Thermodynamics/dynamics coupling in weakly compressible turbulent stratified fluids

In traditional and geophysical fluid dynamics, it is common to describe stratified turbulent fluid flows with low Mach number and small relative density variations by means of the incompressible Boussinesq approximation. Although such an approximation is often interpreted as decoupling the thermodynamics from the dynamics, this paper reviews recent results and derive new ones that show that the reality is actually more subtle and complex when diabatic effects and a nonlinear equation of state are retained. Such an analysis reveals indeed: (1) that the compressible work of expansion/contraction remains of comparable importance as the mechanical energy conversions in contrast to what is usually assumed; (2) in a Boussinesq fluid, compressible effects occur in the guise of changes in gravitational potential energy due to density changes. This makes it possible to construct a fully consistent description of the thermodynamics of incompressible fluids for an arbitrary nonlinear equation of state; (3) rigorous methods based on using the available potential energy and potential enthalpy budgets can be used to quantify the work of expansion/contraction B in steady and transient flows, which reveals that B is predominantly controlled by molecular diffusive effects, and act as a significant sink of kinetic energy.

Excitation and decay of solar-wind driven flows in the magnetosphere-ionosphere system

Basic concepts of the form of high-latitude ionospheric flows and their excitation and decay are discussed in the light of recent high time-resolution measurements made by ground-based radars. It is first pointed out that it is in principle impossible to adequately parameterize these flows by any single quantity derived from concurrent interplanetary conditions. Rather, even at its simplest, the flow must be considered to consist of two basic time-dependent components. The first is the flow driven by magnetopause coupling processes alone, principally by dayside reconnection. These flows may indeed be reasonably parameterized in terms of concurrent near-Earth interplanetary conditions, principally by the interplanetary magnetic field (IMF) vector. The second is the flow driven by tail reconnection alone. As a first approximation these flows may also be parameterized in terms of interplanetary conditions, principally the north-south component of the IMF, but with a delay in the flow response of around 30-60 min relative to the IMF. A delay in the tail response of this order must be present due to the finite speed of information propagation in the system, and we show how "growth" and "decay" of the field and flow configuration then follow as natural consequences. To discuss the excitation and decay of the two reconnection-driven components of the flow we introduce that concept of a flow-free equilibrium configuration for a magnetosphere which contains a given (arbitrary) amount of open flux. Reconnection events act either to create or destroy open flux, thus causing departures of the system from the equilibrium configuration. Flow is then excited which moves the system back towards equilibrium with the changed amount of open flux. We estimate that the overall time scale associated with the excitation and decay of the flow is about 15 min. The response of the system to both impulsive (flux transfer event) and continuous reconnection is discussed in these terms.

The excitation of ionospheric convection

This review presents recent observations of high-latitude ionospheric plasma convection, obtained using the EISCAT radar in the 'Polar' experiment mode. The paper is divided into two main parts. Firstly, the delay in the response of dayside high-latitude flows to changes in the interplanetary magnetic field is discussed. The results show the importance for the excitation of dayside convection of the transfer of magnetic flux from the dayside into the tail lobe. Consequently, ionospheric convection should be thought of as the sum of two intrinsically time-dependent flow patterns. The first of these patterns is directly driven by solar wind-magnetosphere coupling, dominates ionospheric flows on the dayside, is associated with an expanding polar cap area and is the F-region flow equivalent of the DP-2 E-region current system. The second of the two patterns is driven by the release of energy stored in the geomagnetic tail, dominates ionospheric flows on the nightside, is associated with a contracting polar cap and is equivalent to the DP-1, or substorm, current system. In the second half of the paper, various transient flow bursts observed in the vicinity of the dayside cusp are studied. These radar data, combined with simultaneous optical observations of transient dayside aurorae, strongly suggest that momentum is transferred across the magnetopause and into the ionosphere in a series of bursts, each associated with voltages of 30-80 kV. Similarities between these bursts and flux transfer events observed at the magnetopause are discussed.

