Onset Arc Research Articles

Association of High‐Latitude Geomagnetic Perturbations and Pi1 and Pi2 Pulsations With the Three Steps of Auroral Onset Arc Development at Substorm Onset

Association of High‐Latitude Geomagnetic Perturbations and Pi1 and Pi2 Pulsations With the Three Steps of Auroral Onset Arc Development at Substorm Onset

Nightside High‐Latitude Phase and Amplitude Scintillation During a Substorm Using 1‐Second Scintillation Indices

AbstractWe examined evolution of Global Positioning System (GPS) scintillation during a substorm in the nightside high latitude ionosphere, using 1‐s phase and amplitude scintillation indices from the Canadian High Arctic Ionospheric Network (CHAIN) network. The traditional 1‐min scintillation indices showed that the phase scintillation was dominant, while the amplitude scintillation was weak. However, the 1‐s amplitude scintillation occurred more often in association with major auroral structures (polar cap arc, growth phase arc, onset arc, poleward expanding arc, poleward boundary intensification, and diffuse aurora) that were detected by the THEMIS all‐sky imagers (ASIs). The 1‐min index missed much of the amplitude fluctuations because they only lasted ∼10 s near a local peak or at the gradients of the auroral structures. The 1‐s phase scintillation was concurrent with the amplitude scintillation but was much weaker than the 1‐min phase scintillation. The frequency spectral analysis showed that the spectral power above ∼1 Hz was diffractive and below ∼1 Hz was refractive. We suggest that the amplitude scintillation in the high‐latitude ionosphere is much more common than previously considered, and that a short time window of the order of 1 s should be used to detect the scintillation. The 1‐min phase scintillation index is largely influenced by refractive effects due to total electron content (TEC) variations, and the spectral power below ∼1 Hz should be removed to identify diffractive scintillation.

Formation of Beading Auroral Arcs at Substorm Onset: Implications of Its Variability Into the Generation Process

AbstractThe formation of auroral beads, wavy onset arcs, is often discussed in terms of substorm initiation. The present study addresses several aspects of their development and propagation by examining ground all‐sky images for representative events for each aspect. The results are summarized as follows: (a) the expansion and propagation of auroral beads do not depend on whether they form east or west of a preceding equatorward flow; (b) Auroral beads propagate either eastward or westward when the westward auroral electrojet intensifies apparently not preceded by the approach of an equatorward flow in neighboring sectors; (c) A nearby arc does not seem to be affected even if auroral beads form only a few tenths of degree apart in latitude; (d) two wavy arcs occasionally form next to each other and propagate in different directions; (e) the evolution of beading arcs is not always conjugate in two hemispheres. Results (a) and (b) suggest that meso‐ or large‐scale ionospheric convection as inferred from ground magnetic disturbances does not regulate the development and propagation of auroral beads. Results (c) and (d) imply that the development of auroral beads can be attributed to a small‐scale process, possibly a convection flow, at a latitudinal scale of a few tenths of degree or less. Such a small‐scale process may not be conjugate in two hemispheres, which potentially explains Result (e). These results do not necessarily mean that the cause of auroral beads is ionospheric, but suggest that the magnetosphere‐ionosphere coupling plays an important role in their development.

The Onset of a Substorm and the Mating Instability

AbstractThe paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 RE. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high‐beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high‐resolution videos of two substorm onsets. We conclude that they may be low‐entropy content bubbles with a balanced field‐aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non‐MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high‐beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos

Radar Observations of Flows Leading to Longitudinal Expansion of Substorm Onset Over Alaska

AbstractLongitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instability. We used Poker Flat radar campaign auroral observations to identify seven substorm onsets seen east or west of the radar that expanded into the radar field‐of‐view. The radar observations were used to show flows relative to the expanding onset aurora. Flow enhancements were found equatorward (poleward) of the westward (eastward) longitudinal expansion of the onset arc brightening that agree with plasma sheet bubble modeling. This lends significant additional supporting evidence that an incoming reduced entropy flow channel leads to onset. As a flow channel enters the auroral oval (plasma sheet) from the polar cap (magnetotail lobes) and starts to move equatorward (earthward in the plasma sheet) as a bubble, it simultaneously expands azimuthally due to the combination of electric drift along dawnward tilted equipotentials and westward magnetic drift of more energetic ions. As the azimuthally expanding bubble reaches near the equatorward boundary of the electron auroral oval, it leads to the onset instability indicated by the auroral beading and associated flows if conditions are appropriate. The instability grows in the initial onset region, as it simultaneously extends longitudinally with the continued azimuthal expansion of the bubble within the plasma sheet. Additionally, after onset, the auroral bulge expands poleward and can lead to a westward traveling surge. We observe strong flows heading toward the front of the surge that appear to bring low entropy plasma, perhaps from the polar cap, into the surge.

Diagnosing the Time‐Dependent Nature of Magnetosphere‐Ionosphere Coupling via ULF Waves at Substorm Onset

AbstractAzimuthal structuring is usually observed within the brightening auroral substorm onset arc; such structure has been linked to the exponential growth of electromagnetic ultralow‐frequency (ULF) waves. We present a case study investigating the timing and frequency dependence of such ULF waves on the ground and in the near‐Earth magnetotail. In the magnetotail, we observe an increase in broadband wave power across the 10‐ to 100‐s period range. On the ground, the arrival times spread from an epicenter. The onset of longer period waves occurs first and propagates fastest in latitude and longitude, while shorter periods appear to be more confined to the onset arc. The travel time from the spacecraft to the ground is inferred to be approximately 1–2 min for ULF wave periods between 15 and 60 s, with transit times of 60 s or less for longer period waves. This difference might be attributed to preferential damping of the shorter period waves, as their amplitude would take longer to rise above background levels. These results have important consequences for constraining the physics of substorm onset processes in the near‐Earth magnetotail and their communication to the ground.

A New Substorm Onset Mechanism: Increasingly Parallel Pressure Anisotropic Ballooning

AbstractRecent observations demonstrating that auroral beads are observed prior to the majority of substorm onsets (Kalmoni et al., 2017, https://doi.org/10.1002/2016GL071826) have reinforced the potential importance of ballooning instabilities for near‐Earth magnetospheric substorm onset. Here we examine pressure anisotropic ballooning instabilities in stretched magnetotail geometries. Our results show that transition from an initially perpendicular anisotropy toward parallel anisotropy reduces the plasma β threshold for triggering a ballooning instability. Such increasingly parallel anisotropies can form as a direct consequence of tail stretching that occurs during the late substorm growth phase through the well‐known effects of drift shell splitting and the competition between betatron and Fermi processes in the tail. We propose such increasingly parallel ballooning triggers auroral substorm onset on field lines in the transition region between dipolar and tail‐like fields, consistent with observational constraints on the location of the onset arc with respect to the ion isotropy boundary in the magnetotail.

Relativistic Particle Beams as a Resource to Solve Outstanding Problems in Space Physics

The Sun’s connection with the Earth’s magnetic field and atmosphere is carried out through the exchange of electromagnetic and mass flux and is regulated by a complex interconnection of processes. During space weather events, solar flares or fast streams of solar atmosphere strongly disturb the Earth’s environment. Often the electric currents that connect the different parts of the Sun-Earth system become unstable and explosively release the stored electromagnetic energy in one of the more dramatic expressions of space weather – the auroral storm and substorm. Some aspects of the magnetosphere-ionosphere connection that generates auroral arcs during space weather events are well-known. However, several fundamental problems remain unsolved because of the lack of unambiguous identification of the magnetic field connection between the magnetosphere and the ionosphere. The correct mapping between different regions of the magnetosphere and their foot-points in the ionosphere, coupled with appropriate distributed measurements of plasma and fields in focused regions of the magnetosphere, is necessary to establish unambiguously that a given magnetospheric process is the generator of an observed arc. The three most important problems for which the correct magnetic field mapping would provide closure to are the substorm growth phase arcs, the expansion phase onset arcs and the system of arcs that emerge from the magnetosphere-ionosphere connection during the development of the early substorm expansion phase phenomenon known as substorm current wedge (SCW). Energetic electron beams, used as magnetic field tracers, can enable the closure needed. However, the application of beams as tracers require demonstration that the beams can be injected into the loss cone, that the spacecraft potentials induced by the beam emission are manageable, and that sufficient electron flux reaches the atmosphere to be detectable by optical or radio means after the beam has propagated thousands of kilometers under competing effects of beam spread and constriction as well as effects of beam-induced instabilities. In this communication we provide a review of the latest results of synergistic research carried out under the NSF INSPIRE program to address these challenges and discuss the next steps toward the realization of active experiments in space using relativistic electron beams.

