Probing the nature of dissipation in compressible MHD turbulence

Richard, T.; Lesaffre, Pierre; Falgarone, É.; Lehmann, A.

doi:10.1051/0004-6361/202142531

Cited by 6 publications

(16 citation statements)

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“…Intense dissipation in MHD turbulence is therefore seen to lie on thin sheets whether it is incompressible [12,24] or compressible [48]. In incompressible MHD turbulence, viscous and ohmic dissipation always appear disconnected (e.g., [12,24]), and the same probably holds for reduced MHD, as in [52].…”

Section: The Nature Of Coherent Structures In Mhd Turbulencementioning

confidence: 97%

“…The same problem appeared later for HCO + (e.g., [41]) and for CO. Another major power source was needed. Several processes have been proposed, such as thermal conduction and turbulent transport [42] at the WNM/CNM interfaces or extreme dissipative events in interstellar turbulence, such as C-shocks [12,24,28,29,[43][44][45][46][47][48][49][50][51][52][53][54][55], magnetised Burgers vortices [40]. The former two inject the thermal energy of the WNM into the CNM, while the latter feeds the chemistry with the turbulent energy of the CNM, which is an order of magnitude larger than its thermal energy, but is comparable to the thermal energy of the WNM.…”

Section: Specific Molecules As Tracers Of Turbulent Dissipationmentioning

confidence: 99%

“…Note that the extrema of current and vorticity in MHD turbulence are not randomly distributed in space and time but cluster in structures qualified as coherent because, following Moisy and Jiménez [46], their "inertial-range extent, implies a large-scale organization of the small-scale intermittent structures". This paper reports on numerical simulations of compressible MHD turbulence, dedicated to dissipation scales, performed to characterise the nature of the coherent structures in ISM turbulence where dissipation is concentrated [48] and to illustrate the statistics performed on synthetic observations to disclose such Coherent Structures of Intense Dissipation Extrema-hereafter called CSIDE-in diffuse molecular gas. Then, the first example of such a statistical study in a nearby diffuse molecular cloud is presented.…”

Section: Specific Molecules As Tracers Of Turbulent Dissipationmentioning

confidence: 99%

“…The finite size of the mesh and the necessary reconstruction of variables from the center to the edges of each pixel introduces a spurious diffusion length scale for every conservation equation (including mass conservation). To this effect, we built a method to estimate locally the total dissipation including the one inherent to the numerical scheme [24,48]. The method consists of replaying a simulation step while integrating a redundant conservation equation for a variant of the energy (total mechanical energy, for example).…”

Section: Numerical Dissipationmentioning

confidence: 99%

“…For instance, we can plot the los-integrated dissipation as on Figure 2: the regions of highest dissipation are not randomly distributed in space, they display large-scale and thin ridges of intense integrated dissipation. These ridges appear as a caustic effect from the projection of sheets seen almost edge-on where dissipation is intense (see [48]). They are coherent large-scale structures, hereafter called the CSIDE, as opposed to quasi-thermalized small-scale eddies.…”

Section: Numerical Dissipationmentioning

confidence: 99%

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The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

Lesaffre,

Falgarone,

Hily-Blant

2024

Atmosphere

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Turbulent dissipation is a central issue in the star and galaxy formation process. Its fundamental property of space–time intermittency, well characterised in incompressible laboratory experiments, remains elusive in cosmic turbulence. Progress requires the combination of state-of-the-art modelling, numerical simulations and observations. The power of such a combination is illustrated here, where the statistical method intended to locate the extrema of velocity shears in a turbulent field, which are the signposts of intense dissipation extrema, is applied to numerical simulations of compressible magneto-hydrodynamical (MHD) turbulence dedicated to dissipation scales and to observations of a turbulent molecular cloud. We demonstrate that increments of several observables computed at the smallest lags can detect coherent structures of intense dissipation. We apply this statistical method to the observations of a turbulent molecular cloud close to the Sun in our galaxy and disclose a remarkable structure of extremely large velocity shear. At the location of the largest velocity shear, this structure is found to foster 10× more carbon monoxide molecules than standard diffuse molecular gas, an enrichment supported by models of non-equilibrium warm chemistry triggered by turbulent dissipation. In our simulations, we also compute structure functions of various synthetic observables and show that they verify Extended Self-Similarity. This allows us to compute their intermittency exponents, and we show how they constrain some properties of the underlying three-dimensional turbulence. The power of the combination of modelling and observations is also illustrated by the observations of the CH+ cation that provide unique quantitative information on the kinetic energy trail in the massive, multi-phase and turbulent circum-galactic medium of a galaxy group at redshift z=2.8.

