Fine structure of aurora

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Fine structure of auroraIngrid Sandahl a , Urban Brändström a & Tima Sergienko aa Swedish Institute of Space Physics , Kiruna, SwedenPublished online: 24 Jun 2011.

To cite this article: Ingrid Sandahl , Urban Brändström & Tima Sergienko (2011) Finestructure of aurora, International Journal of Remote Sensing, 32:11, 2947-2972, DOI:10.1080/01431161.2010.541507

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International Journal of Remote SensingVol. 32, No. 11, 10 June 2011, 2947–2972

Fine structure of aurora

INGRID SANDAHL� , URBAN BRÄNDSTRÖM and TIMA SERGIENKO*Swedish Institute of Space Physics, Kiruna, Sweden

Fine structure is present in most types of aurora, but much of it has previouslynot been possible to study properly because of instrument limitations. However,recent advances in optical instrumentation have provided considerable improve-ments in temporal and spatial resolution. Optical measurement systems are ableto use a higher resolution than other types of ground-based instruments used inauroral studies. New results have been obtained regarding, for example, elemen-tal structures in discrete auroras, generation of flickering aurora, generation ofAlfvén waves in shear regions, dynamic rayed aurora, fine structure of diffuse auro-ras and fine structure of auroral curls. Outstanding questions are highlighted andrecommendations for future research are given. The importance of a coordinatedinfrastructure for ionospheric research, simultaneous measurements on differ-ent scales, optical calibration facilities and the development of time-dependenthigh-resolution models is stressed.

1. Introduction

The fine structure of optical aurora is an exciting topic that was earlier difficult toexplore because of instrument limitations. This is now changing because of significantimprovements in optical imaging techniques, and the study of auroral fine structureis beginning to receive well-deserved attention. In this paper some recent results aresummarized, outstanding questions highlighted and recommendations for the futurediscussed.

In the magnetosphere we find processes on different scales. We usually talk aboutfour different scales: global, fluid, ion and electron. All of these except the global scaleare illustrated in figure 1. The different scales interact, so to gain a full understandingof a phenomenon, observations need to be carried out on all of them. However, studiesso far have mainly covered one scale at a time. Although it may be of great importance,the electron scale has been hardly studied because of high requirements on spatial andtemporal resolution. In a review paper on multipoint measurements with Cluster andground-based instruments, Amm et al. (2005) note that it has not yet been possible toresolve the important electron-scale plasma regime. The electron scale may hold thekey to understanding an increasing number of wave-wave and wave-particle mecha-nisms. These mechanisms may prove important for energy coupling and triggering oflarge-scale instabilities.

In the aurora the electron scale corresponds to the smallest structures with sizesof the order of 10–100 m. The aurora is very rich in small-scale structure. Figure 2

�Ingrid Sandahl sadly passed away in May 2011.

*Corresponding author. Email: [email protected]

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online © 2011 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431161.2010.541507

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2948 I. Sandahl et al.

Figure 1. In magnetospheric physics processes appear on different scales. It is common todistinguish between four different scales: global, fluid, ion and electron. This figure shows thespatial and temporal scales of the fluid, ion and electron scales. It was prepared during theplanning of the Cross–Scale satellite project. From Schwartz et al. (2008).

shows an example of simultaneous optical imaging of the same event on differentscales presented by Semeter et al. (2008). Figures 2(a)–2(c) show a series of all-skyimages. Two imagers were used to zoom in on details of the auroral arc in the southernpart of the sky. Figures 2(d)–2(f ) are images obtained by an image-intensified auroralTV camera with a 60◦ field of view and figures 2(g)–2(i) are images at even finer scaleobtained with an Electron Multiplying Charge Coupled Device (EMCCD) camera.The field of view of the EMCCD camera was 9◦ × 9◦ and the resolution about 50 m at100 km altitude. The amount of fine structure is impressive.

Another example of a small-scale auroral structure is shown in figure 3. This is afilament with a width of about 100 m between two curls recorded by an intensifiedTV camera by Lanchester et al. (1997). The circle to the left of the filament showsthe beam size of the European Incoherent Scatter (EISCAT) radar at the altitude ofthe aurora, as well as the field of view of a photometer. We note that the radar beamand the photometer field of view are too wide to be able to resolve the small-scaleaurora.

Figure 3 in fact contains one of the narrowest auroral structures so far shown in afigure in the published literature. The width of the structures is of interest, both fromthe point of view of choosing between different mechanisms of their formation andfor the planning of measurements. Most of the work on sizes of auroral forms so far

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Atmospheric studies by optical methods 2949

Figure 2. An auroral event recorded by three different imaging instruments at three dif-ferent scales: (a)–(c) all-sky images; (d)–(f ) images from an image-intensified TV camera;(g)–(i) images from an EMCCD camera. In the top panel the field of view of the TV cam-era is shown and in the middle panel the field of view of the EMCCD camera. The richness inauroral fine structure is clearly demonstrated. From Semeter et al. (2008).

has concerned auroral arcs. Borovsky (1993) carried out a very thorough review onproposed mechanisms for auroral arcs including an estimate of expected widths. Oneof his conclusions was that none of the mechanisms can explain the narrowest auroralstructures.

