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  CLUSTER 13 MSSL, 26th-30th March 2007
Workshop Abstracts The abstracts that have been submitted so far are below, to browse events submitted for discussion at the workshop please see this page.
Kinetic theory and simulations of the tail current sheet: Atypical current sheets, plasma bubbles and reconnection onset As revealed by CLUSTER observation, thin current sheets in the magnetotail may strongly differ from the conventional Harris profile due to effects of embedding and bifurcation. At the same time, they often have no signatures of collisionless magnetic reconnection. A universal phenomenon involving such atypical current sheets, which is connected but not identical to reconnection, may be plasma bubbles. We present a self-consistent kinetic model of plasma bubbles based on a 2D generalization of earlier atypical current sheet models, which also takes into account south-then-north sense of Bz perturbations typical for earthward traveling compression regions and fast flows in the central plasma sheet. The new model predicts that the dominatiing structure element of the earthward flows is a region of the local reduction of the cross-tail current provided by the current bifurcation effect.
Martin Volwerk, Mark Lester, Tony Lui and the ISSI & Cluster teams Fast flows in a Themis-like configuration: Part 1 On 5 September 2005, between 0130 and 0200 UT, the Cluster, DoubleStar and Polar spacecraft were in a Themis-like configuration. The Cluster spacecraft observe two bursts of Earthward plasma flow, 500 < vx < 1000 km/s (~0105 and ~0117 UT), including dipolarizations of the magnetic field. TC1 also observes these dipolarizations, albeit slightly later than Cluster. These two fast flows are accompanied by two poleward boundary intensifications (PBIs).
Comment on Taylor et al., "Cluster encounter with an energetic electron beam during a substorm" [JGR Nov 2006] We have done 1-D particle simulations of an induction electric field in a plasma [Omura et al., 2003]. Due to the acceleration of electrons and ions in the opposite directions, there arises counter-streaming that causes the Buneman instability. However, a major part of the electrons are grouped together at higher energies, forming a distinct bump in their distributions. Beams of high energy electrons and ions are formed. What is surprising is that the low energy electrons are going with the ions! The ions become bunched, and our interpretation is that these low energy electrons are a form of Debye shielding of the ion bunches. Exactly that behaviour has been found by Taylor et al. [2006] during a Cluster encounter with an energetic electron beam on August 11, 2002. Cluster 3 spacecraft made an unprecedented high-resolution measurement of a beam of electrons with energies up to 400 keV. The beam was only fully resolved by combining the energy range coverage of the Plasma Electron and Current Experiment and Research with Adaptive Particle Imaging Detector electron detectors. Its pitch angle distribution evolved from antiparallel, through counterstreaming to parallel over a period of 20 s. They also found that the low energy electrons were antiparallel to the high. Their data are in complete agreement with our simulations. We propose that such beams are the cause of the westward traveling surge (WTS), of omega band auroras, of discharges on the sun, and of various phenomena in space plasma physics. Quasi-periodic dipolarisations in the mid-magnetotail observed by Cluster and Double star M W Dunlop (1), A N Fazakerley (2), C Shen (3), Z Pu (4), M G G T Taylor (5), A Walsh (2), C Perry (1), J A Davies (1), C Carr (6), Z-X Liu (3). Substorm Triggering Instability We propose that the appropriate instability to trigger a substorm is a tailward meander (in the equatorial plane) of the strong current filament that develops during the growth phase. From this single assumption follows the entire sequence of events for a substorm. The main particle acceleration mechanism in the plasma sheet is curvature drift with a dawn-dusk electric field leading to the production of auroral arcs. Eventually the curvature becomes so high that the ions cannot negotiate the sharp turn at the field-reversal region, locally, at a certain place and time. The particle motion becomes chaotic, causing a local outward meander of the cross-tail current. An induction electric held is produced by Lenz\u2019s law, Eind = -dA/dt. An outward meander with Bz > 0 will cause E x B flow everywhere out from the disturbance; this reaction is a macroscopic instability which we designate the electromotive instability. The response of the plasma is through charge separation and a scalar potential, Ees = \u2013grad φ. Both types of electric fields have components parallel to B in a realistic magnetic field. For MHD theory to hold the net Epar must be small; this usually seems to happen (because MHD often does hold), but not always. Part of the response is the formation of field-aligned currents producing the well-known substorm current diversion. This is a direct result of a strong Eind (the cause) needed to overcome the mirror force of the current carriers; this enables charge separation to produce an opposing electrostatic field Ees (the effect). Satellite data confirm the reality of a strong Epar in the plasma sheet by counter-streaming of electrons and ions, and by the inverse ion time dispersion, up to several 100 keV. The electron precipitation is associated with the westward traveling surge (WTS) and the ion with omega bands, respectively. ! However, with zero curl, Ees cannot modify the emf of of the inductive electric held (a property of vector fields). The