Physics

Editorial

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3 pages, 133 KiB

Open AccessEditorial

Personal Reminiscences of Reinhard Schlickeiser

by Ian Lerche

Physics 2021, 3(4), 1051-1053; https://doi.org/10.3390/physics3040066 - 12 Nov 2021

Viewed by 1682

Abstract

In 1979, I arrived back at the University of Chicago from a two-year stint in Australia to find a very large German post-doc eagerly awaiting me, so we could work together on transport of cosmic ray electrons perpendicular to the galactic plane [...] [...] Read more.

In 1979, I arrived back at the University of Chicago from a two-year stint in Australia to find a very large German post-doc eagerly awaiting me, so we could work together on transport of cosmic ray electrons perpendicular to the galactic plane [...] Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

Research

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18 pages, 1060 KiB

Open AccessArticle

Propagation of Cosmic Rays in Plasmoids of AGN Jets-Implications for Multimessenger Predictions

by Julia Becker Tjus, Mario Hörbe, Ilja Jaroschewski, Patrick Reichherzer, Wolfgang Rhode, Marcel Schroller and Fabian Schüssler

Physics 2022, 4(2), 473-490; https://doi.org/10.3390/physics4020032 - 28 Apr 2022

Cited by 7 | Viewed by 1822

Abstract

After the successful detection of cosmic high-energy neutrinos, the field of multiwavelength photon studies of active galactic nuclei (AGN) is entering an exciting new phase. The first hint of a possible neutrino signal from the blazar TXS 0506+056 leads to the anticipation that [...] Read more.

After the successful detection of cosmic high-energy neutrinos, the field of multiwavelength photon studies of active galactic nuclei (AGN) is entering an exciting new phase. The first hint of a possible neutrino signal from the blazar TXS 0506+056 leads to the anticipation that AGN could soon be identified as point sources of high-energy neutrino radiation, representing another messenger signature besides the established photon signature. To understand the complex flaring behavior at multiwavelengths, a genuine theoretical understanding needs to be developed. These observations of the electromagnetic spectrum and neutrinos can only be interpreted fully when the charged, relativistic particles responsible for the different emissions are modeled properly. The description of the propagation of cosmic rays in a magnetized plasma is a complex question that can only be answered when analyzing the transport regimes of cosmic rays in a quantitative way. In this paper, therefore, a quantitative analysis of the propagation regimes of cosmic rays is presented in the approach that is most commonly used to model non-thermal emission signatures from blazars, i.e., the existence of a high-energy cosmic-ray population in a relativistic plasmoid traveling along the jet axis. It is shown that in the considered energy range of high-energy photon and neutrino emission, the transition between diffusive and ballistic propagation takes place, significantly influencing not only the spectral energy distribution, but also the lightcurve of blazar flares. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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15 pages, 1938 KiB

Open AccessArticle

Simulating Three-Wave Interactions and the Resulting Particle Transport Coefficients in a Magnetic Loop

by Seve Nyberg and Rami Vainio

Physics 2022, 4(2), 394-408; https://doi.org/10.3390/physics4020026 - 31 Mar 2022

Viewed by 1795

Abstract

In this paper, the effects of wave–wave interactions of the lowest order, i.e., three-wave interactions, on parallel-propagating Alfvén wave spectra on a closed magnetic field line are considered. The spectra are then used to evaluate the transport parameters of energetic particles in a [...] Read more.

In this paper, the effects of wave–wave interactions of the lowest order, i.e., three-wave interactions, on parallel-propagating Alfvén wave spectra on a closed magnetic field line are considered. The spectra are then used to evaluate the transport parameters of energetic particles in a coronal loop. The wave spectral density is the main variable investigated, and it is modelled using a diffusionless numerical scheme. A model, where high-frequency Alfvén waves are emitted from the two footpoints of the loop and interact with each other as they pass by, is considered. The wave spectrum evolution shows the erosion of wave energy starting from higher frequencies so that the wave mode emitted from the closer footpoint of the loop dominates the wave energy density. Consistent with the cross-helicity state of the waves, the bulk velocity of energetic protons is from the loop footpoints towards the loop apex. Protons can be turbulently trapped in the loop, and Fermi acceleration is possible near the loop apex, as long as the partial pressure of the particles does not exceed that of the resonant waves. The erosion of the Alfvén wave energy density should also lead to the heating of the loop. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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24 pages, 908 KiB

Open AccessArticle

Determining Pitch-Angle Diffusion Coefficients for Electrons in Whistler Turbulence

by Felix Spanier, Cedric Schreiner and Reinhard Schlickeiser

Physics 2022, 4(1), 80-103; https://doi.org/10.3390/physics4010008 - 20 Jan 2022

Viewed by 2358

Abstract

Transport of energetic electrons in the heliosphere is governed by resonant interaction with plasma waves, for electrons with sub-GeV kinetic energies specifically with dispersive modes in the whistler regime. In this paper, particle-in-cell simulations of kinetic turbulence with test-particle electrons are performed. The [...] Read more.

