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The azimuthal evolution of the substorm expansivephase onset aurora

E. Donovan, S. Mende, B. Jackel, M. Syrjasuo, M. Meurant, I. Voronkov, H. U. Frey, V.Angelopoulos, and M. Connors

Abstract: We use data from two white light All-Sky Imagers, deployed as part of the THEMIS Ground-Based Observatoryprogram, to explore the azimuthal evolution of the breakup aurora during a pseudobreakup that occurred on November28, 2005. We find that this breakup occurred on a pre-existing auroral arc. It began with a brightening that formed on anextended region along the arc, and consisted of eastward propagating beads with a wavelength of ∼100 km. During thefive minutes following the breakup, a second pre-existing arc that was poleward of the first remained undisturbed. Theinitial azimuthally extended region of enhanced brightness along the arc did not expand further for at least two minutes,after which it expanded rapidly along the arc. From these observations, we conclude that the breakup in this eventcorresponds to Current Disruption in the inner magnetosphere caused by an instability that creates azimuthally propagatingwaves and that is not triggered by a fast Earthward flow.

1. Introduction

Identifying the macroscale instability responsible for sub-storm expansive phase onset is an important goal in space phys-ics. Bringing closure to this question has been the motivationfor numerous, theoretical, simulation, and observational eventand statistical studies over the years. We now understand thatboth reconnection in the mid-tail and current disruption in theinner magnetosphere occur as integral parts of the substorm,but their interrelationship and in particular whether one startsand then sets in motion a sequence of events that leads to theother is not currently understood [2, 7, 8]. Careful examinationof coordinated in situ and ground observations have providedperhaps the best insights in this direction (see e.g., [13, 9]),however it has become clear that although the expansive phaseonset unfolds very rapidly, the data with which we are able toaddress these questions is fundamentally limited in its spatio-temporal coverage and resolution.

In general, substorm event studies are predicated on the ideathat the correct combination of solar wind, in situ, and remotesensing of the ionospheric electrodynamics from the groundand space would be able to provide sufficient information torule out some proposed onset mechanisms and provide con-straints for models and simulations used to explore mechan-isms that are still viable. For example, the typical substormbreakup arc is embedded in the bright proton aurora whichmaps to the transition between tail-like and dipolar field lines[12, 3, 4], most often in the region between geosynchronousorbit and L�10Re. We also understand that the breakup, sub-storm current wedge, current disruption, dipolarization, and in-jection are all manifestations of the same inner Central PlasmaSheet (CPS) disturbance (see, for example, [9]). The ground-based auroral and magnetic field data in particular point to

Received 9 July 2006.

E. Donovan, B. Jackel, M. Syrjasuo, M. Meurant, and I. Voronkov.Department of Physics and Astronomy, University of CalgaryS. Mende, H. U. Frey, and V. Angelopoulos. Space Physics Labor-atory, University of California, BerkeleyM. Connors. Athabasca University, Athabasca, Alberta, Canada

a disturbance that begins in the inner CPS and evolves azi-muthally and meridionally, the latter reflecting radial evolutionin the magnetosphere [5], consistent with the Current Disrup-tion (CD) paradigm. However, attempts to create a synopticpicture of the evolution of the expansive phase from analysisof combined in situ and ionospheric remote sensing data hasled to a more ambiguous picture, leading to at least three setsof inconsistent conclusions: 1) expansive phase began in theinner CPS after which it progressed tailward (consistent withthe CD paradigm); 2) expansive phase began in the mid-tailwith reconnection, fast Earthward flows, and subsequent innerCPS effects (i.e., consistent with the Near-Earth Neutral Lineor “NENL” paradigm); 3) that the mid-tail and inner CPS ex-pansive phase processes might evolve largely independently ofone another [9, 11, 13, 10, 17].

