Generation of Solenoidal Modes and Magnetic Fields in Turbulence Driven by Compressive Driving. (arXiv:2001.05154v1 [astro-ph.GA])

<a href="http://arxiv.org/find/astro-ph/1/au:+Lim_J/0/1/0/all/0/1">Jeonghoon Lim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cho_J/0/1/0/all/0/1">Jungyeon Cho</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yoon_H/0/1/0/all/0/1">Heesun Yoon</a>

We perform numerical simulations of hydrodynamic (HD) and magnetohydrodynamic

(MHD) turbulence driven by compressive driving to study generation of

solenoidal velocity component and small-scale magnetic field. We mainly focus

on the effects of mean magnetic field ($B_0$) and the sonic Mach number

($M_s$). We also consider two different driving schemes in terms of correlation

timescale of forcing vectors: a finite-correlated driving and a

delta-correlated driving. The former has a longer correlation timescale of

forcing vectors, which is comparable to large-eddy turnover time, than the

latter. Our findings are as follows. First, when we fix the value of $B_0$, the

level of solenoidal velocity component after saturation increases as $M_s$

increases. A similar trend is observed for generation of magnetic field when

$B_0$ is small. Second, when we fix the value of $M_s$, HD and MHD simulations

result in similar level of the solenoidal component when $B_0$ $lesssim$ 0.2

(or Alfven Mach number of $sim$ 5). However, the level increases when $B_0$

$gtrsim$ 0.2. Roughly speaking, the magnetic energy density after saturation

is a linearly increasing function of $B_0$ irrespective of $M_s$. Third,

generation of solenoidal velocity component is not sensitive to numerical

resolution, but that of magnetic energy density is mildly sensitive. Lastly,

when initial conditions are same, the finite-correlated driving always produces

more solenoidal velocity and small-scale magnetic field components than the

delta-correlated driving. We additionally analyze the vorticity equation to

understand why higher $M_s$ and $B_0$ yield larger quantity of the solenoidal

velocity component.

We perform numerical simulations of hydrodynamic (HD) and magnetohydrodynamic

(MHD) turbulence driven by compressive driving to study generation of

solenoidal velocity component and small-scale magnetic field. We mainly focus

on the effects of mean magnetic field ($B_0$) and the sonic Mach number

($M_s$). We also consider two different driving schemes in terms of correlation

timescale of forcing vectors: a finite-correlated driving and a

delta-correlated driving. The former has a longer correlation timescale of

forcing vectors, which is comparable to large-eddy turnover time, than the

latter. Our findings are as follows. First, when we fix the value of $B_0$, the

level of solenoidal velocity component after saturation increases as $M_s$

increases. A similar trend is observed for generation of magnetic field when

$B_0$ is small. Second, when we fix the value of $M_s$, HD and MHD simulations

result in similar level of the solenoidal component when $B_0$ $lesssim$ 0.2

(or Alfven Mach number of $sim$ 5). However, the level increases when $B_0$

$gtrsim$ 0.2. Roughly speaking, the magnetic energy density after saturation

is a linearly increasing function of $B_0$ irrespective of $M_s$. Third,

generation of solenoidal velocity component is not sensitive to numerical

resolution, but that of magnetic energy density is mildly sensitive. Lastly,

when initial conditions are same, the finite-correlated driving always produces

more solenoidal velocity and small-scale magnetic field components than the

delta-correlated driving. We additionally analyze the vorticity equation to

understand why higher $M_s$ and $B_0$ yield larger quantity of the solenoidal

velocity component.

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