The excitation of plasma convection in the high-latitude ionosphere

Recent observations of ionospheric flows by ground-based radars, in particular by the European Incoherent Scatter (EISCAT) facility using the “Polar” experiment, together with previous analyses of the response of geomagnetic disturbance to variations of the interplanetary magnetic field (IMF), suggest that convection in the high-latitude ionosphere should be considered to be the sum of two intrinsically time-dependent patterns, one driven by solar wind-magnetosphere coupling at the dayside magnetopause, the other by the release of energy in the geomagnetic tail (mainly by dayside and nightside reconnection, respectively). The flows driven by dayside coupling are largest on the dayside, where they usually dominate, are associated with an expanding polar cap area, and are excited and decay on ∼10-min time scales following southward and northward turnings of the IMF, respectively. The latter finding indicates that the production of new open flux at the dayside magnetopause excites magnetospheric and ionospheric flow only for a short interval, ∼10 min, such that the flow driven by this source subsequently decays on this time scale unless maintained by the production of more open flux tubes. Correspondingly, the flows excited by the release of energy in the tail, mainly during substorms, are largest on the nightside, are associated with a contracting polar cap boundary, and are excited on ∼1-hour time scales following a southward turn of the IMF. In general, the total ionospheric flow will be the sum of the flows produced by these two sources, such that due to their different response times to changes in the IMF, considerable variations in the flow pattern can occur for a given direction and strength of the IMF. Consequently, the ionospheric electric field cannot generally be regarded as arising from a simple mapping of the solar wind electric field along open flux tubes.

Solar influences on dynamical coupling between the stratosphere and troposphere

We use a simplified atmospheric general circulation model (AGCM) to investigate the response of the lower atmosphere to thermal perturbations in the lower stratosphere. The results show that generic heating of the lower stratosphere tends to weaken the sub-tropical jets and the tropospheric mean meridional circulations. The positions of the jets, and the extent of the Hadley cells, respond to the distribution of the stratospheric heating, with low latitude heating displacing them poleward, and uniform heating displacing them equatorward. The patterns of response to the low latitude heating are similar to those found to be associated with solar variability in previous observational data analysis, and to the effects of varying solar UV radiation in sophisticated AGCMs. In order to investigate the chain of causality involved in converting the stratospheric thermal forcing to a tropospheric climate signal we conduct an experiment which uses an ensemble of model spin-ups to analyse the time development of the response to an applied stratospheric perturbation. We find that the initial effect of the change in static stability at the tropopause is to reduce the eddy momentum flux convergence in this region. This is followed by a vertical transfer of the momentum forcing anomaly by an anomalous mean circulation to the surface, where it is partly balanced by surface stress anomalies. The unbalanced part drives the evolution of the vertically integrated zonal flow. We conclude that solar heating of the stratosphere may produce changes in the circulation of the troposphere even without any direct forcing below the tropopause. We suggest that the impact of the stratospheric changes on wave propagation is key to the mechanisms involved.

Atmospheric electricity coupling between earthquake regions and the ionosphere

We propose a mechanism to explain suggested links between seismic activity and ionospheric changes detected overhead. Specifically, we explain changes in the natural extremely low-frequency (ELF) radio noise recently observed in the topside ionosphere aboard the DEMETER satellite at night, before major earthquakes. Our mechanism utilises increased electrical conductivity of surface layer air before a major earthquake, which reduces the surface-ionosphere electrical resistance. This increases the vertical fair weather current, and (to maintain continuity of electron flow) lowers the ionosphere. Magnitudes of crucial parameters are estimated and found to be consistent with observations. Natural variability in ionospheric and atmospheric electrical properties is evaluated, and may be overcome using a hybrid detection approach. Suggested experiments to investigate the mechanism involve measuring the cut-off frequency of ELF “tweeks”, the amplitude and phase of very low frequency radio waves in the Earth–ionosphere waveguide, or medium frequency radar, incoherent scatter or rocket studies of the lower ionospheric electron density.

On the relationship of normal modes to local modes in molecular vibrations

A simple model for the effective vibrational hamiltonian of the XH stretching vibrations in H2O, NH3 and CH4 is considered, based on a morse potential function for the bond stretches plus potential and kinetic energy coupling between pairs of bond oscillators. It is shown that this model can be set up as a matrix in local mode basis functions, or as a matrix in normal mode basis functions, leading to identical results. The energy levels obtained exhibit normal mode patterns at low vibrational excitation, and local mode patterns at high excitation. When the hamiltonian is set up in the normal mode basis it is shown that Darling-Dennison resonances must be included, and simple relations are found to exist between the xrs, gtt, and Krrss anharmonic constants (where the Darling-Dennison coefficients are denoted K) due to their contributions from morse anharmonicity in the bond stretches. The importance of the Darling-Dennison resonances is stressed. The relationship of the two alternative representations of this local mode/normal mode model are investigated, and the potential uses and limitations of the model are discussed.

