理解天体物理学磁场的几何学
Understanding the Geometry of Astrophysical Magnetic Fields
摘要：
法拉第旋转测量为测量天体物理磁化等离子体的性质提供了一种宝贵的技术。不幸的是，典型的观测只提供了平行于视线的磁场分量的密度加权平均值的信息。结果，沿着视线的磁场几何形状，以及在许多情况下甚至旋转材料的位置，受到很差的约束。通常，法拉第旋转观测的解释取决于被探测的磁场的基础模型（例如，均匀、湍流、均分）。然而，我们发现，在足够低的频率下，特别是在大约13（RM/rad m ^-2）^（1/4）（B/G）^（1/2）MHz以下，法拉第旋转的特性发生了变化，进入了我们所说的"超绝热状态"，在这种状态下，旋转测度与场的视线分量的积分绝对值成正比。因此，将高频率下的旋转测量与这种新机制下的旋转测量进行比较，可以提供有关沿视线的磁场几何形状的直接信息。此外，定义到该新状态的转变的频率nu_SA直接取决于局部电子密度和磁场强度，其中磁场垂直于视线，从而允许在发射区域内和发射区域前方的法拉第旋转之间的明确区分。nu_SA的典型值的范围从10 kHz到10 GHz，这取决于法拉第旋转环境的细节。特别是，对于已解析的活动星系核，包括银河系中心的黑洞（Sgr A *）和M81，nu_SA的范围约为10 MHz至10 GHz，因此可以通过现有的和即将到来的地面射电天文台进行探测。
英文摘要：
Faraday rotation measurements have provided an invaluable technique with which to measure the properties of astrophysical magnetized plasmas. Unfortunately, typical observations provide information only about the density-weighted average of the magnetic field component parallel to the line of sight. As a result, the magnetic field geometry along the line of sight, and in many cases even the location of the rotating material, is poorly constrained. Frequently, interpretations of Faraday rotation observations are dependent upon underlying models of the magnetic field being probed (e.g., uniform, turbulent, equipartition). However, we show that at sufficiently low frequencies, specifically below roughly 13 (RM/rad m^-2)^(1/4) (B/G)^(1/2) MHz, the character of Faraday rotation changes, entering what we term the ``super-adiabatic regime'' in which the rotation measure is proportional to the integrated absolute value of the line-of-sight component of the field. As a consequence, comparing rotation measures at high frequencies with those in this new regime provides direct information about the geometry of the magnetic field along the line of sight. Furthermore, the frequency defining the transition to this new regime, nu_SA, depends directly upon the local electron density and magnetic field strength where the magnetic field is perpendicular to the line of sight, allowing the unambiguous distinction between Faraday rotation within and in front of the emission region. Typical values of nu_SA range from 10 kHz to 10 GHz, depending upon the details of the Faraday rotating environment. In particular, for resolved AGN, including the black holes at the center of the Milky Way (Sgr A*) and M81, nu_SA ranges from roughly 10 MHz to 10 GHz, and thus can be probed via existing and up-coming ground-based radio observatories.
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