Bringing closure the question of how and where expansivephase onset is initiated in the magnetotail is the primary sci-entific motivation for the upcoming NASA Time History ofEvents and Macroscale Interactions in Substorms (THEMIS)MIDEX mission. THEMIS represents the first true constellation-class geospace mission, and will involve five satellites on equat-orial orbits. The orbits will have 1, 2 and 4 day periods, cor-responding to ∼10, 20, and 30 Re apogee distances, respect-ively. The outer two satellites will be at or near apogee formore than ten hours every four days. The inner three will to-gether provide coverage of the ∼10 Re region for those sameten hours. Throughout the mission, the apogee conjunctionswill be over central Canada, on a meridional line bracketed byGOES East and West. During the winter months, apogee willbe in the magnetotail, where the seven THEMIS and GOESsatellites will bracket the NENL and CD regions. Although theTHEMIS constellation will provide numerous and unpreceden-ted opportunities to study the radial evolution of the expansivephase in the magnetotail, the satellites by themselves wouldnot be able to bring closure to the question at hand. While it istrue that the expansive phase evolution is most often couchedin radial terms (as outlined in the previous two paragraphs),the disturbance also evolves azimuthally. Thus, although datafrom the satellites might indicate a clear ordering of events, itmight also be that the process started away from the apogee

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56 Int. Conf. Substorms-8, 2006

meridian and later swept over it.The THEMIS program has a ground-based component which

is specifically designed to deal with the azimuthal uncertaintiesthat would be present if all that was available were satelliteson a more or less radial line through the CPS. The apogee me-ridian was chosen to be over central Canada to take advantageof the fact that a large fraction of the auroral oval is over landin northern Canada and that ground-based space science instru-mentation is already operating in that sector (e.g., instrumentsoperated by Canadian GeoSpace Monitoring, SuperDARN, theUniversity of Alaska Geophysics Institute, MACCS, and otherprograms). In addition, a continent-wide array of fluxgate mag-netometers and white light auroral imagers is being deployedspecifically as part of THEMIS. In Figure 1, we show the loc-ations and fields of view (FOVs) of the 20 All-Sky Imagers(ASIs) that make up the THEMIS ASI array.

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Fig. 1. Fields of view of white light ASIs that will be operatingas part of the THEMIS Ground-Based Observatory network.The fields of view assume 110 km emissions 10 degrees abovethe horizon. The black contours indicate constant magneticlatitude and longitude (Altitude Adjusted Corrected GeomagneticCoordinates or “AACGM” Epoch 2000 - [1]). The magneticlatitude contours are for 60◦ through 80◦ in 5◦ increments.The magnetic longitude contours are separated by one hour ofmagnetic local time.

The THEMIS ASIs will take one image every three seconds.Imaging will be synchronized across the array with Global Po-sitioning System (GPS) timing. While the ASIs and magne-tometers will provide essential contextual information in sup-port of THEMIS meeting its objectives, these new instrumentswill also bring something truly new to substorm studies. Forexample, existing arrays of ground-based optical instrumentshave provided numerous instances of simultaneous observa-tions across all relevant latitudes. The Finnish MIRACLE ASI,Canadian NORSTAR ASI, and CANOPUS Meridian ScanningPhotometer arrays are but three examples of this (see e.g., [5]).The azimuthal evolution of the expansive phase aurora hasbeen extensively studied using global auroral imagers on satel-lites, but the temporal and spatial resolution afforded by thoseinstruments have not been sufficient to explore the details ofthe expansive phase onset (see e.g., [6]). Since we know that, atthe time of the auroral breakup, the auroral features can evolveazimuthally (i.e., east-west) beyond the FOV of a single imager(see [16]) in several tens of seconds, being able to track theevolution of the expansive phase aurora across several hoursMagnetic Local Time (MLT) with three second temporal res-

olution will be an important step forward in constraining sub-storm theories.

One of the great advantages to being involved in the ground-based component of a mission like THEMIS is that we havedata before the satellites are launched. Deployment and opera-tion of the imagers started in 2004, and the array will be com-pletely in place by the fall of 2006. By the end of the northernhemisphere 2005-2006 winter, there were 13 ASIs operatingin Canada and Alaska. Summary keograms and thumbnail im-ages can be seen on the “GAIA” auroral virtual observatory(http://gaia-vxo.org).

