Guided patch-wise nonlocal SAR despeckling

We propose a new method for SAR image despeckling which leverages information drawn from co-registered optical imagery. Filtering is performed by plain patch-wise nonlocal means, operating exclusively on SAR data. However, the filtering weights are computed by taking into account also the optical guide, which is much cleaner than the SAR data, and hence more discriminative. To avoid injecting optical-domain information into the filtered image, a SAR-domain statistical test is preliminarily performed to reject right away any risky predictor. Experiments on two SAR-optical datasets prove the proposed method to suppress very effectively the speckle, preserving structural details, and without introducing visible filtering artifacts. Overall, the proposed method compares favourably with all state-of-the-art despeckling filters, and also with our own previous optical-guided filter

Local helioseismology and the active Sun

The goal of local helioseismology is to elicit three-dimensional information about the sub-surface (or far-side) structure and dynamics of the Sun from observations of the helioseismic wave field at the surface. The physical quantities of interest include flows, sound-speed deviations and magnetic fields. However, strong surface magnetic fields induce large perturbations to the waves making inversions difficult to interpret. The purpose of this paper is to outline the methods of analysis used in local helioseismology, review discoveries associated with the magnetic Sun made using local helioseismology from the past three years, and highlight the efforts towards imaging the interior in the presence of strong magnetic fields.Comment: 6 pages, 4th HELAS International Conference, Lanzarote, Spain, 1-5 February 201

Spin filtering and magnetoresistance in ballistic tunnel junctions

We theoretically investigate magnetoresistance (MR) effects in connection with spin filtering in quantum-coherent transport through tunnel junctions based on non-magnetic/semimagnetic heterostructures. We find that spin filtering in conjunction with the suppression/enhancement of the spin-dependent Fermi seas in semimagnetic contacts gives rise to (i) spin-split kinks in the MR of single barriers and (ii) a robust beating pattern in the MR of double barriers with a semimagnetic well. We believe these are unique signatures for quantum filtering.Comment: Added references + corrected typo

Minimum requirements for feedback enhanced force sensing

The problem of estimating an unknown force driving a linear oscillator is revisited. When using linear measurement, feedback is often cited as a mechanism to enhance bandwidth or sensitivity. We show that as long as the oscillator dynamics are known, there exists a real-time estimation strategy that reproduces the same measurement record as any arbitrary feedback protocol. Consequently some form of nonlinearity is required to gain any advantage beyond estimation alone. This result holds true in both quantum and classical systems, with non-stationary forces and feedback, and in the general case of non-Gaussian and correlated noise. Recently, feedback enhanced incoherent force sensing has been demonstrated [Nat. Nano. \textbf{7}, 509 (2012)], with the enhancement attributed to a feedback induced modification of the mechanical susceptibility. As a proof-of-principle we experimentally reproduce this result through straightforward filtering.Comment: 5 pages + 2 pages of Supplementary Informatio

Deposition of particle pollution in turbulent forced-air cooling

Rotating fans are the prevalent forced cooling method for heat generating equipment and buildings. As the concentration of atmospheric pollutants has increased, the accumulation of microscale and nanoscale particles on surfaces due to advection-diffusion has led to adverse mechanical, chemical and electrical effects that increase cooling demands and reduce the reliability of electronic equipment. Here, we uncover the mechanisms leading to enhanced deposition of particle matter (PM$_{10}$ and PM$_{2.5}$) on surfaces due to turbulent axial fan flows operating at Reynolds numbers, $Re \sim 10^5$. Qualitative observations of long-term particle deposition from the field were combined with \textit{in situ} particle image velocimetry on a telecommunications base station, revealing the dominant role of impingement velocity and angle. Near-wall momentum transport for $10 < y^+ < 50$ were explored using a quadrant analysis to uncover the contributions of turbulent events that promote particle deposition through turbulent diffusion and eddy impaction. By decomposing these events, the local transport behaviour of fine particles from the bulk flow to the surface has been categorised. The transition from deposition to clean surfaces was accompanied by a decrease in shear velocity, turbulent stresses, and particle sweep motions with lower flux in the wall-normal direction. Finally, using these insights, selective filtering of coarse particles was found to promote the conditions that enhance the deposition of fine particle matter

Investigating design features of a computer-mediated communication system

Computer-mediated communication (CMC) is increasingly used in higher education, but it is not without problems. The effectiveness of CMC depends on many factors, including the characteristics of CMC systems themselves. The research reported here therefore aimed to investigate how an educational CMC system might be improved, in order to support learning more effectively. The main context for the research was distance learning at the UK Open University (OU). A two-stage, mixed methods research approach was adopted. In the first stage, interviews and observations were carried out to explore the benefits and problems experienced by users. This revealed two major issues: information overload and lack of social presence. Information overload relates to users’ problems dealing with large numbers of messages. Social presence relates to the need for users to feel connected with each other. The second stage investigated system features aimed at addressing these issues, implemented in a prototype computer conferencing system. Features to address overload included branched message threading and user recommendations. Features to address social presence were ‘résumés’ and instant messaging. These features were evaluated using questionnaires, with several cohorts of students in an OU course. Students expressed approval of the features, although some features were not widely used. Students preferred branched threading to chronological threading because branching helped them to follow ‘conversations’. Students were uncomfortable recommending messages, feeling that the value of a message would vary between people. They were also uncomfortable using instant messaging to contact others whom they did not know. However, the awareness aspect of instant messaging provided a sense of solidarity. The research demonstrated that the problems of overload and lack of social presence are significant, and each has social aspects which must be addressed. Students’ relationships with each other affect whether and how they use the features of CMC systems. We can conclude that particular attention must be paid to the social aspects of online communication, both when designing educational CMC systems and when considering how they are used. To maximise the benefits for learning, students need to feel comfortable with each other online, and there are few short cuts to achieving this

Towards an understanding of jet substructure

We present first analytic, resummed calculations of the rates at which widespread jet substructure tools tag QCD jets. As well as considering trimming, pruning and the mass-drop tagger, we introduce modified tools with improved analytical and phenomenological behaviours. Most taggers have double logarithmic resummed structures. The modified mass-drop tagger is special in that it involves only single logarithms, and is free from a complex class of terms known as non-global logarithms. The modification of pruning brings an improved ability to discriminate between the different colour structures that characterise signal and background. As we outline in an extensive phenomenological discussion, these results provide valuable insight into the performance of existing tools and help lay robust foundations for future substructure studies.Comment: 52 pages, 18 figures. Version to be published in JHEP: added an Appendix about Y-trimming and addressed several points raised by the refere