(3) SUBSTORM TOPOLOGY IN THE IONOSPHERE AND MAGNETOSPHERE DURING A FLUX ROPE EVENT IN THE MAGNETOTAIL

On 13 August 2002, at 23:00 UT, about 10 min after a substorm intensification, Cluster observes a flux rope in the central magnetotail, followed by a localised fast flow event about one minute later. Associated with the flux rope event, a traveling compression region (TCR) is seen by those Cluster spacecraft which reside in the lobe. In the conjugate ionospheric region in Northern Scandinavia, the MIRACLE network observes the ionospheric equivalent currents, and the electron densities and electric fields are measured by the EISCAT radar along a meridional scanning profile. Further, the auroral evolution is observed with the Wideband Imaging Camera (WIC) on the IMAGE satellite. We compare in de- tail the substorm evolution as observed in the ionosphere and in the magnetosphere, and examine whether topological cor- respondences to the flux rope event exist in the ionospheric signatures. The large-scale mapping of both the location and the direction of the flux rope to the ionosphere shows an ex- cellent correspondence to a lens-shaped region of an auroral emission minimum. This region is bracketed by an auroral region equatorward of it which was preexisting to the sub- storm intensification, and a substorm-related auroral region poleward of it. It is characterised by reduced ionospheric conductances with respect to its environment, and down- ward field-aligned current (FAC) observed both in the mag- netosphere and in the ionosphere. As determined from the ionospheric data, this downward FAC area is moving east- ward with a speed of 2 km s 1 , in good agreement with the mapped plasma bulk velocity measured at the Cluster satellite closest to that area. Further southwestward to this leading downward FAC area, a trailing upward FAC area is observed that moves eastward with the same speed. The di- rection of the ionospheric electric field permits a current clo- sure between these two FAC areas through the ionosphere. We speculate that these FAC areas may correspond to the ends of the flux rope in its symmetry direction.

On August 13, 2002, at ~ 2300 UT, about 10 min "plasmoids", played a major role in transporting the after a substorm intensification, Cluster observes a flux substorm energy antisunwards.The minimum energy rope in the central magnetotail, followed by a localised state of such helical magnetic field structures is fast flow event about one minute later.Associated with represented by force-free magnetic flux ropes (Priest, the flux rope event, a traveling compression region 1990).These are elongated magnetic structures in the (TCR) is seen by those Cluster spacecraft which reside in central axis of which the magnetic field is strongest and the lobe.In the conjugate ionospheric region in Northern points along that axis.With increasing radial distance Scandinavia, the MIRACLE network observes the from that axis the field component along the axis ionospheric equivalent currents, and the electron direction diminishes, while the field component densities and electric fields are measured by the EISCAT azimuthal to it increases.In a flux rope with cylindrical radar along a meridional scanning profile.Further, the symmetry, no radial field with respect to the central axis auroral evolution is observed with the Wideband exists.For a special class of flux ropes which can Imaging Camera (WIC) on the IMAGE satellite.We assumed to be force-free, the currents are flowing either compare in detail the substorm evolution as observed in in the direction of the magnetic field, or opposite to it.the ionosphere and in the magnetosphere, and examine The magnetic field solutions for a force-free, cylindrical whether topological correspondences to the flux rope flux rope have been given by Lundquist (1950).event exist in the ionospheric signatures.The large-scale mapping of both the location and the direction of the flux Tailward propagating plasmoids and flux ropes rope to the ionosphere shows an excellent associated with them have been observed since the midcorrespondence to a lens-shaped region of an auroral 1980's in the magnetotail's plasma sheet using data of emission minimum.This region is bracketed by an the ISEE-3 satellite (e.g., Sibeck et al., 1984) and have auroral region equatorward of it which was pre-existing later been extensively studied with Geotail data (e.g., to the substorm intensification, and a substorm-related Nagai et al., 1998;Ieda et al., 1998;Slavin et al., 1998).auroral region poleward of it.It is characterised by In addition to these tailward moving structures, also reduced ionospheric conductances with respect to its earthward moving flux ropes have been observed in the environment, and downward field-aligned current (FAC) near tail (X > ~ -30 R ; e.g., Moldwin and Hughes, observed both in the magnetosphere and in the 1994).Slavin et al. (2003) showed that these flux ropes ionosphere.As determined from the ionospheric data, are often closely associated with earthward bursty bulk this downward FAC area is moving eastward with a flows (BBFs) in the plasma sheet of the magnetotail, i.e., speed of ~ 2 km s , in good agreement with the mapped high-speed plasma flows of several 100 km s (e.g., -1 plasma bulk velocity measured at the Cluster satellite Baumjohann et al., 1990;Angelopoulos et al., 1992), and closest to that area.Further southwestward to this accordingly named them "BBF-type" flux ropes, in leading downward FAC area, a trailing upward FAC contrast to "plasmoid-type" flux ropes moving tailwards.area is observed that moves eastward with the same Both types of flux ropes could in many cases reasonably speed.The direction of the ionospheric electric field be approximated by a force-free model.permits a current closure between these two FAC areas through the ionosphere.We speculate that these FAC While plasmoid-type flux ropes occur tailward of a areas may correspond to the ends of the flux rope in its major active reconnection X-line and therefore are not symmetry direction.
