Auroral Plasma Research Articles

Formation of substorm Pi2: A coherent response to auroral streamers and currents

Pi2 driving has been the subject of long standing debate involving magnetosphere‐ionosphere coupling. Using the THEMIS ground‐based all‐sky imager array, we have examined auroral disturbances evolving in the Pi2 frequency range to help identify where the Pi2 signal propagates from the magnetosphere toward the ionosphere. We demonstrate that expansion‐phase intensifications near the poleward edge of the auroral bulge occur quasiperiodically with one‐to‐one correspondence to Pi2 pulses, neither being strictly periodic. The expansion‐phase intensifications propagate toward the equatorward boundary of the auroral oval, and then often turn azimuthally. Based on the known relation between auroral streamers and plasma sheet flow bursts, the presence of the quasiperiodic streamers indicates that substorm Pi2s are driven by multiple plasma sheet flow bursts, each driving a Pi2 pulse. While it is difficult to tell precisely when auroral zone Pi2 starts because of the modest onset‐related negative bays, we found that midlatitude‐equatorial Pi2 tends to start a few minutes after initial substorm auroral onset, and that each Pi2 pulse starts to rise simultaneously over a wide latitude range from the auroral zone to the magnetic dip equator as well as over a wide longitudinal range. The observations furthermore show that auroral zone Pi2 near the streamer meridian is anti‐correlated with midlatitude‐equatorial Pi2. These features can be explained by an oscillating current wedge, whose upward field‐aligned current (FAC) corresponds to the auroral streamers. The midlatitude‐equatorial Pi2 is likely caused by the FACs of the current wedge, and the oscillating westward electrojet creates the auroral zone Pi2.

Interpretation of vector electric field measurements of bursty Langmuir waves in the cusp

An analysis of auroral Langmuir waves in the cusp observed by two high‐frequency electric field instruments on the TRICE high‐flyer sounding rocket shows many examples of Langmuir wave bursts modulated at approximately 10 kHz. Previous studies have explained these and similar observations as the result of beating between waves with very close frequencies near the Langmuir cutoff, resulting from either wave‐wave interactions or independent linear excitations. The unique three‐dimensional (3‐D) data set provided by the NASA Goddard Space Flight Center TAEFWD instrument shows that up to 25% of waveforms selected from the most intense bursts exhibit anisotropic modulations, i.e., the beat nulls and peaks are not aligned in time across the three perpendicular electric field components. Modulations of this type arise when superposed wave normal modes possess differing polarizations, and simulations using wave modes calculated with the J‐WHAMP numerical dispersion code show (1) that waves with differing polarizations can exist in conditions like those observed by TRICE‐High and (2) that superpositions of such waves can produce anisotropic modulation. Fourier analysis of the 3‐D waveform data suggests that both linear and elliptically polarized waves are present near the Langmuir cutoff at these times. These observations illustrate how 3‐D measurements can give valuable insight into the nature of wave interactions in the auroral plasma environment.

Contribution of Higher-Order Nonlinearity to obliquely electron-acoustic solitary waves in a magnetized auroral zone plasma

Using the Viking Satellite observations data in the dayside auroral zone, a theoretical investigation is carried out for contribution of the higher-order nonlinearity to nonlinear obliquely electron-acoustic solitary waves (EASWs) in a magnetized collisionless plasma consisting of a cold electron fluid and non-thermal hot electrons obeying a non-thermal distribution, and stationary ions. A Zakharov–Kuznetsov (ZK) equation that contains the lowest-order nonlinearity and dispersion is derived from the lowest order of perturbation and a linear inhomogeneous (ZK-type) equation that accounts for the higher-order nonlinearity and dispersion is obtained. A stationary solution for equations resulting from higher-order perturbation theory has been found using the renormalization method. The effects of the external magnetic field and the obliqueness are found to significantly change the higher-order properties (viz. the amplitude, width, electric field and energy) of the EASWs. The effect of higher-order nonlinearity on the amplitude and width of the soliton are also discussed. A comparison with the Viking Satellite observations in the dayside auroral zone are taken into account.

