Particle-in-Cell Simulations of Continuously Driven Mirror and Ion Cyclotron Instabilities in High Beta Astrophysical and Heliospheric Plasmas

Riquelme, Mario; Quataert, Eliot; Verscharen, Daniel

doi:10.1088/0004-637x/800/1/27

Cited by 94 publications

(140 citation statements)

References 53 publications

(76 reference statements)

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“…Astrophysical plasmas that frequently fulfill these requirements include the solar wind and low-luminosity accretion disks. Astrophysical shear flows, for example, can create anisotropies with > R 1 0p (Kunz et al 2014;Riquelme et al 2015). In the presence of compressive fluctuations, the isotropization mechanism described here can limit this equilibrium anisotropy to values that are significantly below the thresholds for the A/IC and mirror-mode instabilities.…”

Section: Discussionmentioning

confidence: 87%

“…For example, the mirror instability largely conserves the magnetic moment unless the mirrors reach large amplitudes | | dB B 1 0 , while the A/IC and FM/W instabilities lead to pitch-angle scattering even at low amplitudes (e.g., Kunz et al 2014;Riquelme et al 2015).…”

Section: Discussionmentioning

confidence: 99%

“…If the temperature anisotropy | | -R 1 p of the protons exceeds a certain threshold, then the plasma becomes unstable, and various kinds of plasma waves and/or nonpropagating structures grow, while the distribution function relaxes toward a stable state. If > R 1 p , the plasma can excite parallel-propagating Alfvén/ion-cyclotron (A/IC) waves or nonpropagating mirror modes (Rudakov & Sagdeev 1961;Sagdeev & Shafranov 1961;Tajiri 1967;Southwood & Kivelson 1993;Gary & Lee 1994;Kunz et al 2014;Riquelme et al 2015;Gary et al 2016). If < R 1 p , the plasma can excite parallel-propagating fastmagnetosonic/whistler (FM/W) waves or nonpropagating oblique firehose modes (Quest & Shapiro 1996;Gary et al 1998;Hellinger & Matsumoto 2000;Hellinger & Trávníček 2008;Rosin et al 2011).…”

Section: Introductionmentioning

confidence: 99%

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Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

Verscharen

Chandran

Klein

et al. 2016

ApJ

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p , whereT p and  T p are the perpendicular and parallel temperatures of the protons, B is the magnetic field strength, and n p is the proton density. If the amplitude of the compressive fluctuations is large enough, R p crosses one or more instability thresholds for anisotropy-driven microinstabilities. The enhanced field fluctuations from these microinstabilities scatter the protons so as to reduce the anisotropy of the pressure tensor. We propose that this scattering drives the average value of R p away from the marginal stability boundary until the fluctuating value of R p stops crossing the boundary. We model this "fluctuating-anisotropy effect" using linear VlasovMaxwell theory to describe the large-scale compressive fluctuations. We argue that this effect can explain why, in the nearly collisionless solar wind, the average value of R p is close to unity.

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Section: Discussionmentioning

confidence: 87%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

Verscharen

Chandran

Klein

et al. 2016

ApJ

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“…This strategy is supported by our previous study of an electron-ion plasma with 2 β e 20 (β e ≡ 8πp e /|B| 2 ), where the electron anisotropy was mostly regulated by the electron-scale whistler instability, with a moderate contribution from the ion-scale mirror instability . For smaller values of β e , the effect of the mirror modes is expected to be even smaller (Riquelme et al 2015). As we will see below, this low β e regime is the most interesting in terms of electron acceleration by whistler waves.…”

Section: Introductionmentioning

confidence: 89%

Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

Riquelme

Osorio

Quataert

2017

ApJ

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We use 2D particle-in-cell (PIC) simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, B. In this setup, an electron pressure anisotropy with p ⊥,e > p ||,e naturally arises due to the adiabatic invariance of the electron magnetic moment (p ||,e and p ⊥,e are the pressures parallel and perpendicular to B). If the anisotropy is large enough, the whistler instability arises, efficiently scattering the electrons and limiting ∆p e (≡ p ⊥,e − p ||,e ). In this context, ∆p e taps into the plasma velocity shear, producing electron heating by the so called anisotropic viscosity. In our simulations, we permanently drive the growth of |B| by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially β e ∼ 1, gradually decreasing its efficiency for larger values of β e (≡ 8πp e /|B| 2 ). If initially β e ∼ 1, the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with spectral index ∼ 3.7. In these cases, the nonthermal tail accounts for ∼ 5% of the electrons, and for ∼ 15% of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.

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“…11 Thus, it is probably a good assumption that growth episodes during which plasma crosses the mirror threshold are typically 11 Some level of scattering can occur if a growth episode lasts so long that the mirrors reach δB/B ∼ 1 Riquelme et al 2015). Then the local effective viscosity of the plasma drops and the "eddy" that caused the growth of the field breaks up into smaller "eddies"-these can produce even faster growth, but also decorrelate very quickly.…”

Section: Plasma Dynamo At Moderate βmentioning

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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Particle-in-Cell Simulations of Continuously Driven Mirror and Ion Cyclotron Instabilities in High Beta Astrophysical and Heliospheric Plasmas

Cited by 94 publications

References 53 publications

Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

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

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