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A Themed Issue in Honor of Professor Lennart Stenflo on...
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A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science

A special issue of Physics (ISSN 2624-8174). This special issue belongs to the section "Condensed Matter Physics".

Deadline for manuscript submissions: closed (15 December 2025) | Viewed by 5028

Special Issue Editors

Prof. Gert Brodin

E-Mail Website
Guest Editor
Department of Physics, Umea University, Umea, Sweden
Interests: classical and quantum plasmas; strong field physics; nonlinear phenomena
Prof. Dr. Hans László Pécseli

E-Mail Website
Guest Editor
Department of Physics, University of Oslo, N-0316 Blindern, Norway
Interests: plasma physics; linear and nonlinear waves; instabilities; turbulence

Special Issue Information

Dear Colleagues,

This Special Issue is dedicated to Professor Lennart Stenflo on the occasion of his 85th birthday. Professor Lennart Stenflo’s distinguished career has profoundly influenced plasma physics in several areas, including nonlinear wave interactions, turbulence, space plasma phenomena, and nonlinear physics. His pioneering research has laid the foundation for many theoretical and experimental advancements in the field.

Beyond his scientific contributions, Professor Lennart Stenflo has played a key role in mentoring young researchers, fostering collaborations across disciplines, and inspiring generations of physicists. His work has had a lasting impact, not only through his direct collaborations but also through the many scientists whose research has been influenced by his ideas and insights.

This Special Issue aims to bring together original contributions from Professor Lennart Stenflo’s collaborators, colleagues, and researchers working in areas connected to his work. The collection will showcase the breadth of his scientific legacy and highlight the ongoing developments that trace back to his pioneering efforts. We hope that this Special Issue will serve as both a tribute to his remarkable career and an inspiration for future research in plasma physics.

Prof. Gert Brodin
Prof. Dr. Hans László Pécseli
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Physics is an international peer-reviewed open access quarterly journal published by MDPI.

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Keywords

  • space plasmas
  • nonlinear phenomena
  • plasma turbulence
  • wave–particle interactions
  • parametric instabilities
  • magnetohydrodynamics
  • dusty plasmas
  • solitary waves and shock waves
  • quantum plasmas
  • kinetic theory of plasmas
  • plasma instabilities
  • acoustic gravity waves

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Published Papers (6 papers)