magnetically connected to the ionosphere, BBF-type flux

INTRODUCTION line, and are thus at least embedded in an environment
Helical magnetic field structures inside the properties of a BBF-type flux rope with respect to its magnetosphere have played an important role for the magnetic field and plasma properties that a satellite understanding of the process of magnetospheric encounters during the passage of such a structure have Hones (1977), in which such structures, termed  field modeling using the stated model and parameter has In the statistical study of Slavin et al. (2003), this independently been confirmed with the Hybrid Input azimuthal orientation of a flux rope has been found to be Algorithm model (Kubyshkina et al., 1999).The quite variable.Further, an increase of the earthward plasma velocity has on average been observed ~ 40s before the flux rope observation, and this velocity peaks at ~ 600 km s within roughly a minute after the flux -1 rope encounter.Ahead of the flux rope, collocated with the earthward flow increase, a compression region with increased ion densities has been observed.In the lobes neighboring to the flux rope, traveling compression regions (TCR) are formed due to the compression of the lobes by the "obstacle" flux rope and are co-moving with it (e.g., Slavin et al., 1984Slavin et al., , 2005)).
The question what features, if any, may topologically correspond to a BBF-type flux rope in the ionospheric evolution of a substorm has not yet been addressed.This question may in the first view look surprising, as a flux rope is ideally thought to be a closed magnetic structure and thus may not directly map magnetically to the ionosphere.Hence, we would like to stress that we do not claim any features seen in the ionosphere to be directly magnetically connected to the flux rope, but rather inspect whether any topological correspondences exist.Such topological relations may follow from the fact that the flux rope deforms the ambient magnetic field, causes compressions in front of it, inhibits plasma flow through it, etc. E.g., in this respect a TCR is a magnetospheric topological consequence of a flux rope caused by the deformation and compression of the lobe magnetic field lines due to its movement.However, we note that the solutions for a force-free cylindrical flux rope by Lundquist (1950) involve Bessel functions that extend to infinity, and there is no way to "cut" these solutions such footprints are located above the northern coast of that both the conditions of the flux rope being force-free Scandinavia, close to the northeastern edge of the and magnetically closed can be preserved.In the real Scandinavian mainland IMAGE stations.Here and in case, the extension of the flux rope is limited at least by the following, spacecraft 1 is marked with black colour, the extension of the magnetosphere.Hence, at one of the spacecraft 2 red, spacecraft 3 green, and spacecraft 4 conditions mentioned will not be valid, particularly at blue. the ends of the flux ropes seen along their major axis, which may allow current to flow in or out of the flux From the different instruments onboard Cluster, for rope region.
