AGU journal highlights - 1 February 2005
The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research-- Atmospheres (JD), Journal of Geophysical Research--Oceans (JC), and Global Biogeochemical Cycles (GB). 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. To obtain the full text of the research paper, see Part II.
1. Detecting landmines with sound without touching the ground
Millions of landmines are buried around the world, causing thousands of injuries each year. Current landmine detectors, using the same technology as metal detectors, can locate mines with metal casings but fail on mines encased in plastic. They also raise false alarms, about 300 for each mine found. Van Wijk et al. have developed a prototype landmine detection system that bombards a minefield with sound waves that become shallow seismic waves underground. A microwave Doppler vibrometer suspended above the field measures the ground displacement and should eventually be able to identify and locate mines. The system, which might also be used to detect underground pipes or map contaminated soils, is currently being tested and improved.
Title: Toward noncontacting seismology
K. van Wijk, J. A. Scales, T. D. Mikesell, and J. R. Peacock,
Colorado School of Mines, Golden, Colorado, USA.
Source: Geophysical Research Letters (GL) paper
2. Aged soot loves water
Coal-burning power plants, diesel trucks, buses and all kinds of fires produce soot. This fine black carbon influences climate by affecting the lifetime of clouds and atmospheric chemistry by playing a role in the breakdown of many pollutants. Little is known, however, about how long soot stays in the atmosphere, so scientists have not been able to fully incorporate soot's effects into climate and atmospheric models. Fresh soot does not absorb water. But Zuberi et al. report that aged soot absorbs water, so that it can be washed out of the atmosphere by rain. The investigators made soot and aged it through oxidation under conditions of high relative humidity. They used a suite of analytical techniques to relate the soot's ability to absorb water to the extent of its aging and found that aged soot readily absorbs water, even though fresh soot repels it. The authors note that understanding the atmospheric processing of soot will improve local, regional, and global models for cloud formation and climate.
Title: Hydrophilic properties of aged soot
Bilal Zuberi, Kirsten S. Johnson, Gretchen K. Aleks, Luisa T.
Molina, Mario Molina, Massachusetts Institute of Technology,
Cambridge, Massachusetts, USA;
Alexander Laskin, Pacific Northwest National Laboratory,
Richland, Washington, USA.
Source: Geophysical Research Letters (GL) paper
3. Natural lightning emits X rays too
In 2003, scientists from Florida discovered that rocket-triggered lightning produces large quantities of highly energetic X rays. Now, Dwyer et al. report observations of X-ray emissions from natural lightning. They describe how intense bursts of many X rays, each about the same energy as those taken in a dentist's office, occur during the initial "stepped leader" phase of the lightning. The stepped leader precedes the return stroke that produces the light we see. Each X-ray burst corresponds to a step in the leader's path. The scientists made these measurements during the summer of 2004 in Florida. They used X-ray detectors mounted inside heavy aluminum cases that block out electromagnetic interference from thunderstorms and lightning. It is not clear what causes the X rays but an atmospheric process called "runaway breakdown" may be promising, they note. In such a process electrons would be accelerated, almost to the speed of light, by strong electrical fields. Collisions between these high-energy electrons and air molecules could create still more electrons and eventually generate X rays.
Title: X-ray bursts associates with leader steps in cloud-to-ground lightning
Authors: J. R. Dwyer, H.K. Rassoul, M. Al-Dayeh, L. Caraway, A.
Chrest, B. Wright, E. Kozak, Florida Institute of Technology,
Melbourne, Florida, USA;
J. Jerauld, M. A. Uman, V. A. Rakov, D. M. Jordan, K. J. Rambo,
University of Florida, Gainesville, Florida, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021782, 2005
4. The "sound" of sprites
Sprites, flashes that fleetingly light up the sky above thunderstorms, are a relatively newly discovered phenomenon, first reported in 1990. Sprites and other transient luminous events (TLEs) occur simultaneously with a very specific type of powerful lightning. Atmospheric scientists know that sprites produce infrasound, sound pitched too low for humans to hear, but they had not been able to simultaneously study the sprites and the low frequency sound waves they produce. Farges et al. report the first such unambiguous identification of infrasound signals emitted by sprites. In Europe during the summer of 2003, the investigators took photographs of sprites at night and measured chirp-like infrasound signals. They combined these observations with data from a network of lightning sensors and electromagnetic wave observations, so that they could correlate the two. By following a sprite's infrasound signature, the investigators were able to monitor daytime sprites, although the infrasound signal was weaker. They speculate that global infrasound monitoring networks might be used to estimate sprite energy dispersion in the atmosphere.
Title: Identification of infrasound produce by sprites during the Sprite2003 campaign.
