Book Reviews

Book 1Book Title: Health Care as Human RightBook Author: Ed. by Anton van NiekerkPp. 115. Stellenbosch: Unit for Bioethics, University of Stellenbosch. 1993. ISBN 0-79720429- 6.Book 2Book Title: Psychosis and its ManagementBook Author: A.E. GangatPp. xvii + 169. Durban: Valentine Bedford. 1992. ISBN 0-620-16940-0.Book 3Book Title: Health Care in South Africa: Structure and DynamicsBook Authors: H.C.J. van Rensburg, A. Fourie & E. PretoriusPp. xix + 438. lllustrated. Pretoria: Academica. 1992. ISBN 0-86874-471-9Book 4Book Title: Guidelines for Cholera ControlBook Author: WHOPp. vi + 61. (in English; French and Spanish in preparation). $16,20. Geneva: WHO. 1993. Order No. 1150398. ISBN 92-4-15444-9X

Dimerization and Incommensurate Spiral Spin Correlations in the Zigzag Spin Chain: Analogies to the Kondo Lattice

Using the density matrix renormalization group and a bosonization approach, we study a spin-1/2 antiferromagnetic Heisenberg chain with near-neighbor coupling $J_1$ and frustrating second-neighbor coupling $J_2$, particularly in the limit $J_2 >> J_1$. This system exhibits both dimerization and incommensurate spiral spin correlations. We argue that this system is closely related to a doped, spin-gapped phase of the one-dimensional Kondo lattice.Comment: 18 pages, with 13 embedded encapsulated Postscript figures, uses epsf.sty. Corrects a misstatement about the pitch angle, and contains additional reference

Phase diagrams of spin ladders with ferromagnetic legs

The low-temperature properties of the spin S=1/2 ladder with anisotropic ferromagnetic legs are studied using the continuum limit bosonization approach. The weak-coupling ground state phase diagram of the model is obtained for a wide range of coupling constants and several unconventional gapless ''spin-liquid'' phases are shown to exist for ferromagnetic coupling. The behavior of the ladder system in the vicinity of the ferromagnetic instability point is discussed in detail.Comment: 11 pages, 4 figure

Stratorotational instability in MHD Taylor-Couette flows

The stability of dissipative Taylor-Couette flows with an axial stable density stratification and a prescribed azimuthal magnetic field is considered. Global nonaxisymmetric solutions of the linearized MHD equations with toroidal magnetic field, axial density stratification and differential rotation are found for both insulating and conducting cylinder walls. Flat rotation laws such as the quasi-Kepler law are unstable against the nonaxisymmetric stratorotational instability (SRI). The influence of a current-free toroidal magnetic field depends on the magnetic Prandtl number Pm: SRI is supported by Pm > 1 and it is suppressed by Pm \lsim 1. For too flat rotation laws a smooth transition exists to the instability which the toroidal magnetic field produces in combination with the differential rotation. This nonaxisymmetric azimuthal magnetorotational instability (AMRI) has been computed under the presence of an axial density gradient. If the magnetic field between the cylinders is not current-free then also the Tayler instability occurs and the transition from the hydrodynamic SRI to the magnetic Tayler instability proves to be rather complex. Most spectacular is the `ballooning' of the stability domain by the density stratification: already a rather small rotation stabilizes magnetic fields against the Tayler instability. An azimuthal component of the resulting electromotive force only exists for density-stratified flows. The related alpha-effect for magnetic SRI of Kepler rotation appears to be positive for negative d\rho/dz <0.Comment: 10 pages, 13 figures, submitted to Astron. Astrophy

Dynamics of Coronal Bright Points as seen by Sun Watcher using Active Pixel System detector and Image Processing (SWAP), Atmospheric Imaging Assembly AIA), and Helioseismic and Magnetic Imager (HMI)

The \textit{Sun Watcher using Active Pixel system detector and Image Processing}(SWAP) on board the \textit{PRoject for OnBoard Autonomy\todash 2} (PROBA\todash 2) spacecraft provides images of the solar corona in EUV channel centered at 174 \AA. These data, together with \textit{Atmospheric Imaging Assembly} (AIA) and the \textit{Helioseismic and Magnetic Imager} (HMI) on board \textit{Solar Dynamics Observatory} (SDO), are used to study the dynamics of coronal bright points. The evolution of the magnetic polarities and associated changes in morphology are studied using magnetograms and multi-wavelength imaging. The morphology of the bright points seen in low-resolution SWAP images and high-resolution AIA images show different structures, whereas the intensity variations with time show similar trends in both SWAP 174 and AIA 171 channels. We observe that bright points are seen in EUV channels corresponding to a magnetic-flux of the order of $10^{18}$ Mx. We find that there exists a good correlation between total emission from the bright point in several UV\todash EUV channels and total unsigned photospheric magnetic flux above certain thresholds. The bright points also show periodic brightenings and we have attempted to find the oscillation periods in bright points and their connection to magnetic flux changes. The observed periods are generally long (10\todash 25 minutes) and there is an indication that the intensity oscillations may be generated by repeated magnetic reconnection

