arXiv subjects

Vadim Urpin

Publications and source records attributed to Vadim Urpin.

26 records · Page 2Linked to original sources

arXiv2008

Magnetic shear-driven instability and turbulent mixing in magnetized protostellar disks

Observations of protostellar disks indicate the presence of the magnetic field of thermal (or superthermal) strength. In such a strong magnetic field, many MHD instabilities responsible for turbulent transport of the angular momentum are suppressed. We consider the shear-driven instability that can occur in protostellar disks even if the field is superthermal. This instability is caused by the combined influence of shear and compressibility in a magnetized gas and can be an efficient mechanism to generate turbulence in disks. The typical growth time is of the order of several rotation periods.

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arXiv2007

Stability of magnetic configurations containing the toroidal and axial fields

Stability properties of magnetic-field configurations containing the toroidal and axial field are considered. The stability is treated by making use of linear analysis. It is shown that the conditions required for the onset of instability are essentially different from those given by the necessary condition $d (s B_ϕ)/ds > 0$, where $s$ is the cylindrical radius. The growth rate of instability is calculated for a wide range of the parameters. We argue that the instability can operate in two different regimes depending on the strength of the axial field and the profile of the toroidal field.

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arXiv2007

Stretching of the toroidal field and generation of magnetosonic waves in differentially rotating plasma

We evaluate the generation of magnetosonic waves in differentially rotating magnetized plasma. Differential rotation leads to an increase of the azimuthal field by winding up the poloidal field lines into the toroidal field lines. An amplification of weak seed perturbations is considered in this time-dependent background state. It is shown that seed perturbations can be amplified by several orders of magnitude in a differentially rotating flow. The only necessary condition for this amplification is the presence of a non-vanishing component of the magnetic field in the direction of the angular velocity gradient.

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arXiv2007

Hydromagnetic Instability in plane Couette Flow

We study the stability of a compressible magnetic plane Couette flow and show that compressibility profoundly alters the stability properties if the magnetic field has a component perpendicular to the direction of flow. The necessary condition of a newly found instability can be satisfied in a wide variety of flows in laboratory and astrophysical conditions. The instability can operate even in a very strong magnetic field which entirely suppresses other MHD instabilities. The growth time of this instability can be rather short and reach $\sim 10$ shear timescales.

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arXiv2007

Compressibility and local instabilities of differentially rotating magnetized gas

We study the stability of compressible cylindrical differentially rotating flow in the presence of the magnetic field, and show that compressibility alters qualitatively the stability properties of flows. Apart from the well-known magnetorotational instability that can occur even in incompressible flow, there exist a new instability caused by compressibility. The necessary condition of the newly found instability can easily be satisfied in various flows in laboratory and astrophysical conditions and reads $B_{s} B_ϕ Ω' \neq 0$ where $B_{s}$ and $B_ϕ$ are the radial and azimuthal magnetic fields, $Ω' =d Ω/ds$ with $s$ being the cylindrical radius. Contrary to the magnetorotational instability that occurs only if $Ω$ decreases with $s$, the newly found instability operates at any sign of $Ω'$. The considered instability can arise even in a very strong magnetic field that suppresses the magnetorotational instability.

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arXiv2003

Mean-field dynamo action in protoneutron stars

We have investigated the turbulent mean-field dynamo action in protoneutron stars that are subject to convective and neutron finger instabilities. While the first one develops mostly in the inner regions of the star, the second one is favoured in the outer regions, where the Rossby number is much smaller and a mean-field dynamo action is more efficient. By solving the mean-field induction equation we have computed the critical spin period below which no dynamo action is possible and found it to be ~1 s for a wide range of stellar models. Because this critical period is substantially longer than the characteristic spin period of very young pulsars, we expect that a mean-field dynamo will be effective for most protoneutron stars.

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arXiv1998

Joule heating and the thermal evolution of old neutron stars

We consider Joule heating caused by dissipation of the magnetic field in the neutron star crust. This mechanism may be efficient in maintaining a relatively high surface temperature in very old neutron stars. Calculations of the thermal evolution show that, at the late evolutionary stage ($t \geq 10$ Myr), the luminosity of the neutron star is approximately equal to the energy released due to the field dissipation and is practically independent of the atmosphere models. At this stage, the surface temperature can be of the order of $3 \times 10^{4} - 10^{5}$K. Joule heating can maintain this high temperature during extremely long time ($\geq 100$ Myr), comparable with the decay time of the magnetic field.

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arXiv1998

Magnetic and spin evolution of isolated neutron stars with the crustal magnetic field

We consider the magnetic and spin evolution of isolated neutron stars assuming that the magnetic field is initially confined to the crust. The evolution of the crustal field is determined by the conductive properties of the crust which, in its turn, depend on the thermal history of the neutron star. Due to this fact, a study of the magnetic field decay may be a powerful diagnostic of the properties of matter in the core where the density is above the nuclear density. We treat the evolution of neutron stars for different possible equations of state and cooling scenarios (standard and, so called, accelerated cooling). The spin evolution is strongly influenced by the behaviour of the magnetic field. Assuming that the spin-down rate of the neutron star is determined by the magnetodipole radiation, we calculated the evolutionary tracks of isolated pulsars in the $B - τ$ and $B-P$ planes, where $B$ and $P$ are the magnetic field and period, respectively, and $τ$ is the spin-down age. The calculated tracks are compared with observational data on the magnetic field and period of pulsars. This comparison allows to infer the most suitable equations of state of nuclear matter and cooling model and to determine the range of parameters of the original magnetic configurations of pulsars.

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