Inverse energy transfer in decaying, three dimensional, nonhelical magnetic turbulence due to magnetic reconnection. (arXiv:2007.07325v1 [astro-ph.CO])

Inverse energy transfer in decaying, three dimensional, nonhelical magnetic turbulence due to magnetic reconnection. (arXiv:2007.07325v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bhat_P/0/1/0/all/0/1">Pallavi Bhat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_M/0/1/0/all/0/1">Muni Zhou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Loureiro_N/0/1/0/all/0/1">Nuno F. Loureiro</a>

It has been recently shown numerically that there exists an inverse transfer
of magnetic energy in decaying, nonhelical, magnetically dominated,
magnetohydrodynamic turbulence in 3-dimensions (3D). We suggest that magnetic
reconnection is the underlying physical mechanism responsible for this inverse
transfer. In the two-dimensional (2D) case, the inverse transfer is easily
inferred to be due to smaller magnetic islands merging to form larger ones via
reconnection. We find that the scaling behaviour is similar between the 2D and
the 3D cases, i.e., the magnetic energy evolves as $t^{-1}$, and the magnetic
power spectrum follows a slope of $k^{-2}$. We show that on normalizing time by
the magnetic reconnection timescale, the evolution curves of the magnetic field
in systems with different Lundquist numbers collapse onto one another.
Furthermore, transfer function plots show signatures of magnetic reconnection
driving the inverse transfer. We also discuss the conserved quantities in the
system and show that the behaviour of these quantities is similar between the
2D and 3D simulations, thus making the case that the dynamics in 3D could be
approximately explained by what we understand in 2D. Lastly, we also conduct
simulations where the magnetic field is subdominant to the flow. Here, too, we
find an inverse transfer of magnetic energy in 3D. In these simulations, the
magnetic energy evolves as $ t^{-1.4}$ and, interestingly, a dynamo effect is
observed.

It has been recently shown numerically that there exists an inverse transfer
of magnetic energy in decaying, nonhelical, magnetically dominated,
magnetohydrodynamic turbulence in 3-dimensions (3D). We suggest that magnetic
reconnection is the underlying physical mechanism responsible for this inverse
transfer. In the two-dimensional (2D) case, the inverse transfer is easily
inferred to be due to smaller magnetic islands merging to form larger ones via
reconnection. We find that the scaling behaviour is similar between the 2D and
the 3D cases, i.e., the magnetic energy evolves as $t^{-1}$, and the magnetic
power spectrum follows a slope of $k^{-2}$. We show that on normalizing time by
the magnetic reconnection timescale, the evolution curves of the magnetic field
in systems with different Lundquist numbers collapse onto one another.
Furthermore, transfer function plots show signatures of magnetic reconnection
driving the inverse transfer. We also discuss the conserved quantities in the
system and show that the behaviour of these quantities is similar between the
2D and 3D simulations, thus making the case that the dynamics in 3D could be
approximately explained by what we understand in 2D. Lastly, we also conduct
simulations where the magnetic field is subdominant to the flow. Here, too, we
find an inverse transfer of magnetic energy in 3D. In these simulations, the
magnetic energy evolves as $ t^{-1.4}$ and, interestingly, a dynamo effect is
observed.

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