Thin electron-scale layers at the magnetopause

We use data from the four Cluster satellites to examine the microphysics of a thin electron-scale layer discovered on the magnetospheric side of the magnetopause. Here the ion and electron motions are decoupled in a layer about 20 km (a few electron scales) wide, including currents and strong electric fields. In this layer the electrons are E x B drifting with the ions as a background, and the region can be described by Hall MHD physics. A unique identification of the source of the thin layer is not possible, but our observations are consistent with recent simulations showing thin layers associated with the separatrix extending far away from a reconnection diffusion region

Cluster electron observations of the separatrix layer during traveling compression regions

[ 1] We present Cluster 4-point observations of electrons during traveling compression regions ( TCRs) on 19 September 2001. The electron and \B\ signatures vary with distance from the plasma sheet, confirming that transient plasma sheet bulges propagate past Cluster. TCRs with \B\ increases have either no electron signature, or unidirectional similar to1 keV electrons at the plasma sheet edge. However, spacecraft initially near the plasma sheet edge are engulfed within the bulge and observe a diamagnetic reduction in \B\. In cases where the underlying plasma sheet bulge moves earthward, electrons at the plasma sheet edge stream tailward. We suggest this represents either a remote observation of electrons closing the Hall current system in an ion diffusion region located farther tailward, or the outflow jets along the separatrix formed by a second neutral line located farther earthward of the spacecraft. The latter case implies the simultaneous action of multiple X-lines in the near-Earth tail

Electron Dynamics in the Diffusion Region of an Asymmetric Magnetic Reconnection

During a magnetopause crossing near the subsolar point Cluster observes the ion diffusion region of antiparallel magnetic reconnection. The reconnecting plasmas are asymmetric, differing in magnetic field strength, density, and temperature. Spatial changes in the electron distributions in the diffusion region are resolved and investigated in detail. Heating of magnetosheath electrons parallel to the magnetic field is observed. This heating is shown to be consistent with trapping of magnetosheath electrons by parallel electric fields

Global MHD simulation of flux transfer events at the high-latitude magnetopause observed by the cluster spacecraft and the SuperDARN radar system

A global magnetohydrodynamic numerical simulation is used to study the large-scale structure and formation location of flux transfer events (FTEs) in synergy with in situ spacecraft and ground-based observations. During the main period of interest on the 14 February 2001 from 0930 to 1100 UT the Cluster spacecraft were approaching the Northern Hemisphere high-latitude magnetopause in the postnoon sector on an outbound trajectory. Throughout this period the magnetic field, electron, and ion sensors on board Cluster observed characteristic signatures of FTEs. A few minutes delayed to these observations the Super Dual Auroral Radar Network (SuperDARN) system indicated flow disturbances in the conjugate ionospheres. These “two-point” observations on the ground and in space were closely correlated and were caused by ongoing unsteady reconnection in the vicinity of the spacecraft. The three-dimensional structures and dynamics of the observed FTEs and the associated reconnection sites are studied by using the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) MHD code in combination with a simple open flux tube motion model (Cooling). Using these two models the spatial and temporal evolution of the FTEs is estimated. The models fill the gaps left by measurements and allow a “point-to-point” mapping between the instruments in order to investigate the global structure of the phenomenon. The modeled results presented are in good correlation with previous theoretical and observational studies addressing individual features of FTEs

Cluster observations of the midaltitude cusp under strong northward interplanetary magnetic field

