Fresh-water lenses and practical limitations of their three-dimensional simulation

Fresh-water lenses are the major sources of water supply in many atoll islands in the Pacific and Indian Oceans, particularly in dry seasons. Several two- and three-dimensional models are currently available for the simulation of atoll-island aquifers; however, 2D models cannot include 3D spatial variability of material properties, they must simplify the boundary conditions, and they cannot correctly simulate pumping wells. In an attempt to overcome these difficulties, a 3D model, SALTFLOW, was adopted for the simulation of Home Island in the Indian Ocean. This exercise required a discretisation on the order of a few metres and time steps of a few hours requiring significantly high CPU times. High CPU demand proved to be a difficult challenge but cannot be considered a serious practical limitation with today's advanced computers. The exhaustive data demands of the model (e.g., 3D distributions of hydraulic conductivity, porosity, dispersivities, and spatial and temporal variations of recharge and extraction rates) proved to be more problematical. Although the Home Island data set is unusually comprehensive by any standards, nonetheless the quality and quantity of the available data proved inadequate to meet the calibration needs of a highly karstic aquifer system. The Home Island modeling demonstrates the practical limitations of 3D models. It raises the concern that our ability to develop computer codes capable of simulating complex systems now exceeds our ability to supply the input data necessary for reliable calibration. Finally, the paper demonstrates the importance of the transient calibration in reliable simulation of various management options and emphasises that transient calibration should be considered as an integral part of any similar 2D or 3D modeling. Electronic Publication     

Active region magnetic fields

The tilt angles of sunspot groups are defined, using the Mount Wilson data set. It is shown that groups with tilt angles greater than or less than the average value (≈ 5 deg) show different latitude dependences. This effect is also seen in synoptic magnetic field data defining plages. The fraction of the total sunspot group area that is found in the leading spots is discussed as a parameter that can be useful in studying the dynamics of sunspot groups. This parameter is larger for low tilt angles, and small for extreme tilt angles in either direction. The daily variations of sunspot group tilt angles are discussed. The result that sunspot tilt angles tend to rotate toward the average value is reviewed. It is suggested that at some depth, perhaps 50 Mm, there is a flow relative to the surface that results from a rotation rate faster than the surface rate by about 60 m/sec and a meridional drift that is slower than the surface rate by about 5 m/sec. This results in a slanted relative flow at that depth that is in the direction of the average tilt angle and may be responsible for the tendency for sunspot groups (and plages) to rotate their magnetic axes in the direction of the average tilt angle.     

Exploring coronal structures with SOHO

We applied advanced image enhancement techniques to explore in detail the characteristics of the small-scale structures and/or the low contrast structures in several Coronal Mass Ejections (CMEs) observed by SOHO. We highlight here the results from our studies of the morphology and dynamical evolution of CME structures in the solar corona using two instruments on board SOHO: LASCO and EIT.     

High Angular Resolution Studies of Coronal Structures with SOHO

Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOHO: the Large Angle Spectrometric Coronagraph (LASCO) and the ExtremeUltraviolet Imaging Telescope (EIT). We have applied advanced image resolutionenhancement techniques to explore in detail the characteristics of thesmall-scale structures and/or the low contrast structures in the solarcorona. We describe here the results from these high-angular resolutionstudies, including of the kinematics of several Coronal Mass Ejections andpolar jets.     

An Issue of Permanence: Assessing the Effectiveness of Temporary Carbon Storage

In this paper, we present a method to quantify the effectiveness of carbon mitigation options taking into account the `permanence' of the emissions reduction. While the issue of permanence is most commonly associated with a `leaky' carbon sequestration reservoir, we argue that this is an issue that applies to just about all carbon mitigation options. The appropriate formulation of this problem is to ask `what is the value of temporary storage?' Valuing temporary storage can be represented as a familiar economic problem, with explicitly stated assumptions about carbon prices and the discount rate. To illustrate the methodology, we calculate the sequestration effectiveness for injecting CO₂ at various depths in the ocean. Analysis is performed for three limiting carbon price assumptions: constant carbon prices (assumes constant marginal damages), carbon prices rise at the discount rate (assumes efficient allocation of a cumulative emissions cap without a backstop technology), and carbon prices first rise at the discount rate but become constant after a given time (assumes introduction of a backstop technology). Our results show that the value of relatively deep ocean carbon sequestration can be nearly equivalent to permanent sequestration if marginal damages (i.e., carbon prices) remain constant or if there is a backstop technology that caps the abatement cost in the not too distant future. On the other hand, if climate damages are such as to require a fixed cumulative emissions limit and there is no backstop, then a storage option with even very slow leakage has limited value relative to a permanent storage option.     