Revisiting substorm events with preonset aurora

Abstract. Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)

The Magnetospheric Source Region of the Bright Proton Aurora

AbstractThe bright proton aurora is generally understood to be the projection of the Central Plasma Sheet where there is sufficient particle energy to cause auroral luminosity and strong pitch angle scattering (presumably due to field line curvature). This region is often interpreted as the transition region between dipolar and tail‐like magnetic topologies. The location of auroral features relative to the peak in the proton aurora has been used, for example, to argue that the substorm onset arc lies on field lines that thread this transition region. In this paper, we present statistics of proton aurora luminosity computed from Time History of Events and Macroscale Interactions in Substorms electrostatic analyzer measurements for various radial distances in the magnetotail. These results are compared to ground observations of proton auroral luminosity and used to derive a statistical source region of the bright proton aurora.

Statistical azimuthal structuring of the substorm onset arc: Implications for the onset mechanism

AbstractThe onset of an auroral substorm is generally thought to occur on a quiet, homogeneous auroral arc. We present a statistical study of independently selected substorm onset arcs and find that over 90% of the arcs studied have resolvable characteristic spatial scales in the form of auroral beads. We find that the vast majority (~88%) of auroral beads have small amplitudes relative to the background, making them invisible without quantitative analysis. This confirms that auroral beads are highly likely to be ubiquitous to all onset arcs, rather than a special case phenomena as previously thought. Moreover, as these auroral beads grow exponentially through onset, we conclude that a magnetospheric plasma instability is fundamental to substorm onset itself.

High‐resolution auroral acceleration signatures within a highly dynamic onset arc

AbstractOn 16 March 2006, the Reimei spacecraft observed dynamic small‐scale vortex‐like structures along an east‐west elongated onset arc, subsequently followed by the auroral expansion. Within a few seconds when Reimei crossed over the arc, detailed comparisons of the simultaneous conjugate auroral imaging and particle observations with high resolution were made to reveal the fine‐scale acceleration processes acting within/around the flux tube of the bright auroral structures. In the surrounding dark region, Alfvénic accelerated electron fluxes—time‐dispersive, field‐aligned electron bursts spanning a broad range of energies—became most noticeable. The dominant contributor to the central bright region, on the other hand, was monoenergetic electron fluxes in 3–7 keV at larger pitch angles (>30°) rather than the Alfvénic electron fluxes. The Reimei observations suggest that the Alfvénic acceleration process operates in the vicinity of small‐scale auroral structures but plays a less significant role in powering the central bright emissions.

Relationship between wave-like auroral arcs and Pi2 disturbances in plasma sheet prior to substorm onset.

Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.

Statistical characterization of the growth and spatial scales of the substorm onset arc.

We present the first multievent study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near‐Earth plasma instability which causes the substorm onset arc. Using data from ground‐based auroral imagers, we study repeatable signatures of along‐arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near‐Earth plasma sheet. We show that the growth and spatial scales of these wave‐like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wave number with the most unstable spatial scales mapping to an azimuthal wavelength λ≈ 1700–2500 km in the equatorial magnetosphere at around 9–12 R E. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the Cross‐Field Current Instability and the Shear Flow Ballooning Instability. We conclude that, although the Cross‐Field Current instability can generate similar magnitude of growth rates, the range of unstable wave numbers indicates that the Shear Flow Ballooning Instability is the most likely explanation for our observations.

