Fluidization of collisionless plasma turbulence

Meyrand, Romain; Kanekar, Anjor; Dorland, W.; Schekochihin, A. A.

doi:10.1073/pnas.1813913116

Cited by 57 publications

(57 citation statements)

References 89 publications

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“…We conjecture that (2.19) and (2.20) may provide a reasonable description of transverse, non-compressive fluctuations and their mutual interactions even when the assumptions δB ≪ B 0 and λ ≪ l fail. For example, if collisionless damping (Barnes 1966) or passive-scalar mixing (Schekochihin et al 2016;Meyrand et al 2019) removes compressive and longitudinal fluctuations, then (2.5) and (2.6) may be reasonable approximations even if δB ∼ B 0 and λ ∼ l. We note that neither our derivation of (2.19) and (2.20), nor the derivation of RMHD as a limit of the Vlasov equation (Schekochihin et al 2009), requires that β = 8πp/B 2 be ordered as either large or small.…”

Section: Transverse Non-compressive Fluctuations In a Radially Stratmentioning

confidence: 99%

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

Chandran

Perez

2019

J. Plasma Phys.

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We present three-dimensional direct numerical simulations and an analytic model of reflection-driven magnetohydrodynamic (MHD) turbulence in the solar wind. Our simulations describe transverse, non-compressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere, through the chromosphere and corona, and out to a heliocentric distance r of 21 solar radii (R ⊙ ). We launch outward-propagating "z + fluctuations" into the simulation domain by imposing a randomly evolving photospheric velocity field. As these fluctuations propagate away from the Sun, they undergo partial reflection, producing inward-propagating "z − fluctuations." Counter-propagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates dynamic alignment, allows for strongly or weakly turbulent nonlinear interactions, and divides the z + fluctuations into two populations with different characteristic radial correlation lengths. The inertial-range power spectra of z + and z − fluctuations in our simulations evolve toward a k −3/2 ⊥ scaling at r > 10R ⊙ , where k ⊥ is the wave-vector component perpendicular to the background magnetic field. In two of our simulations, the z + power spectra are much flatter between the coronal base and r ≃ 4R ⊙ . We argue that these spectral scalings are caused by: (1) high-pass filtering in the upper chromosphere; (2) the anomalous coherence of inertial-range z − fluctuations in a reference frame propagating outwards with the z + fluctuations; and (3) the change in the sign of the radial derivative of the Alfvén speed at r = r m ≃ 1.7R ⊙ , which disrupts this anomalous coherence between r = r m and r ≃ 2r m . At r > 1.3R ⊙ , the turbulent-heating rate in our simulations is comparable to the turbulent-heating rate in a previously developed solar-wind model that agreed with a number of observational constraints, consistent with the hypothesis that MHD turbulence accounts for much of the heating of the fast solar wind. Key words: solar wind -Sun: corona -turbulence -waves † In contrast, Helios radio occultation observations show that the fractional density variations drop to 0.1 − 0.2 at r ∈ (5R⊙, 20R⊙) (Hollweg et al. 2010).

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Section: Transverse Non-compressive Fluctuations In a Radially Stratmentioning

confidence: 99%

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

Chandran

Perez

2019

J. Plasma Phys.

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“…We do not have a quantitative theory that would explain why Qnormali/Qnormale should saturate at the value that we observe numerically (which, based on a resolution study, appears to be converged). Presumably, this is decided by the details of the operation of ion Landau damping in a turbulent environment [a tricky subject (44–46)] and by the efficiency with which energy can be channeled from the MHD scales into the magnetic cascade below ρ* and the KAW cascade below ρnormali. In the absence of a definitive theory, Qnormali/Qnormale≈30 should be viewed as an “experimental” result.…”

Section: Energy Partitionmentioning

confidence: 99%

Thermal disequilibration of ions and electrons by collisionless plasma turbulence

Kawazura

Barnes

Schekochihin

2018

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

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116

106

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SignificanceLarge-scale astrophysical processes inject energy into turbulent motions and electromagnetic fields, which carry this energy to small scales and eventually thermalize it. How this energy is partitioned between ions and electrons is important both in plasma physics and in astrophysics. Here we determine this energy partition via gyrokinetic turbulence simulations and provide a simple prescription for the ion-to-electron heating ratio. We find that turbulence promotes disequilibration of the species: When magnetic energy density is greater than the thermal energy density, electrons are preferentially heated, whereas when it is smaller, ions are. This is a relatively rare example of nature promoting an ever more out-of-equilibrium state in an environment where particle collisions are not frequent enough to equalize the temperatures of the species.

