arXiv subjects

Horst Fichtner

Publications and source records attributed to Horst Fichtner.

At least 37 records · Page 2Linked to original sources

arXiv2022

On the properties of 0.11 keV to 344 MeV ion spectra in the inner heliosheath using regularized $κ$-distributions

The shape of the ion energy spectra plays a critical role toward determining the ion energetics, the acceleration mechanisms and the possible sources of different plasma and suprathermal ion populations. The determination of the exact shape of the total particle spectrum, provide the necessary means to address the inner heliosheath (IHS) dynamics. Apart from various modeling efforts, a direct fit to the measured ion spectra for an extended energy range of $\sim$0.11 to 344 MeV has not been performed to date. We use an extended set of combined 0.11-55 keV ENA measurements from the Interstellar Boundary Explorer (IBEX-Lo and IBEX-Hi) and Cassini/Ion and Neutral Camera (INCA), converted to protons, together with $\sim$28 keV to 344 MeV ion measurements from the Low Energy Charged Particle (LECP) and Cosmic Ray Subsystem (CRS) experiments on Voyager 2, over the declining phase of Solar Cyle 23 (SC23) and ascending phase Solar Cylce 24 (SC24) (2009-2016) to study the characteristics of the particle energy spectrum. We fit the 0.11 keV to 344 MeV composite spectra with a set of regularized isotropic $κ$-distribution functions (RKD) allowing the determination of the macroscopic physical properties. We demonstrate that the 2009-2012 spectrum that corresponds to the declining phase of SC23 is well fitted by three different RKDs, while the 2013-2016 spectrum, associated with the rise of SC24, can only be approximated with six different RKDs. Our results are generally consistent with shock accelerated particles that undergo additional acceleration inside the IHS. We identify a low energy transmitted population of particles, a suprathermal reflected population and a very high energy component that is modulated by GCRs. The 2013-2016 time period is most likely associated with a mixture of particles from SC23 and SC24, which is reflected by the need to employ six RDKs.

astro-ph.SR↗

arXiv2022

Revisiting the Ulysses electron data with a triple fit of velocity distributions

Given their uniqueness, the Ulysses data can still provide us with valuable new clues about the properties of plasma populations in the solar wind, and, especially, about their variations with heliographic coordinates. We revisit the electron data reported by by the SWOOPS instrument on-board of the Ulysses spacecraft between 1990 to early 2008. These observations reveal velocity distributions out of thermal equilibrium, with anisotropies (e.g., parallel drifts or/and different temperatures, parallel and perpendicular to the background magnetic field), and quasi-thermal and suprathermal populations with different properties. We apply a 2D nonlinear least square fitting procedure, using the Levenberg-Marquardt algorithm, to simultaneously fit the velocity electron data (up to a few keV) with a triple model combining three distinct populations: the more central quasi-thermal core and suprathermal halo, and a second suprathermal population consisting mainly of the electron strahl (or beaming population with a major field-aligned drift). The recently introduced $κ$-cookbook is used to describe each component with the following anisotropic distribution functions (recipes): Maxwellian, regularized kappa-, and generalized kappa-distributions. The temperature anisotropy quantified by the best fits is considered as a case study of the main parameters characterizing electron populations. By comparison to the core, both suprathermal populations exhibit higher temperature anisotropies, which slightly increase with the energy of electrons.

astro-ph.SR↗

arXiv2022

Towards a realistic evaluation of transport coefficients in non-equilibrium space plasmas

Recent studies have outlined the interest for the evaluation of transport coefficients in space plasmas, where the observed velocity distributions of plasma particles are conditioned not only by the binary collisions, e.g., at low energies, but also by the energisation of particles from their interaction with wave turbulence and fluctuations, generating the suprathermal Kappa-distributed populations. This paper provides a first estimate of the main transport coefficients based on regularised Kappa distributions (RKDs), which, unlike standard Kappa distributions (SKDs), enable macroscopic parameterisation without mathematical divergences or physical inconsistencies. All transport coefficients derived here, i.e., the diffusion and mobility coefficients, electric conductivity, thermoelectric coefficient and thermal conductivity, are finite and well defined for all values of $κ> 0$. Moreover, for low values of $κ$ (i.e., below the SKD poles), the transport coefficients can be orders of magnitudes higher than the corresponding Maxwellian limits, meaning that significant underestimations can be made if suprathermal electrons are ignored.

