AGU journal highlights - 30 November 2004

11/30/04

The following highlights summarize research papers in Geophysical Research Letters (GL) and the Journal of Geophysical Research--Atmospheres (JD). 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 following 10.1029/ (e.g., 2004GL987654). The doi is found at the end of each Highlight, below.

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1. How hydrogen affects upper mantle melting

An experiment to test the melting conditions in the deep Earth has found that slightly wet minerals can melt from extreme heat approximately 5-20 kilometers [3-10 miles] deeper than dryer formations in the upper mantle. Aubaud et al. report on high-pressure and temperature laboratory tests that measured hydrogen levels in wet and dry rock formations, which have a significant influence on the initiation of melting and on the underground flow of the lithosphere [solid portion of Earth]. The authors analyzed the separation of hydrogen atoms within the minerals and magma, using a newly developed ion mass spectrometry method that allowed them to detect extremely small hydrogen limits for the first time. The technique permits them to measure the hydrogen partition coefficient between major mantle phases and melts from high pressure and high temperature conditions in the upper mantle. They suggest that hydrogen may not be as effective in inducing deep melting beneath ridges as previously believed.

Title: Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts

Authors:
Cyril Aubaud, Marc M. Hirschmann, University of Minnesota, Minneapolis, Minnesota, USA;
Erik H. Hauri, Carnegie Institute of Washington, Washington, D.C., USA.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL021341, 2004

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2. Better monitor for active volcanoes

A new computing technique to analyze seismic data has provided a map of the plumbing system beneath Hawaii's Kilauea Volcano and could provide a method to monitor active volcanoes worldwide. Dawson et al. report that data compiled from a single computer can measure very-long-period seismic energy within a volcano and allow researchers to quickly locate the source of such signals. The authors used near real-time data from a seismic network that has constantly observed the active Hawaiian volcano and calculated the seismic properties of very-long-period events, which allowed them to identify the source of the slight tremors. Their study indicates the conduit that carries magma is a steeply sloped system of sills and dikes near the shallow Halemaumau Pit Crater. The researchers suggest that the new analysis method takes less computing power and could be used to determine the three-dimensional structure of a caldera's magmatic flow system.

Title: Application of near real-time radial semblance to locate the shallow magmatic conduit at Kilauea Volcano, Hawaii

Authors:
Philip Dawson, B. Chouet, U.S. Geological Survey, Menlo Park, California, USA; D. Whilldin, Center for the Study of Active Volcanoes, Hilo, Hawaii, USA.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL021163, 2004

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3. Improving earthquake warning in Costa Rica

A study of the Costa Rica earthquake zone has confirmed the depth of the crust-mantle boundary and mantle seismic velocities in the region, providing new data for researchers to improve earthquake hazard estimates for the area. DeShon and Schwartz report that the crust-mantle interface is located approximately 36 kilometers [22 miles] below sea level and that its seismic properties reveal serpentinization, or a change in the structure of the mantle's rock caused by fluids that can terminate seismic rupture. The authors examined seismic waves from a segment of the Middle America subduction zone below Central America that is capable of producing strong earthquakes and located microseismicity beneath Costa Rica's Nicoya Peninsula. Their analysis also found decreased seismic velocities, likely caused by fluid in the mantle above the tectonic plate interface, which they interpret as evidence that fluid had affected nearly 15-25 percent of the mineral structure.

Title: Evidence for serpentinization of the forearc mantle wedge along the Nicoya Peninsula, Costa Rica

Authors:
Heather R. DeShon, University of California, Santa Cruz, California, and University of Wisconsin, Madison, Wisconsin, USA;
Susan Y. Schwartz, University of Wisconsin, Madison, Wisconsin, USA.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL021163, 2004

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4. Bringing the heliosphere to amateur astronomers

The first identification of the impact between a comet and a fast-moving coronal mass ejection may allow researchers and amateur astronomers to identify such interplanetary solar ejections throughout the solar system. Jones and Brandt present the first evidence for a direct connection between coronal mass ejections and the large-scale disruption of a comet's tail. The authors correlated the appearance, speed, and direction of solar eruptions observed by a powerful satellite instrument with observations by amateur astronomers of scalloping and the subsequent disruption of charged gases in the comet's tail. They suggest that the changes in the tail's shape were caused by coronal mass ejection material wrapping around the charged particles behind the comet. The researchers show that comets can be used to monitor heliospheric conditions and that amateurs and professional astronomers can create a network to track solar ejections.