Tilt-angle variations of active regions

An examination of the tilt angles of multi-spot sunspot groups and plages shows that on average they tend to rotate toward the average tilt angle in each hemisphere. This average tilt angle is about twice as large for plages as it is for sunspot groups. The larger the deviation from the average tilt angle, the larger, on average, is the rotation of the magnetic axis in the direction of the average tilt angle. The rate of rotation of the magnetic axis is about twice as fast for sunspot groups as it is for plages. Growing plages and spot groups rotate their axes significantly faster than do decaying plages and spot groups. There is a latitude dependence of this effect that follows Joy's law. The fact that these tilt angles move toward the average tilt angle and not toward 0 deg (the east-west orientation), combined with other results presented here, suggest that a commonly accepted view of the origin of active region magnetic flux at the solar surface may have to be re-examined.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

The east-west inclination of magnetic field lines in sunspots

Digitized Mount Wilson sunspot data covering the interval from 1917 to 1985 are analyzed to examine the average areas of individual sunspot umbrae over small zones of central meridian distance. Assuming that systematic, east-west differences in these quantities are due to the inclination of the magnetic fields of the spots, one can calculate average east-west inclination angles for all spots and for subsets of the full data set. It is found from such an analysis that on average spot fields are inclined such as to trail the rotation by a few deg. Leading and following spots may show a tendency to be inclined slightly away from each other, in contrast to the results of an earlier study of plage magnetic fields. Growing spots tend to be inclined much more to the east than decaying spots. This is in the opposite sense to the analogous result derived from plage magnetic fields.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Heavy-element abundances from a neutron burst that produces Xe-H

Abstract— We examine quantitatively the suggestion that the heavy anomalous isotopes of Xe-HL found in meteoritic diamonds were produced by a short intense neutron burst and then implanted into the diamonds. Using a large nuclear reaction network we establish one (out of many) neutron irradiation histories that successfully reproduces the heavy isotopes of Xe-HL, and then evaluate what that same history would produce in every heavy element. This has become more relevant following recent measurement of anomalous Ba and Sr in those same diamond samples. Therefore we offer these calculations as a guide to the anomalies to be expected in all elements if this scenario is correct. We also discuss several other aspects of the problem, especially the established contradictions for Ba, the observed Kr pattern, the near normalcy of ¹²⁹Xe, and some related astrophysical ideas. In particular we argue from p-process theory that the observed deficit of ⁷⁸Kr in correlation with ^124–126Xe excess implicates Type II supernovae as the diamond sources. However, our more complete astrophysical conclusions will be published elsewhere. This present work is offered as computational expectation for this class of models and as a guide to considerations that may accelerate the digestion of new experimental results in the diamonds.     

Some characteristics of the development and decay of active region magnetic flux

Mount Wilson synoptic data of both plages and sunspots are examined in an effort to determine in some detail the manner of the appearance and disappearance of the magnetic flux of active regions at the solar surface. Separating regions into leading and following portions by magnetic polarity in the case of the plages and by position in the case of sunspots (for which there is no magnetic information available in this data set), various characteristics of these features are studied, namely their rotation, their relative longitudinal motions, and the east-west inclinations of their magnetic fields. The evidence, taken together, suggests that the magnetic flux loops which comprise a region rise to the surface at the time of its formation, and (at least some of them) sink back below the surface at the time of the decay of the region. It is likely that not all the magnetic flux that arises sinks again below the surface.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.