arXiv subjects

Rudi Gaelzer

Publications and source records attributed to Rudi Gaelzer.

11 recordsLinked to original sources

arXiv2026

Ion firehose and ion cyclotron instability with subtracted-Kappa distributions

In the present paper we discuss numerical solutions of the dispersion relation for electromagnetic waves propagating along the lines of an ambient magnetic field, considering parameters representative of space plasma environments, and considering the ion population described by a subtracted-Kappa distribution. We consider situations in which the ion thermal anisotropy is such that $T_{i\perp} T_{i\parallel}$, and investigate the effects of the same parameters on the ion-cyclotron instability. For both forms of instability, we also investigate the effect of the thermal anisotropy of the electron distribution, and the effect of the occurrence of a drift velocity in the electron distribution function. Among the results obtained, we show that the increase of the loss-cone feature in a bi-kappa distribution leads to a decrease of the growth rates of the firehose instability, and to an increase of the growth rates of the ion-cyclotron instability.

physics.plasm-ph↗

arXiv2026

Wave instabilities in anisotropic regularized Kappa plasmas: New plasma dispersion functions and numerical validation

Plasmas in space and astrophysical environments are frequently found in (quasi-)stationary states not in thermal equilibrium, indicated by velocity distribution functions (VDFs) featuring nonthermal characteristics, such as Lorentzian high-energy tails and kinetic (temperature and strahl/beam) anisotropies. For decades, in-situ observations of electrons and ions in the solar corona, solar wind or planetary magnetospheres have been modelled using so-called Kappa VDFs, whose standard form, however - depending on the considered VDF moment - imposes stringent constraints on the minimum value of the Kappa power-law parameter. Such limitations have posed difficulties on the derivation of a fully-consistent hydrodynamic formulation of suprathermal plasmas. To improve this situation, the recently proposed regularized Kappa VDF avoids strict constraints on the Kappa parameter by introducing an exponential, regularizing cut-off. In this work we assume that the plasma particles are described by regularized-Kappa VDFs that feature anisotropies not only in their thermal velocity spread parameters but also in their exponential cut-offs. The dielectric tensor components for waves propagating along a magnetic field are derived, in terms of new plasma dispersion functions. The resulting dispersion equations are solved with two complementary techniques, namely by using the analytical expressions derived here and by employing the ALPS code. This is done for physical parameters that give rise to electromagnetic and electrostatic instabilities. Both approaches show excellent agreement and the results demonstrate how an anisotropy in the regularizing cut-off of VDFs can serve, depending on an instability's nature, either as a stabilization factor or as an additional free energy source.

physics.plasm-ph↗

arXiv2025

Electrostatic waves in astrophysical Druyvesteyn plasmas: I. Langmuir waves

Plasmas in various astrophysical systems are in non-equilibrium states as evidenced by direct in-situ measurements in the solar wind, solar corona and planetary environments as well as by indirect observations of nonthermal sources of waves and emissions. Specific to observed non-equilibrium plasmas are non-Maxwellian velocity distributions with suprathermal tails, most often described by Kappa (power-law) distributions. In this paper, we introduce an alternative modeling for linear waves in plasmas described by the generalized Druyvesteyn distribution model. This model can reproduce not only high-energy tails, but also low-energy flat-tops of velocity distributions, like those of electrons in interplanetary shocks and the solar transition region. The wave dispersion relation of longitudinal waves is derived in terms of the newly introduced Druyvesteyn dispersion function. The dispersion curves as well as damping rates of high-frequency Langmuir waves are numerically computed for the isotropic case, and their analytical approximations are provided in the limit of weak damping. We thus offer a new tool for modeling longitudinal waves, and in particular Langmuir waves under the specific conditions of Druyvesteyn distributions.

physics.plasm-ph↗

arXiv2025

Collisional charging of dust particles by suprathermal plasmas. II -- Regularized Kappa distributions

