass, the entire Hall domain and one of the Ohm sub-regimes with Brb/|Bφb| < 1 disappear. All viable solutions require the mid-plane neutral–ion momentum exchange time to be shorter than the local orbital time. We also infer that vertical magnetic squeezing always dominates over gravitational tidal compression in this model. In a follow-up paper we will present exact solutions that test the results of this analysis in the Hall regime.
Monthly Notices of the Royal Astronomical Society 12/2009; 401(1):479 - 499. · 4.90 Impact Factor
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ABSTRACT: Supernova remnants interacting with molecular clouds are potentially exciting systems in which to detect evidence of cosmic ray acceleration. Prominent gamma-ray emission is produced via the decay of neutral pions when cosmic rays encounter the nearby dense clouds. In many of the supernova remnants coincident with gamma-ray sources, the presence of OH(1720 MHz) masers is used to identify interaction with dense gas and to provide a kinematic distance to the system. In this paper we use statistical tests to demonstrate that there is a correlation between these masers and a class of GeV- to TeV-energy gamma-ray sources coincident with interacting remnants. For pion decay, the gamma-ray luminosity provides a direct estimate of the local cosmic ray density. We find the cosmic ray density is enhanced by one to two orders of magnitude over the local solar value, comparable to X-ray-induced ionization in these remnants. The inferred ionization rates are sufficient to explain non-equilibrium chemistry in the post-shock gas, where high columns of hydroxyl are observed. Comment: Submitted to ApJ Letters. 5 pages, 1 figure, 3 tables
09/2009;
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ABSTRACT: The nature of the ionizing UV radiation field in the inner Galaxy is investigated by searching for radio continuum emission from the outer envelope of the population of OH/IR stars toward the Galactic center region. Radio continuum observations of a sample of OH/IR stars have been made at 3.6 cm using the VLA in its A, B, and DnC configurations. A highly scattered OH/IR star has been detected with a flux density of ~1 mJy, which suggests that the star is photoionized by an external radiation field. The UV flux responsible for ionization is estimated to be 2.5 × 109(-5)-4 photons cm-2 s-1, where -5 is the mass-loss rate in units of 10-5 M☉ yr-1. We note that the continuum emission should become optically thick below a few hundred megahertz and that measurements at these frequencies will be able to determine both the UV flux and .
The Astrophysical Journal 01/2009; 458(1):L21. · 6.02 Impact Factor
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ABSTRACT: The recent identification of one or two subparsec disks of young, massive stars orbiting the ~4 × 106 M☉ black hole Sgr A* has prompted an in situ scenario for star formation in disks of gas formed from a cloud captured from the Galactic center environment. To date there has been no explanation given for the low angular momentum of the disks relative to clouds passing close to the center. Here we show that the partial accretion of extended Galactic center clouds, such as the 50 km s−1 giant molecular cloud, that temporarily engulf Sgr A* during their passage through the central region of the Galaxy provide a natural explanation for the angular momentum and surface density of the observed stellar disks. The captured cloud material is gravitationally unstable and forms stars as it circularizes, potentially explaining the large eccentricity and range of inclinations of the observed stellar orbits. The application of this idea to the formation of the circumnuclear ring is also discussed.
The Astrophysical Journal 12/2008; 683(1):L37. · 6.02 Impact Factor
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ABSTRACT: ABSTRACTOH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. OH-excitation calculations suggest that the 6049-MHz OH-maser line is excited for higher column densities than for the 1720-MHz line. Previous observations and modelling of 1612-, 1665- and 1667-MHz OH absorption and 1720-MHz OH masers indicated that the column densities in some supernova remnants, ∼1017 cm−2, may be high enough for 6049-MHz OH masers to exist. It is therefore a potentially valuable indicator of remnant–cloud interaction.We present excitation calculations predicting the formation of 6049-MHz OH masers and results of a survey, using the Parkes Methanol Multibeam receiver for 6049-, 6035- and 6030-MHz OH masers toward 35 supernova remnants, a star-forming region and four fields in the Large and Small Magellanic Clouds.Two new sites of 6035- and 6030-MHz OH-maser emission associated with star-forming regions have been discovered, but no 6049-MHz masers were detected to a brightness temperature limit of ∼0.3–0.6 K, even though modelling of the OH excitation suggests that maser emission should have been detected. Our upper limits indicate the OH column density for a typical remnant ≲1016.4 cm−2, which conflicts with observed and modelled column densities. One possible explanation is that 6049-MHz OH masers may be more sensitive to velocity coherence than 1720-MHz OH masers under some conditions.
