Dual Maxwellian-Kappa modeling of the solar wind electrons: new clues on the temperature of Kappa populations

Monthly Notices of the Royal Astronomical Society

Poedts

2018

In the solar wind electron velocity distributions reveal two counter-moving populations which may induce electromagnetic (EM) beaming instabilities known as heat flux instabilities. Depending on plasma parameters two distinct branches of whistler and firehose instabilities can be excited. These instabilities are invoked in many scenarios, but their interplay is still poorly understood. An exact numerical analysis is performed to resolve the linear Vlasov-Maxwell dispersion and characterize these two instabilities, e.g., growth rates, wave frequencies and thresholds, enabling to identify their dominance for conditions typically experienced in space plasmas. Of particular interest are the effects of suprathermal Kappa-distributed electrons which are ubiquitous in these environments. The dominance of whistler or firehose instability is highly conditioned by the beam-core relative velocity, core plasma beta and the abundance of suprathermal electrons. Derived in terms of relative drift velocity the instability thresholds show an inverse correlation with the core plasma beta for the whistler modes, and a direct correlation with the core plasma beta for the firehose instability. Suprathermal electrons reduce the effective (beaming) anisotropy inhibiting the firehose modes while the whistler instability is stimulated.

“…their Maxwellian limits (Lazar et al 2015(Lazar et al , 2017b. Protons (subscript p) are heavier and can be assumed Maxwellian and isotropic, enabling to isolate the effects of electrons.…”

Section: Theorymentioning

confidence: 99%

Clarifying the solar wind heat flux instabilities

Monthly Notices of the Royal Astronomical Society

Poedts

2018

“…Local sources of whistlers are multiple, either the heat-flux instability driven by counterbeaming electrons (Feldman et al 1973;Gary 1993;Shaaban et al 2018b), or the cyclotron instability driven by electrons with anisotropic temperatures, i.e., T ⊥ > T , where , ⊥ denote directions with respect to the magnetic field (Gary & Wang 1996;Lazar et al 2018), or even the interplay of these two instabilities (Shaaban et al 2018a). Both sources of free energy may indeed co-exist in space plasmas (Štverák et al 2008;Viñas et al 2010;Lazar et al 2017), and recent studies (Saeed et al 2017b;Shaaban et al 2018b,a) have unveiled new regimes of whistler heat-flux instability in an attempt to provide an extended linear description in the solar wind conditions.…”

Section: Introductionmentioning

confidence: 99%

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

Monthly Notices of the Royal Astronomical Society

Yoon

et al. 2019

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.

“…Recent attempts to characterize these effects are either limited to a linear analysis (Lazar & Poedts 2009;Lazar et al 2015Lazar et al , 2017aVinãs et al 2017), or simply altered by an idealized (bi-)Maxwellian description of suprathermal populations in a quasilinear analysis. Thus, quasilinear approaches have been successfully proposed for both the whistler and firehose instabilities driven either by a single bi-Maxwellian population of electrons (Yoon et al 2012(Yoon et al , 2017aSarfraz et al 2017), or by dual electrons with a core-halo structure (Sarfraz et al 2016;Sarfraz 2018) minimizing, however, the effects of suprathermals by assuming the halo bi-Maxwellian distributed.…”

Section: Introductionmentioning

confidence: 99%

The Interplay of the Solar Wind Core and Suprathermal Electrons: A Quasilinear Approach for Firehose Instability

Yoon

et al. 2019

ApJ

Self Cite

In the solar wind an equipartition of kinetic energy densities can be easily established between thermal and suprathermal electrons and the instability conditions are markedly altered by the interplay of these two populations. The new thresholds derived here for the periodic branch of firehose instability shape the limits of temperature anisotropy reported by the observations for both electron populations. This instability constraint is particularly important for the suprathermal electrons which, by comparison to thermal populations, are even less controlled by the particle-particle collisions. An extended quasilinear approach of this instability confirms predictions from linear theory and unveil the mutual effects of thermal and suprathermal electrons in the relaxation of their temperature anisotropies and the saturation of growing fluctuations.