Figure 4 shows the results of a number of studies of auroral arc widths. Three peaksexpressed as bar diagrams display data from ground-based optical measurements. Thesmallest scales, all below 1 km, were found by Maggs and Davis (1968). Knudsen et al.(2001) looked at stable auroral arcs with lifetimes of the order of 10 min and found thatmost of these arcs were between 5 and 40 km wide. In between are the measurementsby Partamies et al. (2008b), with scales from 200 m to 2 km and lifetimes of about1 min. The situation regarding the Maggs and Davis (1968) measurements is somewhat

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Figure 3. Auroral filament with a width of about 100 m between two curls recorded by animage-intensified TV camera. The circle shows the field of view of the EISCAT radar beam atan altitude of 100 km, as well as the field of view of a photometer. The total area covered by theimage is 38 km × 28 km at 100 km altitude. From Lanchester et al. (1997).

Figure 4. Arc widths from studies using ground-based optical and satellite data. Basedon original figure by Partamies (personal communication 2008) and modified by Marklund(personal communication 2008).

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unclear. A later examination of their data by Stenbaek-Nielsen et al. (1999) suggestthat some of the structures may be fine structure of diffuse aurora.

Borovsky et al. (1991) calculated a theoretical minimum for widths of arcs, basingthe calculation on the gyro radius of primary electrons. They found a value slightlybelow 10 m and designed an instrument with sufficient resolution to measure suchsizes. The narrowest structure that they found was 40 m, although 100 m was a muchmore common value.

Figure 4 also show arc widths from satellite measurements. Most satellites do nothave sufficient resolution to resolve the narrowest structures. A satellite especiallydesigned for high-resolution studies is the Fast Auroral Snapshot (FAST). Using datafrom FAST, Chaston et al. (2003) found that, for arcs related to Alfvén waves, thewidth distribution had a maximum at 1 km. For inverted V events, Partamies et al.(2008a) obtained typical widths of 20–40 km. The Alfvénic arcs thus correspond mostclosely to the ground-based width distribution obtained by Partamies et al. (2008b),while the stable arcs studied by Knudsen et al. (2001) overlap with the inverted Vevents. Chaston et al. (2003) also looked into the ability of Alfvén waves to producevery narrow structures. They concluded that it would certainly be possible for Alfvénwaves to generate 100 m arcs, but that these would have a very low intensity.

Figure 5. Because auroral forms are usually elongated along the direction of the magneticfield, field-aligned observations are needed to directly measure widths of structures. In thisillustration from de Mairan (1754) the field-aligned direction is marked with x.

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Figure 4 suggests that the widths obtained depend to some extent on the instrumentand on the measurement set-up. Indeed, measuring widths of auroral structures is nottrivial. This can be demonstrated by figure 5, taken from the comprehensive treatiseon the aurora written by de Mairan in 1754. As depicted in this figure, auroral struc-tures tend to be elongated along the direction of the magnetic field and to properlymeasure the width, field-aligned observations are required. In addition to this, at leastthree other factors need to be taken into account. First, the aurora contains intensemetastable transitions with long lifetimes, most prominently the red and green oxygenlines at 630.0 nm (120 s) and 557.7 nm (0.7 s), respectively. For those lines the excitedatoms may have moved a considerable distance before emitting the light. Second, thesmallest structures tend to have the shortest lifetimes so that measurements with hightemporal resolution are needed. Third, most auroral structures move.

A quantitative demonstration of the importance of field-aligned measurements wasgiven by Semeter et al. (2008). If an arc is 100 m wide and 10 km high, the apparenterror in width will be 100% at as little as 1◦ off zenith.

To remedy all the measurement problems listed above, there is a need to use filtersand short exposure times. This means that the amount of light available for each expo-sure is decreased. A limiting factor for what can be measured by a certain instrumentis what Borovsky et al. (1991) termed its ‘light gathering power’. In addition, the timebetween images must be short.

In 2009, half of the Nobel Prize in physics was awarded to William S. Boyle andGeorge E. Smith ‘for the invention of an imaging semiconductor circuit – the CCDsensor’. Indeed, the CCD imager meant a revolution to optical imaging, enabling high-resolution imaging with short exposure times and narrow-band filters (e.g. Janesicket al. 1987). In the more recently developed EMCCD imagers, the signal-to-noise ratiohas been improved considerably, allowing much shorter readout times and combiningthe advantages of the CCDs with very high frame rates (McWhirter 2008). Thus thepossibilities for measurements with very high temporal and spatial resolution are bet-ter than ever before and, in addition, a number of interesting, but weak, emissions cannow be studied.

