Study of near-earth reconnection events with Cluster and Double Star

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007ja012902, 2008 Study of near-earth reconnection events with Cluster and Double Star V. Sergeev, 1 M. Kubyshkina, 1 I. Alexeev, 2 A. Fazakerley, 2 C. Owen, 2 W. Baumjohann, 3 R. Nakamura, 3 A. Runov, 3 Z. Vörös, 3 T. L. Zhang, 3 V. Angelopoulos, 4 J.-A. Sauvaud, 5 P. Daly, 6 J. B. Cao, 7 and E. Lucek 8 Received 23 October 2007; revised 24 January 2008; accepted 19 February 2008; published 5 June [1] Observations made by a unique constellation of Cluster (at R E ), TC2, Goes10, and LANL spacecraft (near 6.6 R E ) have allowed us to study the details of three reconnection events in the middle of a thick plasma sheet with the reconnection X-line located unusually close to Earth (10 12 R E ). We use mapping along field lines with magnetospheric models adapted to magnetic field observations to confirm that the reconnection region mapped onto localized auroral brightenings. Using simultaneous observations in the inflow and outflow regions, we describe an encounter with a localized tailward Alfvénic jet produced by a short isolated reconnection pulse. A good correlation between intense E and ion [BV] indicates that the concurrent strong turbulence could not destroy the frozen-in ion behavior in the reconnection outflow. We find that a steady quadrupole-like distribution of the magnetic B y component in the turbulent reconnection outflow extended far beyond the ion diffusion region and existed for several minutes. We demonstrate an apparent V x flow reversal, formed owing to the reappearance (switch-on) of reconnection at another location, rather than to a continuous motion of the active X-line. Using the Liouville mapping technique, we show that the acceleration of outflow electrons, after the particles passed a potential drop of 180 V, is consistent with Fermi/betatron acceleration. We also suggest another interpretation of the energetic particle bursts at the onsets, to emphasize the role of seed population and explain the sudden burst as a consequence of changing magnetic topology. Citation: Sergeev, V., et al. (2008), Study of near-earth reconnection events with Cluster and Double Star, J. Geophys. Res., 113,, doi: /2007ja Introduction [2] The importance of magnetic reconnection as a universal process in space plasmas that provides rapid energy dissipation in many parts of the plasma universe is widely recognized. Theoretical and simulation studies are important but prove to be very difficult, especially when attempts are made to include the kinetic effects. Laboratory experiments cannot fully reproduce the required physical conditions. Satellite observations are the only way to probe the real behavior of space plasma, and reconnection can be studied systematically only in the Earth s magnetosphere. That approach, however, requires multi-instrumental and 1 Institute of Physics, St. Petersburg State University, St. Petersburg, Russia. 2 Mullard Space Science Laboratory, University College London, Dorking, UK. 3 Space Research Institute, Austrian Academy of Science, Graz, Austria. 4 University of California, Los Angeles, California, USA. 5 Centre d Etude Spatiale des Rayonnements, CNRS, Toulouse, France. 6 MPS, Katlenburg-Lindau, Germany. 7 Laboratory of Space Weather, CSSAR/CAS, Beijing, China. 8 Space and Atmospheric Physics, Imperial College, London, UK. Copyright 2008 by the American Geophysical Union /08/2007JA012902$09.00 multiprobe measurements with good coverage of the reconnection region, which is difficult to achieve with existing spacecraft owing to the highly dynamical and small-scale nature of related phenomena. Until now there have been large controversies on many basic questions, such as: what controls the reconnection rate and what is the role of turbulence; and what is the reconnection geometry and how does it evolve in the case of short reconnection pulses? [3] We address some of these issues in this paper where we present and analyze in more detail observational data for a rare situation of near-earth reconnection events on 26 September 2005, previously studied by Sergeev et al. [2007] (hereafter referred to as Paper 1). These observations are unique because of favorable spacecraft coverage near the central meridian of activation, and because three intense reconnection events with nearly the same configuration were repeatedly observed. The unprecedented coverage during this event makes it imperative to provide a full description of the observations for future reference. Moreover, the reconnection in this case proceeded in specific conditions, namely, in a thin current sheet embedded within a thick closed field-line region of the magnetotail. Such a situation, important for understanding both substorm onsets and reconnection physics, has been little studied theoreti- 1of19

