Auroral Electron Acceleration Research Articles

Turbulent acceleration of auroral electrons

It is shown that the characteristic peak in the auroral electron velocity distribution can be generated stochastically through resonant interactions with lower hybrid electrostatic turbulence. The peak itself is shown to be a direct consequence of restrictions imposed on reflection of electron velocities in the frame of reference of individual wave packets by the limitation in group velocity. A Monte‐Carlo model demonstrates how the various properties of the acceleration region are reflected in the resultant electron distribution. It is shown, in particular, that the width of the peak is governed by the amplitude of the turbulence, while the amplitude of the peak reflects the column density of wave energy. Electron distributions encountered within three auroral arcs are interpreted to yield order of magnitude estimates of the amplitude and rms electric field of lower hybrid wave packets. The velocities and frequencies of the resonant waves, the net electric field, the column density of wave energy, and the electric‐field energy density are also estimated. The results are found to be consistent with available electric‐field measurements. A general broadening of the electron distribution caused by less systematic interactions between electrons and wave packets is shown to have a negligible effect on the peak resulting from the reflection process; it does, though, lead to the creation of a characteristic high‐energy tail.

Auroral electron acceleration

Two theories of auroral electron acceleration are discussed. Section 1 examines the currently widely held view that the acceleration is an ordered process in a quasi-static electric field. It is suggested that, although there are many factors seeming to support this theory, the major qualifications and uncertainties that have been identified combine to cast serious doubt over its validity. Section 2 is devoted to a relatively new interpretation in terms of stochastic acceleration in turbulent electric fields. This second theory, which appears to account readily for most known features of the electron distribution function, is considered to provide a more promising approach to this central question in magnetospheric plasma physics.

Observations of auroral zone processes by the Viking satellite

The scientific results of the Viking project obtained up to the spring of 1988 are reviewed. During solar minimum conditions, when Viking was operated, the dayside auroral oval has been found to be the most active part, except during strong substorms and storms. A number of new auroral morphological features have been seen with the imaging experiment onboard Viking. Large-amplitude slow fluctuations of the electric field heat the ionospheric plasma and pump up the magnetic moment of the ionospheric ions so that they may leave the ionosphere. These fluctuations also accelerate ionospheric electrons upwards along the magnetic field lines. The importance of the acceleration of auroral electrons into the atmosphere by magnetic field-aligned potential differences has been confirmed. The first satellite-borne plasma wave interferometer on Viking has made it possible to determine a number of characteristics of the ‘weak’ double layers, seen first by the S3-3 satellite. A large number of these along the magnetic field lines produce large electric potential differences. Many new results concerning wave-particle interactions have been obtained, of which a few are presented here.

Linear stability of the H+ ‐ O+ two‐stream interaction in a magnetized plasma

The H+ ‐ O+ two‐stream instability in a magnetized plasma is examined for application to ion beam heating in the auroral electron acceleration region. In the fluid limit (zero ion temperature) it is found that the most unstable mode coupling is between the obliquely propagating fast O+ and slow H+ cyclotron modes, even for relative ion beam velocities where parallel propagating modes are unstable. The most unstable mode has properties analogous to those of a beam‐plasma instability. Once a relative velocity is reached where obliquely propagating modes dominate, the growth rate becomes independent of the perpendicular wave vector and so there is no particular angle of maximum wave growth. Upon adding a finite ion temperature it is found that the gross features of the H+ ‐ O+ fluid interaction are retained but finite gyroradius effects cause the appearance of an angle of maximum growth. The tangent of this angle is proportional to the velocity difference between the ion beams, for large enough relative velocity, and is also a function of the H+ to O+ density ratio. The qualitative behavior of the maximum growth rate and its frequency and wave vector versus relative velocity and density ratio are examined. The implications of this behavior for H+ and O+ beam heating in the auroral electron acceleration region are discussed.

Particle simulation of plasma processes in the earth's magnetotail

An electromagnetic code in the Darwin approximation is used to model process in the current sheet of the Earth's magnetotail that may lead to the generation of electrostatic field responsible for auroral electron acceleration. It is shown that the Hamiltonian formulation has advantages over the more commonly used Lagrangian formulation of the Darwin approximation. This paper describes the specific techniques that are needed, to include the magnetic field normal to the current sheet, the presence of a convection electric field and the continuous inflow of particles from the lobe. Simulations incorporating these new features reveal new effects that maybe of significant importance to the understanding of auroral and geomagnetic storm phenomena.