Another useful property of the EMCCD imagers is that they are fairly linear andstable. They can be used for absolute intensity measurements, provided, of course, thatthey are properly calibrated. They can, in fact, be regarded as arrays of photometers.The EMCCD imagers are offering a range of new possibilities to auroral scientists.Some examples of recent results are given in §2. Emphasis is placed on results thathave appeared after the publication of a previous review by Sandahl et al. (2008).

2. Recent results

2.1 Elemental structures in discrete auroras

‘Elemental structure’ is a term describing the smallest elements of discrete auroras(Lanchester et al. 1994, Semeter et al. 2008). In other studies similar features are called‘filaments’ (e.g. Lanchester et al. 2001, Dahlgren et al. 2008).

Figure 6 shows an example of an elemental structure from a study by Semeter andBlixt (2006) using an intensified white light TV camera. The figure demonstrates acommon property of elemental structures, cascading. An elemental structure, indi-cated by the white arrow in the top left frame, duplicates itself a number of timesduring the 0.5 s shown in the figure.

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Figure 6. A cascading elemental structure recorded by an image-intensified white light TVcamera. The original elemental structure is indicated by an arrow in frame 1. As time proceedsthis structure is duplicated several times. Each frame shows an area of 11.7 km × 11.1 km andthe total sequence is 0.5 s. From Semeter and Blixt (2006).

The most common explanation for filamentary structures is that they are causedby shear Alfvén waves. Semeter and Blixt (2006) suggested that cascading is a naturalconsequence of the dispersion of shear Alfvén waves that will inevitably take place inthe ionosphere.

A study with an EMCCD camera (Semeter et al. 2008) could confirm this interpre-tation. In figure 7 we recognize the centre part of the middle picture in the figure 2(h).The time between each frame was 20 ms. The initial elemental structure is first dou-bled, then tripled and then multiplied even more. To prevent the metastable emissionsat 557.7 and 630.0 nm from obscuring the picture, a special blocking filter was used.The characteristic of this filter is shown in figure 8.

The basic idea of how the Alfvén wave dispersion is related to the aurora is shownin figure 9, taken from Semeter et al. (2008). An Alfvén wave hits the top of the iono-sphere. In the figure the ambient magnetic field, B0, is vertical. As the wave disperses,wave energy is spread perpendicular to B0 and the amplitude of the wave decreases.The dispersing wave will be confined within a region called the Alfvén cone. In figure 9only the wave along one of the sides of the Alfvén cone is shown. The field-alignedcomponent of the wave will accelerate electrons. An upward component will accelerateelectrons downwards and give rise to an aurora. Close to the top of the ionosphere theamplitude is still fairly large, giving much acceleration to the electrons. These electronswill give an aurora at low altitudes. Deeper into the atmosphere the Alfvén wave ampli-tude has decreased so that there will be less electron acceleration and a higher altitude

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Figure 7. A cascading elemental structure recorded by an EMCCD imager during a breakupevent. The time between each frame is 20 ms, the same as in figure 6. Each frame correspondsto an area of about 7 km × 7 km. The EMCCD imager gives a clearer picture of the cascadingthan the intensified TV camera. From Semeter et al. (2008).

Figure 8. Transmission of the filter used to obtain the image shown in figure 7. The filterblocks the metastable green and red lines. From Semeter et al. (2008).

of the resulting aurora. When the Alfvén wave has a downward parallel component,electrons will be accelerated upwards and there will be gaps in the aurora.

This explanation for cascading is promising, but there are a number of other basicquestions regarding the elemental structures. Are there single very narrow structuresthat do not cascade? If so, under what circumstances do they exist? Recalling the worksof Chaston et al. (2003) and Borovsky (1993) mentioned in §1, we still have no clear

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Figure 9. Diagram of the mechanism proposed to be responsible for the cascading of auroralelemental structures. The figure is explained in the text. From Semeter et al. (2008).

explanation for single narrow structures. Is it possible to come up with a mechanisminvolving Alfvén waves also for single structures?

2.2 Flickering aurora

Alfvén waves, in this case interfering dispersive Alfvén waves, offer a possible explana-tion for another type of aurora, the flickering aurora (Sakanoi et al. 2005, Gustavssonet al. 2008b, Whiter et al. 2008). This aurora appears as periodic intensity variations insmall-scale columns of aurora in bright auroral forms, usually during breakups. Whenviewed along the field lines, the columns are seen as patches, and in many papers theflickering forms are called patches. The cross-section of the columns is of the orderof 1–10 km and lifetimes are 1–2 s. Typical flickering frequencies are between 2 and20 Hz and the modulation depth 10–20%.