2 Figure 1. Onset times of activations as observed in the auroral zone and near the geosynchronous orbit during events a, b, and c. cally and by using kinetic simulations as compared to a case of an initial 1d plasma sheet confined between antiparallel magnetic fields in a rarefied (lobe) plasma. In addition, the spacecraft constellation on that day allows us to study the reconnection process at 1.5 R E separation, which was not available before. Part of this event was also discussed by Sergeev et al. [2006], with emphasis on onset determinations, and by Vörös et al. [2007], who analyzed turbulence properties using Cluster data during one reconnection episode. [4] Taking advantage of these previous analyses as well as additional data now available, in section 2 we summarize observational information about the event, provide a general context, and specify the magnetospheric magnetic configuration, with the purpose of establishing a frame for further detailed analyses. In section 3 we describe an encounter by Cluster with a localized Alfvénic jet in the tailward outflow region with observations near the separatrix in the earthward outflow region by TC2. In section 4 we look into some details of a reconnection fast outflow region in the case of strong turbulence observed during prolonged Alfvénic outflow, including the appearance of quadrupole magnetic fields, the validity of frozen-in conditions, and how the transition from tailward flow to earthward flow was realized. [5] We try to fully use the advantage of unique spatial coverage as well as multi-instrumental observations provided by Cluster and Double Star (TC2) spacecraft, whose instrumentation was described in special issues (see Annales Geophysicae, 2001 (NN 10 12) and 2005 (N 11)). 2. Event Overview [6] The time interval of interest is between 08 and 10 UT on 26 September 2005, when three auroral brightenings a, b, c occurred at 0842, 0930, and 0940 UT, respectively (see Figure 1 of Paper 1). Each brightening, observed by the IMAGE auroral imager, was localized in 2 h MLT-wide region, being at h MLT for activation a and at h MLT for both b and c. Therefore, the Cluster-TC2 spacecraft constellation aligned along 23 h MLT meridian was near the central meridian of the activity. Because of a too low time resolution (2 min) of IMAGE auroral imager, the ionospheric onsets are better characterized by the ground magnetometers in Canada located beneath the auroral brightening region (shown at the top of Figure 1). Representative near-geosynchronous observations include both magnetic variations recorded at TC2 (and the Goes 10 for event a), and energetic particle flux increases at the Los Alamos spacecraft L084, located closest to the activation meridian (see locations in the Figure 2). The relative timing is simpler for isolated onset: in both cases the earliest signatures were the energetic electron flux increases observed at Cluster C2 at and UT, which were closely accompanied by energetic electron flux increase (within 10 s) at TC2 ( UT in event a) and by L084 proton flux increase ( UT in event b). [7] According to ACE and WIND measurements, the IMF had a small B z component resulting in a weak global auroral zone activity. However, the enhanced SW density (up to 20 cm 3 ) resulted in a high flow pressure up to P d 8 npa between 08 and 0920 UT. According to Paper 1, the high flow pressure was the main reason for the rather stretched magnetotail configuration observed during this period of generally weak activity. Enhanced stretching is evidenced by (1) a depressed geosynchronous magnetic field (down to 50 nt; Figure 1), (2) a high lobe field at 16 R E (Cluster) in excess of 40 nt, and (3) the proton isotropic boundaries at a rather low magnetic latitude 64 CGLat near the midnight (b2i boundaries, observed by DMSP and NOAA spacecraft). Because of strong stretching, the transition region between the quasi-dipolar and tail-like field moved earthward, making it possible for the reconnection to occur very close to the Earth. 2of19

3 Figure 2. Spacecraft configuration in GSM coordinates with schematic of reconnection region location. [8] According to crossings by DMSP F15 around 0843 UT and by DMSP F16 around 0927 UT, both near 23 h MLT (not shown), the auroral oval was wide, with its poleward boundary located at 69 CGLat and the equatorward boundary near 63 CGLat. The plasma sheet precipitation region closest to the pole was characterized by rather dense and cold plasma with electron energy below 1 kev. [9] The spacecraft configuration is shown in Figure 2. Cluster formed a large triangle with separations about 1.5 R E (10000 km) both in X (between C3, C4, and the more tailward C1 C2 pair) and in Y (the largest separation between C1 and C2), allowing us to evaluate the differences at the spatial scale of proton inertial lengths. However, a small separation 1000 km in Z between C3 and C4 at nearly the same X-Y position is useful for probing the thin current sheet conditions expected near the reconnection region. An advantage of this event is that the TC2 spacecraft probed the region near the geostationary distance at the meridian 23 h MLT, where the auroral brightenings were centered, and where the Cluster spacecraft probed the reconnection outflows at the R E distance. The TC1 spacecraft was on the inbound portion of its orbit, probing the lobe at 03 LT, and it was only used for the magnetospheric modeling described below. [10] The accurate knowledge of the magnetic field configuration is a principal factor to integrate the observational information, obtained in the magnetotail and at the low altitudes near the ionosphere. Magnetic configuration and mapping were first explored using the standard T96 models (based on corresponding SW parameters), which gave an insufficiently stretched configuration with a significant difference between the modeled and observed magnetic fields. To obtain a better mapping at the times prior to the auroral brightening and reconnection onsets, we varied model parameters to obtain a best fit to the magnetic fields observed by Cluster, TC2, TC1, and Goes10 spacecraft at 0841 UT and 0930 UT. We used a technique described by Kubyshkina et al. [1999] which allowed us to vary the intensity and thickness of the main current systems (the tail and ring currents) and to include an additional localized tailcurrent sheet to better reproduce the magnetic fields, observed in our case by seven spacecraft. As an additional free parameter we also included the asymptotic tilt-related shift of the neutral sheet. Unfortunately, even though we obtained a much better fit to observations than that provided by the original T96 model, we were unable to make it perfect. The problem is that, when the algorithm is approaching the minimal mismatch between the model and observed fields, an unrealistic magnetic island with large southward B z emerges in the equatorial region at r >8R E, which severely distorts the mapping. [11] Two extreme models obtained for the 0930 UT epoch serve to illustrate that difficulty. The model in Figure 3 (middle row) provided the best fit to all available spacecraft magnetic observations, but it contained the big island with large southward B z at X < 8 R E in the near-earth neutral sheet. Considering possible problems with the B z -offset determination at Goes10 and TC2, we also constructed a model in which a low weight was assigned to observed B z, when finding the best fit function. To compensate for the resulting loss of information, we added the position of DMSP b2i boundary as an additional observational input. That boundary is interpreted as an image of the isotropic boundary of proton precipitation in the current sheet (see Kubyshkina et al. [1999] for the description of how to use it in the modeling). It was observed at 63.6 CGLat at 0927 UT, 3 min before the activation onset. The model produced in that way has a smooth profile with a smaller negative B z and higher latitudes of ionospheric footpoints than those of the previous model. (In this model the geosynchronous B z was overestimated by nt against the observation). Two extreme models (Figure 3, middle and bottom rows) provide a limit window for the ionospheric footpoint: here the TC2 spacecraft maps between 63.4 and 63.6 CGLat (T96 gives 64.0 ) and the Cluster spacecraft map in between 64.7 and 66.0 CGLat (T96 gives 67. ). Also, the poleward oval boundary at 69 CGLat observed by DMSP F16 at 0926 UT is mapped to dz 4 R E northward from the neutral sheet at Cluster distance, suggesting that the plasma sheet was 8 R E thick during the studied events. [12] Concerning the magnetic islands that appear in the reconstructed magnetic configurations just before the activation onsets, we caution against interpreting them as a reconnection signature. We are sure this is due to inability of the standard model to reproduce a configuration with a quasi-neutral tail current sheet associated with a very steep gradient of B in the transition region. [13] The configuration before the activation a (at 0841 UT) has fewer problems when fitting all available measurements: no large magnetic island exists at the 23 h MLT meridian, although a weak negative B z > 5 ntis present near midnight. The ionospheric footpoints in this case are near 63.6 (TC2) and 64.5 (C2), which nicely correspond to the observed latitude of the auroral brighten- 3of19

