Geometry of Frictionless and Frictional Sphere Packings

We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard-spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from $z=6$ as the friction coefficient $\mu$ between two particles is increased.Comment: 6 pages, 9 figures, submitted to Phys. Rev.

Multidecadal variability of potential temperature, salinity, and transport in the eastern subpolar North Atlantic

The Extended Ellett Line (EEL) hydrographic section extends from Scotland to Iceland crossing the Rockall Trough, Hatton-Rockall Basin and Iceland Basin. With 61 full-depth stations at a horizontal resolution of 10 to 50 km, the EEL samples the upper limb of the Atlantic Meridional Overturning Circulation flowing across the Iceland-Scotland Ridge into the Nordic Seas. The Rockall Trough has been sampled nearly four times per year from 1975 to 1996, and the full section annually since 1996. The EEL is an exceptionally long timeseries of deep-ocean temperatures and salinities. This study extends prior work in the Rockall Trough, and examines for the first time 18 year records in the Iceland and Hatton-Rockall Basins. We quantify errors in the timeseries from two sources: observational errors and aliasing. The data quality and annual sampling are suitable for observing interannual to decadal variability because the variability exceeds our error estimates. The upper waters of all 3 basins are cooler/fresher from 1997 to 2001, warmer/more saline 2001 to 2006, and cooler/fresher from 2006 to 2014. A reference level for geostrophic shear is developed heuristically and by comparison with sea-surface altimetry. The mean northward transport in the upper waters is 6.7±3.7 Sv and there is a 6.1±2.5 Sv southward flow below the thermocline. Although the magnitude of the Iceland Basin overturning circulation (4.3±1.9 Sv) is greater than in the Rockall Trough (3.0±3.7 Sv), the variability is greater in the Rockall Trough. We discuss the results in the context of our understanding of drivers of variability

Gravity Dual of Gauge Theory on S^2 x S^1 x R

We (numerically) construct new static, asymptotically AdS solutions where the conformal infinity is the product of time and S^2 x S^1. There always exist a family of solutions in which the S^1 is not contractible and, for small S^1, there are two additional families of solutions in which the S^1 smoothly pinches off. This shows that (when fermions are antiperiodic around the S^1) there is a quantum phase transition in the gauge theory as one decreases the radius of the S^1 relative to the S^2. We also compare the masses of our solutions and argue that the one with lowest mass should minimize the energy among all solutions with conformal boundary S^2 x S^1 x R. This provides a new positive energy conjecture for asymptotically locally AdS metrics. A simple analytic continuation produces AdS black holes with topology S^2 x S^1.Comment: 17 pages, 4 figures, v2: minor changes, added reference

Tachyon Condensation and Black Strings

We show that under certain conditions, closed string tachyon condensation produces a topology changing transition from black strings to Kaluza-Klein "bubbles of nothing." This can occur when the curvature at the horizon is much smaller than the string scale, so the black string is far from the correspondence point when it would make a transition to an excited fundamental string. This provides a dramatic new endpoint to Hawking evaporation. A similar transition occurs for black p-branes, and can be viewed as a nonextremal version of a geometric transition. Applications to AdS black holes and the AdS soliton are also discussed.Comment: 23 pages, 1 figure, v2: references adde

3D evolution of a filament disappearance event observed by STEREO

A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the southern hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories in particular by THEMIS. One day before the disappearance, H$\alpha$ observations showed up and down flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4 degrees, showed quite different views of this untwisting flux rope in He II 304 \AA\ images. Here, we reconstruct the 3D geometry of the filament during its eruption phase using STEREO EUV He II 304 \AA\ images and find that the filament was highly inclined to the solar normal. The He II 304 \AA\ movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km s$^{-1}$, during the rapid evolution phase. Finally, as the flux rope expands into the corona, the filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe XII 195 \AA\ images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope

Statistics of the contact network in frictional and frictionless granular packings

