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On the linear structure of the interlaced Alfvén vortices in the tail of Uranus at solstice

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Abstract

Incompressible vortex flow are observed in a large variety of astrophysical plasmas such as the convection zone and the atmosphere of stars, in astrophysical jets in stellar winds and in planetary magnetospheres. More specifically, magnetohydrodynamic (MHD) simulations have shown that two large scale interlaced Alfvénic vortices structure the magnetic tail of Uranus at solstice time. Assuming identical vortices, we compute the general linear structure of the flow near their centers within the frame of ideal MHD. We then use the analytic results to interpret and qualify the vortices observed in a 3D MHD simulation of a fast rotating Uranus-type planet.

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Acknowledgements

This work has been financially supported by the PLAS@PAR project and by the National Institute of Sciences of the Universe (INSU). Thank you to the anonymous reviewer for valuable comments which helped to improve the paper and to ease the task of the reader.

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FP wrote the manuscript and prepared all figures.

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Correspondence to Filippo Pantellini.

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Appendix:  Explicit form of the matrices of the differential equation system

Appendix:  Explicit form of the matrices of the differential equation system

Recalling the correspondence between the components of the 6 dimensional vector \(\mathbf{X}\) and the components of the magnetic field vector \(\mathbf{B}\) and the velocity vector \(\mathbf{u}\)

$$ {\mathbf{X}}\equiv \begin{pmatrix} X_{1} \\ X_{2} \\ X_{3} \\ X_{4} \\ X_{5} \\ X_{6} \end{pmatrix} \equiv \begin{pmatrix} B_{r} \\ B_{\varphi} \\ B_{z} \\ u_{r} \\ u_{\varphi} \\ u_{z}\end{pmatrix} $$
(A1)

the \(6\times 6\) matrices in the system of differential equations (16) take the form

$$ M_{r}=r \begin{pmatrix} {{X_{4}}} & 0 & 0 & -{{X_{1}}} & 0 & 0 \\ 0 & {{X_{4}}} & 0 & 0 & -{{X_{1}}} & 0 \\ 0 & 0 & {{X_{4}}} & 0 & 0 & -{{X_{1}}} \\ 0 & {{X_{2}}} & {{X_{3}}} & \, {{X_{4}}} & 0 & 0 \\ 0 & -{{X_{1}}} & 0 & 0 & \, {{X_{4}}} & 0 \\ 0 & 0 & -{{X_{1}}} & 0 & 0 & \, {{X_{4}}}\end{pmatrix} $$
(A2)

and

$$ M_{\xi}= \begin{pmatrix} {{X_{5}}}-kr\, {{X_{6}}} & 0 & 0 & kr\, {{X_{3}}}-{{X_{2}}} & 0 & 0 \\ 0 & {{X_{5}}}-kr\, {{X_{6}}} & 0 & 0 & kr\, {{X_{3}}}-{{X_{2}}} & 0 \\ 0 & 0 & {{X_{5}}}-kr\, {{X_{6}}} & 0 & 0 & kr\, {{X_{3}}}-{{X_{2}}} \\ kr\, {{X_{3}}}-{{X_{2}}} & 0 & 0 & \, {{X_{5}}}-kr \, {{X_{6}}} & 0 & 0 \\ {{X_{1}}} & kr\, {{X_{3}}} & {{X_{3}}} & 0 & \, {{X_{5}}}-kr \, {{X_{6}}} & 0 \\ -kr\, {{X_{1}}} & -kr\, {{X_{2}}} & -{{X_{2}}} & 0 & 0 & \, {{X_{5}}}-kr \, {{X_{6}}}\end{pmatrix} . $$
(A3)

As the inverse of \(M_{r}\) is required to compute the matrix \(\tilde{M}_{\xi}=M_{r}^{-1} M_{\xi}\) and the vector \(\tilde{A}_{\xi}=M_{r}^{-1} A_{\xi}\) which appear in the normal form of the system of differential equations (18), we also give its explicit form hereafter:

$$\begin{aligned} &M_{r}^{-1}=\frac{1}{r X_{4}(X_{4}^{2} -X_{1}^{2})}\times \\ &\begin{pmatrix} X_{4}^{2}-X_{1}^{2} & -X_{1} X_{2} & -X_{1} X_{3} & (X_{4}^{2}-X_{1}^{2})X_{1}/X_{4} & -X_{1}^{2} X_{2}/X_{4} & -X_{1}^{2} X_{3}/X_{4} \\ 0 & X_{4}^{2} & 0 & 0 & X_{1} X_{4} & 0 \\ 0 & 0 & X_{4}^{2} & 0 & 0 & X_{1} X_{4} \\ 0 & -X_{2} X_{4} & -X_{3} X_{4} & X_{4}^{2}-X_{1}^{2} & -X_{1} X_{2} & -X_{1} X_{3} \\ 0 & X_{1} X_{4} & 0 & 0 & X_{4}^{2} & 0 \\ 0 & 0 & X_{1} X_{4} & 0 & 0 & X_{4}^{2} \end{pmatrix} . \end{aligned}$$
(A4)

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Pantellini, F. On the linear structure of the interlaced Alfvén vortices in the tail of Uranus at solstice. Astrophys Space Sci 369, 63 (2024). https://doi.org/10.1007/s10509-024-04332-4

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