Pressure anisotropy in global magnetospheric simulations: A magnetohydrodynamics model

Meng, Xing; Tóth, G.; Liemohn, Michael W.; Gombosi, T. I.; Runov, A.

doi:10.1029/2012ja017791

Cited by 36 publications

(42 citation statements)

References 61 publications

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“…Another factor is that global ideal MHD simulations cannot produce as intense a plasma depletion layer (PDL) [Zwan and Wolf, 1976] as observed. This may owe to ion kinetic or pressure anisotropy effects being important to generate PDLs, as noted by Meng et al [2012].…”

Section: Magnetosheath Flows and Magnetopause Shape: Case Studymentioning

confidence: 99%

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

et al. 2013

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[1] Previous works have emphasized the significant influence of the solar wind Alfvén Mach number (M A ) on magnetospheric dynamics. Here we report statistical, observational results that pertain to changes in the magnetosheath flow distribution and magnetopause shape as a function of solar wind M A and interplanetary magnetic field (IMF) clock angle orientation. We use all Cluster 1 data in the magnetosheath during the period 2001-2010, using an appropriate spatial superposition procedure, to produce magnetosheath flow distributions as a function of location in the magnetosheath relative to the IMF and other parameters. The results demonstrate that enhanced flows in the magnetosheath are expected at locations quasi-perpendicular to the IMF direction in the plane perpendicular to the Sun-Earth line; in other words, for the special case of a northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere, while much lower flows are observed above the poles. The largest flows are adjacent to the magnetopause. Using appropriate magnetopause crossing lists (for both high and low M A ), we also investigate the changes in magnetopause shape as a function of solar wind M A and IMF orientation. Comparing observed magnetopause crossings with predicted positions from an axisymmetric semi-empirical model, we statistically show that the magnetopause is generally circular during high M A , while is it elongated (albeit with moderate statistical significance) along the direction of the IMF during low M A . These findings are consistent with enhanced magnetic forces that prevail in the magnetosheath during low M A . The component of the magnetic forces parallel to the magnetopause produces the enhanced flows along and adjacent to the magnetopause, while the component normal to the magnetopause exerts an asymmetric pressure on the magnetopause that deforms it into an elongated shape.

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Section: Magnetosheath Flows and Magnetopause Shape: Case Studymentioning

confidence: 99%

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

et al. 2013

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“…The type of modelling of the pressure-anisotropy effect on dynamics advocated by Mogavero & Schekochihin (2014) (see also Samsonov et al 2001;Schekochihin & Cowley 2006;Lyutikov 2007;Kunz et al 2011), as well as in various numerical implementations of effective closures ensuring that ∆ does not stray into the unstable regimes (3) and (4) (Sharma et al 2006(Sharma et al , 2007Samsonov et al 2007;Chandran et al 2011;Meng et al 2012;Kunz et al 2012;Santos-Lima et al 2014), rests on the assumption that, as the magnetic field is locally increased (decreased), the system will stray into mirror (firehose) unstable regimes, instabilities will instantly produce corresponding fluctuations, those will saturate and adjust the instability thresholds (or the system's position with respect to them). This adjustment is assumed to happen very quickly compared to the time scale on which the system might switch from one instability threshold to the other.…”

Section: Introductionmentioning

confidence: 99%

Pressure-anisotropy-driven microturbulence and magnetic-field evolution in shearing, collisionless plasma

Melville¹,

Schekochihin²,

Kunz³

2016

Mon. Not. R. Astron. Soc.

