dayside pulsed aurora intensifications, observed by itaca...

Dayside pulsed aurora intensifications, observed by ITACA during

constant interplanetary magnetic field Bz �� 0 and By �� 0

S. Massetti, S. Orsini and M. CandidiIstituto di Fisica dello Spazio Interplanetario, Consiglio Nazionale delle Ricerche, Roma, Italy

K. KauristieDepartment of Geophysics, Finnish Meteorological Institute, Helsinki, Finland

Received 4 December 2001; revised 16 January 2002; accepted 16 January 2002; published 27 September 2002.

[1] On 30 November 1999, the Italian all-sky camera ITACA (Ny-Alesund, Svalbard),observed one of the strongest dayside aurora events occurred during the 1999/2000 winterseason. In the postnoon, the red auroral activity was characterized mainly by fourquasiperiodic pulsed activations, with brightness, period and latitudinal extent increasingmonotonically with time. During this period, the Wind satellite was well positionedupstream in the solar wind and measured a IMF Bz close to zero, together with a constantlynegative IMF By � �10 nT. A detailed description of the phenomenon is presented,together with a preliminary analysis of the data. This work underlines that it is difficult toidentify, among the parameters usually involved in the solar wind–magnetospherecoupling, the source of the modulation(s) observed in the aurora development. INDEX

TERMS: 2704 Magnetospheric Physics: Auroral phenomena (2407); 2724 Magnetospheric Physics:

Magnetopause, cusp, and boundary layers; 2736 Magnetospheric Physics: Magnetosphere/ionosphere

interactions; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; KEYWORDS:

magnetosphere, ionosphere, IMF, aurora, dayside aurora, pulsed activity

Citation: Massetti, S., S. Orsini, M. Candidi, and K. Kauristie, Dayside pulsed aurora intensifications, observed by ITACA during

constant interplanetary magnetic field Bz � 0 and By � 0, J. Geophys. Res., 107(A9), 1255, doi:10.1029/2001JA009204, 2002.

1. Introduction

[2] The location and extent of the magnetic merging atthe dayside magnetopause depend on the interplanetarymagnetic field (IMF) orientation with respect to the Earth’smagnetic dipole, and in particular on the IMF Bz and By

components. In the so-called ‘‘antiparallel reconnection’’ (orhigh shear reconnection), the IMF component along the zaxis controls the efficiency of magnetic reconnection andthe latitude of the cusp/cleft structure: when Bz < 0reconnection occurs at low latitudes (sunward of the nom-inal cusp), the cusp/cleft move accordingly toward theequator, and the magnetosheath particles can effectivelyflow to low altitude along the merged field lines whilethese move tailward across the cusp/cleft region. On thecontrary, when Bz > 0 the reconnection can take place athigh latitudes, the cusp/cleft structure shifts poleward, andthe reconnection-related phenomena are usually weak. They component of the IMF influences both the longitudinaldisplacement of the cusp/cleft structure and the draggingdirection of the newly opened magnetic field lines, moremarkedly when Bz is negative. The two effects are somehowopposed: when By > 0 (By < 0) the cusp/cleft region shiftstoward dusk (dawn), while the dragging of the reconnectedfield lines is westward (eastward), due to the Svalgaard-Mansurov effect. The dragging effect drives the ionospheric

signature of dayside reconnection events, so that theresponse of the associated auroras promptly follows the By

sign changes (taking into account the delay time). Thereconfiguration of the dayside magnetosphere that followsreconnection episodes is a dynamical phenomenon occur-ring with a time scale of minutes, which can lead to quickand deep modification of the magnetospheric configuration.Merging can take place also between magnetic fields thatare not exactly antiparallel, in fact there is observationalevidence that reconnection takes place equatorward of thecusp also when the magnetic shear is low [e.g., Fuselier etal., 2000, and references therein], and this is known as‘‘component reconnection.’’ In this view, the neutral line ofthe reconnection may cross the subsolar magnetopause atany clock-angle of the interplanetary magnetic field, definedas

q ¼ arctanðjByj=BzÞ Bz > 0

q ¼ p� arctanðjByj=BzÞ Bz < 0;ð1Þ

where q ranges between 0� (Bz > 0, By � 0), 90� (Bz � 0)and 180� (Bz < 0, By � 0). The IMF clock-angle q and theassociated magnetic shear at the magnetopause, is found tobe a critical parameter for the solar wind - magnetospherecoupling [Phan and Paschmann, 1996]. The low-shearregime corresponds to the clock-angle range q < 45� (Bz > 0,Bz > |By|); in this case reconnection tends to occurpoleward of the cusp at high latitudes. Conversely, when

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A9, 1255, doi:10.1029/2001JA009204, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JA009204$09.00

