ESA Science & Technology - Publication Archive

We use data from four tail seasons (2001-2004) of the Cluster spacecraft to study statistical features of energetic electrons (40-400 keV) in the magnetotail plasma sheet in combination with plasma parameters and indices of geomagnetic activity. We find that the horizontal magnetic field magnitude best orders the energetic electron observations. Using this method we observe a statistical flux gradient toward the neutral sheet. The fluxes also increase with an increase in the vertical magnetic field component (dipolarization). The intensity of supra-thermal electrons is observed to increase strongly as function of plasma temperature. Although the energetic electron fluxes also appear to increase strongly with geomagnetic activity this is found to be mainly due to the increase in plasma temperature with increasing Kp. Investigation of neutral sheet fluxes and phase space densities at fixed first adiabatic invariant (mu) indicates that adiabatic heating can explain observed flux increases during field dipolarization. The spectral slope in the supra-thermal range is observed to be independent of geomagnetic activity (Kp), while there is a significant local time dependence, with harder spectra observed at dawn compared to the dusk side.

Magnetic fluctuations in the solar wind are distributed according to Kolmogorov's power law f ^-5/3 below the ion cyclotron frequency f_ci. Above this frequency, the observed steeper power law is usually interpreted in two different ways, as a dissipative range of the solar wind turbulence, or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of nonlinear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations rises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small-scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ~k^{-7/3 + 2 alpha}, which depends on the degree of plasma compression alpha.

We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the B_y component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi-continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retinò, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small.

We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500 kms^-1 some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the Southern Hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5 mA km^-2. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~18 A km^-2, which is comparable with previously observed ionospheric field-aligned currents associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993). Furthermore, we show that the observations of the BBF are consistent with the creation of the BBF by the reconnection of open field lines Earthward of a substorm associated near-Earth neutral line.

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 R_E. 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. - Remainder of abstract truncated -

The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. - Remainder of abstract truncated -

We investigated the ambient plasma and magnetic field conditions at high latitudes, as well as the macroparameters of the magnetopause. For this purpose we used Cluster spacecraft plasma and magnetic field data when all the interspacecraft distances were less than 300 km. We analyzed 154 magnetosheath-magnetosphere transitions which allow to distinguish different boundaries between the magnetosphere and the magnetosheath. First, we found transitions similar to the low-latitude boundary layer, the plasma mantle, and cusp-associated transitions. Second, we estimated the length of these transitions. Third, we found with high statistical evidence sub-Alfvénic magnetosheath plasma flows just above the plasma mantle. These flows are supposed to stabilize magnetopause reconnection. Fourth, we carried out an analysis of the magnetopause pressure balance. We found a group of 24 transitions during which both the thermal and the magnetic magnetosheath pressure exceeded the magnetospheric pressure, providing conditions for unusual magnetopause formation. Fifth, for 52 magnetopause crossings we obtained the orientation as well as distributions of velocity, thickness, and current density of the magnetopause. It was found that the magnetopause with an attached plasma mantle moves slower, is thinner, and reaches higher current densities than the one with an adjacent low-latitude-like boundary layer. A comparison with the magnetopause at low latitudes revealed that the high-latitude magnetopause is about two times thicker.

Electron velocity distributions in Earth's magnetosheath exhibit a number of nonequilibrium characteristics, including a flat-topped shape at thermal energies and anisotropies relative to the magnetic field, with T_{perp. e} > T_{parallel e}. These features are related to processes at the bow shock and within the magnetosheath. In the present investigation we focus our attention on the development of the electron temperature anisotropy in the magnetosheath, employing the simultaneous multispacecraft data obtained by the Plasma Electron and Current Experiment (PEACE) on board Cluster. Such studies remove the temporal ambiguities introduced by single-spacecraft analyses. We find that the electron velocity space distributions just behind the bow shock are nearly isotropic with a slight T_{perp. e} > T_{parallel e} anisotropy, whereas deeper into the magnetosheath the electrons exhibit a significant T_{perp. e} > T_{parallel e} anisotropy. We find observational evidence for two processes that contribute to the sheath anisotropy. There is a clear decrease of suprathermal electrons at 0° and 180° pitch angles, suggesting that this population suffers losses (e.g., into the upstream region) related to the global configuration. Additionally, an ongoing local mechanism inflates the 90° pitch angle suprathermal electron population.