4 Figure 3. (left) Magnetospheric models and (right) current densities and B z radial profiles at the onsets of events a and b. ing, as well as to the precipitated energy-dispersed energetic proton beam (Paper 1). The modeling also suggests that the TC2 spacecraft could be on the earthward side near the reconnection separatrix, while the C2 spacecraft could be the nearest to the reconnection separatrix on the tailward side. This location explains why the energetic electron flux increases, the earliest indications of the activation a at TC2 and C2, were simultaneous in this case within one 4s spin period. [14] The magnetospheric modeling results shown in Figure 3 and the relative spacecraft ordering based on the B x component and spacecraft potential values (shown below) indicate that in the Cluster group the spacecraft C2 at 16 R E was the closest to the neutral sheet in both isolated onsets a and b when it observed the earliest activity signature, namely, a sudden electron flux increase at energies up to 300 kev (Paper 1; also Figures 5 and 8). In both cases the 50 kev energetic electrons displayed trapped (pancake) distributions before the onset, confirming the closed topology of the magnetic field lines prior to the onset (not shown). Shortly before onset a the C2 spacecraft crossed the neutral sheet. Here it observed a weak magnetic field (B z 1 nt), at 0831 and 0834 UT, in the near-earth part of the tail current sheet center, which is generally required for the reconnection, even though it was embedded into a large area of closed magnetic field lines. [15] At the beginning of the electron burst, it initially displayed a field-aligned distribution with clear tailward streaming, being stronger in event a (see a Supplement in Paper 1). The short timescale (<10 s) of this flux increase combined with the tailward streaming indicates a very abrupt topology change, in which a powerful acceleration source (the reconnection region) suddenly switched on somewhere between Cluster (16 R E ) and geosynchronous orbit. The earliest onset times for these events established from RAPID observations are UT (event a) and UT (event b). [16] PEACE and CIS spectrograms from all Cluster spacecraft (see, e.g., Figure 4) together with the plasma parameters displayed in Figures 5, 8, and 9 confirm that all spacecraft stayed inside the thick plasma sheet. However, while the C2 probe initially observed a population with energies extending to 2 3 kev near the neutral sheet, a colder population was found in the outer plasma sheet at C3 and C4, consistent with the latitudinal pattern of electron spectra observed by the DMSP spacecraft. The spectral variations seen by Cluster PEACE instruments (Figure 4) 4of19

5 Figure 4. PEACE spectrograms at C1, C2, C4, and TC2 spacecraft. are produced by mainly the relative vertical motion of spacecraft in this thick but structured plasma sheet. 3. Probing the Reconnection Onset and Localized Outflow Region With Cluster and TC Cluster Observation of a Localized Alfvénic Outflow Region: Event a [17] This episode, having the best coverage in observations and compared to predictions of an MHD reconnection model [Ivanova et al., 2007], can serve as a textbook example of encountering the localized reconnection region. Its specific features are (1) very intense dissipation signatures, observed only at the C2 spacecraft (electron acceleration up to 300 kev, electric field E y up to 17 mv/m, tailward electron flows exceeding 800 km/s, strong magnetic perturbations), and (2) their short duration, s, depending on a specific signature. Combined, they indicate the short duration (100 s) of the intense reconnection pulse, consistent with the modeling results by Ivanova et al. [2007]. [18] Observations at C2 can be divided into three phases. Between a1 and a2 ( to UT), reconnection started at some distance from the observation point: the tailward beam of energetic electrons propagated fast, but the Alfvénic jet of reconnected plasma tubes had not yet reached the spacecraft. This 40-s propagation delay (assuming a linear change between V x = 0 at the X-line and V x 800 km/s at the observation point observed after a2) is consistent with 3 R E distance to the source, that is, X-line at 13 R E. At the end of this time interval, a smooth increase of the electron temperature and density at C2 probably indicates a compression of the background plasma perturbed by the approaching reconnection-related fast flow [e.g., Ugai and Wang, 1998]. We used a standard Minimum Variance Analyses (MVA) to estimate the lowest variance direction; the eigenvalues were 66./1.4/0.26 for the time interval UT, suggesting a well-developed anisotropy. The well-defined normal to this jet boundary with a normal component B n 15 nt was [ 0.40, 0.05, 0.91] GSM, consistent with the orientation of the plasmoid-like plasma structure shown in Figure 6a. [19] During the time interval between a2 and a3 ( and UT), the C2 spacecraft stayed inside the fast plasma jet in the newly reconnected plasma tube (the electron flow velocity was 800 km/s in the plateau region). Here the southward B z dropped below 15 nt and became comparable to other magnetic components. The B x component decreased by half, indicating a deep C2 penetration into the structure. During that period, E y approached 10 mv/m and peaked at 17 mv/m at the back front of the structure, with only a slight increase of the density. The B y component showed a negative perturbation 5of19