C---H local modes in cyclobutene, part I: FTIR studies, 600 to 10000 cm–1

The vibrational structure of C---H stretching states in gas-phase cyclobutene was studied using FTIR spectroscopy in the range 700–9000 cm−1. The structure was modelled using two effective vibrational Hamiltonians, one for each type of C---H bond present, consisting of local mode basis functions subject to coupling with symmetrically equivalent bonds and to Fermi resonances with suitable low frequency vibrations. Best-fit model parameters were determined using least-squares routines and the model predictions are compared to the observed band positions and intensities. Some discussion is given of the relevance of the observed couplings to intramolecular vibrational redistribution (IVR) which results in the observation of statistical behaviour in cyclobutene isomerization induced by excitation of C---H stretching overtones in the visible region.

C-H and C-D interbond coupling in CHDCl2

Laser photoacoustic spectra of vapour phase CHDCl2 reveal the presence of an interaction which has been ascribed to interbond coupling between C-H and C-D local modes. The absolute value of the interbond coupling parameter for the CHD group, determined from a fit of a model local mode hamiltonian to the experimental data, is shown to be given approximately by the geometric mean of the interbond coupling parameters of the CH2 and CD2 groups recently derived from similar studies of CH2Cl2 and CD2Cl2. Such behaviour is understood in terms of a simple analysis in which kinetic coupling effects dominate. It is suggested that C-H stretch/bend Fermi resonance is responsible for some weaker features in the spectra and modelling calculations are described which allow an order of magnitude estimate of the size of the coupling parameter involved.

Effects of solar wind magnetosphere coupling recorded at different geomagnetic latitudes: Separation of directly-driven and storage/release systems

The effect on geomagnetic activity of solar wind speed, compared with that of the strength of the interplanetary magnetic field, differs with geomagnetic latitude. In this study we construct a new index based on monthly standard deviations in the H-component of the geomagnetic field for all geomagnetic latitudes. We demonstrate that for this index the response at auroral regions correlates best with interplanetary coupling functions which include the solar wind speed while mid- and low-latitude regions respond to variations in the interplanetary magnetic field strength. These results are used to isolate the responsible geomagnetic current systems.

Sonogashira coupling on an extended gold surface in vacuo: reaction of phenylacetylene with iodobenzene on Au(111)

Temperature-programmed reaction measurements supported by scanning tunneling microscopy have shown that phenylacetylene and iodobenzene react on smooth Au(111) under vacuum conditions to yield biphenyl and diphenyldiacetylene, the result of homocoupling of the reactant molecules. They also produce diphenylacetylene, the result of Sonogashira cross-coupling, prototypical of a class of reactions that are of paramount importance in synthetic organic chemistry and whose mechanism remains controversial. Roughened Au(111) is completely inert toward all three reactions, indicating that the availability of crystallographically well-defined adsorption sites is crucially important. High-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy show that the reactants are initially present as intact, essentially flat-lying molecules and that the temperature threshold for Sonogashira coupling coincides with that for C−I bond scission in the iodobenzene reactant. The fractional-order kinetics and low temperature associated with desorption of the Sonogashira product suggest that the reaction occurs at the boundaries of islands of adsorbed reactants and that its appearance in the gas phase is rate-limited by the surface reaction. These findings demonstrate unambiguously and for the first time that this heterogeneous cross-coupling chemistry is an intrinsic property of extended, metallic pure gold surfaces: no other species, including solvent molecules, basic or charged (ionic) species are necessary to mediate the process.

Functional coupling of the human dopamine D-2 receptor with G alpha(i1), G alpha(i2), G alpha(i3) and G alpha(o) G proteins: evidence for agonist regulation of G protein selectivity

1 The human dopamine D-2long (D-2L) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)) the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 mum), demonstrating interaction between the receptor and the different G proteins. 2 The receptor to G protein ratio (R: G ratio) was evaluated using [H-3]-spiperone saturation binding (R) and [S-35]-GTPgammaS saturation binding (G). R: G ratios of 1: 12, 1: 3, 1: 14 and 1: 5 were found for G(i1), G(i2), G(i3), and Go preparations, respectively. However, when R:G ratios of 1:2 and 1: 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [S-35]-GTPgammaS binding. 3 Several agonists were tested for their ability to stimulate [S-35]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [S-35]-GTPyS binding. 4 Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. 5 We conclude that the degree of selectivity of G protein activation by the D-2L receptor can depend on the conformation of the receptor stabilised by an agonist.