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Using three‐dimensional MHD simulations of magnetic reconnection in the magnetotail, we investigate the fate of earthward bursty bulk flows (BBFs). The flow bursts are identified as entropy‐depleted magnetic flux tubes (“bubbles”) generated by the severance of a plasmoid via magnetic reconnection. The onset of fast reconnection coincides closely with a drastic entropy reduction at the onset of lobe reconnection. The fact that, in the simulation, the Alfvén speed does not change significantly at this time suggests that the destabilization of ballooning/interchange modes is important in driving faster reconnection as well as in providing cross‐tail structure. In the initial phase, the BBFs are associated with earthward propagating dipolarization fronts. When the flow is stopped nearer to Earth, the region of dipolarization expands both azimuthally and tailward. Tailward flows are found to be associated with a rebound of the earthward flow and with reversed vortices on the outside of the flow. Earthward and tailward flows are also associated with expansion and contraction of the near plasma sheet. All of these features are consistent with recent satellite observations by Cluster and the Time History of Events and their Macroscopic Interactions during Substorms (THEMIS) mission.

Magnetic flux transfer events (FTEs) are signatures of unsteady magnetic reconnection, often observed at planetary magnetopauses. Their generation mechanism, a key ingredient determining how they regulate the transfer of solar wind energy into magnetospheres, is still largely unknown. We report THEMIS spacecraft observations on 2007‐06‐14 of an FTE generated by multiple X‐line reconnection at the dayside magnetopause. The evidence consists of (1) two oppositely‐directed ion jets converging toward the FTE that was slowly moving southward, (2) the cross‐section of the FTE core being elongated along the magnetopause normal, probably squeezed by the oppositely‐directed jets, and (3) bidirectional field‐aligned fluxes of energetic electrons in the magnetosheath, indicating reconnection on both sides of the FTE. The observations agree well with a global magnetohydrodynamic model of the FTE generation under large geomagnetic dipole tilt, which implies the efficiency of magnetic flux transport into the magnetotail being lower for larger dipole tilt.

Chen and Wolf (1999) used a thin‐filament theory to construct a 2D model of a bursty bulk flow (BBF) motion inside the plasma sheet. The modeling revealed that the low‐entropy filament overshoots its equilibrium position and executes a heavily damped oscillation about that position. In this letter we demonstrate, for the first time, the multiple overshoot and rebound of a BBF observed by the five THEMIS probes on 17 March 2008 just after 10:22 UT. We found that the BBF oscillatory braking was accompanied by interlaced enhancements and depletions of radial pressure gradients. The earthward and tailward flow bursts caused formation of vortices with opposite sense of rotation.

Early magnetopause observations revealed that the magnetic field can rotate across tangential current sheets in the form of C‐and S‐shaped hodograms. We use the four‐spacecraft magnetopause crossings by Cluster in order to study the structure of the C‐ and S‐sheets. We show that both current sheets can be described by analytical equilibria. We employ a force‐free current sheet equilibrium for description of the C‐sheet and develop a new equilibrium to describe the S‐sheet. We suggest that both equilibria be used for setting up initial conditions in the next generation of current sheet simulations. Key Points We use the four‐point Cluster measurements to study the magnetopause structure We show that the found magnetopause structure can be described analytically We suggest to use the analytical solutions to set up numerical models

Stimulated by a recent study of a kinetic ballooning/interchange instability by Pritchett and Coroniti (2010), we present THEMIS events that confirm the predictions of this mechanism. In these events the probes were situated in the plasma sheet at 11 Re, near the presumed location of a B minimum. Prior to substorm onset, they observed strong magnetic oscillations with periods 20–100 s and δBX about 10–20 nT. Associated with these were oscillations of the electric field δEY ∼ 1 mV/m and the field‐aligned electron velocity of several hundreds of km/s. No comparable perturbations in the ion velocity were observed. For two cases cross‐correlation analyses proved duskward propagation of the elongated spatial structures with a cross‐tail width of a few ion gyroradii and a propagation velocity of about the ion drift velocity. In one case THEMIS probes confirmed a sausage‐like geometry of the structures. Key Points Signatures of kinetic BICI observed at 11Re near presumed B‐minimum Electron‐driven sausage(ballooning)‐like westward‐drifting ion gyroscale fingers Observed at off‐equatorial location in the middle of plasma sheet

The Kelvin-Helmholtz Instability (KHI) can drive waves at the magnetopause. These waves can grow to form rolled-up vortices and facilitate transfer of plasma into the magnetosphere. To investigate the persistence and frequency of such waves at the magnetopause we have carried out a survey of all Double Star 1 magnetopause crossings, using a combination of ion and magnetic field measurements. Using criteria originally used in a Geotail study made by Hasegawa et al. (2006) (forthwith referred to as H2006), 17 candidate events were identified from the entire TC-1 mission (covering 623 orbits where the magnetopause was sampled), a majority of which were on the dayside of the terminator. The relationship between density and shear velocity was then investigated, to identify the predicted signature of a rolled up vortex from H2006 and all 17 events exhibited some level of rolled up behavior. The location of the events had a clear dawn-dusk asymmetry, with 12 (71 %) on the post noon, dusk flank suggesting preferential growth in this region.

