arXiv subjects

Kiwan Park

Publications and source records attributed to Kiwan Park.

22 records · Page 2Linked to original sources

arXiv2013

A study of large scale dynamo growth rates from numerical simulations and implications for mean field theories

Understanding large scale magnetic field growth in turbulent plasmas in the magnetohydrodynamic limit is a goal of magnetic dynamo theory. In particular, assessing how well large scale helical field growth and saturation in simulations matches that predicted by existing theories is important for progress. Using numerical simulations of isotropically forced turbulence without large scale shear with the implications, we focus on several aspects of this comparison that have not been previously tested: (1) Leading mean field dynamo theories which break the field into large and small scales predict that large scale helical field growth rates are determined by the difference between kinetic helicity and current helicity with no dependence on the non-helical energy in small scale magnetic fields. Our simulations show that the growth rate of the large scale field from fully helical forcing is indeed unaffected by the presence or absence of small scale magnetic fields amplified in a precursor non-helical dynamo. However, because the precursor non helical dynamo in our simulations produced fields that were strongly sub-equipartition with respect to the kinetic energy, we cannot yet rule out the potential influence of stronger non- helical small scale fields. (2) We have identified two features in our simulations which cannot be explained by the most minimalist versions of two-scale mean field theory: (i) fully helical small scale forcing produces significant non-helical large scale magnetic energy and (ii) the saturation of the large scale field growth is time-delayed with respect to what minimalist theory predicts. We comment on desirable generalizations to the theory in this context and future desired work.

physics.plasm-ph↗

arXiv2012

Comparison Between Turbulent Helical Dynamo Simulations and a Nonlinear Three-Scale Theory

Progress toward understanding principles of nonlinear growth and saturation of large scale magnetic fields has emerged from comparison of theoretical models that incorporate the evolution of magnetic helicity with numerical simulations for problems that are more idealized than expected in astrophysical circumstances, but still fully non-linear. We carry out a new comparison of this sort for the magnetic field growth from forced isotropic helical turbulence in a periodic box. Previous comparisons be- tween analytic theory and simulations of this problem have shown that a two-scale model compares well with the simulations in agreeing that the driver of large scale field growth is the difference between kinetic and current helicities associated with the small scale field, and that the backreaction that slows the growth of the large scale field as the small scale current helicity grows. However, the two-scale model artificially re- stricts the small scale current helicity to reside at the same scale as the driving kinetic helicity. In addition, previous comparisons have focused on the late time saturation regime, and less on the early-time growth regime. Here we address these issues by comparing a three scale model to new simulations for both early and late time growth regimes. We find that the minimalist extension to three scales provides a better model for the field evolution than the two-scale model because the simulations show that the small scale current helicity does not reside at the same scale as that of the driving kinetic helicity. The simulations also show that the peak of the small scale current helicity migrates toward lower wave numbers as the growth evolves from the fast to saturated growth regimes.

astro-ph.IM↗

arXiv2012

Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model

Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large scale field growth to total magnetic helicity evolution best predict the saturation of LSDs seen in simulations. For the simplest so called "α2" LSDs in periodic boxes, the electromotive force driving LSD growth depends on the difference between the time-integrated kinetic and current helicity associated with fluctuations. When the system is helically kinetically forced (KF), the growth of the large scale helical field is accompanied by growth of small scale magnetic (and current) helicity which ultimately quench the LSD. Here, using both simulations and theory, we study the complementary magnetically forced(MF) case in which the system is forced with an electric field that supplies magnetic helicity. For this MF case, the kinetic helicity becomes the back-reactor that saturates the LSD. Simulations of both MF and KF cases can be approximately modeled with the same equations of magnetic helicity evolution, but with complementary initial conditions. A key difference between KF and MF cases is that the helical large scale field in the MF case grows with the same sign of injected magnetic helicity, whereas the large and small scale magnetic helicities grow with opposite sign for the KF case. The MF case can arise even when the thermal pressure is approximately smaller than the magnetic pressure, and requires only that helical small scale magnetic fluctuations dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in accretion discs and Babcock models of the solar dynamo are actually MF LSDs.

physics.plasm-ph↗

arXiv2009

Effect of Plasma Composition on the Interpretation of Faraday Rotation

Faraday rotation (FR) is widely used to infer the orientation and strength of magnetic fields in astrophysical plasmas. Although the absence of electron-positron pairs is a plausible assumption in many astrophysical environments, the magnetospheres of pulsars and black holes and their associated jets may involve a significant pair plasma fraction. This motivates being mindful of the effect of positrons on FR. Here we derive and interpret exact expressions of FR for a neutral plasma of arbitrary composition. We focus on electron-ion-positron plasmas in which charge neutrality is maintained by an arbitrary combination of ions and positrons. Because a pure electron-positron plasma has zero FR, the greater the fraction of positrons the higher the field strength required to account for the same FR. We first obtain general formulae and then specifically consider parameters relevant to active galctic nuclei (AGN) jets to illustrate the significant differences in field strengths that FR measurements from radio frequency measurements. Complementarily, using galaxy cluster core plasmas as examples, we discuss how plasma composition can be constrained if independent measurements of the field strength and number density are available and combined with FR.

astro-ph.HE↗