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Research

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16 pages, 721 KB  
Article
On Cavitons Generated by the Nonlinear Plasma Waves in HF Heating Experiments at HAARP
by Spencer Kuo, Min-Chang Lee, Arnold Snyder and Brenton Watkins
Physics 2025, 7(4), 59; https://doi.org/10.3390/physics7040059 - 12 Nov 2025
Viewed by 533
Abstract
Analysis of nonlinear plasma waves, formulated and applied for ionospheric HF heating experiments, indicates that Langmuir/upper hybrid waves excited by parametric instabilities can evolve into traveling solitary waves accompanied by self-induced cavitons. To explore these cavitons, a digisonde operating in fast mode was [...] Read more.
Analysis of nonlinear plasma waves, formulated and applied for ionospheric HF heating experiments, indicates that Langmuir/upper hybrid waves excited by parametric instabilities can evolve into traveling solitary waves accompanied by self-induced cavitons. To explore these cavitons, a digisonde operating in fast mode was utilized. Significant results were observed in ionograms recorded two minutes after the activation of the O-mode heater. These ionograms displayed two distinct bumps in the virtual height spread, located slightly below both the HF reflection height and the upper hybrid resonance height. It is notable that these heights are also slightly below the excitation regions where Langmuir/upper hybrid Parametric Decay Instabilities (PDIs) are typically generated by an O-mode HF heater. These observations correlate with the theory and provide valuable insights into the dynamics of nonlinear plasma waves and their interaction with the ionosphere during HF heating experiments. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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15 pages, 2026 KB  
Article
Nonlinear Dynamics of Cylindrical Waves in Isentropic Plasma
by Alexander R. Karimov and Grigoriy O. Buyanov
Physics 2025, 7(4), 54; https://doi.org/10.3390/physics7040054 - 3 Nov 2025
Viewed by 419
Abstract
Based on the hydrodynamic description, the dynamics of nonlinear cylindrical waves in an isentropic plasma are investigated. The problem is considered in an electrostatic formulation for a two-dimensional plasma medium where ions form a stationary background. Proceeding from the particular, exact solution of [...] Read more.
Based on the hydrodynamic description, the dynamics of nonlinear cylindrical waves in an isentropic plasma are investigated. The problem is considered in an electrostatic formulation for a two-dimensional plasma medium where ions form a stationary background. Proceeding from the particular, exact solution of hydrodynamic equations, we obtain the system of differential equations which describes the electron’s dynamics, taking into account the finite temperature of electrons. Moreover, we find the conditions when this system is reduced to the generalized Ermakov–Pinney equation which was used for analyzing electron dynamics. In the present calculations, a parabolic-in-radius temperature profile was used, associated with an electron density varying only with time. In the framework of the model that worked out, the influence of initial conditions and thermal effects on the regular and singular dynamics of excited waves are discussed. It is shown that the development of singular behavior due to intrinsic nonlinearity is avoided by taking into account thermal effects and the initial rotation of the electron flow. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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9 pages, 753 KB  
Communication
Spatial Structure and Nonlinear Properties of a Surface Charge Located on a Statically Curved Surface of a Semi-Infinite Plasma
by Oleg M. Gradov
Physics 2025, 7(4), 53; https://doi.org/10.3390/physics7040053 - 30 Oct 2025
Viewed by 352
Abstract
The effect of the curvature of the boundary of semi-infinite cold plasma on the parameters and properties of surface oscillations localized near this boundary is considered. An analytical description of various cases of the impact of static deformation of the plasma boundary on [...] Read more.
The effect of the curvature of the boundary of semi-infinite cold plasma on the parameters and properties of surface oscillations localized near this boundary is considered. An analytical description of various cases of the impact of static deformation of the plasma boundary on the characteristics of the oscillating surface charge is obtained, and the results of the exact numerical solution of the initial equations are found to confirm the reliability of the derived analytical formulas. A significant role of the boundary perturbation shape in the formation of the spatial distribution of surface oscillation parameters is revealed. With the help of analytical formulas and precise numerical calculations, a description of this nonlinear interaction is presented. The availability of such a description is crucial both for determining the possibility of using the examined effect for specific applications and, on the other hand, for exciting it in plasma, which requires knowledge of the field structure features. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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13 pages, 767 KB  
Article
Reinterpretation of Fermi Acceleration of Cosmic Rays in Terms of Ballistic Surfing Acceleration in Supernova Shocks
by Krzysztof Stasiewicz
Physics 2025, 7(4), 51; https://doi.org/10.3390/physics7040051 - 16 Oct 2025
Viewed by 1664
Abstract
The applicability of the first-order Fermi mechanism—a cornerstone of the diffusive shock acceleration (DSA) model—in explaining the cosmic ray spectrum is reexamined in light of recent observations from the Magnetospheric Multiscale (MMS) mission at Earth’s bow shock. It is demonstrated that the Fermi [...] Read more.
The applicability of the first-order Fermi mechanism—a cornerstone of the diffusive shock acceleration (DSA) model—in explaining the cosmic ray spectrum is reexamined in light of recent observations from the Magnetospheric Multiscale (MMS) mission at Earth’s bow shock. It is demonstrated that the Fermi and DSA mechanisms lack physical justification and should be replaced by the physically correct ballistic surfing acceleration (BSA) mechanism. The results show that cosmic rays are energized by the convection electric field during ballistic surfing upstream of quasi-perpendicular shocks, independently of internal shock processes. The spectral index of cosmic rays is determined by the magnetic field compression and shock geometry: the acceleration is strongest in perpendicular shocks and vanishes in parallel shocks. The BSA mechanism reproduces the observed spectral indices, with s=−2.7 below the knee at 1016 eV and s=−3 above it. It is suggested that the spectral knee may correspond to particles whose gyroradii are comparable to the characteristic size of shocks in supernova remnants. The acceleration time to reach the knee energy, as predicted by the BSA, is in the order of 500 years. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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8 pages, 260 KB  
Communication
Generation of Zonal Flows in a Rotating Self-Gravitating Fluid
by Volodymyr M. Lashkin and Oleg K. Cheremnykh
Physics 2025, 7(3), 40; https://doi.org/10.3390/physics7030040 - 8 Sep 2025
Viewed by 703
Abstract
We demonstrate the possibility of generation of zonal (shear) flows in a rotating self-gravitating fluid. A set of equations describing the nonlinear interaction between a large-scale zonal flow (ZF) and a small-scale drift-gravity wave is derived. A nonlinear dispersion relation is obtained, from [...] Read more.
We demonstrate the possibility of generation of zonal (shear) flows in a rotating self-gravitating fluid. A set of equations describing the nonlinear interaction between a large-scale zonal flow (ZF) and a small-scale drift-gravity wave is derived. A nonlinear dispersion relation is obtained, from which the possible instability of the ZF follows. The necessary condition for instability in the space of wave numbers of the drift-gravity wave, as well as the instability threshold for the wave amplitude, are obtained. The growth rate of the modulation instability of ZF is found. The generation of ZFs is due to the Reynolds stresses produced by finite amplitude drift-gravity waves. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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Review