SMP 19 - 1

q > 90� (Bz < 0) magnetic merging fades at the highlatitude and gradually drifts equatorward of the cusp,progressively going to high-shear regime (i.e., antiparallelreconnection) for q ! 180�. In the interval 45� < q < 90�reconnection may appear at both high and low latitude. Inall three cases, strong asymmetries along the east-westdirection are introduced by the By IMF component, andthese give rise to prenoon, noon, and postnoon (magnetic)sectors associated phenomena [Sandholt et al., 1998]. Themagnetic merging at the dayside magnetopause can bebiased by impulsive changes in the dynamic pressure ofthe solar wind. It was suggested that an increase of thedynamic pressure at the magnetopause current sheet maylead to its decay through reconnection, or enhance the rateat which reconnection is occurring [Farrugia et al., 1995].In particular, the pressure pulses exceeding 20–30% ofrelative increase are reported to cause auroral signatures,whereas smaller fluctuations do not seem to producenoticeable effects [Farrugia et al., 1995; Lui and Sibeck,1991]. In this frame, Sandholt et al. [1994] proposed theexistence of two distinct classes of dayside auroral events,‘‘stimulated’’ and ‘‘spontaneous’’, both occurring inassociation with open magnetic flux. In the first one, theauroral intensifications are closely linked to enhanceddynamic pressure variations, while in the second one thedynamic pressure appears to play no role.[3] The coupling of the solar wind to the magnetosphere

is not just a function of the magnetic reconnection effi-ciency, but is thought to result from a complex combinationof magnetic merging and the so-called viscous-type pro-cesses. The relative contribution of these two interactionsvaries in response to many factors, mostly the IMF strengthand orientation. The so-called flux transfer events (FTE)[Russell and Elphic, 1978] have been widely reported [e.g.,Karlson et al., 1996, and reference therein] to be one of the

most characteristic signatures of quasiperiodic reconnectionbursts acting at the dayside magnetopause, predominantlyobserved during southward IMF. Lockwood and Wild[1993] performed an analysis of the FTE recurrence rateand showed that it roughly ranges between 2 and 18 min,with an average value of about 8 min. It is not clear,however, if the rate of FTE occurrence is linked to someintrinsic property of the magnetosphere-ionosphere systemand/or is caused by variations in the solar wind conditions[e.g., Kuo et al., 1995]. The development of Kelvin-Helm-holtz instability (KHI) at the magnetopause and/or at theinner edge of the Low-Latitude Boundary Layer (LLBL)has been long suggested as a candidate viscous-mechanism,likely to produce momentum and energy transfer to themagnetosphere [e.g., Saunders, 1989; Miura, 1992; Farru-gia et al., 1994]. It has been suggested by Saunders [1989]that waves associated with the KHI could be driven unstableby magnetic reconnection at the dayside magnetopause.Moreover, MHD numerical simulations showed the abilityof KHI to guide the growth of plasma vortex structures [Weiet al., 1990]. These structures coalesce and grow in size asthe plasma flows downtail, and the vortices generated in thepostnoon boundary layer can lead to the presence oflocalized upward field-aligned currents, which may accountfor dayside auroras [Wei et al., 1990].[4] In this work, we present an analysis of the dayside

auroral recordings obtained by the high latitude auroralmonitor ITACA (Italian All-sky Camera for Auroral obser-vations). The ITACA station is located at Ny-Alesund (78�550 N, 11� 560 E, Svalbard) and has been working sinceNovember 1999 [Orsini et al., 2000]. ITACA participates inMIRACLE (Magnetometers - Ionospheric Radars- AllskyCameras Large Experiment), a multi-instrument array forionosphere-magnetosphere coupling studies, and it actuallyconstitutes the northernmost all-sky camera of the network.

Figure 1. Gray-coded 630 nm aurora intensity plotted as a function of invariant latitude (ILAT) andtime (UT), as measured by the ITACA observatory during 30 November 1999. The intensity isnormalized and the background is cut off. An auroral emission height of 250 km was assumed for themapping. ITACA is located at about 76� ILAT, and the magnetic local noon is about 0850 UT (MLT =UT + 0310). Two sequences of auroral brightening can be identified in the keogram: a first one occurringbetween 0800 and 0900 UT, and a second one developing in the interval 0930-1140 UT.

SMP 19 - 2 MASSETTI ET AL.: DAYSIDE PULSED AURORAL INTENSIFICATIONS

During the 1999/2000 winter season, ITACA operated for120 days, from 24 November till 23 March. On 30 Novem-ber, between 0800 and 1200 UT (about 1100-1500 MLT),the daytime auroral activity exhibited a relatively uncom-mon strong red (630.0 nm) emission, characterized by asequence of quasiperiodic events with increasing intensity,period and latitudinal extent. Through this interval of time,the interplanetary magnetic field was characterized typicallyby By = �10 nT, and Bz � 0 nT.[5] In the following, we investigate the behavior and the

origin of the pulsed auroral activity: in the next section wedescribe the data used, in section 3 we show an overview of

the auroral activity during the period under study, in section4 the event is discussed in detail, while section 5 is devotedto the discussion and conclusions.

