Inverse transfer of magnetic helicity in supersonic magnetohydrodynamic turbulence

Abstract

The inverse transfer of magnetic helicity is studied through a fourth-order finite volume numerical scheme in the framework of compressible ideal magnetohydrodynamics (MHD), with an isothermal equation of state. Using either a purely solenoidal or purely compressive mechanical driving, a hydrodynamic turbulent steady-state is reached, to which small-scale magnetic helical fluctuations are injected. The steady-state root mean squared Mach numbers considered range from 0.1 to about 11. In all cases, a growth of magnetic structures is observed. While the measured magnetic helicity spectral scaling exponents are similar to the one measured in the incompressible case for the solenoidally-driven runs, significant deviations are observed even at relatively low Mach numbers when using a compressive driving. A tendency towards equipartition between the magnetic and kinetic fields in terms of energy and helicity is noted. The joint use of the helical decomposition in the framework of shell-to-shell transfer analysis reveals the presence of three distinct features in the global picture of a magnetic helicity inverse transfer. Those are individually associated with specific scale ranges of the advecting velocity field and commensurate helical contributions.

Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: helical transfers

“…Thus the velocity field's compressive part, which is not present in the incompressible case, plays an important role in the LDT and the NLIT, which affects the magnetic helicity scaling properties [39].…”

“…The equations are solved using a fourth-order shock-capturing finite-volume solver, which makes use of the constrained-transport approach to ensure the solenoidality of the magnetic field. It is described in detail in [38,40]. The main reconstruction method used is a fourth-order Central Weighted Essentially Non-Oscillatory (CWENO) method [27], with a passage through point values in order to keep fourth-order accuracy [32,14].…”

“…In the present paper, shell-to-shell helical analysis of the least and most compressible runs presented in [38] are considered, which are labelled by "M01s4", "M11s", "M1c" and "M8c". The number in the label stands for the approximate RMS Mach number during the hydrodynamic turbulent steady-state, which is M ≈ 0.116, 11.1, 0.797 and 7.87 respectively, and the letter for the forcing type, either purely solenoidal or compressive.…”

Can a Dynamo Mechanism Act at the Magnetopauses of Magnetic Rapidly Rotating Exoplanets?

“…Their formation may be associated with magnetic helicity, for example [22]. Teissier and Müller [23] have studied the magnetic helicity transferred from small to larger scales in the context of large-scale magnetic fields created by a magnetic dynamo.…”

Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: scaling laws

“…The numerical solver is a shock-capturing fourth-order finite volume method, described in detail in [43,44]. The main reconstruction method used is a fourthorder Central Weighted Essentially Non-Oscillatory (CWENO) procedure [28], with a passage through point values in order to maintain the fourth-order accuracy [34,13].…”