Earth's Auroral Regions Research Articles

AMICal Sat: A Sparse RGB Imager on Board a 2U CubeSat to Study the Aurora

AMICal sat, a dedicated 2U CubeSat, has been developed, in order to monitor the auroral emissions with a dedicated imager. It aims to help reconstructing the low-energy electron fluxes up to 30 keV in the Earth auroral regions. It includes an imager entirely designed in the Grenoble University Space Center. The imager uses a 1.3-Mpixels sparse RGB CMOS detector and a wide-field objective ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f=22.5$ </tex-math></inline-formula> mm). The satellite platform has been built by the Polish company Satrevolution. Launched 3 September 2020 from Kuru (French Guyana) on board the Vega flight 16, it produced its first images in October 2020. The aim of this article is to describe the design of the payload, especially the optics and the proximity electronics, and to describe the use of the payload for space weather purpose. A preliminary analysis of a first image showing the relevance of such an instrument for auroral monitoring is performed. This analysis allowed us to reconstruct from one of the first images the local electron input flux at the top of the atmosphere during the exposure time.

A Review of Cluster Wideband Data Multi‐Spacecraft Observations of Auroral Kilometric Radiation

AbstractWe review important advances in the understanding of auroral kilometric radiation (AKR) resulting from observations by the Wideband Data instruments on the four Cluster spacecraft. AKR is an intense radio emission originating in the Earth's auroral regions with frequencies typically in the range 50–700 kHz, usually observed from space. It is now widely accepted that AKR is generated by the cyclotron maser instability (CMI) in density cavities in the auroral acceleration region. Multi‐point observations by the Cluster spacecraft with a time delay of arrival technique have allowed the source locations of many individual AKR bursts to be determined. The position uncertainty is around 500 km at the source region or about 200 km when mapped on to the auroral zone. AKR is emitted in a narrow beam close to the tangent to the magnetic field vector in the source region. This has important implications for the possible generation mechanisms, being incompatible with filled or hollow cone beaming models. It also implies that an observer at a given location can only see AKR from a fraction of possibly active source regions. The complex frequency time structure of AKR sometimes shows regular striations or pulsations. Cluster observations of these phenomena have been interpreted as modulation of the CMI by disturbances propagating through the generation region. Exceptionally, AKR can sometimes be observed from low altitude spacecraft or even on the ground. Recent work has involved simultaneous observations of AKR on the Cluster spacecraft and on the ground at the South Pole.

Inverted‐V Electron Acceleration Events Concurring With Localized Auroral Observations at Mars by MAVEN

AbstractFrom February to March 2019, the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft repeatedly observed aurora near periapsis over Mars' southern strong crustal fields. During these orbits, the Solar Wind Electron Analyzer observed accelerated electrons at similar locations to where the auroras were observed, resembling the inverted‐V structure observed near Earth's auroral region. In this study, we present a case study of such an acceleration event, where we estimate a field‐aligned electrostatic potential drop of ∼440 V. We determine the field‐aligned current from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing reasonably well with the estimated net electron current carried by acceleration electrons with a maximum of 2.5 μA/m2. Similar to Earth, the potential drop develops when the ambient plasma cannot sustain the imposed field‐aligned current. We also estimate the potential layer to be located above 750‐ to 850‐km altitude and the associated electric field to be ∼0.6 V/m.

Emission of Electrostatic Whistler Waves Associated With Weak Electron‐Beam Excited Langmuir Waves: The 2‐D Particle‐in‐Cell Simulations

AbstractIn this paper, we perform two‐dimensional (2‐D) electrostatic particle‐in‐cell simulations to investigate the emission of electrostatic whistler waves (EWWs) associated with the weak electron‐beam excited Langmuir waves. Both the linear theory and simulations have shown that quasi‐parallel Langmuir waves can be generated during the linear stage of the beam‐plasma interaction process. The 2‐D particle‐in‐cell simulations further demonstrated that when the background magnetic field is strong (Ωe/ωpe &gt; 1, where Ωe and ωpe are electron gyrofrequency and electron plasma frequency, respectively), EWWs will be generated in weak electron‐beam excited Langmuir waves during the nonlinear stage of wave growth, leading to the increment of perpendicular electric field. When the background magnetic field is weak (Ωe/ωpe &lt; 1), EWWs cannot be generated during the nonlinear stage and the perpendicular electric field has no obvious nonlinear increment. In addition, the influences of the electron beam density and the electron beam velocity on the generation of EWWs are also analyzed in this paper. The simulation results may also account for observations in the Earth's Auroral region.