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Several different mechanisms have been proposed for flickering aurora. Sakanoiet al. (2005) suggested that the cause is dispersive Alfvén waves or electromagnetic ioncyclotron waves interfering with each other, an idea originally proposed by Temerinet al. (1986). Sakanoi et al. (2005) found very good agreement between the interferencepattern of two dispersive Alfvén waves and measurements performed with an imagingphotometer system. Gustavsson et al. (2008b) extended the Sakanoi–Temerin modelfrom one to two dimensions and found good agreement with the characteristics offlickering aurora they had measured in white light with the Optical Digital Imager(Odin) (Blixt et al. 2005b). A particular strength of the Odin instrument is its hightemporal resolution, 50 Hz in this case.

Measurements of flickering aurora with the highest spatial resolution so far wereperformed by the Auroral Structure and Kinetics (ASK) instrument and presented byWhiter et al. (2008). ASK contains three EMCCD cameras. This study used one ofthese, equipped with a narrowband filter admitting the prompt N2 1PG emission at673 nm. The spatial resolution was 40 m and the temporal resolution 32 frames persecond.

Figure 10 shows ASK data of a flickering aurora event. The area covered by thepicture corresponds to a 5 km × 5 km square at an altitude of 100 km. The top leftfigure is the average auroral intensity at 673 nm taken over 1 s, that is 32 images. Notethat the scale is absolute in units of kiloRayleigh (kR), not just an arbitrary scale! Thewhole field of view is within one and the same auroral arc. To the right are eight framesshowing the power spectral density in different 2 Hz bins. The bottom left figure showsthe peak flickering frequency for every pixel. Throughout most of the image the peakfrequency was 6–8 Hz. This study gave smaller patch sizes than earlier studies. Themajority of flickering patches were less than 1 km, and as little as 160 m was found.

Whiter et al. (2008) also investigated the correlation between intensity and flickeringpower as a function of time. When each superpixel was treated as one data point, thatis for small spatial scales, the correlation was generally poor. The size of a superpixelwas 20 m × 20 m at 100 km altitude. When the background intensity averaged overthe whole 5 × 5 km image was compared to the total flickering power, the correlationwas very good. Such behaviour is what we expect for a Sakanoi–Temerin mechanism.

2.3 Generation of small-scale Alfvén waves

We now turn to the question of how the Alfvén waves are generated. Results con-cerning this have been obtained by the Japanese satellite Reimei and presented byAsamura et al. (2009). Reimei is able to measure auroral light and particles on thesame field line simultaneously. It was launched in 2005 into a near-circular orbit at analtitude of 630 km. Both the Electron and Ion Spectrum Analyzer (EISA; Asamuraet al. 2003) and the Multichannel Auroral Camera (MAC; Sakanoi et al. 2003) havethe best spatial resolution of any instruments in their respective categories flown ona satellite so far. MAC has three CCD imagers with narrow-band interference filtersat 428, 558 and 672 nm and its resolution corresponds to 1.2 km per pixel at 100 kmaltitude. The temporal resolution is 120 ms. The spatial resolution of the particle mea-surements along the magnetic field line corresponds to 150 or 300 m in the aurora,depending on the number of energy steps.

Figure 11 shows particle measurements in an inverted V structure. During two timeintervals, marked by bars above the spectrogram, dispersed structures appeared inenergies below the peak energy. To show the structures more clearly, data from thesecond interval, marked by a square, are blown up and presented in figure 12. Such

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18:16:30.078 UT

Power Spectral Density (a.u.)

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Figure 10. The ASK instrument has given new detailed information on flickering aurora.These data are of the prompt N2 emission at 673 nm. The whole field of view, 5 km × 5 km,was within the same auroral arc. The top left frame is the brightness (in kiloRayleigh, kR) aver-aged over 1 s, the frames to the right the power spectral density of the fluctuations in differentfrequency bins and the bottom left frame the frequency with the highest flickering power. Someflickering spots (columns) were as small as 160 m. From Whiter et al. (2008).

dispersed structures agree with acceleration by Alfvén waves at the base of the accel-eration region. For this event an average altitude of ∼3000 km was found from thedispersion.

The images of the aurora obtained simultaneously by the MAC were used toremotely sense the spatial and temporal characteristics of the generation region ofthe Alfvén waves (figure 13). The two top rows of images are for the first dispersioninterval and the two bottom rows for the second. Figure 14 is an enlarged versionof the last image for interval 1. The projection of the Reimei orbit is marked with adashed line and the foot point (the projection of the Reimei position at the momentcorresponding to each image in figure 13) with a cross. Reimei crossed a series of arcswith widths of a few kilometres and the images showed movements of bright regionsalong the arcs. The directions of the movements are marked with arrows. The datacould be used to show that these shear regions would be unstable to the generation ofAlfvén waves.

