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The ion composition experiment on Cluster measures 3-D distributions in one spin of the spacecraft (4 s). These distributions often measure field-aligned ion beams (H+, He+ and O+) accelerated out of the ionosphere. The standard model of these beams relies on a quasi-static U-shaped potential model. The beams contain important information about the structure and distribution of the U-shaped potential structures. For example, a simple beam with a narrow velocity range tells us that the particles are accelerated going through a quasi-static U-shaped potential structure localized in space. A more complex beam with a large range of velocities varying smoothly (a few tens of kilometers per second to > 100 km s−1) tells us that the potential structure is extended and distributed along the magnetic field. The Cluster experiment has now revealed new features about the beams. Some beams are broken into many individual structures each with their own velocity. The U-shaped potential model would interpret the new features in terms of particles accelerated by narrow isolated potential structures maintained over an extended region of the magnetic field. Another interpretation is that these features arise as Cluster traverses toward the center of a small-scale U-shaped potential region detecting particles accelerated on different equipotential contours. The estimate of the distance of the adjacent contours is ~ 590–610 m at a Cluster height of ~ 3.5 RE. The observed dimensions map to ~ 295–305 m in the ionosphere, suggesting Cluster has measured the potential structure of an auroral arc.

Numerous observations have shown that ions flow out of the ionosphere during substorms with more fluxes leaving as the substorm intensity increases (Wilson et al., 2004). In this article we show observations of low-energy (few tens of electron volts) ionospheric ions flowing out periods without substorms, determined using the Wideband Imaging Camera (WIC) and Auroral Electrojet (AE) indices. We use Cluster ion composition data and show the outflowing ions are field-aligned H+, He+ and O+ beams accelerated to energies of ~40–80 eV, after correcting for spacecraft potential. The estimated fluxes of the low-energy O+ ions measured at ~20 000 km altitude are >103–105 cm−2 s. Assuming the auroral oval is the source of the escaping ions, the measured fluxes correspond to a flow rate of ~1019–1021 ions s−1 leaving the ionosphere. However, periods without substorms can persist for hours suggesting the low-energy ions flowing out during these times could be a major source of the heavy ion population in the plasma sheet and lobe.

Using Cluster Ion Spectrometry (CIS) data from the spacecraft-4 perigee traversals during the 2001–2006 period (nearly 500 traversals after removing those that are highly contaminated by radiation belt particles), we statistically examined the local time distribution of structured trapped ions at sub- to few-keV range as well as inbound–outbound differences of these ion signatures in intensities and energy–latitude dispersion directions. Since the Cluster orbit during this period was almost constant and approximately north–south symmetric at nearly constant local time near the perigee, inbound–outbound differences are attributed to temporal developments in a 1–2 h timescale. Three types of structured ions at sub- to few keV range that are commonly found in the inner magnetosphere are examined: – Energy–latitude dispersed structured ions at less than a few keV, – Short-lived dispersionless ion stripes at wide energy range extending 0.1–10 keV, – Short-lived low-energy ion bursts at less than a few hundred eV. The statistics revealed that the wedge-like dispersed ions are most often observed in the dawn sector (60% of traversals), and a large portion of them show significant enhancement during the traversals at all local times. The short-lived ion stripes are predominantly found near midnight, where most stripes are significantly enhanced during the traversals and are associated with substorm activities with geomagnetic AL < −300 nT. The low-energy bursts are observed at all local times and under all geomagnetic conditions, with moderate peak of the occurrence rate in the afternoon sector. A large portion of them again show significant enhancement or decay during the traversals.

It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than ~6 RE transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.

A data set of the Cluster cusp crossings over a 5-year period is studied for the interhemispheric comparison of the dipole tilt angle effect on the latitude of the mid-altitude cusp. The result shows that the dipole tilt angle has a clear control of the cusp latitudinal location. Although, the northern cusp moves 0.054° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 5.2 RE, the southern cusp moves 0.051° ILAT for every 1° increase in the dipole tilt angle at the mean altitude of 6.6 RE. The northern cusp dependence agrees with the trend formed by other observations of different satellites for different altitudes, whereas the southern cusp does not. We therefore suggest that there is an inter-hemispheric difference in the dipole tilt angle dependence of cusp, latitudinal location, which has an impact on other observations of different satellites in different altitudes.

The relationship between the average structure of the inner magnetospheric large-scale electric field and geomagnetic activity levels has been investigated by Double Star TC-1 data for radial distances ρ between 4.5 RE and 12.5 RE and MLT between 18:00 h and 06:00 h from July to October in 2004 and 2005. The sunward component of the electric field decreases monotonically as ρ increases and approaches zero as the distance off the Earth is greater than 10 RE. The dawn-dusk component is always duskward. It decreases at about 6 RE where the ring current is typically observed to be the strongest and shows strong asymmetry with respect to the magnetic local time. Surprisingly, the average electric field obtained from TC-1 for low activity is almost comparable to that observed during moderate activity, which is always duskward at the magnetotail (8 RE~12 RE).

On day 7 May 2005, the plasma instruments on board Double Star TC1 and Cluster SC3 spacecraft register inside the magnetosheath, at 19:15:12 and 19:16:20 UT, respectively, a strong pressure pulse due to the impact of an interplanetary shock wave (IS) on the terrestrial bow shock. The analysis of this event provides clear and quantitative evidences confirming and strengthening some results given by past simulations and observational studies. In fact, here we show that the transmitted shock is slowed down with respect to the incident IS (in the Earth's reference frame) and that, besides the transmitted shock, the IS – bow shock interaction generates a second discontinuity. Moreover, supported also by a special set three-dimensional magnetohydrodynamic simulation, we discuss, as further effects of the interaction of the IS with the magnetosphere, other two interesting aspects of the present event, that is: the TC1 double crossing of the bow shock (observed few minutes after the impact of the IS) and the presence, only in the SC3 data, of a third discontinuity produced inside the magnetosheath.

We present observations from two subsolar Cluster magnetopause crossings under southward interplanetary magnetic field and strong mirror mode fluctuations in the magnetosheath. In both events the reconnection outflow jets show strong variations on the timescale of one minute. We show that at least some of the recorded variations are truly temporal, not spatial. On the same timescale, mirror mode fluctuations appear as strong magnetic fluctuations in the magnetosheath next to the magnetopause. This suggests that mirror modes can cause the variations either through modulation of continuous reconnection or through triggering of bursty reconnection. Using a theoretical scaling law for asymmetric reconnection we show that modulation of reconnection at a single x-line can explain the observations of the first event. The second event cannot be explained by a single modulated x-line: there the evidence points to patchy and bursty reconnection.