6 Figure 5. Overview of Cluster observations for event a. of 15 nt, consistent with the expected quadrupole magnetic field on the tailward side of the reconnection region (see also section 4.2). [20] For the trailing edge of the fast jet, the MVA results are well defined in terms of the eigenvalue ratios: for example, they were 125./5.5/0.53 during the UT window, in which the N vector of the normal is [0.37, 0.77, 0.51]. The difference from the leading front is that here the plasma density and temperature sharply dropped in nearly one spin, together with a B increase to resemble a reconnection shock. Unlike the front side, with its well defined start time, here is none, so the inferred orientation varied with varying time window duration and position. Nevertheless, the results were qualitatively the same with respect to both the signs of the components and their relationship. For example, we always had positive Nx and Nz, and the Ny component is the largest in magnitude and negative. This boundary is again tilted in the XZ plane according to the plasmoid-like structure in Figure 6a, but it also displays a stronger tilt toward dawn. This feature indicates that the region near the reconnection separatrix was strongly corrugated, possibly showing a strong kink with a large tilt, a common feature near the flow reversals [Wygant et al., 2005; Runov et al., 2005; Laitinen et al., 2007]. [21] The C2 spacecraft, after having left the fast flow region, found itself in a cold (Te 0.3 kev) and less dense (n e 0.4 cm 3 ) plasma with B = 40 nt, so that the Alfvén velocity V A 1300 km/s. Although it is being 50% higher than the median value of electron flow speed inside the fast jet (800 km/s), the velocity indicates a nearly Alfvénic magnitude of the fast flow jet, consistent with the reconnection theory prediction. These C2 observations provide a well-documented set of observations across the localized Alfvénic jet plasmoid-like structure. Such structures have been previously demonstrated in a transient reconnection model [Semenov et al., 1983; Ivanova et al., 2007], as well as in MHD simulations [Ugai and Wang, 1998]. [22] This episode is equally interesting in the light of observations at the spacecraft C1, C3, and C4, which did not cross the Alfvénic jet but provided comprehensive observations in the reconnection inflow region perturbed 6of19

7 Figure 6. Cluster observations in the inflow region during event a, together with interpretation scheme based on impulsive reconnection model b. by this plasmoid-like structure. These spacecraft did not detect energetic electron bursts, or fast ion outflows (Figure 5), but they all observed similar magnetic and electric perturbations (increase, then decrease of B x, southward B z and positive B y variation, positive E y about 3 4 mv/m), which are nearly simultaneous, although different in shape and weaker as compared to the variations observed at C2. These perturbations propagate tailward (from C3 to C4 to C1) with roughly a 10 s time delay (Figure 6b), consistent with the 1000 km/s propagation velocity of the plasmoid-like structure over the km separation distance between the spacecraft. Such a form of the perturbations is predicted by the impulsive reconnection model, as recently demonstrated by Ivanova et al. [2007]. They used magnetic perturbations in the inflow region as an input in their search procedure to get the parameters of the reconnection pulse. Specifically, they inferred a reconnection pulse duration of 100 s and the location of the reconnection line at X = 10 R E (within ±1 R E margin) in this event Electron Acceleration in the Alfvénic Outflow [23] According to reconnection theory and the scheme in Figure 6a, the trailing boundary of the plasmoid-like struc- 7of19

8 Figure 7. Results of Liouville comparison between the electron field-aligned phase space densities (PSD) measured by C2 PEACE before ( UT, ACC) and after (averaged over UT, SRC) crossing of the Alfvénic jet boundary. (a) Comparison of phase space densities; (b) beam energy E ACC divided by (E SRC 184 ev) computed using source energy E SRC and Liouville mapping results. ture is expected to be analogous to the slow shock in the original Petchek model, where plasma is accelerated inward to become an Alfvénic outflow. A sharp reduction of both electron density and temperature at a3 in Figure 5 could be another signature of this slow shock. If this is the case, then these electrons with reduced density and temperature represent an electron source population for the plasma found inside the Alfvénic structure. This allows us to quantitatively characterize the electron acceleration process by applying Liouville s theorem to relate the phase space densities (PSD) of tailward field-aligned electron populations in two regions. By finding the source energy (E SRC ) for some accelerated population energy (E ACC ) with the same PSD, one may interpret them as the same group of particles. In this way one can establish a relationship between the initial (E SRC ) and final (E ACC ) energies (see Figure 7a). Figure 7b demonstrates that this relationship can be effectively approximated by a linear relationship E ACC = 2.9 (E SRC 183 ev), which implies that the electrons are accelerated proportionally to their starting energy, after having lost the energy that would result from passing through the 183V electrostatic potential barrier. The latter feature is qualitatively consistent with kinetic reconnection models [e.g., Pritchett, 2001; Hoshino et al., 2001] and with a mapping of distribution functions near the diffusion region presented by Egedal et al. [2005], in which the diffusion region was shown to possess a negative electric charge. Observation of this feature in our event therefore provides additional support for the reconnection origin of this Alfvénic outflow structure. The obtained relationship is similar to that previously derived by Alexeev et al. [2006] for the acceleration at the PSBL boundary. 8of19