2. Data

[6] During the period under investigation, 0800-1200 UT,the 3-hourly Kp index ranged between 2 and 3, while theDst index ranged between �3 nT and +24 nT. The data setsused in the present work are the following:1. The ITACA data consisting of digitized all-sky images

(about 180� field-of-view) in both red (630.0 nm) and green

Figure 2. From bottom to top: (a) invariant latitude of both equatorward and poleward boundary of redaurora (EQ and PO, dashed area) together with invariant latitude of the peak intensity (PK, line within thedashed area); (b) peak and mean red aurora intensity (PI and MI); (c) IMF clock angle divided into threeshear regimes (dashed lines); (d) IMF Bz component; (e) IMF By component; (f ) solar wind dynamicpressure. The interplanetary data are plotted keeping into account the travel time to the bow-shock(depending on the satellite position), plus a delay of 16 min. The x axis scales indicate the universal time(UT) and the magnetic local time (MLT = UT + 3:10); the three black arrows mark the passages of DMSPF13 and F14 satellite within the ITACA field-of-view. The two thick segments at the bottom mark theperiod of westward and eastward motions of the auroral forms.

MASSETTI ET AL.: DAYSIDE PULSED AURORAL INTENSIFICATIONS SMP 19 - 3

(557.7 nm) light, with a sampling rate of 1 min for the redimages and 20 s for the green ones. Figure 1 shows thegray-coded keogram of the red aurora activity between 0700and 1200 UT, with the intensity normalized and thebackground cut off. With appropriate algorithms, from the1 min red images we extracted the following parameters:EQ, invariant latitude of the equatorward boundary ofauroral emission (Figure 2a); PO, invariant latitude of thepoleward boundary of auroral emission (Figure 2a); PK,invariant latitude of the peak intensity (Figure 2a); PI, peakintensity (Figure 2b), determined along the local magneticmeridian; MI, mean intensity (Figure 2b), calculated within

field-of-view of 120� centered on the local zenith, and withthe background subtracted.2. The interplanetary parameters as measured by the

Wind satellite, are used. The solar wind speed wasrelatively low, with a mean Vx of about 360 km/s. TheIMF By, IMF Bz, dynamic pressure (Pdyn) values, togetherwith the IMF clock angle are plotted in Figures 2c–2f,taking into account the travel time to the bow-shock plusa lag of 16 min (see section 3). During the periodanalyzed, the Wind spacecraft was in good position formonitoring the solar wind upstream of the bow-shock, inthe postnoon sector Northern Hemisphere (Figure 3, from

Figure 3. Projections of the Wind satellite orbit on the GSE XY, XZ, YZ, and XR planes during 30November 1999. In the period analyzed, Wind was crossing the postnoon sector above the Earth’sequatorial plane, gradually approaching the bow-shock (SSCWeb: http:\\sscweb.gsfc.nasa.gov).

Figure 4. (left) The DMSP F13 satellite pass across the field-of-view of ITACA, between 0822 and 0825UT. (right) The DMSP F13 and F14 passes occurred respectively between 1003 and 1007 UT, and between1031 and 1034 UT. The satellites moved from east (right) to west (left), observing plasma propertiesrelated to the magnetospheric regions indicated in the labels: cusp (CUSP, white areas), mantle (MA,dotted areas), low-latitude boundary layer (LLBL, gray areas), plasma sheet boundary (BPS, black areas)and central plasma sheet (CPS, hatched areas). As reference, in each panel is shown a red aurora image,selected within the period, and projected on the map assuming an auroral emission height of 250 km.


the ‘‘Satellite Situation Center – SSCWeb’’ Web pages:http:sscweb.gsfc.nasa.gov).3. The spectrograms relative to three passes of the

DMSP satellites, F13 at 0822-0825 UT and 1003-1007UT, and F14 at 1031-1034 UT (arrows in Figure 2a), areused to categorize the particle precipitation regions withinthe ITACA field-of-view (Figure 4), by means of theautomated region identification algorithm (JHU/APL).