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Figure 11. Electron measurements of an inverted V event by Reimei. Observed energy fluxesof electrons in: (a) precipitating, (b) perpendicular, and (c) upgoing directions. (d) Pitch angledistributions. (e) Electron energy fluxes in precipitating direction. Solid and dotted lines showthe energy fluxes with energy ranges of 1–12 keV and 0.01–1 keV, respectively. During two inter-vals, marked with blue bars above the top panel, time-dispersed structures were detected atenergies below the peak energy. Such dispersed structures can be considered to be signatures ofacceleration by Alfvén waves. From Asamura et al. (2009).

2.4 Filament caused by primary particles with high energies

Ratios of intensities of different emissions have been used for a long time to giveinformation about the energies of the primary particles. This works because theconditions for creating different emissions depend on factors that vary with alti-tude, such as the atmospheric composition, and density and because particles withhigher energy can penetrate to lower heights in the atmosphere. However, earlierstudies had to make do with the most intense emissions, 472.8 nm (blue), 557.7 nm(green) and 630.0 nm (red). Unfortunately, both the green and the red emissions aremetastable and there are three different major production mechanisms for the greenemission.

EMCCD imagers now allow the use of weaker emissions, so that the long-livedemissions can be avoided. This makes it possible to use the intensity ratio method

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Figure 12. Blow-up of dispersion structures seen in the downgoing electrons in figure 11. Thesestructures are probably caused by acceleration by Alfvén waves. The magnitude of the dispersiongives an acceleration altitude of about 3000 km. Adapted from Asamura et al. (2009).

Figure 13. Images from the MAC imager on Reimei showing the aurora conjugate with themeasurements in figures 11 and 12. The Reimei orbit projected to the altitude of the aurora isshown as a dashed line and the foot point corresponding to each image is marked with a cross.A blow-up of frame 16 in the first sequence is shown in figure 14. From Asamura et al. (2009).

for very small structures as well. Figure 15(a) shows three images of a 200-m wideauroral filament obtained by the ASK instrument (Dahlgren et al. 2008). The leftimage shows the O2

+ first negative band at 562 nm, an emission mainly from the

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Figure 14. Blow-up of a frame from figure 13. The arrows mark flow directions within thearcs. The Alfvén wave-generated regions were found to correspond to flow shears. Adaptedfrom Asamura et al. (2009).

Figure 15. CCD and EMCCD imagers allow the intensity ratio method to be used for small-scale structures and for weaker emissions. (a) Simultaneous images obtained by ASK cameraswith three different filters; (b) I (582 nm)/I (732 nm) intensity ratio plot; (c) ratio plot markedwith three regions with different characteristic intensities; (d) plot of ratio against OI emissionintensity. Here ASK data show that a 200-m-wide filament was caused by electrons having gonethrough additional acceleration and not by a higher electron flux. Adapted from Dahlgren et al.(2008).

E region, and the middle image shows emission mainly coming from the F region,O+ at 732 nm. The right image is of the O emission at 777 nm, which originates fromboth the E and the F regions. The O+ emission has a radiative lifetime of 5 s and the

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intention with this emission is to measure the afterglow of the aurora and thus obtaininformation about its dynamics (Dahlgren et al. 2009).

The intensity ratio between the O2+ and the O emission is given in figure 15(b). In

figure 15(c) the three regions in the image have been marked with different colours. Theintense filament is blue, the less intense aurora equatorwards of the filament is greenand the region north of the filament is red. The O2

+/O ratio as a function of O inten-sity has then been plotted in figure 15(d) for all of the pixels marked in figure 15(c). Theblue points, corresponding to the intense filament, have a lower ratio and thus a lowercontribution of E-region emissions than the more equatorward aurora represented bythe green points. From this, it can be concluded that the intense filament was dueto electrons with higher energies, not just to a greater number of precipitating par-ticles. The electrons responsible for the filament had thus gone through additionalacceleration.

2.5 Auroral curls and ruffs

A fascinating aspect of auroral displays is the different types of vortex structures.Vortices on different scales are common in active auroras. Until recently, curls wereconsidered to be the smallest. Curls are typically 1–2 km perpendicular to the magneticfield and when viewed from the side they look like rays. They wind counter-clockwise,but do not unwind. Often they form vortex streets. The cause has been proposed to bea Kelvin Helmholtz instability resulting from E × B drift, but more recent work hasindicated that the Kelvin Helmholtz instability alone is not sufficient, but that Alfvénwaves are also involved in the mechanism (Vogt et al. 1999).

High-resolution imaging with ASK has now shown that the edges of curls and foldsare adorned with even smaller structures, proposed to be called ruffs (Dahlgren et al.2010). An example of these ruffs is demonstrated in figure 16. The top part showsan auroral vortex street as imaged by the University of Oulu auroral TV system. Thesmall square is the field of view of ASK. The bottom part displays a number of framesfrom ASK. In some of the frames the edges of the ruffs are highlighted. The ruffsappear at edges of both curls and folds and move counter-clockwise along those edges.Their cause is being investigated.