We present a case study of significant heating (up to 8 keV) perpendicular to the geomagnetic field of outflowing oxygen ions at high altitude (12 RE) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2–0.4 RE in altitude). This is a clear example of local ion energization at much higher altitude than usually reported. In contrast to many events at lower altitudes, it is not likely that the locally observed wave fields can cause the observed ion energization. Also, it is not likely that the ions have drifted from some nearby energization region to the point of observation. This suggests that additional fundamentally different ion energization mechanisms are present at high altitudes. One possibility is that the magnetic moment of the ions is not conserved, resulting in slower outflow velocities and longer time for ion energization.

During a long lasting period of northward interplanetary magnetic field and high solar wind speed (above 700 km/s), the Cluster spacecraft go across a number of very large rolled-up Kelvin-Helmholtz (KH) vortices at the dusk magnetopause, close to the terminator. The peculiarity of the present event is a particular sequence of ions and electrons distribution functions observed repeatedly inside each vortex. In particular, whenever Cluster crosses the current layer inside the vortices, multiple field-aligned ion populations appear, suggesting the occurrence of reconnection. In addition, the ion data display a clear velocity filter effect both at the leading and at the trailing edge of each vortex. This effect is not present in the simultaneous electron data. Unlike other KH studies reported in the literature in which reconnection occurs within the vortices, in the present event the observations are not compatible with local reconnection, but are accounted for by lobe reconnection occurring along an extended X-line at the terminator in the Southern Hemisphere. The reconnected field lines "sink" across the magnetopause and then convect tailward-duskward where they become embedded in the vortices. Another observational evidence is the detected presence of solar wind plasma on the magnetospheric side of the vortices, which confirms unambiguously the occurrence of mass transport across the magnetopause already reported in the literature. The proposed reconnection scenario accounts for all the observational aspects, regarding both the transport process and the kinetic signatures.

A large-amplitude excursion of the magnetosheath (MS) in quiet solar wind conditions on 17 March 2004 was recorded simultaneously by the Cluster and TC-1 spacecraft. During this period, the IMF Bz was entirely northward. The coherence between the bow shock motion and magnetopause (MP) motion is revealed and the excursion velocities of the bow shock motion are analyzed. In addition, the relevant plasma transport phenomenon in the form of flux fluctuations below the ion gyrofrequency at the MP is exposed and is interpreted as manifestation of the drift instability. Correlated observations on charge accumulation and electrostatic potential perturbation are recorded by electron measurements in high energy regime, and also the eventual cross-field vortex motion in the nonlinear stage and the consequential mass exchange are exhibited. The present investigation gives some new insight into the MS plasma transport mechanism across the subsolar MP region in quiet solar wind conditions during a period of northward IMF.

During Cluster spacecraft crossing of the magnetotail, a novel density depleted cavity in association with magnetic compressions in the outflow region of reconnection was observed. It contains intense reflected field-aligned particles, which are produced by a generation mechanism similar to that of the terrestrial foreshock, and hence manifests a foreshock-like morphology. In this cavity, reflected field-aligned proton beams were observed and simultaneously the feature of magnetic-mirror loss-cone proton distributions were found. Magnetic field fluctuations, especially quasi-monochromatic oscillations, were recorded. Both the leading egde and the ULF wave boundary of the ion foreshock are identified from the time sequence of proton and magnetic field observations. Just upstream of the leading egde of the ion foreshock, reflected field-aligned electrons were detected, whose distribution has a narrow bump-on-tail pattern. However, close to the shock front, reflected electrons with a broad bump-on-tail pattern was measured. These two different manifestations of reflected electrons reveal the differences in their microscopic physics of the reflecting process. Moreover, a part of incident ions was further accelerated in the cavity due to trans-time magnetic pumping which provides another possible mechanism in the multi-step acceleration processes in reconnection.

In the evening equatorial magnetosphere at about 4 RE geocentric distance and 19 MLT, the four Cluster spacecraft observed a solitary structure with a width of about 1000~2000 km in the propagation direction. The solitary structure propagates sunward with about 5~10 km/s carrying sunward electric field (in the propagation direction) of up to about 10 mV/m (total potential drop of about 5~10 kV), depletion of magnetic field of about 25%, and a duskward E×B convection up to 50 km/s of He+ rich cold plasma without O+. At the same time, auroral images from the IMAGE satellite together with ground based geomagnetic field data showed a westward (sunward at this location) propagating auroral bulge at the magnetically conjugate ionosphere with the solitary structure. The solitary structure is maintained by flux enhancement of selectively 3000 km/s ions (about 50 keV for H+, 200 keV for He+, and 750 keV for O+). These ions are the main carrier of the diamagnetic current causing the magnetic depletion, whereas the polarization is maintained by different behavior of energetic ions and electrons. Corresponding to aurora, field-aligned accelerated ionospheric plasma of several keV appeared at Cluster from both hemispheres simultaneously. Together with good correspondence in location and propagation velocity between the auroral bulge and the solitary structure, this indicates that the sunward moving auroral bulge is caused by the sunward propagation of the solitary structure which is maintained by energetic ions. The solitary structure might also be the cause of Pi2-like magnetic variation that started simultaneously at Cluster location.

Sources of low-energy ring current ions in the early morning sector (eastward drifting energy domain of about 6/m3) is slightly less than that of the hot component (0.5×106/m3), while Cluster observation shows slightly higher density for the wedge-like part than the low-energy band-like part. The hot source component also explains the observed high-energy (>10 keV) ions drifting westward after adiabatic energization in the nightside under time-varying electric field. The wedge-like part has much shorter elapsed time, i.e., less charge-exchange loss, than the band-like part.

The high-latitude boundaries of the plasma sheet (PSBL) are dynamic latitude zones of recurring and transient (minutes to tens of minutes) earthward and magnetic field-aligned bursts of plasma, each being more or less confined in longitude as well, whose ionic component is dominated by protons with flux, energies and density that are consistent with a central plasma sheet (CPS) source at varying distance (varying rates of energy time dispersion), sometimes as close as the ~19 RE Cluster apogees, or closer still. The arguably most plausible source consists of so called "bursty bulk flows" (BBFs), i.e. proton bulk flow events with large, positive and bursty GSE vx. Known mainly from CPS observations made at GSE x>−30 RE, the BBF type events probably take place much further downtail as well. What makes the BBFs an especially plausible source are (1) their earthward bulk flow, which helps explain the lack of distinctive latitudinal PSBL energy dispersion, and (2) their association with a transient strong increase of the local tail Bz component ("local dipolarization"). The enhanced Bz provides intermittent access to higher latitudes for the CPS plasma, resulting in local density reductions in the tail midplane, as illustrated here by proton data from the Cluster CIS CODIF instruments. Another sign of kinship between the PSBL bursts and the BBFs is their similar spatial fine structure. The PSBL bursts have prominent filaments aligned along the magnetic field with transverse flux gradients that are often characterized by local ~10 keV proton gyroradii scale size (or even smaller), as evidenced by Cluster measurements. The same kind of fine structure is also found during Cluster near-apogee traversals of the tail midplane, as illustrated here and implied by recently published statistics on BBFs obtained with Cluster multipoint observations at varying satellite separations. Altogether, the Cluster observations described here mesh rather well with theories about so called plasma sheet "bubbles," i.e. earthward drifting closed magnetic flux tubes with reduced particle pressure and enhanced magnetic field strength at their apex. It is argued that such bubbles may be initiated by localized diamagnetic instabilities.