Solitons and double-layers of electron-acoustic waves in magnetized plasma; an application to auroral zone plasma

A theoretical investigation is carried out for understanding the properties of electron-acoustic potential structures (i.e., solitary waves and double-layers) in a magnetized plasma whose constituents are a cold magnetized electron fluid, hot electrons obeying a nonthermal distribution, and stationary ions. For this purpose, the hydrodynamic equations for the cold magnetized electron fluid, nonthermal electron density distribution, and the Poisson equation are used to derive the corresponding nonlinear evolution equation; modified Zakharov–Kuznetsov (MZK) equation, in the small amplitude regime. The MZK equation is analyzed to examine the existence regions of the solitary pulses and double-layers. It is found that rarefactive electron-acoustic solitary waves and double-layers strongly depend on the density and temperature ratios of the hot-to-cold electron species as well as the nonthermal electron parameter.

Alfvén: magnetosphere—ionosphere connection explorers

The aurorae are dynamic, luminous displays that grace the night skies of Earth’s high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth’s atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The “ideal magnetohydrodynamics” description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfvén concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets.

Numerical simulation of dispersive Alfvén wave turbulence in auroral plasmas affecting the Earth's communications

This paper presents the numerical simulation of the model equation governing the nonlinear dynamics of dispersive Alfvén waves (DAWs) in low-β plasmas. When the nonlinearity arises due to the ponderomotive force and Joule heating-driven density perturbations, the model equation turns out to be a modified nonlinear Schrödinger (MNLS) equation. The nonlinear MNLS is used to investigate the evolution of a finite-amplitude localized initial perturbation. The initial perturbation is of the form of a sinusoidal pulse on a DAW having a Gaussian wavefront. The localized structures of DAWs with sidebands are obtained. Also, we have studied the power spectra of magnetic fluctuations with different scaling laws. These turbulent structures may be responsible for particle acceleration in the aurora region and may affect the Earth's communication.

Auroral Formation and Plasma Interaction Between Magnetized Objects Simulated With the Planeterrella

The Planeterrella is a space plasma simulator, based on Kristian Birkeland's historical experiment, the “Terrella.” This device not only makes it possible to simulate interactions between an electrode and a magnetized sphere in many different geometries but also to simulate interactions between two magnetized spheres. Such configurations allow the visualization of phenomena unknown to Birkeland, such as an emitting body (Io) immersed in a magnetosphere (Jupiter) or the aurora on the night side of a planet where one magnetic pole points toward the Sun (Uranus).

Effect of the presence of excess superthermal hot electrons on electron-acoustic solitary waves in auroral zone plasma

A theoretical investigation is carried out for the nonlinear properties of small amplitude electron acoustic solitary waves (EAWs) in an unmagnetized collisionless plasma consisting of a cold electron fluid and hot electrons obeying κ velocity distribution, and stationary ions. The Korteweg de Vries (KdV) equation that contains the lowest-order nonlinearity and dispersion is derived from the lowest order of perturbation and a linear inhomogeneous (KdV-type) equation that accounts for the higher-order nonlinearity and dispersion is obtained. A stationary solution for equations resulting from higher-order perturbation theory has been found using the renormalization method. The effects of the spectral index κ and the higher-order corrections are found to significantly change the properties (viz. the amplitude, width, electric field ) of the EASWs. A comparison with the Viking Satellite observations in the dayside auroral zone are also discussed.

Volume cross section of auroral radar backscatter and RMS plasma fluctuations inferred from coherent and incoherent scatter data: a response on backscatter volume parameters

Abstract. Norway and Finland STARE radar measurements in the eastward auroral electrojet are combined with EISCAT CP-1 measurements of the electron density and electric field vector in the common scattering volume to investigate the variation of the auroral radar volume cross section (VCS) with the flow angle of observations (radar look direction with respect to the E×B electron drift). The data set available consists of ~6000 points for flow angles of 40–85° and electron drifts between 500 and 2000 m s−1. The EISCAT electron density N(h)-profile data are used to estimate the effective electron density, aspect angle and thickness of the backscattering layer. It is shown that the flow angle variation of the VCS is rather weak, only ~5 dB within the range of the considered flow angles. The VCS values themselves respond almost linearly to the square of both the electron drift velocity magnitude and the effective electron density. By adopting the inferred shape of the VCS variation with the flow angle and the VCS dependence upon wavelength, the relative amplitude of electrostatic electron density fluctuations over all scales is estimated. Inferred values of 2–4 percent react nearly linearly to the electron drift velocity in the range of 500–1000 m s−1 but the rate of increase slows down at electron drifts >1000 m s−1 and density fluctuations of ~5.5 percent due to, perhaps, progressively growing nonlinear wave losses.