The Acceleration of Electrons to High Energies Over the Jovian Polar Cap via Whistler Mode Wave‐Particle Interactions

AbstractUpward traveling electrons with energies from tens of kiloelectron volts to several megaelectron volts have been observed over the Jovian polar regions in association with intense upward propagating whistler mode waves. The electrons have a power law‐like energy distribution, indicative of a stochastic acceleration process. The energy flux of the upward propagating whistler mode waves is comparable to and strongly correlated with the energy flux of the upward traveling energetic electrons, suggesting that the whistler mode waves may be accelerating the electrons. We propose that a downward field‐aligned current over the polar cap generates strong downward parallel electric fields and associated upward electron beams in the low‐density regions of Jupiter's upper ionosphere, a mechanism similar to the formation of inverted‐Vs in Earth's auroral regions. At Jupiter, the upward‐traveling electron beams produce intense upward propagating whistler mode emissions over a broad frequency range, similar to upward propagating auroral hiss at Earth. As the whistler mode waves propagate upward out of the inverted‐V source region, the waves are absorbed by the plasma, thereby accelerating the electrons. We attribute the stochastic power law‐like energy spectrum of the accelerated electrons to the development of Hamiltonian chaos (velocity space diffusion), the signature of which is indicated by the occurrence of spiky soliton‐like variations in the whistler mode electric fields. Acceleration to high energies may be facilitated by the rapid increase in the phase velocity of the whistler mode waves with increasing altitude, which could accelerate some of the electrons trapped in the wavefield to very high relativistic energies.

Electron acoustic nonlinear structures in planetary magnetospheres

In this paper, we have studied linear and nonlinear propagation of electron acoustic waves (EAWs) comprising cold and hot populations in which the ions form the neutralizing background. The hot electrons have been assumed to follow the generalized (r,q) distribution which has the advantage that it mimics most of the distribution functions observed in space plasmas. Interestingly, it has been found that unlike Maxwellian and kappa distributions, the electron acoustic waves admit not only rarefactive structures but also allow the formation of compressive solitary structures for generalized (r,q) distribution. It has been found that the flatness parameter r, tail parameter q, and the nonlinear propagation velocity u affect the propagation characteristics of nonlinear EAWs. Using the plasmas parameters, typically found in Saturn's magnetosphere and the Earth's auroral region, where two populations of electrons and electron acoustic solitary waves (EASWs) have been observed, we have given an estimate of the scale lengths over which these nonlinear waves are expected to form and how the size of these structures would vary with the change in the shape of the distribution function and with the change of the plasma parameters.

Nonlinear radiation generation processes in the auroral acceleration region

Abstract. It is known from laboratory plasma experiments that double layers (DLs) radiate in the electromagnetic spectrum; but this is only known qualitatively. In these experiments, it was shown that the electron beam created on the high-potential side of a DL generates nonlinear structures which couple to electromagnetic waves and act as a sender antenna. In the Earth auroral region, observations performed by auroral spacecraft have shown that DLs occur naturally in the source region of intense radio emissions called auroral kilometric radiation (AKR). Very high time-, spatial-, and temporal-resolution measurements are needed in order to characterize waves and particle distributions in the vicinity of DLs, which are moving transient structures. We report observations from the FAST satellite of a localized large-amplitude parallel electric field (∼ 300 mV m−1) recorded at the edges of the auroral density cavity. In agreement with laboratory experiments, on the high-potential side of the DL, elementary radiation events are detected. They occur substantially above the local electron gyrofrequency and are associated with the presence of electron holes. The velocity of these nonlinear structures can be derived from the measurement of the Doppler-shifted AKR frequency spectrum above the electron gyrofrequency. The generated electron holes appear as the nonlinear evolution of electrostatic waves generated by the electron–electron two-stream instability because they propagate at about half the beam velocity. It is pointed out that, in the vicinity of a DL, the shape of the electron distribution gives rise to a significant power recorded in the left-hand polarized ordinary (LO) mode.