9 Figure 8. Cluster observations of isolated onset b Cluster Observation During Isolated Event b [24] During the second isolated event, the energetic electron flux increase with weak signatures of tailward anisotropy was first observed at C2, then at C3 and C1, and last at C4 with 1 min total time delay (see Figure 8). This sequence arranged in the order of relative spacecraft distance from the neutral sheet suggests that Cluster crossed the expanding separatrix region produced by the progressing reconnection. Note that the spacecraft observed a drop of energetic electron flux when temporarily exiting to the outer part of the plasma sheet (like C2 did between 0935 and 0936 UT and later), but soft electron plasma still had a large density about 0.3 cm 3. This finding suggests that the magnetic separatrix in the reconnection process did not reach the lobe region and that the reconnection was still progressing inside the closed field line domain. The initial phase of this event developed similarly to event a. Again, the major E- and B-field perturbations and the onset of the tailward plasma flow from the C1 HIA instrument started 80 s after the energetic electron burst onset (at b2 in Figure 8, UT), consistent with the propagation of the reconnection jet from the source (X-line) to the spacecraft. Again, the electron tailward flow gradually increased prior to that time, suggesting runaway electrons or a slow compression. Assuming a 300 km/s ion flow speed (observed in the Vx-plateau region near 0932 UT at spacecraft C1) as the propagation speed, the result is a distance of about 4 R E, which suggests the reconnection location at around 12 R E. The electron flow speed at C2 was roughly twice as large as compared to the ion bulk velocity at C1 (we abstain from interpreting this as a feature of the ion diffusion region because of the large 1.5 R E Y separation between C1 and C2). We note that the plasma sheet density in event b exceeded 1 cm 3 (Figures 8 and 9), so the median 600 km/s electron bulk flow speed between 0932 and 0934 is still comparable to the Alfvén velocity in the inflow region. Therefore, the observed southward B z (peaking at 10 nt), considerable electron (C2) and ion (C1) outflow, together with strong positive E y (3 mv/m at C2 spacecraft), can similarly be interpreted as signatures of tailward Alfvénic outflow produced by the reconnection. 9of19

10 Figure 9. TC2 electron moments and magnetic field (in GSM coordinates) during two isolated onsets of reconnection. [25] Although according to energetic electron observations, all spacecraft passed across the magnetic separatrix, the perturbations (E y, B s, etc.) were larger near the neutral sheet (at C2) than at other spacecraft in the outer region. The main difference with event a is that event b lasted longer and showed intense noise-like perturbations, probably with a few activations, with a much stronger event c initiated at this background TC2 Observations Around the Plasma Sheet Expansion Onsets: Events a and b [26] Compared to the electron spectrograms from the Cluster PEACE instruments, a similar PEACE spectrogram from TC2 showed a more dramatic feature: sharp particle energizations associated with the isolated onsets a and b (see Figure 4). The corresponding electron moments and magnetic variations are displayed in Figure 9. The onsets of sharp electron energization are associated with drops of the magnetic field B x component; such events are usually referred to as sudden plasma sheet expansions. In both cases the energization occurred 1 min after the first indication of the reconnection activity. [27] In event a, TC2 spacecraft was in the strong B (100 nt) region near the expected location of the magnetic separatrix (Figure 3). Consistent with this expectation, the density about 0.5 cm 3 and temperature T e 0.7, observed before the earliest activity signature A1, are similar to their values at Cluster C2 (Figure 5). Prior to the plasma sheet expansion, a sharp increase of the 0.2 MeV electron flux at TC2 (from the HEE instrument) started at UT (A1 in Figure 9), simultaneously (within 1 spin) with the energetic electron flux increase at Cluster C2. In terms of the reconnection geometry, this most probably means that TC2 was near the separatrix layer at the reconnection onset. [28] The time interval between A1 and the plasma sheet expansion A3 displays almost no changes in either the 10 of 19

