Properties of magnetohydrodynamic modes in compressively driven plasma turbulence. (arXiv:1907.01853v1 [physics.plasm-ph])
<a href="http://arxiv.org/find/physics/1/au:+Makwana_K/0/1/0/all/0/1">K. D. Makwana</a>, <a href="http://arxiv.org/find/physics/1/au:+Yan_H/0/1/0/all/0/1">Huirong Yan</a>
We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing
the data of MHD simulations into linear MHD modes – namely the Alfven, slow
magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture
of solenoidal and compressive driving, while varying the Alfven Mach number
(MA) and plasma beta. We find that the proportion of fast and slow modes in the
mode mixture increases with increasing compressive forcing. This proportion of
the magnetosonic modes can also become the dominant fraction in the mode
mixture. The anisotropy of the modes is analyzed by means of their structure
functions. The Alfven mode anisotropy is consistent with the Goldreich-Sridhar
theory. We find a transition from weak to strong Alfvenic turbulence as we go
from low to high MA. The slow mode properties are similar to the Alfven mode.
On the other hand the isotropic nature of fast modes is verified in the cases
where the fast mode is a significant fraction of the mode mixture. The fast
mode behavior does not show any transition in going from low to high MA. We
find indications that there is some interaction between the different modes and
the properties of the dominant mode can affect the properties of the weaker
modes. This work identifies the conditions under which magnetosonic modes can
be a major fraction of turbulent astrophysical plasmas and their coexistence
with an Alfvenic cascade will have important implications for cosmic ray
scattering and transport.
We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing
the data of MHD simulations into linear MHD modes – namely the Alfven, slow
magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture
of solenoidal and compressive driving, while varying the Alfven Mach number
(MA) and plasma beta. We find that the proportion of fast and slow modes in the
mode mixture increases with increasing compressive forcing. This proportion of
the magnetosonic modes can also become the dominant fraction in the mode
mixture. The anisotropy of the modes is analyzed by means of their structure
functions. The Alfven mode anisotropy is consistent with the Goldreich-Sridhar
theory. We find a transition from weak to strong Alfvenic turbulence as we go
from low to high MA. The slow mode properties are similar to the Alfven mode.
On the other hand the isotropic nature of fast modes is verified in the cases
where the fast mode is a significant fraction of the mode mixture. The fast
mode behavior does not show any transition in going from low to high MA. We
find indications that there is some interaction between the different modes and
the properties of the dominant mode can affect the properties of the weaker
modes. This work identifies the conditions under which magnetosonic modes can
be a major fraction of turbulent astrophysical plasmas and their coexistence
with an Alfvenic cascade will have important implications for cosmic ray
scattering and transport.
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