Catastrophic eruption of magnetic flux rope in the corona and solar wind with and without magnetic reconnection

It is generally believed that the magnetic free energy accumulated in the corona serves as a main energy source for solar explosions such as coronal mass ejections (CMEs). In the framework of the flux rope catastrophe model for CMEs, the energy may be abruptly released either by an ideal magnetohydrodynamic (MHD) catastrophe, which belongs to a global magnetic topological instability of the system, or by a fast magnetic reconnection across preexisting or rapidly-developing electric current sheets. Both ways of magnetic energy release are thought to be important to CME dynamics. To disentangle their contributions, we construct a flux rope catastrophe model in the corona and solar wind and compare different cases in which we either prohibit or allow magnetic reconnection to take place across rapidly-growing current sheets during the eruption. It is demonstrated that CMEs, even fast ones, can be produced taking the ideal MHD catastrophe as the only process of magnetic energy release. Nevertheless, the eruptive speed can be significantly enhanced after magnetic reconnection sets in. In addition, a smooth transition from slow to fast eruptions is observed when increasing the strength of the background magnetic field, simply because in a stronger field there is more free magnetic energy at the catastrophic point available to be released during an eruption. This suggests that fast and slow CMEs may have an identical driving mechanism.Comment: 7 pages, 4 figures, ApJ, in press (vol. 666, Sept. 2007

Biological Electron Energy Loss Spectroscopy in the Field-Emission Scanning Transmission Electron Microscope

The dedicated scanning transmission electron microscope (STEM) combined with parallel electron energy loss spectroscopy (EELS) provides a very sensitive means of detecting specific elements in small structures. EELS is more sensitive than optimized energy-dispersive X-ray spectroscopy by a factor of about three for calcium. Measurement of such low concentrations requires special processing methods such as difference-acquisition techniques and multiple least squares procedures for fitting reference spectra. By analyzing data recorded at each pixel in a spectrum-image it is possible to map quantitatively the elemental distributions in a specimen. It is possible to prepare cryosections that are sufficiently thin to avoid excessive plural inelastic scattering so analysis can be performed at 100 keV beam energy. Under optimal conditions, a resolution of 10 nm and detection limits of a few atoms are achievable for elements such as calcium, phosphorus and iron. In the field emission STEM certain types of chemical information can be extracted from biological specimens. Valence EELS has been exploited to measure water distributions in frozen hydrated cryosections

Dynamics and Berry phase of two-species Bose-Einstein condensates

In terms of exact solutions of the time-dependent Schrodinger equation for an effective giant spin modeled from a coupled two-mode Bose-Einstein condensate (BEC) with adiabatic and cyclic time-varying Raman coupling between two hyperfine states of the BEC, we obtain analytic time-evolution formulas of the population imbalance and relative phase between two components with various initial states, especially the SU(2)coherent state. We find the Berry phase depending on the number parity of atoms, and particle number dependence of the collapse revival of population-imbalance oscillation. It is shown that self-trapping and phase locking can be achieved from initial SU(2) coherent states with proper parameters.Comment: 18 pages,5 figure

Periodic instanton method and macroscopic quantum tunneling between two weakly-linked Bose-Einstein condensates

A new method is used to investigate the tunneling between two weakly-linked Bose-Einstein condensates confined in double-well potential traps. The nonlinear interaction between the atoms in each well contributes to a finite chemical potential, which, with consideration of periodic instantons, leads to a remarkably high tunneling frequency. This result can be used to interpret the newly found Macroscopic Quantum Self Trapping (MQST) effect. Also a new kind of first-order crossover between different regions is predicted.Comment: 4 pages, 2 eps figures, final version to appear in Phys. Rev.

