Auroral Electron Acceleration Research Articles

The Potential Role of Modified Electron Acoustic Wave and Nonlinear Mode Coupling in Mono‐Energetic Aurora

AbstractResults 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.

Revealing the dynamics of magnetosphere, atmosphere, and interior of solar system objects with the Square Kilometre Array

Abstract Bodies such as planets, moons, and asteroids in our solar system are the brightest objects in the low-frequency radio astronomy at ≲10 GHz. The low-frequency radio emissions from our solar system bodies exhibit various observed characteristics in the spectrum, polarization, periodicity, and flux. The observed characteristics are essential probes for explorations of the bodies’ magnetosphere, atmosphere, surface, and even their interior. Generation and propagation theories of the radio emissions associate the characteristics with fundamental physics embedded in the environments: e.g., auroral electron acceleration, betatron acceleration, and atmospheric momentum transfer. Here we review previous studies on the low-frequency radio emissions from our solar system bodies to unveil some outstanding key questions on the dynamics and evolution of the bodies. To address the key questions by the future observations with the Square Kilometre Array (SKA), we made feasibility studies for detection and imaging of the radio emissions. Possible extensions of the solar system observations with SKA to the exoplanets are also proposed in the summary.

Laboratory measurements of the physics of auroral electron acceleration by Alfv\xe9n waves

While the aurora has attracted attention for millennia, important questions remain unanswered. Foremost is how auroral electrons are accelerated before colliding with the ionosphere and producing auroral light. Powerful Alfvén waves are often found traveling Earthward above auroras with sufficient energy to generate auroras, but there has been no direct measurement of the processes by which Alfvén waves transfer their energy to auroral electrons. Here, we show laboratory measurements of the resonant transfer of energy from Alfvén waves to electrons under conditions relevant to the auroral zone. Experiments are performed by launching Alfvén waves and simultaneously recording the electron velocity distribution. Numerical simulations and analytical theory support that the measured energy transfer process produces accelerated electrons capable of reaching auroral energies. The experiments, theory, and simulations demonstrate a clear causal relationship between Alfvén waves and accelerated electrons that directly cause auroras.

On the Relationship Between Shear Alfvén Waves, Auroral Electron Acceleration, and Field Line Resonances

On the Relationship Between Shear Alfvén Waves, Auroral Electron Acceleration, and Field Line Resonances

Possibilities of a laboratory experiment on investigation of auroral kilometric radiation in the near-Earth plasma

Observations are reported of electromagnetic emissions at frequencies above the electron gyrofrequency in a mirror-confined electron cyclotron resonance plasma. These emissions are interpreted as the excitation of the fast extraordinary wave at the fundamental harmonic of the electron gyrofrequency, similar to the excitation of the auroral kilometric radiation (AKR) of the Earth. The paper proposes a method for studying the temporal characteristics of physical processes occurring in the auroral electron acceleration region, which is applied to the AKR measurements onboard INTERBALL-2 spacecraft and adopted to the measurements of laboratory plasma emissions. The joint data analysis of satellite measurements and the results of laboratory experiments showed similarities in the cyclotron instability development.

Evidence of Alfvenic Poynting Flux as the Primary Driver of Auroral Motion During a Geomagnetic Substorm

AbstractGeomagnetic substorms are major energy transfer events where energy stored in the Earth's magnetotail is released into the ionosphere. Substorm phenomena, including auroral activities, earthward Poynting flux, magnetic field dipolarization, etc, have been extensively studied. However, the complex interplay among them is not fully understood. In a fortuitous event on June 7, 2013, the twin Van Allen Probes (separated by 0.4 h in local time) observed bursts of earthward Alfvenic Poynting flux in the vicinity of the plasma sheet boundary layer (PSBL). The Poynting flux bursts correlate with enhancements of auroral brightness around the footpoints of both spacecraft. This indicates a temporal and spatial correlation between the auroral brightening and Poynting flux bursts, and that the auroral motion is directly linked to the perpendicular expansion of the Alfven wave. These observations suggest that the Alfvenic Poynting flux is a primary driver for the auroral electron acceleration. Around the time of auroral brightening, a dipolarization was seen to propagate more than 4 h in local time during a 20 min period. The azimuthal phase speed of this dipolarization (2 deg/min) is too small to explain the azimuthal motion of the aurora (13.6 deg/min), but the dipolarization could be related to the generation of the Alfvenic Poynting flux through phase mixing at strong density gradients like those in the PSBL.