Data from the Cluster CIS instrument have been used for studying proton and heavy ion (O+ and He+) characteristics of the sub-keV ring current. Thirteen events with dispersed heavy ions (O+ and He+) were identified out of two years (2001 and 2002) of Cluster data. All events took place during rather geomagnetically quiet periods. Three of those events have been investigated in detail: 21 August 2001, 26 November 2001 and 20 February 2002. These events were chosen from varying magnetic local times (MLT), and they showed different characteristics. In this article, we discuss the potential source for sub-keV ring current ions. We show that: (1) outflows of terrestrial sub-keV ions are supplied to the ring current also during quiet geomagnetic conditions; (2) the composition of the outflow implies an origin that covers an altitude interval from the low-altitude ionosphere to the plasmasphere, and (3) terrestrial ions are moving upward along magnetic field lines, at times forming narrow collimated beams, but frequently also as broad beams. Over time, the ion beams are expected to gradually become isotropised as a result of wave-particle interaction, eventually taking the form of isotropic drifting sub-keV ion signatures. We argue that the sub-keV energy-time dispersed signatures originate from field-aligned terrestrial ion energising and outflow, which may occur at all local times and persist also during quiet times.

From 15 February 2004, 20:00 UT to 18 February 2004, 01:00 UT, the solar wind density dropped to extremely low values (about 0.35 cm−3). On 17 February, between 17:45 UT and 18:10 UT, the CLUSTER spacecraft cross the dayside magnetopause several times at a large radial distance of about 16 RE. During each of these crossings, the spacecraft detect high speed plasma jets in the dayside magnetopause and boundary layer. These observations are made during a period of southward and dawnward Interplanetary Magnetic Field (IMF). The magnetic shear across the local magnetopause is ~90° and the magnetosheath beta is very low (~0.15). We evidence the presence of a magnetic field of a few nT along the magnetopause normal. We also show that the plasma jets, accelerated up to 600 km/s, satisfy the tangential stress balance. These findings strongly suggest that the accelerated jets are due to magnetic reconnection between interplanetary and terrestrial magnetic field lines northward of the satellites. This is confirmed by the analysis of the ion distribution function that exhibits the presence of D shaped distributions and of a reflected ion population as predicted by theory. A quantitative analysis of the reflected ion population reveals that the reconnection process lasts about 30 min in a reconnection site located at a very large distance of several tens RE from the Cluster spacecraft. We also estimate the magnetopause motion and thickness during this event. This paper gives the first experimental study of magnetic reconnection during such rare periods of very low solar wind density. The results are discussed in the frame of magnetospheric response to extremely low solar wind density conditions.

Recent observations have uncovered short-duration density holes in the Earth's foreshock region. There is evidence that the formation of density holes involves non-linear growth of fluctuations in the magnetic field and plasma density, which results in shock-like boundaries followed by a decrease in both density and magnetic field. In this study we examine in detail a few such events focusing on their low frequency wave characteristics. The propagation properties of the waves are studied using Cluster's four point observations. We found that while these density hole-structures were convected with the solar wind, in the plasma rest frame they propagated obliquely and mostly sunward. The wave amplitude grows non-linearly in the process, and the waves are circularly or elliptically polarized in the left hand sense. The phase velocities calculated from four spacecraft timing analysis are compared with the velocity estimated from δE/δB. Their agreement justifies the plane electromagnetic wave nature of the structures. Plasma conditions are found to favor firehose instabilities. Oblique Alfvén firehose instability is suggested as a possible energy source for the wave growth. Resonant interaction between ions at certain energy and the waves could reduce the ion temperature anisotropy and thus the free energy, thereby playing a stabilizing role.

Oxygen ion outflow associated with the cusp and cleft give rise to persistent oxygen ion beams which can be observed over the polar cap. For high altitude spacecraft such as Cluster these beams are often observed for several hours on each occasion. This allows for a study of typical temporal structures on the time scale of minutes. We have used 3 years of data from spring, January to May of years 2001 to 2003, for a study of the oxygen number flux variation in the polar cap ion outflow. The source of these oxygen ion beams is the cusp and cleft, and variations in ionospheric upflow on time scales of around 8 min have been reported from ground based studies using incoherent scatter radar. Such upflows typically do not reach escape velocity, and further energization above the ionosphere is required for outflow to occur. Our study shows that a typical time scale between sudden number flux enhancements observed by Cluster in a geocentric distance range of 5 RE to 12 RE is 5 to 10 min. A superposed epoch study does not reveal any significant convection velocity or temperature changes around the flux enhancement events. Sudden temperature enhancements occur with a typical time interval of about 4 min, A superposed epoch study does not reveal any number flux enhancements associated with the temperature enhancements. The clear modulation of the high altitude number flux in a manner which resembles the modulation of the ionospheric upflow indicates that this is the main limiting factor determining the total outflow. The process behind transient upflow events in the ionosphere is therefore important for the total ionospheric outflow. Subsequent heating above the ionosphere appears to be common enough in the cusp/cleft region that it does not significantly modulate the oxygen ion number flux.

Recently, Keiling et al. (2006) showed that periodic (~90 s) traveling compression regions (TCRs) during a substorm had properties of Pi2 pulsations, prompting them to call this type of periodic TCRs "lobe Pi2". It was further shown that time-delayed ground Pi2 had the same period as the lobe Pi2 located at 16 RE, and it was concluded that both were remotely driven by periodic, pulsed reconnection in the magnetotail. In the study reported here, we give further evidence for this association by reporting additional periodic TCR events (lobe Pi2s) at 18 RE all of which occurred in succession during a geomagnetically very quiet, non-substorm period. Each quiet-time periodic TCR event occurred during an interval of small H-bay-like ground disturbance (H bays were superposed by Pi2s. These ground Pi2s are compared to the TCRs in the tail lobe (Cluster) and both magnetic pulsations and flow variations at 9 RE inside the plasma sheet (Geotail). The main results of this study are: (1) Further evidence is given that periodic TCRs in the tail lobe at distances of 18 RE and ground Pi2 are related phenomena. In particular, it is shown that both had the same periodicity and occurred simultaneously (allowing for propagation time delays) strongly suggesting that both had the same periodic source. Since the TCRs were propagating Earthward, this source was located in the outer magnetosphere beyond 18 RE. (2) The connection of periodic TCRs and ground Pi2 also exists during very quiet geomagnetic conditions with PBIs present in addition to the previous result (Keiling et al., 2006) which showed this connection during substorms. (3) Combining (1) and (2), we conclude that the frequency of PBI-associated Pi2 is controlled in the outer magnetosphere as opposed to the inner magnetosphere. We propose that this mechanism is pulsed reconnection based on previous results which combined modeled results and observations of substorm-related periodic TCRs and ground Pi2. (4) We show that TCRs with small compression ratios (ΔB/B1% (which are more commonly studied). (5) Finally, it is noted that both quiet time and substorm-related periodic TCRs had remarkably similar periods in spite of the drastically different geomagnetic conditions prevailing during the events which poses the important question of what causes this periodicity under these different conditions.