charge separation that produces a reduction of Epar must enhance the transverse component. The new plasma flow becomes a switch for access to the free energy of the stressed magnetotail. On the tailward side the dusk-dawn electric field with E\u2022J < 0 will cause tailward motion of the plasma and a plasmoid may be created. It will move in the direction of least magnetic pressure, tailward. On the earthward side the enhanced dawn-dusk induction electric field with E\u2022J > 0 will cause injection into the inner plasma sheet, repeatedly observed at moderate energies of 1-50 keV. This same electric field near the emerging X-line will accelerate particles non-adiahatically to moderate energies. With high magnetic moments in a weak magnetic held, electrons (ions) can benefit from gradient and curvature drift to attain high energies (by the ratio of the magnetic field ! magnitude) in seconds (minutes). Observed tail current systems associated with bursty bulk flows and auroral streamers during a period of multiple substorms During a substorm on the 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a bursty bulk flow (BBF) event nine minutes after expansion phase onset, travelling Earthwards and duskwards with a velocity of 425 km s-1. Coincident with this event, the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the southern hemisphere near the footpoints of the Cluster spacecraft. We present a study of Cluster observations of the ion flows and current system associated with the BBF together with conjugate ground and space-based observations of the ionosphere. Using FluxGate Magnetometer (FGM) data, we infer the currents in the BBF, using the curlometer technique, which when projected into the ionosphere give ionospheric field-aligned currents of ~18 A km-2, comparable with previously observed values of ionospheric field-aligned current associated with BBFs and auroral streamers. The observations of the flow are consistent with an injection of plasma from reconnected open field lines from an X-line close to the Earth. We conclude that the currents detected within the BBF by Cluster are consistent with auroral streamers being the ionospheric manifestation of substorm-time BBFs. The observations of the BBF are consistent with a basic reconnection mechanism for its creation. Tribute to Faraday Throughout the first half of the nineteenth century, the standard English education consisted of the classics of literature plus "pure" mathematics, which included astronomy. The sciences - chemistry, physics, biology, geology - were typically not taught. In the 1850s, Faraday began to use his prestige to lobby for inclusion of the sciences in the secondary-level curriculum and to strengthen its teaching in postsecondary institutions. He reiterated his long-held view that "education has, for its first and its last step, humility". The first step in correction is to learn our deficiencies, and having learned them, the next step is almost complete: for no man who has discovered that his judgment is hasty, or illogical, or imperfect, would go on with the same degree of haste, or irrationality, or presumption, as before. Whatever systemic reforms might be implemented, there was always a place for self-education like his own. "It is necessary that a man examine himself, and that not carelessly. On the contrary, as he advances, he should become more and more strict, till he ultimately prove a sharper critic to himself than any one else can be; and he ought to intend this, for, so far as he consciously falls short of it, he acknowledges that others may have reason on their side when they criticise him." (page 201) "The Electric Life of Michael Faraday" By Alan Hirshfeld, 2006 M. L. Goldstein, C. P. Escoubet, M. Hapgood, M. G. G. T. Taylor The THEMIS mission: background, current status, and coordination with Cluster and Double Star The 5-spacecraft THEMIS mission was launched last month on February 17. The primary objective of THEMIS is to determine the location of the onset of magnetospheric substorms. The spacecraft are currently being commissioned. The orbit of THEMIS affords unique opportunities for collaborations with Cluster and Double Star. This talk will provide an update on the status of THEMIS and will point out some of those opportunities for collaborative science. High energy electron beams in relation with near-Earth reconnection events From a list of near-Earth reconnection events we have searched for the presence of energetic electron "beams" similar to what was observed on August 11, 2002 (Taylor et al., [JGR Nov 2006]). Only a few of the identified events had sufficient data resolution to identify beams (BM, or RAPID L3DD mode since July 2004). In these cases electron beams are for the first time observed by more than one spacecraft and at slightly different times. The beams are observed both north and south of the neutral sheet and are mostly observed streaming away from the reconnection region. Plasma Acceleration around the Magnetic Reconnection Region From the Cluster observations of the magnetic reconnection (flow reversal) events, timing and the spatial distribution of plasma acceleration and thermalization as well as the pitch angle scattering around X lines are examined using PEACE and RAPID electron data and CIS ion data. Relation of the Hall inward beam, isotropic flat-top distribution, and hardening of the supra-thermal components suggests an additional key to the acceleration process inside and around the X line. Reconnection Vs Change in Interconnection Surely there is no doubt that there can be changes in the state of interconnection between the geomagnetic field and the interplanetary magnetic