11 magnetic field or electron density or temperature, but it shows a noticeable electron flow variation. Initially, during 20 s between A1 and A2, the electrons move tailward at 400 km/s bulk velocity with a considerable field-aligned flow component (continued during 4 spins, indicating that time aliasing effects were not a factor). A beam of lowenergy electrons flowing toward the reconnection region near the separatrix was previously identified in observations [Fujimoto et al., 2001], as well as in kinetic simulations [Pritchett, 2001; Hoshino et al., 2001]. However, in our case the energy is different: half of the contribution to the tailward bulk flow is being provided by the energetic (kev) electrons. The outward (toward the lobes) cross-b flow component (corresponding to negative [BV] y, where V is the electron bulk velocity) was also observed at that time. That phenomenon can possibly be interpreted as a signature of a compression wave preceding the arrival of a main body of injected accelerated plasma. [29] During the following 30 s before the sharp energization (A2 A3) the bulk of electron flow was earthward. By analogy with the observations at C2 spacecraft (Figure 5, between a1 and a2) we may interpret this early earthward electron flow as a kind of runaway (high-energy tail) accelerated electrons in the newly reconnected flux tubes. The magnetic field (B x ) is slowly increasing. However the cross-b convection is now toward the neutral sheet, resulting in a positive [BV] y. [30] The inward convection with earthward flow and heat flux continued during the plasma sheet expansion. Applying the MVA for the plasma sheet expansion time interval UT, one obtains a well-defined normal (the eigenvalues 190./37./1.9, with a minimum variance vector N3 [ 0.32, 0.25, 0.91], jb3j << jb1j), which is in line with our expectations. An intense negative B y variation occurred in a narrow outmost part of the plasma sheet. In the case of the expanding plasma sheet, it corresponds to a downward field-aligned current. The outward motion of the plasma boundary concurrent with the inward plasma convection is a feature of the magnetic reconnection process, typically observed during sudden plasma sheet expansions in the near tail [Forbes et al., 1981]. [31] In the second event the B x component was about 20 nt, jb x /B z j << 1 and the plasma density was larger than at Cluster. Here TC2 was in the middle of the current sheet at 6.6 R E. The earliest activity timeline B1 ( UT) in Figure 9 corresponds to Cluster C2 observations (same as b1 in Figure 8). Unlike the previous case, no clear signs of activity were observed for 80 s before the onset of earthward flow, heat flux increase, and positive [BV] Y at UT, which accompanied the beginning of the plasma heating/ compression. This delay can be understood as being caused by the propagation from the activity source (X-line). The plasma density drop at UT may indicate the arrival of a plasma bubble. Strong magnetic perturbations of turbulent character started after 0932 UT. 4. Cluster Observations During Continuous Plasma Sheet Activity 4.1. Plasma Sheet Activity During Events b and c [32] After all four Cluster spacecraft entered the tailward fast outflow, generally with prevailing negative B z, they continued to observe intensely fluctuating fields during the events b and c. A clear intensification of the tailward ion outflow carrying the southward B z, with >1000 km/s peak flow speed value, is observed after 0940 UT (event c). One more activation can also be identified, this time with an earthward flow (event d, after 0950 UT), to be discussed later. Observations by C1 are highlighted in Figure 10, as that spacecraft spent more time in the central part of plasma sheet compared to the other spacecraft. The C2 spacecraft, which was initially in the PS center, soon appeared below the neutral sheet and spent most of its time outside of the fast outflow, still staying in the cold dense thick plasma sheet. [33] There are a few interesting observations during this time period concerning the electric field perturbations and particle acceleration. The first one concerns a good agreement between E YGSE measured by the double probe and the [BV] y observed inside an intense tailward outflow during event c at Cluster C1 and C4 spacecraft. It shows that even a very strong turbulence observed between 0941 and 0943 UT at all spacecraft did not destroy the frozen-in behavior of the ion plasma, contrary to recent conclusions by Lui et al. [2007]; see also a companion paper [Vörös et al., 2008] for a thorough analysis of that turbulence. In terms of the magnetic reconnection pattern the observed frozen-in behavior also places the spacecraft C1 and C4 outside the ion diffusion region in this episode. [34] Another interesting feature is the intermittent appearance of intense short E Y pulses of different polarities at spacecraft C2 between 0935 and 0945 UT, which are in qualitative agreement with the behavior of electron-based [BV] Y at the same spacecraft. Closer inspection shows that these spikes correspond to the crossings of outer parts of the current sheet during its rapid vertical (flapping) motions, where the magnetic field magnitude and, hence, the associated (V z B x ) magnitude, are the strongest. The large spike amplitudes highlight the very fast (hundreds km/s) velocities of the flapping motion, which may be a feature of a nearby reconnection region. [35] A third interesting thing is the behavior of the energetic electron flux. We saw previously that its sudden increase was a remarkable earliest signature of onsets a and b. Surprisingly, the energetic electron flux dropped almost to the background level at all spacecraft during the strongest activation c (including C1 and C3 which stayed near the current sheet center at that time). Even though the electric field in that period was the largest of the whole event (exceeding 10 mv/m at C1 and C4), implying the strongest acceleration, the flux level was lower than, say, between 0945 and 0950, when the dissipation completely faded away. As discussed below (section 5.3), this implies another important controlling factor: the intensity and hardness of the seed population to be accelerated to get the energetic electron burst. [36] When the closely spaced C3 C4 pair moved into the inner plasma sheet after 0933 UT (the intervals marked with circles in the current density panel of Figure 11), it detected high current densities, typical for thin current sheets with a scale size of a few inertial lengths [e.g., Runov et al., 2005]. (For reference, a 40 nt difference of db 43 = B X4 B X3 over 900 km of spacecraft separation along the Z axis is equivalent to a current density of 35 na/m 2 in a 1 d 11 of 19