physics.plasm-ph↗

arXiv2021

Transport coefficients enhanced by suprathermal particles in nonequilibrium heliospheric plasmas

In heliospheric plasmas the transport of energy and particles is governed by various fluxes (e.g., heat flux) triggered by different forces, electromagnetic fields, and gradients in density or temperature. In the outer corona and at relatively low heliocentric distances in the solar wind (i.e., < 1 AU), particle-particle collisions play an important role in the transport of energy, momentum, and matter, described within classical transport theory by the transport coefficients, which relate the fluxes to their sources. The present paper aims to improve the evaluation of the main transport coefficients in such nonequilibrium plasmas, on the basis of an implicit realistic characterization of their particle velocity distributions, in accord with the in situ observations. Of particular interest is the presence of suprathermal populations and their influence on these transport coefficients. Using the Boltzmann transport equation and macroscopic laws for the energy and particle fluxes, we derived electric conductivity, thermoelectric coefficient, thermal conductivity, diffusion, and mobility coefficient. These are conditioned by the electrons, which are empirically well described by the Kappa distribution, with a nearly Maxwellian core and power-law tails enhanced by the suprathermal population. Here we have adopted the original Kappa approach that has the ability to outline and quantify the contribution of suprathermal populations. Without exception, the transport coefficients are found to be systematically and markedly enhanced in the presence of suprathermal electrons, due to the additional kinetic energy with which these populations contribute to the dynamics of space plasma systems. The present results also show how important an adequate Kappa modeling of suprathermal populations is, which is in contrast to other modified interpretations that underestimate the effects of these populations.

astro-ph.SR↗

arXiv2021

Simulating observable structures due to a perturbed interstellar medium in front of astrospheric bow shocks in 3D MHD

Context. While the shapes of many observed bow shocks can be reproduced by simple astrosphere models, more elaborate approaches have recently been used to explain differing observable structures. Aims. By placing perturbations of an otherwise homogeneous interstellar medium in front of the astrospheric bow shock of the runaway blue supergiant $λ$ Cephei, the observable structure of the model astrosphere is significantly altered, providing insight into the origin of perturbed bow shock images. Methods. Three-dimensional single-fluid magnetohydrodynamic (MHD) models of stationary astrospheres were subjected to various types of perturbations and simulated until stationarity was reached again. As examples, simple perturbations of the available MHD parameters (number density, bulk velocity, temperature, and magnetic field) as well as a more complex perturbation were chosen. Synthetic observations were generated by line-of-sight integration of the model data, producing H$α$, $70\,μ$m dust emission, and bremsstrahlung maps of the perturbed astrosphere's evolution. Results. The resulting shock structures and observational images differ strongly depending on the type of the injected perturbation and the viewing angles, forming arc-like protrusions or bifurcations of the bow shock structure, as well as rings, arcs, and irregular structures detached from the bow shock.

astro-ph.GA↗

arXiv2020

Generalized anisotropic $κ$-cookbook: 2D fitting of Ulysses electron data

Observations in space plasmas reveal particle velocity distributions out of thermal equilibrium, with anisotropies (e.g., parallel drifts or/and different temperatures, $T_\parallel$ - parallel and $T_\perp$ - perpendicular, with respect to the background magnetic field), and multiple quasithermal and suprathermal populations with different properties. The recently introduced (isotropic) $κ$-cookbook is generalized in the present paper to cover all these cases of anisotropic and multi-component distributions reported by the observations. We derive general analytical expressions for the velocity moments and show that the common (bi-)Maxwellian and (bi-)$κ-$distributions are obtained as limiting cases of the generalized anisotropic $κ$-cookbook (or recipes). Based on this generalization, a new 2D fitting procedure is introduced, with an improved level of confidence compared to the 1D fitting methods widely used to quantify the main properties of the observed distributions. The nonlinear least-squares fit is \led{applied to electron data sets} measured by the Ulysses spacecraft confirming the existence of three different populations, a quasithermal core and two suprathermal (halo and strahl) components. In general, the best overall fit is given by the sum of a Maxwellian and two generalized $κ$-distributions.

astro-ph.SR↗

arXiv2020

Cosmic-ray propagation around the Sun: investigating the influence of the solar magnetic field on the cosmic-ray Sun shadow

