Formation Of Auroral Arcs Research Articles

Horizontal electric fields from flow of auroral O<sup>+</sup>(<sup>2</sup>P) ions at sub-second temporal resolution

Abstract. Electric fields are a ubiquitous feature of the ionosphere and are intimately linked with aurora through particle precipitation and field-aligned currents. They exhibit order-of-magnitude changes on temporal and spatial scales of seconds and kilometres respectively which are not easy to measure; knowing their true magnitude and temporal variability is important for a theoretical understanding of auroral processes. We present a unique method to estimate ionospheric electric fields in the region close to (kilometre scale) a dynamic auroral arc by solving the continuity equation for the metastable O+(2P) ions, which emit as they move under the influence of electric fields during their 5 s lifetime. The main advantage of this optical method is the increase in temporal resolution over other methods such as ground-based radars. Simultaneous measurements of emission at 732.0 nm (from the O+(2P) ions) and prompt emissions at 673.0 nm (N2) and 777.4 nm (O), all at high spatial (100 m) and temporal (0.05 s) resolution, are used in the solution of the continuity equation, which gives the dynamic changes of the O+ ion population at all heights in a 3D volume close to the magnetic zenith. Perspective effects are taken into account by a new geometric method, which is based on an accurate estimate of the magnetic zenith position. The emissions resulting from the metastable ions are converted to brightness images by projecting them onto the plane of the ground, and the projected images are then compared with the measured images. The flow velocity of the ions is a free parameter in the solution of the continuity equation; the value that minimises the difference between the modelled and observed images is the extracted flow velocity at each time step. We demonstrate the method with an example event during the passage of a brightening arc feature, lasting about 10 s, in which the inferred electric fields vary between 20 and 120 mV m−1. These inferred electric fields are compared with SuperDARN measurements, which have an average value of 30 mV m−1. An excellent agreement is found in the magnitude and direction of the background electric field; an increase in magnitude during the brightening of the arc feature supports theories of small-scale auroral arc formation and electrodynamics.

Strategies for improved global representation of magnetospheric electric potential structure on a polar-capped ionosphere

In some simple models of magnetospheric electrodynamics [e.g., Volland, Ann. Géophys., 31, 154–173, 1975], the normal component of the convection electric field is discontinuous across the boundary between closed and open magnetic field lines, and this discontinuity facilitates the formation of auroral arcs there. The requisite discontinuity in E is achieved by making the scalar potential proportional to a positive power (typically 1 or 2) of L on closed field lines and to a negative power (typically −1/2) of L on open (i.e., polar-cap) field lines. This suggests it may be advantageous to construct more realistic (and thus more complicated) empirical magnetospheric and ionospheric electric-field models from superpositions of mutually orthogonal (or not) vector basis functions having this same analytical property (i.e., discontinuity at L = L*, the boundary surface between closed and open magnetic field lines). The present work offers a few examples of ways to make such constructions. A major challenge in this project has been to devise a coordinate system that simplifies the required analytical expansions of electric scalar potentials and accommodates the anti-sunward offset of each polar cap's centroid relative to the corresponding magnetic pole. For circular northern and southern polar caps containing equal amounts of magnetic flux, one can imagine a geometrical construction of coordinate contours such that arcs of great circles connect points of equal quasi-longitude (analogous to MLT) on the northern and southern polar-cap boundaries. For more general polar-cap shapes and (in any case) to assure mutual orthogonality of respective coordinate surfaces on the ionosphere, a formulation based on harmonic coordinate functions (expanded in solutions of the two-dimensional Laplace equation) may be preferable.

Cross-scale coupling in the auroral acceleration region

[1] High resolution imaging within regions of auroral luminosity reveal complex, highly structured dynamic and often vortical forms which evolve on time scales of the order of several seconds and less. These features are inherently multi-scale in nature with different sizes moving and evolving at different rates. Recent analyses have shown how the scale dependency of these motions can provide new insights into the nature of energy transport across scales occurring in current sheets through the auroral acceleration region. However the processes driving this transport and thus facilitating particle acceleration and the formation of bright and dynamic aurora remain unknown. This is a basic issue not only for advancing understanding of auroral arc formation but moreover for understanding dissipation and particle acceleration in current sheets generally. In this Frontier article we show how dedicated space-borne auroral imagery combined with magnetically conjugate field and particle measurements can be used to advance understanding of this universal physical process. By coupling these measurements with numerical simulations we show how flow shear, magnetic reconnection and tearing may launch a cascade toward smaller scales and conspire to form, shape and structure auroral forms. The simulations show that these processes evolve toward a robust scaling of structured magnetic fields (Bx) with wavenumber (ky) perpendicular to the geomagnetic field where Bx2(ky)/Δky ∼ ky−7/3 as observed.

