ALBERT

Description: Urban surfaces are usually net sources of CO2. Vegetation can potentially have an important role in reducing the CO2 emitted by anthropogenic activities in cities, particularly when vegetation is extensive and/or evergreen. A direct and accurate estimation of carbon uptake by urban vegetation is difficult due to the particular characteristics of the urban ecosystem and high variability in tree distribution and species. Here, we investigate the role of urban vegetation in the CO2 flux from a residential neighbourhood in Singapore using two different approaches. CO2 fluxes measured directly by eddy covariance are compared with emissions estimated from emissions factors and activity data. The latter includes contributions from vehicular traffic, household combustion, soil respiration and human breathing. The difference between estimated emissions and measured fluxes should approximate the flux associated with the aboveground vegetation. In addition, a tree survey was conducted to estimate the annual CO2 sequestration using allometric equations and an alternative model of the metabolic theory of ecology for tropical forests. Palm trees, banana plants and turfgrass were also included in the survey with their annual CO2 uptake obtained from published growth rates. Both approaches agree within 2% and suggest that vegetation sequesters 8% of the total emitted CO2 in the residential neighbourhood studied. An uptake of 1.4 ton km−2 day−1 (510 ton km−2 yr−1) was estimated as the difference between assimilation by photosynthesis minus the aboveground biomass respiration during daytime (4.0 ton km−2 day−1) and release by plant respiration at night (2.6 ton km−2 day−1). However, when soil respiration is added to the daily aboveground flux, the biogenic component becomes a net source amounting to 4% of the total CO2 flux and represents the total contribution of urban vegetation to the carbon flux to the atmosphere.

Description: Urban surfaces are usually net sources of CO2. Vegetation can potentially have an important role in reducing the CO2 emitted by anthropogenic activities in cities, particularly when vegetation is extensive and/or evergreen. Negative daytime CO2 fluxes, for example have been observed during the growing season at suburban sites characterized by abundant vegetation and low population density. A direct and accurate estimation of carbon uptake by urban vegetation is difficult due to the particular characteristics of the urban ecosystem and high variability in tree distribution and species. Here, we investigate the role of urban vegetation in the CO2 flux from a residential neighbourhood in Singapore using two different approaches. CO2 fluxes measured directly by eddy covariance are compared with emissions estimated from emissions factors and activity data. The latter includes contributions from vehicular traffic, household combustion, soil respiration and human breathing. The difference between estimated emissions and measured fluxes should approximate the biogenic flux. In addition, a tree survey was conducted to estimate the annual CO2 sequestration using allometric equations and an alternative model of the metabolic theory of ecology for tropical forests. Palm trees, banana plants and turfgrass were also included in the survey with their annual CO2 uptake obtained from published growth rates. Both approaches agree within 2% and suggest that vegetation captures 8% of the total emitted CO2 in the residential neighbourhood studied. A net uptake of 1.4 ton km−2 day−1 (510 ton km−2 yr−1 ) was estimated from the difference between the daily CO2 uptake by photosynthesis (3.95 ton km−2 ) and release by respiration (2.55 ton km−2). The study shows the importance of urban vegetation at the local scale for climate change mitigation in the tropics.

Description: We consider cross-field plasma flow inside a field-aligned plasma slab embedded in a uniform background in a 1-dimensional geometry. This situation may arise, for instance, when long-lasting reconnection pulses inject plasma into the inner magnetosphere. The present paper presents a detailed analysis of the structure of the interfaces that separate the slab from the background plasma on either side; a fully kinetic model is used to do so. Since the velocity shear across both interfaces has opposite signs, and given the typical gyroradius differences between injected and background ions and electrons, the structure of both interfaces can be very different. The behaviour of the slab and its interfaces depends critically on the flow of the plasma transverse to the magnetic field; in particular, it is shown that there are bounds to the flow speed that can be supported by the magnetised plasma. Further complicating the picture is the effect of the potential difference between the slab and its environment.

Description: Multi-spacecraft data from Cluster allow for a more detailed magnetopause and boundary layer structure determination than ever before. Reconstruction methods, in which the time variability observed during a pass is interpreted as being due to boundary motion and/or spatial structure, are particularly well suited for this task. Such methods rely on the availability of plasma and field data and adopt the tangential discontinuity hypothesis to determine the motion, acceleration, boundary structure, boundary curvature and surface wave speed over an extended time interval. In this paper one- and two-dimensional reconstruction methods are applied to multi-spacecraft data for the first time.

