Public Release:  AGU Journal Highlights -- June 11, 2008

American Geophysical Union

1. Warming could release east Siberia's vast frozen carbon

East Siberia's permafrost contains about 500 Gigatons (1100 trillion pounds) of frozen carbon deposits that are highly susceptible to disturbances as the climate warms. Called the Yedoma, this permafrost has not undergone much alteration by soil microorganisms since its formation, which took place between 20,000 and 40,000 years ago. To investigate how easily this huge carbon stock could be degassed in future warming scenarios, Khvorostyanov et al. use a model of heat transfer and soil organic matter decomposition in frozen soils and find that specific conditions trigger the irreversible thawing of Yedoma, which is maintained by heat production by soil microbial activity. Once started, irreversible thawing could release 2.0-2.8 Gt (4.4-6.2 trillion lb) of carbon per year into the atmosphere between the years 2300 and 2400, transforming 74 percent of the initial carbon stock into carbon dioxide and methane. Further investigations reveal that the faster the planet's surface warms, the sooner fast deep-soil decomposition will start, although the tipping point above which soil carbon starts irreversible mobilization due to permafrost thawing increases slightly with larger external warming rates.

Title: Vulnerability of east Siberia's frozen carbon stores to future warming

Authors: D. V. Khvorostyanov: Laboratoire des Sciences du Climat et l'Environnement, IPSL/CEA-CNRS-UVSQ, Saclay, France; also at A. M. Obukhov Institute of Atmospheric Physics RAS, Moscow, Russia;

P. Ciais: Laboratoire des Sciences du Climat et l'Environnement, IPSL/CEA-CNRS-UVSQ, Saclay, France;

G. Krinner: Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS-UJF-OSUG, Saint Martin d'Hères, France

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033639, 2008;

2. Exploring magnetic holes in the solar wind

Observations of the solar wind have revealed isolated regions with depressed magnetic field magnitude over short durations. Called mirror mode waves, these waves can be thought of as magnetic holes in the solar wind with little or no directional change across them, although little is understood about their structure and evolution. Using magnetic field measurements from the Venus Express spacecraft, Zhang et al. investigate the structure of mirror modes near Venus's orbit and determine the size and shape of mirror modes by examining their durations as a function of the orientation of the magnetic field to the solar wind flow. They find that the mirror mode structure is best fitted with an ellipsoid of revolution, with the polar diameter longer than the equatorial diameter. The authors define two new parameters for mirror modes: the width across the field and the cross-sectional eccentricity. Further, they find that along the direction of the magnetic field, mirror mode structures are more elongated. The authors expect that these new observations will help scientists better describe and classify mirror modes.

Title: Characteristic size and shape of the mirror mode structures in the solar wind at 0.72 AU

Authors: T. L. Zhang: Space Research Institute, Austrian Academy of Sciences, Graz, Austria; also at State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, China;

C. T. Russell and L. K. Jian: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, U.S.A.;

W. Baumjohann, W. Zambellie, M. Volwerk, and M. Delva: Space Research Institute, Austrian Academy of Sciences, Graz, Austria;

M. A. Balikhin: Automatic Control and Systems Engineering, University of Sheffield, Sheffield, U.K.;

J. B. Cao and C. Wang: State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, China;

X. Blanco-Cano: Institute of Geophysics, UNAM, Ciudad Universitaria, Coyoacan, Mexico D.F., Mexico;

K.-H. Glassmeier: Institut für Geophysik und Extraterrestrische Physik, Technical University Braunschweig, Braunschweig, Germany;

Z. Vörös: Institute for Astro- and Particle Physics, University of Innsbruck, Innsbruck, Austria.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033793, 2008;

3. Radio-disrupting bubbles rise high in twilight sky

Equatorial spread F (ESF) is a postsunset phenomenon in which the F region of the ionosphere becomes unstable in an altitude range of roughly 200-1000 kilometers (120-620 miles). Large-scale bubbles of low electron density develop that are tens of kilometers (similar range for miles) wide and elongated along the Earth's magnetic field. These bubbles can disrupt radio signals and degrade communications and navigation systems. To understand the complex and dynamic evolution of ESF, Huba et al. developed a new three-dimensional model at the U.S. Naval Research Laboratory that self-consistently determines electric field strength and direction. By simulating a narrow wedge of the postsunset ionosphere, the authors find that bubbles can rise up to 1600 km (990 mi), significantly higher than previously modeled. Further, extremely steep ion density gradients can develop in both longitude and latitude. Finally, upward plasma velocities approach 1 km (0.6 mi) per second, and the growth time of instabilities is approximately 15 min. These results are consistent with radar and satellite observations.

Title: Three-dimensional equatorial spread F modeling

Authors: J. D. Huba and J. Krall: Plasma Physics Division, Naval Research Laboratory, Washington D.C., U.S.A.

G. Joyce: Icarus Research Inc., Bethesda, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033509, 2008;

4. Titanium may contribute to planets' magnetic anomalies

Magnetite commonly records the Earth's magnetic field in rocks. Magnetite can also have variable amounts of titanium in its structure, in which case it is referred to as titanomagnetite. Noting that the response of the magnetic signals of minerals to very high pressures is poorly understood, Gilder and Le Goff use diamond anvil cells and a superconducting magnetometer to measure changes in titanomagnetite's magnetic properties under the influence of pressure. They find that pressure systematically enhances the magnetic properties of titanomagnetite as a function of titanium concentration. For example, remanent magnetization intensities under pressure can increase by a factor of 2, 3, 13, and 21 for titanomagnetite with 0 percent, 20 percent, 40 percent, and 60 percent titanium, respectively. Further, the resistance of titanomagnetite's magnetization to external magnetic fields (called coercivity) also dramatically rises with pressure for high-titanium titanomagnetite. The authors suggest that such properties could explain deeply rooted magnetic anomalies on Earth and other planets.

Title: Systematic pressure enhancement of titanomagnetite magnetization

Authors: Stuart A. Gilder: Department of Earth and Environmental Sciences, Ludwig Maximilians University, Munich, Germany;

Maxime Le Goff: Laboratoire du Paléomagnétisme, Institut de Physique du Globe, Paris, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033325, 2008;

5. Reservoir size affects flow through porous materials

In aquifers and petroleum reservoirs, the behavior of fluid flow and mass transport is important to extracting resources and monitoring contaminants. This flow is strongly affected by the spatial distribution and continuity of sediments with high permeability, such as sands and gravels (collectively referred to here as "sands"). Using statistical models, Guin and Ritzi analyze fluid flow through percolation lattices, comparing the continuity of sands randomly distributed with those distributed and oriented according to a more orderly pattern. They find that having a more orderly distribution of sand does not affect percolation on an infinite lattice (i.e., a domain of infinite size). However, for finite lattices, the domain size plays an important role in lowering the threshold at which percolation occurs. The finite domain effect is created when cluster-size frequency distributions for either sand or nonsand regions are not fully expressed because they are truncated at domain boundaries. The analytical equations used in this analysis provide an explanation of percolation in finite domains that is clearer than those from prior approaches that use Monte Carlo simulations.

Title: Studying the effect of correlation and finite-domain size on spatial continuity of permeable sediments

Authors: Arijit Guin and Robert W. Ritzi Jr.: Department of Earth and Environmental Sciences, Wright State University, Dayton, Ohio, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032717, 2008;


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