arXiv subjects

Grzegorz Kowal

Publications and source records attributed to Grzegorz Kowal.

42 records · Page 3Linked to original sources

arXiv2008

The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations

We present an analysis of the distribution of H-alpha emission measures for the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude classical H II regions, leaving only sightlines that sample the diffuse WIM. The distribution of EM sin |b| for the full sample is poorly characterized by a single normal distribution, but is extraordinarily well fit by a lognormal distribution, with = 0.146 +/- 0.001 and standard deviation 0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The distribution may widen slightly at low Galactic latitude. We compare the observed EM distribution function to the predictions of three-dimensional magnetohydrodynamic simulations of isothermal turbulence within a non-stratified interstellar medium. We find that the distribution of EM sin |b| is well described by models of mildy supersonic turbulence with a sonic Mach number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predicts the distribution of dispersion measures of pulsars and H-alpha line profiles. In the best fitting model, the turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also discuss the implications of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM.

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arXiv2007

Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear

We analyze the electromotive force (EMF) terms and basic assumptions of the linear and nonlinear dynamo theories in our three-dimensional (3D) numerical model of the Parker instability with cosmic rays and shear in a galactic disk. We also apply the well known prescriptions of the EMF obtained by the nonlinear dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the EMF reconstructed from their prescriptions corresponds to the EMF obtained directly from our numerical models. We show that our modeled EMF is fully nonlinear and it is not possible to apply any of the considered nonlinear dynamo approximations due to the fact that the conditions for the scale separation are not fulfilled.

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arXiv2007

Aspects of Density Fluctuations in Compressible MHD Turbulence

We study scaling relations of compressible isothermal strongly magnetized turbulence using numerical simulations with resolution 512$^3$. We find a good correspondence of our results with the Fleck (1996) model of compressible hydrodynamic turbulence. In particular, we find that the density-weighted velocity, i.e. $\boldsymbol{u} \equiv ρ^{1/3} \boldsymbol{v}$, proposed in Kritsuk et al. (2007) obeys the Kolmogorov scaling, i.e. ${\cal E}_{u}(k)\sim k^{-5/3}$ for the high Mach number turbulence. Similarly, we find that the exponents of the third order structure functions for $\boldsymbol{u}$ stay equal to unity for all Mach numbers studied. The scaling of higher order correlations obeys the She-Lévêque (1994) scalings corresponding to the two-dimensional dissipative structures, and this result does not change with the Mach number either. In contrast to velocity $\boldsymbol{v}$ which exhibits different scaling parallel and perpendicular to the local magnetic field, the scaling of $\boldsymbol{u}$ is similar in both directions. In addition, we find that the peaks of density create a hierarchy in which both physical and column densities decrease with the scale in accordance to the Fleck (1996) predictions. This hierarchy can be related ubiquitous small ionized and neutral structures (SINS) in the interstellar gas. We believe that studies of statistics of the column density peaks can provide both consistency check for the turbulence velocity studies and insight into supersonic turbulence, when the velocity information is not available.

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arXiv2007

Scaling Relations of Compressible MHD Turbulence

We study scaling relations of compressible strongly magnetized turbulence. We find a good correspondence of our results with the Fleck (1996) model of compressible hydrodynamic turbulence. In particular, we find that the density-weighted velocity, i.e. $u \equiv ρ^{1/3} v$, proposed in Kritsuk et al. (2007) obeys the Kolmogorov scaling, i.e. $E_{u}(k)\sim k^{-5/3}$ for the high Mach number turbulence. Similarly, we find that the exponents of the third order structure functions for $u$ stay equal to unity for the all the Mach numbers studied. The scaling of higher order correlations obeys the She-Leveque (1994) scalings corresponding to the two-dimensional dissipative structures, and this result does not change with the Mach number either. In contrast to $v$ which exhibits different scaling parallel and perpendicular to the local magnetic field, the scaling of $u$ is similar in both directions. In addition, we find that the peaks of density create a hierarchy in which both physical and column densities decrease with the scale in accordance to the Fleck (1996) predictions. This hierarchy can be related ubiquitous small ionized and neutral structures (SINS) in the interstellar gas. We believe that studies of statistics of the column density peaks can provide both consistency check for the turbulence velocity studies and insight into supersonic turbulence, when the velocity information is not available.

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arXiv2006

Separation of the Alfvén mode from a turbulent velocity field using wavelets

We report the first attempt to mode decomposition of the velocity field using wavelet transforms in the application to the magnetized turbulence analysis. We compare results of the separation with respect to the global and local mean magnetic field for models with a strong and weak external magnetic field. We show, that for models with a strong external magnetic field our results are relatively consistent for both methods. However, for superAlfvénic turbulence with spatially variable direction of the local mean magnetic field the differences are substantial.

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arXiv2003

Resistive MHD simulations of the Parker instability in galactic disks

Parker instability leads to the formation of tangential discontinuities in a magnetic field and subsequent magnetic reconnection due to a numerical and/or an explicit resistivity. In this paper we investigate the role of the uniform, localized and numerical resistivity on the diffusion of magnetic field lines during the growth phase of Parker instability modes. We propose a new method to quantify the diffusion of magnetic field lines which is attributed to the presence of resistivity in ideal and non-ideal MHD codes. The method relies (1) on integration of magnetic lines in between periodic boundaries, (2) on measurements of the dispersion of magnetic field lines with the left and the right periodic boundaries and (3) on a statistical analysis of shifts of a large set of magnetic lines. The proposed method makes it possible to detect topological evolution of magnetic field. We perform a series of resistive MHD simulations of the Parker instability in uniformly rotating galactic disks. We follow the topological evolution of the magnetic field evolving due to the Parker instability and relate it to the ratio of total to uniform magnetic field in galactic disks. We find that after the onset of the Parker instability, the magnetic field becomes first tangled and later on it evolves toward a uniform state due to the presence of resistivity. A similar effect of a varying contribution of a turbulent magnetic field is observed in arms and inter-arm regions of galaxies.

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