Non-thermal Distribution Function Research Articles

Tripolar vortices in inhomogeneous magnetoplasmas in the presence of non-Maxwellian electron distributions

Nonlinear equations governing the characteristics of tripolar vortices (TPVs) are investigated in an inhomogeneous magnetoplasma having inertialess non-Maxwellian electrons that obey the Cairns, kappa, and (r, q)-distributions. Analytical and numerical solutions of the nonlinear equations are presented for various possible cases. In this regard, the dispersion relation for the drift ion-acoustic waves (IAWs) is derived, and the condition describing the shear flow instability is discussed. It is realized that enhancing the impact of non-Maxwellian electrons in the aforementioned three distributions modifies the size and formation of TPVs. It is found that the increase in the electron concentration in the regions of low-phase space density leads to enhancement in the size of TPVs and the perturbation potential as compared to the effect of increasing concentration of electrons in the regions of high phase space density. The riveting interplay of low and high-energy electrons with spiky distribution and the resulting novel effects on the propagation of vortex structures are also discussed in detail. The present study is useful to understand the (non)linear propagation characteristics of the drift IAWs in space plasmas with special reference to the F-region of the ionosphere and also in laboratory experiments where the nonthermal distribution functions are usually found.

Role of optical rectification in photon-assisted tunneling current

Emission of light and conversely rectification of an optical signal using an all-metallic electronic device is of fundamental and technological importance for nano-optics. However despite recent experimental efforts in the development of electrically-driven plasmonic sources, the interplay between quantum transport and optics is still under debate. Here, we measure the photon-assisted current in a planar tunnel junction under infrared illumination. To address the microscopic mechanism at the origin of the optical rectification, we compare the photon-assisted current and the current-voltage characteristic of the junction measured on a voltage range much greater than {V}_{0}=frac{hc}{elambda }=0.825,{{{{{{{rm{V}}}}}}}}, previously unexplored. The experimental results do not agree with the theory based on the existence of a non-thermal distribution function corresponding to the exchange of energy quanta between electrons and photons. We show instead that the illumination power mainly goes into heating and that the rectification results from the tunneling Seebeck effect.

Modulated positron-acoustic waves and rogue waves in a magnetized plasma system with nonthermal electrons and positrons

A theoretical and numerical study on amplitude modulated positron-acoustic waves (PAWs) in a magnetized four-component space plasma (containing immobile positive ions, inertial cold positrons, and inertia-less hot electrons and positrons following Cairn’s non-thermal distribution function) has been carried out. The reductive perturbation method have been applied to derive the corresponding nonlinear Schrodinger (NLS) equation, whose the nonlinear and dispersion coefficients $Q$ and $P$ are function of the external magnetic field. The criteria for the occurrence of modulational instability (MI) of PAWs is addressed. It is shown that the plasma parameters contribute to enhance substantially the growth rate and the bandwidth of the MI. It is also found from the analysis of the NLS equation that the plasma system under assumption supports the existence of Peregrine solitons and super-rogue waves, whose amplitude are significantly modified by the effects of the external magnetic field, the density ratio of hot positron and cold positron, the density ratio of electron and cold positron, and the non-thermal parameter. Moreover, the various types of localized positron-acoustic excitations exist in the form of bright envelope soliton and dark envelope soliton. It is found that the localized structures’s properties (width and amplitude) are influenced by the presence of magnetic field. The relevance of present study can help researchers to explain the various localized structures and the basic features of PAWs in a magnetized plasmas environments.

Solar coronal heating by Alfvén waves in bi-kappa distributed plasma

ABSTRACT In solar physics, there is a decades-old conundrum that is still unsolved. Why is the temperature of the corona so much larger than that of the surface of the Sun? To solve this, various approaches have been adopted so far, but they have certain limitations. In the present analysis, we invoke the standard Vlasov model and the steady-state Poynting theorem to unlock the mysterious coronal heating mechanism in terms of inertial and kinetic Alfvén waves whose electromagnetic energies turn into heat during wave–particle interaction. The coronal plasmas that support these waves are modelled by a non-thermal bi-kappa velocity distribution function. The non-thermal distribution function, which is assumed to pre-exist in the system, strongly influences the wave-heating process. Particularly, during heating by the waves in the inertial limit, the non-thermal features of the distribution function give rise to a unique competition (which is entirely absent in the usual Maxwellian plasmas) between waves of different perpendicular wavenumbers (kx). For small kx, when either the non-thermal parameter κ or the electron parallel temperature T||e increases, the inertial Alfvén waves can efficiently heat the plasma in their immediate vicinity. However, for relatively large kx, an increase in either κ or T||e enables the inertial Alfvén waves to effectively heat the plasma in remote regions in the corona. Although such competition is not seen in the kinetic limit, the non-thermal features still seem to control the heating process. The possible explanations behind the above-mentioned cases are provided by the bi-kappa velocity distribution function, which holds vital clues as to how the non-thermal features, together with kx, dictate the resonance conditions that play a crucial role in the heating process.

