AGU Journal highlights - 8 July 2004
Highlights, including authors and their institutions
The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research--Atmospheres (JD), and Water Resources Research (WR). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
1. Reading the magnetic writing on the wall
Researchers studying the historical magnetic field alignment in Central America have seen the writing on the wall. Goguitchaichvili et al. show that some of the paint used for wall murals in the Pre-columbian period contained magnetic hematite and magnetite particles, which indicated the Mesoamerican magnetic field before drying. The researchers analyzed nearly 30 samples from mural paintings in four sites in central Mexico that date back to 1200 C.E. and isolated a magnetic form of hematite in the red pigment used to paint the walls. They show that the magnetic particles in the paint assumed the direction of the Earth's magnetic field at the time and locked that information into place when it dried. The authors state that the dried paint can thus provide valuable information about the state of the magnetic field in the pre-recorded past.
Title: Pre-Columbian mural paintings from Mesoamerica as geomagnetic field recorders
Avto Goguitchaichvili, A. M. Soler, J. Urrutia-Fucugauchi, T. Gonzalez, Geophysical Institute, National Autonomous University of Mexico, Mexico City, Mexico;
E. Zanella, R. Lanza, University of Torino, Torino, Italy;
G. Chiari, Getty Conservation Institute, Los Angeles, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020065, 2004
2. Coastal soil breathing caused by sea tides
Air pressure fluctuations in coastal areas can initiate a process of underground "breathing" that causes subsurface soil to absorb or repel air. Jiao and Li created a two-dimensional flow model that reproduces the subsurface air pressure when it is not raining and suggest that such breathing significantly affects aquifer air flow and may be important to the remediation of soil pollutants. Intense rainfall can alter normal subsurface air pressure, creating pressure 2-9 times greater than during dry conditions. The authors found that tidal variations have the most important effect on changing the subsurface air pressure, showing that an elevated water table from rising tides forces air from the ground and raises air pressure. Similarly, a falling water table causes the ground to inhale and absorb greater amounts of air. The natural breathing process studied by the researchers in Hong Kong is relevant to coastal environments worldwide.
Title: Breathing of coastal vadose zone induced by sea level fluctuations
Jui J. Jiao, University of Hong Kong, Hong Kong, China;
Hailong Li, China University of Geosciences, Wuhan, China, and Anshan Normal University, Anshan, China.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019572, 2004
3. Measuring elusive drift waves
Space-based electrostatic instabilities known as drift waves may cause the deterioration of well-defined structures in the Earth's magnetic field. Michael C. Kelley suggests that the small-scale drift waves, ubiquitous in laboratory experiments, but rarely observed in the planetary ionosphere, could cause the breakdown of needle-like irregularities seen along magnetic field lines. The authors analyzed data from a rocket flight through a high-powered high-frequency beam, which raised the electron temperature and stimulated thousands of needle-shaped structures in the Earth's ionosphere. Kelley proposes, however, that the density depletions and temperature enhancements are unstable to the generation of drift waves, which can break down into weaker hybrid waves. He notes that the hybrid waves may be responsible for scattering high-frequency radio signals in the ionosphere that were previously attributed to the needle-like filaments.
Title: Evidence for drift waves in ionospheric heating experiments
Author: Michael C. Kelley, Cornell University, Ithaca, New York.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020105, 2004
4. Generalized law for aftershocks
Researchers have combined three earthquake aftershock laws to create a general theory for estimating the probability of future aftershocks. Shcherbakov et al. combined the Gutenberg-Richter frequency magnitude relation, Bath's law, and the modified Omori's law to provide a generalized law for aftershock decay rates. The authors suggest that the time constant used in the modified Omori's law should instead be a variable, dependent on the magnitude of the quake and lower magnitude cutoff. They then studied several large quakes with readily identified aftershocks and report that the aftershock decay rates depended on several parameters specific to each seismic region. The researchers tested their generalized law against several well documented earthquakes and aftershocks, including the Northridge, Landers, and Hector Mine temblors, and found that the law can be used to forecast the rate of aftershock occurrence after specific information about the main shock's magnitude and the area's land is known.
Title: A generalized Omori's law for earthquake aftershock decay
Robert Shcherbakov, John B. Rundle, Center for Computational Science and Engineering, University of California, Davis, California, USA; Donald L. Turcotte, University of California, Davis, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019808, 2004
5. Mapping a flux transfer event
A two-dimensional map generated from data taken by the four Cluster spacecraft has provided researchers an improved technique to study a mysterious phenomenon known as a flux transfer event. Sonnerup et al. used magnetic field and plasma data from the Cluster group to determine the orientation, cross section, and size of the object that caused the event, a feature often observed near the outer boundary of Earth's magnetic field and believed to be caused by a short interaction between interplanetary plasma and the planetary magnetic field. The authors used a novel reconstruction technique to show that the feature is likely caused by a flux "rope" embedded in the magnetic field boundary that was detected by the spacecraft. Their map shows that the flux rope was moving at supersonic speeds and had a strong core field, which indicates that it originated from magnetic field lines on either side of the magnetic field boundary.
