Cosmic-ray-driven dynamo in galactic disks. A parameter study
Cosmic-ray-driven dynamo in galactic disks. A parameter study
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DOI：
10.1051/0004-6361/200810279
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发表时间：
2008-12
影响因子：
6.5
通讯作者：
M. Hanasz;K. Otmianowska-Mazur;G. Kowal;H. C. F. Astronomy;N. C. University;Astronomical Observatory;J. University;D. Physics;Astronomy;Mcmaster University;M. University
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作者：
M. Hanasz;K. Otmianowska-Mazur;G. Kowal;H. C. F. Astronomy;N. C. University;Astronomical Observatory;J. University;D. Physics;Astronomy;Mcmaster University;M. University
目标。我们提出了一个参数研究的磁流体动力发电机驱动的宇宙射线在星际介质（ISM），专注于磁场放大的效率和磁，动能和宇宙射线（CR）能量之间的能量均分的问题。方法.我们进行数值CR-MHD模拟的ISM使用扩展版本的ZEUS-3D代码在剪切盒近似，并考虑到欧姆电阻率，潮汐力和垂直盘重力的存在。CR是随机分布的超新星（SN）遗迹，并描述了扩散平流方程，其中包括各向异性扩散张量。结果方位磁通量和总磁能在大多数模型中被放大，这取决于模型参数的特定选择。我们发现，磁场放大的最有利条件对应于3 × 10 25 cm 2 s −1数量级的磁扩散率，SN率接近于银河系中观察到的SN率，周期性SN活动对应于螺旋臂，以及高度各向异性和场对准的CR扩散。磁场放大率对SN率的大小相对不敏感，SN率的范围为实际值的10%至100%。在最有利的条件下，磁场放大的时间尺度是1.5亿年，在银河系中心半径等于5千秒差距，这接近银河系旋转的时间尺度。在有效放大的情况下，达到的最终磁场能量波动与气体动能均分附近。在所有模型中，CR能量超过均分值至少一个数量级，与通常预期的均分相反。我们认为，在数值模型中宇宙射线的过剩可以归因于这样一个事实，即剪切盒不允许宇宙射线离开系统的水平磁场沿着，可能是真正的星系的情况。
Aims. We present a parameter study of the magnetohydrodynamical-dynamo driven by cosmic rays in the interstellar medium (ISM), focusing on the efficiency of magnetic-field amplification and the issue of energy equipartition between magnetic, kinetic, and cosmicray (CR) energies. Methods. We perform numerical CR-MHD simulations of the ISM using an extended version of ZEUS-3D code in the shearingbox approximation and taking into account the presence of Ohmic resistivity, tidal forces, and vertical disk gravity. CRs are supplied in randomly-distributed supernova (SN) remnants and are described by the diffusion-advection equation, which incorporates an anisotropic diffusion tensor. Results. The azimuthal magnetic flux and total magnetic energy are amplified in the majority of models depending on a particular choice of model parameters. We find that the most favorable conditions for magnetic-field amplification correspond to magnetic diffusivity of the order of 3 × 10 25 cm 2 s −1 , SN rates close to those observed in the Milky Way, periodic SN activity corresponding to spiral arms, and highly anisotropic and field-aligned CR diffusion. The rate of magnetic-field amplification is relatively insensitive to the magnitude of SN rates spanning a range of 10% to 100% of realistic values. The timescale of magnetic-field amplification in the most favorable conditions is 150 Myr, at a galactocentric radius equal to 5 kpc, which is close to the timescale of galactic rotation. The final magnetic-field energies reached in the efficient amplification cases fluctuate near equipartition with the gas kinetic energy. In all models CR energy exceeds the equipartition values by a least an order of magnitude, in contrast to the commonly expected equipartition. We suggest that the excess of cosmic rays in numerical models can be attributed to the fact that the shearing box does not permit cosmic rays to leave the system along the horizontal magnetic field, as may be the case for true galaxies.