our study mostly data from the Fluxgate magnetometer In order to address the question posed above, in this (CIS, Rème et al., 2001)  have been used for spacecraft 3. Before the event, a

INSTRUMENTATION AND OBSERVATIONS
UT.During this thinning, all Cluster spacecraft exit to During our our event on August 13, 2002, the and 4 later exit to the lobe, while spacecraft 3's position Cluster satellite fleet (Escoubet et al., 2001)  in the PSBL, possibly existing faster and longer lasting After the event, a dipolarization and plasma sheet flows in the central plasma sheet were not recorded.expansion takes place, and the spacecraft re-enter the plasma sheet, lead by spacecraft 3 at 2306 UT.During The other three Cluster spacecraft (1, 2, and 4) which the following ~ 12 min after that re-entry, three distinct are located in the northern lobe encounter a traveling fast flow events take place with velocities in X compression region (TCR), as can be seen from the GSM direction up to ~ 1000 km s (data not shown).increase in B and the dipolar B signature which is -1 In this paper, we concentrate on the discussion and rope in spacecraft 3. From the timing of the B peaks, analysis of the magnetospheric and ionospheric the velocity of this magnetic signature has been inferred observations in the immediate temporal vicinity of the to ~ 1200 km s in an earthward and eastward direction flux rope event at ~ 2259 UT.A detailed plot of the in the X-Y plane, tilted by ~ 29° clockwise from the +X Cluster data for a 6 min interval around that event is axis (if seen from +Z direction).This value is well in shown in Fig. 3  technique (Dunlop et al., 2002; see fourth and fifth panel unite, forming a convex lense-like shape.In the center of from the bottom in Fig. 3).Only a slight increase of the this lense, an area with clearly less auroral emission is earthward flow velocity up to ~ 100 km s is seen by observed, which at 2258:41 UT is located just at the -1 Cluster 3 in association with the flux rope event.We northern coastline of Scandinavia.At 2300:43 UT, this interpret this fact such that due to the Cluster 3's location area is again covered with moderate auroral emission, X Z smaller, but of similar orientation as the one of the flux X -1 magnetogram plot.Following a rather moderately disturbed period, the pre-existing westward electrojet intensifies at 2250 UT in all northern Scandinavian mainland stations (simultaneously with the observed electron injection at LANL; see first vertical dashed line in Fig. 2).The flux rope observation at Cluster occurs during the late expansion phase of the substorm as observed in IMAGE (see second vertical dashed line in Fig. 2).Note that some of the stations in the northeastern Scandinavian mainland, such as Masi (MAS) and Kevo (KEV), show a clear positive excursion which peaks at 2300 UT.Subsequently, the recovery phase begins which lasts until ~ 2325 UT.The ground magnetic data will be analysed in more detail in the following section.
The Far Ultraviolet Wideband Imaging Camera (WIC) on the IMAGE satellite monitors the auroral emissions in the spectral range of 140-190 nm (Mende et al., 2000).On August 13, 2002, the field of view of the WIC covered almost the whole northern auroral region, with Northern Scandinavia located close to the limb of the observation area, and this limb shifting polewards  while the "void" area may have traveled eastwards visible, a clockwise one which is centered at ~ 71.5°( however, this cannot be unambigously concluded latitude, and an anticlockwise one which is centered because of the border of the WIC field of view).
slightly poleward of 68° latitude.If the ionospheric The European Incoherent Scatter Facility mainland correspond to a downward FAC, and the anticlockwise radar (EISCAT; Folkestad et al., 1983) was operating in one to an upward FAC.Both vortices are moving a beam-swinging mode during our event such that the synchronously eastward, with a speed of ~ 2 km s .The main radar beam is moved equatorwards from 72.2°poleward vortex is reaching the Cluster satellites just at latitude to 67° latitude (at 117 km altitude) during 28 the time when a downward FAC is also observed in the min, and then the antenna is directed back to the starting Cluster data (cf lower right panel in Fig. 5 and Fig. 3b).position within the follwing 2 min, in order to complete Comparing the location of the two vortices at 225850 UT a full 30 min cycle.During our period of main interest, with the FUV image which was taken 10 s earlier (Fig. these cycles started at 2230 and 2300 UT, respectively.4), it can be seen that the anticlockwise vortex is well The main beam data are used to deduce, among others, collocated with the lens-shaped emission minimum, the electron density, ion and electron temperatures, and while the clockwise vortex is well collocated with the ion velocity in the beam direction.By using all three emission maximum around 68.5° latitiude, which EISCAT sites including the receiving stations at Kiruna supports the interpretation of the current vortices as and Sodankyla, also the full ion velocity vector in the F downward and upward FAC regions, respectively.We region (at 293 km altitude) and thus the electric field was notice that the tilt of the axis between the two current derived.