SPIN AND CHARGE MODES OF THE t-J LADDER

The spin and charge excitations of the t--J ladder are studied by exact diagonalization techniques for several electron densities. The various modes are classified according to their spin (singlet or triplet excitations) and their parity under a reflection with respect to the symmetry axis along the chains and a finite size scaling of the related gaps is performed. At low doping, formation of hole pairs leads to a spin gap for all $J/t$ ratios. This phase is characterized by (at least) one vanishing energy mode {\it only} in the charge bonding channel when $K_x\rightarrow 0$ consistent with the existence of superconducting pairing correlations. At larger doping the spin gap disappears. Although the anti-bonding modes remain gapped, low energy $K_x\sim 0$ and $K_x\sim 2k_F$ spin and charge bonding modes are found consistent with a single band Luttinger scenario. At sufficient low electron density and above a critical value of J/t we also expect another phase of electron pairs with gapped spin excitations.Comment: 4 pages, 11 figs. included in a uuencoded compressed file

Origins of the Ambient Solar Wind: Implications for Space Weather

The Sun's outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress -- in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue connected with a 2016 ISSI workshop on "The Scientific Foundations of Space Weather." 44 pages, 9 figure

On the Application of the Non Linear Sigma Model to Spin Chains and Spin Ladders

We review the non linear sigma model approach (NLSM) to spin chains and spin ladders, presenting new results. The generalization of the Haldane's map to ladders in the Hamiltonian approach, give rise to different values of the $\theta$ parameter depending on the spin S, the number of legs $n_{\ell}$ and the choice of blocks needed to built up the NLSM fields. For rectangular blocks we obtain $\theta = 0$ or $2 \pi S$ depending on wether $n_{\ell}$, is even or odd, while for diagonal blocks we obtain $\theta = 2 \pi S n_{\ell}$. Both results agree modulo $2 \pi$, and yield the same prediction, namely that even ( resp. odd) ladders are gapped (resp. gapless). For even legged ladders we show that the spin gap collapses exponentially with $n_{\ell}$ and we propose a finite size correction to the gap formula recently derived by Chakravarty using the 2+1 NSLM, which gives a good fit of numerical results. We show the existence of a Haldane phase in the two legged ladder using diagonal blocks and finally we consider the phase diagram of dimerized ladders.Comment: 25 pages, Latex, 7 figures in postscript files, Proc. of the 1996 El Escorial Summer School on "Strongly Correlated Magnetic and Superconducting Systems". Some more references are adde

Research and practice priorities in pilonidal sinus disease: a consensus from the PITSTOP study

Aim Pilonidal sinus disease is a common condition treated by colorectal surgeons. There is a lack of literature in the field to guide optimal management of this condition. As part of the PITSTOP study, we aimed to identify policy and research priorities to provide direction to the field. Method Patients and surgeons were invited to participate. A ‘So what, now what’ exercise was conducted, informed by data from PITSTOP. This generated statements for research and practice priorities. A three-round online Delphi study was conducted, ranking statements based on policy and research separately. Statements were rated 1 (not important) to 9 (important). Statements that were rated 7–9 by more than 70% of participants were entered into the consensus meeting. Personalized voting feedback was shown between rounds. A face-to-face meeting was held to discuss statements, and participants were asked to rank statements using a weighted choice vote. Results Twenty-two people participated in the focus group, generating 14 research and 19 policy statements. Statements were voted on by 56 participants in round 1, 53 in round 2 and 51 in round 3. A total of 15 policy statements and 19 research statements were discussed in the consensus round. Key policy statements addressed treatment strategies and intensity, surgeon training opportunities, need for classification and the impact of treatment on return to work. Research recommendations included design of future trials, methodology considerations and research questions. Conclusion This study has identified research and policy priorities in pilonidal sinus disease which are relevant to patients and clinicians. These should inform practice and future research

Classification and stratification in pilonidal sinus disease: findings from the PITSTOP cohort

Aim Research in pilonidal disease faces several challenges, one of which is consistent and useful disease classification. The International Pilonidal Society (IPS) proposed a four-part classification in 2017. The aim of this work was to assess the validity and reliability of this tool using data from the PITSTOP cohort study. Method Face validity was assessed by mapping the items/domains in the IPS tool against tools identified through a systematic review. Key concepts were defined as those appearing in more than two-thirds of published tools. Concurrent and predictive validity were assessed by comparing key patient-reported outcome measures between groups at baseline and at clinic visit. The outcomes of interest were health utility, Cardiff Wound Impact Questionnaire (CWIQ) and pain score between groups. Significance was set at p = 0.05 a priori. Interrater reliability was assessed using images captured during the PITSTOP cohort. Ninety images were assessed by six raters (two experts, two general surgeons and two trainees), and classified into IPS type. Interrater reliability was assessed using the unweighted kappa and unweighted Gwet's AC1 statistics. Results For face validity items represented in the IPS were common to other classification systems. Concurrent and predictive validity assessment showed differences in health utility and pain between groups at baseline, and for some treatment groups at follow-up. Assessors agreed the same classification in 38% of participants [chance-corrected kappa 0.52 (95% CI 0.42–0.61), Gwet's AC1 0.63 (95% CI 0.56–0.69)]. Conclusion The IPS classification demonstrates key aspects of reliability and validity that would support its implementation