2.6 Dynamic rayed aurora and naturally enhanced ion acoustic lines

During active auroral displays, much of the aurora appears as rapidly moving andchanging rays, aptly described as dynamic rayed aurora. In §2.5 we mentioned thatcurls, when viewed from the side, appear as rays. However, this does not mean thatrays have to be curls. In the previously mentioned study by Semeter et al. (2008), verysharp kinks in small-scale auroral forms were found during auroral breakup. Thesekinks, when viewed from the side, also look like rays. An important property of thesekinks was that, unlike curls, they did not move or bend during their lifetime. Semeteret al. (2008) even suggested that the term ‘dynamic rayed aurora’ may be due to obser-vational bias. However, in our opinion it gives a useful, intuitive description, althoughone should be aware that it may have a number of different physical causes.

Dynamic rayed aurora has been found to be related to a special type of incoher-ent scatter radar echoes, Naturally Enhanced Ion Acoustic Lines (NEIALs; Blixtet al. 2005a). The cause of these echoes has not been established, but studies haveshown that they are frequently associated with fast moving, bright auroral struc-tures. Michell et al. (2009) used an image-intensified CCD (ICCD) imager to study

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Figure 16. A new finding is that edges of curls have even finer structure. The top part of thisfigure is an image from the University of Oulu auroral TV camera showing a vortex street. Thewhite square is the field of view of ASK. The bottom part is a sequence of ASK images. Small-scale structures, ‘ruffs’, move counter-clockwise along the edges of the curls and folds. Thesefeatures were discovered with the ASK instrument. Adapted from Dahlgren et al. (2010).

in detail the generation of NEIALS measured by the Poker Flat Incoherent ScatterRadar (PFISR). Figure 17 shows the radar data and figure 18 the imager data. Theimager was equipped with a filter removing the 557.7 nm emission. The small squarein figure 18 marks the field of view of the radar beam. Figure 17 gives the returned

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Figure 17. Data from Poker Flat Incoherent Scatter Radar (PFISR) with 19 ms resolutionshowing Naturally Enhanced Ion Acoustic Lines (NEIALs). Adapted from Michell et al.(2009).

power from the raw radar data with a temporal resolution of 19 ms. NEIALS appearas short-lived echoes from 200–250 km altitude. The full line shows the auroral inten-sity at the site of the radar beam. It was concluded that the NEIALS were generatedat the boundary of fast moving (∼10 km s–1) auroral structures.

2.7 Fine structure of diffuse aurora

We now leave the discrete auroras and turn to the other main class, the diffuse auro-ras. Although the names discrete and diffuse suggest morphological features, discreteaurora has now generally been taken to mean auroras that involve field-aligned parti-cle acceleration fairly close to the ionosphere, out to a few Earth radii, while diffuseauroras are thought to be due to pitch angle scattering in the plasma sheet (e.g. Lyonset al. 1999). Diffuse auroras are less well understood than discrete auroras and havealso received less scientific attention, although they are in fact responsible for a higherfraction of the energy input to the ionosphere (Newell et al. 2009).

When viewed with sensitive enough imagers it is seen that the diffuse aurora oftencontains fine structure. As mentioned in §1, some of the narrowest structures reportedby Maggs and Davis (1968) may in fact be intensity variations in diffuse aurora. Finestructure of diffuse aurora may sound like a contradiction, but it is easy to imaginethat there may be fine structure in the pitch angle scattering in the generation region.

In February 1997 an interesting coordinated data set was obtained as the FASTsatellite passed over northern Scandinavia and the Auroral Large Imaging System(ALIS; Sergienko et al. 2008). An overview of the aurora was obtained by the

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Figure 18. Examination of images from an ICCD imager showed that the NEIALs were gen-erated at the boundary of bright, fast-moving structures in the aurora. Adapted from Michellet al. (2009).

MIRACLE all-sky camera (Syrjäsuo 2001) in Abisko. To the north there was a brightaurora, to the south there was barely visible diffuse aurora and in between there was aregion that appeared to be fairly empty of aurora.

A mosaic of ALIS images obtained at 557.7 nm is shown in figure 19. The diffuseaurora contained fine structure in the form of parallel stripes with widths of about5 km and typical peak column intensities of between 2 and 5 kR. The whole systemmoved equatorwards with a velocity of about 100 m s–1.

