Structure in the Rotation Measure Sky

Abstract

An analysis of structure in rotation measure (RM) across the sky based on the RM catalog of Taylor et al. (2009) is presented. Several resolved RM structures are identified with structure in the local ISM, including radio loops I, II, and III, the Gum nebula, and the Orion-Eridanus super bubble. Structure functions (SFs) of RM are presented for selected areas, and maps of SF amplitude and slope across the sky are compared with Hα intensity and diffuse polarized intensity. RM variance on an angular scale of 1 • is correlated with length of the line of sight through the Galaxy, with a contribution from local structures. The slope of the SFs is less concentrated to the Galactic plane and less correlated with length of the line of sight through the Galaxy, suggesting a more local origin for RM structure on angular scales ∼ 10 • . The RM variance is a factor ∼ 2 higher towards the SGP than towards the NGP, reflecting a more wide-spread asymmetry between the northern and southern Galactic hemispheres. Depolarization of diffuse Galactic synchrotron emission at latitudes < 30 • can be explained largely by Faraday dispersion related to small-scale variance in RM, but the errors allow a significant contribution from differential Faraday rotation along the line of sight.

Magnetic Fields in the Milky Way

“…This is not surprising, seeing the immense complexity of the magnetic and gaseous structures observed in the Milky Way. Large loops of radio emission such as the North Polar Spur or Loop I to IV [88] show influence of magnetic fields [89,74], created by supernovae blowing bubbles in the ionized interstellar gas, dragging the magnetic field with them. The named Loops are giant structures in the sky because they are located very close to the Sun and are therefore conspicuous on the sky.…”

“…This has been interpreted as an A0 dynamo [18], i.e. a dynamo causing an A0 field configuration (see Section 2.1), but has also been attributed to local structure [81,74]. A0 dynamo models are also strongly inconsistent with modeling comparing near-infrared starlight polarization measurements, based on discrepancies in the Galactic disk.…”

Magnetic Field Disorder and Faraday Effects on the Polarization of Extragalactic Radio Sources

“…Some of the scatter in the RM distributions could be due to the uncertainty of the measurements. (Stil et al 2011). However, the median error on the RM T for the small bright sample of 364 objects in this work is only s = 3.5 rad T m −2 .…”

SKA Deep Polarization and Cosmic Magnetism

“…Toward high galactic latitudes where the Galactic contribution is minimum, the Galactic magnetic field, on the other hand, should produce substantially smaller and steeper SF in angular scale (Akahori et al 2013). Observed SFs toward high latitude, while poorly sampled, are consistent with a flat SF on the smallest scales (Mao et al 2010, Stil et al 2011, suggesting a contribution from the IGMF on scales below 1 • .…”