12 Figure 10. Overview of Cluster observations during events b, c, and d. In the third, fourth, and fifth plots, Eygse (solid lines) comes from double probe whereas the y component of [BV] (circles) is computed on the basis of magnetic and particle observations (HIA for C1 ions, CODIF for C4 protons, PEACE for C2 electrons) with different spacecraft in different plots. Note that only one component ( Vx * Bz) is plotted for C4 spacecraft. In the sixth, seventh, and eighth plots, data from different spacecraft are plotted together with traces being color coded according to the spacecraft number. horizontal current sheet). The db 43 trace in Figure 11 indicates a thin current sheet that persisted during the whole activation c. Spacecraft C1 and C3 spent most of their time in the central part of the current sheet, while C2 and C4 probed its outer part and occasionally visited its central part. The tailward flow faded away by 0945 UT, although both C1 and C4 still stayed in the central plasma sheet (see B x and Us panels in Figure 10) with plasma densities between 0.2 and 1 cm 3 and they observed enhanced flux of energetic electrons throughout the interval UT. [37] After 0950 UT, fast earthward flows were observed by the C1 and C4 spacecraft in two pulses. Before that time there were no signatures of a continuing strong dissipation. The absence of strong ion flows at C1 and C4 cannot be interpreted as being caused by their location in the diffusion region, where they would miss the Alfvénic flow, because the distance between them was larger (25 30 ion inertial 12 of 19

13 Figure 11. Cluster observations during events b and c. Dots in dbx43 (current density) plot show the observations when at least one SC was in the CPS. Circles in two Hall By plots show the predicted differences (only when they are >5 nt in the absolute value), whereas the traces show the By differences observed by the spacecraft pairs. lengths) than the expected length of the diffusion region. Moreover, the onsets of the earthward flow pulses after 0950 UT were first detected at C1 and, then, at C4 spacecraft (located earthward from C1). Such a delay agrees with a leading front of the structure propagating inward from the midtail, but it is inconsistent with the tailward motion of the active reconnection region. These observations indicate that in our case the 5-min time interval between the fast tailward flow and the fast earthward flow should be interpreted as a real pause in the reconnection, which later reappeared impulsively at a much farther distance in the midtail after it faded away in the near-earth region. This is at variance with most commonly cited scenarios of the tailward shift of a continuously operating reconnection region Appearance of Quadrupole Magnetic Perturbations During Events b and c [38] A quadrupole pattern of the magnetic field component directed along the tail current (By component in a reconnection coordinate system) is generated in the ion diffusion region and may possibly extend into a more distant region of the reconnection outflow. This pattern is a basic magnetic field property and an important distinctive signature of the reconnection process, which has been identified in the magnetotail flow reversal regions [e.g., Nagai et al., 2001; Oieroset et al., 2001; Runov et al., 2003]. Its relatively widespread nature and large magnitude make it attractive for monitoring purposes (together with Alfve nic outflows) to infer the temporal history and the relative location of the reconnection. However, it is spatially inhomogeneous and its presence is masked by other processes/effects that also cause the By component (e.g., fieldaligned currents and 3-D effects of the flank interaction, tail flaring effects, and the IMF By-related net By across the tail). It can also be significantly modified by severe flapping motions. Therefore it is not easy to separate the Hall-related quadrupole By from contributions of other sources, especially in the case of having only one spacecraft available. Previously, its demonstration relied upon statistical distributions, for example, by separately ordering By against Bx 13 of 19

14 Figure 12. Spatial distributions characterizing the quadrupole magnetic fields at tailward Cluster pair (C1 and C2) and earthward pair (C3 and C4) during activations b and c. (a) Characterization of the vertical By gradients by comparing the B y differences at two spacecraft against their (B x -based) predicted model values. (b, c) The B y distributions against B x, with the model of quadrupole B y shown by dashed magenta line and grey solid line corresponding to the tail flaring effect. (used as a proxy of z coordinate) in the tailward and earthward fast flow regions. Here we would like to investigate how the monitoring capability to detect the quadrupole B y can be expanded using the Cluster-like spacecraft configurations. [39] We start from showing the statistical B y (B x ) distributions for events b and c in Figure 12. The distinct patterns, shown schematically with dashed lines, are characteristic for a quadrupole B y in the tailward outflow (with addition of the flaring effect). By averaging the B y values in B x bins as in Figure 3 of Paper 1, the resulting regression relationship B y = B x for jb x j28 nt gives the average peak jb y j value of 13 nt at jb x j = 28 nt. This value (about 1/3 of BL = 40 nt found outside the current sheet) is comparable to previously cited peak values B y of the lobe field magnitude BL [Nagai et al., 2003; Runov et al., 2003; Laitinen et al., 2007]. This Hall-related effect is superposed onto the flaring-related regression line connecting the lobe field values (jb y j6 nt with the lobe field jb x j40 nt, resulting from the tail flaring). The comparisons in Figure 12 are made separately for two pairs of spacecraft C1 C2 (distant pair) and C3 C4 (earthward pair). Observation of similar relationships suggests a stable pattern existing on the spatial scale in X exceeding at least the 1.5 R E separation between the Cluster spacecraft. [40] The above relationships (shown by the dashed lines in Figure 12) provide a formal model for predicting the Hall-related B y based on the observed B x component. According to this model, one may compute the expected B y -field gradients for each of two spacecraft pairs at any time. Using the pairs instead of a single spacecraft is more convenient since it suppresses some effects (net B y, etc.), 14 of 19