The cosmic-ray Sun shadow, which is caused by high-energy charged cosmic rays being blocked and deflected by the Sun and its magnetic field, has been observed by various experiments, such as Argo-YBJ, HAWC, Tibet, and IceCube. Most notably, the shadow's size and depth was recently shown to correlate with the 11-year solar cycle. The interpretation of such measurements, which help to bridge the gap between solar physics and high-energy particle astrophysics, requires a solid theoretical understanding of cosmic-ray propagation in the coronal magnetic field. It is the aim of this paper to establish theoretical predictions for the cosmic-ray Sun shadow in order to identify observables that can be used to study this link in more detail. To determine the cosmic-ray Sun shadow, we numerically compute trajectories of charged cosmic rays in the energy range of 5-316 TeV for five different mass numbers. We present and analyze the resulting shadow images for protons and iron, as well as for typically measured cosmic-ray compositions. We confirm the observationally established correlation between the magnitude of the shadowing effect and both the mean sunspot number and the polarity of the magnetic field during the solar cycle. We also show that during low solar activity, the Sun's shadow behaves similarly to that of a dipole, for which we find a non-monotonous dependence on energy. In particular, the shadow can become significantly more pronounced than the geometrical disk expected for a totally unmagnetized Sun. For times of high solar activity, we instead predict the shadow to depend monotonously on energy, and to be generally weaker than the geometrical shadow for all tested energies. These effects should become visible in energy-resolved measurements of the Sun shadow, and may in the future become an independent measure for the level of disorder in the solar magnetic field.

astro-ph.HE↗

arXiv2020

On the Generation of Compressible Mirror-mode Fluctuations in the Inner Heliosheath

Measurements made with the Voyager 1 spacecraft indicate that significant levels of compressive fluctuations exist in the inner heliosheath. Some studies have already been performed with respect to the mirror-mode instability in the downstream region close to the solar wind termination shock, and here we extend the investigation to the whole inner heliosheath. We employ quasilinear theory and results from a global magnetohydrodynamic model of the heliosphere to compute the time evolution of both the temperature anisotropy and the energy density of the corresponding magnetic fluctuations, and we demonstrate their likely presence in the inner heliosheath. Furthermore, we compute the associated, locally generated density fluctuations. The results can serve as inputs for future models of the transport of compressible turbulence in the inner heliosheath.

physics.space-ph↗

arXiv2020

The $κ$-cookbook: a novel generalizing approach to unify $κ$-like distributions for plasma particle modeling

In the literature different so-called $κ$-distribution functions are discussed to fit and model the velocity (or energy) distributions of solar wind species, pickup ions or magnetospheric particles. Here we introduce a generalized (isotropic) $κ$-distribution as a "cookbook", which admits as special cases, or "recipes", all the other known versions of $κ$-models. A detailed analysis of the generalized distribution function is performed, providing general analytical expressions for the velocity moments, Debye length, and entropy, and pointing out a series of general requirements that plasma distribution functions should satisfy. From a contrasting analysis of the recipes found in the literature, we show that all of them lead to almost the same macroscopic parameters with a small standard deviation between them. However, one of these recipes called the regularized $κ$-distribution provides a functional alternative for macroscopic parameterization without any constraint for the power-law exponent $κ$.

astro-ph.SR↗

arXiv2020

On the Diversity of M-Star Astrospheres and the Role of Galactic Cosmic Rays Within

With upcoming missions such as the James Webb Space Telescope (JWST), the European Extremely Large Telescope (ELT), and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), we soon will be on the verge of detecting and characterizing Earth-like exoplanetary atmospheres for the first time. These planets are most likely to be found around smaller and cooler K- and M-type stars. However, recent observations showed that their radiation environment might be much harsher than that of the Sun. Thus, the exoplanets are most likely exposed to an enhanced stellar radiation environment, which could affect their habitability, for example, in the form of a hazardous flux of energetic particles. Knowing the stellar radiation field, and being able to model the radiation exposure on the surface of a planet is crucial to assess its habitability. In this study, we present 3D magnetohydrodynamic (MHD)-based model efforts investigating M-stars, focusing on V374 Peg, Proxima Centauri, and LHS 1140, chosen because of their diverse astrospheric quantities. We show that V374 Peg has a much larger astrosphere (ASP) than our Sun, while Proxima Centauri and LHS 1140 most likely have ASPs comparable or even much smaller than the heliosphere, respectively. Based on a 1D transport model, for the first time, we provide numerical estimates of the modulation of Galactic cosmic rays (GCRs) within the three ASPs. We show that the impact of GCRs on the Earth-like exoplanets Proxima Centauri b and LHS 1140 b cannot be neglected in the context of exoplanetary habitability.

