Magnetostatic equilibria for coronal loops on rotating stars

We have investigated magnetostatic equilibria for coronal loops embedded in a potential magnetic field on a rotating star. We find that for any given star, there is a maximum value of the plasma pressure inside a single loop, above which no equilibrium exists. This maximum internal pressure depends on the ratio of the temperatures inside and outside the loop, and on the ratio of the plasma pressure to the magnetic pressure at the base of the external field. Thus, any loop of a large-scale field which is heated or cooled to a different temperature from its immediate surroundings, or which experiences a change in its internal pressure may eventually lose equilbrium. For some values of the base pressure and temperature ratio the relation between summit height and footpoint separation is double-valued. As the summit height of a loop is increased, its footpoint separation increases to a critical value, then decreases to zero at the maximum possible summit height. At the critical footpoint separation the slope of the loop height-footpoint separation relation becomes infinite, and no equilibrium solution exists for greater footpoint separations.We find also that the strength and scale of the field external to the flux tube is the most important factor in determining its maximum height. The effects of varying the stellar rotation rate - and, hence, the variation in pressure with height - are comparatively unimportant, even for very high rotation rates at which the point of balance between gravitational and centrifugal forces lies close to the stellar surface. In this case it is possible to find equilibrium loop solutions whose summits lie outside the centrifugal balance point.We have also investigated the effects of varying the stellar surface gravity. For stellar of fixed mass and rotation rate, the loop dimensions scale approximately linearly with the stellar radius.     

On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy

We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection, pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/B:SOLA.0000021801.83038.aa     

Anisotropy and depth‐related heterogeneity of hydraulic conductivity in a bog peat. II: modelling the effects on groundwater flow

Anisotropy and heterogeneity of hydraulic conductivity (K) are seldom considered in models of mire hydrology. We investigated the effect of anisotropy and heterogeneity on groundwater flow in bog peat using a steady‐state groundwater model. In five model simulations, four sets of K data were used. The first set comprised measured K values from an anisotropic and heterogeneous bog peat. These data were aggregated to produce the following simplified data sets: an isotropic and heterogeneous distribution of K; an isotropic and homogeneous distribution; and an anisotropic and homogeneous distribution. We demonstrate that, where anisotropy and heterogeneity exist, groundwater flow in bog peat is complex. Fine‐scale variations in K have the potential to influence patterns and rates of groundwater flow. However, for our data at least, it is heterogeneity and not anisotropy that has the greater influence on producing complex patterns of groundwater flow. We also demonstrate that patterns and rates of groundwater flow are simplified and reduced when measured K values are aggregated to create a more uniform distribution of K. For example, when measured K values are aggregated to produce isotropy and homogeneity, the rate of modelled seepage is reduced by 28%. We also show that when measured K values are used, the presence of a drainage ditch can increase seepage through a modelled cross‐section. Our work has implications for the accurate interpretation of hydraulic head data obtained from peat soils, and also the understanding of the effect of drainage ditches on patterns and rates of groundwater flow. Copyright © 2002 John Wiley & Sons, Ltd.     

An experimental test of Henry's Law in solid metal‐liquid metal systems with implications for iron meteorites

Abstract— Experimental solid metal‐liquid metal partition coefficients have been used to model the crystallization of magmatic iron meteorites and understand the evolution of asteroid cores. However, the majority of the partitioning experiments have been conducted with trace elements doped at levels that are orders of magnitude higher than measured in iron meteorites. Concern about Henry's Law and the unnatural doping levels have been cited as one reason that two recent iron meteorite studies have dismissed the experimental partition coefficients in their modeling. Using laser ablation ICP‐MS analysis, this study reports experimentally determined solid metal‐liquid metal trace element partition coefficients from runs doped down to the levels occurring in iron meteorites. The analyses for 12 trace elements (As, Co, Cr, Cu, Ga, Ge, Ir, Os, Pd, Pt, Re, and W) show no deviations from Henry's Law, and these results support decades of experimental work in which the partition coefficients were assumed to be independent of trace element concentration. Further, since our experiments are doped with natural levels of trace elements, the partitioning results are directly applicable to iron meteorites and should be used when modeling their crystallization. In contrast, our new Ag data are inconsistent with previous studies, suggesting the high Ag‐content in previous studies may have influenced the measured Ag partitioning behavior.