Pulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$, where RM and DM are the rotation and dispersion measure of the pulsar. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and trans-sonic flows, but for supersonic turbulence, the field strength can be severely overestimated by using $1.232 \, \text{RM}/\text{DM}$. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density and magnetic fields probed by 12CO data of molecular clouds, magnetic fields from the Zeeman splitting of the 21 cm line, neutral hydrogen column density, and H α observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$ provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.

中文翻译：

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来自脉冲星观测的银河系磁场：热电子和磁场之间相关性的影响

脉冲星可以作为银河系磁场的极好探测器。平行于视线的银河磁场分量的平均强度可以估计为 $\langle B_\parallel \rangle = 1.232 \, \text{RM}/\text{DM}$，其中 RM 和 DM 是脉冲星的旋转和色散测量。然而，这是假设星际介质的热电子密度和磁场不相关。使用数值模拟和观察，我们测试了这个假设的有效性。基于驱动湍流的磁流体动力学模拟，我们表明热电子密度与小尺度磁场之间的相关性随着湍流马赫数的增加而增加。我们发现热电子密度和磁场不相关的​​假设仅对亚音速和跨音速流动有效，但对于超音速湍流，使用 $1.232 \, \text{RM}/\text{ 可以严重高估场强马克}$。然后，我们将来自澳大利亚国家望远镜的现有脉冲星观测与分子云的 12CO 数据探测到的热电子密度和磁场增强的区域、21 厘米线的塞曼分裂产生的磁场、中性氢柱密度和 H α 相关联观察。使用这些观测数据，我们表明热电子密度和磁场在 kpc 尺度上基本不相关。因此，我们得出关系 $\langle B_\parallel \rangle = 1.232 \,