Kelvin-Helmholtz instabilities at the magnetic cavity boundary of comet 67P/Churyumov-Gerasimenko

We investigate the plasma environment of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta mission. Rosetta will rendezvous with the comet in 2014 at almost 3.5 AU and follow it all the way to and past perihelion at 1.3 AU. During its journey towards the inner solar system the comet's environment will significantly change. The interaction of the solar wind with a well developed neutral coma leads to the formation of an upstream bow shock and, closer to the comet, the inner shock separating the solar wind, with cometary pick-up ions mass-loaded, from the inner cometary ions which are dragged outward through abundant collisions and charge exchange with the expanding neutral gas. As a consequence the interplanetary magnetic field is prevented from penetrating the innermost region of the comet, the so-called magnetic cavity. We use our magnetohydrodynamics model BATSRUS (Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme) to simulate the solar wind - comet interaction. The model includes photoionization, ion-electron recombination, and charge exchange. Under certain conditions our model predicts an unstable plasma flow at the inner shock. We show that the plasma shear flow around the magnetic cavity can lead to Kelvin-Helmholtz instabilities. We investigate the onset of this phenomenon with change of heliocentric distance and furthermore show that a previously stable magnetic cavity boundary can become unstable when the neutral gas is predominately released from the dayside of the comet.

Tunable mechanical resonator with aluminum nitride piezoelectric

The electromechanical response of piezoelectrically-actuated AlN micromachined bridge resonators has been characterized using laser interferometry and electrical admittance measurements. We compare the response of microbridges with different dimensions and buckling (induced by the initial residual stress of the layers). The resonance frequencies are in good agreement with numerical simulations of the electromechanical behavior of the structures. We show that it is possible to perform a rough tuning of the resonance frequencies by allowing a determined amount of builtin stress in the microbridge during its fabrication. Once the resonator is made, a DC bias added to the AC excitation signal allows to fine-tune the frequency. Our microbridges yield a tuning factor of around 88 Hz/V for a 500 ?m-long microbridge.

Sound and music: an elementary treatise on the physical constitution of musical sounds and harmony, including the chief acoustical discoveries of Professor Helmholtz.

Mode of access: Internet.

Handbuch der physiologischen Optik / von H. von Helmholtz.

"Issued in 17 parts, 1886-1896." - L.C.

Synchrotron resonator for the Illinois 300-MeV betatron.

Mode of access: Internet.

The problem of human life : containing the fundamental principles of the substantial philosophy; with a review of the six great modern scientists: Darwin, Tyndall, Huxley, Haeckel, Helmholtz, and Mayer ... /

Printed in two columns.

Hermann von Helmholtz als mensch und gelehrter /

Mode of access: Internet.

Kant und Helmholtz über den Ursprung und die Bedeutung der Raumanschauung und der geometrischen Axiome /

Bibliographical footnotes.

Amplitude response of a unilaterally constrained nonlinear micromechanical resonator

Dynamical systems that involve impacts frequently arise in engineering. This Letter reports a study of such a system at microscale that consists of a nonlinear resonator operating with an unilateral impact. The microresonators were fabricated on silicon-on-insulator wafers by using a one-mask process and then characterised by using the capacitively driving and sensing method. Numerical results concerning the dynamics of this vibro-impact system were verified by the experiments. Bifurcation analysis was used to provide a qualitative scenario of the system steady-state solutions as a function of both the amplitude and the frequency of the external driving sinusoidal voltage. The results show that the amplitude of resonant peak is levelled off owing to the impact effect and that the bandwidth of impacting is dependent upon the nonlinearity and the operating conditions.

Nonlinear dynamics of a periodically driven Duffing resonator coupled to a Van der Pol oscillator

We explore the dynamics of a periodically driven Duffing resonator coupled elastically to a van der Pol oscillator in the case of 1?:?1 internal resonance in the cases of weak and strong coupling. Whilst strong coupling leads to dominating synchronization, the weak coupling case leads to a multitude of complex behaviours. A two-time scales method is used to obtain the frequency-amplitude modulation. The internal resonance leads to an antiresonance response of the Duffing resonator and a stagnant response (a small shoulder in the curve) of the van der Pol oscillator. The stability of the dynamic motions is also analyzed. The coupled system shows a hysteretic response pattern and symmetry-breaking facets. Chaotic behaviour of the coupled system is also observed and the dependence of the system dynamics on the parameters are also studied using bifurcation analysis.

Characterization of a multiresonance ring resonator-based optical device

We describe the linear and nonlinear transfer characteristics of a multi-resonance optical device consisting of two ring resonators coupled one to another and to a waveguide. The propagation effects displayed by the device are compared with those of a sequence of a waveguide-coupled fundamental ring resonators.