Galactic and Extragalactic Magnetic Fields
Galactic and Extragalactic Magnetic Fields
复制标题
银河系和河外磁场
DOI：
10.1023/a:1013805401252
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发表时间：
2000
影响因子：
10.3
通讯作者：
R. Beck
中科院分区：
文献类型：
--
作者：
R. Beck
评述了银河系磁场的研究现状。在能量均分假设下，由射电同步加速器数据得到的总场平均强度在局部为6±2 μG，在3kpc银河系半径处约为10±3 μG。这些值与利用当地测量的宇宙射线能谱和γ射线质子径向变化的估计值吻合得很好。光学和同步加速器偏振数据产生4±1 μG的局部规则场强度，但如果场强在光束内波动或存在各向异性场，则该值是上限。另一方面，脉冲星自转测量仅给出1.4±0.2 μG，如果规则场强和热电子密度的波动沿路径长度沿着方向是相关的，则这是一个下限。局部规则场可以是光学臂之间的“磁臂”的一部分。然而，规则银河场的全球结构尚不清楚。从旋转测量数据中检测到银河系中的几个大尺度场反转，但在外部星系中没有观察到类似的现象。银河场可能是年轻的发电机行动，使逆转从混沌种子场被保存下来，或发电机模式的混合物造成的逆转，或逆转是大规模的各向异性场环的签名。银河系被一个厚的射电连续辐射盘所包围，其范围与边缘螺旋星系相似。虽然薄盘中的局部场相对于平面（四极）是偶对称的，但全局厚盘场可能是偶极型的。银河系中心区域拥有高度规则的磁场，强度高达毫高斯，方向垂直于平面。一个主要的扩展数据库的脉冲星旋转措施和塞曼分裂测量需要确定的银河场的结构。在6厘米或更短的波长上对银河系平面进行进一步的偏振测量，可以直接揭示局部磁场的精细结构。
The current state of research of the Galactic magnetic field is reviewed critically. The average strength of the total field derived from radio synchrotron data, under the energy equipartition assumption, is 6±2 μG locally and about 10±3 μG at 3 kpc Galactic radius. These values agree well with the estimates using the locally measured cosmic-ray energy spectrum and the radial variation of protons derived from γ-rays. Optical and synchrotron polarization data yield a strength of the local regular field of 4±1 μG, but this value is an upper limit if the field strength fluctuates within the beam or if anisotropic fields are present. Pulsar rotation measures, on the other hand, give only 1.4±0.2 μG, a lower limit if fluctuations in regular field strength and thermal electron density are anticorrelated along the pathlength. The local regular field may be part of a `magnetic arm’ between the optical arms. However, the global structure of the regular Galactic field is not yet known. Several large-scale field reversals in the Galaxy were detected from rotation measure data, but a similar phenomenon was not observed in external galaxies. The Galactic field may be young in terms of dynamo action so that reversals from the chaotic seed field are preserved, or a mixture of dynamo modes causes the reversals, or the reversals are signatures of large-scale anisotropic field loops. The Galaxy is surrounded by a thick disk of radio continuum emission of similar extent as in edge-on spiral galaxies. While the local field in the thin disk is of even symmetry with respect to the plane (quadrupole), the global thick-disk field may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane. A major extension of the data base of pulsar rotation measures and Zeeman splitting measurements is required to determine the structure of the Galactic field. Further polarization surveys of the Galactic plane at wavelengths of 6 cm or shorter may directly reveal the fine structure of the local magnetic field.