Response of the Magnetosphere to Microscopic Effects in Auroral Arc Formation

A process of quiet auroral arc formation, in particular, the response of the magnetosphere to the effect of auroral energetic electrons, is studied using a holistic auroral simulation code. It consists of a three-dimensional magnetohydrodynamic code for a dipole magnetosphere-ionosphere coupling system and a one-dimensional electrostatic plasma particle code for auroral energetic electron production. Results of the holistic auroral simulation indicate that drastic variation in the ionospheric electric field, which is induced by auroral energetic electrons and propagates into the magnetosphere, influences the magnetospheric field-aligned current distribution. Furthermore, the characteristic time scale of this process is investigated.

CHROMOSPHERIC EVAPORATION VIA ALFVÉN WAVES

This paper presents a scenario for the chromospheric evaporation during solar flares, which is inspired by the chain of events leading to the formation of auroral arcs and ionospheric evacuation during magnetospheric substorms. The plasma, ejected from high coronal altitudes during a flare reconnection event, accumulates at the tops of coronal loops by braking of the reconnection flow, possibly by fast shock formation. A high-beta layer forms and distorts the magnetic field. Energy contained in magnetic shear stresses is transported as Alfven waves from the loop-top toward the chromosphere. It is shown that under these conditions the Alfven waves carry enough energy to feed the chromospheric evaporation process. The second subject of this investigation is identification of the most effective energy dumping or wave dissipation process. Several processes are being analyzed: ion-neutral collisions, classical and anomalous field-aligned current dissipation, and critical velocity ionization. All of them are being discarded, either because they turn out to be insufficient or imply very unlikely physical properties of the wave modes. It is finally concluded that turbulent fragmentation of the Alfven waves entering the chromosphere can generate the required damping. The basic process would be phase mixing caused by a strongly inhomogeneous distribution of Alfvenic phase speed and laminar flow breakup by Kelvin-Helmholtz (K-H) instability. The filamentary (fibril) structure of the chromosphere thus appears to be essential for the energy conversion, in which the K-H instability is the first step in a chain of processes leading to ion thermalization, electron heating, and neutral particle ionization. Quantitative estimates suggest that a transverse structure with scales not far below 100 km suffices to produce strong wave damping within a few seconds. Nonthermal broadening of some metallic ion lines observed during the pre-impulsive rise phase of a flare might be a residue of the turbulent breakup process.

Tackling Substorm Problems: New Observational and Modeling Capabilities: Ninth International Conference on Substorms; Graz, Austria, 5–9 May 2008

Geomagnetic substorms, responsible for creating brightly lit aurorae that can disrupt satellite communications and electric power grids, comprise a wealth of dynamic plasma processes. These processes and their interplay were discussed by about 130 scientists at a conference at Austria's Seggau Castle, organized by the Space Research Institute of the Austrian Academy of Sciences.Recent advances in observational and computational capabilities have allowed closer comparison between observation and theory on current sheet instabilities associated with reconnection, ballooning modes, and different wave properties. For example, presentations at the meeting showed how theory combined with observations from ESA's Cluster satellite helped scientists conclude that the formation of an embedded current sheet occurred before the growth of the instability rather than from thinning the entire plasma sheet. Additionally, the evolution of instabilities leading to magnetic reconnection was reported by several theoretical studies at the meeting, which were complemented by presentations of in situ observations seen from Cluster and NASA's Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission that detailed the characteristics of the reconnecting current sheet. Observations at the inner magnetosphere where the magnetic field configuration changes from tail‐like to dipolar fields showed that instabilities such as ballooning modes are needed to explain the initial auroral arc formation near the equatorward edge of plasma sheet electron precipitation.