Transport of energetic electrons in the heliosphere is governed by resonant interaction with plasma waves, for electrons with sub-GeV kinetic energies specifically with dispersive modes in the whistler regime. In this paper, particle-in-cell simulations of kinetic turbulence with test-particle electrons are performed. The pitch-angle diffusion coefficients of these test particles are analyzed and compared to an analytical model for left-handed and right-handed polarized wave modes. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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12 pages, 5940 KiB

Open AccessArticle

Dissipative Ion-Acoustic Solitary Waves in Magnetized κ-Distributed Non-Maxwellian Plasmas

by Sharmin Sultana and Ioannis Kourakis

Physics 2022, 4(1), 68-79; https://doi.org/10.3390/physics4010007 - 20 Jan 2022

Cited by 5 | Viewed by 2621

Abstract

The propagation of dissipative electrostatic (ion-acoustic) solitary waves in a magnetized plasma with trapped electrons is considered via the Schamel formalism. The direction of propagation is assumed to be arbitrary, i.e., oblique with respect to the magnetic field, for generality. A non-Maxwellian (nonthermal) [...] Read more.

The propagation of dissipative electrostatic (ion-acoustic) solitary waves in a magnetized plasma with trapped electrons is considered via the Schamel formalism. The direction of propagation is assumed to be arbitrary, i.e., oblique with respect to the magnetic field, for generality. A non-Maxwellian (nonthermal) two-component plasma is considered, consisting of an inertial ion fluid, assumed to be cold for simplicity, and electrons. A (kappa)

κ

-type distribution is adopted for the electron population, in addition to particle trapping taken into account in phase space. A damped version of the Schamel-type equation is derived for the electrostatic potential, and its analytical solution, representing a damped solitary wave, is used to examine the nonlinear features of dissipative ion-acoustic solitary waves in the presence of trapped electrons. The influence of relevant plasma configuration parameters, namely the percentage of trapped electrons, the electron superthermality (spectral) index, and the direction of propagation on the solitary wave characteristics is investigated. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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36 pages, 12985 KiB

Open AccessArticle

A Perspective on the Solar Modulation of Cosmic Anti-Matter

by Marius S. Potgieter, O. P. M. Aslam, Driaan Bisschoff and Donald Ngobeni

Physics 2021, 3(4), 1190-1225; https://doi.org/10.3390/physics3040076 - 7 Dec 2021

Cited by 9 | Viewed by 2899

Abstract

Global modulation studies with comprehensive numerical models contribute meaningfully to the refinement of very local interstellar spectra (VLISs) for cosmic rays. Modulation of positrons and anti-protons are investigated to establish how the ratio of their intensity, and with respect to electrons and protons, [...] Read more.

Global modulation studies with comprehensive numerical models contribute meaningfully to the refinement of very local interstellar spectra (VLISs) for cosmic rays. Modulation of positrons and anti-protons are investigated to establish how the ratio of their intensity, and with respect to electrons and protons, are changing with solar activity. This includes the polarity reversal of the solar magnetic field which creates a 22-year modulation cycle. Modeling illustrates how they are modulated over time and the particle drift they experience which is significant at lower kinetic energy. The VLIS for anti-protons has a peculiar spectral shape in contrast to protons so that the total modulation of anti-protons is awkwardly different to that for protons. We find that the proton-to-anti-proton ratio between 1–2 GeV may change by a factor of 1.5 over a solar cycle and that the intensity for anti-protons may decrease by a factor of ~2 at 100 MeV during this cycle. A composition is presented of VLIS for protons, deuteron, helium isotopes, electrons, and particularly for positrons and anti-protons. Gaining knowledge of their respective 11 and 22 year modulation is useful to interpret observations of low-energy anti-nuclei at the Earth as tests of dark matter annihilation. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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15 pages, 2108 KiB

Open AccessArticle

Proton-Alpha Drift Instability of Electromagnetic Ion-Cyclotron Modes: Quasilinear Development

by Shaaban M. Shaaban, Marian Lazar, Peter H. Yoon, Stefaan Poedts and Rodrigo A. López

Physics 2021, 3(4), 1175-1189; https://doi.org/10.3390/physics3040075 - 1 Dec 2021

Cited by 2 | Viewed by 2221

Abstract

The ability of space plasmas to self-regulate through mechanisms involving self-generated fluctuations is a topic of high interest. This paper presents the results of a new advanced quasilinear (QL) approach for the instability of electromagnetic ion-cyclotron modes driven by the relative alpha-proton drift [...] Read more.

