Global variation in the long-term seasonal changes observed in ionospheric F-region data

Long-term variability has previously been observed in the relative magnitude of annual and semi-annual variations in the critical frequency (related to the peak electron concentration) of the ionospheric F2 layer (foF2). In this paper we investigate the global patterns in such variability by calculating the time varying power ratio of semi-annual to annual components seen in ionospheric foF2 data sequences from 77 ionospheric monitoring stations around the world. The temporal variation in power ratios observed at each station was then correlated with the same parameter calculated from similar epochs for the Slough/Chilton dataset (for which there exists the longest continuous sequence of ionospheric data). This technique reveals strong regional variation in the data which bear a striking similarity to the regional variation observed in long-term changes to the height of the ionospheric F2 layer. We argue that since both the height and peak density of the ionospheric F2 region are influenced by changes to thermospheric circulation and composition, that the observed long-term and regional variability can be explained by such changes. In the absence of long-term measurements of thermospheric composition, detailed modelling work is required to investigate these processes

Spontaneously modulated spin textures in a dipolar spinor Bose-Einstein condensate

Helical spin textures in a $^{87}$Rb F=1 spinor Bose-Einstein condensate are found to decay spontaneously toward a spatially modulated structure of spin domains. This evolution is ascribed to magnetic dipolar interactions that energetically favor the short-wavelength domains over the long-wavelength spin helix. This is confirmed by eliminating the dipolar interactions by a sequence of rf pulses and observing a suppression of the formation of the short-range domains. This study confirms the significance of magnetic dipole interactions in degenerate $^{87}$Rb F=1 spinor gases

Coherence-enhanced imaging of a degenerate Bose gas

We present coherence-enhanced imaging, an in situ technique that uses Raman superradiance to probe the spatial coherence properties of an ultracold gas. Applying this method, we obtain a spatially resolved measurement of the condensate number and more generally, of the first-order spatial correlation function in a gas of $^{87}$Rb atoms. We observe the enhanced decay of propagating spin gratings in high density regions of a Bose condensate, a decay we ascribe to collective, non-linear atom-atom scattering. Further, we directly observe spatial inhomogeneities that arise generally in the course of extended sample superradiance.Comment: 4 pages, 4 figure

Building a Socio-technical Perspective of Community Resilience with a Semiotic Approach

Situated in the diversity and adversity of real-life contexts facing crisis situations, this research aims at boosting the resilience process within communities supported by digital and social technology. In this paper, eight community leaders in different parts of the world are invited to express their issues and wishes regarding the support of technology to face social challenges. Methods and artefacts based on the Organisational Semiotics (OS) and the Socially-Aware computing have been applied to analyse and consolidate this data. By providing both a systemic view of the problem and also leading to the identification of requirements, the analysis evidences some benefits of the OS-based approach to consolidate perspectives from different real-life scenarios towards building a socio-technical solution

Low energy dynamics of spinor condensates

We present a derivation of the low energy Lagrangian governing the dynamics of the spin degrees of freedom in a spinor Bose condensate, for any phase in which the average magnetization vanishes. This includes all phases found within mean-field treatments except for the ferromagnet, for which the low energy dynamics has been discussed previously. The Lagrangian takes the form of a sigma model for the rotation matrix describing the local orientation of the spin state of the gas

Quantum Dynamics of Atomic Coherence in a Spin-1 Condensate: Mean-Field versus Many-Body Simulation

We analyse and numerically simulate the full many-body quantum dynamics of a spin-1 condensate in the single spatial mode approximation. Initially, the condensate is in a ``ferromagnetic'' state with all spins aligned along the $y$ axis and the magnetic field pointing along the z axis. In the course of evolution the spinor condensate undergoes a characteristic change of symmetry, which in a real experiment could be a signature of spin-mixing many-body interactions. The results of our simulations are conveniently visualised within the picture of irreducible tensor operators.Comment: Accepted for publication for the special issue of "Optics Communications" on Quantum Control of Light and Matte

Kinetic theory and dynamic structure factor of a condensate in the random phase approximation

We present the microscopic kinetic theory of a homogeneous dilute Bose condensed gas in the generalized random phase approximation (GRPA), which satisfies the following requirements: 1) the mass, momentum and energy conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this approach, the condensate influences the binary collisional process between the two normal atoms, in the sense that their interaction force results from the mediation of a Bogoliubov collective excitation traveling throughout the condensate. Furthermore, as long as the Bose gas is stable, no collision happens between condensed and normal atoms. In this paper, we show how the kinetic theory in the GRPA allows to calculate the dynamic structure factor at finite temperature and when the normal and superfluid are in a relative motion. The obtained spectrum for this factor provides a prediction which, compared to the experimental results, allows to validate the GRPA. PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc