Auroral Electron Acceleration Research Articles

A magnetospheric generator driving ion and electron acceleration and electric currents in a discrete auroral arc observed by Cluster and DMSP

Simultaneous observations on April 28, 2001 by Cluster and DMSP‐F14 reveal a stable discrete auroral arc and fluxes of field‐aligned accelerated electrons and ions coincident with a magnetospheric plasma interface at an altitude of 4.5 RE in the dusk sector. We compare satellite data with a quasi‐stationary magnetosphere‐ionosphere coupling model based on a Vlasov solution for the magnetospheric generator. The model provides a self‐consistent magnetospheric electric potential matching the Cluster observations. The ionospheric potential is derived from the current continuity equation and gives a field‐aligned potential drop and a flux of precipitating energy in agreement with the DMSP data. Model results and data analysis suggest a quasi‐stationary field‐aligned acceleration of auroral electrons and ions with a magnetospheric generator. We associate the generator with the convergent perpendicular electric field at the interface of the plasma sheet boundary layer with the lobe or at the inner edge of the low latitude boundary layer.

Simulation of auroral electron acceleration by inertial Alfvén waves

A two‐dimensional particle code, similar to that used in the work of Swift (2007), is used to model the acceleration of auroral electrons by inertial Alfvén waves. The simulation domain is 20,000 km parallel to the magnetic field and some tens of kilometers across. Effects of a variable magnetic field and magnetic mirror force are included. The Alfvén wave is launched by a moving bipolar potential waveform with peak amplitudes of ±200 V imposed at the top of the simulation domain. Electrons are accelerated to 3 keV energy. Two types of acceleration processes are seen: one is impulsive acceleration from shock structures, and the other is from weaker parallel electric fields extending over several thousand kilometers. The shock structures are very small, and electrons accelerated by these structures can produce auroral forms with thicknesses considerably smaller than an electron inertial length. Electrons also tend to undergo multiple acceleration events. Electrons are more strongly accelerated upward. Half of these are destined to precipitate in the opposite hemisphere, and half are destined to become trapped. This implies that much of the aurora seen in the Northern Hemisphere is due to electrons accelerated in the Southern Hemisphere. The simulations also suggest that auroral acceleration processes provide a likely source for radiation belt particles.

Generation of short‐burst radiation through Alfvénic acceleration of auroral electrons

A comparison of modeled simulation results with in situ observations is presented in an effort to better understand the generation mechanism of short‐burst radiation (AKR) observed in the Alfvénic acceleration region. The simulation is performed using a linear gyrofluid Alfvén model to generate propagating inertial Alfvén waves with a test particle scheme to study the behavior of electrons under the influence of waves. The observations are taken from the wave and plasma instruments on board the Fast Auroral Snapshot (FAST) satellite. We find that single‐ and multiple‐electron shell distributions are produced as a result of parallel electric fields generated from inertial Alfvén waves. Electrons outside the loss cone are reflected back up the flux tube because of the mirror force forming electron conics. Distributions obtained from this simulation resemble the observed electron distributions in the Alfvénic auroral acceleration region where short‐burst radiations were present. Multiple‐shell distributions as well as electron conics are unstable, making them candidates for triggering the electron‐cyclotron maser instability. Hence we suggest that these unstable distributions, resulting from the effects of inertial Alfvén wave, could be the source of the observed short‐burst radiation in the terrestrial auroral zone magnetosphere‐ionosphere coupling region. We also briefly discuss the application of this general mechanism to explain the Io‐associated S‐bursts observed emanating from the Jovian magnetosphere.

Reply to Comments on “A turning point in auroral physics”

Haerendel's conviction (An Apparent Controversy in Auroral Physics, Eos, this issue) that a potential difference is responsible for the downward acceleration of auroral electrons is based on there being an upward acceleration of positive ions, caused apparently by a corresponding electric field. However, even if one accepts on face value this indication of an upward directed quasi‐static electric field, the inevitable presence of a downward directed field farther along the line, where the equipotentials must close and the ions are slowed down again, prevents the negative potential well having any net effect on the energy of either ions or electrons traversing it. The conviction is therefore unfounded.Stern (Origin of Auroral Arcs, Eos, this issue) is swayed to the same conclusion by the fact that in order to deliver the necessary power, electrons need to be forced through the magnetic mirror field. Again, though, even setting aside the fact that static fields cannot produce net acceleration, the conclusion overlooks the fact that any process leading to downward acceleration will accomplish the same feat. Similarly, his claim that a voltage drop has been shown to exist is false. The measurements in question were of electrons, not electric fields. They demonstrate that downward acceleration occurs but do not reveal the process responsible.