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19 pages, 3999 KB  
Review
A Review of Whistler Wave Propagation and Interaction Experiments at Arecibo Observatory, Puerto Rico
by Min-Chang Lee
Physics 2025, 7(4), 62; https://doi.org/10.3390/physics7040062 - 1 Dec 2025
Viewed by 305
Abstract
BU–MIT whistler wave injection experiments, which were conducted at Arecibo Observatory, started with the joint US–USSR Active Space Plasma Program Experiment on 24 December 1989. In this experiment, a satellite-borne VLF transmitter injected radio waves at the frequency and power of 10 kHz [...] Read more.
BU–MIT whistler wave injection experiments, which were conducted at Arecibo Observatory, started with the joint US–USSR Active Space Plasma Program Experiment on 24 December 1989. In this experiment, a satellite-borne VLF transmitter injected radio waves at the frequency and power of 10 kHz and 10 kW. A series of controlled whistler wave experiments with the Arecibo HF heater were subsequently carried out during 1990–1998 until the HF heater was damaged by Hurricane Georges in 1998. In these ionospheric HF heating experiments, 28.5 kHz whistler waves were launched from the nearby naval transmitter (code-named NAU) located at Aguadilla, Puerto Rico. HF heater waves were used to create ionospheric ducts (in the form of parallel-plate waveguides) to facilitate the entry of NAU whistler waves from the neutral atmosphere into the ionosphere. Conjugate whistler wave propagation experiments were conducted between Arecibo, Puerto Rico and Trelew, Argentina in 1997. After 1999, whistler wave experiments in the absence of an HF heater had been conducted. Naturally-occurring large-scale ionospheric irregularities due to spread F or Traveling Ionospheric Disturbances (TIDs) were relied on to guide NAU launched 40.75 kHz whistler waves to propagate from the ionosphere further into the radiation belts, to cause 390 keV charged-particle precipitation. A train of TIDs, resulting from the 9.2 Mw earthquake off the west coast of Sumatra, Indonesia, was observed in our 26 December 2004 Arecibo experiments, about a day after the earthquake-launched tsunami waves traveled across the Indian Ocean, then into remote parts of the Atlantic Ocean. The author’s recent research efforts, motivated by Arecibo experiments, focus on Solar Powered Microwave Transmitting Systems, to simulate Solar Energy Harvesting via Solar Power Satellite (SPS) (also known as Space Based Solar Power (SBSP)) These experiments involved a large number of the author’s BU and MIT students working on theses and participating in the Undergraduate Research Opportunities Program (UROP), in collaboration with other colleagues at several universities and national laboratories. Full article
(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)
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