Incorporating topography into the multiscale systems for the atmosphere and oceans

Recently, new hyperbolic systems of equations that can be used to describe smooth flows accurately in both the atmosphere and oceans have been developed. These ‘approximate systems’ are derived by slowing down the speed of the fast waves instead of increasing their speed to infinity as in the primitive equations. The approximate systems have a number of theoretical advantages over the traditional systems. The practical implications of some of these advantages have already been demonstrated for the oceanic case. There is another advantage of the new systems that has not been discussed extensively. A model based on either of the new systems can be used to describe different scales of motion, e.g. the large, medium, or small scale. In addition, a mechanism is provided for a smooth transition between these scales. The incorporation of topography into the approximate systems has also not been discussed. To demonstrate the multiscale nature of the transformed systems in the presence of topography, numerical results from a model based on the approximate system for meteorology are compared with analytic solutions for three topographic scales. Removing the horizontal means of the density and pressure, which was necessary to obtain the proper scaling of the equations in the original papers, reduces the truncation error associated with a transformed system near steep mountains. For example, in the atmospheric case a second-order method requires only approximately 10 points across the base of the mountain to achieve a 1% relative error for any of the three topographic solutions during the relevant time scale of the associated motion.     

Energy dissipation and helicity in coronal loops of variable cross-section

A model is developed to describe a coronal loop, which may originate from a photospheric source of smaller size than the coronal radius of the loop. The energy and relative helicity of the loop are evaluated, as are two alternative estimates of the energy available for coronal heating. Both of these estimates are strongly dependent on the size of the photospheric footprint of the loop. A coronal heating rate is then deduced, based on a nanoflare-type scenario, where slowly accumulated energy is rapidly released as heat. An explicit calculation is carried out for one particular choice of loop length and coronal radius, with dissipation timescale and photospheric radius as parameters. Two main conclusions are reached. Firstly, the proposed mechanism can make a significant contribution to coronal heating. Secondly, the mechanism is more effective for a more concentrated photospheric flux source.     

The way forward for coronal heating

Ineke De Moortel, Philippa K Browning, Stephen J Bradshaw, Balázs Pintér and Eduard P Kontar consider approaches to the longstanding and enigmatic problem of coronal heating, as presented at the RAS discussion meeting on 11 January 2008.     

Cation substitution in synthetic meridianiite (MgSO4·11H2O) II: variation in unit-cell parameters determined from X-ray powder diffraction data

We have prepared aqueous MgSO₄ solutions doped with various divalent metal cations (Ni²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Fe²⁺, and Co²⁺) in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. In a companion paper (Fortes et al., in Phys Chem Min 39) we reported the identification of various phases using X-ray powder diffraction, including meridianiite-structured undecahydrates, melanterite- and epsomite-structured heptahydrates, novel enneahydrates and a new octahydrate. In this work we report the changes in unit-cell parameters of these crystalline products where they exist over sufficient dopant concentrations. We find that there is a linear relationship between the rate of change in unit-cell volume as a function of dopant concentration and the ionic radius of the dopant cation; large ions such as Mn²⁺ produce a substantial inflation of the hydrates’ unit-cell volume, whereas smaller ions such as Ni²⁺ produce a modest reduction in unit-cell volume. Indeed, when the data for all hydrates are normalised (i.e., divided by the number of formula units per unit-cell, Z, and the hydration number, n), we find a quantitatively similar relationship for different values of n. Conversely, there is no relationship between the degree of unit-cell inflation or deflation and the limit to which a given cation will substitute into a certain hydrate structure; for example, Co²⁺ and Zn²⁺ affect the unit-cell volume of MgSO₄·11H₂O to a very similar degree, yet the solubility limits inferred in our companion paper are >60 mol. % Co²⁺ and <30 mol. % Zn²⁺.     

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESA's SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.     

Global magnetohydrostatic fields in stellar atmosphere

Abstract

The equilibrium properties of the magnetic field of an axisymmetric star are studied. A family of analytical solutions to the magnetohydrostatic equations is found, which are used to model the slow evolution of the field through a series of equilibria.