In this paper, we present data from a small substorm that oc-curred over the Alaska-Canada border region. The initial au-roral breakup was in the FOV of one ASI and quickly spreadeast into that of an adjacent imager. We have two objectives inthis paper. First, we want to highlight some of the new capab-ilities that the THEMIS ASI array will bring to the THEMISprogram and substorm studies in general. Second, we want toexplore the azimuthal evolution of the breakup aurora on a timescale of tens of seconds, and comment on what the auroral dataindicates in terms of the substorm onset mechanism. We pointout in advance that we are presenting data from only one event.Further, as the auroral expansion lasts only ∼10 minutes, andas it does not appear to lead to lobe flux reconnection, thisevent is a pseudobreakup (see [17] and references therein forwhat we mean by “pseudobreakup”).

2. The Event

The event that we focus on occurred between 10:00 and10:30 UT on November 28, 2005. In the several hours lead-ing up to the event, the solar wind speed and dynamic pres-sure were relatively steady, and consistently below 400 km/sand 4 nPa, respectively. As well, during the time leading upto the event the Interplanetary Magnetic Field was consistentlysouthward. In short, the IMF and solar wind plasma observa-tions do not indicate any trigger for the event. At ∼10:10 UT, a∼ 200 nT negative H-bay was observed in western Canada andAlaska (see below). The event does not have any noticeablesignature in the geosynchronous GOES East or GOES Westmagnetic field data. In Figure 2, we show FUV auroral imagesfrom the IMAGE WIC instrument for this event. The auroralbreakup is evident in the 23:00-24:00 MLT sector, starting firstat roughly ∼10:14 UT, and ending by ∼10:20 UT.

On that night, there were relatively clear skies over two ofthe THEMIS ASIs under the region where the auroral breakupshown in Figure 2 occurred. These were the Fort Yukon (Alaska,USA) and Whitehorse (Yukon, Canada) ASIs, the FOVs ofwhich are shown in the expanded map in Figure 3. In Figure 4,we show a sequence of auroral images at 12 second intervals(top) from the Fort Yukon ASI, and the X-component magneticfield data from the Fort Yukon magnetometer (bottom). Look-ing first at the ASI images, we see the onset arc was in thesouth of the imager FOV, located at roughly 64◦ magnetic lat-itude (see Figure 3). The image sequence begins at 10:12 UT, atwhich time the substorm brightening was already visible in thesouth. As well, there was a quasi-stable auroral arc just slightlynorth of overhead in the imager. Over the next ∼5 minutes, thebrightening evolved into a vortex which grew poleward andazimuthally. It was only after ∼10:17 UT (i.e., more than 5

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minutes after the initial brightening) that the more polewardarc was noticeably disturbed. The image sequence spans thetime period bracketed by the two dashed lines in the lowerpanel, and hence the time during which there is an enhancedwestward electroject and corresponding negative H-bay.

In Figure 5, we focus in on the step-by-step developmentof the auroral breakup from 10:11:12 until 10:12:54 UT. Asthe arc that brightened was in the southern part of the FOVof the Fort Yukon ASI, we restrict our attention to partial im-ages, which comprise the left-hand column of the figure. Theimage at 10:11:12 UT is the first image in which there is un-mistakeable evidence of the onset. Note that this is more thana minute before the negative H-Bay is noticeable in the FortYukon magnetometer data, and a full three minutes prior to thepseudobreak being clearly evident in the IMAGE WIC data. Inthe right-hand panel of Figure 5, we show differences betweensuccessive images (see figure caption), which highlight the wavedisturbance more clearly than do the raw images. The differ-

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ence image sequence shows a bead-like structure that emergeson the pre-existing breakup arc. Over the nearly two minutesspanned by the image sequence, the beads brightened, and propag-ated eastward (note that examination of the three second datashows that there is no problem with aliasing). The wavelengthand azimuthal propagation speeds of the beads were estimatedat ∼100 km, and ∼5 km/s, respectively (presuming 110 kmaltitude emissions).