We report on a multispacecraft cusp observation lasting more than 100 min. We determine the cusp boundary motion and reveal the effect on the cusp size of the interplanetary magnetic field (IMF) changing from southward to northward. The cusp shrinks at the beginning of the IMF rotation and it reexpands at the rate of 0.40° invariant latitude per hour under stable northward IMF. On the basis of plasma signatures inside the cusp, such as counterstreaming electrons with balanced fluxes, we propose that pulsed dual lobe reconnection operates during the time of interest. SC1 and SC4 observations suggest a long-term regular periodicity of the pulsed dual reconnection, which we estimate to be ~1–5 min. Further, the distances from the spacecraft to the reconnection site are estimated on the basis of observations from three satellites. The distance determined using SC1 and SC4 observations is ~15 RE and that determined from SC3 data is ~8 RE. The large-scale speed of the reconnection site sunward motion is ~16 km s-1. We observe also a fast motion of the reconnection site by SC1, which provides new information about the transitional phase after the IMF rotation. Finally, a statistical study of the dependency of plasma convection inside the cusp on the IMF clock angle is performed. The relationship between the cusp stagnation, the dual lobe reconnection process, and the IMF clock angle is discussed

Lymphoproliferative disease in mice infected with murine gammaherpesvirus 68

Murine gammaherpesvirus is a natural pathogen of wild rodents. In the laboratory it establishes an infection of epithelial cells and persists in B lymphocytes in a latent form. Inbred mice chronically infected with the virus develop a lymphoproliferative disease (LPD) similar to that seen in patients infected with Epstein-Barr virus. The frequency of LPD over a period of 3 years was 9% of all infected animals, with 50% of these displaying high grade lymphomas. The incidence of LPD was greatly increased when infected mice were treated with cyclosporin A. The majority of mice used in the experiments were BALB/c, although lymphomas were detected in mice on other genetic backgrounds, ie, CBA and B10Br. Lymphomas were associated with both lymphoid and nonlymphoid tissues (liver, lung, and kidney). In all cases of lymphomas studied thus far, there was a mixed B cell (B220+ve) and T cell (CD3+ve) phenotype. The B cells were light chain restricted, indicative of a clonal origin. Variable numbers of virus genome-positive cells were detected by in situ hybridization in and around the lymphomas. In contrast, no lytic antigen-positive cells were detected, indicating that genome-positive cells were either latently infected or undergoing an abortive infection. These observations suggest that murine gammaherpesvirus-infected mice may be an important model to study the pathogenesis of LPD associated with other gammaherpesviruses, such as Epstein-Barr virus

Mechanism for the formation of the high-altitude stagnant cusp: Cluster and SuperDARN observations

On 16 March 2002, Cluster moved from nightside to dayside, across the high-altitude northern cusp during an extended period of relatively steady positive IMF BY and BZ. Combined Cluster and SuperDARN data imply the existence of two reconnection sites: in the high- latitude northern hemisphere dusk and southern hemisphere dawn sectors. Within the cusp, Cluster encounters 3 distinct plasma regions. First, injections of magnetosheath-like plasma associated with dawnward and sunward convection suggest Cluster crosses newly- reconnected field lines related to the dusk reconnection site. Second, Cluster observes a Stagnant Exterior Cusp (SEC), characterized by nearly isotropic and stagnant plasma. Finally, Cluster crosses a region with significant antifield-aligned flows. We suggest the observed SEC may be located on newly re-closed field lines, reconnected first poleward of the northern hemisphere cusp and later reconnected again poleward of the southern hemisphere cusp. We discuss how the Cluster observations correspond to expectations of ’double reconnection’ model

On the location of dayside magnetic reconnection during an interval of duskward oriented IMF

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

Electron velocity distribution and lion roars in the magnetosheath

International audienceWhistler waves which are termed "lion roars" in the magnetosheath are studied using data obtained by the Spectrum Analyser (SA) of the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment aboard Cluster. Kinetic theory is then employed to obtain the theoretical expression for the whistler wave with electron temperature anisotropy which is believed to trigger lion roars in the magnetosheath. This allows us to compare theory and data. This paper for the first time studies the details of the electron velocity distribution function as measured by the Plasma Electron And Current Experiment (PEACE) in order to investigate the underlying causes for the different types of lion roars found in the data. Our results show that while some instances of lion roars could be locally generated, the source of others must be more remote regions of the magnetosheath