Identification of plasma modes in Galactic turbulence with synchrotron polarization


Magnetohydrodynamic turbulence is a ubiquitous and fundamental ingredient underlying many astrophysical phenomena. The multiphase nature of the interstellar medium and the diversity of driving mechanisms give rise to spatial variation of turbulence properties, particularly plasma properties. There has been no observational diagnosis of the plasma modes beyond the solar system so far. Here we report the identification of different plasma modes in various Galactic environments, including active star-forming zones and supernova remnants, on the basis of our synchrotron polarization analysis. The observed high degree of consistency between the γ-ray excess in the Cygnus cocoon and the location of magnetosonic modes provides strong observational evidence for the long-advocated theory that magnetosonic modes dominate the cosmic ray (CR) scattering and acceleration. Our results open up a new avenue for the study of interstellar turbulence and demonstrate the indispensability of accounting for their plasma properties in all the relevant processes, including CR transport and star formation.

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Fig. 1: Illustration for SPA recipe.
Fig. 2: Numerical simulations on total MHD turbulence data cubes with different compositions of plasma modes.
Fig. 3: Applying SPA to observational data.
Fig. 4: Turbulence modes identified in Cygnus X region.
Fig. 5: SPA analysis in the vicinity of the Rosette nebula.

Data availability

We used the synchrotron polarization data from the Urumqi 6-cm polarization survey ( The data represented in Figs. 4 and 5 are provided with the paper as source data.

Code availability

We opt not to make the SPA code publicly available now because it is still under development and maintenance. We intend to publish the code at a later time. The codes used to generate the plots presented in this paper are available from the corresponding author upon reasonable request.

Change history

  • 18 June 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.


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We thank the following colleagues for helpful communications on various topics discussed in this paper: F. Boulanger, M. Gangi, S. Gao, A. Lazarian, H. B. Liu, R. Liu, J. Liu, M. Pohl, I. Sushch, A. Taylor, J. Volmer, M. Vorster, X. Wu and Q. Zhu. S.A. acknowledges support from the DESY summer student programme.

Author information




H.Y. oversaw the project. A.C., H.Y. and H.Z. contributed to the theoretical analysis of SPA. K.M. and R.S.-L. prepared the turbulence data. H.Z. carried out the numerical simulation, performed the probability distribution analysis on the simulation results and led the establishment of the SPA recipe. A.C., S.A. and H.Z. analysed the data from real observations. H.Z. and H.Y. interpreted the observational results and led the manuscript preparation. H.Y. and A.C. designed the project. All authors discussed the project and commented on the manuscript.

Corresponding author

Correspondence to Huirong Yan.

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Extended data

Extended Data Fig. 1 Theoretical analysis and decomposed modes test.

(ac) Theoretical Analyses. The different colours represent different Alfvénic-Mach numbers MA. For MS modes, the solid lines are for the high-β cases and the dashed lines refer to the low-β cases. The dashed-dotted lines marks rxx=-2/3 (df) Numerical tests. The color code for signatures: Green- “Alfvénic”; Red- “MS”; Purple- “Ambiguous”. For slow modes with high-β and MA~1, all the observed results are rejected according to the classification threshold (Fig. 3 in the main text).

Extended Data Fig. 2 MS and Alfvénic signature percentages in clustering.

The spot and cluster core scales are marked on the map. Each grid shown here corresponds the information of the cluster core whose center is the grid, as depicted on the top left part of each figure. The color on each grid reveals the percentage of (a)(c) MS and (b)(d) Alfvénic signature at the corresponding cluster. The background is the synchrotron intensity map.

Supplementary information

Supplementary Information

Supplementary discussion.

Source data

Source Data Fig. 4

Statistical source data.

Source Data Fig. 5

Statistical source data.

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Zhang, H., Chepurnov, A., Yan, H. et al. Identification of plasma modes in Galactic turbulence with synchrotron polarization. Nat Astron (2020).

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