Cluster observations of currents in the plasma sheet during reconnection (original) (raw)
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Cluster observations near reconnection X lines in Earth's magnetotail current sheet
J. Geophys. Res. Space Physics, 2013
Magnetic reconnection is an efficient way to convert magnetic energy into particle energy. In this paper, we use Cluster thermal electron and ion measurements in the vicinity of a reconnection X line to delineate the structure of the reconnection current sheet. Multispacecraft observations made by Cluster on 18 August 2002 indicate that an X line drifted close to the spacecraft, about 3.4 R E earthward of the position where another X line had been observed earlier. Comparison of the Hall magnetic and electric field geometry and the observed properties of energetic electron beams streaming along the separatrix between the Cluster spacecraft indicates that the second X line formed within 20 s of the observation of the first X line. Repeated flow reversals and Hall field geometry together with the presence of a magnetic island embedded in the outflow region downstream of the first X line suggest that the initial current sheet was unstable, perhaps to the tearing mode. We identify a region with a thickness of 0.72 ion inertial lengths (29 electron inertial lengths, d e) of super-Alfvénic electron outflow (greater than the ion inflow Alfvén speed) during the period when the spacecraft was in the vicinity of the neutral sheet. Slightly below the neutral sheet, Cluster observed asymmetric counter-streaming electrons with a loss of axisymmetry in the electron (V ?1 , V ?2) distribution functions over a thin boundary with a thickness of several d e. This electron-scale transition layer was embedded in a much wider region where both the ion and electron Walén tests failed, and the electron super-Alfvénic bulk outflow jets with high-energy electron beams were detected. Those phenomena provide details of the substructure of the reconnection current sheet and suggest that the spacecraft traversed or skimmed the tailward edge of an elongated electron current layer. We also note that this event differs from a previously reported reconnection event in that strong electron temperature anisotropy (T k > T ?) is observed both in the inflow region and in the exhaust, where the anisotropy appears to be associated with the elongated electron outflow jets.
Multispacecraft observation of electron beam in reconnection region
Journal of Geophysical Research, 2008
, during a crossing of the near-Earth plasma sheet Cluster observed an ion flow burst, caused by a near-Earth reconnection event. Cluster observed a tailward bulk flow which reverse to earthward flow, indicating a close passage of the diffusion region. We show that reversals in B Z and B Y are consistent with reconnection. During the event, a short duration burst of electrons in the range of a few keV up to more than 100 keV are observed streaming away from the reconnection region. The accelerated electrons were aligned with the magnetic field, streaming tailward, and were observed simultaneously by all four spacecraft located on both the northern and southern side of the current sheet. The four Cluster spacecraft, separated by 3700km,observetheelectronsforatimeperiodof3700 km, observe the electrons for a time period of 3700km,observetheelectronsforatimeperiodof60 s, indicating the burst to be a temporal rather than localized feature. A second burst of tailward accelerated electrons observed for $40 s was observed by Cluster 1 and 2 upon entering the earthward outflow region. The second beam thus appear to be directed toward the X-line. The flux levels of the energetic electron bursts exceed those of the ambient plasma sheet by a factor 2-4. In general, the highest energetic electron fluxes during this event were observed in the earthward outflow region. Observations indicate that reconnection operates on closed plasma sheet field lines in this event and does not reach lobe field lines.
Physics of Plasmas, 2009
Open questions concerning structures and dynamics of diffusion regions and electron acceleration in collisionless magnetic reconnection are addressed based on data from the four-spacecraft mission Cluster and particle-in-cell simulations. Using time series of electron distribution functions measured by the four spacecraft, distinct electron regions around a reconnection layer are mapped out to set the framework for studying diffusion regions. A spatially extended electron current sheet (ecs), a series of magnetic islands, and bursts of energetic electrons within islands are identified during magnetotail reconnection with no appreciable guide field. The ecs is collocated with a layer of electron-scale electric fields normal to the ecs and pointing toward the ecs center plane. Both the observed electron and ion densities vary by more than a factor of 2 within one ion skin depth north and south of the ecs, and from the ecs into magnetic islands. Within each of the identified islands, t...
Satellite observations of plasma physics near the magnetic field reconnection X line
Journal of Geophysical Research: Space Physics, 2011
Satellite observations near the X line are required to understand electromagnetic energy conversion and particle acceleration resulting from magnetic field reconnection. More than 900 orbits of Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft across the low-latitude dayside magnetopause, involving more than 4000 magnetopause crossings and 5000 h of data, were searched for examples of magnetic field reconnection within a few electron skin depths of the X line. Evidence that the X line was crossed in the best of these events comes from observations of DC electric and magnetic fields, electrostatic and electromagnetic lower hybrid waves, magnetosheath electrons flowing along the separatricies, and a super-Alfvenic electron jet flowing perpendicular to the magnetic field. A dispersion analysis identifies properties of the wave that are in agreement with the experiment. Neither these waves nor the DC electric field were sufficient to account for acceleration of the electron jet. The anomalous drag was not an important source of the observed DC electric field. The observed pressure gradient is a possible candidate for maintaining the electric field.