The dispersive Alfvén wave in the time-stationary limit with a focus on collisional and warm-plasma effects

A nonlinear, collisional, two-fluid model of uniform plasma convection across a field-aligned current (FAC) sheet, describing the stationary Alfvén (StA) wave, is presented. In a previous work, Knudsen showed that, for cold, collisionless plasma [D. J. Knudsen, J. Geophys. Res. 101, 10761 (1996)], the stationary inertial Alfvén (StIA) wave can accelerate electrons parallel to a background magnetic field and cause large, time-independent plasma-density variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. Knudsen suggested that these fundamental properties of the StIA wave may play a role in the formation of discrete auroral arcs. Here, Knudsen’s model has been generalized for warm, collisional plasma. From this generalization, it is shown that nonzero ion-neutral and electron-ion collisional resistivity significantly alters the perpendicular ac and dc structure of magnetic-field-aligned electron drift, and can either dissipate or enhance the field-aligned electron energy depending on the initial value of field-aligned electron drift velocity. It is also shown that nonzero values of plasma pressure increase the dominant Fourier component of perpendicular wavenumber.

MHD–PIC connection model in a magnetosphere–ionosphere coupling system

A new simulation model is developed, in which the interaction between the macroscopic and microscopic plasma processes is able to be taken into account self-consistently. Magnetohydrodynamic (MHD) simulation and particle-in-cell (PIC) simulation are directly connected and performed simultaneously. This MHD–PIC connection model is first applied to the study of the quiet auroral arc formation process. It is an example of our challenge to create the holistic simulation framework, that is, the heterogeneous schemes that can be unified by the state-of-the-art numerical technique. In this paper, we will explain the algorithm of our new model and show the results which have been calculated on the Earth Simulator.

Heater-induced phenomena in a coupled ionosphere–magnetosphere system

Experimental results from HF pumping experiments in the nightside auroral E and F region are reported. The experiments were carried out by the use of the EISCAT HF heating facility located near Tromsø, Norway, allowing HF pumping the ionosphere in a near magnetic field-aligned direction. We present experimental results from multi-instrument observations related to heater-induced phenomena in a coupled ionosphere–magnetosphere system. The following results have been observed on different occasions: a reverberation effect in scattered signals observed simultaneously on two diagnostic paths which is an indication of Alfvén wave generation. This phenomenon was seen under specific disturbed background geophysical conditions, namely, a high electron density in the F region up to 8 MHz produced by soft electron precipitation from the magnetosphere along with low electron density in lower ionosphere; increased ionospheric electric fields; ion outflows from the ionosphere. On another occasion a magnetospheric response to heater turning on and off was found from magnetic pulsation observations over a frequency range up to 5 Hz (the upper frequency limit of the sensitive magnetometer at Kilpisjarvi, located near Tromsø). The response manifests itself about 1 min after the heater is turned on and off. Other results have shown the modification of a natural auroral arc and local spiral-like formation. It is thought that a local heater-driven current system is formed. An interesting feature is the generation of the heater-induced ion outflows from the ionosphere. They are observed in night hours under both quiet and disturbed magnetic conditions.

Phenomena in the ionosphere-magnetosphere system induced by injection of powerful HF radio waves into nightside auroral ionosphere

Abstract. Experimental results from three ionospheric HF pumping experiments in overdense E or F regions are summarized. The experiments were conducted by the use of the EISCAT HF Heating facility located near Tromsø, Norway, allowing HF pumping the ionosphere in a near geomagnetic field-aligned direction. Distinctive features related to auroral activations in the course of the experiments are identified. Typical features observed in all experiments are the following: generation of scattered components in dynamic HF radio scatter Doppler spectra; strong increase of ion temperatures Ti and local ionospheric electric field E0; modification of the auroral arc and local spiral-like formation. However, some effects were observed only when the HF pump wave was reflected from the F2 layer. Among them are the generation of intense field-aligned ion outflows, and a strong increase in the electron temperature Te with altitude. A possible scenario for the substorm triggering due to HF pumping into an auroral ionosphere is discussed. The authors present their interpretation of the data as follows. It is suggested that two populations of charged particles are at play. One of them is the runaway population of electrons and ions from the ionosphere caused by the effects of the powerful HF radio wave. The other is the population of electrons that precipitate from the magnetosphere. It is shown that the hydrodynamical equilibrium was disrupted due to the effects of the HF pumping. We estimate that the parallel electric field can reach values of the order of 30mV/m during substorm triggering.

