An indirect mechanism for the production of O(1S) in the aurora

Recent laboratory measurements of the deactivation rate constants for O(¹S) have suggested that the dominant production mechanism for the green line in the nightglow is a two-step process. A similar mechanism involving energy transfer from an excited state of molecular oxygen is considered as a potential source of the OI (5577 Å) emission in the aurora. It is shown that the mechanism, $\text{O}{}_2\text{+ e → O}{}_2{}^1\text{+ e O}{}_2{}^1\text{+ O → O}{}_2\text{+ O(}{}^1\text{S}\text{)}$, is consistent with auroral observations; the intermediate excited state has been tentatively identified as the O₂(c¹∑^?_u) state. For the proposed energy transfer mechanism to be the primary source of the auroral green line, the peak electron impact cross-section for O₂¹ production must be approximately 2 × 10^?17 cm².     

Winter seabird distribution and abundance off south-western Greenland, 1999

South-western Greenland constitutes an internationally important wintering area for many seabird species. Several species of management concern have a predominantly near-coastal distribution, though available information about seabird numbers is mostly confined to offshore waters. Here we report on extensive aerial surveys conducted in March 1999, covering the coastal waters (up to 15-20 km from the mainland coast) and fjords of south-west Greenland. The most widespread and numerous species were estimated as 463 000 common eiders ( Somateria mollissima ), 153000 king eiders ( S. spectabilis ), 125000 thick-billed murres ( Uria lomvia ), 94 000 long-tailed ducks ( Clangula hyemails ), and 12 000 black guillemots ( Cepphus grylle ). A total of 19 bird species were recorded. The estimates for common eider and long-tailed duck approximately represent the entire winter population in south-western Greenland while estimates for the other species represent only an unknown proportion since their distribution continues further offshore. Waters around Nuuk and within the Julianehåbsbugten (Julianehåb Bay) area were identified as areas of high seabird density. A large proportion of the common eider population was aggregated in the fjord systems (22%), calling attention to the importance of fjords for this species. In contrast, pelagic seabird species appear to be absent from the fjords. The large winter population of common eider reveals the importance of south-western Greenland as a key wintering area for the eastern Canadian breeding population. The western Greenland breeding population is the only other contributor, probably amounting to no more than 15 000 pairs.     

The Barents Sea Ice Sheet — A model of its growth and decay during the last ice maximum

On the basis of geomorphological and sedimentological data, we believe that the entire Barents Sea was covered by grounded ice during the last glacial maximum. ¹⁴C dates on shells embedded in tills suggest marine conditions in the Barents Sea as late as 22 ka BP; and models of the deglaciation history based on uplift data from the northern Norwegian coast suggest that significant parts of the Barents Sea Ice Sheet calved off as early as 15 ka BP. The growth of the ice sheet is related to glacioeustatic fall and the exposure of shallow banks in the central Barents Sea, where ice caps may develop and expand to finally coalesce with the expanding ice masses from Svalbard and Fennoscandia.The outlined model for growth and decay of the Barents Sea Ice Sheet suggests a system which developed and existed under periods of maximum climatic deterioration, and where its growth and decay were strongly related to the fall and rise of sea level.     

On redshifts and number counts of extragalactic objects in two cosmological models

Observed redshifts and magnitudes of seven classes of extragalactic objects, and number counts of QSRS are compared with predictions made by cosmological models. The uniform model U 1 and the proposed non-expanding model RHL 5 represent the observations very well. The latter offers numerous advantages. Absolute magnitudes and colors are determined.     

A measurement of the O2 ultra-violet nightglow emission

New measurements of the Herzberg I emission height profile in the night airglow are reported and indicate a peak emission height near 96 km in agreement with previous measurements. Using an atomic oxygen concentration profile determined from the oxygen green line profile measured on the same rocket it is concluded that the O₂(A³Σ_u⁺) state is not excited in the direct three body recombination of atomic oxygen. It is suggested that the excitation mechanism is a two step process, similar to the Barth mechanism for the atomic oxygen green lineand that the excited intermediate state is C³Δ_u.     