The ability of space plasmas to self-regulate through mechanisms involving self-generated fluctuations is a topic of high interest. This paper presents the results of a new advanced quasilinear (QL) approach for the instability of electromagnetic ion-cyclotron modes driven by the relative alpha-proton drift observed in solar wind. For an extended parametric analysis, the present QL approach includes also the effects of intrinsic anisotropic temperatures of these populations. The enhanced fluctuations contribute to an exchange of energy between proton and alpha particles, leading to important variations of the anisotropies, the proton-alpha drift and the temperature contrast. The results presented here can help understand the observational data, in particular, those revealing the local variations associated with the properties of protons and alpha particles as well as the spatial profiles in the expanding solar wind. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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11 pages, 363 KiB

Open AccessArticle

A Shock-in-Jet Synchrotron Mirror Model for Blazars

by Markus Böttcher

Physics 2021, 3(4), 1112-1122; https://doi.org/10.3390/physics3040070 - 22 Nov 2021

Cited by 1 | Viewed by 2079

Abstract

Reinhard Schlickeiser has made groundbreaking contributions to various aspects of blazar physics, including diffusive shock acceleration, the theory of synchrotron radiation, the production of gamma-rays through Compton scattering in various astrophysical sources, etc. This paper, describing the development of a self-consistent shock-in-jet model [...] Read more.

Reinhard Schlickeiser has made groundbreaking contributions to various aspects of blazar physics, including diffusive shock acceleration, the theory of synchrotron radiation, the production of gamma-rays through Compton scattering in various astrophysical sources, etc. This paper, describing the development of a self-consistent shock-in-jet model for blazars with a synchrotron mirror feature, is therefore an appropriate contribution to a Special Issue in honor of Reinhard Schlickeiser’s 70th birthday. The model is based on our previous development of a self-consistent shock-in-jet model with relativistic thermal and non-thermal particle distributions evaluated via Monte-Carlo simulations of diffusive shock acceleration, and time-dependent radiative transport. This model has been very successful in modeling spectral variability patterns of several blazars, but has difficulties describing orphan flares, i.e., high-energy flares without a significant counterpart in the low-frequency (synchrotron) radiation component. As a solution, this paper investigates the possibility of a synchrotron mirror component within the shock-in-jet model. It is demonstrated that orphan flares result naturally in this scenario. The model’s applicability to a recently observed orphan gamma-ray flare in the blazar 3C279 is discussed and it is found that only orphan flares with mild (≲ a factor of 2–3) enhancements of the Compton dominance can be reproduced in a synchrotron-mirror scenario, if no additional parameter changes are invoked. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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14 pages, 731 KiB

Open AccessArticle

Studying the Influence of External Photon Fields on Blazar Spectra Using a One-Zone Hadro-Leptonic Time-Dependent Model

by Michael Zacharias

Physics 2021, 3(4), 1098-1111; https://doi.org/10.3390/physics3040069 - 18 Nov 2021

Cited by 4 | Viewed by 1834

Abstract

The recent associations of neutrinos with blazars require the efficient interaction of relativistic protons with ambient soft photon fields. However, along side the neutrinos,

γ

-ray photons are produced, which interact with the same soft photon fields producing electron-positron pairs. The strength of [...] Read more.

The recent associations of neutrinos with blazars require the efficient interaction of relativistic protons with ambient soft photon fields. However, along side the neutrinos,

γ

-ray photons are produced, which interact with the same soft photon fields producing electron-positron pairs. The strength of this cascade has significant consequences on the photon spectrum in various energy bands and puts severe constraints on the pion and neutrino production. In this study, we discuss the influence of the external thermal photon fields (accretion disk, broad-line region, and dusty torus) on the proton-photon interactions, employing a newly developed time-dependent one-zone hadro-leptonic code OneHaLe. We present steady-state cases, as well as a time-dependent case, where the emission region moves through the jet. Within the limits of this toy study, the external fields can disrupt the “usual” double-humped blazar spectrum. Similarly, a moving region would cross significant portions of the jet without reaching the previously-found steady states. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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10 pages, 862 KiB

Open AccessArticle

Cylindrical and Spherical Nucleus-Acoustic Solitary and Shock Waves in Degenerate Electron-Nucleus Plasmas

by A A Mamun

Physics 2021, 3(4), 1088-1097; https://doi.org/10.3390/physics3040068 - 16 Nov 2021

Cited by 4 | Viewed by 2331

Abstract

The basic characteristics of cylindrical as well as spherical solitary and shock waves in degenerate electron-nucleus plasmas are theoretically investigated. The electron species is assumed to be cold, ultra-relativistically degenerate, negatively charged gas, whereas the nucleus species is considered a cold, non-degenerate, positively [...] Read more.