A magnetotail event on 15 September 2001 is analyzed, during which the Cluster spacecraft observed both embedding and bifurcation of the Thin Current Sheet (TCS). It is indicated that the ion anisotropy and nongyrotropy are responsible for those new equilibrium features that represent deviations from the conventional Harris model. Measurements show that an embedded proton TCS manifests a pressure anisotropy with p||>p⊥, simultaneously associated with a density embedding; while a bifurcated oxygen ion TCS exhibits a pressure anisotropy mainly with p||p⊥ is the necessary marginal firehose stability condition. The local flapping motion of the TCS was observed, and some particular features such as the solitary wave-like behavior and the kink motion in the plane perpendicular to the Sun-Earth direction were revealed. The present investigation intimates the complexity of equilibria of the magnetotail current sheet and declares the crucial role played by the ion kinetics in the dynamics of TCSs.

A detailed analysis of successive tailward flow bursts in the near-Earth magnetotail (X~−19 RE) plasma sheet is performed on the basis of in-situ multi-point observations by the Cluster spacecraft on 15 September 2001. The tailward flows were detected during a northward IMF interval, 2.5 h after a substorm expansion. Each flow burst (VxBx≈0) and the level of Bx≈10–15 nT simultaneously, giving a possibility to distinguish between closed plasmoid-like structures and open NFTE-like surges. The data analysis shows NFTE-like structures and localized current filaments embedded into the tailward plasma flow. 3-D shapes of the structures were reconstructed using the four-point magnetic filed measurements and the particle data.

The role of the centrifugal acceleration mechanism for ion outflow at high altitude above the polar cap has been investigated. Magnetometer data from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients. This is combined with ion moment data of the convection drift and the field-aligned particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration are directly obtained from observations. The temporal variation of the unit vector of the magnetic field is estimated by predicting consecutive measurement-points through the use of observed estimates of the magnetic field gradients, and subtracting this from the consecutively observed value. The calculation has been performed for observations of outflowing O+ beams in January to May for the years 2001–2003, and covers an altitude range of about 5 to 12 RE. The accumulated centrifugal acceleration during each orbit is compared with the observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration. Finally the observed spatial terms (parallel and perpendicular) of the centrifugal acceleration are compared with the results obtained when the magnetic field data was taken from the Tsyganenko T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may explain a large fraction of the parallel velocities observed at high altitude above the polar cap. The magnetic field model results underestimate the centrifugal acceleration at the highest altitudes investigated and show some systematic differences as compared to the observations in the lower altitude ranges investigated. Our results indicate that for altitudes corresponding to magnetic field values of more than 50 nT a test particle model with a steady state magnetic field model, a realistic convection model and an initial velocity of about 20 k m s−1 at 5 RE should be able to reproduce the main part of our observational results.

Bursty bulk flow associated magnetic fluctuations exhibit at least three spectral scaling ranges in the Earth's plasma sheet. Two of the three scaling ranges can be associated with multi-scale magnetohydrodynamic turbulence between the spatial scales from ~100 km to several RE (RE is the Earth's radius). These scales include the inertial range and below ~0.5 RE a steepened scaling range, theoretically not fully understood yet. It is shown that, in the near-Earth plasma sheet, the inertial range can be robustly identified only if multi-scale quasi stationary (MSQS) data intervals are selected. Multiple bursty flow associated magnetic fluctuations, however, exhibit 1/f type scaling indicating that large-scale fluctuations are controlled by multiple uncorrelated driving sources of the bulk flows (e.g. magnetic reconnection, instabilities).

Simultaneous observations by the Cluster spacecraft and SuperDARN radars are presented of magnetotail flux transport during northward, but BY-dominated IMF. Two events are discussed, which occurred on 14 August 2004 and 17 September 2005, during intervals of negative and positive IMF BY, respectively. During both intervals the Cluster spacecraft observed isolated bursts of Earthward plasma convection in the central plasma sheet. During the first event, the flows observed by Cluster also had a significant V⊥Y component in the duskward direction, consistent with westward azimuthal flows observed in the midnight sector by the Northern Hemisphere SuperDARN radars. During the second event, Cluster 4 observed a significant dawnward V⊥Y component, again consistent with the Northern Hemisphere SuperDARN observations which revealed eastward azimuthal flow. In this instance, however, Cluster 3 observed a duskward V⊥Y component which was more consistent with the duskward sense of the convection observed by the Southern Hemisphere SuperDARN radars. This implies that Cluster 3 and Cluster 4 were located on different field lines which experienced opposite net azimuthal forces and hence observed oppositely directed convection. These observations are consistent with previous SuperDARN studies of nightside flows under northward IMF and, more importantly, provide the first simultaneous in-situ evidence for a mode of tail reconnection occurring during non-substorm intervals in an asymmetric tail.

The simple model of reconnected field line motion developed by Cooling et al. (2001) has been used in several recent case studies to explain the motion of flux transfer events across the magnetopause. We examine 213 FTEs observed by all four Cluster spacecraft under a variety of IMF conditions between November 2002 and June 2003, when the spacecraft tetrahedron separation was ~5000 km. Observed velocities were calculated from multi-spacecraft timing analysis, and compared with the velocities predicted by the Cooling model in order to check the validity of the model. After excluding three categories of FTEs (events with poorly defined velocities, a significant velocity component out of the magnetopause surface, or a scale size of less than 5000 km), we were left with a sample of 118 events. 78% of these events were consistent in both direction of motion and speed with one of the two model de Hoffmann-Teller (dHT) velocities calculated from the Cooling model (to within 30° and a factor of two in the speed). We also examined the plasma signatures of several magnetosheath FTEs; the electron signatures confirm the hemisphere of connection indicated by the model in most cases. This indicates that although the model is a simple one, it is a useful tool for identifying the source regions of FTEs.