Late Cenozoic depositional history of the western Svalbard continental shelf, controlled by subsidence and climate

Based on a grid of high resolution, single channel seismic lines, this paper addresses the Late Cenozoic evolution of the western Svalbard continental shelf. The seismic structure of the shelf includes at least 16 erosional unconformities, each representing a glacial advance. The evolution during the last approximately one million years has been divided into six main erosional and depositional phases. Differential margin subsidence around a hinge zone is an important controlling mechanism for the accumulation of the sedimentary wedge at the outer shelf. The most significant depositional change appears to be related to a general climatic shift, globally recorded to be centred around 1 Ma. At this level, corresponding to the Upper Regional Unconformity (URU) on the shelf, the depositional regime changed from net erosion to net deposition and shelf aggradation. Of major significance is probably a shift from thick, eroding glaciers with steep ice profiles, to low profile fast flowing ice streams maintained by an increased amount of interglacial and interstadial sediments. The relationship between climatic fluctuations, glacial dynamics and depositional regime is discussed.     

Discovery of the 'missing' mode in HR 1217 by the Whole Earth Telescope

HR 1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of non-radial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacings of 33.3 and 34.6 μHz, with a sixth mode at a problematic spacing of 50.0 μHz (which equals  1.5×33.3 μHz)  to the high-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 μHz, but Hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha predicted that there should be a previously undetected mode 34 μHz higher than the main group, with a smaller spacing between it and the highest one. In this Letter, we present preliminary results from a multisite photometric campaign on the rapidly oscillating Ap star HR 1217 using the 'Whole Earth Telescope'. While a complete analysis of the data will appear in a later paper, one outstanding result from this run is the discovery of a newly detected frequency in the pulsation spectrum of this star, at the frequency predicted by Cunha.     

Improving modelling and inversion in refraction seismics with a first-order Eikonal solver

A first-order Eikonal solver is applied to modelling and inversion in refraction seismics. The method calculates the traveltime of the fastest wave at any point of a regular grid, including head waves as used in refraction. The efficiency, robustness and flexibility of the method give a very powerful modelling tool to find both traveltimes and raypaths. Comparisons with finite-difference data show the validity of the results. Any arbitrarily complex model can be studied, including the exact topography of the surface, thus avoiding static corrections. Later arrivals are also obtained by applying high-slowness masks over the high-velocity zones. Such an efficient modelling tool may be used interactively to invert for the model, but a better method is to apply the refractor-imaging principle of Hagedoorn to obtain the refractors from the picked traveltime curves. The application of this principle has already been tried successfully by previous authors, but they used a less well-adapted Eikonal solver. Some of their traveltimes were not correct in the presence of strong velocity variations, and the refractor-imaging principle was restricted to receiver lines along a plane surface. With the first-order Eikonal solver chosen, any topography of the receiving surface can be considered and there is no restriction on the velocity contrast. Based on synthetic examples, the Hagedoorn principle appears to be robust even in the case of first arrivals associated with waves diving under the refractor. The velocities below the refractor can also be easily estimated, parallel to the imaging process. In this way, the model can be built up successively layer by layer, the refractor-imaging and velocity-mapping processes being performed for each identified refractor at a time. The inverted model could then be used in tomographic inversions because the calculated traveltimes are very close to the observed traveltimes and the raypaths are available.     

Piecewise 1D laterally constrained inversion of resistivity data

In a sedimentary environment, layered models are often capable of representing the actual geology more accurately than smooth minimum structure models. Furthermore, interval thicknesses and resistivities are often the parameters to which non‐geophysicist experts can relate and base decisions on when using them in waste site remediation, groundwater modelling and physical planning. We present a laterally constrained inversion scheme for continuous resistivity data based on a layered earth model (1D). All 1D data sets and models are inverted as one system, producing layered sections with lateral smooth transitions. The models are regularized through laterally equal constraints that tie interface depths and resistivities of adjacent layers. Prior information, e.g. originating from electric logs, migrates through the lateral constraints to the adjacent models, making resolution of equivalences possible to some extent. Information from areas with well‐resolved parameters will migrate through the constraints in a similar way to help resolve the poorly constrained parameters. The estimated model is complemented by a full sensitivity analysis of the model parameters, supporting quantitative evaluation of the inversion result. Examples from synthetic 2D models show that the model recognition of a sublayered 2D wedge model is improved using the laterally constrained inversion approach when compared with a section of combined 1D models and when compared with a 2D minimum structure inversion. Case histories with data from two different continuous DC systems support the conclusions drawn from the synthetic example.