A two‐dimensional particle code simulation of inertial Alfvén waves and auroral electron acceleration

A two‐dimensional particle code used to simulate the propagation of inertial Alfvén waves down auroral field lines. The simulation domain is 20,000 km parallel to the magnetic field and some tens of kilometers perpendicular to the field. Electrons are tied to the magnetic field. Effects of a variable magnetic field and magnetic mirror force are included. A spatially narrow Alfvén pulse is launched by application of a variable potential with peak amplitude of 100 V at the top of the simulation domain. If the thickness of the pulse is less than the electron inertial length, the pulse tends to broaden as it propagates down the field line. The result is a parallel electric field considerably smaller than might be expected, assuming a wave whose perpendicular wavelength maps along magnetic field lines. In this model the strongest interaction between the electrons and the Alfvén wave occurs in the 3000 to 7000 km altitude range with an energy gain of the order 100 eV. The downward acceleration was strongest when the applied excitation consisted of a moving positive pulse followed by a negative pulse. Strongest upward acceleration was seen when the polarity was reversed. When the magnitude of the driving potential was doubled, there appeared to be some acceleration in the 9000 to 18,000 km altitude range and acceleration of some particles to 1.5 keV.

Particle-fluid simulation of the auroral current circuit

The incompatibility between stationary kinetic and dynamic fluid descriptions of auroral electron acceleration has been an outstanding problem in space physics for decades. In this study we introdu ...

Current-voltage and kinetic energy flux relations for relativistic field-aligned acceleration of auroral electrons

Abstract. Recent spectroscopic observations of Jupiter's "main oval" auroras indicate that the primary auroral electron beam is routinely accelerated to energies of ~100 keV, and sometimes to several hundred keV, thus approaching the relativistic regime. This suggests the need to re-examine the classic non-relativistic theory of auroral electron acceleration by field-aligned electric fields first derived by Knight (1973), and to extend it to cover relativistic situations. In this paper we examine this problem for the case in which the source population is an isotropic Maxwellian, as also assumed by Knight, and derive exact analytic expressions for the field-aligned current density (number flux) and kinetic energy flux of the accelerated population, for arbitrary initial electron temperature, acceleration potential, and field strength beneath the acceleration region. We examine the limiting behaviours of these expressions, their regimes of validity, and their implications for auroral acceleration in planetary magnetospheres (and like astrophysical systems). In particular, we show that for relativistic accelerating potentials, the current density increases as the square of the minimum potential, rather than linearly as in the non-relativistic regime, while the kinetic energy flux then increases as the cube of the potential, rather than as the square.

Auroral electron acceleration induced by Cherenkov damped waves

We discuss afresh problem of the auroral electron acceleration based on the controversial reports by Bryant et al. [1992. Double layers are not particle accelerators. Physical Review Letters 68(1), 37–39] and Borovsky [1992. Double layers do accelerate particles in the Auroral Zone. Physical Review Letters 69(7), 1054–1056] that refer to the mechanism, which is the most plausible to accelerate auroral electrons: dc electric field generated by double layers or wave–particle interaction due to wave turbulence? Here, we show that both mechanisms are important, and what is more likely to happen in auroras is that turbulence and double layers assist each other so as to synergetically accelerate electrons.

Auroral electron dispersion below inverted‐V energies: Resonant deceleration and acceleration by Alfvén waves

Rocket observations of electron time dispersion which occurs at energies below the primary inverted‐V electron precipitation exhibit a variety of characteristics in the dispersion timescale and flux intensity distribution. We present simulation results that illustrate how different dispersion signatures can be produced by resonant acceleration and deceleration of auroral electrons by inertial Alfvén waves. We investigate the individual effects of relevant parameters such as the altitude of the inverted‐V potential, Alfvén wave amplitude, wave velocity, and source electron population. The energies to which Alfvén waves can accelerate electrons are primarily determined by the peak Alfvén speed below the DC potential drop and to a lesser extent by the amplitude of the wave parallel electric field. The observed electron signatures can be used to obtain information about the peak Alfvén speed, the wave parallel electric field, the DC electric field, and plasma environment through which the waves propagate.