We study the effects of the velocity distribution functions of the plasma particles on the equilibrium charge of dust grains, acquired through inelastic collisions of the particles with the grains. This paper is the second in a series of two papers on the subject. Here, we consider the charging process when the plasma particles are statistically described by the recently proposed regularized Kappa distribution functions, which allow for extreme suprathermal states, characterized by extremely low values of the kappa index, previously forbidden to the standard Kappa distributions, whose effects on dust charging were studied in Paper I of this series. We analyse the effects that extreme suprathermal states of the plasma particles have on dust charging and verify conditions for the uncommon result of positive equilibrium charge, employing two different models for the regularized Kappa distributions, one with kinetic temperature dependent on the kappa index, and another where the temperature is kappa-independent.

physics.plasm-ph↗

arXiv2024

Single-Mode Wave Decay

The usual approach on electrostatic wave decay process for a weak beam-plasma system considers two different wave modes interplaying, the Langmuir and ion-sound mode. In the present paper, a single-mode approach is shown to be feasible for conditions where the respective dispersion relations undergo topological changes. Numerical solutions for the dispersion relation of a beam-plasma system are presented, supporting the modeling of an analytic dispersion relation of a single wave mode. This wave mode is accounted for in the kinetic equations for particles and waves, which rule the evolution of the system. The results are compared against the two-wave mode approach using Langmuir and ion-sound waves, within the context of weak turbulence theory. It is found that the single-mode approach can account for the basic features of particles and waves, since the single mode exhibits both low and high frequency regions, which ultimately play the roles of ion-sound and Langmuir modes, respectively.

physics.plasm-ph↗

arXiv2024

Collisional charging of dust particles by suprathermal particles. I -- Standard anisotropic Kappa distributions

We study the effect of the velocity distributions of the plasma particles on the equilibrium charge of dust particles which suffer collisional charging, considering different forms of both isotropic and anisotropic Kappa distributions for ions and electrons. This paper is the first of a series of two papers on this subject. Here, we consider two different forms of Kappa distributions which are widely used in the literature, and show that effects on the dust charge associated to one of these forms are much more significant than the effects associated to the other basic form. The results obtained also show that changes in the electron distribution can have effect on the dust charge which is much more significant than the effect which can be associated to changes in the ion distribution.

physics.plasm-ph↗

arXiv2024

A dispersion function for the regularized kappa distribution function

In this work we define the plasma dispersion function for a suprathermal plasma described with a regularized kappa distribution. As is known from Maxwellian as well as (standard) kappa plasmas, the respective Fried-Conte and the modified plasma dispersion functions are valuable tools for various analytical studies. For the latter is has been proven very useful to know about the mathematical properties, as analytical continuation, series expansions as well as asymptotic expressions. Given the growing popularity of the regularized kappa distribution, as indicated by its increasing number of applications to various problems related to suprathermal plasmas, we extend its theoretical treatment here by providing the corresponding plasma dispersion function along with various of its properties.

physics.plasm-ph↗

arXiv2018

The oblique firehose instability in a bi-kappa magnetized plasma

In this work, we derive a dispersion equation that describes the excitation of the oblique (or Alfvén) firehose instability in a plasma that contains both electron and ion species modelled by bi-kappa velocity distribution functions. The equation is obtained with the assumptions of low-frequency waves and moderate to large values of the parallel (respective to the ambient magnetic field) plasma beta parameter, but it is valid for any direction of propagation and for any value of the particle gyroradius (or Larmor radius). Considering values for the physical parameters typical to those found in the solar wind, some solutions of the dispersion equation, corresponding to the unstable mode, are presented. In order to implement the dispersion solver, several new mathematical properties of the special functions occurring in a kappa plasma are derived and included. The results presented here suggest that the superthermal characteristic of the distribution functions leads to reductions to both the maximum growth rate of the instability and of the spectral range of its occurrence.

