Anisotropies in the HI gas distribution toward 3C 196

Abstract

Context. The local Galactic Hi gas was found to contain cold neutral medium (CNM) filaments that are aligned with polarized dust emission. These filaments appear to be dominated by the magnetic field and in this case turbulence is expected to show distinct anisotropies. Aims. We use the Galactic Effelsberg-Bonn Hi Survey (EBHIS) to derive 2D turbulence spectra for the Hi distribution in direction to 3C 196 and two more comparison fields. Methods. Prior to Fourier transform we apply a rotational symmetric 50% Tukey window to apodize the data. We derive average as well as position angle dependent power spectra. Anisotropies in the power distribution are defined as the ratio of the spectral power in orthogonal directions. Results. We find strong anisotropies. For a narrow range in position angle, in direction perpendicular to the filaments and the magnetic field, the spectral power is on average more than an order of magnitude larger than parallel. In the most extreme case the anisotropy reaches locally a factor of 130. Anisotropies increase on average with spatial frequency as predicted by Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains almost unchanged. The strongest anisotropies are observable for a narrow range in velocity and decay with a power law index close to -8/3, almost identical to the average isotropic spectral index of −2.9 < γ < −2.6. Conclusions. Hi filaments, associated with linear polarization structures in LOFAR observations in direction to 3C 196, show turbulence spectra with marked anisotropies. Decaying anisotropies appear to indicate that we witness an ongoing shock passing the Hi and affecting the observed Faraday depth.

Magnetohydrodynamic turbulence and turbulent dynamo in partially ionized plasma

“…Over scales larger than the damping scales, neutrals are strongly coupled with ions and magnetic fields, and thus the turbulence properties measured in neutrals can also reflect the properties of turbulent magnetic fields. As expected, in the diffuse magnetized ISM, the H I gas distribution exhibits turbulence anisotropy (e.g., Kalberla and Kerp 2016), and both the velocity gradient (González-Casanova and Lazarian 2017, Yuen and Lazarian 2017a, 2017b) and density structure (Clark et al 2015) of H I gas are found to well trace the magnetic field orientation.…”

H i column density statistics of the cold neutral medium from absorption studies

“…In this work, we studied the effect of anisotropic structures in H i in the isotropic power spectrum using a particular model for the anisotropic power spectrum. Kalberla & Kerp (2016) and Kalberla et al (2017) measure a maximum power anisotropy of a few times 10, which translate to a factor of ∼ 0.5 for the quantity R. This suggests that in presence of anisotropic structures in H i , an estimation of only isotropic power results in an additional degeneracy in the amplitude of the measured optical depth autocorrelation. This degeneracy, however, can be lifted by probing the statistical nature of anisotropic structures.…”

“…In this work we are interested in cold neutral medium structures at length scales ranging sub parsecs to a few tens of parsecs. Recent works by Martin et al (2015); Kalberla & Kerp (2016); Kalberla et al (2017); Blagrave et al (2017) and Clark et al (2019) find the existence of anisotropic structures at scales of a few pcs to 100s of pcs in certain directions in the Galaxy with significant alignments along the magnetic field lines. Here, we restrict ourselves to the isotropic estimators of the two point correlation function and hence Ξ depends only on U = | U |.…”

“…In a more recent study on the anisotropy in ISM turbulence, Kandel et al (2017) quantify the anisotropic structures in terms of the multipole moments of the structure function. Muller et al (2004), Kalberla & Kerp (2016) and Kalberla et al (2017) measure the anisotropy associated with H i distribution along different directions in our Galaxy and the Magellanic bridge.…”

Magnetized filament models for diverging plasma lenses

“…Examples of field aligned HI fibers in the cold neutral medium (Stanimirovic & Zweibel 2018) have been discovered in the diffuse ISM (Clark, Peek & Putman 2014). These linear features vary in scale up to a maximum length ∼ 10 4 AU and are aligned along the poloidal magnetic field lines (Kalberla & Kerp 2016). If the HI fibers can act as refractive lenses, they are most similar to the poloidal magnetic filament model (Gwinn 2019).…”