field. It is often said that this must imply magnetic reconnection. However, there seem be two connotations of the word reconnection, a physical [colloquial] one and a mathematical [modeling] one. With the first, the above statement is a tautology. The quotation from Sonnerup [1995] clearly uses the physical definition: 'A large amount of indirect evidence in support of an open, or reconnecting magnetosphere has accumulated.' With the mathematical definition this is by no means so, as it depends on the actual model, input conditions, boundary conditions, etc. This difference is never openly expressed, nor even appreciated. Cluster measurements of the divergence of the electron pressure tensor and J × B: Relative contributions to Ohm's law On the 17th August 2003 Cluster observed a flow reversal in the tail plasma sheet, consistent with the passage of a reconnection X-line. Henderson et al. (2006) have investigated the divergence of the observed electron pressure tensor, derived solely from measurements measured from the PEACE electron spectrometer instruments on each of the 4 Cluster spacecraft, during a current sheet crossing in this period. This calculation is possible when the spacecraft are in burst mode and carefully calibrated, full 3D particle distributions can be used to determine the electron moments. The Curlometer technique, applied to data from the Cluster magnetometers, was used to assess j × B, and thus the relative contributions of the electron pressure divergence and Hall terms in the generalised Ohm's law were determined. An anti-correlation between these 2 terms in the component normal to the current sheet was reported. In this paper we report an extension to Henderson et al.'s (2006) case study, in which we investigate the generality of those results by investigating further current sheet crossings which occurred on the 17th August 2003, as well as a number of additional periods of burst mode data available from the 2003 tail season. Data from this period are particularly useful for this study as the inter-spacecraft separations are at their smallest for the Cluster mission. We confirm the generality of the observed anti-correlation between the Hall and electron pressure tensor divergence terms in Ohm's law, and consider the theoretical background in an attempt to explain the relative magnitude of their contributions to the total electric field. Henderson et al., "Cluster PEACE observations of electron pressure tensor divergence in the magnetotail", Geophys, Res, Lett. (2006), 33, L22106Near-Simultaneous Magnetotail Flux Rope Observations with Cluster and Double Star We present observations of three magnetic flux ropes in the tail of the Earth?s magnetosphere on 7 August 2004 by the Cluster and Double Star TC-1 spacecraft. The first two flux rope signatures were observed, near simultaneously, by Cluster and TC-1, which were located at (-16.3, -8.7, 0.10)RE GSM and (-10.3, -7.11, 0.81)RE GSM respectively, a separation of 6.3RE. A third signature was observed some four minutes later by two of the four Cluster spacecraft, while the other two spacecraft observed a feature resembling a Travelling Compression Region (TCR). These observations are interpreted as three individual flux ropes existing in the magnetotail, the first two, at least, simultaneously. The formation mechanism of the flux ropes and the consequences of their presence for the structure of the magnetotail are discussed in the context of multiple X-point reconnection. Observations and Simulations of Plasma Sheet Boundary Layer Structures We have investigated the occurrence and variability of ion beams in the plasma sheet boundary layer (PSBL) by using Large Scale Kinetic (LSK) simulations in which we calculate the trajectories of millions of ions in the electric and magnetic fields from global magnetohydrodynamic (MHD) simulations. We used upstream solar wind and interplanetary magnetic field data from the WIND and Ace spacecraft to drive MHD simulations of the events. Recent observations from the Cluster and Double Star spacecraft have shown that the PSBL can be a region with the expected ion beams, and can, at times, contain fine-scale sub-structures called beamlets. At other times, the transition from the CPS to the lobes is very abrupt in the observations, suggesting that the PSBL is missing. We have selected two events during which both Cluster and Double Star traversed the PSBL in opposite hemispheres and with a separation of less than one hour.On October 8, 2004, Cluster and Double Star were located close to the current sheet at comparable distances downtail. Cluster and Double Star sampled the plasma sheet boundary layer (PSBL) multiple times, and both observed high-speed beams at the lobeward edge of the PSBL that contained small-scale structures. Differences in these observations demonstrate the variability and scale of the observed structures. For this event, we launched ions near the boundary of open and closed field lines on trajectories incident on the magnetotail current sheet. On October 10, 2005, during a period of northward IMF, Cluster, in the southern hemisphere, and Double Star, in the northern hemisphere, crossed into the PSBL nearly simultaneously. Here, we launched ions near the flanks, in the low latitude boundary layer. As a result of the non-adiabatic interaction of ions with the current sheet, we obtain a complex and spatial- and time-dependent PSBL structures for both events. We will present a detailed analysis of these structures that are caused by both global processes and local kinetic processes. By Andrew Walsh Last updated on 23-Mar-2007 by Andrew Walsh |
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