Semi-analytical inspection of the quasi-linear absorption of lower hybrid wave in presence of -particles in tokamak reactor

In a reactor plasma like demonstration power station (DEMO), when using the radio frequency (RF) for heating or current drive in the lower hybrid (LH) frequency range (Frankeet al.,Fusion Engng Des., vol. 96–97, 2015, p. 46; Cardinaliet al.,Plasma Phys. Control. Fusion, vol. 59, 2017, 074002), a large fraction of the ion population (the continuously born$\unicode[STIX]{x1D6FC}$-particle, and/or the neutral beam injection (NBI) injected ions) is characterized by a non-thermal distribution function. The interaction (propagation and absorption) of the LH wave must be reformulated by considering the quasi-linear approach for each species separately. The collisional slowing down of such an ion population in a background of an electron and ion plasma is balanced by a quasi-linear diffusion in velocity space due to the propagating electromagnetic wave. In this paper, both propagations are considered by including the ion distribution function, solution of the Fokker–Planck equation, which describes the collisional dynamics of the$\unicode[STIX]{x1D6FC}$-particles including the effects of frictional slowing down, energy diffusion and pitch-angle scattering. Analytical solutions of the Fokker–Planck equation for the distribution function of$\unicode[STIX]{x1D6FC}$-particles with a background of ions and electrons at steady state are included in the calculation of the dielectric tensor. In the LH frequency domain, ray tracing (including quasi-linear damping), can be analytically solved by iterating with the Fokker–Planck solution, and the interaction of the LH wave with$\unicode[STIX]{x1D6FC}$-particles, thermal ions and electrons can be accounted self-consistently and the current drive efficiency can be evaluated in this more general scenario.

Cosmological implications of light sterile neutrinos produced after the QCD phase transition

We study the production of sterile neutrinos in the early Universe from $\ensuremath{\pi}\ensuremath{\rightarrow}l{\ensuremath{\nu}}_{s}$ shortly after the QCD phase transition in the absence of a lepton asymmetry, while including finite-temperature corrections to the $\ensuremath{\pi}$ mass and decay constant ${f}_{\ensuremath{\pi}}$. Sterile neutrinos with masses $\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{MeV}$ produced via this mechanism freeze out at ${T}_{f}\ensuremath{\simeq}10\text{ }\text{ }\mathrm{MeV}$ with a distribution function that is highly nonthermal and that features a sharp enhancement at low momentum, thereby making this species cold even for very light masses. Dark matter abundance constraints from the cosmic microwave background and phase space density constraints from the most dark-matter-dominated dwarf spheroidal galaxies provide upper and lower bounds, respectively, on combinations of mass and mixing angles. For $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}{\ensuremath{\nu}}_{s}$, the bounds lead to a narrow region of compatibility with the latest results from the 3.55-keV line. The nonthermal distribution function leads to free-streaming lengths (today) in the range of a few kpc, consistent with the observation of cores in dwarf galaxies. For sterile neutrinos with mass $\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{eV}$ that are produced by this reaction, the most recent accelerator and astrophysical bounds on ${U}_{ls}$ combined with the nonthermal distribution function suggests a substantial contribution from these sterile neutrinos to ${N}_{\text{eff}}$.

Interstellar Probe: Impact of the Voyager and IBEX results on science and strategy

The ongoing Voyager Interstellar Mission (VIM) and recent observations from the Interstellar Boundary Explorer (IBEX) and Cassini missions are providing significant new information about the interaction of the heliosphere with the very local interstellar medium (VLISM). With new observations have come significant new puzzles for describing the interaction physics. Direct measurements of the shocked, solar-wind flow speed are now possible (from Voyager 2) and show the flow remains supersonic. This is one more piece of evidence supporting the idea that the bulk of the energy density in the plasma resides in a non-thermal component that extends to very high energies. There are both quantitative and qualitative implications for the overall heliospheric structure. Observations of energetic neutral atoms (ENAs) by IBEX (in Earth orbit) from the interaction region(s) of the solar wind and the VLISM show unexpected structure on a variety of scales. In addition to the general “glow” of the sky in ENAs, IBEX data show a relatively narrow “ribbon” of atomic hydrogen emission from ∼200 to ∼6 keV, roughly circular, but asymmetric in intensity, and centered on an ecliptic longitude ∼221 degrees and ecliptic latitude of 39 degrees. The ribbon may be ordered by the interstellar magnetic field. It passes through, rather than being centered on, the “nose” from which the local, neutral interstellar wind enters the Heliosphere, indicating that the flow is not the primary driver of the system as had been thought previously. The neutrals from both the glow and ribbon are also characterized by non-thermal distribution functions. ENAs are observed at higher energies as well by the Ion and Neutral Camera (INCA) on Cassini (in orbit about Saturn). A “belt” of emission, broader than the ribbon but similar to it, is seen up to ∼50 keV. These observations emphasize the need for in situ measurements to understand the global nature of our local galactic environment, which is much more complex than previously thought. Only an interstellar probe with modern instruments and measurement requirements better defined by these recent observations can provide the new information required. Even more importantly, the broader scale of the interaction as revealed in these measurements suggests much greater flexibility in scientifically allowable, asymptotic trajectories from the heliosphere for the probe. This is a significant relaxation in the trajectory requirements that open up the trade space for Jupiter gravity assists to increase the flyout speeds.

