3D triangulation of a Sun-grazing comet

The bright Kreutz Comet C/2007 L3 (SOHO) entered the fields of view of the twin Solar Terrestrial Relations Observatory (STEREO) COR1 telescopes on 7–8 June 2007. The 12° separation between the two spacecraft at the time afforded the opportunity to derive the position of the comet's tail in three-dimensional space using direct triangulation. The track of the comet's orbit is compared against more traditional orbital calculations using observations from the STEREO COR2 telescopes, and from the Large Angle and Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO). The shape of the comet's tail shows that it is composed of dust particles released when the comet was between 18 and 22 solar radii, with no significant dust production after that. The comet did not survive perihelion passage, but a rare faint remnant of the comet tail persisted for several hours after the break-up, and was seen by both the SOHO and STEREO coronagraphs to drift slowly away from the Sun. This tail remnant was found to be composed of particles far back from the head of the comet. The motion of the tail remnant shows a loss of angular momentum during the passage through the solar corona. Atmospheric drag is estimated to account for a significant fraction of this change in angular momentum, but indications are that other mechanisms may be required to completely account for the total amount of change.     

Bootstrapped discrete scale invariance analysis of geomagnetic dipole intensity

The technique of bootstrapped discrete scale invariance allows multiple time-series of different observables to be normalized in terms of observed and predicted characteristic timescales. A case study is presented using the SINT2000 time-series of virtual axial dipole moment, which spans the past 2 Myr. It is shown that this sequence not only bears a clear signature of a preferred timescale of about 55.6 Ka, but additionally predicts similar features (of shorter and longer duration) that are actually observed on the timescales of historical secular variation and dipole reversals, respectively. In turn, the latter two empirical sources both predict the characteristic timescale found in the dipole intensity sequence. These communal scaling characteristics suggest that a single underlying process could be driving dynamo fluctuations across all three observed timescales, from years to millions of years.     

Understanding the surface hydrology of the Lake Eyre Basin: Part 1—Rainfall

This is the first of two papers that describe the surface hydrology of the Lake Eyre Basin (LEB) (1,140,000 km²) in central Australia and compares some key characteristics with those observed from arid regions globally. This paper concentrates on annual rainfall, whereas the second paper is devoted to streamflow. The first part describes the LEB's climate (arid to semi-arid), which is dominated by a subtropical high pressure ridge stretching latitudinally across central Australia. Then follow major analyses that include the characteristics of rainfall, wet and dry spell lengths and cumulative surpluses and deficiencies, rainfall trends and intra- and inter-decadal fluctuations, and the relationship between rainfall and El Niño-Southern Oscillation (ENSO). The paper concludes with six conclusions, the key ones being: the variability of the annual rainfall (based on the coefficient of variation) in the LEB is approximately 60% greater than that found for stations located in arid regions in the rest of the world; there is a bias towards longer lengths of dry years than observed in the rest of Australia; and, there is a significant lag correlation between rainfall and ENSO, particularly in the east and in the latter part of a year.     

Palaeolimnological records of shallow lake biodiversity change: exploring the merits of single versus multi-proxy approaches

Shallow lakes are among the most threatened ecosystems in the world and many contemporary studies have demonstrated declines in biodiversity due to anthropogenic forcing. Mostly, however, these studies have not covered the full period of human-induced diversity change in lakes which is typically over decades-centuries. Here we provide two examples of palaeoecological studies focussed on reconstructing biodiversity changes in contrasting shallow lake environments that demonstrate the efficacy of the approach—a shallow UK lake and a suite of floodplain lakes (called billabongs) in the Murray-Darling basin, Australia. In the Murray-Darling billabongs, complex sedimentary processes operate, sediment chronologies are less certain and replication of sites is needed to confirm patterns. The combination of sediment records from 10 billabongs showed that diatom diversity changes pre- and post-European (>1850) disturbance were inconsistent; however, reductions in diversity were more common and appear to reflect reductions in macrophyte abundance. At Felbrigg Lake, a multi-proxy study with strong chronological control demonstrated divergent responses of macrophyte, diatom, cladoceran and chironomid richness and diversity to a century of eutrophication. Eutrophication of the site was qualitatively inferred from changes in the macrophyte community in turn reconstructed from plant macrofossils. Benthic cladocerans showed a consistent decline in richness through the record, reflecting the gradual reduction in their macrophyte associated habitat over the past century. Diatom richness and diversity responses were complex, with increases in diversity and richness linked to both increases and decreases in macrophyte species richness and abundance. Chironomid richness and diversity patterns were less consistently linked to eutrophication. The loss of the dominant zooplanktivore (perch) in the 1970s was reflected in the richness and diversity profiles for all groups. Our study reveals clear potential for using sediment cores to infer biodiversity change in shallow lakes and shallow lake regions. However, given the contrasting patterns of diversity change for the different biological groups both in Felbrigg Lake and between Felbrigg and the billabongs, caution is required when interpreting whole-ecosystem biodiversity changes (or the absence of change) based on single as opposed to multi-proxy studies.     

Annual and sub-annual effects of euv heating—I. Harmonic analysis

In this paper we compute the rate of solar EUV heating in the upper atmosphere by photo-dissociation and photo-ionization, taking care to include properly the effects of oblique incidence of solar flux, sphericity of the atmosphere and ellipticity of the Earth's orbit. The time and latitudinal variations of the computed heat function are revealed by numerical Fourier analysis of the heat function into harmonics of the yearly cycle. It is shown that EUV absorption contains a ‘latitude independent’ semi-annual component of heating with the ‘proper phase’ to account for the semi-annual density variations. Further, the annual component of the heat function predicts the existence of ‘summer polar’ density increases in the northern and southern hemispheres.     

Impact of Gas Adsorption Induced Coal Matrix Damage on the Evolution of Coal Permeability

It has been widely reported that coal permeability can change from reduction to enhancement due to gas adsorption even under the constant effective stress condition, which is apparently inconsistent with the classic theoretical solutions. This study addresses this inconsistency through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking induced damage and fracture aperture alteration, and translations of these interactions to permeability evolution under the constant effective stress condition. We develop a coupled coal–gas interaction model that incorporates the material heterogeneity and damage evolution of coal, which allows us to couple the progressive development of damage zone with gas adsorption processes within the coal matrix. For the case of constant effective stress, coal permeability changes from reduction to enhancement while the damage zone within the coal matrix develops from the fracture wall to further inside the matrix. As the peak Langmuir strain is approached, the decrease of permeability halts and permeability increases with pressure. The transition of permeability reduction to permeability enhancement during gas adsorption, which may be closely related to the damage zone development in coal matrix, is controlled by coal heterogeneity, external boundary condition, and adsorption-induced swelling.