Three-dimensional structure of the magnetic field in the disk of the Milky Way

Abstract

Aims. We present Rotation Measures (RM) of the diffuse Galactic synchrotron emission from the Canadian Galactic Plane Survey (CGPS) and compare them to RMs of extragalactic sources in order to study the large-scale reversal in the Galactic magnetic field (GMF). Methods. Using Stokes Q, U and I measurements of the Galactic disk collected with the Synthesis Telescope at the Dominion Radio Astrophysical Observatory, we calculate RMs over an extended region of the sky, focusing on the low longitude range of the CGPS ( = 52• to = 72 • ). Results. We note the similarity in the structures traced by the compact sources and the extended emission and highlight the presence of a gradient in the RM map across an approximately diagonal line, which we identify with the well-known field reversal of the Sagittarius-Carina arm. We suggest that the orientation of this reversal is a geometric effect resulting from our location within a GMF structure arising from current sheets that are not perpendicular to the Galactic plane, as is required for a strictly radial field reversal, but that have at least some component parallel to the disk. Examples of models that fit this description are the three-dimensional dynamo-based model of Gressel et al. (2013) and a Galactic scale Parker spiral (Akasofu & Hakamada 1982), although the latter may be problematic in terms of Galactic dynamics. Conclusions. We emphasize the importance of constructing three-dimensional models of the GMF to account for structures like the diagonal RM gradient observed in this dataset.

Pulsar Rotation Measures and Large-scale Magnetic Field Reversals in the Galactic Disk

“…Most recent models (Brown et al 2007;Sun et al 2008;Sun & Reich 2010;Van Eck et al 2011;Jansson & Farrar 2012) with the benefit of a larger sample of EGRS RMs (Brown et al 2003;Van Eck et al 2011) are dominated by a general clockwise field but with counterclockwise fields in the Sagittarius/Scutum -Crux spiral zone. Recently, Ordog et al (2017) used both EGRS and Galactic continuum background data to suggest that the outer boundary of this spiral zone is not perpendicular to the Galactic plane but is sloping toward later longitudes at positive latitudes and earlier longitudes at negative latitudes.…”

Practical Modeling of Large-Scale Galactic Magnetic Fields: Status and Prospects

“…Though it remains difficult to determine the distance to these reversals, Han et al [13] use Galactic pulsars with distance estimates to model the reversals with alternating magnetic arms and inter-arm regions. Ordog et al [75] recently added the analysis of the RM gradient in the diffuse polarized radio emission. Their findings highlight the importance of modeling these reversals in 3D and connecting them to dynamo theory.…”

Faraday Rotation of Extended Emission as a Probe of the Large-Scale Galactic Magnetic Field

“…Considering the diagonal orientation of the boundary across which the reversal occurs, it may be argued that it is artificial to average over vertical bins in the longitude-latitude maps. Averaging the XE and EG RMs in bins parallel to the boundary was illustrated in Figure 3 of [22]. However, calculating the ratios of those diagonally binned EG and XE RM values still does not yield a constant value in this region.…”

“…However, calculating the ratios of those diagonally binned EG and XE RM values still does not yield a constant value in this region. This may be due to the possibility suggested in [22] that the current sheet defining the boundary between opposing magnetic field directions is tilted not only in the plane of the sky-projection, but also along the line of sight, leading to complicated three-dimensional variations in the ratio of the detected EG to XE RMs near the reversal.…”