Observation and interpretation of energetic ion conics in Jupiter's polar magnetosphere

AbstractNASA's Juno spacecraft successfully completed its first science polar pass over Jupiter's northern and southern aurora, with all the instruments powered, on 27 August 2016. Observations of conical energetic proton distributions at low altitudes (&lt;6 RJ) over the northern polar region are interpreted as resulting from transversely (to the local magnetic field lines) accelerated H+ at a position planetward of the point of observation. The proton conics were observed within a broad region of upward beaming electrons and were accompanied by broadband low‐frequency wave emissions as well as low‐altitude trapped magnetospheric protons and heavy ions. The characteristic energies associated with these accelerated ion conics are ~100 times more energetic than similar distributions observed in the Earth's auroral region and similar in energy to those found at Saturn. In addition, the ion conics also exhibited pitch angle dispersion with time that is interpreted as a consequence of the structure of the source location. Mapping these distributions along magnetic field lines connected from the spacecraft to the ionosphere suggests that the source region exists at altitudes between ~3 and 5 RJ. These new and exciting observations of accelerated ions over the polar region of Jupiter open up new areas for comparative planetary auroral physics.

Backward wave cyclotron-maser emission in the auroral magnetosphere.

In this Letter, we present theory and particle-in-cell simulations describing cyclotron radio emission from Earth's auroral region and similar phenomena in other astrophysical environments. In particular, we find that the radiation, generated by a down-going electron horseshoe distribution is due to a backward-wave cyclotron-maser emission process. The backward wave nature of the radiation contributes to upward refraction of the radiation that is also enhanced by a density inhomogeneity. We also show that the radiation is preferentially amplified along the auroral oval rather than transversely. The results are in agreement with recent Cluster observations.

A particle-in-cell approach to obliquely propagating electrostatic waves

The electron-acoustic and beam-driven modes associated with electron beams have previously been identified and studied numerically. These modes are associated with Broadband Electrostatic Noise found in the Earth's auroral and polar cusp regions. Using a 1-D spatial Particle-in-Cell simulation, the electron-acoustic instability is studied for a magnetized plasma, which includes cool ions, cool electrons and a hot, drifting electron beam. Both the weakly and strongly magnetized regimes with varying wave propagation angle, θ, with respect to the magnetic field are studied. The amplitude and frequency of the electron-acoustic mode are found to decrease with increasing θ. The amplitude of the electron-acoustic mode is found to significantly grow at intermediate wavenumber ranges. It reaches a saturation level at the point, where a plateau forms in the hot electron velocity distribution after which the amplitude of the electron-acoustic mode decays.

Investigation of Io's auroral hiss emissions due to its motion in Jupiter's magnetosphere

The left-hand side of the auroral hiss emission observed by Galileo has a frequency time profile shaped very similar to the funnel shape observed in the Earth's auroral region. This close similarity indicates that we can use the theory of whistler-mode propagation near the resonance cone to locate the emission source. The general characteristics of the whistler mode are discussed. Then the position of the emission source is investigated using a geometrical method that takes into account the trajectory of Galileo. Initially a point source is assumed. Then the possibility of a sheet source aligned along the magnetic field lines which are tangent to the surface of Io is investigated. Both types of sources show that the whistler mode radiation originates very close to the surface of Io.

Ion-acoustic vortex formation in a non-uniform two-electron-temperature magnetoplasma with sheared ion flow

AbstractWe have investigated some linear as well as nonlinear properties of ion-acoustic wave (IAW) propagation in a non-uniform two-electron-temperature magnetoplasma in the presence of ion-sheared flows. In the linear limit, new dispersion relation has been derived and analyzed in some interesting limiting cases. It is shown that ion-sheared flow can drive the IAW unstable under a certain limit.Whereas, in the nonlinear case, dipolar and vortex street type solutions can appear. It is observed that by choosing some typical parameters of Earth's auroral region, the scalelength of the nonlinear structures are significantly modified. The present investigation might be very useful to explain various types of nonlinear structure formation which has been observed in space environments where two-electron-temperature plasma exist.