vortices is comparable with the tilt of the ionospheric

ANALYSIS RESULTS AND DISCUSSION
are marked by the blue and red lines in the 225950 UT The picture changes when we inspect the 2D 2.4 km s , the mapped eastward bulk velocity observed distributions of the ionospheric quantities: Fig. 4 shows at Cluster 4 (which is located closest to the central the WIC intensities over Northern Scandinavia together latitude of the anticlockwise vortex) agrees well with the with the mapped location and direction of the flux rope, eastward speed of the current vortices.as well as the Cluster magnetic footprints.For the flux rope mapping, the blue line shows the estimate from the 90° rotated direction of the TCR motion, and the red line the estimate as determined from the Cluster 3 magnetic data.In both cases, for the mapping the estimated directions were linearly continued in the magnetospheric X-Y plane.The flux rope maps into the lens-shaped area of weak FUV emissions below 1000 R.Moreover, also the orientation of the mapped flux rope coincides well with that of the weak auroral emission region, which are both northeast-southwest aligned in the ionosphere.Note that although the 90° rotated direction of the TCR motion is expected to correspond best to the flux rope core direction, this conclusion is qualitatively not sensitive to the difference between the two estimates of the flux rope orientation.Since a flux rope effectively acts as an "obstacle" for particles precipitating from the tail to the ionosphere, it is likely that this signature is caused by an inhibition of such particles due to the presence of the flux rope.
A further topological correspondence to the flux rope is found in the 2D ionospheric current pattern.For that purpose, we calculate the 2D ionospheric equivalent currents using the 2D SECS technique for the upward continuation of the ground magnetic field (Amm and Viljanen, 1999).Further, we decompose this currents according to , where Thus we conclude that in a 2D view, there are several is the part of the current which is divergence-and clear topological ionospheric correspondences to the flux curl-free within our analysis area, while denotes rope event: A lens-shaped decrease of the ionospheric the part of the current which is divergence-free, but not FUV emissions which is collocated with a downward curl-free inside that area (e.g., Amm, 1997).(Note that FAC area that moves eastward with a speed close to the is by definition divergence-free, cf., e.g., Untiedt bulk velocity of the plasma as measured in the and Baumjohann, 1993).With such a decomposition, the magnetosphere.This leading downward FAC area is main background westward electrojet will be contained balanced by a trailing upward FAC area which moves in , while local features of the current system are eastward at the same speed at lower latitudes.On the seen in .The evolution of this latter current system other hand, no ionospheric topological correspondence is is shown in 1-min steps in Fig. 5, between 225650 and found to the mentioned fast earthward motion of the B 225950 UT.Two oppositely directed current vortices are peaks of the TCR, which would map to a ~ 22 km s conductances were uniform, the clockwise vortex would -1 projection of the flux rope direction (the two estimates panel of Fig. 5, similarly to Fig. 4).Furthermore, with -1 Fig. 5 Evolution of divergence-free equivalent currents (only part which has curls in the analysis area) between 2256:50-2259:50 UT, and Cluster magnetic footprints (colours like in Fig. 1).Blue and green dashed circles mark the location of clockwise and anticlockwise current vortices, respectively.Mapped flux rope directions as in Fig. 4.
eastward velocity component in the ionosphere. is at the present stage speculative, and more work needs A tentative interpretation of the topological be consistent with the topology of the horizontal currents correspondence might be that the two current vortices in and FAC, as observed by Cluster and in the ionosphere.are the "edges" of a flux rope in its symmetry direction where part of the current flowing in its central area is In order to inspect whether or not the two FAC diverted into upward and downward FAC, respectively.
regions can be connected by ionospheric currents, the Both in the ionosphere and in the flux rope (where electric field observations by EISCAT are shown in a ), the currents are flowing southwestwards.(Note superposed vector plot in Fig. 6.The measurements of that the currents in Fig. 5 would in case of the 2230-2300 UT scan are shown at their actual uniform conductances correspond to Hall currents, but observation points, while measurements of later scans the total currents related to the double vortex structure have been shifted eastward in space, and those of earlier are expected to flow from the downward FAC area in the scans westwards.Again, the observation times are northeast to the upward FAC area in the southwest.This indicated at the vectors.We note that before and later is also discussed below when looking at the ionospheric equatorward of the substorm region, large southeastward electric field.)For the flux rope, this current direction pointing electric field vectors with magnitudes partly also corresponds to the prevailing positive IMF B well above 100 mV m are seen.This high electric field Y component (data not shown).Hence, in this region, which is independently also confirmed by a interpretation there is no current circuit between ends of comoving area of enhanced ion temperatures (data not the flux rope and the ionosphere, since in this case the shown), moves equatorward with a speed of ~ 0.1 km s .current direction in both domains should be opposite, However, as this interesting feature is pre-existing to the and such a circuit would miss a current generator.