126

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The nonlinear state of a high-beta collisionless plasma is investigated in which an externally imposed linear shear amplifies or diminishes a uniform mean magnetic field, driving pressure anisotropies and, therefore, firehose or mirror instabilities. The evolution of the resulting microscale turbulence is considered when the external shear changes, mimicking the local behaviour of a macroscopic turbulent plasma flow, viz., the shear is either switched off or reversed after one shear time, so that a new macroscale configuration is superimposed on the microscale state left behind by the previous one. It is shown that there is a threshold value of plasma beta: when β ≪ Ω/S (ion cyclotron frequency/shear rate), the emergence of firehose or mirror fluctuations when they are driven unstable by shear and their disappearance when the shear is removed or reversed are quasi-instantaneous compared to the macroscopic (shear) timescale. This is because the free-decay time scale of these fluctuations is ∼ β/Ω ≪ S −1 , a result that arises from the free decay of both types of fluctuations turning out to be constrained by the same marginal-stability thresholds as their growth and saturation in the driven regime. In contrast, when β Ω/S ("ultra-high" beta), the old microscale state can only be removed on the macroscopic (shear) timescale. Furthermore, it is found that in this ultra-high-beta regime, when the firehose fluctuations are driven, they grow secularly to order-unity amplitudes (relative to the mean field), this growth compensating for the decay of the mean field, with pressure anisotropy pinned at marginal stability purely by the increase in the fluctuation energy, with no appreciable scattering of particles (which is unlike what happens at moderate β). When the shear reverses, the shearing away of this firehose turbulence compensates for the increase in the mean field and thus prevents growth of the pressure anisotropy, stopping the system from going mirror-unstable. Therefore, at ultra-high β, the system stays close to the firehose threshold, the mirror instability is almost completely suppressed, while the mean magnitude of the magnetic field barely changes at all. Implications of the properties of both ultra-high-and moderate-beta regimes for the operation of plasma dynamo and thus the origin of cosmic magnetism are discussed, suggesting that collisionless effects are broadly beneficial to fast magnetic-field generation.

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“…The simplest collisionless MHD-type approximation is the CGL-MHD model (Chew et al 1956). A modified CGL-MHD model taking into account the anisotropy constraints due to kinetic instabilities has been used for modeling the solar wind and magnetosphere in numerical simulations (Denton et al 1994;Pudovkin et al 1999;Samsonov & Pudovkin 2000;Samsonov et al 2001Samsonov et al , 2007Meng et al 2012aMeng et al , 2012b; see also Chandran et al 2011 where a higher order fluid model is used).…”

Section: Introductionmentioning

confidence: 99%

Magnetic Field Amplification and Evolution in Turbulent Collisionless Magnetohydrodynamics: An Application to the Intracluster Medium

Santos-Lima

Pino

Kowal

et al. 2014

ApJ

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The amplification of magnetic fields (MFs) in the intracluster medium (ICM) is attributed to turbulent dynamo (TD) action, which is generally derived in the collisional-MHD framework. However, this assumption is poorly justified a priori, since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. The present study uses an anisotropic plasma pressure that brings the plasma within a parametric space where collisionless instabilities take place. In this model, a relaxation term of the pressure anisotropy simulates the feedback of the mirror and firehose instabilities, in consistency with empirical studies. Our three-dimensional numerical simulations of forced transonic turbulence, aiming the modeling of the turbulent ICM, were performed for different initial values of the MF intensity and different relaxation rates of the pressure anisotropy. We found that in the high-β plasma regime corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results that are similar to the collisional-MHD model, as far as the statistical properties of the turbulence are concerned. Also, the TD amplification of seed MFs was found to be similar to the collisional-MHD model. The simulations that do not employ the anisotropy relaxation deviate significantly from the collisional-MHD results and show more power at the small-scale fluctuations of both density and velocity as a result of the action of the instabilities. For these simulations, the large-scale fluctuations in the MF are mostly suppressed and the TD fails in amplifying seed MFs.

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Pressure anisotropy in global magnetospheric simulations: A magnetohydrodynamics model

Cited by 36 publications

References 61 publications

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

Asymmetry of magnetosheath flows and magnetopause shape during low Alfvén Mach number solar wind

Pressure-anisotropy-driven microturbulence and magnetic-field evolution in shearing, collisionless plasma

Magnetic Field Amplification and Evolution in Turbulent Collisionless Magnetohydrodynamics: An Application to the Intracluster Medium

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