| 1. |
- Amata, E., et al.
(författare)
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Experimental study of nonlinear interaction of plasma flow with charged thin current sheets : 1. Boundary structure and motion
- 2006
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Ingår i: Nonlinear processes in geophysics. - 1023-5809 .- 1607-7946. ; 13:4, s. 365-376
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 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 M-A=3 and beta> 10 (peak value 23). The magnetic field clock angle rotates by 70 degrees across the MP. E-x 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 similar to 300 V electric potential jump across the TCS. The E x 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 (<350 eV), being transparent for ions with larger gyroradius. As a result, the penetration of plasma by finite gyroradius is considered as a possible source for the jet. The role of reconnection is briefly discussed. The electrodynamics of the TCS along with mass and momentum transfer across it are further discussed in the companion paper by Savin et al. (2006).
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| 2. |
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| 3. |
- Apatenkov, S. V., et al.
(författare)
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Conjugate observation of sharp dynamical boundary in the inner magnetosphere by Cluster and DMSP spacecraft and ground network
- 2008
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Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 26:9, s. 2771-2780
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Tidskriftsartikel (refereegranskat)abstract
- We investigate an unusual sharp boundary separating two plasma populations (inner magnetospheric plasma with high fluxes of energetic particles and plasma sheet) observed by the Cluster quartet near its perigee on 16 December 2003. Cluster was in a pearl-on-string configuration at 05:00 MLT and mapped along magnetic field lines to similar to 8-9 R-E in the equatorial plane. It was conjugate to the MIRACLE network and the DMSP F16 spacecraft passed close to Cluster footpoint. The properties of the sharp boundary, repeatedly crossed 7 times by five spacecraft during similar to 10 min, are: (1) upward FAC sheet at the boundary with similar to 30 nA/m(2) current density at Cluster and similar to 2000 nA/m(2) at DMSP; (2) the boundary had an embedded layered structure with different thickness scales, the electron population transition was at similar to 20 km scale at Cluster (<7 km at DMSP), proton population had a scale similar to 100 km, while the FAC sheet thickness was estimated to be similar to 500 km at Cluster (similar to 100 km at DMSP); (3) the boundary propagated in the earthward-eastward direction at similar to 8 km/s in situ (equatorward-eastward similar to 0.8 km/s in ionosphere), and then decelerated and/or stopped. We discuss the boundary formation by the collision of two different plasmas which may include dynamical three-dimensional field-aligned current loops.
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| 4. |
- Apatenkov, S. V., et al.
(författare)
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Multi-spacecraft observation of plasma dipolarization/injection in the inner magnetosphere
- 2007
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Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 25:3, s. 801-814
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Tidskriftsartikel (refereegranskat)abstract
- Addressing the origin of the energetic particle injections into the inner magnetosphere, we investigate the 23 February 2004 substorm using a favorable constellation of four Cluster (near perigee), LANL and Geotail spacecraft. Both an energy-dispersed and a dispersionless injection were observed by Cluster crossing the plasma sheet horn, which mapped to 9-12 R-E in the equatorial plane close to the midnight meridian. Two associated narrow equatorward auroral tongues/streamers propagating from the oval poleward boundary could be discerned in the global images obtained by IMAGE/WIC. As compared to the energy-dispersed event, the dispersionless injection front has important distinctions consequently repeated at 4 spacecraft: a simultaneous increase in electron fluxes at energies similar to 1.300 keV, similar to 25 nT increase in B-Z and a local increase by a factor 1.5-1.7 in plasma pressure. The injected plasma was primarily of solar wind origin. We evaluated the change in the injected flux tube configuration during the dipolarization by fitting flux increases observed by the PEACE and RAPID instruments, assuming adiabatic heating and the Liouville theorem. Mapping the locations of the injection front detected by the four spacecraft to the equatorial plane, we estimated the injection front thickness to be similar to 1 R-E and the earthward propagation speed to be similar to 200-400km/s (at 9-12 RE). Based on observed injection properties, we suggest that it is the underpopulated flux tubes (bubbles with enhanced magnetic field and sharp inner front propagating earthward), which accelerate and transport particles into the strong-field dipolar region.
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| 5. |
- Chen, Li-Jen, et al.
(författare)
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Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A12, s. A12213-
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Tidskriftsartikel (refereegranskat)abstract
- We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two-dimensional particle-in-cell simulations with a mass ratio m(i)/m(e) = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy (Te-parallel to/Te-perpendicular to > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth (d(i)) above and below the ECS, the electron density decreases toward the ECS by a factor of 3-4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A di-scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron-scale structures in response to this electric field.
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| 6. |
- Chen, Li-Jen, et al.
(författare)
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Multispacecraft observations of the electron current sheet, neighboring magnetic islands, and electron acceleration during magnetotail reconnection
- 2009
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 16:5, s. 056501-
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Tidskriftsartikel (refereegranskat)abstract
- 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, there is a burst of suprathermal electrons whose fluxes peak at density compression sites [L.-J. Chen , Nat. Phys. 4, 19 (2008)] and whose energy spectra exhibit power laws with indices ranging from 6 to 7.3. These results indicate that the in-plane electric field normal to the ecs can be of the electron scale at certain phases of reconnection, electrons and ions are highly compressible within the ion diffusion region, and for reconnection involving magnetic islands, primary electron acceleration occurs within the islands.
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| 7. |
- Chen, Li-Jen, et al.
(författare)
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Observation of energetic electrons within magnetic islands
- 2008
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Ingår i: Nature Physics. - : Nature Publishing Group. - 1745-2473 .- 1745-2481. ; 4:1, s. 19-23
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic reconnection is the underlying process that releases impulsively an enormous amount of magnetic energy(1) in solar flares(2,3), flares on strongly magnetized neutron stars(4) and substorms in the Earth's magnetosphere(5). 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(6-8). 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(9). 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.
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| 8. |
- Hasegawa, H., et al.
(författare)
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Kelvin-Helmholtz waves at the Earth's magnetopause : Multiscale development and associated reconnection
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:12, s. A12207-
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Tidskriftsartikel (refereegranskat)abstract
- We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at similar to 1500 magnetic local time (MLT) by the Geotail spacecraft and at similar to 1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 x 10(4) km) is considerably longer than predicted by the linear theory from the thickness (similar to 1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length similar to 100 km and of magnetic shear similar to 60 degrees existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvenic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.
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| 9. |
- Hasegawa, H., et al.
(författare)
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Reconstruction of a bipolar magnetic signature in an earthward jet in the tail : Flux rope or 3D guide-field reconnection?
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A11, s. A11206-
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 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 similar to 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.
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| 10. |
- Hasegawa, H., et al.
(författare)
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Retreat and reformation of X-line during quasi-continuous tailward-of-the-cusp reconnection under northward IMF
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:15, s. L15104-
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Tidskriftsartikel (refereegranskat)abstract
- We present observations on 19-20 November 2006 by the Cluster spacecraft that were skimming the high-latitude dusk-flank magnetopause, which are consistent with more than one reconnection X-line present on the tailward side of the cusp under northward IMF. Evidence of quasi-continuous reconnection over 16 hours exists in the form of Alfvenic acceleration of magnetosheath ions found almost always when either of the satellites traversed the boundary. The data indicate that a dominant X-line was sunward of Cluster for most of the time, but ion velocity distributions consisting of two magnetosheath populations demonstrate that for part of the time, more than one X-line existed. Further, the motion of reconnected field lines shows that some X-line(s) retreated tailward. It is inferred that following the X-line retreat, another X-line reformed sunward of Cluster, leading tomultiple X-lines.
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