3. Auroral Activity Overview

[7] On 30 November 1999, dayside auroral observationwas partly disturbed by moonlight (phase = 0.5). In spite ofthat, the ITACA instrumentation recorded strong red emis-sion (630.0 nm) during daytime, between 0800 and1200 UT(Figure 1). A faint auroral activity started at 0715 UTbetween 76� and 78.5� invariant latitude (ILAT), polewardthe local zenith (76� ILAT); this beginning phase waswobbly and then faded after �20–25 min. Then, reddominated auroras suddenly brightened at 0805 UT, lastingfor about one hour and exhibiting two distinct intensitypeaks. During this phase, auroral emission spread in lat-itude, with its poleward and equatorward boundariesexpanded respectively to about 80� and 75.5� ILAT. After0830 UT, auroral forms suddenly started to move westward,most likely in response to the sharp reversal of the IMF By

component (Figure 2c and left thick mark at the bottom ofFigure 2) at 0813-0814 UT. Taking this time as reference wecan estimate a lag of about 16–17 min between IMF atbow-shock and ITACA recordings, value that is compatiblewith the time delay reported in literature for dayside auroralphenomena [e.g., Lockwood et al., 1989]. It is useful todivide the total time delay �T, here considered from thebow-shock, into the contribution of several terms:

�T ¼ TBS�MP þ TMP�IO þ TIO�FOV þ T630; ð2Þ

where �TBS-MP is the delay from the bow-shock to themagnetopause, �TMP-IO from the magnetopause to theionosphere,�TIO-FOV from the ionosphere to the instrumentfield-of-view, and�T630 that is the lifetime of the metastablestate that generates the red auroral emission (630.0 nm).Since the mean velocity in the magnetosheath is about oneeighth of the solar wind speed, we have �TBS-MP � 8–9min; the second term is roughly equal to the Alfven wavetravel time along a field line, so that �TMP-IO � 2–3 min;the third term depends upon the position of the observer andthe width of its field of view and we can estimate �TIO-FOV

� 2–3 min; while the last term is equal to 110 s. The sum ofall the terms leads to �T � 14–17 min, in agreement withthe value we have found.[8] From 0855 UT, roughly corresponding to magnetic

local noon (MLT � UT + 0310), auroral intensity decreasedabruptly, down to a dim diffuse emission. From 0920 UTon, again red aurora intensified progressively, starting with afaint narrow band centered around 77� ILAT and thenexhibiting a complex sequence of events growing instrength and latitudinal width. At 0939 UT, the red auroraintensified markedly and began to move persistently towardEast (right thick mark at the bottom of Figure 2). From theall-sky camera recordings it is possible to identify four main

auroral pulses, characterized by both increasing intensity(PI, MI) and expansion of poleward (PO) and equatorward(EQ) boundary as well (panels a and b, Figure 2), eachpulse was composed of a number of subintensifications.These four events crossed the local magnetic meridian atabout 0945 UT, 1000 UT, 1015 UT and 1043 UT. After thestrongest event peaked at 1043 UT red emission graduallyshifted southward to 75� ILAT and then, after 1100 UT, itssouthern boundary reached 72� ILAT in response to asouthward turning of IMF Bz. This sharp equatorwarddisplacement disrupted the nearly steady IMF conditioncharacterized by Bz � 0 nT and By � �10 nT. Then, thered aurora activity continued to exhibit pulsed intensifica-tions, with a gradual decreasing intensity, until about 1300UT. Due to this enhanced southward shift, the auroralactivity during this interval is not easy to compare withthe previous period, and it will not be considered in themost of the following study.

4. Detailed Analysis of Auroral Events

4.1. Cusp Displacement Between 0800 and 0900 UT

[9] In Figures 2a–2c, we first note the disappearance (orat least the strong decrease in intensity) of the auroral redemission when the IMF clock angle was in the low-shearregime, i.e., when both IMF Bz and By were close to zero.This is particularly noticeable at 0900 UT, roughly corre-sponding to the local magnetic noon, when on the contrarythe probability to observe the cusp aurora is maximum[Newell and Meng, 1998]. Moreover, between 0830 and0855 UT, the large positive By is expected to shift the cuspregion toward dusk, in the postnoon sector. When the IMFclock angle exited from the low-shear regime (thus favoring

Figure 5. Schemes of the dayside particle precipitationregions for (top) IMF By > 0 and (bottom) IMF By < 0, theblack arrows indicate the motion of the auroral forms(adapted from Sandholt et al. [1998]).


the recovery of auroral emission), the By componentbecomes more and more negative, and this pushes the cuspregion toward dawn, in the prenoon sector. This second shiftof the cusp is confirmed by the second DMSP F13 pass(1003-1007 UT, right panel of Figure 4, upper trace), whichindicates cusp-like particle precipitation above Greenland at1007 UT, corresponding to about 1100 MLT in the eastcoast of Greenland (1320 MLT at Ny-Alesund). Hence, thecombination of low magnetic shear regime with the IMF By

sign changes seems to have pushed the cusp away from thefield-of-view of ITACA for most of the time. The twopanels of Figure 5 (adapted from Sandholt et al. [1998])should roughly illustrate the situation before (upper panel)and after (lower panel) the By sign reversals. Furthermore,looking at the first DMSP F13 passage (0822-0825 UT,left panel of Figure 4) it is possible to observe four distinctsmall regions characterized by cusp-like signatures with-in LLBL precipitation, between 0822 UT and 0824 UT(� 1130 MLT). The strongly negative value of By (about�15 nT) suggests a shift of the cusp in the prenoon, whilethe small positive value of Bz would imply that the cusp islocated slightly poleward of Ny-Alesund. The almost reg-ular displacement of the four cusp-like areas encountered byDMSP F13 gives the idea of spikes or bursts of daysidemerging, or of a wavy boundary between the cleft/LLBLand cusp regions.