Title: The interaction of comet 153P/Ikeya-Zhang with interplanetary coronal mass ejections: Identification of fast ICME signatures

Authors:
Geraint H. Jones, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;
John C. Brandt, University of New Mexico, Albuquerque, New Mexico, USA.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL021166, 2004

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5. Stopping stick-slip seismic events?

Detailed information from a seismic experiment in Japan has provided researchers with an improved technique to observe frictional melting along fault lines and a potential mechanism to stop such seismic slip. Koizumi et al. analyzed data from a simulated intermittent (stick-slip) seismic event, where underground pressure builds and initiates ground movement that is eventually stopped by friction, and detected the exact moment the slip began and the maximum speed of the ground motion. Such stick-slip events are relatively common in fault lines across the globe, although it is unclear what effects the melting from subsurface shifts have on seismic behavior. The authors modeled underground frictional resistance to determine the amount of energy needed to weaken a fault plane enough to initiate ground motion and simulated the speed of the recovery after the pressure subsides. Their study indicates that stick-slip events stop spontaneously when frictional melting begins, suggesting a potential way to stop the seismic events.

Title: Frictional melting can terminate seismic slips: Experimental results of stick-slips

Authors:
Yosuke Koizumi, Kenshiro Otsuki, Hiroyuki Nagahama, Tohoku University, Sendai, Japan;
Akihiro Takeuchi, Niigata University, Niigata, Japan.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL020642, 2004

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6. Using GPS for earthquake imaging

A dense array of Global Positioning System (GPS) stations operating at a frequency of 1 Hertz has been used to image an earthquake rupture for the first time. Usually such analyses are made with seismometers. Advances in GPS instrumentation and capabilities allowed Miyazaki et al. to use the satellite system to model how the Earth slipped during the 8.0-magnitude Tokachi-Oki earthquake near Japan in 2003. The strong quake caused the fault to move up to nine meters [30 feet] near the island of Hokkaido, on the tectonic plate boundary between Japan and the Pacific Plate. Their analysis found that the earthquake also caused displacements of more than one meter [three feet] in approximately 20 seconds on Hokkaido. The researchers found that while the satellite data are less precise than traditional seismic data, GPS has the advantage of measuring displacement, whereas seismometers cannot distinguish between the ground's acceleration and rotation.

Title: Modeling the rupture process of the 2003 September 25 Tokachi-Oki (Hokkaido) earthquake using 1-Hz GPS data

Authors:
Shin'ichi Miyazaki, Earthquake Research Institute, University of Tokyo, Tokyo, Japan, and Stanford University, Stanford, California, USA;
Kristine M. Larson, Kyuhong Choi, University of Colorado, Boulder, Colorado, USA;
Kazuhito Hakima, Kazuki Koketsu, Earthquake Research Institute, University of Tokyo, Tokyo, Japan;
Paul Bodin, University of Memphis, Memphis, Tennessee, USA;
Jennifer Haase, Gordon Emore, Purdue University, West Lafayette, Indiana, USA;
Atsushi Yamagiwa, Geographical Survey Institute, Tsukuba, Japan.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL021457, 2004

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7. Diurnal changes in ocean's carbon cycle

Daytime heating and nighttime cooling affects the carbon cycle in the oceans such that the global oceans' uptake of carbon dioxide is likely more than twice as large during the evening than during the day. Olsen et al. analyzed the effects of diurnal variations of sea surface temperature and wind speed on the pressure of the oceans' carbon dioxide and on the sea-air carbon dioxide exchange. The authors combined satellite observations and existing data to examine the movement of carbon dioxide over a 24-hour period and found that although variations in sea surface temperature over the course of a day affect the air-sea carbon dioxide flux worldwide, the effects are largest in the southern latitudes, and wind speed effects are restricted to warmer tropical ocean zones. They then created new equations that would capture the magnitude of the variations and test whether sea surface temperature and wind speed had a significant effect on the ocean-carbon cycle.