Description: The magnetospheric boundary is always moving, making it difficult to establish its structure. This paper presents a novel method for tracking the motion of the boundary, based on in-situ observations of the plasma velocity and of one or more additional observables. This method allows the moving boundary to be followed for extended periods of time (up to several hours) and aptly deals with limitations on the time resolution of the data, with measurement errors, and with occasional data gaps; it can exploit data from any number of spacecraft and any type of instrument. At the same time the method is an empirical reconstruction technique that determines the one-dimensional spatial structure of the boundary. The method is illustrated with single- and multi-spacecraft applications using data from Ampte/Irm and Cluster.

Description: We consider sheared flows in magnetospheric boundary layers of tangential discontinuity type, forming a structure that is embedded in a large-scale convergent perpendicular electric field. We construct a kinetic model that couples the magnetospheric structure with the topside ionosphere. The contribution of magnetospheric electrons and ionospheric electrons and ions is taken into account into the current-voltage relationship derived for an electric potential monotonically decreasing with the altitude. The solution of the current continuity equation gives the distribution of the ionospheric potential consistent with the given magnetospheric electric potential. The model shows that a sheared magnetospheric flow generates current sheets corresponding to upward field-aligned currents, field-aligned potential drops and narrow bands of precipitating energy, as in discrete auroral arcs. Higher velocity magnetospheric sheared flows have the tendency to produce brighter and slightly broader arcs. An increase in arc luminosity is also associated with enhancements of magnetospheric plasma density, in which case the structures are narrower. Finally, the model predicts that an increase of the electron temperature of the magnetospheric flowing plasma corresponds to slightly wider arcs but does not modify their luminosity.

Description: The Spatio Temporal Analysis of Field Fluctuations (STAFF) experiment is one of the five experiments, which constitute the Cluster Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, a waveform unit (up to either 10 Hz or 180 Hz) and a Spectrum Analyser (up to 4 kHz). The Spectrum Analyser combines the 3 magnetic components of the waves with the two electric components measured by the Electric Fields and Waves experiment (EFW) to calculate in real time the 5 × 5 Hermitian cross-spectral matrix at 27 frequencies distributed logarithmically in the frequency range 8 Hz to 4 kHz. The time resolution varies between 0.125 s and 4 s. The first results show the capabilities of the experiment, with examples in different regions of the magnetosphere-solar wind system that were encountered by Cluster at the beginning of its operational phase. First results obtained by the use of some of the tools that have been prepared specifically for the Cluster mission are described. The characterisation of the motion of the bow shock between successive crossings, using the reciprocal vector method, is given. The full characterisation of the waves analysed by the Spectrum Analyser, thanks to a dedicated program called PRASSADCO, is applied to some events; in particular a case of very confined electromagnetic waves in the vicinity of the equatorial region is presented and discussed.Key words. Magnetospheric physics (magnetopause, cusp and boundary layer) – Space plasma physics (waves and instabilities; shock waves)

Description: From satellite data sampling the top ionosphere in the Northern Hemisphere we have identified strong eastward ion drifts, with speeds larger than 1 km/s, widths of 1°–2°, occurring at similar temporal and spatial locations as rapid westward ion drifts known as sub-auroral ion drifts (SAID). We have called these events "abnormal sub-auroral ion drifts" (ASAID). Two events observed in the 20:00–22:00 MLT interval are discussed: the first occurring on 21 September 2003 and the other on 12 October 2003. Tomographic reconstructions of the electron density in the F-region, based on satellite data, provided by the Scandinavian tomography chain, were also available. We have observed that ASAID are accompanied by upward flows with a speed of the same order as that of the zonal ion drift. They coincide with deep, narrow troughs in the total ion density, both at the altitude of the F15 DMSP satellite (850 km) and in the F-region of the ionosphere, but do not seem to be a feature of the convective transport. During the entire duration of ASAID the electron temperature is very high while, contrary to SAID, the ion temperature has no clear variation. Both events described in this paper end up turning into classical SAID. Satellite data indicate that the generator of ASAID could be located inside the plasmasphere close to the plasmapause and we suggest a possible mechanism for their formation.

Description: We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.