astro-ph.SR↗

arXiv2020

Linear Dispersion Theory of Parallel Electromagnetic Modes for Regularized Kappa-Distributions

The velocity particle distributions measured in-situ in space plasmas deviate from Maxwellian (thermal) equilibrium, showing enhanced suprathermal tails which are well described by the standard Kappa-distribution (SKD). Despite its successful application, the SKD is frequently disputed due to a series of unphysical implications like diverging velocity moments, preventing a macroscopic description of the plasma. The regularized Kappa-distribution (RKD) has been introduced to overcome these limitations, but the dispersion properties of RKD-plasmas are not explored yet. In the present paper we compute the wavenumber dispersion of the frequency and damping or growth rates for the electromagnetic modes in plasmas characterized by the RKD. This task is accomplished by using the grid-based kinetic dispersion solver LEOPARD developed for arbitrary gyrotropic distributions [P. Astfalk and F. Jenko, J. Geophys. Res. 122, 89 (2017)]. By reproducing previous results obtained for the SKD and Maxwellian, we validate the functionality of the code. Furthermore, we apply the isotropic as well as the anisotropic RKDs to investigate stable electromagnetic electron-cyclotron (EMEC) and ion-cyclotron (EMIC) modes as well as temperature-anisotropy-driven instabilities, both for the case $T_\perp / T_\parallel > 1$ (EMEC and EMIC instabilities) and for the case $T_\perp / T_\parallel < 1$ (proton and electron firehose instabilities), where $\parallel$ and $\perp$ denote directions parallel and perpendicular to the local time-averaged magnetic field. Provided that the cutoff parameter $α$ is small enough, the results show that the RKDs reproduce the dispersion curves of the SKD plasmas at both qualitative and quantitative levels. For higher values, however, physically significant deviation occurs.

physics.plasm-ph↗

arXiv2020

A new class of discontinuous solar wind solutions

A new class of one-dimensional solar wind models is developed within the general polytropic, single-fluid hydrodynamic framework. The particular case of quasi-adiabatic radial expansion with a localized heating source is considered. We consider analytical solutions with continuous Mach number over the entire radial domain while allowing for jumps in the flow velocity, density, and temperature, provided that there exists an external source of energy in the vicinity of the critical point which supports such jumps in physical quantities. This is substantially distinct from both the standard Parker solar wind model and the original nozzle solutions, where such discontinuous solutions are not permissible. We obtain novel sample analytic solutions of the governing equations corresponding to both slow and fast wind.

astro-ph.SR↗

arXiv2019

An empirical model of Energetic Neutral Atom imaging of the heliosphere and its implications for future heliospheric missions at great heliocentric distances

Several concepts for heliospheric missions operating at heliocentric distances far beyond Earth orbit are currently investigated by the scientific community. The mission concept of the Interstellar Probe (McNutt et al. 2018), e.g., aims at reaching a distance of 1000 au away from the Sun within this century. This would allow the coming generation to obtain a global view of our heliosphere from an outside vantage point by measuring the Energetic Neutral Atoms (ENAs) originating from the various plasma regions. It would also allow for direct sampling of unperturbed interstellar medium, and for many observation opportunities beyond heliospheric science, such as visits to Kuiper Belt Objects, a comprehensive view on the interplanetary dust populations, and infrared astronomy free from the foreground emission of the Zodiacal cloud. In this study, we present a simple empirical model of ENAs from the heliosphere and derive basic requirements for ENA instrumentation onboard a spacecraft at great heliocentric distances. We consider the full energy range of heliospheric ENAs from 10 eV to 100 keV because each part of the energy spectrum has its own merits for heliospheric science. To cover the full ENA energy range, two or three different ENA instruments are needed. Thanks to parallax observations, some insights about the nature of the IBEX Ribbon and the dimensions of the heliosphere can already be gained by ENA imaging from a few au heliocentric distance. To directly reveal the global shape of the heliosphere, measurements from outside the heliosphere are, of course, the best option.

