A comment on ‘A comment on the damping of magnetohydrodynamic waves’ by Bibhas R. De

We point out that in the case of sunspots, when Coulomb collisions are included, the reduction in lundqvist number due to anisotropic conductivity is several orders of magnitude less than the reduction estimated by De. This result suggests that, contrary to De's conclusion, sunspot conditions do permit efficient MHD wave propagation.     

Enhanced emission of Alfvén waves from sunspots during proton flares

Thomas (1978) has shown that, if Alfvén waves exist in a sunspot umbra, they are normally reflected so strongly by the temperature minimum as to be essentially undetectable in the upper solar atmosphere. However, it is known that in many proton flares, chromospheric emission overlies the umbra of a sunspot, indicating that the transition region (TR) between chromosphere and corona in the umbral flux tube has moved down to lower altitudes. As a result of this lowering, umbral Alfvén waves have readier access to the corona: the coronal leakage depends exponentially on the altitude of the TR. We find that the Alfvén wave flux which leaks out of the umbra into the corona can exceed 10⁷ ergs cm^-2 s^-1. A flux of this magnitude is expected to dissipate rapidly in the corona, thereby contributing to a positive feedback loop which ensures prolonged (1 hr) leakage of the umbral Alfvén waves into the corona. We propose that these Alfvén waves may contribute significantly to prolonged energization of proton flares in which umbral coverage occurs.     

Motion of solar cosmic rays in the coronal magnetic field

Trajectories of solar cosmic rays have been calculated in a static ninth-order coronal magnetic field. It is found that as a result of field curvature and gradients, protons drift across the field lines at a rate of up to 200 ² deg hr^–1. These drift rates are of the same order as, but somewhat smaller than, empirically derived rates. Localized enhancements of magnetic field have been inserted into the ninth-order field in order to model (in a highly idealized manner) the effects of the small-scale magnetic features which give rise to X-ray bright points. The motions of the particles in the presence of these scattering centers can be parameterized approximately by a cross-field diffusion coefficient. Our estimates of this coefficient, although crude, overlap with empirical values which have been deduced over a wide range of energies.We propose that coronal propagation of solar cosmic rays has two components. One is independent of particle velocity, and is associated with dynamic field phenomena (such as an expanding magnetic bottle): this is the only component which is important in flares which occur close to the foot-point of the Sun-Earth field line. The second component is velocity dependent, but is independent of mass, and is associated with scattering off (relatively static) magnetic inhomogeneities with scale sizes of at least 500 km: the second component contributes to coronal propagation if the flare occurs more than about 50–60 deg away from the Sun-Earth field line.     

Periodic Modulation of X-Ray Intensity from Coronal Loops – Heating by Resonant Absorption?

We have applied time-series analysis to sequences of coronal loop images obtained with the Yohkoh soft X-ray telescope (SXT). Using images obtained in fast-cadence mode ( t = 4 s), we have generated X-ray light curves and power spectra for multiple subregions of a number of coronal loops. Our analysis indicates that out of 544 spectra of loops which range in length L from 40 Mm to 150 Mm, there are 16 cases where the X-ray brightness is modulated periodically with 99.5% confidence. The periods range from 9.6 s to 61.6 s, and the amplitudes range from 0.4% to 1.8%. Our result is statistically significant at the 8 level.The observed periods can hardly be due to fluctuations in gas pressure: temperatures estimated from image ratios indicate sound speeds of 200–280 km s^-1, with loop crossing times of hundreds of seconds. On the other hand, the ratio 2 L/ yields velocities of 2–14 Mm s^-1: these overlap with published estimates of Alfvén speeds v _A in coronal loops, 3.5–37 Mm s^-1 (Schmelz et al., 1994). Now, in the context of coronal loop heating by resonant absorption of MHD waves, the period _g 2L/v _A plays a key role: loops which are oscillating in their global mode (with period _g) are maximally efficient at absorbing energy (Steinolfson and Davila, 1993). We suggest that the loops which we have found to be modulated periodically are oscillating in their global mode. With this interpretation, we find that the plasma beta in the loops lies in the range _p = 0.001–0.045. According to the oscillating loop model of Zaitsev and Stepanov (1989), loops with such _p values should be modulated with amplitudes of 1.1% or less: our data are consistent with at least the upper limit of this prediction.     

An open source,server-side framework for analytical web mapping and its application to health

In this paper, we detail the design and the implementation of an open source, server-side web mapping framework for the analysis of health data. The framework forms part of a larger project, the goal of which is to provide an analytical web geographical information system (GIS) that enables health experts to analyse spatial aspects of health data. The aim of the framework is to provide a method for the dynamic and flexible spatial visualisation of health data to facilitate data exploration and analysis. Consequently, a dynamic thematic web mapping technique, an extension to the Open Geospatial Consortium (OGC) web map service standard, was developed. The technique combines a data query, processing technique and styling methodology on the fly to generate a thematic map. The resulting thematic map represents a virtual map layer that enables a user to rapidly visually summarise properties of a data-set. A test web interface was developed to assess the efficacy of the web mapping technique. As the dynamic web mapping method builds on existing OGC web mapping standards, it can be readily integrated with the existing lightweight slippy map web clients and virtual globes.