15 reduces the amount of information to analyze, and allows concentration on time periods when the effect was intense and can therefore be easily detected. The model predictions for dby ij (By i By j ) are compared with the observed dby ij differences in Figure 12a. These hodograms confirm that required gradients really exist during most of the time period considered. The hodograms also show that the vertical motions (recognized by the large B x variations) contribute most strongly to the large B-field variability observed (with associated db y up to nt). Another (more isotropic) source of magnetic perturbations is also evident from the observed scatter. [41] Time plots comparing the modeled and observed B y gradients are shown in Figure 11. Intervals highlighted by yellow show when the predicted B y difference was large enough and the observed and the model values behaved similarly. During the entire event c both spacecraft pairs demonstrate a nice agreement. During event b the predicted quadrupole B y is clearly observed by the tailward pair (C1/ C2) simultaneously with the observation of a tailward ion outflow at C1. At the same time, between 0935 and 0939 UT, the B y difference observed at C3/C4 frequently has the opposite sign, and during that time the ion outflows at C4 also show sporadic earthward flows, suggesting that the X- line could sporadically appear between C1/C2 and C3/C4 pairs. Simultaneous observation of quadrupole-like fields of similar magnitude at the spacecraft pairs separated by km in X indicates that this is a lower estimate for the spatial scale of the region of quadrupole B y perturbations. The comparison of predicted and observed B y gradients can be used in practice as a method to determine when the Hall effect is switched on or off. 5. Discussion 5.1. Intense Near-Earth Reconnection on Closed Field Lines as Origin of Activations [42] A basic conclusion of Paper 1 that on 26 September 2005 Cluster registered in the near tail intense magnetic reconnection events embedded inside a thick region of closed plasma sheet field lines, is considerably strengthened in this paper by analyzing electron moments from C2 and TC2 spacecraft, available after a careful ground calibration, as well as by a number of new facts obtained. We have now verified that all three activations a, b, and c originated deep in the closed field-line region, as seen, for example, from pancake electron distributions on Cluster and TC2 before the onsets, and from a large oval width which gives a 8 R E - thick plasma sheet at Cluster distance being mapped to the tail (section 2). The large plasma density outside the embedded thin current sheet (always >0.2 cm 3 ) provides two important advantages for the study of reconnection in that (1) the source population outside the reconnecting sheet could be reliably controlled and (2) the double probe E-field measurements, which become uncertain in the tenuous lobe plasma, were more reliable than usually. [43] Identification of the Petschek-type magnetic reconnection in these cases is now more robust than ever before. Now we have directly measured with Cluster: (1) the tailward plasma outflows carrying the southward magnetic field, which are nearly Alfvénic (taking into account the relatively low V A outside the embedded sheet); (2) the quadrupole B y, extended into the outflow region; (3) the E-B variations in the inflow region consistent with measurements in the tailward outflow region; and (4) field-aligned energetic electron bursts with a tailward anisotropy in the tailward outflow. Also, (5) the Liouville mapping of electron distributions between inflow and outflow regions suggested a negative electric charge in the acceleration region (section 3.2), and (6) the intense precipitated energetic proton beam with a steep energy dispersion observed at low altitudes near the separatrix (see Paper 1) provided additional evidence for the reconnection. In addition, (7) the demonstrated frozen-in plasma behavior (outside the diffusion region) provided another characteristic feature of the Petschek-like reconnection, as distinct from the Sweet- Parker reconnection, or from the turbulence-based current disruption scenario. Important simultaneous observations in the earthward outflow region, particularly the measurements of simultaneous earthward flow and heat flux, are now provided by TC2. [44] Locations of the activation (reconnection) onsets are now better specified. First, they are certainly within the 7 R E to 15 R E margin, defined by TC2 and Cluster positions. Simultaneous detection (within one 4s spin) of energetic electron bursts at C2 and TC2 at onset a implies that they were on the neighboring field lines close to the reconnection separatrix, which is consistent with the modeling results in Figure 3. The estimate [11 R E ±2R E ] is consistent with (1) inversion of magnetic perturbations observed in the inflow region by Cluster using a transient reconnection model [Ivanova et al., 2007] and (2) a time delay of about 1 min when the plasma sheet expansion was observed at TC2 (section 3.4) and the open plasmoid structure start to be seen at Cluster 2 (sections 3.1 and 3.3). [45] In these cases the earliest signatures of enhanced dissipation were the reconnection-related energetic electron bursts shown in Figures 5, 8, and 9 (see also section 5.3), whereas the Alfvénic outflows at Cluster and the injection of energized plasma at TC2 were observed with the 1 min delay, consistent with their propagation from the source. Spatially, the reconnection region is certainly mapped onto the auroral brightening region, although the accuracy of comparison in the latitude (1 InvLat) is not yet sufficiently high. This unprecedented data collection provides clear support for the view that, both spatially and temporally, reconnection was the source of activations in these cases. We remind that all three events a, b, and c demonstrated negative magnetic bays and auroral brightening in the auroral zone, injections of energetic particles to geostationary orbit, turbulent magnetic field dipolarization, and the plasma sheet expansion in the near tail, as well as severe perturbations of the plasma sheet at >10 R E, where the reconnection develops. The events studied clearly belong to the NENL category [Baker et al., 1996], although they were not full-scale substorms Localized Perturbation During Isolated Short-Duration Reconnection Event [46] No detailed observations of localized plasma structure related to impulsive single X-line reconnection, like those presented above for the event a, were available before, and this is a major result of our paper. The transient character of the dissipation process in the tail and its 15 of 19