Simulated granular packings with different particle friction coefficient mu are examined. The distribution of the particle-particle and particle-wall normal and tangential contact forces P(f) are computed and compared with existing experimental data. Here f equivalent to F/F-bar is the contact force F normalized by the average value F-bar. P(f) exhibits exponential-like decay at large forces, a plateau/peak near f = 1, with additional features at forces smaller than the average that depend on mu. Computations of the force-force spatial distribution function and the contact point radial distribution function indicate that correlations between forces are only weakly dependent on friction and decay rapidly beyond approximately three particle diameters. Distributions of the particle-particle contact angles show that the contact network is not isotropic and only weakly dependent on friction. High force-bearing structures, or force chains, do not play a dominant role in these three dimensional, unloaded packings.Comment: 11 pages, 13 figures, submitted to PR

Multiwavelength Study of M8.9/3B Solar Flare from AR NOAA 10960

We present a multi-wavelength analysis of a long duration white-light solar flare (M8.9/3B) event that occurred on 4 June 2007 from NOAA AR 10960. The flare was observed by several spaceborne instruments, namely SOHO/MDI, Hinode/SOT, TRACE and STEREO/SECCHI. The flare was initiated near a small, positive-polarity, satellite sunspot at the centre of the AR, surrounded by opposite-polarity field regions. MDI images of the AR show considerable amount of changes in a small positive-polarity sunspot of delta configuration during the flare event. SOT/G-band (4305 A) images of the sunspot also suggest the rapid evolution of the positive-polarity sunspot with highly twisted penumbral filaments before the flare event, which were oriented in the counterclockwise direction. It shows the change in orientation and also remarkable disappearance of twisted penumbral filaments (~35-40%) and enhancement in umbral area (~45-50%) during the decay phase of the flare. TRACE and SECCHI observations reveal the successive activations of two helical twisted structures associated with this sunspot, and the corresponding brightening in the chromosphere as observed by the time-sequence images of SOT/Ca II H line (3968 A). The secondary-helical twisted structure is found to be associated with the M8.9 flare event. The brightening starts 6-7 min prior to the flare maximum with the appearance of secondary helical-twisted structure. The flare intensity maximizes as this structure moves away from the AR. This twisted flux-tube associated with the flare triggering, is found to be failed in eruption. The location of the flare is found to coincide with the activation site of the helical twisted structures. We conclude that the activations of successive helical twists in the magnetic flux tubes/ropes plays a crucial role in the energy build-up process and triggering of M-class solar flare without a CME.Comment: 22 pages, 12 figures, Accepted for Publication in Solar Physic

Rotating Black Holes which Saturate a Bogomol'nyi Bound

We construct and study the electrically charged, rotating black hole solution in heterotic string theory compactified on a $(10-D)$ dimensional torus. This black hole is characterized by its mass, angular momentum, and a $(36-2D)$ dimensional electric charge vector. One of the novel features of this solution is that for $D >5$, its extremal limit saturates the Bogomol'nyi bound. This is in contrast with the $D=4$ case where the rotating black hole solution develops a naked singularity before the Bogomol'nyi bound is reached. The extremal black holes can be superposed, and by taking a periodic array in $D>5$, one obtains effectively four dimensional solutions without naked singularities.Comment: 13 pages, no figure

Supergravity pp-wave solutions with 28 and 24 supercharges

We conduct an exhaustive search for solutions of IIA and IIB supergravity with augmented supersymmetry. We find a two-parameter family of IIB solutions preserving 28 supercharges, as well as several other IIA and IIB families of solutions with 24 supercharges. Given the simplicity of the pp-wave solution, the algorithm described here represents a systematic way of classifying all such solutions with augmented supersymmetry. By T-dualizing some of these solutions we obtain exact non-pp wave supergravity solutions (with 8 or 16 supercharges), which can be interpreted as perturbations of the AdS-CFT correspondence with irrelevant operators

A measurement of the tau mass and the first CPT test with tau leptons

We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV using tau pairs from Z0 decays. To test CPT invariance we compare the masses of the positively and negatively charged tau leptons. The relative mass difference is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.