4.2. Pulsed Red Aurora Intensifications, 0930-1100 UT

4.2.1. Magnetospheric Regions Identification[10] The dayside region pattern illustrated in the right

panel of Figure 4 (DMSP F13 and F14 transits, respectivelyat 1003-1007 UT and 1031-1034 UT), together with theprevious considerations about the cusp position during0800-0900 UT, suggest that the auroral activity observedby ITACA after the 0930 UT (� 1240MLT) occurred mostlywithin the cleft/LLBL region or on its poleward boundary.This conclusion seems to be confirmed by a preliminary lookat the SuperDARN field of view plots, in particular of thePykkvibaer radar pointing toward the Svalbard archipelagofrom the Iceland http://superdarn.jhuapl.edu): in fact, severalsignatures of tailward convection northward of Ny-Alesund

and sunward convection southward of it, some in coinci-dence with the major auroral intensifications, are recogniz-able during the period considered. This would indicate thatthe convection reversal boundary (CRB), defined as theonset of steady antisunward convection and usually posi-tioned near the equatorward edge of the LLBL [Newell et al.,1991], often stayed close to the zenith of Ny-Alesund.4.2.2. Auroral Signatures[11] The auroral activity occurred between 0930 and

1100 UT (1240-1410 MLT) is characterized by the fourdistinct spots visible in the keogram (Figure 1) and in thelower panel of Figure 2; a detail of the 630.0 nm peak andmean intensity during this interval is plotted in Figure 6.During this period, several dim green (557.7 nm) auroraltransients were also recorded on the equatorial edge of thered emission, generally in coincidence with the brightestfeatures of the latter. The four pulses, labeled with A, B, Cand D, are visible both on peak and mean red intensity(Figure 6), even if in the first one (A) the two parametersdo not match closely. Looking at the peak intensity trend,it is interesting to stress that (1) each pulse has stepincrease followed by a gradual decrease (apart from themaximum occurred at 1046 UT), (2) each pulse has astructure of secondary peaks, with a periodic recurrence ofabout 5–6 min, (3) the intensity (maximum peak andmean intensity) increase monotonically from A to D, (4)the period also increases monotonically from A to D,respectively 18, 19, 22, 25 min, with mean recurrence ofabout 20 min.[12] The impression that arises looking at Figure 6 is a

short-term periodic signal (peak intensity, 5–6 min) modu-lated by a longer pulsation with increasing period andamplitude (both peak and mean intensity, period from 18to 25 min). In Figure 7 six all-sky camera images illustratethe later part of the whole sequence: two auroral formsappear to move in parallel in the eastward direction, with thebrightest one showing an apparent clockwise rotation.[13] A detailed analysis of ITACA images shows that

these four pulses actually correspond to four events ofsubintensifications moving approximately along 76� ILAT:the auroral motion is mainly zonal (in the eastward

Figure 6. A detail of the 630.0 nm peak (solid line) and mean (dashed line) intensity reported in Figure2. The four auroral pulses dominated by red emission, labeled with A, B, C and D, are visible in bothparameters. The peak intensity presents a short-term periodicity (5–6 min) modulated by a quasiperiodiclong-term trend (18–25 min).


direction), although, a small equatorward componentseems to be present.4.2.3. Data Analysis[14] A simple linear correlation, with variable time lag,

between the peak/mean red intensity (PI - MI), and inter-planetary parameters, Bz - Pdyn, gives the following results(See Table 1):[15] The anticorrelation with Bz is generally expected to

exist since it is linked with the enhancement of magneticmerging, even if in this case the Bz component is close tozero. Moreover, the time lag of 18 min is close to the value(16 min) we derive by looking at the auroral flow motionattributed to the By sign change (see section 3). However,the rather small value of the correlation coefficients impliesthat Bz variations cannot account for all the modulationobserved. The same considerations can be drawn for theclock-angle q and the modulation associated with thecomponent reconnection (q was roughly equal to Bz dueto the constancy of By during this interval of time, seeFigure 2). The anticorrelation with the solar wind dynamicpressure seems to be largely fortuitous: first, because apositive correlation is expected in the case of magneticreconnection induced by pressure enhancements; second,

because a time lag of 25–27 min is about 10 min greaterthan the expected delay for dayside events (see section 3). Ifthe correlation between the peak/mean red intensity and thedynamic pressure is calculated with a lag of 16 min, we getonly �0.22 (PI - Pdyn) and �0.35 (MI - Pdyn). The aboveresult indicates that the small pressure variations present inthe solar wind during this period do not influence signifi-cantly the dayside auroral emission, in line with pastfindings [e.g., Farrugia et al., 1995].[16] Although the relationship between the above men-

tioned solar wind parameters and the auroral intensificationslooks to be weak, it may be worthwhile inspecting thisresult further. Since it is not possible to show here the fullsequence of all-sky images, and hence illustrate its time