Monthly Notices of the Royal Astronomical Society 10/2008; 390(1):49 - 58. · 4.90 Impact Factor
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ABSTRACT: We investigate the linear growth and vertical structure of the MRI in protoplanetary discs when dust grains are well mixed with the gas over the entire disc thickness. All the grains have the same radius (a = 0.1, 1 or 3 micron) and constitute 1 % of the total mass of the gas. Solutions are obtained at R = 5 and 10 AU for a minimum-mass solar nebula model and different choices of the initially vertical magnetic field strength (B), configuration of the diffusivity tensor and grain sizes. We find that when no grains are present, or they are > 1 micron, the midplane remains magnetically coupled for B up to a few gauss at both radii. In contrast, when a population of small grains (a = 0.1 micron) is present, the disc is magnetically inactive for z/H < 2 and only B < 50 mG couple to the fluid. At 5 AU, Ohmic diffusion dominates for z/H < 1 when B < a few mG, irrespective of the properties of the grain population. Conversely, at 10 AU this diffusion term is unimportant in all the scenarios studied here. For z/H > 5, ambipolar diffusion is severe and prevents the field from coupling to the gas for all B. Hall diffusion is dominant for a wide range of field strengths at both radii when dust grains are present. The growth rate, wavenumber and range of magnetic field strengths for which MRI-unstable modes exist are all drastically diminished when dust grains are present, particularly when they are small (a ~ 0.1 micron). We conclude that in protoplanetary discs, the magnetic field is able to couple to the gas and shear over a wide range of fluid conditions even when small dust grains are well mixed with the gas. Despite the low magnetic coupling, MRI modes grow for an extended range of magnetic field strengths and Hall diffusion largely determines the properties of the perturbations in the inner regions of the disc (abridged). Comment: 17 pages, 11 figures. Submitted to MNRAS
01/2008;
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ABSTRACT: We report on Australia Telescope Compact Array observations of the massive star-forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non-local thermodynamic equilibrium excitation calculation suggests a kinematic temperature of the order of 200 K. A time-dependent chemical model is also used to model the gas-phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between 2 × 104 and 1.5 × 105 yr. We also report on a feature to the south-east of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3 arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two H ii regions. The implications of the non-detection of radio continuum emission towards G305A and G305B are also discussed.
Monthly Notices of the Royal Astronomical Society 09/2007; 380(4):1703 - 1714. · 4.90 Impact Factor
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ABSTRACT: We report on Australia Telescope observations of the massive star forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non-LTE excitation calculation suggests a kinematic temperature of order 200 K. A time dependent chemical model is also used to model the gas phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between 2 x10^4 and 1.5 x10^5 years. We also report on a feature to the SE of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3\arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two \hii regions. The implications of the non-detection of radio continuum emission toward G305A and G305B are also discussed.