Energy dependent pitch angle distributions of auroral primary electrons

The acceleration of auroral primary electrons by dc parallel electric field is often questioned on the ground that the pitch angle distribution of the primary electrons shows a complex behavior; the relatively low energy electrons are found to be field-aligned while the relatively energetic ones are found to be “isotropized” over all pitch angles directed into the atmosphere. In the central regions of arcs or potential structures the field-aligned and the “isotropized” components are found simultaneously. On the other hand, near the arc edges, where electrons have relatively low energies, the field-aligned distributions prevail. It is shown here that double-layer/parallel-electric field acceleration and the subsequent electron-beam plasma interactions involving Cerenkov and anomalous cyclotron resonances yield pitch angle distributions as observed from rockets and satellites. It is highlighted that the electron acceleration by weak parallel electric fields forming a runaway electron tail is limited to a critical parallel energy determined by the anomalous cyclotron resonance. On the other hand, with acceleration by a localized parallel electric field, like that in a double layer, such a limitation does not occur. The beam-plasma interactions after the acceleration by a strong double layer reshape the electron distribution function. During the early stage of the interaction an extended plateau forms. The electrons with parallel energies above the critical energy undergo anomalous cyclotron resonance and thereby their parallel energy is partly transferred to their perpendicular energy. This causes the plateau to shrink towards the critical energy, and also a pitch angle scattering of the electrons leading towards isotropization. The extent of isotropization depends on the time that the electrons spend in the interaction zone. The electrons having parallel energies less than the critical energy are likely to remain field aligned.

Energy spectra and pitch angle distributions of auroral electrons observed in active and quiet auroras.

Energy spectra and pitch angle distributions of auroral electrons were obtained by means of two sounding rockets launched respectively into stable auroral arcs in geomagnetically quiet condition, and into active auroral arcs during the expansion phase of a substorm. These rockets were launched from Syowa Station, Antarctica in the austral winter in 1985.It was found that the observed energy spectra of downward electrons show a plateau extending from about 1keV to several keV in the center of an auroral arc, while they have a mono-energetic peak at the edge, or just outside an auroral arc. This feature suggests that strong wave-particle interaction processes are occurring on auroral field lines.The pitch angle distribution of auroral electrons at the mono-energetic peak was isotropic in the range from 0° to 90° (0° is upward along the magnetic field line), and was almost isotropic in the energy below the peak. The former feature suggests acceleration of auroral electrons at high altitude, and the latter feature is attributable to secondary electrons produced by auroral electrons.Just outside an auroral arc, the peak energy decreased with the distance from the arc. This feature can be considered to be an inverted-V structure of precipitating electrons observed at the rocket altitude.

Some aspects of double layer formation in a plasma constrained by a magnetic mirror

The discussion of parallel electric fields in the earth's magnetosphere has undergone a notable shift of emphasis in recent years, away from wave-generated anomalous resistivity towards the more large-scale effects of magnetic confinement of current carrying plasmas. This shift has been inspired in large part by the more extensive data on auroral particle distribution functions that have been made available, data that may often seem consistent with a dissipation-free acceleration of auroral electrons over an extended altitude range.Efforts to interpret these data have brought new vigor to the concept that a smooth and static electric field can be self-consistently generated by suitable pitch-angle anisotropies among the high-altitude particle populations, different for electrons and ions, and that such an electric field is both necessary and sufficient to maintain the plasma in a quasi-neutral steady state. This paper reviews and criticizes certain aspects of this concept, both from a general theoretical standpoint and from the standpoint of what we know about the magnetospheric environment. It is argued that this concept has flaws and that the actual physical problem is considerably more complicated, requiring a more complex electric field, possibly including double layer structures.