Effect of nonthermal electrons on the propagation characteristics and stability of two-dimensional nonlinear electrostatic coherent structures in relativistic electron positron ion plasmas

Two-dimensional propagation of nonlinear ion acoustic shock and solitary waves in an unmagnetized plasma consisting of nonthermal electrons, Boltzmannian positrons, and singly charged hot ions streaming with relativistic velocities are investigated. The system of fluid equations is reduced to Kadomtsev-Petviashvili-Burgers and Kadomtsev-Petviashvili (KP) equations in the limit of small amplitude perturbation. The dependence of the ion acoustic shock and solitary waves on various plasma parameters are explored in detail. Interestingly, it is observed that increasing the nonthermal electron population increases the wave dispersion which enervates the strength of the ion acoustic shock wave; however, the same effect leads to an enhancement of the soliton amplitude due to the absence of dissipation in the KP equation. The present investigation may be useful to understand the two-dimensional propagation characteristics of small but finite amplitude localized shock and solitary structures in planetary magnetospheres and auroral plasmas where nonthermal populations of electrons have been observed by several satellite missions.

Cavitation by Nonlinear Interaction Between Inertial Alfvén Waves and Magnetosonic Waves in Low Beta Plasmas

This paper presents the model equations governing the nonlinear interaction between dispersive Alfven wave (DAW) and magnetosonic wave in the low-β plasmas (β≪me/mi; known as inertial Alfven waves (IAWs); here \(\upbeta = 8\pi n_{0}T /B_{0}^{2}\) is thermal to magnetic pressure, n0 is unperturbed plasma number density, T(=Te≈Ti) represents the plasma temperature, and me(mi) is the mass of electron (ion)). This nonlinear dynamical system may be considered as the modified Zakharov system of equations (MZSE). These model equations are solved numerically by using a pseudo-spectral method to study the nonlinear evolution of density cavities driven by IAW. We observed the nonlinear evolution of IAW magnetic field structures having chaotic behavior accompanied by density cavities associated with the magnetosonic wave. The relevance of these investigations to low-β plasmas in solar corona and auroral ionospheric plasmas has been pointed out. For the auroral ionosphere, we observed the density fluctuations of ∼ 0.07n0, consistent with the FAST observation reported by Chaston et al. (Phys. Scr.T84, 64, 2000). The heating of the solar corona observed by Yohkoh and SOHO may be produced by the coupling of IAW and magnetosonic wave via filamentation process as discussed here.

Storm-enhanced plasma density (SED) features, auroral and polar plasma enhancements, and rising topside bubbles of the 31 March 2001 superstorm

[1] This study focuses on the 31 March 2001 superstorm's evening and nighttime sectors. We specified forward fountain circulation and equatorial ionization anomaly (EIA), storm-enhanced density (SED) plume plasma flows, auroral zones, polar cap regions, and traveling ionospheric disturbances (TIDs). These permitted analyzing low-latitude plasma structures, and middle- and high-latitude plasma buildups. Results reveal significant low-latitude plasma structuring around Ancon due to topside rising bubbles during the initial phase under disturbance conditions. The deep equatorial plasma depletion, appearing over the Pacific during the main phase, is an EIA-trough-bubble structure. Topside bubbles and TID-related drift perturbations structured SED plume plasma. Plasma distribution maximized when TIDs were in phase with fountain circulation and SED plume plasma flows, and equatorward wind surges maintained high plasma densities. Plasma density and plasma flow measurements demonstrate the SED plume plasma's poleward migration through the auroral zone into the polar region and the maintenance and increase of auroral and polar enhancements forming a polar tongue of ionization (TOI). Our results contradict previous studies on this superstorm reporting the suppression of bubble formation over Ancon and the absence of TOI and explaining the development of equatorial depletion with no E × B drift action. Our results oppose the current hypothesis that plasma enhancements are transported over many hours into the polar region. Indeed, SED plume plasma (produced by plasmaspheric detachment processes) creates multiple downward plasma flows (stretching from the midlatitude trough's equatorward edge to the polar cap region) that in turn create plasma buildups at various latitudes where flow stagnates.

