AGU journal highlights - 4 May 2006

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GRL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Himalayan topography influences the spatial distribution of monsoon rainfall

The onset of the Indian summer monsoon in early June marks the beginning of the principal rainy season for the Himalayan region. This rain causes heavy flooding and landslides along the southern Himalayan front and is the most important factor in understanding Himalayan erosion on long and short timescales, making the spatial distribution of Indian summer monsoon rainfall critically important to hazard assessments. To investigate the influence of topography and relief on rainfall generation and resultant erosion, Bookhagen and Burbank analyzed rainfall amounts collected between 1988-2005 by the Tropical Rainfall Measurement Mission (TRMM) satellite. They found that high rainfall rates occur near the toe of the Greater Himalaya, as expected. However, a more continuous band of high rainfall stretches along the southern edge of the Lesser Himalaya, near the junction with the Indo-Gangetic foreland basin, showing that heavy rainfall is induced by the first significant topography it encounters. The authors note that changes in topographic relief can be used to identify areas of focused rainfall in other mountain belts influenced by monsoons.

Title: Topography, relief, and TRMM-derived rainfall variations along the Himalaya

Authors: Bodo Bookhagen: Geological and Environmental Sciences, Stanford University, Stanford, California, USA;Douglas W. Burbank: Institute for Crustal Studies, University of California Santa Barbara, Santa Barbara, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026037, 2006

2. Density and gravity variations within Saturn's A ring

Images from Earth-based observatories and the Voyager spacecraft's pass near Saturn in the early 1980s showed the brightness of the A ring varies around the ring, with two bright patches separated by two darker patches. These were attributed to density alignments within the orbiting matter of these rings; if ring mass density is large enough, particles can agglomerate for brief periods before they are torn apart by rotational forces and the gravitational pull of Saturn. Colwell et al. term these gravitational instabilities "self-gravity wakes" and confirmed their presence through analyzing the patterns of light transmission and interruption as viewed by the Cassini spacecraft, as it traveled between Saturn's rings. They also calculated the shape and spacing of the self-gravity wakes using a model, and found that they were highly flattened structures with thickness, frequency, and opacity dependent on the radial distance from Saturn. The authors expect that future data from Cassini will help refine their results.

Title: Self-gravity wakes in Saturn's A ring measured by stellar occultations from Cassini

Authors: J. E. Colwell, L. W. Esposito, and M. Sremcevic: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025163, 2006

3. The composition of Titan's ionosphere

Titan, Saturn's largest satellite, has a dense atmosphere, composed of molecular nitrogen and methane, with minor amounts of many hydrocarbon and nitrile species. Past studies have shown that solar radiation and energetic plasma from Saturn's magnetosphere ionizes neutral molecules in Titan's atmosphere, creating an ionosphere above 800 kilometers [500 miles]. To study the composition and features of Titan's ionosphere, Cravens et al. used data from the Cassini's Ion and Neutral Mass Spectrometer (INMS) and found that a substantial ionosphere, housing complex ion chemistry, exists on the nightside of Titan. Measurements from INMS and Cassini's Radio and Plasma Wave experiments suggest that highly energetic electrons are raining out of Saturn's magnetosphere into Titan's ionosphere. Using mass spectra measured by INMS, the authors confirmed the presence of several hydrocarbon ion species in Titan's ionosphere that were previously suggested by models. They also showed that INMS saw high densities at mass numbers not predicted by models, including mass 18, which they suggest are ammonia ions produced by reaction of other ion species with neutral ammonia.

Title: The composition of Titan's ionosphere

Authors: T. E. Cravens and I. P. Robertson: Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA; J. H. Waite Jr. and V. De La Haye: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA; R. V. Yelle and V. Vuitton: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA; W. T. Kasprzak and H. B. Niemann: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; C. N. Keller: Cornerstone University, Grand Rapids, Michigan, USA S. A. Ledvina and J. G. Luhmann: Space Sciences Laboratory, University of California, Berkeley, California, USA; R. L. McNutt: Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA; W.-H. Ip: National Central University, Chung-Li, Taiwan; I. Mueller-Wodarg: Space and Atmospheric Physics Group, Imperial College, London, United Kingdom; J.-E. Wahlund: Swedish Institute of Space Physics, Uppsala, Sweden; V. G. Anicich: (Retired) NASA Jet Propulsion Lab, Pasadena, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025575, 2006

4. The seasonal electrical nature of Saturn's rings

Cassini's orbit about Saturn allowed the spacecraft's Radio and Plasma Wave Science instrument to directly detect the electron density in the vicinity of Saturn's rings. Using this information and a model of the photoemission flux, Farrell et al. derived the ring charging that showed that the rings' surface potential undergoes seasonal changes. With one year on Saturn equaling roughly 30 years on Earth, the authors discovered that the electrical configuration of the rings varied from being primarily unipolar (one charge state) during the Voyager flyovers in the early 1980s to its current bipolar (two separate regions of charge) condition, as observed by Cassini. The authors suggest that the transition from one state to the other occurs when Saturn's rings, with a tilt that wobbles back and forth across the orbital plane, reaches a threshold opening angle. Ring spokes (finger-shaped, radially oriented dusty plasma structures) are considered negatively charged, and the authors indicate that these should now be absent on the positively-charged Sun-facing side of the B-ring, which is consistent with Cassini observations. The authors suggest future times and locations to look for spoke activity.

