Polarization Gradient Study of Interstellar Medium Turbulence Using the Canadian Galactic Plane Survey

Abstract

We have investigated the magneto-ionic turbulence in the interstellar medium through spatial gradients of the complex radio polarization vector in the Canadian Galactic Plane Survey (CGPS). The CGPS data cover 1300 square-degrees, over the range 53 • ≤ ≤ 192 • , −3 • ≤ b ≤ 5 • with an extension to b = 17.5 • in the range 101 • ≤ ≤ 116 • , and arcminute resolution at 1420 MHz. Previous studies found a correlation between the skewness and kurtosis of the polarization gradient and the Mach number of the turbulence, or assumed this correlation to deduce the Mach number of an observed turbulent region. We present polarization gradient images of the entire CGPS dataset, and analyze the dependence of these images on angular resolution. The polarization gradients are filamentary, and the length of these filaments is largest towards the Galactic anti-center, and smallest towards the inner Galaxy. This may imply that small-scale turbulence is stronger in the inner Galaxy, or that we observe more distant features at low Galactic longitudes. For every resolution studied, the skewness of the polarization gradient is influenced by the edges of bright polarization gradient regions, which are not related to the turbulence revealed by the polarization gradients. We also find that the skewness of the polarization gradient is sensitive to the size of the box used to calculate the skewness, but insensitive to Galactic longitude, implying that the skewness only probes the number and magnitude of the inhomogeneities within the box. We conclude that the skewness and kurtosis of the polarization gradient are not ideal statistics for probing natural magneto-ionic turbulence.

Advanced Diagnostics for the Study of Linearly Polarized Emission. II. Application to Diffuse Interstellar Radio Synchrotron Emission

“…Each simulation is assigned a code of the form Ms0.8Ma1.7, for example, which means that the simu- The mean magnetic field is in the x direction, and we view the cube along the x, y, or z directions, where the latter is shown above. The inset image shows polarization gradients from the Canadian Galactic Plane Survey (see Herron et al 2017c), where black denotes a large amplitude of the polarization gradient, and white denotes a small amplitude.…”

“…Gaensler et al (2011) found that the polarization gradient traces spatial variations in the magnetoionic medium, and Burkhart et al (2012) found that statistics of the polarization gradient, such as the skewness and genus, were sensitive to the sonic Mach number of their simulations. However, Herron et al (2017c) cast doubt on the ability of the skewness of the polarization gradient to probe the regime of turbulence, as they found that the skewness of the gradient was very sensitive to angular resolution, and the size of the evaluation box used to calculate the skewness, in the Canadian Galactic Plane Survey dataset (CGPS, Landecker et al 2010).…”

S-band Polarization All-Sky Survey (S-PASS): survey description and maps

“…This has previously been applied to limited areas (e.g. Gaensler et al 2011;Herron et al 2017), but the area, sensitivity, and resolution of S-PASS render it possible to construct an image of turbulence throughout the Milky Way (Iacobelli et al 2014;Robitaille et al 2017).…”

Advanced Diagnostics for the Study of Linearly Polarized Emission. I. Derivation

“…The path in the image plane corresponds to a path in the Q-U plane, and we calculate the speed at which we traverse this path. The background image is of polarization gradients from a portion of the Canadian Galactic Plane Survey, see Herron et al (2017) for more information.…”

“…Burkhart et al (2012) calculated the amplitude of the polarization gradient for mock observations of synchrotron emission propagating through a turbulent magnetoionic medium, and found that statistics of polarization gradient structures, such as the genus, were sensitive to the regime of turbulence. The polarization gradient has been applied to observations by Iacobelli et al (2014), Sun et al (2014) and Herron et al (2017), to constrain the sonic Mach number of observed magnetoionic turbulence, and Robitaille & Scaife (2015) found that there were different networks of filaments in the images of the amplitude of the polarization gradient on different angular scales in a field of the Canadian Galactic Plane Survey (Landecker et al 2010). Robitaille et al (2017) also compared the polarization gradient to the E-and B-modes (Zaldarriaga & Seljak 1997) of diffuse synchrotron emission in the S-band Polarization All Sky Survey (Carretti 2010;Carretti et al 2013), and found that the two had similar properties, and provide complementary information on observed magnetoionic turbulence.…”