Micro-scale Fluxes Research Articles

<i>In situ</i> detection of the electron diffusion region of collisionless magnetic reconnection at the high-latitude magnetopause

Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles. For reconnection to occur, the frozen-in condition must break down in a localized region, commonly called the ‘diffusion region’. In Earth’s magnetosphere, ion diffusion regions have already been observed, while electron diffusion regions have not been detected due to their small scales (of the order of a few km) ( Paschmann, 2008 ). In this paper we report, for the first time, in situ observations of an active electron diffusion region by the four Cluster spacecraft at the Earth’s high-latitude magnetopause. The electron diffusion region is characterized by nongyrotropic electron distribution, strong field-aligned currents carried by electrons and bi-directional super-Alfvenic electron jets. Also observed were multiple micro-scale flux ropes, with a scale size of about 5 c/ωpe (12 km, with c/ωpe the electron inertial length), that are crucial for electron acceleration in the guide-field reconnection process ( Drake et al., 2006a ). The data demonstrate the existence of the electron diffusion region in collisionless guide-field reconnection at the magnetopause.

Identification of Micro-scale Anthropogenic CO2, heat and moisture sources – Processing eddy covariance fluxes for a dense urban environment

Anthropogenic emissions of heat and exhaust gases play an important role in the atmospheric boundary layer, altering air quality, greenhouse gas concentrations and the transport of heat and moisture at various scales. This is particularly evident in urban areas where emission sources are integrated in the highly heterogeneous urban canopy layer and directly linked to human activities which exhibit significant temporal variability. It is common practice to use eddy covariance observations to estimate turbulent surface fluxes of latent heat, sensible heat and carbon dioxide, which can be attributed to a local scale source area. This study provides a method to assess the influence of micro-scale anthropogenic emissions on heat, moisture and carbon dioxide exchange in a highly urbanized environment for two sites in central London, UK. A new algorithm for the Identification of Micro-scale Anthropogenic Sources (IMAS) is presented, with two aims. Firstly, IMAS filters out the influence of micro-scale emissions and allows for the analysis of the turbulent fluxes representative of the local scale source area. Secondly, it is used to give a first order estimate of anthropogenic heat flux and carbon dioxide flux representative of the building scale. The algorithm is evaluated using directional and temporal analysis. The algorithm is then used at a second site which was not incorporated in its development. The spatial and temporal local scale patterns, as well as micro-scale fluxes, appear physically reasonable and can be incorporated in the analysis of long-term eddy covariance measurements at the sites in central London. In addition to the new IMAS-technique, further steps in quality control and quality assurance used for the flux processing are presented. The methods and results have implications for urban flux measurements in dense urbanised settings with significant sources of heat and greenhouse gases.

Spark plasma sintering of MgB2 in the two-temperature route

Superconducting bulks of MgB2 were obtained by an ex-situ two-temperature route applied to spark plasma sintering (SPS). Processing of samples was performed at lower temperatures than previously reported. Samples produced by the two-temperature route show a higher morphological uniformity, a higher density (above 98%), a higher Vickers hardness, and undesirable stronger microscale flux jumps, as indicated by magnetic relaxation measurements when compared to a sample obtained by the one-temperature route (95.3% relative density). At the same time, all sintered samples show approximately constant crystallite size, critical current density, irreversibility field, critical temperature, weight fraction of impurity phases (MgB4 and MgO), and the amount of carbon accidentally introduced during SPS processing.

Improvement of the critical current density of spark plasma sinteredMgB2 byC60 addition

We investigated the influence of fullerene (C60) addition on the superconducting parameters ofMgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt%C60 addition leads to the reduction of the critical temperature by a few K,whereas the magnetically determined critical current density increasesby approximately one order of magnitude in an applied magnetic fieldH = 70 kOe attemperature T = 10 K. This indicates substitution of B by C inC60 addedMgB2 processed by SPS, contrary to some data reported in the literatureobtained using conventional methods. For our relatively large specimens,the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that themacroscale flux jumps transform into microscale flux jumps with increasingH andT.

