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| Electron flat-top distributions around the magnetic reconnection region |
| 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. |
| Publication date: 24 Jan 2008 |
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| Observation of energetic electrons within magnetic islands |
| 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. |
| Publication date: 15 Jan 2008 |
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| Flux transfer events simultaneously observed by Polar and Cluster: Flux rope in the subsolar region and flux tube addition to the polar cusp |
| 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. |
| Publication date: 12 Jan 2008 |
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| Characterization of waves in the vicinity of an interplanetary directional discontinuity |
| 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. |
| Publication date: 29 Dec 2007 |
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| Evidence for an Elongated (>60 Ion Skin Depths) Electron Diffusion Region during Fast Magnetic Reconnection |
| 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. |
| Publication date: 21 Dec 2007 |
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| Response of the magnetosheath-cusp region to a coronal mass ejection |
| 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 By and Bz 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 Bz remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region. |
| Publication date: 19 Dec 2007 |
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| SAID: Plasmaspheric short circuit of substorm injections |
| 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. |
| Publication date: 18 Dec 2007 |
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| Statistical properties of small-amplitude Langmuir waves in the Earth's electron foreshock |
| 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. |
| Publication date: 15 Dec 2007 |
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| The distribution characteristics of the flows in the near-Earth region: TC-1 observational results |
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| Publication date: 15 Dec 2007 |
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| The distribution characteristics of the flows in the near-Earth region: TC-1 observational results |
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| Publication date: 15 Dec 2007 |
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| Spontaneous Generation of Self-Organized Solitary Wave Structures at Earth's Magnetopause |
| 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. |
| Publication date: 16 Nov 2007 |
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| Reconstruction of a bipolar magnetic signature in an earthward jet in the tail: Flux rope or 3D guide-field reconnection? |
| Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar Bz (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 Bz 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 RE 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 Bz can be produced through localized (3D) reconnection in the presence of guide field By (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 Bz 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. |
| Publication date: 10 Nov 2007 |
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| Beam-plasma interaction in randomly inhomogeneous plasmas and statistical properties of small-amplitude Langmuir waves in the solar wind and electron foreshock |
| A numerical model for wave propagation in an unstable plasma with inhomogeneities is developed. This model describes the linear interaction of Langmuir wave packets with an electron beam and takes into account the angular diffusion of the wave vector due to wave scattering on small-amplitude density fluctuations, as well as suppression of the instability caused by the removal of the wave from the resonance with particles during crossing density perturbations of relatively large amplitude. Using this model, the evolution of the wave packets in inhomogeneous plasmas with an electron beam is studied. To analyze data obtained both in space experiments and numerical modeling, a Pearson technique was used to classify the spectral density distributions. It was shown that both experimental distributions obtained within the Earth's foreshock aboard the CLUSTER spacecraft and model distributions for the logarithm of wave intensity belong to Pearson type IV rather than normal. The main reason for deviations of empirical distributions from the normal one is that the effective number of regions where the waves grow is not very large and, as a consequence, the central limit theorem fails to be true under the typical conditions for the Earth's electron foreshock. For large amplitudes, it is suggested that power law tails can result from variations of wave amplitudes due to changes of group velocity in the inhomogeneous plasma, in particular due to reflection of waves from inhomogeneities. |
| Publication date: 26 Oct 2007 |
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| Mode Conversion and Anomalous Transport in Kelvin-Helmholtz Vortices and Kinetic Alfvén Waves at the Earth's Magnetopause |
| Observations at the Earth's magnetopause identify mode conversion from surface to kinetic Alfvén waves at the Alfvén resonance. Kinetic Alfvén waves radiate into the magnetosphere from the resonance with parallel scales up to the order of the geomagnetic field-line length and spectral energy densities obeying a kperp.-2.4 power law. Amplitudes at the Alfvén resonance are sufficient to both demagnetize ions across the magnetopause and provide field-aligned electron bursts. These waves provide diffusive transport across the magnetopause sufficient for boundary layer formation. |