physics.plasm-ph↗

arXiv2016

Electromagnetic ion-cyclotron instability in a dusty plasma with product-bi-kappa distributions for the plasma particles

We study the dispersion relation for parallel propagating ion-cyclotron (IC) waves in a dusty plasma, considering situations where the velocity dispersion along perpendicular direction is greater than along the parallel direction, and considering the use of product-bi-kappa (PBK) velocity distributions for the plasma particles. The results obtained by numerical solution of the dispersion relation, in a case with isotropic Maxwellian distributions for electrons and PBK distribution for ions, show the occurrence of the electromagnetic ion-cyclotron instability (EMIC), and show that the decrease in the kappa indexes of the PBK ion distribution leads to significant increase in the magnitude of the growth rates and in the range of wavenumber for which the instability occurs. On the other hand, for anisotropic Maxwellian distribution for ions and PBK distribution for electrons, the decrease of the kappa index in the PBK electron distribution contributes to reduce the growth rate of the EMIC instability, but the reduction effect is less pronounced than the increase obtained with ion PBK distribution with the same $κ$ index. The results obtained also show that, as a general rule, the presence of a dust population contributes to reduce the instability in magnitude of the growth rates and range, but that in the case of PBK ion distribution with small kappa indexes the instability may continue to occur for dust populations which would eliminate completely the instability in the case of bi-Maxwellian ion distributions. It has also been seen that the anisotropy due to the kappa indexes in the ion PBK distribution is not so efficient in producing the EMIC instability as the ratio of perpendicular and parallel ion temperatures, for equivalent value of the effective temperature.

physics.plasm-ph↗

arXiv2016

The general dielectric tensor for bi-kappa magnetized plasmas

In this paper we derive the dielectric tensor for a plasma containing particles described by an anisotropic superthermal (bi-kappa) velocity distribution function. The tensor components are written in terms of the two-variables kappa plasma special functions, recently defined by Gaelzer and Ziebell [Phys. Plasmas 23, 022110 (2016)]. We also obtain various new mathematical properties for these functions, which are useful for the analytical treatment, numerical implementation and evaluation of the functions and, consequently, of the dielectric tensor. The formalism developed here and in the previous paper provides a mathematical framework for the study of electromagnetic waves propagating at arbitrary angles and polarizations in a superthermal plasma.

physics.plasm-ph↗

arXiv2016

Obliquely propagating electromagnetic waves in magnetized kappa plasmas

Velocity distribution functions (VDFs) that exhibit a power-law dependence on the high-energy tail have been the subject of intense research by the plasma physics community. Such functions, known as kappa or superthermal distributions, have been found to provide a better fitting to the VDFs measured by spacecraft in the solar wind. One of the problems that is being addressed on this new light is the temperature anisotropy of solar wind protons and electrons. In the literature, the general treatment for waves excited by (bi-)Maxwellian plasmas is well-established. However, for kappa distributions, the wave characteristics have been studied mostly for the limiting cases of purely parallel or perpendicular propagation, relative to the ambient magnetic field. Contributions to the general case of obliquely-propagating electromagnetic waves have been scarcely reported so far. The absence of a general treatment prevents a complete analysis of the wave-particle interaction in kappa plasmas, since some instabilities can operate simultaneously both in the parallel and oblique directions. In a recent work, Gaelzer and Ziebell [J. Geophys. Res. 119, 9334 (2014)] obtained expressions for the dielectric tensor and dispersion relations for the low-frequency, quasi-perpendicular dispersive Alfvén waves resulting from a kappa VDF. In the present work, the formalism introduced by Ref. 1 is generalized for the general case of electrostatic and/or electromagnetic waves propagating in a kappa plasma in any frequency range and for arbitrary angles. An isotropic distribution is considered, but the methods used here can be easily applied to more general anisotropic distributions, such as the bi-kappa or product-bi-kappa.

physics.plasm-ph↗