This article describes observations of a bursty bulk flow (BBF) in the outer central plasma sheet. The observations are made with the Cluster satellites, located approximately 19 RE downtail, close to the midnight sector in the Southern Hemisphere. 40–60 s after Cluster first detected the BBF, there was a large bipolar perturbation in the magnetic field. A Grad-Shafranov reconstruction has revealed that this is created by a field-aligned current at the flank of the BBF. Further analysis of the plasma moments has shown that the BBF has the properties of a depleted flux tube. Depleted flux tubes are an important theoretical model for how plasma and magnetic flux can be transported Earthward in the magnetotail as part of the Dungey cycle. The field aligned current is directed Earthward and is located at the dawn side of the BBF. Thus, it is consistent with the magnetic shear at the flank of an Earthward moving BBF. The total current has been estimated to be about 0.1 MA.

A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 RE downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 RE downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction.

We examine Cluster observations of a reconnection event at xGSM=−15.7 RE in the magnetotail on 11 October 2001, when Cluster recorded the current sheet for an extended period including the entire duration of the reconnection event. The onset of reconnection is associated with a sudden orientation change of the ambient magnetic field, which is also observed simultaneously by Goes-8 at geostationary orbit. Current sheet oscillations are observed both before reconnection and during it. The speed of the flapping motions is found to increase when the current sheet undergoes the transition from quiet to active state, as suggested by an earlier statistical result and now confirmed within one single event. Within the diffusion region both the tailward and earthward parts of the quadrupolar magnetic Hall structure are recorded as an x-line passes Cluster. We report the first observations of the Hall structure conforming to the kinks in the current sheet. This results in relatively strong fluctuations in Bz, which are shown to be the Hall signature tilted in the yz plane with the current sheet.

During the last 30 years, several magnetospheric missions have recorded the presence of narrow proton structures in the ring current region. These structures have been referred as "nose-like" structures, due to their appearance when represented in energy-time spectrograms, characterized by a flux value increase for a narrow energy range. Cluster's polar orbit, with a 4 RE perigee, samples the ring current region. The ion distribution functions obtained in-situ by the CIS experiment (for energies of ~5 eV/q to 40 keV/q) reveal the simultaneous presence of several (up to 3) narrow nose-like structures. A statistical study (over one year and a half of CIS data) reveals that double nose structures are preferentially observed in the post-midnight sector. Also, the characteristic energy of the nose (the one observed at the lower energy range when several noses occur simultaneously) reveals a clear MLT dependence during quiet events (Kp Numerical simulations of particles trajectories, using large-scale electric and magnetic field models are also presented. Most of the features have been accurately reproduced (namely the single and double noses), but the triple noses cannot be produced under these conditions and require to consider a more complex electric field model.

We present space- and ground-based observations of the signatures of magnetic reconnection during an interval of duskward-oriented interplanetary magnetic field on 25 March 2004. In situ field and plasma measurements are drawn from the Double Star and Cluster satellites during traversals of the pre-noon sector dayside magnetopause at low and high latitudes, respectively. These reveal the typical signatures of flux transfer events (FTEs), namely bipolar perturbations in the magnetic field component normal to the local magnetopause, enhancements in the local magnetic field strength and mixing of magnetospheric and magnetosheath plasmas. Further evidence of magnetic reconnection is inferred from the ground-based signatures of pulsed ionospheric flow observed over an extended interval. In order to ascertain the location of the reconnection site responsible for the FTEs, a simple model of open flux tube motion over the surface of the magnetopause is employed. A comparison of the modelled and observed motion of open flux tubes (i.e. FTEs) and plasma flow in the magnetopause boundary layer indicates that the FTEs observed at both low and high latitudes were consistence with the existence of a tilted X-line passing through the sub-solar region, as suggested by the component reconnection paradigm. While a high latitude X-line (as predicted by the anti-parallel description of reconnection) may have been present, we find it unlikely that it could have been responsible for the FTEs observed in the pre-noon sector under the observed IMF conditions. Finally, we note that throughout the interval, the magnetosphere was bathed in ULF oscillations within the solar wind electric field. While no one-to-one correspondence with the pulsed reconnection rate suggested by the ground-based observation of pulsed ionospheric flow has been demonstrated, we note that similar periodicity oscillations were observed throughout the solar wind-magnetosphere-ionosphere system. These findings are consistent with previously proposed mechanisms of solar wind modulation of the dayside reconnection rate.

We combine in situ measurements from Cluster with high-resolution 557.7 nm all-sky images from South Pole to investigate the spatial and temporal evolution of merging on the dayside magnetopause. Variations of 557.7 nm emissions were observed at a 6 s cadence at South Pole on 29 April 2003 while significant changes in the Interplanetary Magnetic Field (IMF) clock angle were reaching the magnetopause. Electrons energized at merging sites are the probable sources for 557.7 nm cusp emissions. At the same time Cluster was crossing the pre-noon cusp in the Northern Hemisphere. The combined observations confirm results of a previous study that merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 10 s to 3 min (Maynard et al., 2004). The intensity of the emissions and the merging rate appear to vary with changes in the IMF clock angle, IMF BX and the dynamic pressure of the solar wind. Most poleward moving auroral forms (PMAFs) reflect responses to changes in interplanetary medium rather than to local processes. The changes in magnetopause position required by increases in dynamic pressure are mediated by merging and result in the generation of PMAFs. Small (15–20%) variations in dynamic pressure of the solar wind are sufficient to launch PMAFs. Changes in IMF BX create magnetic flux compressions and rarefactions in the solar wind. Increases (decreases) in IMF BX strengthens |B| near northern (southern) hemisphere merging sites thereby enhancing merging rates and triggering PMAFs. When correlating responses in the two hemispheres, the presence of significant IMF BX also requires that different lag-times be applied to ACE measurements acquired ~0.1 AU upstream of Earth. Cluster observations set lag times for merging at Northern Hemisphere sites; post-noon optical emissions set times of Southern Hemisphere merging. All-sky images and magnetohydrodynamic simulations indicate that merging occurs in multiple discrete locations, rather than continuously, across the dayside for southward IMF conditions in the presence of dipole tilt. Matching optical signatures with clock-angle, BX, and dynamic pressure variations provides new insights about interplanetary control of dayside merging and associated auroral dynamics.