Decay of electrostatic hydrogen cyclotron waves into ion acoustic modes on auroral field lines

The coherent three‐wave decay of a linearly unstable electrostatic hydrogen cyclotron (EHC) wave into stable EHC and ion acoustic modes is considered. Possible triads of parent and daughter waves are identified, and the coupling strength between the waves is calculated in two plasma models. The first model is the fluid limit of an electron‐hydrogen plasma, and the second is a three‐species kinetic model of the plasma in the evening auroral electron acceleration region. It is found that EHC waves commonly observed on auroral field lines are of sufficient amplitude to exceed the threshold for decay to weakly damped EHC modes. Further, frequency and wave number matching can be satisified only if the acoustic mode is oblique to the geomagnetic field. This decay is proposed to be a saturation mechanism for the linearly unstable coherent EHC mode. The temporal dependence of the amplitudes of the three interacting modes, as predicted by the four coupled wave kinetic equations, indicates that the coherent decay can act to saturate the parent wave. Further, it indicates that observed EHC waves may actually be linearly marginally stable daughter EHC modes.

Anomalous resistivity and AE‐D observations of auroral electron acceleration

Electron fluxes measured with high time resolution by AE‐D during narrow bursts of intense electron precipitation fluxes within inverted‐V events are compared with electron distributions obtained from particle simulations based on an anomalous resistivity model. The qualitative agreement between the AE‐D observations and the computer simulations is excellent. When the potential energy is small, both the AE‐D observations and the simulations indicate strongly field‐aligned electron beams; both also agree that the accelerated electrons undergo large pitch angle scattering when the potential drop increases above a certain critical value. Furthermore, the simulations show that electron distributions are isotropized progressively from low to high energy, in agreement with the AE‐D observation that the electron flux isotropization occurred first at lower energies. These results suggest that anomalous resistivity may play an important role in the acceleration of auroral particles.

Paired electrostatic shocks

Examination of 21 paired electrostatic shocks from a portion of the S3‐3 satellite data set has shown that in 20 cases the electric field in the equatorward half of the paired shock pointed poleward while the electric field in the poleward half of the paired shock pointed equatorward. At the same time the electron, ion, and wave data indicated the presence of an electric field parallel to the magnetic field in the region between the paired shocks. This parallel electric field was at an altitude below that of the satellite and pointed out of the ionosphere so as to accelerate ions upward. A comparison of the potential across the electrostatic shock and the energy of the corresponding ion beam at the energy of maximum flux gives reasonable agreement. This provides convincing evidence that the potential contours associated with paired electrostatic shocks close across the magnetic field to produce the auroral electron acceleration responsible for inverted‐V particle distributions and auroral arcs.

Auroral electron acceleration and atmospheric interactions: (1) Rocket‐borne observations and (2) Scattering calculations

A Terrier‐Malemute rocket was launched from Poker Flat, Alaska, on March 9, 1978, over a stable premidnight auroral arc. It carried instrumentation to measure electric and magnetic fields and particle fluxes. The electron intensity in 13 fixed energy intervals from 0.04 to 40 keV was measured over 180° of pitch angle every half spin at ∽2.5 rps. The intensity was generally isotropic over the upper hemisphere with the total energy flux and the spectral hardness greater in the visible arc than elsewhere. Field alignment of low‐energy electrons occurred at the edges of the arc, with higher energies involved toward the center, and in two bursts within the arc. The interaction of the measured downward intensity with the neutral atmosphere (below apogee altitude ∼320 km) was modeled by transport theory, including inelastic and multiple‐elastic scattering of electrons by neutrals. The upcoming intensity calculated there‐from agrees with the measured upcoming intensity so that no collective effects, including parallel electric fields, need be postulated. The downward intensity at high energy within the discrete arc was fit to a thermal plasma accelerated (at some high altitude) through a parallel potential drop V0 (Evans, 1974). Electrons with energy E ≤|eV0| ⩽6 keV are produced, in this model, by scattering on neutrals and specular reflection from the potential barrier above. The calculated downward intensity of electrons with E ⩽|eV0| fits the measured intensity well enough, in the arc, that no other interactions need be postulated. We infer that in this type of aurora the interaction of electrons with the atmosphere and ionosphere below several hundreds of kilometers is satisfactorily described by collisional scattering.