The Effects of Mesoscale Surface Heterogeneity on the Fair-Weather Convective Atmospheric Boundary Layer

Large-eddy simulation (LES) is used to examine the impact of heterogeneity in the surface energy balance on the mesoscale and microscale structure of the convective atmospheric boundary layer (ABL). A long (16 or 32 km) and narrow (5 km) domain of the convective ABL is forced with an imposed surface heat flux consisting of a constant background flux of 0.20 K m s−1 (250 W m−2) added to a sinusoidal perturbation of 16 or 32 km and whose amplitude varies from 0.02 to 0.20 K m s−1 (25–250 W m−2). The output is analyzed using a spatial filter, spectral analyses, and a wave-cutoff filter to show how the mesoscale and microscale components of the ABL respond to surface heterogeneity. The ABL response is divided by amplitude of heterogeneity into oscillatory and nonoscillatory mesoscale flows, with amplitudes of 0.08 K m s−1 (100 W m−2) and greater being oscillatory. Although mean ABL structure is disturbed relative to the homogeneous case for all heterogeneous cases, the microscale structure of the ABL in the quasi-steady flows retains characteristics of mixed-layer similarity. The vertical sensible heat flux is dominated in all cases by the microscale flux, with an interscale term becoming significant for high-amplitude cases and the mesoscale flux remaining small in all cases. Prior observations of ABLs over heterogeneous surfaces are consistent with the lower-amplitude cases. These results contradict past studies that suggest that heterogeneous surfaces lead to large mesoscale fluxes. The interscale flux and oscillatory microscale structures raise questions about the ability of mesoscale models to properly simulate the ABL in high-amplitude heterogeneity.

Tracer budgets in the warm water sphere

In global equilibrium numerical solutions for the oceanic general circulation, we examine the time-mean budgets for the tracers potential temperature and salinity integrated over volumes bounded by interior surfaces of constant tracer values and lying within the warm water sphere. In this domain, the budgets have surface fluxes primarily of one sign that must be balanced by mesoscale and microscale fluxes through its lower boundary, because advection makes a zero integral contribution. The mesoscale fluxes are represented by the isopycnally orientated, quasiadiabatic parameterization of Gent and McWilliams and contribute little to the integral budgets where isopycnals are nearly tangent to the volume boundary. The microscale fluxes occur with a small vertical diffusivity ( κ v = O(10 −5 ) m 2 s −1 ) in the predominantly stably stratified warm water sphere, yet they are shown to be sufficient to provide the primary balance against surface forcing in all ocean basins and over a wide range of tracer values. This is especially true for potential temperature because of the close alignment of isotherms and isopycnals. For salinity, however, the mesoscale isopycnal diffusion also contributes significantly to the budget. The budgets are dominated by the surface and interior fluxes from the time-mean circulation, although there are also modest contributions from the rectification of the seasonal cycle. These results are in contrast to previous analyses that concluded that much larger vertical diffusivities are required for budget balance. We do not attempt to fully resolve the relatively smaller role of vertical diffusion in other budget volumes outside the warm water sphere. DOI: 10.1034/j.1600-0870.1996.00010.x

Handover in Scale of the Fluxes of Momentum, Heat, etc. in the Atmospheric Boundary Layer

The handover of the vertical fluxes of momentum, heat, etc., from the small-scale turbulence close to the earth's surface into the larger-scale motions above is becoming an important question in the context of numerical treatment of atmospheric dynamics. In convective conditions, a decoupling of heat flow from the surface value is suggested at a height of some 5 to 10 times the Obukhov scale height, at times lower. Some changes in regime are already well documented below this level. Above it, the need for direct measurement of the eddy heat flux on the cumulus scale, and location of the roots of these circulations, is emphasized. For momentum flux the handover from microscale direct into the synoptic scale is considered primarily. At a level where the microscale flux has fallen from its surface value τ0 to τ, the vertical motion in unaccelerated flow is ρw = curl [(τ0 − τ)/f], where f is the Coriolis parameter. When this is viewed as a component of eddy motion on the global scale, the resultant eddy transport of momentum is found to be insignificant. But the asymmetry of acceleration patterns in the westerlies, particularly that broadly associated with the transient frontal systems, appears to yield significant momentum flux in the required direction. This defines the need for a clearer identification of the typical acceleration patterns and for numerical solutions of the appropriately modified boundary layer equations. The momentum flux attributable to thermal wind and the handover into mean meridional circulation are also discussed.