Title: Anatomy of a flux transfer event seen by Cluster
Bengt U. O. Sonnerup, H. Hasegawa, Dartmouth College, Hanover, New Hampshire, USA;
G. Paschmann, Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020134, 2004
6. Global sea level rise likely overestimated
An analysis of recent sea level change indicates that freshwater added to the global ocean, combined with ocean warming, leads to a rise of nearly 1.1 millimeters [0.043 inches] per year, a figure slightly lower than previous estimates. Wadhams and Munk estimated that approximately 650 cubic kilometers [170 million gallons] of fresh water is added into the world's oceans each year, resulting in a sea level increase that is slightly below previously published estimates between 1.5-2 millimeters [0.059-0.079 inches] per year. The authors note that melting sea ice, seen in the shrinkage and thinning observed on the edges of the Arctic ice sheet, contributes nearly two thirds of the new freshwater and also leads to reducing the salinity of seawater, but does not cause any changes to the sea level. They also suggest that any sea level increase from fresh water running from the land is offset by a growth in the Antarctic ice sheet. Their study used estimates from the World Ocean database, containing millions of records of ocean temperature profiles, and submarine sonar data to examine salinity reductions in the oceans.
Title: Ocean freshening, sea level rising, sea ice melting
Peter Wadhams, University of Cambridge, Cambridge, United Kingdom, and Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, United Kingdom;
Walter Munk, Scripps Institution of Oceanography, University of California, La Jolla, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020039, 2004
7. First analysis of GRACE gravity field data
Newly available gravity field data have allowed researchers improved information to estimate monthly changes in water levels worldwide. Wahr et al. report on 11 recently released monthly GRACE gravity field data sets and suggest that the new information can provide immense improvements to the accuracy of water storage predictions. The data come from the Gravity Recovery And Climate Experiment (GRACE) satellite mission to provide detailed monthly maps of the Earth's gravity field, which can be used to study changes in the planetary mass distribution. The authors found that water storage in the global ocean can be determined down to approximately 1.5 centimeters [0.59 inches] in amplitude, including small errors that can come from model and satellite mistakes. They used the data to refine the water storage estimates for three large drainage basins: the Mississippi and Amazon River basins and the union of two systems that flow into the Bay of Bengal.
Title: Time-variable gravity from GRACE: First results
John Wahr, Sean Swenson, Isabella Velicogna, Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, and University of Colorado, Boulder, Colorado, USA;
Victor Zlotnicki, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019779, 2004
8. Analyzing recent surface energy trends
An analysis of surface temperature and solar radiation records from the late 20th century indicates that the observed intensification of the hydrological cycle outside the tropics was likely caused by the transfer of moist air from the oceans rather than from evaporation. Wild et al. analyzed more than 30 years of recent global surface energy balance data in an attempt to account for the seemingly counterintuitive observations of reduced surface sunshine and increased surface temperatures. The authors' database analysis, which included records from more than 2,000 sites around the world, found no indications of increased radiative heating between 1960 and 1990, thus ruling out increased atmospheric moisture from evaporation. Their results indicate that the inferred decline in surface evaporative cooling may have been responsible for the observed temperature increase, despite the reduction in subsurface radiative heating over that period.
Title: On the consistency of trends in radiation and temperature records and implications for the global hydrological cycle
Martin Wild, Atsumu Ohmura, Hans Gilgen, Institute for Atmsopheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland;
Daniel Rosenfeld, Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019188, 2004
9. Atmospheric ions and aerosol formation
A new study quantifies the role of cosmic rays and fossil fuel combustion in the formation of atmospheric particles. Lovejoy et al. show that sulfuric acid and water condense on atmospheric ions to generate the new particles. The authors developed a model of cosmic ray-induced particle formation based on laboratory data and then compared the model predictions with field observations of small particles. They report that the model reproduced observed particle formation in the middle and upper troposphere, which suggests a direct link between atmospheric ionization and small particle formation. The study can help researchers better understand the influence of sources of ions, like cosmic rays, and sources of sulfuric acid, like fossil fuel combustion, on the formation of new atmospheric particles.
Title: Atmospheric ion-induced nucleation of sulfuric acid and water
Edward R. Lovejoy, Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA; J. Curtius, Institute for Atmospheric Physics, University of Mainz, Mainz, Germany;
K. D. Froyd, Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, and Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA.
Source: Journal of Geophysical Research-Atmospheres (JD) paper 10.1029/2003JD004460, 2004
10. New method to estimate deforestation impacts on water cycle
A new technique to compare long-term data records of the same water basins suggests that water cycle changes can persist for nearly four decades following deforestation. Jones and Post used a novel "paired-basin" experimental method to compare streamflow changes over the past 70 years in the U.S. Pacific Northwest and Eastern forests, including the water response from seasonal changes, climate warming and forest regrowth. The authors note that while the extent of the daily increases in water flow after the forest removal ranged from 2-3 millimeters [0.08-0.1 inches] per day in eastern deciduous forests to 6-8 millimeters [0.2-0.3 inches] in the western conifer forest sites, the age of the forest before it was cut also had a significant influence on the flow changes. They propose that the paired-basin technique, which includes continuous monitoring of growing vegetation and climate patterns, provides the best estimates of water cycle changes from forest removal.
Title: Seasonal and successional streamflow response to forest cutting and regrowth in the northwest and eastern United States
Julia A. Jones, David A Post, Oregon State University, Corvallis, Oregon.
Source: Water Resources Research (WR) paper 10.1029/2003WR002952, 2004
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