Annales Geophysicae, 2004
On 26 January 2001, the Cluster spacecraft detected high-speed plasma jets at multiple crossings of the high-latitude duskside magnetopause (MP) and boundary layer (BL) over a period of more than 2 h. The 4 spacecraft combined spent more than half of this time in the MP/BL and jets were observed whenever a spacecraft was in the MP. These observations were made under steady southward and dawnward interplanetary magnetic field (IMF) conditions. The magnetic shear across the local MP was ∼100 • and β∼1 in the adjacent magnetosheath. The jet velocity is in remarkable agreement with reconnection prediction throughout the entire interval, except for one crossing that had no ion measurements inside the current layer. The flow speed measured in the deHoffmann Teller frame is 90% of the Alfvén speed on average for the 10 complete MP current layer crossings that are resolved by the ion measurements. These findings strongly suggest that reconnection was continuously active for more than two hours. The jets were directed persistently in the same northward and anti-sunward direction, implying that the X-line was always below the spacecraft. This feature is inconsistent with patchy and random reconnection or convecting multiple X-lines. The majority of MP/BL crossings in this two-hour interval were partial crossings, implying that they are caused by bulges sliding along the MP, not by inward-outward motion of a uniformly thin MP/BL. The presence of the bulges suggests that, although reconnection is continuously active under steady IMF conditions, its rate may be modulated. The present investigation also reveals that (1) the predicted ion D-shaped distributions are absent in all reconnection jets on this day, (2) the electric field fluctuations are larger in the reconnecting MP than in the magnetosheath proper, but their amplitudes never exceed 20 mV/m, (3) the ion-electron differential motion is ∼20 km/s for the Correspondence to:
Journal of Geophysical Research-Space Physics, 2013
1] Fortunate positioning of Cluster and TC-1 in the plasma sheet (PS) of the Earth's magnetotail has allowed studies of the current sheet (CS) structure and particle dynamics in mesoscale and microscale in both sides of the near-Earth reconnection, which took place between 03:42 and 03:55 UT on 22 September 2004. The distinctive feature of this event was the presence of a strong negative B Y field forming a "bell-like" spatial profile with the maximum absolute value near the neutral plane. The magnitude of this B Y field was almost two times larger than the interplanetary magnetic field (IMF) and therefore could not be explained solely by the IMF penetration into the magnetotail. We propose a possible intrinsic mechanism of the B Y field enhancement near the neutral plane based on peculiarities of the nonadiabatic ion interaction with the thin CS. An analysis of test particle trajectories shows that in the presence of a guide field with the "bell-like" spatial profile, a pronounced north-south asymmetry appears in the refraction/reflection properties of nonadiabatic ions from the CS. In a region tailward of the reconnection (B Z < 0), this asymmetry results in an increase of the density of the keV ions ejected into the northern PS and moving tailward. These ions can carry the tailward current which may be responsible for the strong negative B Y near the neutral plane, i.e., self-consistent enhancement of a B Y field could occur near the neutral plane. Citation: Grigorenko, E. E., et al. (2013), Current sheet structure and kinetic properties of plasma flows during a near-Earth magnetic reconnection under the presence of a guide field,
Annales Geophysicae, 2011
Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming fluxrope-type structures. In this study we examine an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet. The magnetic structure has a velocity of (99, 154, −31) km s −1 in GSM (|V | = 186 km s −1 ), an estimated size of 1.19 R E along the direction of travel and a size between 1.94 and 2.86 R E in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in B Z , plasma-sheet-like plasma and fieldaligned electron flows. Cluster 3 passed closest to the centre of the structure and observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 Correspondence to: S. Beyene (sb2@mssl.ucl.ac.uk) and Cluster 4 and the TCR. Our observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasmasheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.
2006
On March 18, 2002, the Cluster satellites traveled from the earth's northern mantle into the magnetosheath. During this time, the IMAGE spacecraft observed a long-lived proton emission northward of the auroral zone. The Cluster electron and magnetic field data suggest Cluster passed within 1 km of an active reconnection line, entering the ion diffusion region and the edge of the electron diffusion region. We present the current structure, velocity, orientation, and size of the reconnection line. The functional fit to the data also gives an estimate of 100 km for the thickness of the current sheet. We propose that the x-line, though wavering over the spacecraft, is globally stable during Cluster's passage through the magnetopause.
Satellite observations of separator-line geometry of three-dimensional magnetic reconnection
Nature Physics, 2007
Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the threedimensional (3D) magnetic reconnection process 1-9 . However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of the Earth using data from the four Cluster spacecraft. With d i and d e designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ∼0.7 ± 0.3d i and an associated oscillation is identified as a lower-hybrid wave with wavelength ∼d e . This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step towards establishing an observational framework of 3D reconnection.
Journal of Geophysical Research, 1991
The Coordinated Data Analysis Workshop (CDAW 6) substorm event at 1054 UT on March 22, 1979, is studied using energetic particle (• 20 keV) data from the mediumenergy particle experiment of ISEE I as well as high-resolution magnetometer data of ISEE I and 2. These analyses cast new light on the structure and temporal evolution of the plasma sheet near 15/•E during the substorm. These changes are generally consistent with predictions from the near-Earth reconnection model of substorms, modified to include subsidiary plasma and field structure on scales of a few Earth radii. In particular, the onset of streaming flow, nearly aligned with magnetic field lines, of energetic ions and electrons in all available energy channels and its reversal and subsidence are closely correlated with substorm phases. The energetic particles (which are isotropic during the growth phase) begin streaming tailward right after the expansion onset. They reverse their sense of flow in the midexpansion phase. The magnetic field variations suggest that two small magnetic island structures embedded within the plasma sheet moved tailward past the spacecraft shortly after the substorm expansion onset. These structures may be signatures of smallscale multisite reconnection associated with the formation of the near-Earth neutral line.