Mesoscale PIC simulation of double layers and electron holes affecting parallel and transverse accelerations of electrons and ions

[1] We study the dynamical behavior of potential structures in the auroral upward current region. The study is based on a large two-dimensional particle-in-cell simulation. A double layer (DL) forms in the auroral potential structure in the counterstreaming expansion of cold and hot plasmas from bottom (ionospheric side) and top (magnetospheric side), respectively. Transversely nonuniform converging perpendicular electric field drives the plasma from the top, generating V-shaped potential structure within the expanding plasmas. The dynamical features include (1) recurring formation of the DL, (2) downward motion of the DL over the distance of thousands of Debye lengths, (3) collapse of the existing double layer after the reformation of a new one near the top in the hot magnetospheric plasma, and (4) generation of electron holes (EHs) on the high-potential side of the DL and their downward propagation over a few thousand of Debye lengths, where they are dissipated. The EHs are associated with broadband plasma waves with spectrum peaked near the lower hybrid frequency. The EH turbulence is temporally modulated by low-frequency plasma waves near the ion cyclotron frequency Ωi. Electrostatic ion cyclotron waves above Ωi occur on the low-potential side of the DL. Transverse and parallel acceleration/heating of both electrons and ions are studied. The fast moving electron holes are found effective in transverse heating of the cold ionospheric electrons trapped below the DL. Relevance of our results to satellite observations in the auroral plasma is discussed.

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.

Theoretical constraints on the generation mechanism of auroral medium frequency burst radio emissions

[1] Auroral medium frequency (MF) burst is an impulsive auroral radio emission observed at ground level between ∼1.3 and 4.5 MHz for a few minutes at substorm onsets. Despite multiple suggested theories, the exact generation mechanism for MF burst remains unknown. The foremost theory suggests that MF burst originates from Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside or topside F region, which linearly mode convert to the electromagnetic LO mode, gaining access to ground level. Analysis of plasmas with two electron components implies the existence of additional normal modes, commonly referred to as the electron acoustic (EA) and electron cyclotron sound (ECS) modes, which have been suggested to potentially play a role in the generation of MF burst. Analytical calculations using Waves in Homogeneous, Anisotropic Multicomponent Plasmas, or WHAMP, and other dispersion-solving calculations are applied to applicable ionospheric normal modes (RX, LO, Langmuir or upper hybrid, EA, and ECS), compared to previous results, and applied to MF burst and the auroral plasma environment. Computation of resonant parallel energies suggests that, in the presence of an auroral electron distribution, instability of EA, ECS, LX, and Langmuir or upper hybrid modes is possible. The same analysis suggests that LO and RX modes are unlikely to be directly excited. Excitation of the EA or ECS mode in the frequency range applicable to MF burst is found to be more favorable for higher ionospheric plasma densities (fpe/fce > 2.36) and higher secondary electron temperatures (Th ≈ 100s of eV). Growth rate calculations suggest that growth of the EA or ECS mode is excitable by the auroral electron beam with higher growth rates for the ECS mode.

Radar detection of a localized 1.4 Hz pulsation in auroral plasma, simultaneous with pulsating optical emissions, during a substorm

Abstract. Many pulsating phenomena are associated with the auroral substorm. It has been considered that some of these phenomena involve kilometer-scale Alfvén waves coupling the magnetosphere and ionosphere. Electric field oscillations at the altitude of the ionosphere are a signature of such wave activity that could distinguish it from other sources of auroral particle precipitation, which may be simply tracers of magnetospheric activity. Therefore, a ground based diagnostic of kilometer-scale oscillating electric fields would be a valuable tool in the study of pulsations and the auroral substorm. In this study we attempt to develop such a tool in the Poker Flat incoherent scatter radar (PFISR). The central result is a statistically significant detection of a 1.4 Hz electric field oscillation associated with a similar oscillating optical emission, during the recovery phase of a substorm. The optical emissions also contain a bright, lower frequency (0.2 Hz) pulsation that does not show up in the radar backscatter. The fact that higher frequency oscillations are detected by the radar, whereas the bright, lower frequency optical pulsation is not detected by the radar, serves to strengthen a theoretical argument that the radar is sensitive to oscillating electric fields, but not to oscillating particle precipitation. Although it is difficult to make conclusions as to the physical mechanism, we do not find evidence for a plane-wave-like Alfvén wave; the detected structure is evident in only two of five adjacent beams. We emphasize that this is a new application for ISR, and that corroborating results are needed.