A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping-type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the "kink"-like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet.

Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat-top-type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole-like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat-top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross-tail current density is sometimes very small. These results indicate that the flat-top-type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat-top distributions, strong field-aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off-equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 410 keV, which is comparable to the upper edge of flat-top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high-energy end, and it is found that they are not correlated with the increase of the higher-energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat-top-type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Magnetic reconnection is the underlying process that releases impulsively an enormous amount of magnetic energy in solar flares, flares on strongly magnetized neutron stars and substorms in the Earth's magnetosphere. Studies of energy release during solar flares, in particular, indicate that up to 50% of the released energy is carried by accelerated 20-100 keV suprathermal electrons. How so many electrons can gain so much energy during reconnection has been a long-standing question. A recent theoretical study suggests that volume-filling contracting magnetic islands formed during reconnection can produce a large number of energetic electrons. Here we report the first evidence of the link between energetic electrons and magnetic islands during reconnection in the Earth's magnetosphere. The results indicate that energetic electron fluxes peak at sites of compressed density within islands, which imposes a new constraint on theories of electron acceleration.

In this paper we present observational evidence of a flux transfer event observed simultaneously at low-latitude by Polar and at high-latitude by Cluster. This event occurs on 21 March 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE's two open flux tubes: Polar encounters the southward moving flux tube near the equator and Cluster encounters the northward moving flux tube at high latitude. The low-latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bipolar variation. Unlike the low-latitude FTE, the high-latitude FTE observed by Cluster does not exhibit the characteristic bipolar perturbation in the magnetic field. However, the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low and high latitudes. This event provides a unique opportunity to understand high-latitude FTE signatures and the nature of time-varying reconnection. It shows that existing FTE models cannot accommodate all the features in global observations, and coordinated measurements from largely spaced multiple spacecraft place important constraints which are crucial to the development and refinement of FTE models.

Directional discontinuities are frequently encountered in the solar wind. This study utilizes the possibility of simultaneous four-point measurements that the Cluster satellites provide for an investigation of the waves near one such interplanetary discontinuity event. In particular, the k-filtering technique has for the first time been applied for the wave characterization in terms of frequency, wave vector, wave power, and polarization at different distances from a discontinuity. The advantages of the k-filtering method are that these parameters can be transformed into the plasma frame of reference to allow comparison with theory and that it is possible to detect more than one wave mode at each frequency in the spacecraft frame of reference. The discontinuity event in this study was chosen because its wave activity had unusually high power and because it also contains signatures of large-scale magnetic reconnection, such as a reconnection exhaust. Two wave modes are found: one which has the features of a shear Alfvén wave with propagation direction ordered by the background magnetic field direction and one which behaves like a compressional Alfvén wave and is ordered by the discontinuity normal. Both wave modes become weaker farther away from the discontinuity, but the compressional Alfvén mode is more suppressed. The shear mode dominates for long wavelengths at all distances from the discontinuity. The wave field is also found to be asymmetric on the two sides of the event.

Observations of an extremely elongated electron diffusion region occurring during fast reconnection are presented. Cluster spacecraft in situ observations of an expanding reconnection exhaust reveal a broad current layer (~10 ion skin depths thick) supporting the reversal of the reconnecting magnetic field together with an intense current embedded at the center that is due to a super-Alfvénic electron outflow jet with transverse scale of ~9 electron skin depths. The electron jet extends at least 60 ion skin depths downstream from the X-line.

