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A statistical survey using 3 years of Van Allen Probes data from 2013 to 2015 is conducted to investigate the impact of broadband kinetic Alfvén waves (KAWs) on the pitch angle distributions (PADs) of relativistic electrons. 62 events exhibiting distinct KAW signatures, identified when other wave modes known to generate butterfly distributions were absent, are examined along with the corresponding PADs of electrons. The results reveal a relationship between the spectral energy density of KAWs and PAD of relativistic electrons, with butterfly PAD features becoming more pronounced and showing larger dip‐sizes as the spectral energy density of KAWs increases, particularly for electrons in 0.5–3.4 MeV energy range. At these times the magnetopause sub‐solar stand‐off distance renders magnetopause shadowing an unlikely formation mechanism. This suggests the interaction of relativistic electrons with broadband KAWs could be a significant mechanism, alongside drift‐shell splitting, contributing to the formation of butterfly PADs in the night‐side outer radiation belt of Earth.

The propagation of kinetic Alfvén waves (assumed sourced from intermittent dayside reconnection) is investigated with a gyrofluid‐kinetic electron model compared with Cluster observations. These observations reveal electron distributions that are preferentially field‐aligned or field‐opposed, with signatures that are unidirectional or counterstreaming and skews that vary with the current sense. The simulations reproduce, with good fidelity, the observed local characteristics when the conditions match the observed local plasma conditions. The wave energy conversion is predominantly positive at mid‐altitudes, indicating a transfer of wave to electron energy. This conversion rate increases significantly at low‐altitudes (in the inertial Alfvén wave regime) and is accompanied by the formation of highly field‐aligned electron beams peaking at several hundred eV in energy, with a directionality that is opposite to the current sense. This low‐altitude energization results in the dissipation of the majority of the wave Poynting flux and would lead to substantial soft electron precipitation.

Small-scale dynamic auroras have spatial scales of a few km or less, and temporal scales of a few seconds or less, which visualize the complex interplay among charged particles, Alfvén waves, and plasma instabilities working in the magnetosphere-ionosphere coupled regions. We summarize the observed properties of flickering auroras, vortex motions, and filamentary structures. We also summarize the development of fundamental theories, such as dispersive Alfvén waves (DAWs), plasma instabilities in the auroral acceleration region, ionospheric feedback instabilities (IFI), and the ionospheric Alfvén resonator (IAR).

Results from a 1D kinetic simulation, for the first time, reveal the important role of modified electron acoustic wave (MEAW) in auroral electron acceleration. Parallel electric fields, generated due to the mode coupling between kinetic Alfven waves (KAWs) and MEAWs in the transition region from the magnetosphere and the ionosphere, can be sustained by continuous energy input carried by Alfven waves from the magnetosphere. Under the incidence of long‐period Alfven waves carrying upward field‐aligned currents, a parallel potential drop can be formed in the transition region, leading to mono‐energetic electron acceleration. Such a mechanism provides a possible link between shear Alfven waves and the mesoscale mono‐energetic auroral electron acceleration. Plain Language Summary Discrete aurora is supposed to be caused by precipitated electrons accelerated by parallel electric field. The mode coupling between modified electron acoustic waves (MEAWs), which received little attention, and kinetic Alfven waves is an effective mechanism of the generation of parallel electric field. The present simulation results show that the generation of parallel electric field, due to the mode coupling in the transition region, could be important or even dominant under certain circumstances. Our findings point out the importance of the MEAWs on the auroral electron acceleration. Key Points Simulation results indicate that modified electron acoustic waves (MEAWs) strongly affect the auroral electron acceleration Kinetic Alfven waves under certain circumstances lead to mono‐energetic acceleration through mode coupling to MEAWs Such a mechanism provides a possible link between Alfven waves and mono‐energetic electron acceleration

Juno satellite observations have illustrated substantial examples of broadband electron energization up to 105–106 eV levels. In order to explain these observations, we use a hybrid gyrofluid kinetic‐electron model in an untilted dipolar topology to illustrate energization to high levels in weak current conditions by inertial Alfvén waves close to the Jupiter ionosphere for ambient plasma densities and magnetic field perturbations inferred from Juno satellite observations. The key to the high energization is the extremely low densities evident in the observations which necessitates the acceleration of electrons to very high velocities in order to carry the field‐aligned current. Plain Language Summary Juno satellite observations have illustrated broadband energization that is associated with Alfvén waves which are electromagnetic waves with field vectors perpendicular to the background magnetic field that have common analogy to waves on a string. Specifically, the energization is associated with inertial Alfvén waves (IAWs) which are the limit of Alfvén waves when the perpendicular scale length of the wave is on the order of the electron skin depth whereby the waves can support significant parallel electric fields to accelerate electrons that lead to aurora. This energization is described as broadband since the range of energies extends from tens of eV to MeV levels. At Earth, IAWs are associated with electron energization to keV levels and so it has been a puzzle to explain the very high electron energies evident at Jupiter. In this work, we illustrate that for observed parameters, IAWs can energize electrons to much higher energies at Jupiter than in the terrestrial analogue. This is because the very low plasma densities evident in the high latitude Jupiter magnetosphere necessitates the IAW to generate a sufficiently large parallel electric field to accelerate a limited supply of electrons to correspondingly higher energies in order to carry the parallel current. Key Points In Jupiter's density depleted flux tubes, inertial Alfvén waves can accelerate electrons to very high energies for modest parallel currents Due to the high energies, this energization can be manifest in primarily unidirectional electron beams It is expected that inertial Alfvén waves can explain observed electron energizations to 105−106 eV levels

