Exploring magnetic fields on the largest scales

磁场遍布宇宙，包括在星系团中，它们延伸了数百万光年。最近，在所谓的“巨晕”星系团周围，在更大的尺度上发现了磁场。这些磁巨晕之所以重要，一方面是因为它们的存在挑战了磁场起源的理论，另一方面是因为它们可能在宇宙大尺度结构的形成中发挥了作用。探测磁场方向的传统技术在如此巨大的尺度上失效了。威斯康辛大学麦迪逊分校的一个研究小组开发了一种新方法——梯度技术（GT），用于测量超级日冕中的磁场方向。GT利用受磁场影响的流体湍流运动的性质来揭示磁场方向。钱德拉x射线望远镜和低频阵列（LOFAR）射电望远镜获得的磁场图将用于(1)验证巨晕磁场起源的理论，以及(2)巨晕磁场在高能粒子加速和星系团结构发展中的作用。研究生和本科生都将接受培训，学习如何将新技术应用于观测数据、分析观测结果和检验理论预测。该小组将积极参与涉及高中生的外展活动。该团队将采用广泛的数值模拟来衡量GT测量磁场的准确性。此目的将使用星系团合并目录中的多维数据集。数值模拟的结果将用于更好地理解关键参数对由GT方法得到的磁场图的影响。这些包括等离子体磁化、湍流阿尔芬数、星团的演化阶段和望远镜的分辨率。可以测量偏振的星系团部分的GT磁图，即无线电遗迹，将与现有的偏振数据进行比较。通过比较光晕和遗迹中合并团簇和松弛团簇的磁场结构，评价合并团簇的效果。分析了具有不同红移、动态状态和有无冷核的团簇的磁性结构，并将结果与湍流发电机理论的理论预测进行了比较。根据得到的磁场结构，重新评价宇宙射线的再加速和传播效应。对冷核的热浮力不稳定性和星系团外围的磁热不稳定性的预测将与获得的磁场数据进行比较。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Magnetic fields permeate the cosmos, including in clusters of galaxies where they extend for millions of light years. Recently, magnetic fields were discovered on even larger scales around clusters of galaxies in so-called "megahalos". These magnetic megahalos are important both because their existence challenges theories of the origins of magnetic fields and because they may play a part in the growth of large-scale structure in the universe. Traditional techniques to probe the magnetic field direction fail on these gigantic scales. A team from University of Wisconsin, Madison, have developed a new method, the Gradient Technique (GT), to measure the direction of magnetic fields in megahalos. The GT reveals the magnetic field direction by employing the properties of fluid turbulent motions that are affected by the magnetic field. Maps of magnetic fields obtained using the Chandra X-ray telescope and the LOw Frequency ARray (LOFAR) radio telescope will be used to (1) test theories of the origin of the magnetic field in megahalos, and (2) the part megahalo magnetic fields play in the acceleration of energetic particles and the development of structures within galaxy clusters. Students, both graduate and undergraduates, will be trained in applying the new technique to observational data, analyzing observations and testing theoretical predictions. The team will be actively involved in outreach involving high school students.The team will employ extensive numerical simulations to gauge the accuracy of GT in measuring magnetic fields. Cubes from Galaxy Cluster Merger Catalog will be used for this purpose. The results of the numerical simulations will be used to better understand the effect of key parameters on maps of the magnetic field derived from the GT method. These include plasma magnetization, turbulent Alfven number, the evolutional stage of the cluster and the resolution of the telescope. The GT magnetic maps of the parts of clusters where polarization measurements are possible, i.e., radio relics, will be compared with the available polarization data. The structure of magnetic fields in halos and relics will be compared for merging and relaxed clusters to evaluate the effect of cluster merger. The magnetic structure of clusters with different redshifts, dynamical states, with/without cool cores will be analyzed with the results compared to the theoretical predictions of turbulent dynamo theory. The effects of cosmic ray re-acceleration and propagation will be re-evaluated in view of the obtained magnetic field structure. The predictions of the heat-buoyancy instability in cool cores and magnetothermal instability in outskirts of the galaxy clusters will be compared with the obtained magnetic field data.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.