Influence of the ionosphere on the altitude of discrete auroral arcs

Abstract. The altitude of the maximum luminosity of single, discrete auroral arcs was measured by photometric triangulation from two stations (College and Fort Yukon, Alaska) located 226km apart on nearly the same magnetic meridian. The average height of the evening aurora decreases smoothly with increasing solar depression angle (sda) from 160km near 12° sda to 100km after 18° sda. The average height remains constant until around 12° sda in the morning. This diurnal variation is similar to that of the electron density in the F region of the ionosphere. Thus, the behavior is consistent with the concept that the mean auroral electron energy increases as the ionospheric conductivity decreases due to ionospheric recombination in the evening twilight. However, the mean electron energy decreases in magnitude at dawn when the solar ionizing radiation returns and the electron density in the F region increases. The magnetospheric acceleration mechanism associated with discrete auroral arcs thus appears to be inversely proportional to the ionospheric conductivity, because the time variation of the acceleration mechanism coincides with the local F region electron density and not with any obvious magnetospheric process. Previous auroral altitude observations, using similar triangulation methods, showed that the altitude of discrete auroral arcs increases as a function of latitude. When these data are corrected for the twilight effect, the dependence of altitude on latitude disappears. Thus, the average altitude of discrete auroral arcs and, by inference the magnetospheric auroral electron acceleration mechanism, is significantly influenced by the initial ionospheric conductance.

Electron pressure effects on driven auroral Alfvén waves

Fluid models for the auroral electron acceleration processes have almost exclusively been derived by assuming cold or isothermal electrons. The consequences of these assumptions have never been thoroughly analyzed. In this study we compare results from an isothermal simulation with those obtained when the pressure is calculated from a double adiabatic approximation and from stationary kinetic theory. We find that the reflection of shear Alfvén waves, as well as the current‐voltage relation, is very sensitive to the description of the electron pressure variations. Using pressures calculated from steady‐state kinetic theory, we find that driven shear Alfvén waves can build up auroral currents and fields that are consistent with a linear current‐voltage relation.

Recent progress in nonlinear kinetic Alfvén waves

Abstract. This paper presents a review of recent progress in nonlinear kinetic Alfvén wave (KAW hereafter). We start with the two-fluid theory of KAWs and show how the difference between the motions of electrons and ions in small-scale fields of KAWs modifies the Alfvén wave properties. Then, we focus on nonlinear solitary structures of KAWs. A general criterion of the existence for solitary KAW (SKAW hereafter) and its exact analytical solution in a low-β plasma (β<me/mi are presented, where the electron drift velocity along the background magnetic field is larger than the thermal speed within a SKAW, and hence can excite, for instance, ion acoustic turbulence as showed by in situ observations of satellites in space plasmas. In consequence, the turbulence results in kinetic dissipation of the SKAW and dynamical evolution in its structure. We further discuss the structure of the dissipated SKAW (DSKAW hereafter) that evolves from the SKAW due to the dissipation. The result shows that the DSKAW has a local shock-like structure in its density profile and a net electric potential drop over the shock-like structure. In particular, the electric potential drop of the DSKAW can be expected to accelerate electrons efficiently to the order of the local Alfvén speed. The application of the DSKAW acceleration mechanism to the auroral electron acceleration is also discussed. Finally, a few perspectives of KAW studies in future are presented.

