Three-dimensional coronal structures using clark lake observations

We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph. Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with related observations using white-light coronagraphs.     

A 20‐ka climate record from Central Himalayan loess deposits

The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd.     

The Preboreal climate reversal and a subsequent solar‐forced climate shift

Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric ¹⁴C; (ii) a temporary decline of atmospheric CO₂; and (iii) an increase in the GRIP ¹⁰Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.     

A model for the constant-head pumping test conducted in vertically fractured media

An analytical model for the constant-head pumping test is developed for a partially penetrating well that has a finite thickness skin, and intersects a single vertical fracture. In the model, the fracture is fully confined and flow occurs only in the fracture. The model is developed using Laplace transform and finite Fourier transform methods. The model is to be used for analysing well test data from vertically fractured media and for verifying numerical models. Dimensionless curves are used to study the effects of a finite thickness skin and a partially penetrating wellbore. In the presence of a finite thickness skin, or a partially penetrating wellbore, a typical flow response for the constant-head pumping test has three distinct periods of flow corresponding to small-, intermediate- and large-time. Small- and large-time approximations are presented for the model. For tests where the wellbore is fully penetrating, or the partial penetration ratio is known, these approximations can be used to analyse field data.     

Wavelet analysis of a magnetogram

I analyze a quiet-Sun magnetogram with an orthogonal wavelet transform, which allows me to define an entropy measure. The entropy measure of the magnetogram as a function of spatial scale obeys a scaling law, which leads to a fractal dimension ofD _f = 1.7. Furthermore, the entropy scaling law is directly related to the intermittency of magnetic features, which increases for decreasing spatial scales, as expected for a turbulent signal. In this context, the scaling law parameter can be interpreted as a fractional reduction in volume from one step of the turbulent cascade to the next.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.     

Spectroscopy of CN Orionis

Two outbursts and a minimum phase of the dwarf nova CN Orionis have been observed spectroscopically. One outburst was covered almost completely. The outburst spectra show periodic variations of the absorption lines which are interpreted with the formation of an elliptic disc during outburst stage. During decline from outburst a narrow emission line appears in the core of the broad H absorption line. The balmer decrement in the outburst phase is much steeper than in the minimum phase. This implies that during the outburst the emission line region is located more outward in the disk. The semi amplitude of the radial velocity curve was determined to K1=152 km/s±10 km/s. Using the photometric orbital period and an assumption about the inclination angle the approximate system parameters could be derived.Based on observations colleted at the European Southern Observatory, La Silla, Chile and at Mount Stromlo Observatory, Canberra, Australia.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

The nature and significance of the feedback of changes in terrestrial vegetation on global atmospheric and climatic change

The potential feedback on global atmospheric and climate change of climate-driven changes in terrestrial vegetation is examined by systematically relating the surface exchanges of energy, mass and momentum to two dimensions of vegetation, structure and taxonomy, such that the significance of climate driven changes in these characteristics can be assessed. A detailed quantitative understanding of this feedback is an important prerequisite to realistic and dynamic representations of the Earth's surface within general circulation and biological models (GCMs and GBMs). Without realistic representations of terrestrial vegetation within these models, any forecasts of future climates by these models must be suspect.Several general conclusions are drawn. The first is that the indirect feedbacks, those associated with the clouds and aerosols of the planetary boundary layer, appear to be very powerful but as yet their behaviour and connections with the underlying surface are both poorly understood and captured within GCMs.The physical structure of vegetation, the disposition of biomass in 3-D, is the characteristic that most strongly influences the exchange of momentum (via aerodynamic roughness) and solar radiation (via albedo). Vegetation structure and species composition determine the most important of the mass exchanges, evapotranspiration. Of all of the surface exchanges, the parameterization of evapotranspiration (E ) and the simulation of the water balance over time is the most critical.Lastly, the problems of scaling and spatial heterogeneity, the sub-grid variability of the modellers, looms as a difficult, but not insoluble, problem. It remains a critical problem however, and the detailed parameterization of the various big leaf models stands in absurd contrast to the simplistic generalization of the spatial heterogeneity of terrestrial landscapes.Plant ecologists can contribute to the task of improving the representation of vegetated landscapes within GCMs. There is need to simply and unify the way in which vegetation can be grouped at landscape scales. A classification that is based on function rather than phylogeny is required. The definition of Vegetation Functional Types (VFTs) would expedite research on both the impact of, and feedback on, climate change.