Title: The changing electrical nature of Saturn's rings: Implications for spoke formation

Authors: W. M. Farrell, M. D. Desch, and M. L. Kaiser: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; W. S. Kurth and D. A. Gurnett: University of Iowa, Iowa City, Iowa, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024922, 2006

5. China's Three-Gorges Dam is affecting the nutrient supply to the East China Sea

Dam construction on major rivers has been documented to impact marine ecology by changing nutrient concentrations in the coastal zone. In 1994, construction began on the Three-Gorges Dam (TGD) on the Changjiang (Yangtze) River, the fifth largest river in the world, with a discharge that supports a very productive fishing ground in the East China Sea. From 1998 to 2004, Gong et al. coordinated the LORECS (Long-term Observation and Research of the East China Sea) project and conducted investigations before and after the TGD first filling phase in June 2003. They found that the ratio of silicon to nitrogen in a mixed section of river water and coastal seawater changed from 1.5 in 1998 to 0.4 in 2004, with sediment loading in some areas reduced to about half of pre-dam values. Moreover, primary production during the high flood season declined by 86 percent between 1998 and 2003. These results imply that the ecosystem of the East China Sea may have responded sensitively to changes in nutrient supply arising from the TGD project. Future influences of TGD on the East China Sea ecosystem will be under continuous study by the LORECS project.

Title: Reduction of primary production and changing of nutrient ratio in the East China Sea: Effect of the Three Gorges Dam?

Authors: Gwo-Ching Gong and Chin-Chang Hung: Institute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, Keelung, Taiwan; Jeng Chang: Institute of Marine Environmental Chemistry and Ecology, and Institute of Marine Biology, National Taiwan Ocean University, Keelung, Taiwan; Kuo-Ping Chiang: Institute of Marine Environmental Chemistry and Ecology, and Department of Environmental Biology and Fishery Science, National Taiwan Ocean University, Keelung, Taiwan; Tung-Ming Hsiung: Institute of Bioscience and Biotechnology, National Taiwan Ocean University, Keelung, Taiwan; Shui-Wang Duan: Department of Marine Science, Texas A&M University at Galveston, Galveston, Texas, USA; L. A. Codispoti: Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025800, 2006

6. Autonomous underwater vehicle explores cavity beneath Antarctic ice shelf

Antarctica is fringed by ice shelves, the floating extension of the Antarctic Ice Sheet. Often more than one kilometer [more than half a mile] thick and several hundred thousand square kilometers tens of thousands of square miles] in area, Antarctic ice shelves are a critical link between the ice sheet and the Southern Ocean. Very cold water masses, which cool and freshen the deep waters of the world's oceans, form within sub ice-shelf cavities. To learn more about these poorly-understood regions, Nicholls et al. studied the cavity beneath the Fimbul Ice Shelf, located off Queen Maud Land, by analyzing data from an autonomous underwater vehicle. Their data revealed that rather than smooth, as previously thought, much of the base of the ice shelf was rough. In addition, the cavity was periodically exposed to water above the surface freezing point. The authors note that the discovery of such a spatially complex oceanographic environment will likely reform views of the topographic and oceanographic conditions beneath ice shelves. They expect that the success of this mission will prompt further studies with autonomous underwater vehicles.

Title: Measurements beneath an Antarctic ice shelf using an autonomous underwater vehicle

Authors: K.W. Nicholls, E. P. Abrahamsen, and C. J. Pudsey, British Antarctic Survey, Cambridge, United Kingdom; J. J. H. Buck and G. F. Lane-Serff: School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, United Kingdom; P. A. Dodd, C. Goldblatt, K. J. Heywood, K.I.C. Oliver, and M. R. Price: School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom; G. Griffiths, S. D. McPhail, N. W. Millard, J. Perrett, K. Saw, K. Stansfield, and A. T. Webb: National Oceanographic Centre, Southampton, United Kingdom; N. E. Hughes and J. P. Wilkinson: Scottish Society for Marine Sciences, Dunstaffnage Laboratory, Oban, United Kingdom; A. Kaletzky and P. Wadhams: Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Cambridge, United Kingdom;M. J. Stott: Retired, formerly at the Department of Mathematics, Keele University, Keele, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025998, 2006

