Untersuchung der Elektrodynamik eines Aurorabogens mit FAST-Satellitendaten und optischen Beobachtungen

The work is devoted to the examination of a wide, stable, wintertime evening arc. Particle and field data measured by the FAST satellite at ~4000km altitude, as well as ground optical data, are used to get a detailed description of the arc electrodynamics. In the vicinity of the arc FAST detected several ion beams, indicating field-aligned potential drop below the satellite, which precludes the mapping of the measured electric field to the ionosphere. In order to derive the ionospheric electric field we introduce a new method, based on the current continuity at ionospheric level and on a parametric arc model. The minimum set of parameters necessary to obtain consistent results includes polarization, a longitudinal electric field constant across the arc, and the coupling between the field-aligned current (FAC) and the electrojet (EJ). The current topology proves to be completely atypical. Although the magnetic field signature suggests the common pattern, with upward and downward FAC sheets connected through Pedersen current, we find that the two current sheets are actually decoupled in the ionosphere. The upward FAC is fed by the westward EJ while most of the downward FAC feeds the eastward EJ. The results point to the prospect of a systematic surveillance of the polar ionosphere by medium altitude satellites, on a spatial scale (~1km) unattainable from ground measurements, and including time intervals when the measured electric field cannot be mapped to the ionosphere. By extending the application of the developed method other peculiar auroral features might be unraveled.Die Arbeit ist der Untersuchung eines breiten, stabilen, Winterabendbogens gewidmet. Teilchen- und Felddaten, vom Satelliten FAST in ~4000km Höhe gemessen, sowie optische Bodenbeobachtungen werden verwendet, um eine detaillierte Beschreibung der Electrodynamik des Bogens zu erhalten. In der Nähe des Bogens wurden mit FAST mehrere Ionenstrahlen entdeckt. Dies weist auf eine parallele Potentialdifferenz unter dem Satelliten hin und schließt damit die mapping des gemessenen elektrischen Feldes zur Ionosphäre aus. Um das ionosphärische elektrische Feld abzuleiten führen wir eine neue Methode ein, die auf der Stromerhaltung auf ionosphärischem Niveau und auf einem parametrischen Bogenmodell basiert. Die minimale Anzahl der Parameter, die notwendig sind übereinstimmende Ergebnisse zu erreichen, umfaßt Polarisation, ein elektrisches Longitudinalfeld das quer durch den Bogen konstant ist und die Kopplung zwischen dem feldparallelen Strom (FAC) und dem Elektrojet (EJ). Die Stromtopologie stellt sich als sehr untypisch heraus. Obwohl die magnetische Feldsignatur das gewöhnliche Muster aufzeigt - sie deutet aufwärts- und abwärtsgerichtete FAC Schichten an, die normalerweise durch den Pedersen Strom verbunden sind - finden wir eigentlich, dass die zwei Stromschichten in der Ionosphäre entkoppelt sind. Das aufwärtsgerichtete FAC wird von dem nach Westen fließenden EJ gespeist, während der größte Teil des abwärtsgerichteten FAC den nach Osten fließenden EJ speist. Die Ergebnisse deuten auf die Möglichkeit einer systematischen Überwachung der polaren Ionosphäre mit Satelliten in mittlere Höhe hin. Dabei kann eine vom Boden aus unerreichbare Auflösung von ~1km erzielt werden. Darüber hinaus können auch Zeiten untersucht werden, in denen eine Projektion des elektrischen Feldes zur Ionosphäre nicht möglich ist. Durch weitere Anwendungen der entwickelten Methode könnte man auch andere bisher nicht erforschte Polarlicht-Phänomene untersuchen

Experimental investigation of auroral generator regions with conjugate Cluster and FAST data

Here and in the companion paper, Hamrin et al. (2006), we present experimental evidence for the crossing of auroral generator regions, based on conjugate Cluster and FAST data. To our knowledge, this is the first investigation that concentrates on the evaluation of the power density, <i><b>E</b></i>·<i><b>J</b></i>, in auroral generator regions, by using in-situ measurements. The Cluster data we discuss were collected within the Plasma Sheet Boundary Layer (PSBL), during a quiet magnetospheric interval, as judged from the geophysical indices, and several minutes before the onset of a small substorm, as indicated by the FAST data. Even at quiet times, the PSBL is an active location: electric fields are associated with plasma motion, caused by the dynamics of the plasma-sheet/lobe interface, while electrical currents are induced by pressure gradients. In the example we show, these ingredients do indeed sustain the conversion of mechanical energy into electromagnetic energy, as proved by the negative power density, <i><b>E</b></i>·<i><b>J</b></i><0. The plasma characteristics in the vicinity of the generator regions indicate a complicated 3-D wavy structure of the plasma sheet boundary. Consistent with this structure, we suggest that at least part of the generated electromagnetic energy is carried away by Alfvén waves, to be dissipated in the ionosphere, near the polar cap boundary. Such a scenario is supported by the FAST data, which show energetic electron precipitation conjugated with the generator regions crossed by Cluster. A careful examination of the conjunction timing contributes to the validation of the generator signatures

EVALUATING MOVEMENT CONSISTENCY IN WHEELCHAIR PROPULSION USING FLOQUET MULTIPLIERS AND QUATERNIONS

Wheelchair users may develop repetitive stress injuries that are associated with the wheelchair-stroke. Evaluating if a stroke pattern is consistent may reveal the existence of a possible injury. However, there is no standardized method to evaluate the stability of the wheelchair-stroke. The purpose of this paper was to develop a method to measure movement consistency during manual wheelchair propulsion without the need of normreferenced tests. The method is based (a) on the Floquet theory, (b) on Poincaré, and (c) on quaternion transformations. The method was evaluated with experimental data and the results showed that the method is sensitive enough to identify instabilities in the wheelchair stroke cycle. Furthermore, the method can be generalized to study other aspects of human motor control

Analytical and Experimental Analysis of a Free Link in Contact with a Granular Medium

In this study, the experimental and the simulation results for a planar free link impacting a granular medium are analyzed. The resistance force of the granular medium on the body from the moment of the impact until the body stops is very important. Horizontal and vertical static resistance forces developed by theoretical and empirical approaches are considered. The penetrating depth of the impacting end of the free link increases with the increase of the initial impacting velocity. We define the stopping time as the time interval from the moment of impact until the vertical velocity of the link end is zero. The stopping time of the end decreases as the initial velocity increases. The faster the end of the link impacts the surface of the granular medium, the sooner it will come to a stop. This phenomenon involves how rapidly a free link strikes the granular medium and how it slows down upon contact