We present a statistical study of four years of Cluster crossings of the mid-altitude cusp. In this first part of the study, we start by introducing the method we have used a) to define the cusp properties, b) to sort the interplanetary magnetic field (IMF) conditions or behaviors into classes, c) to determine the proper time delay between the solar wind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261 fulfill our criteria and are considered as cusp crossings. We look at the size, location and dynamics of the mid-altitude cusp under various IMF orientations and solar wind conditions. For southward IMF, Bz rules the latitudinal dynamics, whereas By governs the zonal dynamics, confirming previous works. We show that when |By| is larger than |Bz|, the cusp widens and its location decorrelates from By. We interpret this feature in terms of component reconnection occurring under By-dominated IMF. For northward IMF, we demonstrate that the location of the cusp depends primarily upon the solar wind dynamic pressure and upon the Y-component of the IMF. Also, the multipoint capability of Cluster allows us to conclude that the cusp needs typically more than ~20 min to fully adjust its location and size in response to changes in external conditions, and its speed is correlated to variations in the amplitude of IMF-Bz. Indeed, the velocity in °ILAT/min of the cusp appears to be proportional to the variation in Bz in nT: Vcusp=0.024 ΔBz. Finally, we observe differences in the behavior of the cusp in the two hemispheres. Those differences suggest that the cusp moves and widens more freely in the summer hemisphere.

The nature of particle precipitations at dayside mid-altitudes can be interpreted in terms of the evolution of reconnected field lines. Due to the difference between electron and ion parallel velocities, two distinct boundary layers should be observed at mid-altitudes between the boundary between open and closed field lines and the injections in the cusp proper. At lowest latitudes, the electron-dominated boundary layer, named the "electron edge" of the Low-Latitude Boundary Layer (LLBL), contains soft-magnetosheath electrons but only high-energy ions of plasma sheet origin. A second layer, the LLBL proper, is a mixture of both ions and electrons with characteristic magnetosheath energies. The Cluster spacecraft frequently observe these two boundary layers. We present an illustrative example of a Cluster mid-altitude cusp crossing with an extended electron edge of the LLBL. This electron edge contains 10–200 eV, low-density, isotropic electrons, presumably originating from the solar wind halo population. These are occasionally observed with bursts of parallel and/or anti-parallel-directed electron beams with higher fluxes, which are possibly accelerated near the magnetopause X-line. We then use 3 years of data from mid-altitude cusp crossings (327 events) to carry out a statistical study of the location and size of the electron edge of the LLBL. We find that the equatorward boundary of the LLBL electron edge is observed at 10:00–17:00 magnetic local time (MLT) and is located typically between 68° and 80° invariant latitude (ILAT). The location of the electron edge shows a weak, but significant, dependence on some of the external parameters (solar wind pressure, and IMF BZ- component), in agreement with expectations from previous studies of the cusp location. The latitudinal extent of the electron edge has been estimated using new multi-spacecraft techniques. The Cluster tetrahedron crosses the electron and ion boundaries of the LLBL/cusp with time delays of 1–40 min between spacecraft. We reconstruct the motion of the electron boundary between observations by different spacecraft to improve the accuracy of the estimation of the boundary layer size. In our study, the LLBL electron edge is distinctly observed in 87% of mid-altitude LLBL/cusp crossings with clear electron and ion equatorward boundaries equivalent to 35% of all LLBL/cusp crossings by Cluster. The size of this region varied between 0°–2° ILAT with a median value of 0.2° ILAT. Generally, the size of the LLBL electron edge depends on the combination of many parameters. However, we find an anti-correlation between the size of this region and the strength of the IMF, the absolute values of the IMF BY- and BZ-components and the solar wind dynamic pressure, as is expected from a simple reconnection model for the origin of this region.

On 3 July 2001, the four Cluster satellites traversed along the dawnside magnetospheric flank and observed large variations in all plasma parameters. The estimated magnetopause boundary normals were oscillating in the z-direction and the normal component of the magnetic field showed systematic 2–3 min bipolar variations for 1 h when the IMF had a small positive bz-component and a Parker-spiral orientation in the x,y-plane. Brief 33 s intervals with excellent deHoffman Teller frames were observed satisfying the Walén relation. Detailed comparisons with 2-D MHD simulations indicate that Cluster encountered rotational discontinuities generated by Kelvin-Helmholtz instability. We estimate a wave length of 6 RE and a wave vector with a significant z-component.

Observations by the Cluster spacecraft of VLF/ELF wave activity show distinct signatures for different regions in the vicinity of high altitude polar cusps, which are identified by using magnetic field and plasma data along spacecraft trajectories. These waves include: (1) Broad band magnetic noise observed in the polar cusp at frequencies from several Hz to ~100 Hz, below the local electron cyclotron frequency, fce. Similar magnetic noise is also observed in the high latitude magnetosheath and the magnetopause boundary layer. (2) Strong broad band electrostatic emissions observed in the cusp, in the magnetosheath, and in the high latitude magnetopause boundary layer, at frequencies extending from several Hz to tens of kHz, with maximum intensities below ~100 Hz. (3) Narrow-band electromagnetic whistler waves at frequencies ~0.2–0.6 fce, frequently observed in the closed boundary layer (CBL) adjacent to the polar cusp. These waves are for the first time observed in this region to be accompanied by counter-streaming electron beams of ~100 eV, which suggests that the waves are excited by these electrons through wave-particle interaction. (4) Narrow-band electrostatic waves observed slightly above the local fce in the CBL. (5) Lion roars, observed in the high latitude magnetosheath, often in magnetic troughs of mirror mode oscillations. The above wave signatures can serve as indicators of the regions in the vicinity of the magnetospheric cusp.

We investigate in detail a reversal of plasma flow from tailward to earthward detected by Cluster at the downstream distance of ~19 RE in the midnight sector of the magnetotail on 22 August 2001. This flow reversal was accompanied by a sign reversal of the Bz component and occurred during the late substorm expansion phase as revealed by simultaneous global view of auroral activity from IMAGE. We examine the associated Hall current system signature, current density, electric field, Lorentz force, and current dissipation/dynamo term, the last two parameters being new features that have not been studied previously for plasma flow reversals. It is found that (1) there was no clear quadrupole Hall current system signature organized by the flow reversal time, (2) the x-component of the Lorentz force did not change sign while the other two did, (3) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source, (4) the electric field was occasionally dawnward, producing a dynamo effect, and (5) the electric field was occasionally larger at the high-latitude plasma sheet than near the neutral sheet. These observations are consistent with the current disruption model for substorms in which these disturbances are due to shifting dominance of multiple current disruption sites and turbulence at the observing location.