Title: Diurnal variations of surface ocean pCO2 and sea-air CO2 flux evaluated using remotely sensed data

Authors:
Are Olsen, Abdirahman M. Omar, University of Bergen, Bergen, Norway, and Bjerknes Centre for Climate Research, Bergen, Norway;
Alice C. Stuart-Menteth, Southampton Oceanography Centre, Southampton, United Kingdom;
Joaquin A. Trinanes, University of Santiago de Compostela, Santiago de Compostela, Spain.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL020583, 2004

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8. Better estimates for tropospheric pollution

A new method to analyze carbon monoxide observations in the troposphere [lower atmosphere] can help researchers estimate the pollutant gas levels and may improve their ability to predict changes in the chemical composition of the troposphere. Petron et al. examined monthly global estimates of natural and manmade surface emissions from fossil fuel, biomass burning and natural emissions for one year and provide estimates for the total carbon monoxide output by Asia, Africa, Europe, and North, South, and Central America. The authors modeled the emissions based on data from NASA's Terra satellite. They report that their results provide a substantial improvement in the agreement between model predictions, compared to previous carbon monoxide observations, and suggest that their mathematical techniques can be used to constrain future surface pollutant levels in the lower atmosphere.

Title: Monthly CO surface sources inventory based on the 2000-2001 MOPITT satellite data

Authors:
Gabrielle Petron, Boris Khattatov, Varery Yudin, Jean-Francois Lamarque, Louisa Emmons, John Gille, David P. Edwards, National Center for Atmospheric Research, Boulder, Colorado, USA;
Claire Granier, Pierre-Simon Laplace Institute, University of Paris, and National Center for Scientific Research, Paris, France.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL020560, 2004

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9. Improving water cycle estimates

Gravity observations from NASA's GRACE satellites can be used to infer monthly changes in water stored in the land surface, providing new insight into global weather and evaporation. Rodell et al. present equations for estimating the amount of water transferred from land to air, or evapotranspiration, by combining the satellite information with other water cycle observations. Evapotranspiration from sources like groundwater, surface water, soil moisture, and snow replenishes atmospheric humidity that helps initiate and sustain storms. The authors used the new equations to analyze the water budget of the Mississippi River basin and found that their results compared favorably with other modeling systems. They suggest that the results from their study could be used in future models and could lead to improvements in weather predictions and a better understanding of Earth's climate and water cycle.

Title: Basin scale estimates of evapotranspiration using GRACE and other observations

Authors:
Matthew Rodell, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
J.S. Famiglietti, S. Holl, University of California, Irvine, California, USA;
J. Chen, C. R. Wilson, University of Texas, Austin, Texas, USA;
S. I. Seneviratne, Goddard Earth Sciences and Technology Center, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
P. Viterbo, European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom.

Source: Geophysical Research Letters (GL) paper: 10.1029/2004GL020873, 2004

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10. New method to locate lightning

A new method to locate lightning can pinpoint the location of intense lightning flashes around the globe and may allow researchers to track global thunderstorm activity from a single point on Earth's surface. Greenberg and Price developed an improved algorithm that can determine the source location of unusually intense lightning strokes around the globe from a single observation station. The authors tracked the extremely low frequency radiation that is a hallmark of unusually strong lightning and analyzed nearly 150 such events by comparing their lightning locations with satellite observations of the type of storms that generate such strong lightning. They found that while the most intense lightning flashes tend to occur in the tropical regions, the majority of them occurred over the ocean. The researchers suggest that the new technique may eventually replace existing very low frequency data, which require more observation stations to generate accurate results.

Title: A global lightning location algorithm based on the electromagnetic signature in the Schumann resonance band

Authors:
Eran Greenberg, Colin Price, Tel Aviv University, Tel Aviv, Israel.

Source: Journal of Geophysical Research-Atmospheres (JD) paper: 10.1029/2004JD004845, 2004

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