External Confinement and Surface Modes in Magnetised Force-Free Jets
External Confinement and Surface Modes in Magnetised Force-Free Jets
复制标题
磁化无力射流中的外部约束和表面模式
DOI：
10.1093/mnras/stx2592
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Y. Lyubarsky
中科院分区：
文献类型：
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作者：
E. Sobacchi;Y. Lyubarsky
在天体物理喷流的磁发射范例中，MHD流中的不稳定性是将坡印廷通量转换为等离子体动能的良好候选者。如果磁化的等离子体几乎充满整个空间，喷流就不稳定，因为它的螺旋扰动。然而，当磁场的极向分量具有消失的梯度时，这些模式的增长率被抑制，这可能是现实配置的实际情况。在这里，我们表明，如果磁化等离子体被限制到一个有限的区域由一些外部介质的压力，在接触表面的速度剪切激发不稳定的模式，可以影响一个显着的部分射流的身体。我们发现，当射流的洛伦兹因子为$\Gamma\sim 10 $（$\Gamma\sim 100$）时，这些扰动通常是在沿射流沿着传播几十（几百）个射流半径后发展起来的。因此，表面模式可能在将喷流的能量从坡印亭通量转换为等离子体的动能方面发挥重要作用，特别是在活动星系核中。讨论了色散关系与（i）场线角速度和（ii）封闭气体中声速的标度关系。
In the paradigm of magnetic launching of astrophysical jets, instabilities in the MHD flow are a good candidate to convert the Poynting flux into the kinetic energy of the plasma. If the magnetised plasma fills the almost entire space, the jet is unstable to helical perturbations of its body. However, the growth rate of these modes is suppressed when the poloidal component of the magnetic field has a vanishing gradient, which may be the actual case for a realistic configuration. Here we show that, if the magnetised plasma is confined into a limited region by the pressure of some external medium, the velocity shear at the contact surface excites unstable modes which can affect a significant fraction of the jet's body. We find that when the Lorentz factor of the jet is $\Gamma\sim10$ ($\Gamma\sim 100$), these perturbations typically develop after propagating along the jet for tens (hundreds) of jet's radii. Surface modes may therefore play an important role in converting the energy of the jet from the Poynting flux to the kinetic energy of the plasma, particularly in AGN. The scaling of the dispersion relation with (i) the angular velocity of the field lines and (ii) the sound speed in the confining gas is discussed.