Interchange instability in the presence of the field‐aligned current: Application to the auroral arc formation

We have revisited the ideal MHD interchange instability for the case of the nonaligned gradients of the plasma pressure p and unit magnetic flux tube volume V, i.e., the background field‐aligned current (FAC) included. Several possibilities of a directional difference between ∇p and ∇V in the Earth's plasma sheet are considered. The growth of an interchange perturbation leading to the pressure‐gradient‐driven FAC striation is illustrated through numerical simulation in a simple model of the nightside region 1 FAC by Karty et al. [1984] and Yang et al. [1994]. It is shown that the FAC is being striated by the interchange process on a timescale from minutes to tens of minutes. The application to the quiet time discrete arc formation is discussed.

Energetics of the ionospheric feedback interaction

The ionospheric feedback instability has been invoked as a possible mechanism for the formation of narrow auroral arcs. This instability can excite eigenmodes of both field line resonances and the ionospheric Alfvén resonator, producing narrow‐scale structures. Although the basic dispersion relation of this instability has been discussed for both of these cases, the energetics of this instability has not been discussed quantitatively and questions remain as to the nonlinear evolution of this instability. The free energy for this instability comes from the reduction of Joule heating due to the preexisting convection caused by the self‐consistent changes in ionization and conductivity due to Alfvénic perturbations on the ionosphere. In an active ionosphere, narrow‐scale Alfvén waves can be overreflected; i.e., the reflected wave can have a larger amplitude than the incident wave, with the extra energy coming from a local reduction of Joule heating. Recombination produces a damping of this instability, particularly for high background conductivity, indicating that this instability operates best in a dark background ionosphere. This feedback interaction produces narrow‐scale currents when strong gradients in the conductivity are produced, and effects from parallel resistivity or possibly kinetic effects will become important in its evolution. Theoretical constraints on low‐spatial resolution observations of the energy dissipated by precipitation as opposed to Joule heating will be discussed.

Models of two‐dimensional embedded thin current sheets from Vlasov theory

This paper uses the steady state Vlasov theory to obtain an explicit description of the properties of thin current sheets in two space dimensions. The method is general; in particular, it is not restricted to local Maxwellian distribution functions. The treatment includes chaotic particle motion, and the plasma is treated as quasi‐neutral. Such general equilibria are necessary for modeling observed structures in the Earth's magnetotail. In particular, this applies to thin current sheets embedded in wider sheets. Abandoning local Maxwellian distribution functions implies that the electric potential can no longer be transformed to zero such that additional efforts are required to include the electric field. Critical input parameters are the gyroscales of the different particle species and the ion/electron temperature ratio. The results illustrate how finite ion gyroscales affect the current sheet structure. Further aspects that are analyzed include the contributions of electrons and ions to the total current density, Hall currents, the occurrence of significant electric fields and the role of the ion/electron temperature ratio. The necessity of electric potentials in the self‐consistent structure of thin current sheets of the present type may provide a link between the presence or formation of thin current sheets and auroral arcs.

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfven wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 RE. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 RE). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.

Mechanisms of auroral particle acceleration

The paper deals with the conditions enabling auroral acceleration, the coupling of the various processes involved, and the feedback mechanisms on the source of energy and mass. Particular emphasis is given to the energy flow, for which four different paths are identified, whose efficiencies and time-scales are being compared. Furthermore, the necessary ingredients of auroral arc formation are discussed, among which current filamentation and dynamical feedback on the source plasma are still largely unknown elements. The paper closes with a conjecture about the origin of auroral folds and curls, which is attributed to the evacuation process of the topside ionosphere, the ensuing modification of the electric potential distribution, generation of small-scale shear flows and their instability.

Auroral electron acceleration by Alfvén waves and electrostatic fields

We present a two‐dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfvén waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfvén waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 μAm−2. We also find that the electrostatic potential drop is proportional to the square of the current density.

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Abstract. Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)

The importance of a dark ionosphere for ion heating and auroral arc formation

We present observations from the Freja Satellite to show that density reductions and ion heating at Freja heights are anticorrelated with solar illumination of the ionosphere. When the ionospheric foot‐point of a flux‐tube is in shadow, the ambient density is lower, transverse ion energization is more common and more intense, and the associated density cavities are deeper. In combination with the suggestion that the electrons must be accelerated to keV energies to carry an imposed current in a low density auroral cavity, these observations may explain the recent observation that auroras are more common when the ionosphere below is in darkness.