Referring to Figure 3, the arc visible to the south of FortYukon must have extended into the poleward part of the White-horse FOV. In Figure 6, we show a time sequence of partialimages from Fort Yukon and Whitehorse side by side. Before10:12:00 UT, the brightening was restricted to the Fort YukonFOV, although it did expand rapidly in the azimuthal direc-tion, involving the entire arc within the FOV in less than 30seconds. By 10:12:24, the brightening was clearly visible onthe arc at the western edge of the Whitehorse FOV. For thenext 90 seconds, the eastern edge of the disturbance remainedfixed. That is, the brightening extended azimuthally along thearc until it appeared just inside the Whitehorse field of view,at which time the azimuthal growth stalled for 90 seconds ormore. By 10:14:24, the brightening was spreading rapidly east-ward along the pre-existing arc. The speed of this expansionwas ∼0.5 km/s.

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Fig. 4. The negative H-bay associated with the pseudobreakupis clearly evident in the Fort Yukon magnetometer data. Imagesare from the Fort Yukon THEMIS ASI, separated by 12 seconds,and spanning the time range indicated by the two vertical dashedlines. UT for each image shown is indicated in the grey boxesand is given in hhmmss format.

3. Summary

This event was a pseudobreakup that does not appear to havebeen triggered by changes in the interplanetary medium. Theevent unfolded in just over ten minutes. Seen in ASI data, theevent began with a brightening that spread rapidly along thepre-existing arc. While this azimuthal expansion was occur-ring, there was a well-defined wave form in the brightnessalong the arc that grew in intensity and also propagated rap-idly eastward. The azimuthal expansion stalled for 90 secondsor more, and then proceeded further with the brightening ex-tending across the entire FOV of the Whitehorse ASI over thenext two minutes. While this expansion phase was unfolding,there was a pre-existing arc poleward of the breakup arc thatremained undisturbed until at least five minutes after the initialbrightening.

As discussed in the Introduction, there is intense debate overwhat macroscale instability leads to the expansive phase onset.While the data that we present here in this paper is only for

one event, and utilizes data from only two of the soon to be20 THEMIS white light ASIs, it brings forward some interest-ing points. First of all, the pre-existing arc that brightened wasequatorward of another pre-existing arc. That arc remained un-disturbed for at least five minutes while the expansive phaseis clearly unfolding. Although we do not have proton auroraldata available for this event, the onset arc was at only 64◦ mag-netic latitude, and so in this event was likely embedded in thebright proton aurora (see the relevant discussion in the intro-duction). Further, the auroral brightening occurred at the sametime as the negative H-Bay, and so it is compelling to associateit with CD in the inner CPS. Thus, if the CD was initiated bythe braking of a fast flow that was in turn launched by mid-tail reconnection, then all of the dynamics leading up to theCD created no auroral signatures that were detectable by theTHEMIS ASIs, and had no detectable effect on the second,poleward arc. Thus, subject to the caveats discussed below,these observations are (in our opinion) more consistent withthe CD rather than the NENL model.

The azimuthal propagation of the beads that formed duringthe first few minutes of the brightening point to the growth ofa wave in the CPS magnetically conjugate to the arc. Althoughthis is only one event, the azimuthal structuring and propaga-tion were very clear, and occurred during the first 1.5 minutesof the brightening, and commensurate with the formation ofthe negative H-bay. It is thus compelling to associate thesewaves with an instability that leads to a decrease in the cross-tail current. In a recent study, [11] used Cluster observations toshow the presence of eastward propagating waves during theinitial several minutes of expansive phase onset, and arguedthat these waves were a manifestation of ballooning mode in-stability. Those authors went on to argue that the azimuthalstructuring would lead to filamentation of the perpendicularcurrent in the current sheet, and via ∇ · �J = 0 would gen-erate periodic and eastward propagating filaments of parallelcurrent. This could account for the bead-like structuring of thebreakup arc shown in Figure 5.