Nonlinear dust acoustic waves in a charge varying dusty plasma with suprathermal electrons

Arbitrary amplitude dust acoustic waves in a dusty plasma with a high-energy-tail electron distribution are investigated. The effects of charge variation and electron deviation from the Boltzmann distribution on the dust acoustic soliton are then considered. The dust charge variation makes the dust acoustic soliton more spiky. The dust grain surface collects less electrons as the latter evolves far away from their thermodynamic equilibrium. The dust accumulation caused by a balance of the electrostatic forces acting on the dust grains is more effective for lower values of the electron spectral index. Under certain conditions, the dust charge fluctuation may provide an alternate physical mechanism causing anomalous dissipation, the strength of which becomes important and may prevail over that of dispersion as the suprathermal character of the plasma becomes important. Our results may explain the strong spiky waveforms observed in auroral plasmas.

Ion-beam driven dust ion-acoustic solitary waves in dusty plasmas

The nonlinear propagation of small but finite amplitude dust ion-acoustic waves (DIAWs) in an ion-beam driven plasma consisting of Boltzmannian electrons, positive ions, and stationary negatively charged dust grains is studied by using the standard reductive perturbation technique. It is shown that there exist two critical values (γc1 and γc2) of ion beam to ion phase velocity ratio (γ), above and below which the beam generated solitons are not possible. The effects of the parameters, namely, γ, the ratio of the ion beam to plasma ion density (μi), the dust to ion density ratio (μd), and the ion beam to plasma ion mass ratio (μ) on both the amplitude and width of the stationary DIAWs, are analyzed numerically, and applications of the results to laboratory ion beam as well as space plasmas (e.g., auroral plasmas) are explained.

Dynamic variability in F-region ionospheric composition at auroral arc boundaries

Abstract. The work presents a data-model synthesis examining the response of the auroral F-region ion temperature, composition, and density to short time scale (<1 min) electric field disturbances associated with auroral arcs. Ion temperature profiles recorded by the Sondrestrom incoherent scatter radar (ISR) are critically analyzed with the aid of theoretical calculations to infer ion composition variability. The analyses presented include a partial accounting for the effects of neutral winds on frictional heating and show promise as the groundwork for future attempts to address ion temperature-mass ambiguities in short-integration ISR data sets. Results indicate that large NO+ enchancements in the F-region can occur in as little as 20 s in response to impulsive changes in ion frictional heating. Enhancements in molecular ion density result in recombination and a depletion in plasma, which is shown to occur on time scales of several minutes. This depletion process, thus, appears to be of comparable importance to electrodynamic evacuation processes in producing auroral arc-related plasma depletions. Furthermore, the potential of ionospheric composition in regulating the amounts and types of ions supplied to the magnetosphere is outlined.

Nonlinear low-frequency structures in the auroral plasma in the presence of an oxygen beam including charge separation

Observations from the Fast Auroral SnapshoT (FAST) satellite indicate that the parallel and perpendicular (to the Earth’s magnetic field) electric field structures exhibit a spiky appearance. In this study, a magnetized plasma system consisting of protons, electrons, and a cold oxygen ion beam is considered. Both background electrons and protons are treated as hot species with Boltzmann density distributions. The dynamics of the oxygen ion beam is governed by the fluid equations. Effect of charge separation is studied on nonlinear fluctuations arising from a coupling of ion cyclotron and ion-acoustic waves. A scan of parameter space reveals a range of solutions for the parallel electric field from sinusoidal to sawtooth to highly spiky waveforms. The inclusion of charge separation effects tends to in most cases increase the frequency of oscillation of the nonlinear structures. In the case of a weakly magnetized plasma, the amplitude of the oscillations are found to be constant while they are modulated for a strongly magnetized plasma. The findings are compared with satellite observations.