Observation of conical electron distributions over Martian crustal magnetic fields

[1] Electron angular distributions similar to bidirectional electron conics (BECs) near Earth's auroral regions have been previously reported at Mars. They are almost always symmetric about 90° pitch angle, having peaks between 35°–70° and 110°–145°. Signatures of Martian BECs are clearly observable from ∼90 eV to ∼640 eV and they are mainly observed in darkness (60% of 150,000 conic events identified globally). Statistical analysis shows that BECs mostly occur on horizontal magnetic field lines over moderate crustal fields (∼15 nT). They are surrounded by regions containing electrons with trapped/mirroring pitch angle distributions, which suggests that BECs form on closed field lines. The energy spectra of the conics exhibit substantial decreases in all energy levels in relation to neighboring regions, which mostly have access to the Martian magnetotail or magnetosheath. Upstream conditions (draped IMF direction, solar wind pressure, and EUV flux) do not affect the observation of the events. Therefore stability inferred from observations of similar BECs over the same geographical locations suggests that the driving conditions resulting in their formation operate on the crustal fields. We propose that conical electron distributions may be generated by merging of neighboring open crustal magnetic field lines resulting in the trapping of the incident plasma they carry initially. Electrons at ∼90° pitch angle may subsequently be pushed into the loss cone by wave-particle interactions, static, or time varying electric fields, resulting in the conics. BECs may also be generated by mirroring of the particles that are streamed to lower altitudes on nearby open field lines, which then diffuse and/or scatter onto inner closed field lines.

Auroral hiss, electron beams and standing Alfvén wave currents near Saturn's moon Enceladus

Observations from the Cassini spacecraft have shown that Saturn's small icy moon Enceladus ejects a plume of water vapor and small ice particles into Saturn's rapidly co-rotating magnetosphere. In this paper we show that the interaction of the moon with the magnetospheric plasma produces a number of electrodynamics effects that are remarkably similar to those observed in Earth's auroral regions and near Jupiter's moon Io. These include whistler-mode emissions similar to terrestrial auroral hiss, magnetic-field-aligned electron beams, and currents associated with a standing Alfven wave excited by the moon. Ray path analyses of the auroral hiss show that the electron beams responsible for the emissions are accelerated very close to the moon, most likely by parallel electric fields associated with the Alfven wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered. Citation: Gurnett, D. A., et al. (2011), Auroral hiss, electron beams and standing Alfven wave currents near Saturn's moon Enceladus, Geophys. Res. Lett., 38, L06102, doi:10.1029/2011GL046854.

Whistler mode auroral hiss emissions observed near Jupiter's moon Io

On 16 October 2001, the Galileo spacecraft made a close flyby of Jupiter's moon Io. During the flyby the plasma wave instrument detected an electric field emission with spectral characteristics very similar to a type of whistler‐mode noise called “auroral hiss” that is commonly observed in Earth's auroral region. This paper gives a detailed analysis of the “auroral hiss” observed near Io. The frequency‐time spectrum of the emission has a sharp high‐frequency cutoff near the electron cyclotron frequency and a V‐shaped low‐frequency cutoff. On a frequency‐time spectrogram these cutoffs give the emission a characteristic funnel shape that is very similar to the spectrum of terrestrial auroral hiss. Strong magnetic field perturbations occurred near the vertex of the funnel indicating the presence of a field‐aligned current. To explain the origin of the emission, a brief review is given of whistler‐mode wave propagation and the unipolar inductor model of Io's interaction with the magnetosphere of Jupiter. Assuming propagation near the whistler‐mode resonance cone, ray‐tracing analyses show that the radiation originates from a source very close to the surface of Io. The source is located in the same region where field‐aligned currents are believed to originate in the ionosphere of Io. Since terrestrial auroral hiss is known to be produced by beams of low‐energy auroral electrons, these observations suggest that the auroral hiss at Io is generated by an electron beam that is part of the field‐aligned current system induced by the interaction of Io with the rapidly rotating magnetosphere of Jupiter.

Generation mechanism of electrostatic solitary structures in the Earth's auroral region

Satellite observations have revealed electrostatic solitary structures in the Earth's auroral region. These structures have positive electrostatic potentials and move along the ambient magnetic field. In this paper we performed one‐dimensional electrostatic particle simulations of electrostatic solitary waves (ESW) in plasma composed of three electron components: cold, hot, and beam electrons. First, the nearly monochromatic electrostatic acoustic waves are excited. When the amplitude of the electron acoustic (EA) waves is sufficiently large, part of hot and beam electrons are trapped by the electron acoustic waves. These waves coalesce each other during their nonlinear evolution, and at last the solitary structures with travel speed related to the beam velocity are formed at the quasi‐equilibrium stage. These structures have positive potential signatures, and they seem to be stable. Electron density cavities for cold electron component are always accompanied with these structures. In addition, the corresponding electric fields have a bipolar structure, which has also been observed in the Earth's auroral region recently. The conditions for existence of such solitary structures are investigated through our simulations, and the comparisons between our simulated results and satellite observations in the Earth's auroral regions are also discussed.