T. Farges, E. Blanc, A. Le Pichon, Commissariat a l'Energie
Atomique, DASE/LDG, Bruyeres le Chatel, France;
T. Neubert, Danish Space Research Institute, Copenhagen,
T.H. Allin, Danish Technical University, Lyngby, Denmark
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021212, 2005
5. Will sunspot cycle 24 be the smallest in 100 years?
Sunspots occur on the surface of the Sun on an approximately 11-year cycle related to the Sun's magnetic field. At the solar maximum, sunspots pepper the solar surface and there are more solar storms that can cause power outages, disable satellites, and stop GPS receivers from working. Astronomers know that sunspot cycles vary in intensity, and they would like to be able to anticipate it. Svalgaard et al. predict that the next solar cycle could be the smallest in 100 years. The investigators analyzed measurements of the Sun's polar magnetic field for the past four sunspot cycles and looked for properties of one cycle that predict those of its successor. If they are right, then the Ulysses space probe will measure weaker fields when it passes over the Sun's poles in 2007 and 2008.
Title: Sunspot cycle 24: Smallest cycle in 100 years?
Authors: Leif Svalgaard, Yohsuke Kamide, Nagoya University, Toyokawa,
Edward W. Cliver, Air Force Research Laboratory, Hanscom Air
Force Base, Massachusetts, USA.
Source: Geophysical Research Letters (GL) paper
6. Tracking U.S. pollution to Europe
Plumes of pollution have been traced as they cross the Pacific Ocean from Asia to North America, but what about pollution that travels across the Atlantic from North America to Europe? Huntrieser et al. followed such a pollution plume from the eastern United States until it showed up as high levels of ozone and carbon monoxide in the Alps and died out about a week after leaving the U.S. The investigators used model predictions to direct a research aircraft into the polluted layer above Europe. By combining model predictions with trace gas measurements made on the ground and in the air, the scientists followed a plume that left the United States on 14 November 2001 and intercepted it over Scandinavia five days later at an altitude of about three kilometers [two miles]. They found that the impact of the plume was most noticeable at mountain sites in the Alps; at lower elevations the pollution signatures of the plume were more difficult to detect, due to local pollution.
Title: Intercontinental air pollution transport from North America to
Europe: Experimental evidence from airborne measurements and
H. Huntrieser, J. Heland, H Schlanger, Deutsches Zentrum fur
Luftund Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany;
C. Forster, Technische Universitat Munchen (TUM), Freising,
A. Stohl, O. Cooper, University of Colorado/Aeronomy Laboratory,
NOAA, Boulder, Colorado, USA;
H. Aufmhoff, F. Arnold, Max-Planck-Institut fur Kernphysik,
H.E. Scheel, Forschungszentrum Karlsruhe, IMK-IFU,
M. Campana, Eidgenosische Technische Hochschule, Zurich,
S. Gilge, Deutscher Wetterdienst (DWD) Hohenpeissenberg,
R. Eixmann, Leibniz-Institut fur Atmospharenphysik, Rostock,
Source: Journal of Geophysical Research--Atmospheres (JD) paper 10.1029/2004JD005045, 2005
7. Arctic warmth causes thin ice
Scientists have realized that polar bears are living on thin ice since 1999, when Drew Rothrock and colleagues analyzed radar soundings made underneath the ice by submarines and found that the volume of Arctic ice declined by over 20 percent from 1958 to 1999. Researchers have identified two likely causes of this thin ice: stronger Arctic winds or warmer springtime temperatures. Computer simulations can produce thin ice in the mid-1990s, but they differ over the explanations. Some point to an increase in surface temperatures, but others focus on the concentration of older and thicker ridged ice. Rothrock and J. Zhang have now conducted a computer simulation that allows them to separately consider wind and temperature effects. They used a model of the Arctic ice and ocean with actual measurements of daily surface temperatures and sea level pressure. They find that only the measured increases in Arctic temperatures can produce a decades-long decreasing trend in ice thickness and that the change shows up most strongly in the thinning of undeformed rather than ridged ice. The wind can account for large changes in ice thickness but causes the ice to swing from thick to thin and back again
Title: Arctic Ocean sea ice volume: What explains its recent depletion?
Authors: D.A. Rothrock and J. Zhang, University of Washington, Seattle, Washington, USA.
Source: Journal of Geophysical Research--Oceans (JC) paper 10.1029/2004JC002282, 2005
8. Dead zones likely to increase
Every summer in the Gulf of Mexico, a vast area becomes a dead zone bereft of living creatures, because the oxygen in the waters of the zone is severely depleted. The cause is excess nitrogen and phosphorous washed into the Gulf from the Mississippi River. Currently there are about 150 known dead zones throughout the world's oceans. Such zones are likely to increase in the future, according to an analysis by Bouwman et al. who predict that nitrogen inputs will increase in large parts of the world over the next three decades. The predictions are based on results of a global model that tracks the fate of nitrogen from its entry into the environment to a river and finally to the sea. Developing countries will see the biggest increase: 27 percent, due to higher nitrogen discharges that accompany urbanization, sanitation, and food production. Phosphorus discharges are also likely to mirror nitrogen increases. These changes will most severely impact the Indian and Pacific Oceans.
Title: Exploring changes in river nitrogen export to the world's oceans.
A.F. Bouwman, G. Van Drecht, J.M. Knoop, A.H.W. Beusen and
C.R. Meinardi, Netherlands Environmental Assessment Agency,
National Institute for Public Health and the Environment,
Bilthoven, The Netherlands.
Source: Global Biogeochemical Cycles (GB) paper
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