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The equatorial plasma bubble (EPB) phenomenon is an important component of space weather as the ionospheric irregularities that develop within EPBs can have major detrimental effects on the operation of satellite-based communication and navigation systems. Although the name suggests that EPBs occur in the equatorial ionosphere, the nature of the plasma instability that gives rise to EPBs is such that the bubbles may extend over a large part of the global ionosphere between geomagnetic latitudes of approximately ±15°. The scientific challenge continues to be to understand the day-to-day variability in the occurrence and characteristics of EPBs, such as their latitudinal extent and the development of irregularities within EPBs. In this paper, basic theoretical aspects of the plasma processes involved in the generation of EPBs, associated ionospheric irregularities, and observations of their characteristics using different techniques will be reviewed. Special focus will be given to observations of scintillations produced by the scattering of VHF and higher frequency radio waves while they propagate through ionospheric irregularities associated with EPBs, as these observations have revealed new information about the non-linear development of Rayleigh–Taylor instability in equatorial ionospheric plasma, which is the genesis of EPBs.

In this work, the development of two-dimensional current sheets with respect to tearing modes, in collisionless plasmas with a strong guide field, is analysed. During their nonlinear evolution, these thin current sheets can become unstable to the formation of plasmoids, which allows the magnetic reconnection process to reach high reconnection rates. We carry out a detailed study of the effect of a finite $\\beta _e$, which also implies finite electron Larmor radius effects, on the collisionless plasmoid instability. This study is conducted through a comparison of gyrofluid and gyrokinetic simulations. The comparison shows in general a good capability of the gyrofluid models in predicting the plasmoid instability observed with gyrokinetic simulations. We show that the effects of $\\beta _e$ promotes the plasmoid growth. The effect of the closure applied during the derivation of the gyrofluid model is also studied through the comparison among the variations of the different contributions to the total energy.

We examine with a particle-in-cell (PIC) simulation the collision of two equally dense clouds of cold pair plasma. The clouds interpenetrate until instabilities set in, which heat up the plasma and trigger the formation of a pair of shocks. The fastest-growing waves at the collision speed $c/5$ , where $c$ is the speed of light in vacuum, and low temperature are the electrostatic two-stream mode and the quasi-electrostatic oblique mode. Both waves grow and saturate via the formation of phase space vortices. The strong electric fields of these nonlinear plasma structures provide an efficient means of heating up and compressing the inflowing upstream leptons. The interaction of the hot leptons, which leak back into the upstream region, with the inflowing cool upstream leptons continuously drives electrostatic waves that mediate the shock. These waves heat up the inflowing upstream leptons primarily along the shock normal, which results in an anisotropic velocity distribution in the post-shock region. This distribution gives rise to the Weibel instability. Our simulation shows that even if the shock is mediated by quasi-electrostatic waves, strong magnetowaves will still develop in its downstream region.

We analyzed six Kelvin‐Helmholtz (K‐H) vortex events from Mars Atmosphere and Volatile EvolutioN (MAVEN) measurements. We found that fully developed vortices can occur at Mars' equatorial flanks and in the southern hemisphere, while they were previously observed only in the northern hemisphere. This implies that they do not exhibit a hemispheric asymmetry, and may occur globally as long as onset conditions are satisfied. We also estimated growth rates of 10−3 $1{0}^{-3}$–10−2 $1{0}^{-2}$ s−1 ${\\mathrm{s}}^{-1}$, and found that the inclusion of heavy planetary ions reduces growth rates while increasing the directions over which K‐H instability occurs. We calculated instantaneous ion loss rates due to detachment of K‐H vortices of 1025 $1{0}^{25}$–1027 $1{0}^{27}$ s−1 ${\\mathrm{s}}^{-1}$, rivaling other loss mechanisms in contributing to Mars' global atmospheric escape. The inferred higher occurrence rate of K‐H instability at Mars over a larger spatial domain strongly suggests a more significant contribution to overall atmospheric loss than previously thought.

An electrostatic parallel particle‐in‐cell (EPPIC) code that allows for particle beam injections and multiple boundary conditions is used to investigate the beam‐plasma interaction and its manifestations in the incoherent scatter (IS) spectrum. Specifically, the code is used to investigate anomalous enhancements in the ion acoustic line through the destabilization of the plasma by injection (or precipitation) of low‐energy electron beams. This enhancement of the ion acoustic line is a form of IS distortion commonly observed in the vicinity of auroral arcs called the naturally enhanced ion‐acoustic line (NEIAL). Simulations confirm the parametric decay of Langmuir waves as a plausible mechanism, assuming a mechanism for the formation of dense low‐energy (<10 eV) electron beams in the ionosphere. The spectral distortions are similar at aspect angles as large as ±15° from the beam direction. Simulations also show that the first Langmuir harmonic can have a power intensity higher than that of the ion acoustic line of a thermal plasma. Conditions which would allow the detection of Langmuir harmonics with existing incoherent scatter radars are discussed. Key Points Langmuir decay process is a feasible explanation to ion acoustic enhancements Langmuir harmonics may be detectable with current ISRs Langmuir harmonics may be useful to discriminate between explanations of NEIALs

This article demonstrates the generation of enhanced ion‐acoustic waves by beam‐plasma instability in a density cavity. The self‐consistent equations of weak turbulence theory that include quasi‐linear, decay, and scattering processes as well as convective and dispersive effects are numerically solved for a one‐dimensional density cavity. It is shown that significant enhancements of ion‐acoustic waves occur in the presence of counterstreaming electron beams and that the enhanced ion‐acoustic waves are initially localized near the center of the density cavity at large wavelengths. Later in the evolution, the enhancement in the spectrum of ion‐acoustic waves spreads out toward the edges of the cavity, with a shift to smaller wavelengths, while the enhancement near the center of the cavity tends to decrease in magnitude. The significance of the present findings is discussed.