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Section: The Nature Of Coherent Structures In Mhd Turbulencementioning

confidence: 97%

Section: Specific Molecules As Tracers Of Turbulent Dissipationmentioning

confidence: 99%

Section: Specific Molecules As Tracers Of Turbulent Dissipationmentioning

confidence: 99%

Section: Numerical Dissipationmentioning

confidence: 99%

Section: Numerical Dissipationmentioning

confidence: 99%

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The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

Lesaffre,

Falgarone,

Hily-Blant

2024

Atmosphere

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Shock excitation of H₂ in the James Webb Space Telescope era

Kristensen

Godard

Guillard

et al. 2023

A&A

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Context. Molecular hydrogen, H2, is the most abundant molecule in the Universe. Thanks to its widely spaced energy levels, it predominantly lights up in warm gas, T ≳ 102 K, such as shocked regions externally irradiated or not by interstellar UV photons, and it is one of the prime targets of James Webb Space Telescope (JWST) observations. These may include shocks from protostellar outflows, supernova remnants impinging on molecular clouds, all the way up to starburst galaxies and active galactic nuclei. Aims. Sophisticated shock models are able to simulate H2 emission from such shocked regions. We aim to explore H2 excitation using shock models, and to test over which parameter space distinct signatures are produced in H2 emission. Methods. We here present simulated H2 emission using the Paris-Durham shock code over an extensive grid of ~14 000 plane-parallel stationary shock models, a large subset of which are exposed to a semi-isotropic external UV radiation field. The grid samples six input parameters: the preshock density, shock velocity, transverse magnetic field strength, UV radiation field strength, the cosmic-ray-ionization rate, and the abundance of polycyclic aromatic hydrocarbons, PAHs. Physical quantities resulting from our self-consistent calculations, such as temperature, density, and width, have been extracted along with H2 integrated line intensities. These simulations and results are publicly available on the Interstellar Medium Services platform. Results. The strength of the transverse magnetic field, as quantified by the magnetic scaling factor, b, plays a key role in the excitation of H2. At low values of b (≲0.3, J-type shocks), H2 excitation is dominated by vibrationally excited lines; whereas, at higher values (b ≳ 1, C-type shocks), rotational lines dominate the spectrum for shocks with an external radiation field comparable to (or lower than) the solar neighborhood. Shocks with b ≥ 1 can potentially be spatially resolved with JWST for nearby objects. H2 is typically the dominant coolant at lower densities (≲104 cm−3); at higher densities, other molecules such as CO, OH, and H2O take over at velocities ≲20 km s−1 and atoms, for example, H, O, and S, dominate at higher velocities. Together, the velocity and density set the input kinetic energy flux. When this increases, the excitation and integrated intensity of H2 increases similarly. An external UV field mainly serves to increase the excitation, particularly for shocks where the input radiation energy is comparable to the input kinetic energy flux. These results provide an overview of the energetic reprocessing of input kinetic energy flux and the resulting H2 line emission.

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Shocks in the warm neutral medium

Godard,

des Forêts,

Bialy

2024

A&A

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Atomic and molecular line emissions from shocks may provide valuable information on the injection of mechanical energy into the interstellar medium (ISM), the generation of turbulence, and the processes of phase transition between the warm neutral medium (WNM) and the cold neutral medium (CNM). In this series of papers, we investigate the properties of shocks propagating in the WNM. Our objective is to identify the tracers of these shocks, use them to interpret ancillary observations of the local diffuse matter, and provide predictions for future observations. Shocks propagating in the WNM are studied using the Paris-Durham shock code, a multi-fluid model built to follow the thermodynamical and chemical structures of shock waves at steady-state in a plane-parallel geometry. The code, designed to take into account the impact of an external radiation field, is updated to treat self-irradiated shocks at intermediate ($30 < V_S < 100$ and high velocity ($V_S which emit ultraviolet (UV), extreme-ultraviolet (EUV), and X-ray photons. The couplings between the photons generated by the shock, the radiative precursor, and the shock structure are computed self-consistently using an exact radiative-transfer algorithm for line emission. The resulting code is explored over a wide range of parameters ($0.1 V_S and $0.1 B 10$ mu G), which covers the typical conditions of the WNM in the solar neighborhood. The explored physical conditions lead to the existence of a diversity of stationary magnetohydrodynamic solutions, including J-type, CJ-type, and C-type shocks. These shocks are found to naturally induce phase transition between the WNM and the CNM, provided that the postshock thermal pressure is higher than the maximum pressure of the WNM and that the maximum density allowed by magnetic compression is greater than the minimum density of the CNM. The input flux of mechanical energy is primarily reprocessed into line emissions from the X-ray to the submillimeter domain. Intermediate- and high-velocity shocks are found to generate a UV radiation field that scales as $V_S^3$ for $V_S < 100$ and as $V_S^2$ at higher velocities, and an X-ray radiation field that scales as $V_S^3$ for $V_S Both radiation fields may extend over large distances in the preshock depending on the density of the surrounding medium and the hardness of the X-ray field, which is solely driven by the shock velocity. This first paper presents the thermochemical trajectories of shocks in the WNM and their associated spectra. It corresponds to a new milestone in the development of the Paris-Durham shock code and a stepping stone for the analysis of observations that will be carried out in forthcoming works.

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Probing the nature of dissipation in compressible MHD turbulence

Cited by 6 publications

References 75 publications

The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

Shock excitation of H₂ in the James Webb Space Telescope era

Shocks in the warm neutral medium

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Probing the nature of dissipation in compressible MHD turbulence

Cited by 6 publications

References 75 publications

The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

The Intermittency of Turbulence in Magneto-Hydrodynamical Simulations and in the Cosmos

Shock excitation of H2 in the James Webb Space Telescope era

Shocks in the warm neutral medium

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Product

Resources

About

Shock excitation of H₂ in the James Webb Space Telescope era