Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales

Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well-known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: i) it is much faster, developing on scales of the order of the Alfv\'en time, and ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed circulation patterns where the jets going out of the reconnection regions turn around and forces their way back in, carrying along copious amounts of magnetic flux

On the nature of reconnection at a solar coronal null point above a separatrix dome

Three-dimensional magnetic null points are ubiquitous in the solar corona, and in any generic mixed-polarity magnetic field. We consider magnetic reconnection at an isolated coronal null point, whose fan field lines form a dome structure. We demonstrate using analytical and computational models several features of spine-fan reconnection at such a null, including the fact that substantial magnetic flux transfer from one region of field line connectivity to another can occur. The flux transfer occurs across the current sheet that forms around the null point during spine-fan reconnection, and there is no separator present. Also, flipping of magnetic field lines takes place in a manner similar to that observed in quasi-separatrix layer or slip-running reconnection.Comment: Accepted for publication in the Astrophysical Journa

The Role Of Molecular Motors In Peripheral Nerve Regeneration

Following injury, axons of the peripheral nervous system have retained the capacity for regeneration. While it is well established that injury signals require molecular motors for their transport from the injury site to the nucleus, whether kinesin and dynein motors play additional roles in peripheral nerve regeneration is not well understood. Here we use genetic mutants of motor proteins in a zebrafish peripheral nerve regeneration model to visualize and define in vivo roles for kinesin and dynein. We find that both kinesin-1 and dynein are required for zebrafish peripheral nerve regeneration. While loss of kinesin-1 reduced the overall robustness of axonal regrowth, loss of dynein dramatically impaired axonal regeneration and also reduced injury-induced Schwann cell remodeling. Chimeras between wild type and dynein mutant embryos demonstrate that dynein function in neurons is sufficient to promote axonal regrowth. Finally, by simultaneously monitoring actin and microtubule dynamics in regenerating axons we find that dynein appears dispensable to initiate axonal regrowth, but is critical to stabilize microtubules, thereby sustaining axonal regeneration. These results reveal two previously unappreciated roles for dynein during peripheral nerve regeneration, initiating injury induced Schwann cell remodeling and stabilizing axonal microtubules to sustain axonal regrowth

Geotechnical aspects of tunnelling in discontinuous rock, with particular reference to the lower chalk

Discontinuities, defined as breaks or interruptions of the mechanical properties of a solid, are recognised in the literature as being of vital importance in controlling the behaviour of rock during tunnelling. This thesis presents a detailed study of an experimental tunnel excavated in discontinuous rock (Lower Chalk) at Chinnor in Oxfordshire. The work falls into three main areas of study. Prior to tunnel excavation, detailed discontinuity surveys were carried out using an orthogonal scanline technique on exposed faces in the Chinnor quarry and in a 3m diameter shaft. This work, supported by subsequent surveys in the tunnel during its excavation- established - in numerical terms - the discontinuity characteristics of the Lower Chalk at Chinnor.p Ground behaviour during tunnel excavation was monitored in boreholes, and in a trench excavated across the tunnel line. Ground movement results, when viewed in the light of data obtained from finite element analyses, have emphasised the non-elastic discontinuous response of the ground. The actual behaviour of the ground at Chinnor was found to be equivalent to that of a material having a deformation modulus that was approximately l/20th of the Young's modulus of chalk determined from laboratory tests on intact samples. This modulus reduction was a direct result of the interaction between the tunnel excavation system and the discontinuous ground. Having established the role of discontinuities in controlling the behaviour of the ground during tunnelling, and recognising the importance of discontinuity spacing as an index of discontinuity frequency, a detailed study of various statistical and geotechnical aspects of discontinuity spacings in rock is presented. The distribution of discontinuity spacing values obtained from measurements in three tunnels in the UK was found to closely follow a negative exponential distribution. The implications of this relation are that a single parameter expression can be, adopted to describe the discontinuity spacing characteristics. By integrating this expression between the appropriate limits, a formula giving a precise value for the RQD (Rock Quality Designation) of the reck exposed can be obtained. In addition, important conclusions concerning optimum sample size and precision can be made once the discontinuity spacing distribution has been described in these simple mathematical terms. Subsidiary to the three main areas of study outlined above, three additional peripheral subjects are considered. These include an evaluation of the hydrological characteristics of the chalk and an examination of chalk cutability. In addition, data concerning the shear strength characteristics of discontinuity surfaces in the chalk (obtained from laboratory tests) are incorporated in an analysis of stability near the tunnel at Chinnor, using a computer program on file at the University of Durham

Role of the heat shock transcription factor, Hsf1, in a major fungal pathogen that is obligately associated with warm-blooded animals

Peer reviewedPublisher PD

Flood risk management and ‘fairness’: aspirations and reality

Flood risk management in United Kingdom has been going through a process of rapid change in the last decade or so, no doubt spurred on by a series of very serious floods since the year 2000. These changes affect flood defence and non-structural flood risk management measures alike, and involve a degree of devolution from central government to local communities and regional organisations, as central government seeks to shed responsibilities for policy implementation. This paper discusses three case studies concerning flood defence, property level protection, approaches to social justice. The results show a different pattern in each area, with flood defence moving somewhat towards a Rawlsian approach, but flood insurance and property level protection showing signs of both inefficiency and poor penetration, respectively, particularly with regard to low income residents, especially those in social housing