Auroral particle acceleration by strong double layers: The upward current region

Satellite observations have established that parallel electric fields of both upward and downward current regions of the aurora are supported, at least in part, by strong double layers. The purpose of this article is to examine the role of double layers in auroral electron acceleration using direct measurements of parallel electric fields and the accompanying particle distributions, electrostatic waves, and nonlinear structures; the concentration is on the upward current region. Direct observations of the ionospheric boundary of the auroral cavity suggest that a stationary, oblique double layer carries a substantial, albeit a minority fraction (∼10% to ∼50%) of the auroral potential. An order of magnitude density gradient results in an asymmetric electric field signature. Oblique double layers with amplitudes greater than 100 mV/m have been verified in ∼3% and may occur in up to 11% of auroral cavity crossings, so it is feasible that strong double layers are a principal acceleration mechanism. In this article we also present a second type of double layer that has a symmetric electric field signature and is seen inside of the auroral cavity. These structures are a possible signature of a midcavity or high‐altitude acceleration mechanism. Numerical solutions of the Vlasov‐Poisson equations support the possibility of midcavity double layers and indicate that trapped electrons can play an important role in the double‐layer structure.

Energy‐flux relationship in the FUV Jovian aurora deduced from HST‐STIS spectral observations

Far ultraviolet spectral observations of the Jovian aurora have been made since 1997 with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope at low spectral resolution. The combination of the spectral resolution with the intensity variation along the STIS slit provides information on the latitudinal variation of the precipitating auroral electron energy flux and the mean electron energy, from which the electron current density at the top of the atmosphere can also be deduced. It is found that the mean electron energies associated with the main oval lie in the range 30–200 keV and show a tendency to increase with the precipitating energy flux. The current densities lie in the range ∼0.04–0.4 μA m−2, consistent with previous estimates, and are also positively correlated with the energy flux. The observed relationship between the auroral time‐integrated energy fluxes and the electron energies in the main oval is compatible with that expected from Knight's theory of field‐aligned currents. The best agreement between the observed data and the Knight curves is obtained for an electron temperature of Te = 2.5 keV and a source density N = 0.003 cm−3, that is within the range of values observed in the equatorial plane during the Voyager flybys. No systematic dependence of the electron energy with magnetic local time is found, but the morning sector around 0800 MLT shows greater variability than other regions of the oval. Analysis of time‐tagged data shows that the main oval energy flux usually varies steadily over the several minute intervals of observation and that the mean electron energy usually undergoes correlated variations such that the current density remains relatively constant. It is shown that these overall properties are also consistent with Knight's theory of auroral electron acceleration associated with field‐aligned current flow, from which it is inferred that the temporal variations observed are often due to slow changes in the magnetospheric “source” electron parameters in the presence of near‐steady magnetosphere‐ionosphere coupling currents. By contrast, time‐integrated emissions in the polar region are found to be associated with similar mean electron energies to the main oval but with typically smaller energy fluxes and current densities. Pressure balance arguments are advanced, which indicate that the brighter of these emissions must be associated with an auroral acceleration mechanism perhaps similar to that operative in the main oval, while it remains possible that the weaker emissions could result from precipitation from a quasi‐isotropic hot magnetospheric electron source.

Ion shell distributions as free energy source for plasma waves on auroral field lines mapping to plasma sheet boundary layer

Abstract. Ion shell distributions are hollow spherical shells in velocity space that can be formed by many processes and occur in several regions of geospace. They are interesting because they have free energy that can, in principle, be transmitted to ions and electrons. Recently, a technique has been developed to estimate the original free energy available in shell distributions from in-situ data, where some of the energy has already been lost (or consumed). We report a systematic survey of three years of data from the Polar satellite. We present an estimate of the free energy available from ion shell distributions on auroral field lines sampled by the Polar satellite below 6 RE geocentric radius. At these altitudes the type of ion shells that we are especially interested in is most common on auroral field lines close to the polar cap (i.e. field lines mapping to the plasma sheet boundary layer, PSBL). Our analysis shows that ion shell distributions that have lost some of their free energy are commonly found not only in the PSBL, but also on auroral field lines mapping to the boundary plasma sheet (BPS), especially in the evening sector auroral field lines. We suggest that the PSBL ion shell distributions are formed during the so-called Velocity Dispersed Ion Signatures (VDIS) events. Furthermore, we find that the partly consumed shells often occur in association with enhanced wave activity and middle-energy electron anisotropies. The maximum downward ion energy flux associated with a shell distribution is often 10mWm-2 and sometimes exceeds 40mWm-2 when mapped to the ionosphere and thus may be enough to power many auroral processes. Earlier simulation studies have shown that ion shell distributions can excite ion Bernstein waves which, in turn, energise electrons in the parallel direction. It is possible that ion shell distributions are the link between the X-line and the auroral wave activity and electron acceleration in the energy transfer chain for stable auroral arcs.