Artificial stimulation of auroral electron acceleration by intense field aligned currents

A cesium doped high explosion was detonated at 165 km altitude in the auroral ionosphere during quiet conditions. An Alfvén wave pulse with a 200 mV/m electric field was observed with the peak occurring 135 ms after the explosion at a distance of about 1 km. The count rate of fixed energy 2 keV electron detectors abruptly increased at 140 mspeaked at 415 ms and indicated a downward field aligned beam of accelerated electrons. An anomalously high field aligned beam of backscattered electrons was also detected. We interpret the acceleration as due to a production of an electrostatic shock or double layer between 300 and 800 km altitude. The structure was probably formed by an instability of the intense field aligned currents in the Alfvén wave launched by the charge separation electric field due to the explosion.

Relationship of noise in the frequency range 100 < f < 500 kHz to auroral arcs and field‐aligned current and implications regarding acceleration of auroral electrons

The magnetic signatures of field‐aligned and ionospheric currents can be used to determine the geometry and strength of the electrical current systems which couple the ionosphere to the outer regions of the earth's magnetosphere. It is now recognized that the currents flowing in the evening sector are an important clue to the understanding of processes leading to explosive substorm instabilities and episodes of intense ionospheric disruption. In this report we have used a combination of ground‐based magnetometer data and Isis 2 satellite data in the form of energetic particles, auroral luminosity, and electromagnetic and/or electrostatic noise measurements to provide a comprehensive picture of processes which are taking place on field lines penetrating the auroral oval in the evening and midnight sectors. We shall show that noise measurements in the frequency range 100 < f < 500 kHz are characteristic of regions of upward current flow which are found in the region of discrete auroral features. We find that the higher‐energy electrons (a few keV) are associated with the higher‐frequency noise. In addition, the highest noise frequencies are found in the center of discrete auroral arc regions with the frequency decreasing on either side of the center of the arc region. We believe that the source of noise in this frequency range is associated with electron beams in discrete arcs whose velocity space distribution contains a region of positive slope. We shall finally discuss the physics of the relationship among the various electromagnetic and energetic particle signatures presented in this study.

On the structure of auroral arcs: The results of numerical simulations

The subject of this paper is the structure of discrete auroral arcs and bands. The mechanism for acceleration of auroral electrons is taken to be the V‐shock structures characterized by electric fields oblique to the magnetic field. The starting point in the analysis is that the accelerating voltages are generated by electrostatic MHD turbulence. A numerical simulation is used to illustrate how the large potential differences, necessary for acceleration of auroral electrons, can be generated across magnetic fields by the turbulent cascade. It is shown that this generation mechanism requires a flow of enstrophy to short wavelengths as well as a flow of energy to long wavelengths. The most important result is that it is shown that the limitation of upward field‐aligned current by the V shocks can result in a clustering of precipitation regions and the imposition of a long‐range order on patterns of auroral precipitation. It is argued that this effect is likely responsible for the multiple, regular auroral bands observed during quiet conditions. When the auroral electric fields become sufficiently intense, turbulent cascade is initiated which breaks up the well‐ordered structure. It is concluded that the regions of energy input into the cascade must be loosely coupled to the region where the electrons are accelerated.

Mechanisms for the discrete aurora ? A review

The V-shock is identified as the primary mechanism for the acceleration of electrons responsible for the discrete aurora. A brief review of the evidence supporting the V-shock model is given, including the dynamics of auroral striations, anomalous motion of barium plasma at high altitudes and in-situ observations of large electric fields. The V-shock is a nonlinear, n = 0 ion cyclotron mode soliton, Doppler shifted to zero frequency. The V-shock is also shown to be a generalization of the one-dimensional double layer model, which is an ion acoustic soliton Doppler shifted to zero frequency. The essential difference between the double layer theory and the theory for the oblique, current-driven, laminar electrostatic shock is that the plasma dielectric constant in directions perpendicular to the magnetic field is c 2/V /2 , where V a is the Alfven velocity; but the plasma dielectric constant parallel to the magnetic field is unity. Otherwise, in the limit that the shock thickness perpendicular to the magnetic field is much larger than an ion gyroradius, the equations describing the double layer and the oblique shock are the same. The V-shock, while accounting for the acceleration of auroral electrons, requires an energy source and mechanism for generating large potential differences perpendicular to the magnetic field. An energy source is the earthward streaming protons coming from the distant magnetospheric tail. It is shown how these protons can be energized by the cross-tail electric field, which is the tailward extension of the polar cap dawn-to-dusk electric field. The local, large cross-field potential differences associated with the V-shock are seen to be the result of a non-linear, E × B drift turbulent cascade which transfers energy from small- to large-scale sizes. Energy at the smallest scale sizes comes from the kinetic energy in the ion cyclotron motion of the earthward streaming protons, which are unstable against the zero-frequency flute-mode instability. The review points out the gaps in our understanding of the mechanism of the diffuse aurora and the mechanism of the auroral substorm.