Cluster made an unusual magnetosheath-exterior cusp crossing during the first 2.5 hours of a coronal mass ejection (CME) that flowed past Earth for about 7 hours on 24 October 2003. During the first 2.5 hours (1525-1802 UT) the solar wind dynamic pressure remained high and stable though the CME had a discontinuity after 40 min (1605 UT), when the azimuthal flow turned dawnward up to -100 kms^-1 and IMF B_y and B_z changed from highly negative to positive up to 25 nT. The responses of the magnetosheath-cusp region during the unusual event are presented using Cluster and ground-based (EISCAT VHF radar; 69.6°N, 19.2°E) observations. The unusual Cluster crossing (compared to the usual midaltitude cusp crossing at this time of the year) occurred owing to a large compression of the magnetosphere. Cluster, which was in the southern magnetospheric lobe, suddenly found itself in the magnetosheath at the arrival of the CME at 1524:45 UT. Cluster then crossed through the compressed magnetosheath (highly compressed after the discontinuity in the CME) for about 1.5 hours (1525-1655 UT), magnetopause with strong signatures of lobe reconnection (~1655 UT), stagnant but compressed exterior cusp for about an hour (1700-1802 UT), and then entered the dayside magnetosphere. The observations also show strong signatures of magnetosphere-ionosphere coupling through a late afternoon (~17 MLT) cusp during the first 40 min (1525-1605 UT) of the event when IMF B_z remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region.

We present magnetically-conjugate Cluster/DMSP observations of subauroral ion drifts (SAID) after the onset of a weak substorm on 18 March 2002 that confirm and expand on the previous Cluster/DMSP results. The outer side of the SAID channel is aligned with the plasmapause and its outer edge marks the dispersionless boundary of the electron precipitation. The SAID's inner edge co-locates with a sharp decrease in the flux of the injected ions whose minimum energy increases with the electric potential. Downward ionospheric FACs flow within the channel, mainly near its outer side, concentrating near the density maxima. The SAID peak co-locates either with that in the density or lies on the outer wall of the trough. The overall SAID features are consistent with a short circuiting of the substorm injection front over the plasmasphere and subsequent formation of a turbulent overlap region.

We present the results of a statistical study of Langmuir waves observed by the Cluster spacecraft in the Earth's electron foreshock. To classify the probability density distributions of the logarithms of the wave energies, a Pearson technique is used. We show that experimental distributions can be better approximated, in a statistical sense, by Beta distribution or Pearson Type IV distribution rather than by a normal distribution predicted by the stochastic growth theory. This conclusion agrees with the results of numerical simulations that were previously performed with the use of a model for Langmuir wave propagation in unstable plasma with random inhomogeneities. The main reason for deviations of empirical distributions from a normal distribution is probably related to the effective number of regions, where the waves grow, which is not sufficiently large for the central limit theorem to be applicable under typical conditions in the Earth's electron foreshock. For two of seven events such deviations may be partially attributed to the effects of thermal Langmuir waves.

Spontaneous formation of solitary wave structures has been observed in Earth's magnetopause, and is shown to be caused by the breakup of a zonal flow by the action of drift wave turbulence. Here we show matched observations and modeling of coherent, large-scale solitary electrostatic structures, generated during the interaction of short-scale drift wave turbulence and zonal flows at the Earth's magnetopause. The observations were made by the Cluster spacecraft and the numerical modeling was performed using the wave-kinetic approach to drift wave-zonal flow interactions. Good agreement between observations and simulations has been found, thus explaining the emergence of the observed solitary structures as well as confirming earlier theoretical predictions of their existence.

Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B_z (magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At ~2313 UT on 13 August 2002, Cluster encountered a bipolar B_z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov (GS) reconstruction, the technique for recovery of two-dimensional (2D) magnetohydrostatic structures, suggests that a flux rope with diameter of ~2 R_E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional (3D) MHD simulation, in which a bipolar B_z can be produced through localized (3D) reconnection in the presence of guide field B_y (Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B_z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some (e.g., force-free, or magnetohydrostatic) model-based results.