On 17 March 2008 around 0912 UT the five THEMIS spacecraft P1–P5 were in the plasma sheet between 2200 and 2300 h magnetic local time (MLT), covering radial distances between 15 Earth radii (Re) (P1) and 9 Re (P5). All the spacecraft consecutively observed a bursty bulk flow (BBF) that traveled earthward, slowed down from 400 km/s to 50 km/s between P1 and P5, and then turned in the opposite direction. The most tailward‐located spacecraft, P1 and P2, detected thinning and then thickening of the plasma sheet around the time of the flow direction change. Meanwhile, the other three THEMIS spacecraft, which were located in a more dipolar region, observed plasma sheet thickening and then thinning. Observations indicated that the thinning/thickening was stronger around the BBF funnel. Further, during the interaction of the earthward‐flowing BBF plasma with the Earth's dipolar field lines, the BBF was deflected by about 70° at a scale of about 5 Re. The radial pressure gradient was substantially increased when the BBF reached the shortest radial distance to the Earth and substantially decreased after the tailward plasma flow. We conclude that the tailward pressure pulse produced by the enhanced radial pressure gradients after the earthward BBF stopped could be responsible for the observed tailward plasma flows.

Tailward flows in the plasma sheet boundary layer (PSBL) were observed simultaneously by the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes at down‐tail distances X ∼ 7.6 to ∼17.6 RE for ∼10 min during an interval of successive substorm intensification in a storm time on 9 March 2008. The flows first occurred close to Earth and then propagated along the magnetic field lines in the PSBL with a speed of ∼150–350 km s−1. We show that the occurrence of tailward flows is highly dependent on proximity to the plasma sheet boundary. Higher speeds occurred in the outer part of the PSBL, while either lower tailward speeds (or even earthward flows) were seen in the inner part of the PSBL. The tailward flow occurrence increased during magnetotail stretching and decreased or ceased during magnetic field dipolarizations. These PSBL tailward flows near the Earth can be understood as an outflow of the earthward flows that essentially empty the central plasma sheet. The tailward flows along the field line filled the void left behind by the dipolarization front. Key Points The tailward flows in the PSBL are observed by THEMIS from X = −7.6 to 17.6 RE The occurrence of tailward flows is highly dependent on proximity to PSBL The tailward flow occurrence increased during magnetotail stretching

We develop a method to estimate the reconnected magnetic flux and the location of the reconnection site using properties of magnetic field and plasma velocity disturbances in the regions surrounding the reconnection plasma flow. Our analysis is based on a 3-D non-steady reconnection model with a finite-sized X-line length. In this framework, we obtain a system of equations capturing the relationships between the disturbances of the magnetic field and plasma flow from one side and the reconnection characteristics from another side. These equations allow us to determine the reconnection characteristics from one-point remote observations of the reconnection fast flow, propagated in the magnetotail current sheet. We apply the model to magnetic field and plasma observations at (−43, −11.2, −6.9) RE GSM obtained by the THEMIS/ARTEMIS spacecraft, located in the tail lobe during a substorm event. We found that the reconnection region was located at ~ (−27, 3.5, 0) RE GSM. The X-line appeared to be close to the local time of the substorm current wedge identified from ground-based observations. We estimated the total magnetic flux, which was reconnected in the event as ~5 MWb. That corresponds to a small fraction of the total amount of magnetic flux transferred during a substorm.

Violent releases of space plasma energy from the Earth’s magnetotail during substorms produce strong electric currents and bright aurora. But what modulates these currents and aurora and controls dissipation of the energy released in the ionosphere? Using data from the THEMIS fleet of satellites and ground-based imagers and magnetometers, we show that plasma energy dissipation is controlled by field-aligned currents (FACs) produced and modulated during magnetotail topology change and oscillatory braking of fast plasma jets at 10–14 Earth radii in the nightside magnetosphere. FACs appear in regions where plasma sheet pressure and flux tube volume gradients are non-collinear. Faster tailward expansion of magnetotail dipolarization and subsequent slower inner plasma sheet restretching during substorm expansion and recovery phases cause faster poleward then slower equatorward movement of the substorm aurora. Anharmonic radial plasma oscillations build up displaced current filaments and are responsible for discrete longitudinal auroral arcs that move equatorward at a velocity of about 1 km s −1 . This observed auroral activity appears sufficient to dissipate the released energy. Substorms in the Earth’s magnetosphere lead to bright aurorae, releasing energy into the surrounding ionosphere. Ground- and space-based observations now reveal how that energy is dissipated and controlled by strong electric currents.