A Streamwise Constant Model of Turbulence in Plane Couette Flow

Streamwise and quasi-streamwise elongated structures have been shown to play a significant role in turbulent shear flows. We model the mean behavior of fully turbulent plane Couette flow using a streamwise constant projection of the Navier Stokes equations. This results in a two-dimensional, three velocity component ($2D/3C$) model. We first use a steady state version of the model to demonstrate that its nonlinear coupling provides the mathematical mechanism that shapes the turbulent velocity profile. Simulations of the $2D/3C$ model under small amplitude Gaussian forcing of the cross-stream components are compared to DNS data. The results indicate that a streamwise constant projection of the Navier Stokes equations captures salient features of fully turbulent plane Couette flow at low Reynolds numbers. A system theoretic approach is used to demonstrate the presence of large input-output amplification through the forced $2D/3C$ model. It is this amplification coupled with the appropriate nonlinearity that enables the $2D/3C$ model to generate turbulent behaviour under the small amplitude forcing employed in this study.Comment: Journal of Fluid Mechanics 2010, in pres

Trans-sonic cusped shaped, periodic waves and solitary waves of the electrostatic ion-cyclotron type

International audienceBy adopting an essentially fluid dynamic viewpoint we derive the wave structure equation for stationary, fully nonlinear, electrostatic, ion-cyclotron waves. The existence of two fundamental constants of the motion, namely, conservation of momentum flux parallel to the ambient magnetic field, and energy flux parallel to the direction of wave propagation, enables the wave structure equation to be reduced to a first order differential equation, which has solutions that are physically transparent. The analysis shows that sufficiently oblique waves, propagating at sub-ion acoustic speeds, form soliton pulse-like solutions whose amplitudes are greatest for perpendicular propagation. Waves that propagate supersonically have periodic cnoidal waveforms, which are asymmetric about the compressive and rarefactive phases of the wave. It is also shown that there exist critical driver fields for which the end point of the compressive phase goes sonic (in the wave frame), with the consequence that the wave form develops a cusp. It is possible that this trans-sonic, choked flow feature provides a mechanism for the "spiky" waveforms observed in auroral electric field measurements

Explosive events associated with a surge

The solar atmosphere contains a wide variety of small-scale transient features. Here, we explore the inter-relation between some of them such as surges, explosive events and blinkers via simultaneous spectral and imaging data taken with the TRACE imager, the SUMER, and CDS spectrometers on board SoHO, and SVST La Palma. The alignment of all data both in time and solar XY shows that SUMER line profiles, which are attributed to explosive events, are due to a surge phenomenon. The surge is triggered, most probably, by one or more Elerman bombs which are best visible in Halpha +-350 A but were also registered by TRACE Fe IX/X 171 A and correspond to a strong radiance increase in the CDS Mg IX 368.07 A line. With the present study we demonstrate that the division of small-scale transient events into a number of different subgroups, for instance explosive events, blinkers, spicules, surges or just brightenings, is ambiguous, implying that the definition of a feature based only on either spectroscopic or imaging characteristics as well as insufficient spectral and spatial resolution can be incomplete.Comment: 17 pages, 7 figures, 1 tabl

Valence and magnetic instabilities in Sm compounds at high pressures

We report on the study of the response to high pressures of the electronic and magnetic properties of several Sm-based compounds, which span at ambient pressure the whole range of stable charge states between the divalent and the trivalent. Our nuclear forward scattering of synchrotron radiation and specific heat investigations show that in both golden SmS and SmB6 the pressure-induced insulator to metal transitions (at 2 and about 4-7 GPa, respectively) are associated with the onset of long-range magnetic order, stable up to at least 19 and 26 GPa, respectively. This long-range magnetic order, which is characteristic of Sm(3+), appears already for a Sm valence near 2.7. Contrary to these compounds, metallic Sm, which is trivalent at ambient pressure, undergoes a series of pressure-induced structural phase transitions which are associated with a progressive decrease of the ordered 4f moment.Comment: 15 pages (including 7 figures) submitted to J. Phys.: Condens. Matte

X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri

Using the XMM-Newton X-ray observatory to monitor the nearest star to the Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically active star ever observed. Correlated X-ray and optical variability provide strong support for coronal energy and mass supply by a nearly continuous sequence of rapid explosive energy releases. Variable emission line fluxes were observed in the He-like triplets of OVII and NeIX during a giant flare. They give direct X-ray evidence for density variations, implying densities between 2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the volume of the line-emitting plasma. We discuss the data in the context of the chromospheric evaporation scenario.Comment: 10 pages, 2 figures, accepted by The Astrophysical Journal, Letters; improved calculations of radiative loss of cool plasma (toward end of paper

Astrometric Discovery of GJ 164B

We discovered a low-mass companion to the M-dwarf GJ 164 with the CCD-based imaging system of the Stellar Planet Survey (STEPS) astrometric program. The existence of GJ 164B was confirmed with Hubble Space Telescope NICMOS imaging observations. A high-dispersion spectral observation in V sets a lower limit of delta m> 2.2 mag between the two components of the system. Based upon our parallax value of 0.082 +/- 0.008, we derive the following orbital parameters: P = 2.04 +/- 0.03 y, a = 1.03 +/- 0.03 AU, and Mtotal = 0.265 +/- 0.020 MSun. The component masses are MA = 0.170 +/- 0.015 MSun and MB = 0.095 +/- 0.015 MSun. Based on its mass, colors, and spectral properties, GJ 164B has spectral type M6-8 V.Comment: pdf file 14 pages with 6 fig

Collisional properties of cold spin-polarized nitrogen gas: theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of 14N and 15N atoms with densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at milli-Kelvin temperatures. Rigorous quantum scattering calculations based on accurate ab initio interaction potentials for the 7Sigma_u electronic state of N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.Comment: 48 pages, 17 figures, 3 table