The initial brightening occured in the FOV of the Fort YukonASI. The brightening rapidly expanded azimuthally until it wasin the FOV of the Whitehorse ASI (we have no images fromwest of Fort Yukon so we do not know how far expansion wentbeyond the western edge of the FOV of the imager). At thattime, the azimuthal expansion stalled, only to pick up againseveral minutes later. Our interpretation of this is that the in-stability that gave rise to the eastward propagating structuresdiscussed in the previous paragraph formed virtually instantan-eously (i.e., in less than 30 seconds) in an azimuthally extendedbut radially localized region in the CPS. Again, in our view,this region in which the initial instability formed was magnet-ically conjugate to the part of the arc that initially brightened.It is interesting to note that a separate study found that the ra-dial and azimuthal extent of the initial dispersionless injectionregion is azimuthally extended and radially limited in muchthe same way [15]. Further, that study also found that, on aver-age, the initial dispersionless injection region forms outside ofgeosynchronous distance, most likely 8-10 Re from the Earth.

These results are obviously preliminary, and are subject toseveral important caveats. First, we have no idea what an au-roral arc corresponds to in the magnetosphere. This is a tre-mendously troubling issue given our field’s reliance on the

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evolution of discrete aurora in studying the expansive phaseonset. Second, in this particular case, we do not have some fun-damentally important complementary observations that wouldallow us to place the two arcs in the Fort Yukon FOV in a bet-ter magnetospheric context. On the one hand, if we had goodcomplementary proton auroral observations, then we would beable to place the breakup arc relative to the ionospheric foot-print of the transition between dipolar and tail-like field lines(which is where we believe that arc maps to). On the otherhand, if we had good 630 nm auroral “redline” observations,we would know where the open-closed boundary was relativeto the poleward arc. This latter point would allow us to assesshow strongly the lack of disturbance of that arc supports thenotion that formation of a NENL and subsequent fast flowscannot have preceded this auroral breakup. Subject to thesecaveats, we assert that the data presented here supports the fol-lowing conclusions for this event:

1. the breakup corresponded to CD in the inner CPS.

2. the CD was not triggered by fast Earthward flows.

3. the breakup was initiated by an instability that producedazimuthally propagating waves.

4. the initial instability formed in a radially localized butazimuthally extended region in the CPS.

5. our results are consistent with the CD rather than theNENL substorm model.

As we come into the THEMIS era, it is clear the the THEMISASI array will provide synoptic white light images with unpre-cedented spatial and temporal resolution. Future work alongthe lines of this study will involve the building up of a reper-toire of events, comparing wavelengths and propagation withthose predicted for magnetospheric waves and instabilities, andattempts to better constrain the mapping between the magneto-sphere and ionosphere. Finally, we point out that we need todevelop an understanding of what auroral arcs correspond toin the magnetosphere.

Acknowledgements

Operational support for the THEMIS All-Sky Imager array isprovided by NASA and the Canadian Space Agency. The FortYukon magnetometer is operated by the Alaska GeophysicsInstitute. The IMAGE WIC data was mapped to the northernhemisphere using a technique developed by Nikolai Ostgaard.We used NASA CDAWeb to survey complementary data re-ferred to but not included in the paper.

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Fig. 5. The left column consists of partial Fort Yukon white lightASI images (roughly the bottom quarter of the image in eachcase) separated in time by 6 seconds with time increasing downon the plot. The time corresponding to each image is indicatedin hhmmss format. The images go from 10:11:12 UT through10:12:54 UT, and so span 1 minute and 42 seconds (as an aside,we note that the THEMIS ASIs will operate at a cadence of oneimage every three seconds. The right hand column consists ofdifferences between successive images in false color. We havemapped a range of differences around zero to white so as tosuppress the effects of CCD readout noise.

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Fig. 6. The onset and subsequent expansion can be seen inthe images collected by both the Fort Yukon and Whitehorseimagers (see Figure 3). The onset arc is in the southern partof the Fort Yukon field of view and the northern part of theWhitehorse field of view. In this figure, we show subsequent(time increasing downwards) partial images from the Fort Yukon(left) and Whitehorse (right) ASIs. The Fort Yukon partial imagesare roughly the bottom third of the field of view. The whitehorseimages are roughly the top one third of the field of view.

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