E and B Polarizations from Inhomogeneous and Solar Surface Turbulence

Brandenburg, Axel; Bracco, A.; Kahniashvili, Tina; Mandal, Sayan; Pol, Alberto Roper; Petrie, Gordon; Singh, Nishant K.

doi:10.3847/1538-4357/aaf383

Cited by 19 publications

(32 citation statements)

References 35 publications

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“…We produced synthetic observations from the models to qualitatively compare with the dust polarization observational results of Planck. A different approach based on fully helical MHD numerical simulations is employed in a separate paper to probe the role of anisotropic turbulence in the dust-polarization power spectra (Brandenburg et al 2019).…”

Section: Discussionmentioning

confidence: 99%

“…Helical turbulence in the ISM could possibly affect our results concerning the T -B and T -E correlations, which mainly focus on the large-scale structure of the Galactic magnetic field around the Sun. Brandenburg et al (2019) explore the link between fully helical MHD turbulence and the polarization power spectra, but their study lacks the interplay with a mean field on large scales.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

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

Bracco

Candelaresi

Sordo

et al. 2019

A&A

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

Bracco

Candelaresi

Sordo

et al. 2019

A&A

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“…17 represents the forward magnetic cascading of MHD turbulence from the injection scale k 0 to small scales (k > k 0 ), while the first two terms describe the inverse cascading (Pouquet et al 1976) in MHD turbulence from k 0 to scale k 1 1/L which corresponds to the physical size L of the MHD system. According to the simulation results from Brandenburg et al (2019), we set k 1 = 0.1 kpc −1 and α 1 = 0.0 by default in this work if not specified. In terms of more physical parameterization, we are interested in realizing theoretical descriptions of turbulence in compressible plasma recently discussed by Cho & Lazarian (2002), Caldwell et al (2016) and Kandel et al (2017).…”

Section: Power Spectrummentioning

confidence: 99%

“…In addition to the technical improvements, we also keep up with recent progress in physical modelling of Galactic foreground emission with the turbulent Galactic magnetic field (GMF), e.g. phenomenological research carried out by Beck et al (2016), analytic estimations calculated by Cho & Lazarian (2002); Caldwell et al (2016); Kandel et al (2017Kandel et al ( , 2018, and heavy simulations analyzed by Akahori et al (2013); Kritsuk et al (2018); Brandenburg et al (2019). For future work about inferring the GMF configuration from observational data (e.g., Galactic synchrotron and dust emission, dispersion measure and Faraday rotation measure) we need physically rational and numerically fast magnetic field simulators instead of setting up trivial random fields or directly adopting expensive magnetohydrodynamics (MHD) simulators.…”

Section: Introductionmentioning

confidence: 99%

hammurabi X: Simulating Galactic Synchrotron Emission with Random Magnetic Fields

Wang

Jaffe

Enßlin

et al. 2020

ApJS

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We present version X of the hammurabi package, the HEALPix-based numeric simulator for Galactic polarized emission. Improving on its earlier design, we have fully renewed the framework with modern C++ standards and features. Multi-threading support has been built in to meet the growing computational workload in future research. For the first time, we present precision profiles of hammurabi line-of-sight integral kernel with multi-layer HEALPix shells. In addition to fundamental improvements, this report focuses on simulating polarized synchrotron emission with Gaussian random magnetic fields. Two fast methods are proposed for realizing divergence-free random magnetic fields either on the Galactic scale where a field alignment and strength modulation are imposed, or on a local scale where more physically motivated models like a parameterized magneto-hydrodynamic (MHD) turbulence can be applied. As an example application, we discuss the phenomenological implications of Gaussian random magnetic fields for high Galactic latitude synchrotron foregrounds. In this, we numerically find B/E polarization mode ratios lower than unity based on Gaussian realizations of either MHD turbulent spectra or in spatially aligned magnetic fields.

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“…Hence, a means of inferring the helicity of magnetic fields, independent of the π ambiguity, is desired. Brandenburg et al (2019) introduced a possible proxy for helical magnetic fields, which could circumvent the uncertainty introduced by the π ambiguity. They used Stokes Q and U polarisation measurements, and decomposed them into rotationally invariant E and B polarisations (Kamionkowski et al 1997;Durrer 2008).…”

Section: Introductionmentioning

confidence: 99%

Helicity proxies from linear polarisation of solar active regions

Prabhu

Brandenburg

Käpylä

et al. 2020

A&A

Self Cite

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Context. The α effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs both between the two hemispheres as well as between small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the π ambiguity of spectropolarimetric observations. Aims. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the π ambiguity. Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields. Methods. We analysed polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then computed the parity-odd EB and TB correlations and tested for a systematic preference of their sign based on the hemisphere of the active regions. Results. We find that: (i) EB and TB correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores; and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals that Faraday rotation does not have a significant influence on the computed parity-odd correlations. Conclusions. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the π ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.

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E and B Polarizations from Inhomogeneous and Solar Surface Turbulence

Cited by 19 publications

References 35 publications

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

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

hammurabi X: Simulating Galactic Synchrotron Emission with Random Magnetic Fields

Helicity proxies from linear polarisation of solar active regions

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