Enhancing Optical Up-Conversion Through Electrodynamic Coupling with Ancillary Chromophores

In lanthanide-based optical materials, control over the relevant operating characteristics–for example transmission wavelength, phase and quantum efficiency–is generally achieved through the modification of parameters such as dopant/host combination, chromophore concentration and lattice structure. An alternative avenue for the control of optical response is through the introduction of secondary, codoped chromophores. Here, such secondary centers act as mediators, commonly bridging the transfer of energy between primary absorbers of externally sourced optical input and other sites of frequency-converted emission. Utilizing theoretical models based on experimentally feasible, three-dimensional crystal lattice structures; a fully quantized theoretical framework provides insights into the locally modified mechanisms that can be implemented within such systems. This leads to a discussion of how such effects might be deployed to either enhance, or potentially diminish, the efficiency of frequency up-conversion

The Influence of Molecular Adsorption on Elongating Gold Nanowires

Using molecular dynamics simulations, we study the impact of physisorbing adsorbates on the structural and mechanical evolution of gold nanowires (AuNWs) undergoing elongation. We used various adsorbate models in our simulations, with each model giving rise to a different surface coverage and mobility of the adsorbed phase. We find that the local structure and mobility of the adsorbed phase remains relatively uniform across all segments of an elongating AuNW, except for the thinning region of the wire where the high mobility of Au atoms disrupts the monolayer structure, giving rise to higher solvent mobility. We analyzed the AuNW trajectories by measuring the ductile elongation of the wires and detecting the presence of characteristic structural motifs that appeared during elongation. Our findings indicate that adsorbates facilitate the formation of high-energy structural motifs and lead to significantly higher ductile elongations. In particular, our simulations result in a large number of monatomic chains and helical structures possessing mechanical stability in excess of what we observe in vacuum. Conversely, we find that a molecular species that interacts weakly (i.e., does not adsorb) with AuNWs worsens the mechanical stability of monatomic chains.Comment: To appear in Journal of Physical Chemistry

Exact calculation of the skyrmion lifetime in a ferromagnetic Bose condensate

The tunneling rate of a skyrmion in ferromagnetic spin-1/2 Bose condensates through an off-centered potential barrier is calculated exactly with the periodic instanton method. The prefactor is shown to depend on the chemical potential of the core atoms, at which level the atom tunnels. Our results can be readily extended to estimate the lifetime of other topological excitations in the condensate, such as vortices and monopoles.Comment: 16 pages, 4 figures, to appear Phys. Rev.

Submillimetric GPS distance measurement over short baselines: noise mitigation by global robust estimation

