Is there a left-handed magnetic field in the solar neighborhood?

Abstract

Context. The analysis of the full-sky Planck polarization data at 850 μm revealed unexpected properties of the E- and B-mode power spectra of dust emission in the interstellar medium (ISM). The positive cross-correlations over a wide range of angular scales between the total dust intensity, T, and both E and (most of all) B modes has raised new questions about the physical mechanisms that affect dust polarization, such as the Galactic magnetic field structure. This is key both to better understanding ISM dynamics and to accurately describing Galactic foregrounds to the polarization of the cosmic microwave background (CMB). In particular, in the quest to find primordial B modes of the CMB, the observed positive cross-correlation between T and B for interstellar dust requires further investigation towards parity-violating processes in the ISM. Aims. In this theoretical paper we investigate the possibility that the observed cross-correlations in the dust polarization power spectra, and specifically the one between T and B, can be related to a parity-odd quantity in the ISM such as the magnetic helicity. Methods. We produce synthetic dust polarization data, derived from 3D analytical toy models of density structures and helical magnetic fields, to compare with the E and B modes of observations. We present several models. The first is an ideal fully helical isotropic case, such as the Arnold-Beltrami-Childress field. Second, following the nowadays favored interpretation of the T–E signal in terms of the observed alignment between the magnetic field morphology and the filamentary density structure of the diffuse ISM, we design models for helical magnetic fields wrapped around cylindrical interstellar filaments. Lastly, focusing on the observed T–B correlation, we propose a new line of interpretation of the Planck observations advocating the presence of a large-scale helical component of the Galactic magnetic field in the solar neighborhood. Results. Our analysis shows that: I) the sign of magnetic helicity does not affect E and B modes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce the Planck results; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain the T–B correlation seen in the Planck data. Such a magnetic-field configuration would also account for the observed large-scale T–E correlation. Conclusions. This work suggests a new perspective for the interpretation of the dust polarization power spectra that supports the imprint of a large-scale structure of the Galactic magnetic field in the solar neighborhood.

Link between E–B polarization modes and gas column density from interstellar dust emission

“…aanda Another result that extends the recent finding of P18XI is that r T B , and maybe r EB , may indeed differ from zero, with a stronger positive signal at large scale compared to small scales. Bracco et al (2019) suggested that the T B positive correlation at very large scale (for multipoles l < 50) may be principally caused by the Galactic-magnetic field structure in the Solar neighborhood, which would leave an imprint of a left-handed helical component on the T B correlation on scales of a few hundred parsecs. However, at the angular scales probed in this work, other processes may be at play, since for the closest Gould Belt clouds we would be probing physical scales of a few parsecs.…”

“…Displacing the view point within a kpc-scale shearing box, they found large fluctuations of E-B asymmetry and T -E correlation depending both on the observer's position and on temporal fluctuations of ISM properties due to bursts of star formation. The observer's environment, and the role of the large-scale Galactic magnetic field in the Solar neighborhood, were also considered in Bracco et al (2019) as a possible explanation for the positive T -E and T -B correlations at very low multipoles ( < 50) via a left-handed helical component.…”

A Global Two-scale Helicity Proxy from π-ambiguous Solar Magnetic Fields

“…This decomposition yields a field that is parity even, i.e., statistically mirror symmetric, and another one that is parity odd, i.e., statistically mirror antisymmetric (Kamionkowski et al 1997;. The relevant diagnostic quantity is usually the cross correlation of the spectral representations of E and B (Kahniashvili & Ratra 2005;Kahniashvili et al 2014;Bracco et al 2019).…”

hammurabi X: Simulating Galactic Synchrotron Emission with Random Magnetic Fields

“…The alignment of the random GMF around local filaments (including helicity) and non-Gaussianity will be interesting extensions, through which we can study the joint effect of the magnetic field alignment and its spectral anisotropy. In hammurabi X, both the global and local generators are designed to allow in the future the addition of non-Gaussianity, e.g., with the method introduced by Vio et al (2001), helicity, e.g., with the method instructed by Kitaura & Enßlin (2008) and more realistic modeling, e.g., with local filaments studied by Bracco et al (2018). We intend to extend hammurabi X for further studies of Galactic Faraday rotation, dust emission and free-free absorption by including (where possible) the coupling between the random GMF and the thermal electron and dust distributions implemented in similarly calibrated numeric implementations.…”