Response to sub-threshold stimulus is enhanced by spatially heterogeneous activity

Sub-threshold stimuli cannot initiate excitations in active media, but surprisingly as we show in this paper, they can alter the time-evolution of spatially heterogeneous activity by modifying the recovery dynamics. This results in significant reduction of waveback velocity which may lead to spatial coherence, terminating all activity in the medium including spatiotemporal chaos. We analytically derive model-independent conditions for which such behavior can be observed.Comment: 5 pages, 5 figure

Some new features of electron density irregularities over SHAR during strong spread F

International audienceAn RH-560 rocket flight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip latitude 5.5°N) to study electron density and electric field irregularities during spread F. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of electron density fluctuations are presented here. Two extremely sharp layers of very high electron density were observed at 105 and 130 km. The electron density increase in these layers was by a factor of 50 in a vertical extent of 10 km. Large depletions in electron density were observed around 175 and 238 km. Both sharp layers as well as depletions were observed also during the descent. The presence of sharp layers and depletions during the ascent and the descent of the rocket as well as an order of magnitude less electron density, in 150-300 km region during the descent, indicate the presence of strong large-scale horizontal gradients in the electron density. Some of the valley region irregularities (165-178 km), in the intermediate scale size range, observed during this flight, show spectral peaks at 2 km and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of new type. The growth rate of intermediate scale size irregularities, produced through generalized Rayleigh Taylor instability, was calculated for the 200-330 km altitude, using observed values of electron density gradients and an assumed vertically downward wind of 20 ms-1. These growth rate calculations suggest that the observed irregularities could be produced by the gradient drift instability

Role of the equatorial ionization anomaly in the development of the evening prereversal enhancement of the equatorial zonal electric field

1] During the evening prereversal enhancement of the zonal electric field (EPRE) that begins around 1700 LT when the F region neutral winds turn eastward, as assumed here, and continues till the postsunset zonal electric field reversal time, an overall positive feedback is shown to occur between the eastward electric field in the lower side of the flux tube integrated (LSFTI) F region and the increased flux tube integrated Pedersen conductivity (FTIC) of the tropical F region. The increase in this FTIC can take place because of the increase in electron density through the increase in solar flux and the intensification of the equatorial ionization anomaly (EIA). While the influence of EIA on EPRE is immediate, the growth time for EIA is 2 to 3 h. Therefore, for a strong EPRE to occur, a fairly strong EIA is required at 1700 LT which is then sustained by the electric field associated with EPRE during its growth period. This study suggests that the postsunset eastward electric field is due to the combined currents in the equatorial electrojet and the LSFTI F regions that get diverted from the daytime Sq current system and flow from the presunset region toward the postsunset zonal electric field reversal region. Thereafter these currents turn and flow poleward to meet the current continuity requirement of the F region dynamo followed by a westward turn to rejoin the daytime Sq current system in midlatitudes. Thus the currents responsible for EPRE are an extension of the daytime Sq current system

Core-tube morphology of multiwall carbon nanotubes

The present paper investigates the cross-sectional morphology of Multiwalled Carbon Nanotubes (MWNTs) restrained radially and circumferentially by an infinite surrounding elastic medium, subjected to uniform external hydrostatic pressure. In this study, a two-dimensional plane strain model is developed, assuming no variation of load and deformation along the tube axis. We find some characteristic cross-sectional shapes from the elastic buckling analysis. The effect of the surrounded elastic medium on the cross-sectional shape which occurs due to pressure buckling is focused on by the comparison with the shape for no elastic medium case in our discussion. It is suggested that in no embedded elastic medium cases, the cross-sectional shapes of inner tubes maintain circle or oval; on the other hand, an embedded medium may cause inner tube corrugation modes especially when the number of shells for MWNTs is small.Comment: 7 figures, 2 figure

Collective behavior of stock price movements in an emerging market

To investigate the universality of the structure of interactions in different markets, we analyze the cross-correlation matrix C of stock price fluctuations in the National Stock Exchange (NSE) of India. We find that this emerging market exhibits strong correlations in the movement of stock prices compared to developed markets, such as the New York Stock Exchange (NYSE). This is shown to be due to the dominant influence of a common market mode on the stock prices. By comparison, interactions between related stocks, e.g., those belonging to the same business sector, are much weaker. This lack of distinct sector identity in emerging markets is explicitly shown by reconstructing the network of mutually interacting stocks. Spectral analysis of C for NSE reveals that, the few largest eigenvalues deviate from the bulk of the spectrum predicted by random matrix theory, but they are far fewer in number compared to, e.g., NYSE. We show this to be due to the relative weakness of intra-sector interactions between stocks, compared to the market mode, by modeling stock price dynamics with a two-factor model. Our results suggest that the emergence of an internal structure comprising multiple groups of strongly coupled components is a signature of market development.Comment: 10 pages, 10 figure

Self-Organized Dynamical Equilibrium in the Corrosion of Random Solids

Self-organized criticality is characterized by power law correlations in the non-equilibrium steady state of externally driven systems. A dynamical system proposed here self-organizes itself to a critical state with no characteristic size at ``dynamical equilibrium''. The system is a random solid in contact with an aqueous solution and the dynamics is the chemical reaction of corrosion or dissolution of the solid in the solution. The initial difference in chemical potential at the solid-liquid interface provides the driving force. During time evolution, the system undergoes two transitions, roughening and anti-percolation. Finally, the system evolves to a dynamical equilibrium state characterized by constant chemical potential and average cluster size. The cluster size distribution exhibits power law at the final equilibrium state.Comment: 11 pages, 5 figure

Hyperspherical entanglement entropy

The coefficient of the log term in the entanglement entropy associated with hyperspherical surfaces in flat space-time is shown to equal the conformal anomaly by conformally transforming Euclideanised space--time to a sphere and using already existing formulae for the relevant heat--kernel coefficients after cyclic factoring. The analytical reason for the result is that the conformal anomaly on the lune has an extremum at the ordinary sphere limit. A proof is given. Agreement with a recent evaluation of the coefficient is found.Comment: 7 pages. Final revision. Historical comments amended. Minor remarks adde