In figure 20 particle data from FAST are shown and compared to the ground-basedoptical data. At the top is the downward electron energy flux measured by FAST. Themiddle panel shows the volume emission rate of 557.7 nm calculated from the FASTdata. The bottom panel shows the column emission rate resulting from this calcula-tion and the column emission rate measured by ALIS. The absolute agreement is very

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Figure 19. Mosaics of ALIS images showing fine structure in diffuse aurora during a pass byFAST. The red and blue line shows the projected orbit and the dot the foot point of FAST. FromSergienko et al. (2008).

good. Careful examination of the pitch angle distributions inside and outside the auro-ral stripes and comparison with theoretical and modelling work led to the followingconclusions: (1) everywhere there was a background of particles with energies up to3–4 keV. These were probably due to pitch angle scattering by electron cyclotron har-monic (ECH) waves. (2) In the stripes there was precipitation of electrons with energiesabove 3–4 keV caused by pitch angle scattering by whistler mode waves. (3) There wasno sign of particle acceleration in the stripes. Thus they cannot be classified as discreteauroral forms.

An important question that remains to be answered is what causes the modulationin pitch angle scattering. One hitherto little explored wave mode in the equatorialplane is the so-called internal gravity wave that was proposed by Safargaleev andMaltsev (1986). This mode should be able to give a velocity in the equatorial planecorresponding to the observed equatorward velocity, 100 m s–1, of the stripes. Furtherwork is needed to establish if this mode is a possible mechanism.

2.8 Black aurora

Our final topic is black aurora. Black aurora describes regions with significantly lowerluminosity than the surrounding aurora. In earlier works they were defined as lowluminosity regions in diffuse aurora or in regions intermediate between diffuse and

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Figure 20. Analysis of electron data from FAST together with the ground-based imager datashows that the fine structure was probably caused by modulation of the pitch angle scatteringby whistler mode waves of electrons above 3–4 keV. Everywhere there was also a background,probably due to pitch angle scattering of lower energy electrons by electron cyclotron harmonicwaves. See the text for further explanation. Adapted from Sergienko et al. (2008).

discrete aurora (Trondsen and Cogger (1997), and references therein), but now manyresearchers also include dark regions related to discrete aurora.

Proposed mechanisms for the creation can be divided into two classes, those inwhich the black aurora is associated with a downward field-aligned potential dropand ionospheric density depletion and those explaining it as a cause of reduced pitchangle scattering. In data from satellites, mainly Freja and Cluster, several eventsof the first type have been found. Most ground-based optical data agree with thesecond type.

Figure 21 was taken from a study by Gustavsson et al. (2008a) using a number ofoptical imagers and EISCAT. To the left are consecutive images taken by ALIS at 4 sinterval. The top and bottom images are of the N2

+ emission at 427.8 nm and themiddle image is of the O emission at 844.6 nm. Several dark stripes were present inthe aurora. To the right are luminosity profiles along the vertical lines marked in theimages. The luminosity decrease in the dark stripes was 25% in the 427.8 nm emissionand 28% in the 844.6 nm emission.

Using the electron density profile measured by EISCAT, the primary electron spec-trum could be calculated. The resulting luminosity variations from the two typesof mechanisms were then modelled. A mechanism involving a downward, retardingpotential drop would give a proportionally larger decrease of the 844.6 nm emissioncompared to the 427.8 nm emission than would the reduction of the pitch angle dif-fusion. The observations agreed much better with the reduced pitch angle diffusionmechanism.

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Figure 21. In a study of non-sheared black aurora, the luminosity reduction in the 427.8 nmemission was found to be about equal to that in the 844.6 nm emission. Modelling using a pri-mary electron spectrum derived from EISCAT data showed that this fits best with a productionmechanism involving reduced pitch angle scattering. For a retarding potential mechanism, thereduction in the 844.6 nm emission would have been larger. The figure shows data from ALIS.Further explanation is given in the text. From Gustavsson et al. (2008a).

Thus, this study comes to a similar conclusion to most earlier ground-based studies,that black aurora is an effect of a reduction in the pitch angle diffusion, presumablytaking place in the equatorial plane region. However, the satellite data of downwardpotential drops and reduced ionospheric plasma density are also very convincing, andshould correspond to auroral signatures. It is of great interest to find those signatures

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in optical data or, alternatively, to find an explanation for the lack of such signatures.It appears most likely that the two types of mechanisms represent different types ofaurora. One idea is that the pitch angle diffusion mechanisms are related to diffuseaurora and that the retarding potential mechanisms are related to discrete aurora, butthis needs to be investigated. With this comes the issue of terminology. If the differ-ent types of low luminosity regions have quite different generation mechanisms, theyshould also have different names.

3. Outstanding questions

The overarching question is how near-Earth space works, but in the context treatedhere we address small-scale structures more specifically and can list four generalquestions:

1. What are the mechanisms responsible for different small-scale auroralstructures?

2. How and to what extent do small-scale structures couple to the magnetosphere?3. What is the role of small-scale structures in magnetosphere–ionosphere inter-

action?4. What can we learn from observing small-scale structures?