substorm, its detailed analysis lies outside the scope of Rather, the current generated further tailward in the this paper.We concentrate on the electric field structure magnetosphere could close either via the ionosphere, or after the activation and inside the substorm area (i. the electric field direction required to direct the total magnetic field in the flux rope points westwards, i.e., in horizontal ionospheric current into a west-southwestward +Y direction like in our case.We emphasize that at the direction, in good correspondence with the direction present stage this interpretation is a hypothesis, and between the downward and upward FAC regions in Fig.
more work needs to be done in order to either verify or 5.During the 2330-0000 UT scan, i.e., already at the end falsify it.However, the interpretation appears to be and after the recovery phase of the substorm, the electric consistent with the magnetospheric and ionospheric data field magnitudes somewhat increase again and the and analysis presented in this study.directions become more variable.Hence we conclude that the EISCAT electric field data during the substorm event ACKNOWLEDGMENTS is compatible with the current structure found from the magnetic field data, and thus further confirms our The authors would like to thank James Slavin for interpretation of the two current vortices as oppositely valuable discussions.We are indebted to the director and directed FAC regions.
staff of EISCAT for operating the facility and supplying

SUMMARY AND CONCLUSIONS
supported by Finland (SA), France (CNRS), Germany During the expansion phase of a substorm on August and the United Kingdom (PPARC).13, 2002, around 2259:50 UT Cluster 3 observed an earthward traveling BBF-type flux rope in the PSBL, REFERENCES while the other three Cluster spacecraft experienced the signature of a TCR in the northern lobe.Our search for Amm, O., and Viljanen, A.: Ionospheric disturbance possible topological correspondences in the conjugate magnetic field continuation from the ground to the ionosphere to these magnetospheric substorm features ionosphere using spherical elementary current has yielded the following results: systems, Earth, Planets and Space, 51, 431, 1999.
97, 4027, 1992.$ This FUV emission minimum region is collocated Balogh, A., Carr, C.M., Acuña, M.H., Dunlop, M.W., with a downward FAC area as both measured with Beek, T.J., Brown, P., Fornaçon, K.H., Georgescu, Cluster and inferred from the ground-based data.The E., Glassmeier, K.-H., Harris, J., Musmann, G., downward FAC area is moving eastward with a Oddy, T., and Schwingenschuh K.: The Cluster speed of ~ 2 km s which is in good agreement with Magnetic Field Investigation: overview of in-flight -1 the mapped bulk velocity measured at the Cluster performance and initial results, Annales spacecraft closest to that area.Geophysicae, 19, 1207Geophysicae, 19, -1217Geophysicae, 19, , 2001.$ Comoving with this leading downward FAC area at Baumjohann, W., Paschmann, G., and Lühr, H.: ~ 71.5° latitude is a trailing upward FAC area Characteristics of high speed ion flows in the plasma slightly poleward of 68° latitude.The tilt angle between these two FAC areas is in good agreement with the tilt angle of the flux rope mapped to the ionosphere.The electric field in the region is consistent with a current flow from the downward to the upward FAC area in southwestward direction.This is the same direction in which the current in the flux rope is flowing if it is assumed to be close to force-free.$ No ionospheric topological correspondence can be found to the fast (~ 1200 km s ) earthward -1 movement of the B peaks in the magnetosphere X related to the TCR.
We suggest a possible interpretation of these topological correspondences as follows: The two FAC regions of opposite polarity may correspond to the ends of the flux rope in the direction of its central axis, where the magnetic topology is not closed and current can partly be diverted into FAC (or vice versa).If this hypothesis applies, it would mean that the current generated further downtail in the magnetosphere can either close via the ionosphere, or "tunnel" through the flux rope in the same direction.Such a current closure during a substorm would only be possible if the core the data.EISCAT is an International Association (MPG), Japan (NIPR), Norway (NFR), Sweden (NFR) Amm, O.: Ionospheric elementary current systems in

Fig. 1
Fig. 1 Mapped positions of Cluster spacecraft to the ionosphere (dots, colours as in ?), and IMAGE magnetometer stations (squares with station abbreviations).
instruments are used.With paper we analyse Cluster satellite data, IMAGE WIC respect to the CIS, data from the Hot Ion Analyser (HIA) optical data, and conjugate MIRACLE and EISCAT are shown for spacecraft 1 and 4, while data from the ground-based data for a flux rope event observed by Composition Distribution Function (CODIF) instrument Cluster on August 13, 2002, at ~ 2259 UT.