The basic characteristics of cylindrical as well as spherical solitary and shock waves in degenerate electron-nucleus plasmas are theoretically investigated. The electron species is assumed to be cold, ultra-relativistically degenerate, negatively charged gas, whereas the nucleus species is considered a cold, non-degenerate, positively charged, viscous fluid. The reductive perturbation technique is utilized in order to reduce the basic equations (governing the degenerate electron-nucleus plasmas under consideration) to the modified Korteweg-de Vries and Burgers equations. The latter are numerically solved and analyzed to detect the basic characteristics of solitary and shock waves in such electron-nucleus plasmas. The nonlinear nucleus-acoustic waves are found to be propagated in the form of solitary as well as shock waves in such degenerate electron-nucleus plasmas. Their basic properties as well as their time evolution are significantly modified by the effects of cylindrical as well as spherical geometries. The results of this study is expected to be applicable not only to astrophysical compact objects, but also to ultra-cold dense plasmas produced in laboratory. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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15 pages, 307 KiB

Open AccessArticle

Landau Damping of Langmuir Waves: An Alternative Derivation

by Andreas Shalchi

Physics 2021, 3(4), 940-954; https://doi.org/10.3390/physics3040059 - 29 Oct 2021

Viewed by 2991

Abstract

In this paper, a discussion of the Landau damping of Langmuir waves is presented together with a simple derivation which does not require the application of methods of complex analysis. A general dispersion relation is derived systematically which corresponds to a nonlinear equation. [...] Read more.

In this paper, a discussion of the Landau damping of Langmuir waves is presented together with a simple derivation which does not require the application of methods of complex analysis. A general dispersion relation is derived systematically which corresponds to a nonlinear equation. The latter equation is solved numerically but asymptotic limits are also discussed. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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Review

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28 pages, 665 KiB

Open AccessReview

A Review on Scene Prediction for Automated Driving

by Anne Stockem Novo, Martin Krüger, Marco Stolpe and Torsten Bertram

Physics 2022, 4(1), 132-159; https://doi.org/10.3390/physics4010011 - 1 Feb 2022

Cited by 4 | Viewed by 4135

Abstract

Towards the aim of mastering level 5, a fully automated vehicle needs to be equipped with sensors for a 360

^{\circ}

surround perception of the environment. In addition to this, it is required to anticipate plausible evolutions of the traffic scene such that [...] Read more.

Towards the aim of mastering level 5, a fully automated vehicle needs to be equipped with sensors for a 360

^{\circ}

surround perception of the environment. In addition to this, it is required to anticipate plausible evolutions of the traffic scene such that it is possible to act in time, not just to react in case of emergencies. This way, a safe and smooth driving experience can be guaranteed. The complex spatio-temporal dependencies and high dynamics are some of the biggest challenges for scene prediction. The subtile indications of other drivers’ intentions, which are often intuitively clear to the human driver, require data-driven models such as deep learning techniques. When dealing with uncertainties and making decisions based on noisy or sparse data, deep learning models also show a very robust performance. In this survey, a detailed overview of scene prediction models is presented with a historical approach. A quantitative comparison of the model results reveals the dominance of deep learning methods in current state-of-the-art research in this area, leading to a competition on the cm scale. Moreover, it also shows the problem of inter-model comparison, as many publications do not use standardized test sets. However, it is questionable if such improvements on the cm scale are actually necessary. More effort should be spent in trying to understand varying model performances, identifying if the difference is in the datasets (many simple situations versus many corner cases) or actually an issue of the model itself. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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Other

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5 pages, 226 KiB

Open AccessBrief Report

Plasma Flows in Solar Filaments as Electromagnetically Driven Vortical Flows

by Yuri E. Litvinenko

Physics 2021, 3(4), 1046-1050; https://doi.org/10.3390/physics3040065 - 10 Nov 2021

Cited by 2 | Viewed by 1808

Abstract

Electromagnetic expulsion acts on a body suspended in a conducting fluid or plasma, which is subject to the influence of electric and magnetic fields. Physically, the effect is a magnetohydrodynamic analogue of the buoyancy (Archimedean) force, which is caused by the nonequal electric [...] Read more.

Electromagnetic expulsion acts on a body suspended in a conducting fluid or plasma, which is subject to the influence of electric and magnetic fields. Physically, the effect is a magnetohydrodynamic analogue of the buoyancy (Archimedean) force, which is caused by the nonequal electric conductivities inside and outside the body. It is suggested that electromagnetic expulsion can drive the observed plasma counter-streaming flows in solar filaments. Exact analytical solutions and scaling arguments for a characteristic plasma flow speed are reviewed, and their applicability in the limit of large magnetic Reynolds numbers, relevant in the solar corona, is discussed. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Reinhard Schlickeiser on the Occasion of His 70th Birthday)

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