Figure 7. Six all-sky camera images (1041–1047 UT, from left to right and from top to bottom)illustrate part of the sequence of the eastward moving red auroras, occurred between 0930 and 1100 UT.The intensity is scaled to enhance the brightest auroral form. The images have been projected over ageographic map assuming an auroral height equal to 250 km. Two auroral forms move across the ITACAfield-of-view, and the brightest one seems to rotate clockwise while approaching Ny-Alesund. It must benoted that the elongation of the auroral form and/or its rotation could be in part produced by the mapprojection. The dashed line marks 76� invariant latitude, while the spot on the left border of each image isthe Moon.

Table 1. Linear Correlations in the Interval 0935–1100 UT

Parameters Correlation Coefficient Time Lag, min

MI-Bz �0.53 ± 0.08 18PI-Bz �0.48 ± 0.08 18

MI-Pdyn �0.68 ± 0.06 25PI-Pdyn �0.63 ± 0.06 27


evolution, we have tried to solve the problem by calculatingthe 630.0 nm peak intensity at three equidistant locations,corresponding to about �5�, �10� and �15� (westward)with respect to the local magnetic meridian (that we willlabel as 0�), roughly along the 76� invariant latitude.Assuming an aurora emission altitude of 250 km, theselocations are about 110 km from each other. It must beunderlined that these assumptions are arbitrary to someextent, since we have no information on the real verticaldistribution of the auroral emission. The computed peakintensity values are plotted in Figure 8, together with theIMF By, IMF Bz and the solar wind dynamic pressure, alllagged by 16 min. The IMF clock-angle is not shown in thefigure, since its trend practically coincides with the one ofBz due to the small variability of By. As a reference, theoblique segments illustrate the time displacement of twoauroral forms, moving eastward with an apparent velocity of1.8 km s�1 (the value may be overestimated by theprojection distortion).

[17] As previously noted, even if the average increase ofthe clock-angle associated with the main Bz variations isalmost certainly decisive in the general activation ofauroral manifestations, a careful inspection of Figure 8leads to the conclusion that the Bz variations cannot beregarded as the source of the long-term modulation of theaurora during the period considered. In particular, it can benoted that the third auroral pulse (labeled as C) is not inphase with the southward turnings of Bz. This conclusionis supported by the observation of the effect induced uponthe auroral activity by the strong Bz southward turningoccurred between 1100 and 1200 UT: the red auroramoves equatorward, as expected, but its intensity decreasesconversely to what expected (Bz < 0, q > 90�). During thislast phase the pulsating activations of the aurora graduallydisappeared.[18] Besides, Figure 8 shows that a one-to-one coinci-

dence between pressure pulses and auroral brighteningseems unlikely. In fact, the three pressure peaks indicated

Figure 8. The 630 nm peak intensity profiles measured on the magnetic meridian of ITACA (labeled as0�) and on the magnetic meridians located at �5�, �10�, �15� westward of ITACA in the direction ofGreenland, approximately along 76� ILAT (bottom, left scales). The �5�, �10� and �15� profiles hasbeen derived assuming an aurora mean emission height of 250 km. These four lines, from top to bottom(i.e., from �15� to 0�), show the time evolution of the auroral activity coming from west (i.e., fromGreenland) while it travel toward Ny-Alesund. All the four auroral trends shown are characterized by ageneral constant increase. It can be noticed that some events fade, some others grow, before reaching theITACA meridian (0�). The oblique segments delineate at least two auroral forms clearly travelingeastward, with a mean velocity of about 1.8 km/s (�330 km / 180 s). The solar wind dynamic pressurePdyn, lagged by 16 min, is shown in the middle. The arrows indicated the pressure pulses with relativeincrease >20� over the background level: it can be noticed that just two or three auroral enhancements canbe related to pressure pulses, and that a drastic reduction of the lag to about 10 min would be required forthis. The IMF Bz and By components are shown on the top.


