The AuScope geodetic VLBI array   总被引：1，自引：1，他引：0  

J. E. J. Lovell  J. N. McCallum  P. B. Reid  P. M. McCulloch  B. E. Baynes  J. M. Dickey  S. S. Shabala  C. S. Watson  O. Titov  R. Ruddick  R. Twilley  C. Reynolds  S. J. Tingay  P. Shield  R. Adada  S. P. Ellingsen  J. S. Morgan  H. E. Bignall 《Journal of Geodesy》2013,87(6):527-538

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Radio Intra-Day Variability: Answers and Questions     

David L. Jauncey  Lucyna Kedziora-Chudczer  James E.J. Lovell  Jean-Pierre Macquart  George D. Nicolson  Rick A. Perley  John E. Reynolds  Anastasios K. Tzioumis  Mark H. Wieringa  Hayley E. Bignall 《Astrophysics and Space Science》2001,278(1-2):87-92

Intra-day variability (IDV) of active galactic nuclei (AGN) has been detected from gamma-ray energies to radio wavelengths. At high energies, such variability appears to be intrinsic to the sources themselves. However, at radio wavelengths, brightness temperatures as high as10¹⁸ to 10²¹ K are encountered if the IDV is intrinsic to the source. We discuss here the accumulating evidence showing that, at radio wavelengths where the highest brightness temperatures are encountered, interstellar scintillation (ISS) is the principal mechanism causing IDV. While ISS reduces the implied brightness temperatures, they still remain uncomfortably high. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Magnetic field limits and spectral variability in the Circinus galaxy H₂O megamasers     

J. N. McCallum  S. P. Ellingsen  J. E. J. Lovell 《Monthly notices of the Royal Astronomical Society》2007,376(2):549-556

We have used the Australia Telescope Compact Array (ATCA) to search for polarized emission from the Circinus galaxy H₂O megamasers. No linear or circular polarized emission was detected with 3σ upper limits of 24 and 39 mJy, respectively, corresponding to fractional polarizations of 0.6 and 0.9 per cent for the strongest line. These results allow us to place upper limits on the strength of the nuclear magnetic field of ≤150 mG near the outer edge of the masing disc and ≤360 mG at the inner edge.
Rapid variability, thought to be due to interstellar scintillation, is a well-known feature of the Circinus H₂O megamasers. Our Stokes-I data show evidence for changes in the shape of the maser line spectral profiles, as well as their intensity. In addition to the rapid variations, the maser light curves also exhibit variations with a time-scale of the order of days (although our observations are too short to characterize this well). These behaviours support the hypothesis that the Circinus H₂O megamasers undergo diffractive interstellar scintillation, which has previously only been observed in pulsars and in one active galactic nucleus.  相似文献   

Towards improved ground models for slope instability evaluations through better characterization of sediment-hosted gas-hydrates     

D. A. Gunn  L. M. Nelder  C. A. Rochelle  K. Bateman  P. D. Jackson  M. A. Lovell  P. R. N. Hobbs  D. Long  J. G. Rees  P. Schultheiss  J. Roberts  T. Francis 《地学学报》2002,14(6):443-451

ABSTRACT Recently developed effective stress-controlled geophysical property models are used in passive-margin slope instability analyses including simulated earthquake motion. The pressure–temperature (P–T) history of sediment-hosted gas hydrate may significantly alter the geophysical property profile of the sediment column (e.g. metastable cement or increased pore pressures). This can result in significant amplification of earthquake ground motion, and thus seabed instability, where hydrates are present. Published studies suggest destabilization of these high-pressure/low-temperature sediment-hosted hydrates could trigger catastrophic slope failures with consequent liberation of 'greenhouse' gases and significant effects on global climate. To provide improved ground models for slope instability analyses we are investigating the influence of P–T history on hydrate distribution in sediments through the development of laboratory techniques to enable geophysical quantification of hydrate morphology and fabric on hydrate stability.  相似文献   

The Pedagogical Benefits of Participatory GIS for Geographic Education     

Gaurav Sinha  Thomas A. Smucker  Eric J. Lovell  Kgosietsile Velempini  Samuel A. Miller  Daniel Weiner 《The Journal of geography》2017,116(4):165-179

In this article, participatory GIS (PGIS) is explored and established as a powerful platform for geographic education. PGIS pedagogy can help educators meet diverse learning objectives pertaining to: (1) local knowledge and place-based thinking; (2) community engagement; (3) field mapping with geospatial technologies; (4) mixed-methods research; and (5) fostering of critical reflexivity in students. The discussion is supported with insights from multiple student-facilitated PGIS projects organized in rural Tanzania. There also is a thorough discussion of the challenges and caveats associated with involving students in PGIS projects, and a call for new research for assessing and advancing PGIS pedagogy.  相似文献   

Probing the microarcsecond structure of the Circinus megamasers through diffractive interstellar scintillation     

J. N. McCallum  S. P. Ellingsen  J. E. J. Lovell  C. J. Phillips  J. E. Reynolds 《Monthly notices of the Royal Astronomical Society》2009,392(4):1339-1362