08/2007;
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ABSTRACT: The Hall effect arises in a plasma when electrons are able to drift with the magnetic field but ions cannot. In a fully-ionized plasma this occurs for frequencies between the ion and electron cyclotron frequencies because of the larger ion inertia. Typically this frequency range lies well above the frequencies of interest (such as the dynamical frequency of the system under consideration) and can be ignored. In a weakly-ionized medium, however, the Hall effect arises through a different mechanism -- neutral collisions preferentially decouple ions from the magnetic field. This typically occurs at much lower frequencies and the Hall effect may play an important role in the dynamics of weakly-ionised systems such as the Earth's ionosphere and protoplanetary discs. To clarify the relationship between these mechanisms we develop an approximate single-fluid description of a partially ionized plasma that becomes exact in the fully-ionized and weakly-ionized limits. Our treatment includes the effects of ohmic, ambipolar, and Hall diffusion. We show that the Hall effect is relevant to the dynamics of a partially ionized medium when the dynamical frequency exceeds the ratio of ion to bulk mass density times the ion-cyclotron frequency, i.e. the Hall frequency. The corresponding length scale is inversely proportional to the ion to bulk mass density ratio as well as to the ion-Hall beta parameter. Comment: 11 page, 1 figure, typos removed, numbers in tables revised; accepted for publication in MNRAS
07/2007;
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ABSTRACT: Angular momentum in protostellar discs can be transported either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field. We present a model of steady-state discs where these two mechanisms operate at the same radius and derive approximate criteria for their occurrence in an ambipolar diffusion dominated disc. We obtain "weak field'' solutions - which we associate with the MRI channel modes in a stratified disc - and transform them into accretion solutions with predominantly radial angular-momentum transport by implementing a turbulent-stress prescription based on published results of numerical simulations. We also analyze "intermediate field strength'' solutions in which both radial and vertical transport operate at the same radial location. Our results suggest, however, that this overlap is unlikely to occur in real discs. Comment: 5 pages, 2 figures, 1 table, aastex.cls. Accepted for publication in Astrophysics & Space Science
06/2007;
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ABSTRACT: OH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. Recent calculations suggest that the 6049 MHz OH maser line is excited for higher column densities than for the 1720 MHz line. It is therefore a potentially valuable indicator of remnant-cloud interaction. We present preliminary results of a survey using the Parkes Methanol Multibeam receiver for 6049 MHz and 6035/6030 MHz OH masers towards 36 supernova remnants and 4 fields in the Large and Small Magellanic Clouds. While no 6049 MHz masers have been found, three new sites of 6035 and 6030 MHz OH maser emission have been discovered in star-forming regions. Comment: 2 pages, 1 fig, iaus.cls. To appear in IAU 242, Astrophysical Masers and Their Environments, eds. J. Chapman & W. Baan
04/2007;
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Mark Wardle
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ABSTRACT: OH (1720 MHz) masers serve as indicators of SNR - molecular cloud interaction sites. These masers are collisionally excited in warm (50-100K) shocked gas with densities of order 1e5 cm^-3 when the OH column density is in the range 1e16-1e17 cm^-2. Here I present excitation calculations which show that when the OH column density exceeds 1e17 cm^-2 at similar densities and temperatures, the inversion of the 1720 MHz line switches off and instead the 6049 MHz transition in the first excited rotational state of OH becomes inverted. This line may serve as a complementary signal of warm, shocked gas when the OH column density is large. Comment: 2 pages, 2 figs, iaus.cls. To appear in IAU 242, Astrophysical Masers and Their Environments, eds. J. Chapman & W. Baan. Addition of two references and minor changes to wording
03/2007;
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ABSTRACT: Angular momentum transport in protostellar discs can take place either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field that threads the disc. Using semi-analytic and numerical results, we construct a model of steady-state discs that includes vertical transport by a centrifugally driven wind as well as MRI-induced turbulence. We present approximate criteria for the occurrence of either one of these mechanisms in an ambipolar diffusion-dominated disc. We derive ‘strong field’ solutions in which the angular momentum transport is purely vertical and ‘weak field’ solutions that are the stratified-disc analogues of the previously studied MRI channel modes; the latter are transformed into accretion solutions with predominantly radial angular momentum transport when we implement a turbulent-stress prescription based on published results of numerical simulations. We also analyse ‘intermediate field strength’ solutions in which both modes of transport operate at the same radial location; we conclude, however, that significant spatial overlap of these two mechanisms is unlikely to occur in practice. To further advance this study, we have developed a general scheme that incorporates also the Hall and Ohm conductivity regimes in discs with a realistic ionization structure.
Monthly Notices of the Royal Astronomical Society 02/2007; 375(1):177 - 183. · 4.90 Impact Factor