Compressional plasma perturbations may cause thermal misbalance between plasma-heating and -cooling processes. This misbalance significantly affects the dispersion properties of compressional waves providing a feedback between the perturbations and plasmas. It has been shown that Alfvén waves may induce longitudinal (compressional) plasma motions. In the present study, we analyze the effects of thermal misbalance caused by longitudinal plasma motions induced by shear Alfvén waves. We show that thermal misbalance leads to appearance of exponential bulk flows, which themselves modify the Alfvén-induced plasma motions. In the case of sinusoidal Alfvén waves, we show how the amplitude and phase shift of induced longitudinal motions gain dependence on the Alfvén wave frequency while shedding light on its functionality. This feature has been investigated analytically in application to coronal conditions. We also consider the evolution of longitudinal plasma motions induced by the shear sinusoidal Alfvén wave by numerical methods before comparing the results obtained with our presented analytical predictions to justify the model under consideration in the present study.

The torsional Alfvén wave is highly regarded as the carrier of the energy from the photosphere to the corona in the solar atmosphere. This paper presents a comprehensive linear analysis of the wave behavior and energy transfer within an open, twisted, divergent magnetic flux tube configuration, considering the impact of wave guide structure on the propagation of these waves using the magneto-hydrodynamic approach. The study shows that waves with frequencies between 0.001 Hz and 1 Hz can effectively penetrate the transition region, with the most efficient energy transfer occurring in the 0.1 Hz to 1 Hz frequency range. The research findings suggest that waves with certain intermediate frequencies are able to transmit energy to the coronal region of the Sun, contributing to its active heating.

The Alfvén wave (AW) is the most common fluctuation present within the solar wind emitted from the Sun. Whether or not AWs can originate after the collision of an Interplanetary Coronal Mass Ejection (ICME) and a High-Speed Stream (HSS) remains an open question. To find an answer to this question, we have investigated an ICME-HSS interaction event observed on 21 s t October 1999 at 1 AU by the WIND spacecraft. We have used the Walén test to identify AWs and estimated the Elsässer variables to find its characteristics. We explicitly find dominant sunward AWs within the ICME, whereas the trailing HSS has strong anti-sunward AWs. We suggest that the ICME-HSS interaction deforms the Magnetic Cloud (MC) of the ICME, resulting in the generation of AWs inside the MC. Additionally, the existence of reconnection within the ICME’s early stage could also contribute to the origin of AWs within it.

We present new results towards the explanation of the chromospheric-heating problem and the solar-wind origin, using a two-fluid model that takes into account the collisional interaction between ions (protons) and neutrals (hydrogen atoms). Our aim is to further reveal the mechanism behind chromospheric heating and plasma outflows. We simulate and analyse the propagation and evolution of Alfvén waves in the partially ionised solar chromosphere, consisting of ions + electrons and neutral fluids. The simplified model chromosphere is permeated by a vertical, uniform magnetic field. We perform numerical simulations in the framework of a quasi-1.5-dimensional (1.5D), two-fluid model in which Alfvén waves are excited by a harmonic driver in the transverse component of the ion and neutral velocities, operating in the chromosphere. In the case of a small-amplitude driver, Alfvén waves are weakly damped, and for the chosen wave periods of a few seconds, Alfvén waves manage to propagate through the chromosphere and enter the solar corona. Non-linear Alfvén waves excited by a large-amplitude driver cause significant chromospheric heating and plasma outflows. We thus conclude that two-fluid Alfvén waves with larger amplitudes can contribute to chromospheric heating and plasma outflows, which may result higher up in the solar-wind origin.

This article describes the relationship between shear Alfvén waves and auroral electron acceleration, with an emphasis on long-period standing waves that correlate with redline auroral arcs in the Earth’s magnetosphere. Discrete auroral arcs were correlated with high-latitude field line resonances in the early 1990’s. The past decade has seen advances in all-sky camera technology improve the detection and categorization of “FLR arcs” and establish them as a distinct population. We review observations of redline arcs and discuss estimates of wave amplitudes, wavelengths perpendicular to the geomagnetic field, and saturation times obtained within the framework of two-fluid theories. The two-fluid theory explains the spatial and temporal evolution of FLR optical signatures, but the estimated parallel electric field strengths are insufficient to accelerate electrons and produce 6300 Å auroral emissions. A kinetic theory of FLRs is necessary since electron bounce motion in long-wavelength standing waves affects the ac conductivity and hence the strength of parallel electric fields. In the kinetic theory, the current-voltage relation comprises a conductivity kernel that is a function of the wave frequency, field line length, electron thermal speed, and the number of electron trajectories nearly parallel to geomagnetic field lines close to the ionosphere. The ensuing nonlocal relationship between wave parallel currents and parallel electric fields provides a feasible explanation of the correlation between long-period field line resonances and redline arcs in the terrestrial magnetosphere. The mirror force and particle trapping in the wave fields of shear Alfvén waves are demonstrated to be important aspects of the kinetics of FLRs.