Monitoring of the Electron-Acceleration Region with Auroral Kilometric Radiation

A method is proposed for the study of the height–time characteristics of physical processes in the region of auroral electron acceleration. Auroral kilometer radiation, which is generated at the local electron gyrofrequency, can be used to obtain the height distribution of the properties of a radiation source in the auroral region, and long-term satellite measurements of the auroral kilometric radiation allows the evolution of these features to be tracked over time. This approach to the analysis of auroral kilometric radiation makes it possible to obtain data remotely on variations in the features of its source concurrently along the magnetic field line of the acceleration region. This approach has been used here to analyze the auroral kilometric radiation recorded on the INTERBALL-2 satellite within the POLRAD experiment. The analysis has revealed a regular change in the wavelet spectra of fluctuations in the auroral kilometric radiation along the magnetic field line. In the given frequency range with an almost constant radiation intensity, the fluctuation spectrum follows a power law at high altitudes and becomes uniform over all characteristic times at low altitudes. This spectral transformation indicates that the processes in the source of auroral kilometric radiation are nonlinear.

Planetary Period Oscillations in Saturn's Magnetosphere: Comparison of Magnetic and SKR Modulation Periods and Phases During Northern Summer to the End of the Cassini Mission

AbstractWe compare periods and phases of Saturn planetary period oscillations determined from Cassini magnetic field and Saturn kilometric radiation (SKR) data from the beginning of 2016 to the end of mission in mid‐September 2017, encompassing northern summer solstice in May 2017. Both data sets show that the periods are almost unchanging, varying by only ~ ±0.01 hr about 10.79 hr for the northern system and 10.68 hr for the southern system, close to values attained by mid‐2015 after period coalescence between mid‐2013 and mid‐2014. The mean absolute differences between the magnetic and SKR periods are ~0.0036 hr (~13 s), consistent with estimated magnetic measurement uncertainties, while the overall mean difference is less than 0.001 hr (~2–3 s), at the limit of resolution. The relative phasing between magnetic and SKR modulations is correspondingly near constant and such that the equatorial planetary period oscillation fields of the northern/southern systems point radially outward near‐oppositely at ~14.3/2.5 hr local time at corresponding SKR maxima, with upward planetary period oscillation currents located ~2 hr postdawn for both systems, consistent with previous intervals having dawnside spacecraft apoapsides. Southern SKR emissions are found to be significantly dual modulated at both southern and northern periods in data limited to lie well within the southern shadow zone of the northern sources. These northern period modulations are shown to be approximately in phase with those in the northern emissions, consistent with a recent suggestion that bidirectional auroral electron acceleration may generate in phase SKR emissions in both hemispheres.

Electric Mars: A large trans‐terminator electric potential drop on closed magnetic field lines above Utopia Planitia

AbstractParallel electric fields and their associated electric potential structures play a crucial role in ionospheric‐magnetospheric interactions at any planet. Although there is abundant evidence that parallel electric fields play key roles in Martian ionospheric outflow and auroral electron acceleration, the fields themselves are challenging to directly measure due to their relatively weak nature. Using measurements by the Solar Wind Electron Analyzer instrument aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Mars Scout, we present the discovery and measurement of a substantial (ΦMars=7.7 ± 0.6 V) parallel electric potential drop on closed magnetic field lines spanning the terminator from day to night above the great impact basin of Utopia Planitia, a region largely free of crustal magnetic fields. A survey of the previous 26 orbits passing over a range of longitudes revealed similar signatures on seven orbits, with a mean potential drop (ΦMars) of 10.9 ± 0.8 V, suggestive that although trans‐terminator electric fields of comparable strength are not ubiquitous, they may be common, at least at these northerly latitudes.