Ground-based optical observations have a key role when searching for answers to thesequestions. High spatial and temporal resolutions are essential and ground-based opti-cal instruments are able to give the best resolution of all types of instruments. Thesame region can be observed for an extended period of time, thus making it possibleto resolve the temporal–spatial ambiguity. By combining images from different sitesit is even possible to get the three-dimensional luminosity distribution in a volume(e.g. Frey et al. 1998, Gustavsson 1998, Aso et al. 2008). By using inversion methods,either from emission intensity ratios or from the three-dimensional luminosity distri-bution, the spatial and temporal pattern of precipitating electrons can be derived. Ofcourse, information from other ground-based instruments as well as in situ informa-tion from satellites and sounding rockets is also very important and the best resultscan be expected when measurements from different platforms can be combined.

4. Recommendations for the future

The following recommendations aim to answer the questions given in §3.

1. We need optical instruments measuring different scales simultaneously.Instruments with high temporal and spatial resolution need to be combinedwith instruments giving an overview. In many cases this means increased collab-oration between different research groups. A problem during the years aheadcould be a lack of global imaging (Donovan et al. 2007), but global imagingis necessary if we are to understand the role of small-scale processes in thelarge-scale behaviour of the aurora.

2. We need excellent calibration facilities. For detailed physical questions absolutemeasurements are needed. For multipoint studies instruments need to be inter-calibrated. The calibration of imagers is particularly difficult, requiring accessto a light source with absolutely homogeneous luminosity over an extendedarea. The best way to achieve this is by a large integrating sphere, and the only

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large integrating sphere openly available to the optical aurora community is theone at the National Institute of Polar Research in Tokyo.

3. We need a coordinated infrastructure for ionospheric research. Small-scalestructures in the optical aurora are accompanied by small-scale structures inother ionospheric properties such as particles, fields and electrodynamics. Thestrength of combining optical and incoherent scatter radar measurements hasbeen demonstrated and phased array systems such as the Advanced ModularIncoherent Scatter Radar (AMISR) in North America and the even more pow-erful proposed EISCAT_3D in Northern Europe make it possible to measureseveral parameters in three dimensions with very high temporal and spatialresolution. The Super-DARN system gives an excellent overview of convec-tion patterns, but it is desirable to extend the field of view to cover the NorthEuropean mainland, which is otherwise well instrumented. The infrastructurealso needs to include sounding rockets for detailed in situ measurements.

4. To interpret and understand our data we need comprehensive models ofauroral particle interaction with the upper atmosphere. Most of the existingmodels are steady state. There has been one attempt to develop a time-dependent model for the auroral electron transport, published by Peticolasand Lummerzheim (2000). This model has a number of simplifications suchas continuous energy loss for primary high-energy electrons that implies noangular scattering, and local energy loss for secondary electrons. As a conse-quence, a correct result for temporal and spatial modifications of the energyand angular electron distributions cannot be obtained by this model. However,it gives a very important result, namely that the timescale involved for theauroral electron transport process to come to a steady state is longer than0.5 s. Time-scales shorter than 0.5 s occur frequently in many types ofaurora, in particular for small-scale structures. Thus, a time-dependent modelis needed for quantitative description of small-scale processes in the auroralionosphere and for understanding their role in the magnetosphere–ionospherecoupling.

5. A roadmap for studies of small-scale structures would help to optimize the useof existing resources and focus future work. Many recent new findings suggestthat the classification of small-scale auroral structures should be revisited andpossibly revised.

5. Conclusions

In this paper we have reviewed recent results, mainly from optical instruments, regard-ing small-scale auroral structures. Optical instruments offer the best temporal andspatial resolution for such studies.

The aurora is very rich in small-scale structures. Their study should provideimportant information on magnetospheric and ionospheric physics. Thanks to newtechnology, breakthroughs are within reach. New results have been obtained regardingseveral phenomena including:

(i) Alfvén waves, their cascading to create multiplication of elemental structures,their interference to create flickering aurora, their generation in shear regions.

(ii) The use of intensity ratios for investigation of primary electrons at smallscales.

(iii) Fine structure of auroral curls or ‘ruffs’.

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(iv) Dynamic rayed aurora and NEIALs.(v) Fine structure of diffuse aurora and black aurora.

Recommendations for future work on small-scale auroral structures are:

(i) Fine-structure measurements should be combined with measurements givingan overview of the aurora.

(ii) Better and more accessible optical calibration facilities are needed.(iii) A coordinated infrastructure of ionospheric research including advanced

optical instruments, phased array incoherent scatter radars, backscatterradars, other ground-base instruments and sounding rockets would be thebest way to make progress on the physics of small-scale structures and theirimportance.

(iv) Time-dependent high-resolution models need to be developed.(v) A road-map for future studies should be developed and the classification of

small-scale structures should be revised.

AcknowledgementsWe thank the participants at the 36th Annual European Meeting for AtmosphericStudies by Optical Methods for valuable discussions regarding small-scale auroras.

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Fine structure of aurora

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