Characteristics of Poker Flat Incoherent Scatter Radar (PFISR) naturally enhanced ion-acoustic lines (NEIALs) in relation to auroral forms

Abstract. Naturally enhanced ion-acoustic lines (NEIALs) have been observed with the Poker Flat Incoherent Scatter Radar (PFISR) ever since it began operating in 2006. The nearly continuous operation of PFISR since then has led to a large number of NEIAL observations from there, where common-volume, high-resolution auroral imaging data are available. We aim to systematically distinguish the different types of auroral forms that are associated with different NEIAL features, including spectral shape and altitude extent. We believe that NEIALs occur with a continuum of morphological characteristics, although we find that most NEIALs observed with PFISR fall into two general categories. The first group occurs at fairly low altitudes – F region or below – and have power at, and spread between, the ion-acoustic peaks. The second group contains the type of NEIALs that have previously been observed with the EISCAT radars, those that extend to high altitudes (600 km or more) and often have large asymmetries in the power enhancements between the two ion-acoustic shoulders. We find that there is a correlation between the auroral structures and the type of NEIALs observed, and that the auroral structures present during NEIAL events are consistent with the likely NEIAL generation mechanisms inferred in each case. The first type of NEIAL – low altitude – is the most commonly observed with PFISR and is most often associated with active, structured auroral arcs, such as substorm growth phase, and onset arcs and are likely generated by Langmuir turbulence. The second type of NEIAL – high altitude – occurs less frequently in the PFISR radar and is associated with aurora that contains large fluxes of low-energy electrons, as can happen in poleward boundary intensifications as well as at substorm onset and is likely the result of current-driven instabilities and in some cases Langmuir turbulence as well. In addition, a preliminary auroral photometry analysis revealed that there is an anticorrelation between the altitude of the NEIALs and the calculated energy of the electrons, which is consistent with the hypotheses presented here regarding generation mechanisms.

Westward traveling surges: Sliding along boundary arcs and distinction from onset arc brightening

After onset, auroral brightening rapidly spreads azimuthally along the onset arc, typically, but not always, extending substantially farther westward than eastward. Expansion‐phase auroras also expand poleward and can contact an arc along the auroral poleward boundary. For such events, we find that poleward expansion of the region of active aurora accelerates after contact, forming a bulge region that protrudes into the preexisting polar cap and expands westward as the westward traveling surge. This westward motion is slower than, and initiates later than, the azimuthal spreading of the onset arc brightening. The poleward boundary arc remains connected to the surge as the surge slides along the equatorward moving edge of the boundary arc, and this connection appears to set off the westward traveling, poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc. Ground magnetic signatures show a relatively weak H component depression moving westward with the onset arc brightening, and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current. The clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetometer current signatures implies that their underlying processes are very different. Evidence suggests that flow channels adjacent to the boundary arc possibly feed surge development. These results are consistent with current wedge formation being more associated with post‐onset activity than with onset, and with flow channels from the polar cap being important for surge/bulge development.

On the formation of pre‐onset azimuthal pressure gradient in the near‐Earth plasma sheet

Xing et al. (2011) demonstrated by case study that the plasma sheet pressure gradient at ∼11 REnear the substorm onset meridian undergoes a substantial duskward enhancement shortly before substorm onset as identified from the auroral poleward expansion. It was suggested that the increased upward FAC driven by this pressure gradient enhancement lead to the thin onset arc intensification from which the poleward expansion initiates. In the present study, we employ the multiTHEMIS spacecraft in azimuthal conjunction ‐at ∼−11 REand examine the ion flux and distributions during the period of pressure gradient enhancement, and identify two categories of events. For events with pressure gradient change ≤2 min prior to onset, strong earthward ion flux enhancements covering the energy range from several keV to above 25 keV were observed by the spacecraft identifying the higher pressure increase, while at the same time the ion distributions show a substantial earthward shift in velocity space. These resemble the ion acceleration ahead of earthward moving dipolarization fronts. On the other hand, the spacecraft observing the lower pressure increase found weaker or no ion flux enhancements and had nearly isotropic distributions, indicating that longitudinally localized dipolarization fronts are responsible for the pressure gradient development. These pressure gradient enhancements were in the duskward (dawnward) direction for measurements to the east (west) of the onset region. For pressure gradient enhancement >2 min prior to onset, stronger plasma sheet thinning was observed by the spacecraft identifying the larger pressure increase and the ion distributions appear nearly isotropic, which suggests plasma redistribution in the undisturbed plasma sheet. These two categories suggest that both dipolarization fronts and growth phase plasma sheet localized thinning can drive azimuthal pressure gradient enhancement near the onset meridian and cause pre‐onset thin auroral arc intensification, though the dipolarization front events appear to be substantially more common based on our 14 events.

Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008

Abstract. We present a new M-I coupling model of substorm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-1) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ∼71 ± 3 s, but the breakup arcs faded within ∼5 min without substantial poleward expansion and the dipolarization stopped and reversed to thinning. The second event (E-2) was identified as a substorm: brightening of the second onset arc preceded the second dipolarization onset by ∼80 ± 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ∼20 min. The Alfvén travel time from the ionosphere to the dipolarization onset region is estimated at ∼69.3 s in E-1; at ∼80.3 s in E-2, which matched well with the observed time delay of the dipolarization onset after the brightening of the onset arc, respectively in E-1 and E-2. Brightening of the onset arc precedes the dipolarization onset suggest that the onset arc brightening is caused by the intense upward field-aligned currents originating from the divergence of the Cowling electrojet in the ionosphere. The Cowling electrojet current loop (CECL) is formed to close the field-aligned currents at all times. The closure current in the Alfvén wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfvén wavefront incident on the near-Earth plasma sheet to disrupt the cross-tail current in the dipolarization region. Slow MHD waves dominate the disruption of the cross-tail current in the dipolarization region.

Near-Earth plasma sheet azimuthal pressure gradient and associated auroral development soon before substorm onset

[1] The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the premidnight region of the near-Earth plasma sheet during the substorm growth phase owing to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FAC variations associated with the substorm growth phase and may set up a plasma sheet precondition for the substorm onset being triggered near this region. We take advantage of two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient before substorm onset in the R ∼10–12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Five events with the spacecraft footprints mapped very close to the aurora onset region were selected. These events show substantial duskward pressure gradient enhancement 1–5 min before onset. The onset arc, which results from enhanced energetic electron precipitation, was found to intensify simultaneously with the pressure gradient enhancement before onset breakup occurs. Since the energy and energy flux of precipitating electrons reflect the upward FAC strength, these results indicate that the duskward azimuthal pressure gradient enhancement is associated with enhanced upward FAC during the late growth phase and leads to the intensification of the onset auroral arc soon before it breaks up. It is possible that this pressure gradient enhancement may lead to ballooning mode instability and thus substorm onset along the intensifying arc.

Pi1B pulsations as a possible driver of Alfvénic aurora at substorm onset

[1] Ground-based observations have shown Pi1B magnetic pulsations are associated with substorm onset. These pulsations can also be observed at geosynchronous orbit, suggesting that they propagate from (or beyond) geosynchronous orbit to the ionosphere at substorm onset. Independently, investigations have shown that the initial brightening of an arc at subtorm onset is Alfvenic in nature (i.e., that the aurora during the initial brightening is wave-driven). These results raise the question of whether Pi1B pulsations might drive Alfvenic aurora at substorm onset. In this paper, data from a single event are presented that show Pi1B pulsations observed simultaneously at geosynchronous orbit, by FAST just above the ionosphere and by various ground stations. The event is observed by FAST within a few minutes of the onset of Pi1B pulsations, with an electron signature of the onset arc that is quite characteristic of Alfvenic aurora, showing that at least a portion of the initial brightening is wave-driven and associating this with the presence of Pi1B pulsations. The implication of this work is that Pi1B pulsations propagate to the ionosphere from beyond geosynchronous orbit and provide the wave power to drive Alfvenic aurora at substorm onset, at least for this single, isolated event. The luminosity associated with the Alfvenic aurora, however, may only provide a small contribution to the total brightness.