Features of Pc5 pulsations in the geomagnetic field, auroral luminosity, and Riometer absorption

Simultaneous morning Pc5 pulsations (f ~ 3–5 mHz) in the geomagnetic field, aurora intensities (in the 557.7 and 630.0 nm oxygen emissions and the 471.0 nm nitrogen emission), and riometer absorption, were studied based on the CARISMA, CANMOS, and NORSTAR network data for the event of January 1, 2000. According to the GOES-8 satellite observations, these Pc5 geomagnetic pulsations are observed as incompressible Alfven waves with toroidal polarization in the magnetosphere. Although the Pc5 pulsation frequencies in auroras, the geomagnetic field, and riometer absorption are close to one another, stable phase relationships are not observed between them. Far from all trains of geomagnetic Pc5 pulsations are accompanied by corresponding auroral pulsations; consequently, geomagnetic pulsations are primary with respect to auroral pulsations. Both geomagnetic and auroral pulsations propagate poleward, and the frequency decreases with increasing geomagnetic latitude. When auroral Pc5 pulsations appear, the ratio of the 557.7/630.0 nm emission intensity sharply increases, which indicates that auroral pulsations result from not simply modulated particle precipitation but also an additional periodic acceleration of auroral electrons by the wave field. A high correlation is not observed between Pc5 pulsations in auroras and the riometer absorption, which indicates that these pulsations have a common source but different generation mechanisms. Auroral luminosity modulation is supposedly related to the interaction between Alfven waves and the region with the field-aligned potential drop above the auroral ionosphere, and riometer absorption modulation is caused by the scattering of energetic electrons by VLF noise pulsations.

Electrostatic double layers as auroral particle accelerators – a problem

Abstract. A search of the Annales Geophysicae database shows that double layers and other quasi-static electric potential structures have been invoked hundreds of times since the year 2000 as being the agents of auroral electron acceleration. This is despite the fact that energy transfer by conservative fields has been known for some 200 years to be impossible. Attention is drawn to a long-standing interpretation of the acceleration process in terms of the dynamic fields of electrostatic waves.

A unified model of auroral arc growth and electron acceleration in the magnetosphere‐ionosphere coupling

AbstractA unified physical model of auroral arc growth and electron acceleration is constructed from the gyrokinetic and two‐fluid equations for the magnetosphere‐ionosphere (M‐I) coupling system. The present theory describes destabilization of kinetic Alfvén waves (KAWs) in the M‐I coupling, where development of upward and downward field‐aligned currents carried by the KAWs leads to ionospheric density enhancement and depletion, respectively. The feedback M‐I coupling through KAWs elucidates growth of auroral arcs and field‐aligned acceleration of electrons self‐consistently and provides a possible explanation to the Alfvénic auroras observed by the FAST spacecraft.

Survey of the ULF wave Poynting vector near the Earth's magnetic equatorial plane

Ultralow frequency (ULF) waves transfer energy in the Earth's magnetosphere through a variety of mechanisms that impact the Earth's ionosphere, radiation belts, and other plasma populations. Measurements of the electromagnetic portion of the energy transfer rate are an important source of information for assessing the importance of ULF waves relative to other energy transfer mechanisms as well as a diagnostic for studying the behavior of ULF waves. Using Time History of Events and Macroscale Interactions during Substorms satellite data, we examine the time‒averaged electromagnetic energy transfer rate, or Poynting vector, as a function of frequency and region of the magnetosphere; for this study, we focus on the direction and rate of energy transfer relative to the background magnetic field, comparing perpendicular and parallel transfer rates. This study extends earlier studies of the ULF wave Poynting vector that focused on narrower frequency ranges or specific regions of the magnetosphere; here we consider the 3–50 mHz frequency range, all local time sectors, radial distances from 3 to 13 Re, and magnetic latitudes close to the equatorial plane. We measure time‒averaged Poynting vectors that range from 10−11 to 10−5 W/m2 , with larger Poynting vector magnitudes occurring at larger radial distances and smaller frequencies. In every spatial region and frequency we examined, we found a large degree of scatter in both the Poynting vector magnitude and direction. The Poynting vector tends to be anisotropic at all frequencies, with more energy transferred along rather than across the background magnetic field. This preference for parallel energy transfer near the magnetic equator suggests that Joule dissipation in the ionosphere and the acceleration of auroral electrons are the largest sinks of ULF wave energy in the magnetosphere.