Nonlinear effects associated with dispersive Alfvén waves in plasmas

Large amplitude Alfvén waves are frequently found in magnetized space and laboratory plasmas. Our objective here is to discuss the linear and nonlinear properties of dispersive Alfvén waves (DAWs) in a uniform magnetoplasma. We first consider finite frequency (ω/ωci) and ion gyroradius effects on inertial and kinetic Alfvén waves, where ωci is the ion gyrofrequency. Next, we focus on nonlinear effects caused by DAWs. Such effects include plasma density enhancement and depression by the Alfvén wave ponderomotive force, electron Joule heating by the thermal Alfvén wave force, the generation of zonal flows due to the shear Alfvén wave mode couplings as well as the formation of localized Alfvénic structures and Alfvénic vortices. The relevance of our investigation to the appearance of nonlinear Alfvén waves in the Earth's auroral acceleration region, in the solar corona and in the large plasma device at UCLA is discussed.

The position of the auroral energetic electron precipitation region obtained from PIXIE global X-ray observations

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on NASA's POLAR spacecraft has been making global observations of X-rays from the Earth's auroral regions since March, 1996. PIXIE is a remote-sensing global monitor of multi-keV electrons which have precipitated into the ionosphere from the magnetosphere. This study examines nine months (May, 1997 - January, 1998) of X-ray observations from the PIXIE instrument of the northern auroral oval. X-ray flux is obtained from the PIXIE observations, allowing us to determine a synoptic auroral precipitation pattern of energetic particles. In addition, we examine the mean magnetic latitude at ionospheric altitude of the detected X-rays as a function of geomagnetic indices.

Laboratory experiments on shear Alfvén waves and their relationship to space plasmas

Alfvén waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfvén waves radiated from sources with cross‐field scale size of the order of the electron inertial length, δ = c/ωpe, have properties which differ considerably from planar magnetohydrodynamic waves. Currents of cross‐field size, δ, are common in space plasmas. A series of experiments in the Large Plasma Device at the University of California, Los Angeles is presented which illustrates that waves generated by small‐scale fluctuating currents have a parallel electric field and radiate across magnetic field lines. Data and theory are presented for varying plasma collisionality and ratio of Alfvén speed to electron thermal velocity. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. The shear Alfvén wave is important in the Earth's auroral regions and may play a role in auroral dynamics.

Spatial modulation of electron energy and density by nonlinear stationary inertial Alfvén waves

A cold, nonlinear, quasi‐neutral, two‐fluid model of field‐aligned current (FAC) sheets with a uniform tangential electric field describes stationary, spatially periodic electromagnetic perturbations (stationary inertial Alfvén (SIA) waves) which carry enhanced current, which can accelerate electrons parallel to a strong background magnetic field B0, and which cause large variations in plasma density. The waves propagate along B0 at speeds less than the Alfvén speed VA. Waves can be divided into two branches according to the sign of their field‐aligned phase velocity with respect to the direction of field‐aligned electron drift. Waves propagating parallel to electron drift can accelerate low‐energy electrons up to VA within steady state parallel sheets with widths of the order of the electron inertial length λe = c/ωpe, where ωpe is the electron plasma frequency. Waves propagating antiparallel to electron drift can accelerate electrons to velocities much greater than VA in steady state sheets which are orders of magnitude wider than λe and which are depleted in density. SIA waves also form when only an energetic subpopulation of electrons carries FAC through an ambient population initially at rest and can accelerate the ambient electrons to approach the energy of the subpopulation. These results extend the phenomenon of steady state electron acceleration by inertial Alfvén waves to include much higher energies and wider spatial scales than previously recognized. Although the relatively simple model described here omits certain effects which are known to be important in the Earth's auroral regions, several properties of SIA wave solutions suggest they may play a role in auroral arc formation, including steady state field‐aligned electron acceleration over a broad range of spatial scales and energies, density variation, and the formation of multiple parallel structures.