arXiv subjects

Adolfo F. Viñas

Publications and source records attributed to Adolfo F. Viñas.

4 recordsLinked to original sources

arXiv2026

Knudsen number as a non-thermal parameter: possible origin of skewness in space plasma distributions

Non-Maxwellian distributions and their origins in space plasma have attracted significant attention due to their prevalence and impact on various astrophysical and space-related phenomena. This paper presents a theoretical study of the consequences of incorporating a Skew-Kappa distribution to describe the non-thermal electron distribution in the solar wind. By introducing a Krook-like term into the Boltzmann equation to represent collision effects, we investigate the dependence of the skewness parameter on plasma macro-dynamics. Our analysis focuses on understanding the departure from thermal equilibrium and the statistical behavior of the plasma under the influence of collisional processes. By analyzing the Boltzmann transport equation adapted to space plasma, we derive expressions for the skewness parameter as a function of plasma parameters and the collision effect. Our results provide valuable information on the relationship between skewness, collisional dynamics, and the statistical properties of space plasmas, namely $δ\sim K_N$, the relationship between the skewness parameter and the effective Knudsen number. This study contributes to a deeper understanding of non-Maxwellian distributions and their role in astrophysical and space plasma phenomena.

physics.plasm-ph↗

arXiv2021

Skew-Kappa distribution functions & whistler-heat-flux instability in the solar wind: the core-strahlo model

Electron velocity distributions in the solar wind are known to have field-aligned skewness, which has been characterized by the presence of secondary populations such as the halo and strahl. Skewness may provide energy for the excitation of electromagnetic instabilities, such as the whistler heat-flux instability (WHFI), that may play an important role in regulating the electron heat-flux in the solar wind. Here we use kinetic theory to analyze the stability of the WHFI in a solar-wind-like plasma where solar wind core, halo and strahl electrons are described as a superposition of two distributions: a Maxwellian core, and another population modeled by a Kappa distribution to which an asymmetry term has been added, representing the halo and also the strahl. Considering distributions with small skewness we solve the dispersion relation for the parallel propagating whistler-mode and study its linear stability for different plasma parameters. Our results show that the WHFI can develop in this system, and provide stability thresholds for this instability, as a function of the electron beta and the parallel electron heat-flux, to be compared with observational data. However, since different plasma states, with different stability level to the WHFI, can have the same moment heat-flux value, it is the skewness (i.e. the asymmetry of the distribution along the magnetic field), and not the heat-flux, the best indicator of instabilities. Thus, systems with high heat-flux can be stable enough to WHFI, so that it is not clear if the instability can effectively regulate the heat-flux values through wave-particle interactions.

physics.plasm-ph↗

arXiv2017

Kinetic Theory and Fast Wind Observations of the Electron Strahl

We develop a model for the strahl population in the solar wind -- a narrow, low-density and high-energy electron beam centered on the magnetic field direction. Our model is based on the solution of the electron drift-kinetic equation at heliospheric distances where the plasma density, temperature, and the magnetic field strength decline as power-laws of the distance along a magnetic flux tube. Our solution for the strahl depends on a number of parameters that, in the absence of the analytic solution for the full electron velocity distribution function (eVDF), cannot be derived from the theory. We however demonstrate that these parameters can be efficiently found from matching our solution with observations of the eVDF made by the Wind satellite's SWE strahl detector. The model is successful at predicting the angular width (FWHM) of the strahl for the Wind data at 1 AU, in particular by predicting how this width scales with particle energy and background density. We find the strahl distribution is largely determined by the local temperature Knudsen number $γ\sim |T dT/dx|/n$, which parametrizes solar wind collisionality. We compute averaged strahl distributions for typical Knudsen numbers observed in the solar wind, and fit our model to these data. The model can be matched quite closely to the eVDFs at 1 AU, however, it then overestimates the strahl amplitude at larger heliocentric distances. This indicates that our model may be improved through the inclusion of additional physics, possibly through the introduction of "anomalous diffusion" of the strahl electrons.

physics.space-ph↗

arXiv2015

Dissipation of parallel and oblique Alfvén-cyclotron waves: implications for minor ion heating in the solar wind

We perform 2.5D hybrid simulations with massless fluid electrons and kinetic particle-in-cell ions to study the temporal evolution of ion temperatures, temperature anisotropies and velocity distribution functions in relation to the dissipation and turbulent evolution of a broad-band spectrum of parallel and obliquely propagating Alfvén-cyclotron waves. The purpose of this paper is to study the relative role of parallel versus oblique Alfvén-cyclotron waves in the observed heating and acceleration of minor ions in the fast solar wind. We consider collisionless homogeneous multi-species plasma, consisting of isothermal electrons, isotropic protons and a minor component of drifting $α$ particles in a finite-$β$ fast stream near the Earth. The kinetic ions are modeled by initially isotropic Maxwellian velocity distribution functions, which develop non-thermal features and temperature anisotropies when a broad-band spectrum of low-frequency non-resonant, $ω\leq 0.34 Ω_p$, Alfvén-cyclotron waves is imposed at the beginning of the simulations. The initial plasma parameter values, such as ion density, temperatures and relative drift speeds, are supplied by fast solar wind observations made by the \textit{Wind} spacecraft at 1AU. The imposed broad-band wave spectra is left-hand polarized and resembles \textit{Wind} measurements of Alfvénic turbulence in the solar wind. The imposed magnetic field fluctuations for all cases are within the inertial range of the solar wind turbulence and have a Kraichnan-type spectral slope $α=-3/2$. We vary the propagation angle from $θ= 0^\circ$ to $θ=30^\circ$ and $θ=60^\circ$, and find that the minor ion heating is most efficient for the highly-oblique waves propagating at $60^\circ$, whereas the protons exhibit perpendicular cooling at all propagation angles.

physics.plasm-ph↗