Firstly, a model is set up for a force-free dipole-like field, which has a toroidal field component; it is found that, as such a field is twisted up, a critical point is reached, at which the field topology changes. If the twist is increased beyond this point, there is no physically reasonable equilibrium. Next, an untwisted magnetostatic dipole-like field is studied, with an increasing pressure differential between pole and equator. A critical point again occurs when the pressure differential becomes too large. Finally a force-free quadrupole-like field is modelled, which is being twisted up, for example by differential rotation; this has similar properties to the dipole-like field. In each case, it is suggested that, when the critical point is reached, the field will no longer evolve smoothly, but will change catastrophically to a new stable, releasing energy. Such an event could represent the onset of a stellar flare or some other dynamic stellar process.     

Low abundance of the dinoflagellates,Pfiesteria piscicida, P. shumwayae, andCryptoperidiniopsis spp., in South Carolina tidal creeks and open estuaries

The toxicPfiesteria complex are a group of dinoflagellates that have received considerable attention in recent years as causative factors in fish kill or lesion events in North Carolina estuaries and in the Pocomoke River of Chesapeake Bay. In response to the potentialPfiesteria threat, the South Carolina Task Group on Harmful Algae was formed in late 1997 and implemented programs to monitor harmful algal blooms and respond to fish kills or lesion events with particular emphasis on the Bushy Park (Cooper River, Charleston) region, a site of annually recurrent menhaden lesion events.Pfiesteria piscicida, Pfiesteria shumwayae, andCryptoperidiniopsis spp. were documented in South Carolina estuaries. Routine monitoring and fish kill or lesion event sampling consistently indicated low abundances compared to estimates from similar programs in North Carolina and Maryland that sampled areas with a history ofPfiesteria toxic activity. The finding thatPfiesteria-like organism (PLO) abundances were always low in samples collected during menhaden lesion events in Bushy Park suggested other causes for lesion progression, althoughPfiesteria spp. could not be ruled out as a factor in lesion initiation. Based on the previously demonstrated positive relationship between PLO abundance, chlorophylla, and inorganic nutrient concentrations (in laboratory experiments and North Carolina field observations), we hypothesized that the relatively low abundance ofPfiesteria spp. and other PLO (e.g.,Cryptoperidiniopsis) in South Carolina estuaries is related to the relatively low supply of phytoplankton prey, as supported by interstate comparisons in chlorophylla concentrations. Nitrate concentrations were generally much lower in South Carolina estuaries. Estuarine eutrophication may be an important consideration in explaining interstate differences in susceptibility toPfiesteria-related toxic events.     

Coronal magnetic field equilibrium with discrete flux sources

A model of the equilibrium structure of the coronal magnetic field is developed, taking account of the fact that field lines are rooted in the photosphere, where field is concentrated into isolated flux tubes. The field is force-free, described by ×B = B, with constant; this field has special physical significance, being the state of mininum energy after small-scale reconnections, and is also mathematically convenient in that the principle of superposition can be used to construct complex geometries. First a model of a single loop is presented, with a flux source and sink pair at the photosphere; both point flux tubes and finite radius flux tubes are considered. Then more complex topologies with multiple sources and sinks are investigated. It is shown that significant topology changes arise for different values of, indicating the possibility that there can be energy changes through magnetic reconnection if the field evolves ideally and then relaxes to a linear state.     

Discovery of the “Phantom” dinoflagellate in Chesapeake Bay

Since its discovery in natural estuarine habitat of North Carolina in 1991, the widespread impact of the toxic dinoflagellate, Pfiesteria piscicida (gen. et sp. nov.), popularly called the “phantom” dinoflagellate, on North Carolina fish stocks has been established, yet little is known about its influence outside of North Carolina estuaries. Here, we document the presence of P. piscicida in Chesapeake Bay. A fish kill was observed after inoculating an aquarium containing mummichogs with sediment samples from Jenkins Creek, a brackish creek (salinity 11‰) of the Chesapeake Bay system. P. piscicida was the cause of the kill, as supported by morphological, physiological, and histological evidence. The appearance and behavior of the algae and symptoms associated with fish mortality were consistent with those previously observed in P. piscicida-associated aquaria fish kills in North Carolina. The discovery of P. piscicida in Chesapeake Bay supports the speculation that these toxic dinoflagellates have a dramatic and far-reaching impact on fish stocks in shallow, eutrophic estuaries along the eastern United States.