by arrows (�Pdyn/Pdyn � 20% over the background level),which could be good candidates for stimulating magneticmerging [Farrugia et al., 1995; Lui and Sibeck, 1991], donot match with the major auroral intensifications. To have areasonable agreement, the time lag should be reduced toabout 10–11 min, values that should be too small forallowing the travel of a signal from the bow-shock to theinstrument field-of-view (see section 3). This would implythat the four dayside auroral events were ‘‘spontaneous’’(according to Sandholt et al. [1994]) and not directlyinduced by solar wind pressure variations. However,throughout the whole period under study (0800-1200 UT),the auroral intensifications generally occur in coincidencewith an increase/decrease of the dynamic pressure in therange 3–6 nPa, and it cannot be excluded that the associatedcompression/decompression of the dayside magnetopause(about 10%) could play a role in triggering the phenom-enon.[19] Finally, it is interesting to note (Figure 8) that,

between 0935 and 1045 UT, the IMF By was characterizedby periodic fluctuations (1–2 nT) with a recurrence ratevery close to the short-term modulation of the peak intensity(section 4.2.2): within this interval (70 min), 14 distinctrelative maxima are identifiable in both parameters, leadingto an average period of 5 min. Even if it is difficult todefinitely prove a direct link between this 5-min variationsshaping of the two parameters, when By is strongly negativea partial penetration of the interplanetary magnetic fieldacross the magnetopause is expected to exist in the post-noon sector (see the next section). In this context, quasi-periodic By fluctuations could drive forced oscillations in

those regions magnetically connected across the magneto-pause on the postnoon/dusk sector.4.2.4. Sketch of the Magnetospheric Configuration[20] To clarify the previous speculations, we have calcu-

lated the magnetic interconnection geometry that shouldhave characterized the dayside magnetosphere during theperiod with By < 0, between 0930 and 1100 UT, by meansof the T96 model [Tsyganenko, 1996]. This version of themagnetospheric model includes an explicitly defined real-istic magnetopause, large-scale region 1 and 2 Birkelandcurrent systems, and the IMF penetration across the mag-netospheric boundary. In the calculations we used thefollowing input values: Pdyn = 4.5 nPa, Dst index = 20.0nT, IMF By = �10 nT and Bz = 0 nT. The configurationderived from the T96 model is illustrated in Figure 9: thetwo panels illustrate a set of geomagnetic field lines close tothe cusp region, between 1000 and 1400 MLT in steps of 20min. The left panel depicts a 3-D view of both theunperturbed configuration (By = 0 nT, gray lines) and theperturbed one (By = �10 nT, black lines), while the rightpanel shows the perturbed geomagnetic configuration asseen from North (GSM XYplane). On both panels, the graythick lines mark the intersection of the magnetopause on theGSM equatorial plane, and the locus where the geomagneticfield lines cross the magnetopause at postnoon. The per-turbed configuration was calculated to match approximatelythe particle precipitation regions reported in Figure 4 (rightpanel), and schematized in Figure 5 (lower panel). Thedistortion of the magnetosphere due to the negative By

causes the magnetic noon and the cusp to precede, i.e., toshift toward dawn (the magnetic noon actually lies in the

Figure 9. Theoretical configuration of the dayside magnetosphere, during the period 1000-1400 MLT,calculated by means of the T96 Tsyganenko magnetospheric model with the following input parameters:solar wind dynamic pressure = 4.5 nPa, Dst index = 20.0 nT, IMF By = �10 nT and Bz = 0 nT. The leftpanel depicts a 3-D view of both the unperturbed configuration (By = 0 nT, gray lines) and the perturbedone (By = �10 nT, black lines), while the right panel shows the perturbed geomagnetic configuration asseen from North (GSM XY plane). On both panels, the gray thick lines mark the intersection of themagnetopause on the GSM equatorial plane, and the locus where the geomagnetic field lines cross themagnetopause at postnoon. The distortion due to the negative By component causes the magnetic noonand the cusp related region to precede (i.e., to move toward dawn, see the lower panel of Figure 5), and itdrives also a strong duskward bending of the field lines in the postnoon sector.


postnoon), and it drives also a deep duskward bending ofthe field lines in the postnoon sector. Figure 9 shows howthe geomagnetic field lines, which in the unperturbedsituation would stay within the LLBL (i.e., closed fieldlines), are shifted toward dusk by the negative IMF By

component and then reconnected with the IMF through themagnetopause, in the postnoon.[21] Using the values derived from the T96 model, the

field-of-view of the ITACA all-sky camera, approximately 2hours in longitude, would map to about 8�9 RE at themagnetopause. With this scale ratio, the large auroral formshown in Figure 7, which has an estimated longitudinalextension of roughly 200 km, would scale to about 1.3�1.5RE at the magnetopause; while, the quasiperiodic modula-tions visible in Figure 6 (about 5 and 20 min) would lead awavelength of about 4 and 15 RE at the magnetopause, if weassume an auroral longitudinal velocity equal to 1.8 km s�1

as derived before.