astro-ph.SR↗

arXiv2019

Moments of the anisotropic regularized $κ$-distributions

For collisionless (or collision-poor) plasma populations which are well described by the $κ$-distribution functions (also known as the Kappa or Lorentzian power-laws) a macroscopic interpretation has remained largely questionable, especially because of the diverging moments of these distributions. Recently significant progress has been made by introducing a generic regularization for the isotropic $κ$-distribution, which resolves this critical limitation. Regularization is here applied to the anisotropic forms of $κ$-distributions, commonly used to describe temperature anisotropies, and skewed or drifting distributions of beam-plasma systems. These regularized distributions admit non-diverging moments which are provided for all positive $κ$, opening promising perspectives for a macroscopic (fluid-like) characterization of non-ideal plasmas.

physics.comp-ph↗

arXiv2019

On the applicability of $κ$-distributions

The standard (non-relativistic) $κ$-distribution is widely used to fit data and to describe macroscopic thermodynamical behavior, e.g.\ the pressure (temperature) as the second moment of the distribution function. By contrast to a Maxwellian distribution, for small relevant values $κ< 2$ there exists a significant, but unphysical contribution to the pressure from unrealistic, superluminal particles with speeds exceeding the speed of light. Similar concerns exist for the entropy. We demonstrate here that by using the recently introduced regularized $κ$-distribution one can avoid such unphysical behaviour.

physics.plasm-ph↗

arXiv2018

The interaction of multiple stellar winds in stellar clusters: potential flow

While several studies have investigated large-scale cluster winds resulting from an intra-cluster interaction of multiple stellar winds, as yet they have not provided details of the bordering flows inside a given cluster. The present work explores the principal structure of the combined flow resulting from the interaction of multiple stellar winds inside stellar clusters. The theory of complex potentials is applied to analytically investigate stagnation points, boundaries between individual outflows, and the hydrodynamic structure of the asymptotic large-scale cluster wind. In a second part, these planar considerations are extended to fully three-dimensional, asymmetric configurations of wind-driving stars. We find (i) that one can distinguish regions in the large-scale cluster wind that are determined by the individual stellar winds, (ii) that there are comparatively narrow outflow channels, and (iii) that the large-scale cluster wind asymptotically approaches spherical symmetry at large distances. The combined flow inside a stellar cluster resulting from the interaction of multiple stellar winds is highly structured.

astro-ph.SR↗

arXiv2018

Uncertainties in the heliosheath ion temperatures

The Voyager plasma observations show that the physics of the heliosheath is rather complex, and especially that the temperature derived from observation differs from expectations. To explain this fact the temperature in the heliosheath should be based on $κ$ distributions instead of Maxwellians because the former allows for much higher temperature. Here we show an easy way to calculate the $κ$ temperatures when those estimated from the data are given as Maxwellian temperatures. We use the moments of the Maxwellian and $κ$ distributions to estimate the $κ$ temperature. Moreover, we show that the pressure (temperature) given by a truncated $κ$ distribution is similar to that given by a Maxwellian and only starts to increase for higher truncation velocities. We deduce a simple formula to convert the Maxwellian to $κ$ pressure or temperature. We apply this result to the Voyager-2 observations in the heliosheath.

physics.space-ph↗

arXiv2017

CRPropa 3.1 -- A low energy extension based on stochastic differential equations

The propagation of charged cosmic rays through the Galactic environment influences all aspects of the observation at Earth. Energy spectrum, composition and arrival directions are changed due to deflections in magnetic fields and interactions with the interstellar medium. Today the transport is simulated with different simulation methods either based on the solution of a transport equation (multi-particle picture) or a solution of an equation of motion (single-particle picture). We developed a new module for the publicly available propagation software CRPropa 3.1, where we implemented an algorithm to solve the transport equation using stochastic differential equations. This technique allows us to use a diffusion tensor which is anisotropic with respect to an arbitrary magnetic background field. The source code of CRPropa is written in C++ with python steering via SWIG which makes it easy to use and computationally fast. In this paper, we present the new low-energy propagation code together with validation procedures that are developed to proof the accuracy of the new implementation. Furthermore, we show first examples of the cosmic ray density evolution, which depends strongly on the ratio of the parallel $κ_\parallel$ and perpendicular $κ_\perp$ diffusion coefficients. This dependency is systematically examined as well the influence of the particle rigidity on the diffusion process.

astro-ph.IM↗