EXCITATION OF KINETIC ALFVÉN WAVES BY DENSITY STRIATION IN MAGNETO-PLASMAS

Field-aligned density striation is one of the most common inhomogeneity phenomena in magneto-plasmas, such as in the solar coronal plasma and terrestrial auroral plasma. Kinetic Alfven waves (KAWs) can play an important role in the inhomogeneous heating of coronal magneto-plasmas as well as in the local acceleration of auroral energetic electrons. In this paper, we study the dispersion and instability of KAWs in a magneto-plasma with density striation structures. Results show that KAWs become unstable in the presence of the density striation and the corresponding instability has a maximal growth rate at the perpendicular wavelength close to the spatial scale of the density gradient. Related experimental phenomena in both laboratory and space plasmas are discussed. It is suggested that the excitation of KAWs by the density striation of magneto-plasmas can be of potential importance in understanding the physics of the formation of magneto-plasma filaments and their heating mechanisms, which are often present in the terrestrial auroral plasma, the solar coronal plasma, and other astrophysical plasmas.

Auroral counterpart of magnetic field dipolarizations in Saturn's tail

Following magnetic reconnection in a planetary magnetotail, newly closed field lines can be rapidly accelerated back towards the planet, becoming “dipolarized” in the process. At Saturn, dipolarizations are initially identified in magnetometer data by looking for a southward turning of the magnetic field, indicating the transition from a radially stretched configuration to a more dipolar field topology. The highly stretched geometry of the kronian magnetotail lobes gives rise to a tail current which flows eastward (dusk to dawn) in the near equatorial plane across the centre of the tail. During reconnection and associated dipolarization of the field, the inner edge of this tail current can be diverted through the ionosphere, in a situation analogous to the substorm current wedge picture at Earth. We present a picture of the current circuit arising from this tail reconfiguration, and outline the equations which describe the field–current relationship. We show a new in situ example of a dipolarization identified in the Cassini magnetometer data and use this formalism to estimate the ionospheric current density that would arise based on in situ tail measurements of the magnetic field and the implications for corresponding auroral electron acceleration in regions of upward directed field-aligned current. We then present a separate example of data from the Cassini UVIS instrument where we observe small ‘spots’ of auroral emission lying near the main oval; features suggested to be associated with dipolarizations in the tail. In the example shown, such auroral features are the precursor to more intense activity associated with recurrent energisation via particle injections from the tail following reconnection.

Kinetic Alfven wave in the presence of parallel electric field with general loss-cone distribution function: A kinetic approach

In past, kinetic Alfven waves are widely investigated regarding the transfer of energy from the magnetosphere to the ionosphere. These waves are supposed to be responsible for the acceleration of auroral electrons to exhibit auroral phenomena. The existence of parallel electric fields is always reported in association with the kinetic Alfven waves. In the present investigation, assuming parallel electric fields developed by different mechanism, the dispersion relation and growth/damping rate of the kinetic Alfven Wave (KAW) in the presence of parallel electric field are evaluated with general loss-cone distribution using the kinetic approach. The wave frequency w and growth/damping rate gL are obtained for two regimes of propagation k^ri 1, where k^ is perpendicular wave number andri is the ion-gyro radius. The results of the present study show that the frequency w and growth/damping rate of wave depend upon the distribution indices, parallel electric field and k^ri. The generation of kinetic Alfven wave by the combined effect of parallel electric field and loss-cone distribution indices (J), at a particular range of k^ri is noticed. Thus the propagation of kinetic Alfven wave and loss of the Poynting flux from plasma sheet boundary layer (PSBL) to the ionosphere can be explained more effectively on the basis of present investigation. The results of the present study show that the parallel electric field in the presence of loss-cone distribution index is an important parameter to study KAW in the PSBL. The finding of the present investigation is to report that auroral phenomena are suitably described by kinetic Alfven wave in the presence of parallel electric field and the general loss-cone distribution function. Key words: Kinetic Alfven wave, Plasma-sheet-boundary-layer, Parallel electric field, Kinetic theory, Loss-cone distribution function.