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“…Around the ion Larmor scale, an energetically subdominant population of non-GK perturbations, viz., whistler and ion-cyclotron waves with large k , is observed in the solar wind (Wicks et al 2010;Podesta & Gary 2011;He et al 2011He et al , 2012Klein et al 2014;Lion et al 2016) and may be due to pressure-anisotropy instabilities, which are not captured by GK, but are not a significant danger at low beta (e.g., Hellinger et al 2006;Bale et al 2009;Kunz et al 2018). 3 Interestingly, it turns out that even at βi ∼ 1, Landau damping in the inertial range can be effectively suppressed by a nonlinear effect, the stochastic echo (Meyrand et al 2019), and the compressive free energy cascades mostly unimpeded towards the Larmor scale. via various dissipation effects at or below the electron Larmor scale.…”

Section: Epitomementioning

confidence: 99%

“…They are at low frequencies because they are typically excited by large-scale mechanisms and because their cascade to smaller scales is anisotropic with respect to their local magnetic field, k ≪ k ⊥ , implying that the Larmor scales (ρ i , ρ e ) in the perpendicular direction are reached before the Larmor frequencies (Ω i , Ω e ) (see Schekochihin et al 2009, and references therein; this paper is henceforth referred to as S09). Thus, opportunities for transferring the energy of the turbulent cascade into ion thermal energy via the Landau damping of compressive fluctuations (throughout the inertial range; see S09- §6 and Meyrand et al 2019) or via the ion entropy cascade (starting at the ion Larmor scale; see S09- §7 and Kawazura et al 2019) occur before (i.e., at larger scales than) the cyclotron heating can take place (Howes et al 2008a). All of these low-frequency heating routes can be treated in the so-called gyrokinetic (GK) approximation (Frieman & Chen 1982;Howes et al 2006, the latter paper is henceforth referred to as H06).…”

Section: Introductionmentioning

confidence: 99%

Constraints on ion versus electron heating by plasma turbulence at low beta

Schekochihin

Kawazura²,

Barnes

2019

J. Plasma Phys.

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It is shown that in low-beta, weakly collisional plasmas, such as the solar corona, some instances of the solar wind, the aurora, inner regions of accretion discs, their coronae, and some laboratory plasmas, Alfvénic fluctuations produce no ion heating within the gyrokinetic approximation, i.e., as long as their amplitudes (at the Larmor scale) are small and their frequencies stay below the ion Larmor frequency (even as their spatial scales can be above or below the ion Larmor scale). Thus, all low-frequency ion heating in such plasmas is due to compressive fluctuations ("slow modes"): density perturbations and non-Maxwellian perturbations of the ion distribution function. Because these fluctuations energetically decouple from the Alfvénic ones already in the inertial range, the above conclusion means that the energy partition between ions and electrons in low-beta plasmas is decided at the outer scale, where turbulence is launched, and can be determined from magnetohydrodynamic (MHD) models of the relevant astrophysical systems. Any additional ion heating must come from non-gyrokinetic mechanisms such as cyclotron heating or the stochastic heating owing to distortions of ions' Larmor orbits. An exception to these conclusions occurs in the Hall limit, i.e., when the ratio of the ion to electron temperatures is as low as the ion beta (equivalently, the electron beta is order unity). In this regime, slow modes couple to Alfvénic ones well above the Larmor scale (viz., at the ion inertial or ion sound scale), so the Alfvénic and compressive cascades join and then separate again into two cascades of fluctuations that linearly resemble kinetic Alfvén and ion cyclotron waves, with the former heating electrons and the latter ions. The two cascades are shown to decouple, scalings for them are derived, and it is argued physically that the two species will be heated by them at approximately equal rates. †

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Fluidization of collisionless plasma turbulence

Cited by 57 publications

References 89 publications

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind

Thermal disequilibration of ions and electrons by collisionless plasma turbulence

Constraints on ion versus electron heating by plasma turbulence at low beta

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