| Publication date: 26 Oct 2007 |
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| Global view of dayside magnetic reconnection with the dusk-dawn IMF orientation: A statistical study for Double Star and Cluster data |
| Double Star/TC-1 and Cluster data show that both component reconnection and anti-parallel reconnection occur at the magnetopause when the interplanetary magnetic field (IMF) is predominantly dawnward. The occurrence of these different features under these very similar IMF conditions are further confirmed by a statistical study of 290 fast flows measured in both the low and high latitude magnetopause boundary layers. The directions of these fast flows suggest a possible S-shaped configuration of the reconnection X-line under such a dawnward dominated IMF orientation. |
| Publication date: 16 Oct 2007 |
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| Global view of dayside magnetic reconnection with the dusk-dawn IMF orientation: A statistical study for Double Star and Cluster data |
| Double Star/TC-1 and Cluster data show that both component reconnection and anti-parallel reconnection occur at the magnetopause when the interplanetary magnetic field (IMF) is predominantly dawnward. The occurrence of these different features under these very similar IMF conditions are further confirmed by a statistical study of 290 fast flows measured in both the low and high latitude magnetopause boundary layers. The directions of these fast flows suggest a possible S-shaped configuration of the reconnection X-line under such a dawnward dominated IMF orientation. |
| Publication date: 16 Oct 2007 |
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| Thinning and stretching of the plasma sheet |
| With Cluster observations in the magnetotail we study dynamics of plasma sheet thinning and stretching during 39 intervals associated with substorm growth phases. The cross-tail current density and normal magnetic field generally scale as Bn ~ TpNp 1/2/J0, but with frequent transient variations. Typical pre-onset values are Bz ~1-2 nT, Jo ~ 4-8 nA/m², thickness (Harris estimate) >3000 km. A current density increase in each particular event is not accompanied with a corresponding number density increase. About 30% of the events are characterized by a large (>5 nT) field component parallel to the current (in most of cases equal to By), implying adiabatic particle dynamics even with small Bz. Most local onsets, associated with the ends of thin sheet intervals, were accompanied with tailward plasma flows. In some cases embedded current sheet structure was detected and, therefore, estimation of thickness requires caution. |
| Publication date: 16 Oct 2007 |
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| Two-Scale Structure of the Electron Dissipation Region during Collisionless Magnetic Reconnection |
| Particle-in-cell simulations of collisionless magnetic reconnection are presented that demonstrate that reconnection remains fast in very large systems. The electron dissipation region develops a distinct two-scale structure along the outflow direction. Consistent with fast reconnection, the length of the electron current layer stabilizes and decreases with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. Surprisingly, the electrons form a super-Alfvénic outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x line. |
| Publication date: 09 Oct 2007 |
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| Taylor scale and effective magnetic Reynolds number determination from plasma sheet and solar wind magnetic field fluctuations |
| Cluster data from many different intervals in the magnetospheric plasmas sheet and the solar wind are employed to determine the magnetic Taylor microscale from simultaneous multiple point measurements. For this study we define the Taylor scale as the square root of the ratio of the mean square magnetic field (or velocity) fluctuations to the mean square spatial derivatives of their fluctuations. The Taylor scale may be used, in the assumption of a classical Ohmic dissipation function, to estimate effective magnetic Reynolds numbers, as well as other properties of the small scale turbulence. Using solar wind magnetic field data, we have determined a Taylor scale value of 2400 ± 100 km, which is used to obtain an effective magnetic Reynolds number of about 260,000 ± 20,000, and in the plasma sheet we calculated a Taylor scale of 1900 ± 100 km, which allowed us to obtain effective magnetic Reynolds numbers in the range of about 7 to 110. The present determination makes use of a novel extrapolation technique to derive a statistically stable estimate from a range of small scale measurements. These results may be useful in magnetohydrodynamic modeling of the solar wind and the magnetosphere and may provide constraints on kinetic theories of dissipation in space plasmas. |
| Publication date: 03 Oct 2007 |
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| Coordinated ground-based, low altitude satellite and Cluster observations on global and local scales during a transient post-noon sector excursion of the magnetospheric cusp |
| On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Greenland between 13:00 and 14:00 UT. During that interval, a sudden reorganisation of the high-latitude dayside convection pattern accurred after 13:20 UT, most likely caused by a direction change of the Solar wind magnetic field. The result was an eastward and poleward directed flow-channel, as monitored by the SuperDARN radar network and also by arrays of ground-based magnetometers in Canada, Greenland and Scandinavia. |
| Publication date: 26 Sep 2007 |
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