Recent progress in astrophysical plasma turbulence from solar wind observations

Chen, C. H. K.

doi:10.1017/s0022377816001124

Cited by 219 publications

(247 citation statements)

References 139 publications

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“…SA perturbations above the limit (1) rapidly transfer their mechanical energy from the largest scales to plasma microscales and thermal energy, without the help of a turbulent cascade. This paradigm is at odds with standard theories of Alfvénic turbulence in collisionless systems [11], and may be crucial for understanding turbulent energy dissipation in astrophysical plasmas ranging from the intracluster medium (ICM) [12][13][14][15] to hot accretion flows [16] and high-β regions of the solar wind [5,[17][18][19].…”

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confidence: 99%

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

et al. 2017

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Using two-dimensional hybrid-kinetic simulations, we explore the nonlinear "interruption" of standing and traveling shear-Alfvén waves in collisionless plasmas. Interruption involves a self-generated pressure anisotropy removing the restoring force of a linearly polarized Alfvénic perturbation, and occurs for wave amplitudes δB ⊥ =B 0 ≳ β −1=2 (where β is the ratio of thermal to magnetic pressure). We use highly elongated domains to obtain maximal scale separation between the wave and the ion gyroscale. For standing waves above the amplitude limit, we find that the large-scale magnetic field of the wave decays rapidly. The dynamics are strongly affected by the excitation of oblique firehose modes, which transition into long-lived parallel fluctuations at the ion gyroscale and cause significant particle scattering. Traveling waves are damped more slowly, but are also influenced by small-scale parallel fluctuations created by the decay of firehose modes. Our results demonstrate that collisionless plasmas cannot support linearly polarized Alfvén waves above δB ⊥ =B 0 ∼ β −1=2 . They also provide a vivid illustration of two key aspects of low-collisionality plasma dynamics: (i) the importance of velocity-space instabilities in regulating plasma dynamics at high β, and (ii) how nonlinear collisionless processes can transfer mechanical energy directly from the largest scales into thermal energy and microscale fluctuations, without the need for a scale-by-scale turbulent cascade.

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confidence: 99%

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

et al. 2017

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“…The reason is that high-β, weakly collisional plasmas are susceptible to several kinetic instabilities when |∆p/p| 1/β, such as the firehose (Rosenbluth 1956;Chandrasekhar et al 1958;Hellinger & Matsumoto 2000) and mirror (Barnes 1966;Hasegawa 1969;Southwood & Kivelson 1993;Hellinger 2007) instabilities. These instabilities regulate the pressure anisotropy to values near the instability thresholds, an effect that has been diagnosed in various kinetic particle-in-cell simulations (Kunz et al 2014;Riquelme et al 2015;Hellinger & Trávníček 2015;Melville et al 2016) and directly observed using in situ measurements of particle distribution functions and magnetic fluctuations in the solar wind (Kasper et al 2002;Hellinger et al 2006;Bale et al 2009;Chen et al 2016;Chen 2016). Thus, as the magnitudes of pressure anisotropy (and thus parallel viscosity) specified by (1.4) are often unphysically large in weakly collisional, high-β plasmas, any fluid model of the fluctuation dynamo must adopt some form of microphysical closure to account for the otherwise absent regulation of the pressure anisotropy.…”

Section: (Compiled On 24 March 2020)mentioning

confidence: 89%

Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma

St-Onge

Kunz

2018

ApJL

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The turbulent amplification of cosmic magnetic fields depends upon the material properties of the host plasma. In many hot, dilute astrophysical systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity of particle-particle collisions allows departures from local thermodynamic equilibrium. These departures -pressure anisotropies -exert anisotropic viscous stresses on the plasma motions that inhibit their ability to stretch magnetic-field lines. We present an extensive numerical study of the fluctuation dynamo in a weakly collisional plasma using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous (Braginskii) stress. When the stress is limited to values consistent with a pressure anisotropy regulated by firehose and mirror instabilities, the Braginskii-MHD dynamo largely resembles its MHD counterpart, particularly when the magnetic field is dynamically weak. If instead the parallel viscous stress is left unabated -a situation relevant to recent kinetic simulations of the fluctuation dynamo and, we argue, to the early stages of the dynamo in a magnetized ICM -the dynamo changes its character, amplifying the magnetic field while exhibiting many characteristics reminiscent of the saturated state of the large-Prandtl-number (Pm 1) MHD dynamo. We construct an analytic model for the Braginskii-MHD dynamo in this regime, which successfully matches simulated dynamo growth rates and magnetic-energy spectra. A prediction of this model, confirmed by our numerical simulations, is that a Braginskii-MHD plasma without pressure-anisotropy limiters will not support a dynamo if the ratio of parallel and perpendicular viscosities is too large. This ratio reflects the relative allowed rates of field-line stretching and mixing, the latter of which promotes resistive dissipation of the magnetic field. In all cases that do exhibit a viable dynamo, the generated magnetic field is organized into folds that persist into the saturated state and bias the chaotic flow to acquire a scale-dependent spectral anisotropy.

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“…Several mechanisms may contribute to the nonadiabatic temperature profile of the solar wind. For instance, the plasma may be heated as a result of instabilities and turbulent fluctuations that extract energy from the streaming motion and convert it into kinetic energy of particles (e.g., Richardson & Smith 2003;Cranmer et al 2007Cranmer et al , 2009Chen 2016;Vech et al 2017;Tang et al 2018;Berčič et al 2019;Verscharen et al 2019a,b;López et al 2019;Shaaban et al 2019;Vasko et al 2019;Roberg-Clark et al 2019). The analysis of such local instabilities, however, does not allow for a definitive prediction of the global radial temperature profile.…”

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confidence: 99%

Electron temperature of the solar wind

Boldyrev

Forest

Egedal

2020

Proc. Natl. Acad. Sci. U.S.A.

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Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as T e (r) ∼ r −0.3 . . . r −0.7 , which is significantly slower than the adiabatic expansion law ∼ r −4/3 . Motivated by such observations, we propose a kinetic theory that addresses the non-adiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic flux tube. Due to conservation of their magnetic moments, the electrons form a beam collimated along the magnetic field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as T e (r) ∝ r −2/5 . This is close to the observed scaling of the solar wind temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind. More broadly, it may be useful for describing magnetized winds from G-type stars.

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Recent progress in astrophysical plasma turbulence from solar wind observations

Cited by 219 publications

References 139 publications

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High- β Plasma

Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma

Electron temperature of the solar wind

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