Model of auroral electron acceleration by dissipative nonlinear inertial Alfvén wave

In a recent work [Wu, 2003a, 2003b], a dissipative nonlinear inertial Alfvén wave (DNIAW) were proposed as the physical explanation for the formation of the strong electric spikes often observed in the auroral ionosphere and the magnetosphere. DNIAW can also lead to the field_aligned electron acceleration. In the present paper, dynamical characteristics of DNIAW acceleration are discussed and its possible role in auroral electron acceleration is further investigated. The effective acceleration region for auroral electrons with energies of the order of keV produced by DNIAW acceleration is between 0.5 and 2.5 RE above the ionosphere, and the most efficient acceleration occurs around 0.8 RE where both the Alfvén velocity and the produced auroral electron energy peak, and the peak energy is around 10 keV. We suggest that this could explain the precipitous decrease of the auroral electron energy spectrum toward energies above 10 keV, which can be inferred from measurements of energy distribution of precipitating auroral electrons. Typical widths of auroral arcs caused by the DNIAW acceleration are in scales of the order of 1 km.

A linear auroral current-voltage relation in fluid theory

Abstract. Progress in our understanding of auroral currents and auroral electron acceleration has for decades been hampered by an apparent incompatibility between kinetic and fluid models of the physics involved. A well established kinetic model predicts that steady upward field-aligned currents should be linearly related to the potential drop along the field line, but collisionless fluid models that reproduce this linear current-voltage relation have not been found. Using temperatures calculated from the kinetic model in the presence of an upward auroral current, we construct here approximants for the parallel and perpendicular temperatures. Although our model is rather simplified, we find that the fluid equations predict a realistic large-scale parallel electric field and a linear current-voltage relation when these approximants are employed as nonlocal equations of state. This suggests that the concepts we introduce can be applied to the development of accurate equations of state for fluid simulations of auroral flux tubes.Key words. Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions) – Space plasma physics (kinetic and MHD theory)

Comment on ‘Kinetic effects in the acceleration of auroral electrons in small scale Alfvén waves: A FAST case study’ by Chaston et al.

We reconsider the underlying physics and the dispersion properties of inertial and kinetic Alfvén waves in plasmas, accounting for the inertial electron and kinetic ion responses. In that way, we clarify the ambiguities in a recently published paper by Chaston et al. [Geophys. Res. Lett. 30(6), 1289, doi:10.1029/2002GL015777, 2003].

Reply to “Comment by P. K. Shukla and L. Stenflo on ‘Kinetic effects in the acceleration of auroral electrons in small scale Alfvén waves: A FAST case study’”

Reply to “Comment by P. K. Shukla and L. Stenflo on ‘Kinetic effects in the acceleration of auroral electrons in small scale Alfvén waves: A FAST case study’”

Some recent developments in understanding auroral electron acceleration processes

Discrete auroral arcs and the associated inverted-V-type electron precipitation are accompanied by potential structures, upgoing ion beams and plasma density depletions in the acceleration region altitude and above. However, exactly how these phenomena depend on altitude in various Kp and solar illumination conditions as well as at various magnetic local time (MLT) and invariant latitude (ILAT) is not so well known. We review the main results of our recent large statistical studies of the altitude dependence of these parameters using Polar data. We also include similar statistical results on some additional parameters that appear to have major importance in inverted-V auroral acceleration. These parameters include middle-energy (/spl sim/50-500 eV) electron anisotropies, certain types of electrostatic wave bursts as well as ion shell distributions. One of the most interesting results of the statistical studies is that many of the parameters mentioned change their behavior rather abruptly at around 4 R/sub E/ radial distance. We draw the conclusion that the region for auroral energization processes seems to take place within a closed potential structure below /spl sim/3-4 R/sub E/. These conclusions lead to a new scenario for auroral plasma physics and the energy flow in the auroral acceleration process. The observational results are in agreement with simulations that we also summarize in this paper.