Nuclear fusion power delivered by magnetic-confinement tokamak reactors holds the promise of sustainable and clean energy 1 . The avoidance of large-scale plasma instabilities called disruptions within these reactors 2 , 3 is one of the most pressing challenges 4 , 5 , because disruptions can halt power production and damage key components. Disruptions are particularly harmful for large burning-plasma systems such as the multibillion-dollar International Thermonuclear Experimental Reactor (ITER) project 6 currently under construction, which aims to be the first reactor that produces more power from fusion than is injected to heat the plasma. Here we present a method based on deep learning for forecasting disruptions. Our method extends considerably the capabilities of previous strategies such as first-principles-based 5 and classical machine-learning 7 – 11 approaches. In particular, it delivers reliable predictions for machines other than the one on which it was trained—a crucial requirement for future large reactors that cannot afford training disruptions. Our approach takes advantage of high-dimensional training data to boost predictive performance while also engaging supercomputing resources at the largest scale to improve accuracy and speed. Trained on experimental data from the largest tokamaks in the United States (DIII-D 12 ) and the world (Joint European Torus, JET 13 ), our method can also be applied to specific tasks such as prediction with long warning times: this opens up the possibility of moving from passive disruption prediction to active reactor control and optimization. These initial results illustrate the potential for deep learning to accelerate progress in fusion-energy science and, more generally, in the understanding and prediction of complex physical systems. Using data from plasma-based tokamak nuclear reactors in the US and Europe, a machine-learning approach based on deep neural networks is taught to forecast disruptions, even those in machines on which the algorithm was not trained.

Ion‐acoustic enhancements are investigated within the context of turbulent beam‐plasma interaction processes. The analysis assumes a pair of counterstreaming electron beams interacting with the background plasma. Two‐dimensional velocity space and two‐dimensional wave number space are assumed for the analysis, with physical parameters that characterize typical ionospheric conditions. The solutions of the electrostatic weak turbulence theory show that the ion‐acoustic wave levels are significantly enhanced when the computation is initialized with a pair of counterstreaming beams in contrast to a single beam. We suggest that this finding is highly relevant for the observed ion‐acoustic enhancements in the Earth's ionosphere that are known to be correlated with auroral activity.

First-principles simulations of tokamak turbulence have proven to be of great value in recent decades. We develop a pseudo-spectral velocity formulation of the turbulence equations that smoothly interpolates between the highly efficient but lower resolution three-dimensional (3-D) gyrofluid representation and the conventional but more expensive 5-D gyrokinetic representation. Our formulation is a projection of the nonlinear gyrokinetic equation onto a Laguerre–Hermite velocity-space basis. We discuss issues related to collisions, closures and entropy. While any collision operator can be used in the formulation, we highlight a model operator that has a particularly sparse Laguerre–Hermite representation, while satisfying conservation laws and the H theorem. Free streaming, magnetic drifts and nonlinear phase mixing each give rise to closure problems, which we discuss in relation to the instabilities of interest and to free energy conservation. We show that the model is capable of reproducing gyrokinetic results for linear instabilities and zonal flow dynamics. Thus the final model is appropriate for the study of instabilities, turbulence and transport in a wide range of geometries, including tokamaks and stellarators.

Protoplanetary discs are made of gas and dust orbiting a young star. They are also the birth place of planetary systems, which motivates a large amount of observational and theoretical research. In these lecture notes, I present a review of the magnetic mechanisms applied to the outer regions ($R\\gtrsim 1\\ \\mathrm {AU}$) of these discs, which are the planet-formation regions. In contrast to usual astrophysical plasmas, the gas in these regions is noticeably cold ($T < 300\\ \\mathrm {K}$) and dense, which implies a very low ionisation fraction close to the disc midplane. In these notes, I deliberately ignore the innermost$(R\\sim 0.1\\ \\mathrm {AU})$region, which is influenced by the star–disc interaction and various radiative effects. I start by presenting a short overview of the observational evidence for the dynamics of these objects. I then introduce the methods and approximations used to model these plasmas, including non-ideal magnetohydrodynamics, and the uncertainties associated with this approach. In this framework, I explain how the global dynamics of these discs is modelled, and I present a stability analysis of this plasma in the local approximation, introducing the non-ideal magneto-rotational instability. Following this mostly analytical part, I discuss numerical models that have been used to describe the saturation mechanisms of this instability, and the formation of large-scale structures by various saturation mechanisms. Finally, I show that local numerical models are insufficient because magnetised winds are also emitted from the surface of these objects. After a short introduction on wind physics, I present global models of protoplanetary discs, including both a large-scale wind and the non-ideal dynamics of the disc.