This paper presents a study of nearly field-aligned outflowing ion beams observed on the Cluster satellites over the polar cap. Data are taken at geocentric radial distances of the order of 5–9 RE. The distinction is made between ion beams originating from the polar cusp/cleft and beams accelerated almost along the magnetic field line passing by the spacecraft. Polar cusp beams are characterized by nearly field-aligned proton and oxygen ions with an energy ratio EO+ / EH+, of the order of 3 to 4, due to the ion energy repartition inside the source and to the latitudinal extension of the source. Rapid variations in the outflowing ion energy are linked with pulses/modifications of the convection electric field. Cluster data allow one to show that these perturbations of the convection velocity and the associated ion structures propagate at the convection velocity. In contrast, polar cap local ion beams are characterized by field-aligned proton and oxygen ions with similar energies. These beams show the typical inverted V structures usually observed in the auroral zone and are associated with a quasi-static converging electric field indicative of a field-aligned electric field. The field-aligned potential drop fits well the ion energy profile. The simultaneous observation of precipitating electrons and upflowing ions of similar energies at the Cluster orbit indicates that the spacecraft are crossing the mid-altitude part of the acceleration region. In the polar cap, the parallel electric field can thus extend to altitudes higher than 5 Earth radii. A detailed analysis of the distribution functions shows that the ions are heated during their parallel acceleration and that energy is exchanged between H+ and O+. Furthermore, intense electrostatic waves are observed simultaneously. These observations could be due to an ion-ion two-stream instability.

The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H+) and oxygen ions (O+) from 3 years (2001-2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5–12 RE geocentric distance. It was found that O+ is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O+ parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O+ parallel bulk velocities in excess of 60 km s-1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O+ the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H+ and O+ was found to typically be close to the same throughout the observation interval when the H+ bulk velocity was calculated for all pitch-angles. When the H+ bulk velocity was calculated for upward moving particles only the H+ parallel bulk velocity was typically higher than that of O+. The parallel bulk velocity is close to the same for a wide range of relative abundance of the two ion species, including when the O+ ions dominates. The thermal velocity of O+ was always well below that of H+. Thus perpendicular energization that is more effective for O+ takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

Electromagnetic low-frequency waves in the magnetotail lobe close to the PSBL (Plasma Sheet Boundary Layer) are studied using the Cluster spacecraft. The lobe waves show Alfvénic properties and transport their wave energy (Poynting flux) on average toward the Earth along magnetic field lines. Most of the wave events are rich with oxygen (O+) ion plasma. The rich O+ plasma can serve to enhance the magnetic field fluctuations, resulting in a greater likelihood of observation, but it does not appear to be necessary for the generation of the waves. Taking into account the fact that all events are associated with auroral electrojet enhancements, the source of the lobe waves might be a substorm-associated instability, i.e. some instability near the reconnection site, or an ion beam-related instability in the PSBL.

High-resolution magnetic field data from Cluster Flux Gate Magnetometer (FGM) and the Spatio-Temporal Analysis of Field Fluctuations (STAFF) instruments are used to study turbulent magnetic field fluctuations during the high-altitude cusp crossing on 17 March 2001. Despite the quiet solar wind conditions, the cusp was filled with magnetic field turbulence whose power correlates with the field-aligned ion plasma flux. The magnetic field wave spectra shows power law behavior with both double and single slopes with break in the spectra usually occurring in the vicinity of the local ion cyclotron frequency. Strong peaks in the wave power close to local ion cyclotron frequency were sometimes observed, with secondary peaks at higher harmonics indicative of resonant processes between protons and the waves. We show that the observed spectral break point may be caused partly by damping of obliquely propagating kinetic Alfvén (KAW) waves and partly by cyclotron damping of ion cyclotron waves.

Thirty rapid crossings of the magnetotail current sheet by the Cluster spacecraft during July-October 2001 at a geocentric distance of 19 RE are examined in detail to address the structure of the current sheet. We use four-point magnetic field measurements to estimate electric current density; the current sheet spatial scale is estimated by integration of the translation velocity calculated from the magnetic field temporal and spatial derivatives. The local normal-related coordinate system for each case is defined by the combining Minimum Variance Analysis (MVA) and the curlometer technique. Numerical parameters characterizing the plasma sheet conditions for these crossings are provided to facilitate future comparisons with theoretical models. Three types of current sheet distributions are distinguished: center-peaked (type I), bifurcated (type II) and asymmetric (type III) sheets. Comparison to plasma parameter distributions show that practically all cases display non-Harris-type behavior, i.e. interior current peaks are embedded into a thicker plasma sheet. The asymmetric sheets with an off-equatorial current density peak most likely have a transient nature. The ion contribution to the electric current rarely agrees with the current computed using the curlometer technique, indicating that either the electron contribution to the current is strong and variable, or the current density is spatially or temporally structured.

Here and in the companion paper, Hamrin et al. (2006), we present experimental evidence for the crossing of auroral generator regions, based on conjugate Cluster and FAST data. To our knowledge, this is the first investigation that concentrates on the evaluation of the power density, E·J, in auroral generator regions, by using in-situ measurements. The Cluster data we discuss were collected within the Plasma Sheet Boundary Layer (PSBL), during a quiet magnetospheric interval, as judged from the geophysical indices, and several minutes before the onset of a small substorm, as indicated by the FAST data. Even at quiet times, the PSBL is an active location: electric fields are associated with plasma motion, caused by the dynamics of the plasma-sheet/lobe interface, while electrical currents are induced by pressure gradients. In the example we show, these ingredients do indeed sustain the conversion of mechanical energy into electromagnetic energy, as proved by the negative power density, E·J

Standing Alfvén waves of 1.1 mHz (~15 min in period) were observed by the Cluster satellites in the mid-tail during 06:00-07:00 UT on 8 August 2003. Pulsations with the same frequency were also observed at several ground stations near Cluster's footpoint. The standing wave properties were determined from the electric and magnetic field measurements of Cluster. Data from the ground magnetometers indicated a latitudinal amplitude and phase structure consistent with the driven field line resonance (FLR) at 1.1 mHz. Simultaneously, quasi-periodic oscillations at different frequencies were observed in the post-midnight/early morning sector by GOES 12 (l0≈8.7), Polar (l0≈11-14) and Geotail (l0≈9.8). The 8 August 2003 event yields rare and interesting datasets. It provides, for the first time, coordinated in situ and ground-based observations of a very low frequency FLR in the mid-tail on stretched field lines.

Here and in the companion paper, Marghitu et al. (2006), we investigate plausible auroral generator regions in the nightside auroral magnetosphere. In this article we use magnetically conjugate data from the Cluster and the FAST satellites during a 3.5-h long event from 19-20 September 2001. Cluster is in the Southern Hemisphere close to apogee, where it probes the plasma sheet and lobe at an altitude of about 18 RE. FAST is below the acceleration region at approximately 0.6 RE. Searching for clear signatures of negative power densities, E·JE·JEyJy. The electric field Ey is weakly negative during most of our entire event and we conclude that the CGRs occur when the duskward current Jy grows large and positive. We find that our observations are consistent with a local southward expansion of the plasma sheet and/or rather complicated, 3-D wavy structures propagating over the Cluster satellites. We find that the plasma is working against the magnetic field, and that kinetic energy is being converted into electromagnetic energy. Some of the energy is transported away as Poynting flux.