(next page).The flux rope event is accordance with the results of the TCR statistics by bracketed by the two vertical dashed lines.The magnetic Slavin et al. (2005).As the direction of the core of the data shown have a 22 Hz resolution.The most flux rope is expected to be roughly 90° tilted with respect pronounced feature in the magnetic data during that to the motion of the associated TCR, this leads to an period is a positive peak in B of ~ 9 nT in spacecraft 3, estimate for the flux rope tilt of ~ 29° clockwise from Y coincident with a bipolar signature of comparable the +Y axis (if seen from +Z direction), which is amplitude in B .This signature, with a first equatorward, somewhat smaller than the value of 42° deduced from Z then poleward B component, corresponds to what is the Cluster 3 data above.However, since Cluster 3 is Z expected for a earthward moving or BBF-type flux rope located northward of the core of the flux rope, for the as described in the introduction.Simultaneously with the given geometry this increase of the apparent tilt angle B increase, a sharp increase of B on top of a smoother with increasing distance from the core in +Z direction is Y X decrease is measured at spacecraft 3.While the smooth expected.decrease corresponds to a deeper entry of the spacecraft into the plasma sheet boundary layer, the sharp increase The X (geographic north) component of the IMAGE is interpreted as a tilt of the flux rope in the X-Y plane.magnetometers are shown in Fig. 2 in a conventional By comparison of the peak values in B and B with during the time of interest around 2300 UT.Two major X Y respect to the background magnetic field, a tilt of ~ 42°auroral regions are visible (data not shown): An in clockwise direction (if seen from +Z direction) with equatorward, fainter, auroral structure is seen located at respect to the Y axis is inferred.Also |B| peaks during ~ 68° of latitude.This structure has been pre-existing the flux rope, as expected (not shown in the Figure, but before the substorm intensification at 2250 UT (data not can indirectly be deduced from the peak in the magnetic shown).Starting from that substorm onset, a second, pressure, bottom panel of Fig. 3).The duration of the B brighter auroral region has developed and moved Y and |B| peaks lies with ~ 20 s well in the range of the polewards.At 2256:38 UT, this regions peaks at ~ 71.2°s tatistical results of Slavin et al. (2003).Simultaneously latitude, while at 2300:43 UT, it has reached ~ 72.7°w ith the flux rope, a downward field-aligned current latitude at 16° longitude.To the west and to the east of (FAC) of ~ 3 nA m is calculated using the curlometer Scandinavia, these two auroral regions appear to re--2 Fig. 3 Cluster FGM and CIS data of August 13, 2002, for the interval 2257-2303 UT.The CODIF instrument has been used for the CIS data of Cluster 4, HIA for CIS data of Cluster 1 and 3; colours for the different Cluster spacecraft as in Fig. 1.Panels from top to bottom: First to third panel: Magnetic field components in nT, in GSM coordinates; Fourth to sixth panel: Ion bulk velocities in km s , in GSM coordinates; Seventh panel: Currents -1 determined with the curlometer technique in nA m , in GSM coordinates: black: X component, red: Y component, -2 green: Z component; magneta: div (quality parameter, should ideally be zero; plotted with an offset of -6 nA m , -2 for convenience); Eight panel: Currents parallel (black) and perpendicular (red) to the magnetic field, in nA m ; -2 Ninth panel: Plasma density at the four different spacecraft, in ccm ; Tenth panel: Temperature perpendicular to the -1 magnetic field, in eV; Eleventh panel: Total (thick line) and magnetic (thin line) pressure in nPa; the flux rope event is bracketed by the two vertical dashed lines.

Fig. 4
Fig. 4 Cluster footprints (dots, colours as in ?), and direction estimates of the flux rope as mapped to the ionosphere (blue line: based on 90° rotated TCR direction of motion; red line: based on the orientation measured by Cluster 3; in both cases linearly continued in the magnetospheric X-Y plane) at 2259:50UT, together with WIC emissions in R at 2258:41 UT (iso contour lines).

Fig. 6
Fig. 6 Electric field measured by EISCAT for four consecutive scans from north to south between 2200-0000 UT (two vectors have been deleted for data quality reasons).Data of the scan 2230-2300 UT are shown at the actual observation points, data of earlier scans are shifted westward, of later scans eastward.The area enclosed by the red dashed line marks the spatiotemporal domain of the substorm intensification and recovery phase.