Energy source for auroral electrons from two proposed substorm onset processes

[1] There are two main processes in the tail that are proposed to link electron precipitation for the initial brightening auroral arc at substorm onset. These are current disruption/dipolarization (CDD) and magnetic reconnection (MR). To provide further clues in differentiating these two processes, we examine the electron energy spectrum and the associated total field-aligned energy flux during six CDD events and compare them with five MR events. Data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE), Geotail, and Time History of Events and Macroscale Ineractions during Substorms (THEMIS) missions are used in this study. It is found that (1) the total field-aligned electron energy flux from CDD sites is generally high enough to account for the energy flux required to produce bright auroral arcs at substorm onsets and (2) the total field-aligned electron energy flux from MR sites is generally a factor of one to two orders of magnitude lower than that from CDD sites. These results favor a direct link between the CDD sites and the substorm onset arcs, but they do not rule out further acceleration of auroral electrons away from these sites by other energy sources (e.g., field-aligned potential drops and kinetic Alfven waves) to produce even higher-intensity auroral arcs.

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

[1] With the help of a 2.5-D particle-in-cell simulation code, we investigate the physics of the acceleration of auroral electrons, through the interaction of an isolated Alfven wave packet with a plasma density cavity. The cavity is edged by density gradients perpendicular to the magnetic field. We show that a single passing of an isolated wave packet over a (infinite) cavity creates an electron beam. It triggers local current and beam-plasma instabilities and small-scale coherent electric structures. The energy flux of downgoing electrons is significantly increased, whereas upgoing electrons are also accelerated, even if no beam is formed. Accelerated electrons remain after the passage of the Alfvenic pulse, allowing the observation of energetic particles without any significant electromagnetic perturbation. The dependence of this process on the electron to ion mass ratio is consistent with its control by inertial effects.

Excitation and steepening of ion‐acoustic waves in the ionospheric Alfvén resonator

A nonlinear two‐dimensional fluid model describing excitation of the ionospheric Alfvén resonator by a shear Alfvén wave coming from the magnetosphere is developed. Initially, the plasma is in an equilibrium defined by a balance between the gravity, electric field, and pressure gradient forces. This equilibrium is perturbed when a standing Alfvén wave is excited in the resonator. The nonlinear Lorentz force of the wave creates converging and diverging plasma flows along the geomagnetic field, thus producing compressions and rarefactions in the plasma density. Simulation reveals that density perturbations evolve into ion‐acoustic shock waves in a process similar to the nonlinear steepening of sound waves in neutral gases. A shock associated with compression of hydrogen ions propagates faster than a shock associated with compression of oxygen ions. One‐dimensional shock‐capturing Poisson simulation reveals that the shocks appear as double layers at first, but then they decay into ion‐acoustic wave packets. The drop of potential across each shock is negligible at any stage of shock's development, making these shocks unfavorable for auroral electron acceleration.

Observations of the auroral width spectrum at kilometre-scale size

Abstract. This study examines auroral colour camera data from the Canadian Dense Array Imaging SYstem (DAISY). The Dense Array consists of three imagers with different narrow (compared to all-sky view) field-of-view optics. The main scientific motivation arises from an earlier study by Knudsen et al. (2001) who used All-Sky Imager (ASI) combined with even earlier TV camera observations (Maggs and Davis, 1968) to suggest that there is a gap in the distribution of auroral arc widths at around 1 km. With DAISY observations we are able to show that the gap is an instrument artifact and due to limited spatial resolution and coverage of commonly used instrumentation, namely ASIs and TV cameras. If the auroral scale size spectrum is indeed continuous, the mechanisms forming these structures should be able to produce all of the different scale sizes. So far, such a single process has not been proposed in the literature and very few models are designed to interact with each other even though the range of their favourable conditions do overlap. All scale-sizes should be considered in the future studies of auroral forms and electron acceleration regions, both in observational and theoretical approaches.