7. Intense gravity waves recorded in the polar mesosphere

Though normally associated with ocean dynamics, gravity waves frequently occur in the atmosphere as oscillations in density, pressure, temperature, and winds. Gravity waves originate mainly in the troposphere [lowest part of the atmosphere] and propagate up into the stratosphere and mesosphere [higher levels of the atmosphere], growing in amplitude due to decreasing air density, where they break, transferring large amounts of energy and momentum into the upper atmosphere. They can be detected as bright undulations in the glowing gas layers of the upper mesosphere using sensitive imaging systems. Nielsen et al. searched for gravity waves in the mesosphere above Halley Station, Antarctica. On 27 May 2001, they documented an unusual wave event exhibiting several features of a fast-moving "bore" pulse, which enhanced emissions in the hydroxyl, sodium, and oxygen mesospheric layers. Additional waves were created in its wake. Relatively common at mid- and low-latitudes, this event was the first documented case of a polar mesospheric bore. The authors suggeste that studying bores and other wave phenomena at all latitudes will help to better understand their dynamical impact on the mesosphere and refine global atmospheric circulation models.

Title: An unusual mesospheric bore event observed at high latitudes over Antarctica

K. Nielsen and M. J. Taylor: Center for Atmospheric and Space Sciences, Physics Department, Utah State University, Logan, Utah, USA; R. G. Stockwell: Colorado Research Associates Division, Northwest Research Associates Inc., Boulder, Colorado, USA; M. J. Jarvis: British Antarctic Survey, Cambridge, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025649, 2006

8. Earthquakes on the southern San Andreas may cause strong shaking in Los Angeles

The southernmost segments of the San Andreas Fault have a high probability of rupturing in the next few decades with an earthquake of a magnitude greater than 7.5. Olsen et al. simulated shaking in an area from northern Santa Barbara County down to northern Mexico using a model, called TeraShake, with high spatial resolution. These simulations showed that the chain of sedimentary basins between the city of San Bernardino and downtown Los Angeles form an effective waveguide that channels surface (Love) waves along the southern edge of the San Bernardino and San Gabriel mountains for earthquake scenarios with northward rupture. The authors showed that such earthquakes could produce unusually high long-period ground motions over much of the greater Los Angeles basin for several minutes, including intense and localized amplification of the seismic energy. The authors point out that future simulations should take into account the critical role of sedimentary basins in the southern San Andreas region, to accurately estimate the seismic hazards.

Title: Strong shaking in Los Angeles expected from southern San Andreas earthquake

Authors: K. B. Olsen and S. M. Day: Department of Geological Sciences, San Diego State University, San Diego, California, USA; J. B. Minster: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA; Y. Cui, A. Chourasia, M. Faerman, and R. Moore: San Diego Supercomputer Center, University of California, San Diego, La Jolla, California, USA; P. Maechling and T. Jordan: Department of Earth Sciences, University of Southern California, Los Angeles, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025472, 2006

9. Using surface topography to predict groundwater flow patterns

Land surface topography governs groundwater flow patters at both large and small scales, playing an important role in nutrient and contaminant transport, and in the water resources available to aquatic ecosystems. The interaction between surface water and groundwater flows is controlled by surface topography through pressure variations that are induced by stream flow over sediment bedforms and, at the landscape scale, the fact that groundwater surface generally follows the ground surface. To study this, Wörman et al. developed a Fourier-series spectrum that estimates groundwater flows in three dimensions, based on known surface topography. This provides a practical tool for quickly calculating the subsurface flow field and offers a theoretical platform for advancing conceptual understanding of the effect of landscape topography on subsurface flow, the authors note. Their method also shows how the residence time of subsurface flows is affected by surface topography and that hydrologic head in the subsurface decays exponentially with depth, faster than it would in a two dimensional model. This results in shallower interaction between surface water and groundwater.

Title: Exact three-dimensional spectral solution to surface groundwater interactions with arbitrary surface topography

Authors: Anders Wörman and Lars Marklund: Department of Biometry and Engineering, Swedish University of Agricultural Sciences, Uppsala, Sweden; Aaron I. Packman and Susan H. Stone: Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois, USA; Judson W. Harvey: U.S. Geological Survey, Reston, Virginia, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025747, 2006

II. Ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
1. Copy the portion of the digital object identifier (doi) of the paper following "10.1029/" (found under "Source" at the end of each Highlight).
2. Paste it into the second-from-left search box at http://www.agu.org/pubs/search_options.shtml and click "Go."
3. This will take you to the citation for the article, with a link marked "Abstract + Article."
4. Clicking on that link will take you to the paper's abstract, with a link to purchase the full text: "Print Version (Nonsubscribers may purchase for $9.00)."

The Highlights and the papers to which they refer are not under AGU embargo.

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