On the formation of electrostatic shock waves associated with auroral electron precipitation

Models of acceleration of auroral electrons by electrostatic shock waves are considered based on the model electron beam, calculated by Evans (1974), to account for the observed precipitating electron fluxes. Electron populations in our models include a primary accelerated beam, originating from the plasma sheet, the secondary electrons and the energy-degraded and backscattered primary electrons produced by precipitating electrons of that beam. We find a feasible electrostatic shock model with appropriate ion populations from considerations on the conditions for the existence of shock solutions.

Characteristics of auroral electron acceleration regions observed by Atmosphere Explorer C

Satellite measurements of electron precipitation and ion drift velocities showed that electron acceleration regions (or inverted V's) in the 1200 to 1800 MLT quadrant exhibit the following systematic behavior: electron distribution functions in the accelerated region can be well described by Maxwellian primary electron beams accelerated through an electrostatic potential; the typical inverted V latitudinal structure is always observed in the accelerated regions, the electrostatic potential reaching a maximum and consequently decreasing to near zero over distances of 100 to 250 km; the Maxwellian temperature of the primary electron beam increases systematically with increasing electrostatic potential; rather weak acceleration regions, characterized by values of the electrostatic potential below 1 keV and values of the Maxwellian temperature between 100 and 350 eV, occur in the cusp and in the highest-latitude portion of the dusk side electron precipitation zone.

Acceleration of auroral electrons in parallel electric fields

Rocket observations of auroral electrons are compared with the predictions of a number of theoretical acceleration mechanisms that involve an electric field parallel to the earth's magnetic field. The theoretical models are discussed in terms of required plasma sources, the location of the acceleration region, and properties of necessary wave-particle scattering mechanisms. We have been unable to find any steady state scatter-free electric field configuration that predicts electron flux distributions in agreement with the observations. The addition of a fluctuating electric field or wave-particle scattering several thousand kilometers above the rocket can modify the theoretical flux distributions so that they agree with measurements. The presence of very narrow energy peaks in the flux contours implies a characteristic temperature of several tens of electron volts or less for the source of field-aligned auroral electrons and a temperature of several hundred electron volts or less for the relatively isotropic 'monoenergetic' auroral electrons. The temperature of the field-aligned electrons is more representative of the magnetosheath or possibly the ionosphere as a source region than of the plasma sheet.

On the formation of auroral arcs and acceleration of auroral electrons

It is suggested that the highly structured auroral arc is caused by a current-driven laminar electrostatic shock oblique to the geomagnetic field. Electrons are accelerated by the potential jump associated with the shock. The shock is assumed to be confined to a plane. Self-consistent solutions to the Poisson-Vlasov systems are calculated for the electrostatic potential. Adiabatic theory is used to calculate the ion number density in terms of the electrostatic potential and its derivatives. The electrons are assumed to be highly magnetized so they can only move parallel to the magnetic field. Solutions are exhibited for two plasma models: (1) streaming electrons and a two-temperature distribution of ions and (2) streaming electrons and ions and thermal electrons and ions. In the latter model, solutions can be obtained for an arbitrary potential jump across the shock. The shock is identified with the linear electrostatic ion cyclotron wave, and stability of these waves is examined to determine conditions for the formation of oblique shocks. Finally, the theory is discussed in the context of the magnetosphere, and possible model shocks are exhibited and discussed in terms of auroral arc formation.