5. Discussion and Conclusions

[22] Previous deductions lead to hypothesize that thesequence of dayside auroral events, observed between0930 and 1100 UT, was presumably induced and/or drivenby a prolonged period (about 90 min) with IMF By � �10nT and IMF Bz � 0 nT. The corresponding IMF clock-angle(q � 90�) lies in the transition region that separates high-and low- shear regimes. The aurora activity was dominatedby red emission with dim green transients, indicating thepredominance of low energy magnetosheath plasma; thisfact together with the presence of intensifications of theionospheric flow (section 4.2.1), and the accordance of theaurora east-west motion with the Svalgaard-Mansuroveffect (IMF By sign), would imply that the activity observedwas generally associated with the magnetic reconnection atthe dayside magnetosphere, and supporting the effective-ness of the so-called ‘‘component reconnection,’’ At thesame time, the DMSP data support both the two ideas thatpart or most of the auroral activity took place inside theLLBL (i.e., within closed magnetic field lines), and that thetraveling auroral forms originated from or near the cuspregion. The auroral activity was characterized by a sequenceof four quasiperiodic main pulses composed by severalrecurrent subintensifications (section 4.2.2); these mainauroral pulses exhibited brightness, latitudinal extent andperiod increasing monotonically with time, i.e., withincreasing distance from the local magnetic noon. None ofthese features look to be to be directly linkable to thevariations observed in the solar wind ahead the bow-shock,and the same seems to be true also for both the short- andthe long-term modulation of the red aurora activity (section4.2.3). Nevertheless, the broad increase/decrease of the solarwind dynamic pressure that occurred during the periodobserved, and the subsequent inflation/deflation of themagnetopause (about 10%), could have played a role intriggering the red auroral emission.[23] Although more data and a wider analysis are surely

required to describe the phenomenon, there are someinteresting indications that are worth commenting on. Thesequence of red aurora intensifications under analysis hassome analogies with the periodic auroral forms movingalong the polar cap boundary, which are widely reported to

be one of the most recurrent phenomena during southwardIMF conditions [e.g., Moen et al., 1995, and referencestherein]. These manifestations are often put in relation withthe so-called flux transfer events. The expected FTE dimen-sion at the magnetopause is close to the value we derivedfor the most intense auroral brightening (� 1 RE). On theother hand, the observed auroral intensifications could berelated to viscous mechanisms acting at the magnetopauseor at its inner boundary, since these intensifications: (1)could have entirely or partially occurred within LLBLclosed field lines; (2) exhibit a wave-like modulation; (3)generally increase with increasing distance from the sub-solar point. The magnetopause surface waves have been putin relation to postnoon/dusk auroras (see for exampleKauristie et al. [2001] for a recent analysis of combinedCluster satellites and ground based measurements), and theKelvin-Helmholtz instability can drive auroral activity aswell from the inner boundary layer. Results derived bynumeric simulation of the KHI [e.g., Wei et al., 1990] showfeatures that are to some extent comparable with the aurorasignatures observed by ITACA. The magnetopause surfacewaves can be generated by the Kelvin-Helmholtz instabilityitself or, otherwise, amplified by it: ‘‘a surface wave mayenter a KH active region where its amplitude is furtherenhanced’’ [Farrugia et al., 2000, p. 7658]. It was alsosuggested that Kelvin-Helmholtz waves could be drivenunstable by the magnetic reconnection that takes place at thedayside magnetopause [Saunders, 1989]. In this context, itis interesting to consider the event discussed by Clauer etal. [1995, 1997], which presents many analogies with theone described here. The period examined by Clauer andcoworkers was characterized by IMF Bz � 0 nT, IMF By �10 nT and Pdyn � 4�5 nPa, i.e., the same as the present caseexcept for the sign of By. The authors, analyzing data fromthe Greenland chains of magnetometers, found long-periodmagnetic pulsations (34 min) consistent with ionosphericconvection vortices moving tailward (1.3 km s�1), acrossthe local magnetic noon. These magnetic variations wereassociated with wave-like displacements of the convectionreversal boundary, which the authors put in relation withKH waves developing near local noon under large IMF By

conditions. Finally, another candidate mechanism could berepresented by the plasma transfer events (PTE) across thedayside magnetopause, which should produce ionosphericsignatures that are expected to propagate equatorwardacross the LLBL and the convection reversal boundary,with a subsequent tailward motion [cf. Heikkila et al., 1989](Table 1).

[24] Acknowledgments. The authors wish to acknowledge: theGSFC/NASA SSCWeb (Satellite Situation Centre) for the Wind satelliteorbit plots (Figure 3); R.P. Lepping (MFI) and K.W. Ogilvie (SWE) for theWind satellite data, the Johns Hopkins University Applied Physics Labo-ratory (JHU/APL); D. Hardy, F. Rich, and P. Newell for the DMSPspectrograms; the NASA/GSFC and N. Tsyganenko for the T96 magneto-spheric model.[25] Arthur Richmond thanks Charles Farrugia and Per Even Sandholt

for their assistance in evaluating this paper.

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��M. Candidi, S. Massetti, and S. Orsini, Istituto di Fisica dello Spazio

Interplanetario CNR/IFSI, Roma, 00133, Italy.K. Kauristie, Finnish Meteorological Institute FMI/GEO, P. O. Box 503,

FIN-00101Helsinki, Finland.


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