On the generation of proton beams in fast solar wind in the presence of obliquely propagating Alfvén waves

Over the past few decades, satellite investigations have revealed that the solar wind contains a plethora of distribution function signatures (proton beam, anisotropic cores) requiring models departing from the conventional fluid theory and necessitating the inclusion of wave‐particle interactions. Numerous theoretical models and explanations have been proposed, but several questions concerning time and heliocentric evolution as well as the formation of beam components and anisotropic cores remain unanswered. We propose a simple solution for the generation of beam components, as a result of physical trapping due to obliquely propagating Alfvén modes, based on a previously reported dynamical system describing the wave interaction with a single ion in the absence of dissipation mechanisms. We have found that beams with significant densities (7%–8%) and with velocities of the order of the Alfvén speed can be generated. The beams' properties, including density, velocity, and temperature, are dependent on the wave propagation angle. The results are in qualitative agreement with the observations and could provide an explanation for the evolution of nonthermal distribution functions observed in fast solar wind and other space plasmas regimes.

Sterile neutrinos produced near the electroweak scale: Mixing angles, MSW resonances, and production rates

We study the production of sterile neutrinos in the region $T\ensuremath{\sim}{M}_{W}$ in an extension beyond the standard model with the seesaw mass matrix originating in Yukawa couplings to Higgs-like scalars with masses and vev's of the order of the electroweak scale. Sterile neutrinos are produced by the decay of scalars and standard model vector bosons. We obtain the index of refraction, dispersion relations, mixing angles in the medium and production rates including those for right-handed sterile neutrinos, from the standard model and beyond the standard model self-energies. For $1\ensuremath{\lesssim}{M}_{W}/T\ensuremath{\lesssim}3$ we find narrow MSW resonances with $k\ensuremath{\lesssim}T$ for both left- and right-handed neutrinos even in absence of a lepton asymmetry in the (active) neutrino sector, as well as very low energy ($k/T\ensuremath{\ll}|\ensuremath{\xi}|$) narrow MSW resonances in the presence of a lepton asymmetry consistent with the bounds from Wilkinson Microwave Anisotropy Probe and Big Bang Nucleosynthesis. For small vacuum mixing angle, consistent with observational bounds, the absorptive part of the self-energies lead to a strong damping regime very near the resonances resulting in the exact degeneracy of the propagating modes with a concomitant breakdown of adiabaticity. We argue that cosmological expansion sweeps through the resonances, resonant and nonresonant sterile neutrino production results in a highly nonthermal distribution function enhanced at small momentum $k<T$, with potentially important consequences for their free-streaming length and transfer function at small scales.

Hot-Electron Instability in Superconductors

High flux velocities in a superconductor can distort the quasiparticle distribution function and elevate the electronic temperature. Close to T c , a non-thermal distribution function shrinks the vortex core producing the well-known Larkin-Ovchinnikov flux instability. In the present work we consider the opposite limit of low temperatures, where electron-electron scattering is more rapid than electron-phonon, resulting in an electronic temperature rise with a thermal-like distribution function. This produces a different kind of flux instability, due to a reduction in condensate and expansion of the vortex core. Measurements in YBCO films confirm the distinct predictions ofthis mechanism.

Nonequilibrium stationary states in the earth's magnetotail: Stochastic acceleration processes and nonthermal distribution functions

A self-consistent kinetic description of turbulent plasma and fields at the nonequilibrium stationary states (NESS's) of the Earth's magnetotail is proposed. We argue that the inherent dynamics of the NESS's is manifested in the low-frequency power law fluctuation spectrum ∼ ƒ −1 (i.e., the flicker noise). At the higher frequebcies, the fluctuation spectrum has a steeper shape ∼ ƒ −7 3 and is related to the structural characteristics of the magnetotail turbulence. The basic dynamical processes operating at the NESS are described by a nonlinear fractional kinetic equation which includes the following principal effects: (1) stochastic particle acceleration in the turbulent magnetic field varying with time, and (2) self-interaction of the medium associated with the reproduction of the turbulent field by the energetic particles accelerated therein. We find that the particle energy distribution in presence of the self-interaction events reveals a high-energy nonthermal tail ƒ(ε) ∝ ε −η . The slope of the distribution, η, obeys the condition 6 ≤ η < 7; this reflects the fundamental dynamical properties underlying magnetic field and plasma coupling in self-organized electromagnetic systems. The results obtained are in close agreement with spacecraft observational data on the Earth's magnetotail.