Whistler Fan Instability Driven by Strahl Electrons in the Solar Wind

Vasko, I. Y.; Krasnoselskikh, V.; Tong, Yuguang; Bale, S. D.; Bonnell, J. W.; Mozer, F. S.

doi:10.3847/2041-8213/ab01bd

Cited by 84 publications

(106 citation statements)

References 42 publications

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“…It is also claimed that a small anisotropy A b 1 may prevent the whistler heat-flux instability even in the presence of a considerable drift velocity of the core, which can also be explained by the quasi-stable states after relaxation (case 3). To conclude, these observations are in agreement with our results in case 3, which (i) show that the saturation of the whistler heat-flux may occur via the induced temperature anisotropies for the core and beam, and (ii) confirm a minor implication of this instability in the regulation of electron heat-fluxes, as suggested by recent studies (Horaites et al 2018;Vasko et al 2019) indicating instabilities of oblique modes, e.g., kinetic Alfvén and magnetosonic waves, as potentially more efficient in scattering and suppressing the heat-flux of electron strahl.…”

Section: Discussionsupporting

confidence: 93%

Quasi-linear approach of the whistler heat-flux instability in the solar wind

Shaaban

Lazar

Yoon

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The hot beaming (or strahl) electrons responsible for the main electron heat-flux in the solar wind are believed to be self-regulated by the electromagnetic beaming instabilities, also known as the heat-flux instabilities. Here we report the first quasilinear theoretical approach of the whistler unstable branch able to characterize the long-term saturation of the instability as well as the relaxation of the electron velocity distributions. The instability saturation is not solely determined by the drift velocities, which undergo only a minor relaxation, but mainly from a concurrent interaction of electrons with whistlers that induces (opposite) temperature anisotropies of the core and beam populations and reduces the effective anisotropy. These results might be able to (i) explain the low intensity of the whistler heat-flux fluctuations in the solar wind (although other explanations remain possible and need further investigation), and (ii) confirm a reduced effectiveness of these fluctuations in the relaxation and isotropization of the electron strahl and in the regulation of the electron heat-flux.

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Section: Discussionsupporting

confidence: 93%

Quasi-linear approach of the whistler heat-flux instability in the solar wind

Shaaban

Lazar

Yoon

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…We have compared the observed frequencies to predictions of the WHFI theory with electron VDFs consisting of core and halo electron populations. The presence of the anti-sunward strahl population typical for the fast solar wind (Pilipp et al 1987;Štverák et al 2009) would not affect any characteristics of the WHFI, because whistler waves produced by the WHFI propagate anti-sunward and do not resonate with the strahl (e.g., Vasko et al 2019, for discussion).…”

Section: Discussionmentioning

confidence: 99%

Statistical Study of Whistler Waves in the Solar Wind at 1 au

Tong

Vasko

Artemyev

et al. 2019

ApJ

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107

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Whistler waves are intermittently present in the solar wind, while their origin and effects are not entirely understood. We present a statistical analysis of magnetic field fluctuations in the whistler frequency range (above 16 Hz) based on about 801,500 magnetic field spectra measured over three years aboard ARTEMIS spacecraft in the pristine solar wind. About 13,700 spectra (30 hours in total) with intense magnetic field fluctuations satisfy the interpretation in terms of quasi-parallel whistler waves. We provide estimates of the whistler wave occurrence probability, amplitudes, frequencies and bandwidths. The occurrence probability of whistler waves is shown to strongly depend on the electron temperature anisotropy. The whistler waves amplitudes are in the range from about 0.01 to 0.1 nT and typically below 0.02 of the background magnetic field. The frequencies of the whistler waves are shown to be below an upper bound that is dependent on β e . The correlations established between the whistler wave properties and local macroscopic plasma parameters suggest that the observed whistler waves can be generated in local plasmas by the whistler heat flux instability. The whistler wave amplitudes are typically small, which questions the hypothesis that quasi-parallel whistler waves are capable to regulate the electron heat flux in the solar wind. We show that the observed whistler waves have sufficiently wide bandwidths and small amplitudes, so that effects of the whistler waves on electrons can be addressed in the frame of the quasi-linear theory.

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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.…”

Section: Introductionmentioning

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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Whistler Fan Instability Driven by Strahl Electrons in the Solar Wind

Cited by 84 publications

References 42 publications

Quasi-linear approach of the whistler heat-flux instability in the solar wind

Quasi-linear approach of the whistler heat-flux instability in the solar wind

Statistical Study of Whistler Waves in the Solar Wind at 1 au

Electron temperature of the solar wind

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