The potential use of GPS technology for precise length determination is currently a topic of extensive research. A prior work was dedicated to submillimetric length determination over short baselines and under ideal conditions of data availability and a clear environment. This paper presents a new computation method suited to the less favourable working conditions that are usually encountered in practice. It is based on both robust estimation theory and the use of an ambiguity-free estimation method. As the experimental comparisons with the standard procedure based on least-squares ambiguity determination show, it provides more stable values and permits results to be obtained significant to the submillimetre level with time spans of a few hours.This research is funded by the Spanish Ministry of Science and Innovation (AYA2011-23232).Baselga Moreno, S.; García-Asenjo Villamayor, L.; Garrigues Talens, P. (2014). Submillimetric GPS distance measurement over short baselines: noise mitigation by global robust estimation. Measurement Science and Technology. 25(10):1-6. https://doi.org/10.1088/0957-0233/25/10/105004S162510Altamimi, Z., Collilieux, X., & Métivier, L. (2011). ITRF2008: an improved solution of the international terrestrial reference frame. Journal of Geodesy, 85(8), 457-473. doi:10.1007/s00190-011-0444-4Amiri-Simkooei, A. R., & Tiberius, C. C. J. M. (2006). Assessing receiver noise using GPS short baseline time series. GPS Solutions, 11(1), 21-35. doi:10.1007/s10291-006-0026-8Baire, Q., Bruyninx, C., Legrand, J., Pottiaux, E., Aerts, W., Defraigne, P., … Chevalier, J. M. (2013). Influence of different GPS receiver antenna calibration models on geodetic positioning. GPS Solutions, 18(4), 529-539. doi:10.1007/s10291-013-0349-1Baselga, S. (2007). Global Optimization Solution of Robust Estimation. Journal of Surveying Engineering, 133(3), 123-128. doi:10.1061/(asce)0733-9453(2007)133:3(123)Baselga, S. (2010). Global optimization applied to GPS positioning by ambiguity functions. Measurement Science and Technology, 21(12), 125102. doi:10.1088/0957-0233/21/12/125102Baselga, S. (2014). Ambiguity-Free Method for Fast and Precise GNSS Differential Positioning. Journal of Surveying Engineering, 140(1), 22-27. doi:10.1061/(asce)su.1943-5428.0000111Baselga, S., & García-Asenjo, L. (2008). GNSS Differential Positioning by Robust Estimation. Journal of Surveying Engineering, 134(1), 21-25. doi:10.1061/(asce)0733-9453(2008)134:1(21)Baselga, S., & García-Asenjo, L. (2008). Multipath Mitigation by Global Robust Estimation. Journal of Navigation, 61(3), 385-392. doi:10.1017/s0373463308004803Baselga, S., García-Asenjo, L., & Garrigues, P. (2013). Submillimetric GPS distance measurement over short baselines: case study in inner consistency. Measurement Science and Technology, 24(7), 075001. doi:10.1088/0957-0233/24/7/075001Dow, J. M., Neilan, R. E., & Rizos, C. (2009). The International GNSS Service in a changing landscape of Global Navigation Satellite Systems. Journal of Geodesy, 83(3-4), 191-198. doi:10.1007/s00190-008-0300-3Griffiths, J., & Ray, J. R. (2012). Sub-daily alias and draconitic errors in the IGS orbits. GPS Solutions, 17(3), 413-422. doi:10.1007/s10291-012-0289-1Huber, P. J. (1981). Robust Statistics. Wiley Series in Probability and Statistics. doi:10.1002/0471725250Koivula, H., Häkli, P., Jokela, J., Buga, A., & Putrimas, R. (2011). GPS Metrology: Bringing Traceable Scale to a Local Crustal Deformation GPS Network. International Association of Geodesy Symposia, 105-112. doi:10.1007/978-3-642-20338-1_13Niu, X., Chen, Q., Zhang, Q., Zhang, H., Niu, J., Chen, K., … Liu, J. (2013). Using Allan variance to analyze the error characteristics of GNSS positioning. GPS Solutions, 18(2), 231-242. doi:10.1007/s10291-013-0324-xRay, J., Altamimi, Z., Collilieux, X., & van Dam, T. (2007). Anomalous harmonics in the spectra of GPS position estimates. GPS Solutions, 12(1), 55-64. doi:10.1007/s10291-007-0067-7Snay, R. A., & Soler, T. (2008). Continuously Operating Reference Station (CORS): History, Applications, and Future Enhancements. Journal of Surveying Engineering, 134(4), 95-104. doi:10.1061/(asce)0733-9453(2008)134:4(95)Wieser, A., & Brunner, F. K. (2002). Short Static GPS Sessions: Robust Estimation Results. GPS Solutions, 5(3), 70-79. doi:10.1007/pl00012901Yang, Y. (1999). Robust estimation of geodetic datum transformation. Journal of Geodesy, 73(5), 268-274. doi:10.1007/s001900050243Yang, Y., Song, L., & Xu, T. (2002). Robust estimator for correlated observations based on bifactor equivalent weights. Journal of Geodesy, 76(6-7), 353-358. doi:10.1007/s00190-002-0256-

Security challenges of small cell as a service in virtualized mobile edge computing environments

Research on next-generation 5G wireless networks is currently attracting a lot of attention in both academia and industry. While 5G development and standardization activities are still at their early stage, it is widely acknowledged that 5G systems are going to extensively rely on dense small cell deployments, which would exploit infrastructure and network functions virtualization (NFV), and push the network intelligence towards network edges by embracing the concept of mobile edge computing (MEC). As security will be a fundamental enabling factor of small cell as a service (SCaaS) in 5G networks, we present the most prominent threats and vulnerabilities against a broad range of targets. As far as the related work is concerned, to the best of our knowledge, this paper is the first to investigate security challenges at the intersection of SCaaS, NFV, and MEC. It is also the first paper that proposes a set of criteria to facilitate a clear and effective taxonomy of security challenges of main elements of 5G networks. Our analysis can serve as a staring point towards the development of appropriate 5G security solutions. These will have crucial effect on legal and regulatory frameworks as well as on decisions of businesses, governments, and end-users