We have used the Long Baseline Array to observe the Circinus Galaxy H₂O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission.
The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers.
In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s⁻¹ for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success.  相似文献   

Microearthquakes in the TDP swarm, Turkey: clustering in space and time     

John Lovell  Stuart Crampin  Russ Evans  S. Balamir Üçer 《Geophysical Journal International》1987,91(2):313-330

Summary. The third occupation (experiment TDP3) of recording sites above a persistent swarm of microearthquakes near the North Anatolian Fault, with a larger seismic network and over a longer period of time, confirms and refines previous observations with greater resolution. The greater resolution in earthquake locations has revealed marked clustering in time and space. Many, perhaps most, of the earthquakes belong to clusters, where successive earthquakes originate in a very small volume and have similar fault mechanisms. Such studies allow the progression of seismic activity of small earthquakes to be followed in some detail, and may reveal features which are hidden in larger and more complex earthquake sequences.  相似文献   

Observations of asteroids with ALMA     

Amy J. Lovell 《Astrophysics and Space Science》2008,313(1-3):191-196

Thermal observations of large asteroids at millimeter wavelengths have revealed high amplitude rotational lightcurves. Such lightcurves are important constraints on thermophysical models of asteroids, and provide unique insight into the nature of their surface and subsurface composition. A better understanding of asteroid surfaces provides insight into the composition, physical structures, and processing history of these surviving remnants from the formation of our solar system. In addition, detailed observations of the larger asteroids, accompanied by thermophysical models with appropriate temporal and spatial resolution, promise to decrease uncertainties in their flux predictions. Of particular interest are the near-Earth objects, which can be observed at large phase angles, permitting better assessment of the thermal response of their unilluminated surfaces. The high sensitivity of ALMA will enable us to detect many small bodies in all the major groups, to obtain lightcurves for a large sample of main-belt and near-Earth objects, to resolve the surfaces of some large objects, and to separate the emission from primary and secondary objects in binary pairs. In addition to the science goals of asteroid studies, these bodies may also prove useful operationally because those with known shapes and well-characterized lightcurves could be employed for flux calibration by ALMA and other high frequency instruments.  相似文献   

XTE J1550–564: a superluminal ejection during the September 1998 outburst     

Diana Hannikainen  Duncan Campbell-Wilson  Richard Hunstead  Vince McIntyre  Jim Lovell  John Reynolds  Tasso Tzioumis  Kinwah Wu 《Astrophysics and Space Science》2001,276(1):45-48

In 1998 September, the X-ray transient XTE J1550–564 underwent amajor outburst in soft and hard X-rays, followed by a radio flare. Australian Long Baseline Array imagesobtained shortly after the peak in the radio flare showed evolving structure.The components observed have an apparent separation velocity of >2c.  相似文献   

Surface geochemistry at Roosevelt Springs Kgra, Utah     

J.S. Lovell  W.T. Meyer  D. Atkinson 《Journal of Geochemical Exploration》1983,19(1-3)

As part of a research program conducted on behalf of the Department of Energy, available data on the Roosevelt Springs KGRA were synthesized to determine the spatial arrangement of the rocks, and the patterns of mass and energy flow within them. The resulting model led to a new interpretation of the geothermal system, and provided “ground truth” for evaluating the application of soil geochemistry to exploration for concealed geothermal fields. Preliminary geochemical studies comparing the surface micro-layer to conventional soil sampling methods indicated both practical and chemical advantages for the surface micro-layer technique.The elements arsenic, antimony and cesium in the surface microlayer samples in particular, gave a strong expression of one of the principal faults in the geothermal field. In contrast the analysis of soil samples from only 20 cm below the surface gave little or no expression of the geothermal field.As a consequence, the surface micro-layer was the chosen sampling medium for the second field program, which entailed the collection of a total of some 300 samples on both a regional and detailed pattern covering about 250 km². These samples were subsequently analyzed by a variety of methods yielding data on 41 elements and ions.Computer contouring revealed that, on a single-element basis, cesium, antimony and arsenic provided the best expression of the KGRA, and indicated other interesting areas of geothermal leakage. Elements such as beryllium and lithium, which are present in highly anomalous concentrations in the opaline sinter deposited by geothermal leakage, do not have an expression in the overlying soils or the surface micro-layer. Computer manipulation of the multi-element data using R-mode factor analysis provided the optimum method of interpretation of the surface micro-layer data. A single factor in which the principal contributors were arsenic, antimony and cesium provided the best indication of the leakage of geothermal solutions within the KGRA. Anomalies in the Escalante Desert to the west of the geothermal field are associated with the trace of a fault zone, and may, therefore, be an activity. Trend surface analysis of the soil mercury data has indicated a regional high in this element in the Mineral Mountains to the east of the KGRA, which may indicate the position of a dry heat source at depth.These data demonstrate that surface micro-layer sampling on a regional scale can serve as a prospecting tool for geothermal resource areas. However, it is possible that the optimum pathfinder elements may vary with the nature of heat source, the geochemistry of the local rocks and the local surficial environment. It is therefore recommended that a multi-element approach should be adopted, with subsequent computer processing of the data.  相似文献