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 oneminute 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 detail the substorm evolution as observed in the ionosphere and in the magnetosphere, and examine whether topological correspondences 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 excellent 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 substorm intensification, and a substorm-related auroral region poleward of it. It is characterised by reduced ionospheric conductances with respect to its environment, and downward field-aligned current (FAC) observed both in the magnetosphere and in the ionosphere. As determined from the ionospheric data, this downward FAC area is moving eastward 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 direction of the ionospheric electric field permits a current closure 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.

An investigation of the 2003 Cluster tail season has revealed small flux ropes in the near-tail plasma sheet of Earth. These flux ropes manifest themselves as a bipolar magnetic field signature (usually predominantly in the Z-component) associated with a strong transient peak in one or more of the other components (usually the Y-component). These signatures are interpreted as the passage of a cylindrical magnetic structure with a strong axial magnetic field over the spacecraft position. On the 2 October 2003 all four Cluster spacecraft observed a flux rope in the plasma sheet at X (GSM) ~-17 RE. The flux rope was travelling Earthward and duskward at ~160 kms-1, as determined from multi-spacecraft timing. This is consistent with the observed south-then-north bipolar BZ signature and corresponds to a size of ~0.3 RE (a lower estimate, measuring between the inflection points of the bipolar signature). The axis direction, determined from multi-spacecraft timing and the direction of the strong core field, was close to the intermediate variance direction of the magnetic field. The current inside the flux rope, determined from the curlometer technique, was predominantly parallel to the magnetic field. However, throughout the flux rope, but more significant in the outer sections, a non-zero component of current perpendicular to the magnetic field existed. This shows that the flux rope was not in a "constant α" force-free configuration, i.e. the magnetic force, J×B was also non-zero. In the variance frame of the magnetic field, the components of J×B suggest that the magnetic pressure force was acting to expand the flux rope, i.e. directed away from the centre of the flux rope, whereas the smaller magnetic tension force was acting to compress the flux rope. The plasma pressure is reduced inside the flux rope. A simple estimate of the total force acting on the flux rope from the magnetic forces and surrounding plasma suggests that the flux rope was experiencing an expansive total force. On 13 August 2003 all four Cluster spacecraft observed a flux rope at X (GSM) ~-18 RE. This flux rope was travelling tailward at 200 kms-1, consistent with the observed north-then-south bipolar BZ signature. The bipolar signature corresponds to a size of ~0.3 RE (lower estimate). In this case, the axis, determined from multi-spacecraft timing and the direction of the strong core field, was directed close to the maximum variance direction of the magnetic field. The current had components both parallel and perpendicular to the magnetic field, and J×B was again larger in the outer sections of the flux rope than in the centre. This flux rope was also under expansive magnetic pressure forces from J×B, i.e. directed away from the centre of the flux rope, and had a reduced plasma pressure inside the flux rope. A simple total force calculation suggests that this flux rope was experiencing a large expansive total force. The observations of a larger J×B signature in the outer sections of the flux ropes when compared to the centre may be explained if the flux ropes are observed at an intermediate stage of their evolution after creation by reconnection at multiple X lines near the Cluster apogee. It is suggested that these flux ropes are in the process of relaxing towards the force-free like configuration often observed further down the tail. The centre of the flux ropes may contain older reconnected flux at a later evolutionary stage and may therefore be more force-free.

The structure and formation mechanism of a total of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster spacecraft, with separation of ~5000 km, are studied by applying the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assumption that the structure is two-dimensional (2-D) and time-independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggesting that multiple X-line reconnection was involved in generating the observed FTEs. The dimension of the flux ropes in the direction normal to the magnetopause ranges from about 2000 km to more than 1 RE. The orientation of the flux rope axis can be determined through optimization of the GS map, the result being consistent with those from various single-spacecraft methods. Thanks to this, the unambiguous presence of a strong core field is confirmed, providing evidence for component merging. The amount of magnetic flux contained within each flux rope is calculated from the map and, by dividing it by the time interval between the preceding FTE and the one reconstructed, a lower limit of the reconnection electric field during the creation of the flux rope can be estimated; the estimated value ranges from ~0.11 to ~0.26 mV m-1, with an average of 0.19 mV m-1. This can be translated to the reconnection rate of 0.038 to 0.074, with an average of 0.056. Based on the success of the 2-D model in recovering the observed FTEs, the length of the X-lines is estimated to be at least a few RE.

We study plasma transport at a thin magnetopause (MP), described hereafter as a thin current sheet (TCS), observed by Cluster at the southern cusp on 13 February 2001 around 20:01 UT. The Cluster observations generally agree with the predictions of the Gas Dynamic Convection Field (GDCF) model in the magnetosheath (MSH) up to the MSH boundary layer, where significant differences are seen. We find for the MP a normal roughly along the GSE x-axis, which implies a clear departure from the local average MP normal, a ~90 km thickness and an outward speed of 35 km/s. Two populations are identified in the MSH boundary layer: the first one roughly perpendicular to the MSH magnetic field, which we interpret as the "incident" MSH plasma, the second one mostly parallel to B. Just after the MP crossing a velocity jet is observed with a peak speed of 240 km/s, perpendicular to B, with MA=3 and β>10 (peak value 23). The magnetic field clock angle rotates by 70° across the MP. Ex is the main electric field component on both sides of the MP, displaying a bipolar signature, positive on the MSH side and negative on the opposite side, corresponding to a ~300 V electric potential jump across the TCS. The E×B velocity generally coincides with the perpendicular velocity measured by CIS; however, in the speed jet a difference between the two is observed, which suggests the need for an extra flow source. We propose that the MP TCS can act locally as an obstacle for low-energy ions (

On 23 March 2002, the four Cluster spacecraft crossed in close configuration (~100 km separation) the high-altitude (10 RE) cusp region. During a large part of the crossing, the STAFF and EFW instruments have detected strong electromagnetic wave activity at low frequencies, especially when intense field-aligned proton fluxes were detected by the CIS/HIA instrument. In all likelihood, such fluxes correspond to newly-reconnected field lines. A focus on one of these ion injection periods highlights the interaction between waves and protons. The wave activity has been investigated using the k-filtering technique. Experimental dispersion relations have been built in the plasma frame for the two most energetic wave modes. Results show that kinetic Alfvén waves dominate the electromagnetic wave spectrum up to 1 Hz (in the spacecraft frame). Above 0.8 Hz, intense Bernstein waves are also observed. The close simultaneity observed between the wave and particle events is discussed as an evidence for local wave generation. A mechanism based on current instabilities is consistent with the observations of the kinetic Alfvén waves. A weak ion heating along the recently-opened field lines is also suggested from the examination of the ion distribution functions. During an injection event, a large plasma convection motion, indicative of a reconnection site location, is shown to be consistent with the velocity perturbation induced by the large-scale Alfvén wave simultaneously detected.