Destabilizing effects of the suprathermal populations in the solar wind

Abstract

Context. Suprathermal populations are ubiquitous in the solar wind, indicating plasma states out of thermal equilibrium, and an excess of free energy expected to enhance the kinetic instabilities. However, recent endeavors to disclose the effects of these populations on the electromagnetic instabilities driven by the temperature anisotropy do not confirm this expectation, but mainly show that these instabilities are inhibited by the suprathermals. Aims. In an attempt to clarify the effect of the suprathermals, we propose to revisit the existing models for the anisotropic velocity distributions of plasma particles and to provide an alternative comparative analysis that unveils the destabilizing effects of the suprathermal populations. Methods. Suprathermal tails of the observed distributions are best fitted by the Kappa power laws (with the bi-Kappa variant to model temperature anisotropies), which are nearly Maxwellian at low speeds (thermal core) and decrease as a power law at high speeds (suprathermal halo). To unveil the destabilizing effects of the suprathermal populations, the existing methods (A) compare Kappa and Maxwellian distributions of the same effective temperature, while the alternative comparative method (B) proposed in this paper allows for an increase of the effective temperature with increasing the suprathermal populations. Both of these two methods are invoked here to quantify and compare the effects of suprathermal electrons on the electromagnetic electron-cyclotron (EMEC) instability, driven by the temperature anisotropy T e,⊥ > T e, of the electrons (where , ⊥ are directions with respect to the magnetic field). Results. Only the Maxwellian limit of lower effective temperature shapes the Kappa model at low energies (method B), enabling a realistic comparison between the Maxwellian core and the global best-fitting Kappa, which incorporates both the core and suprathermal tails. In this case, the EMEC instability is found to be markedly and systematically enhanced by the suprathermal populations for any level of the temperature anisotropy. The results of the present study may provide valuable premises for a realistic description of the suprathermal populations and their destabilizing effects for the whole spectrum of kinetic instabilities in the solar wind.

The Electron Temperature and Anisotropy in the Solar Wind. Comparison of the Core and Halo Populations

“…These variations are indeed confirmed by the observations in Fig. 3, and they come to provide an important observational support for the recent studies (Lazar et al, 2015b) which suggest that Kappa modelling of the plasma particles and their kinetic effects in the solar wind must consider a κ-dependent temperature, increasing with decreasing the κ-index.…”

“…The core show a modest increase of both the temperature components with the κ-index, while the halo seems to compensate, exhibiting a very clear decrease of the parallel and perpendicular temperatures with increasing κ. These evolutions of the halo temperatures appear to be in perfect agreement with the recent theoretical studies (Lazar et al, 2015b), which indicate an increase of the Kappa temperature with a decrease of κ, i.e., an enhance of suprathermal populations, rather than a κ-independent temperature (Livadiotis and McComas, 2013). An increase of T h with decreasing κ is also in accord with the radial profiles of these two quantities provided by our analysis above.…”

Shaping the solar wind temperature anisotropy by the interplay of electron and proton instabilities

“…In their study the WI growth rates were found to decrease with increasing the electron suprathermal population as a result of a different Kappa approach with a κ−independent temperature that recently was proven inappropriate for such an analysis . More results on the kinetic instabilities using approaches with a κ−dependent temperature, as used in the present paper, can be found in Schupfer (2000, 2001); Lazar et al (2015), and Shaaban et al (2016).…”

“…Suprathermals enhance the electron temperature, and implicitly the plasma beta parameter Schupfer 2000, 2001;Lazar et al 2015) T K e, ,⊥ = 2κ e 2κ e − 3 T M e, ,⊥ > T M e, ,⊥ , β K e, ,⊥ = 2κ e 2κ e − 3 β e, ,⊥ > β e, ,⊥ ,…”

Debye screening under non-equilibrium plasma conditions

“…1) κ-functions with a locally best-fitting Kappa index κ = κ(r) that develops from high κ-values higher than 10 at 1 AU (i.e., quasi-Maxwellian limit) and decrease in different forms dependent on the effectiveness of the velocity-space diffusion, to systematically lower κ-values (asymptotically approaching κ = 1, 5, i.e., v −5 power-law distributions). This may express how far the range of Kappa values covered at the evolution of the solar wind ion distribution function extends outward from the corona to the termination shock and what it means in terms of κ = κ(r) (for a basic interpretation of what this means in terms of a non-equilibrium state see, e.g., Lazar et al 2015).…”