AGU journal highlights - 23 June 2004

06/23/04

The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research--Solid Earth (JB), Journal of Geophysical Research--Atmospheres (JD), and Radio Science (RS). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.


1. Why nitric oxide lights up the sky

A new study shows that electron interactions may have nearly the same importance as chemical luminescence in exciting the atmospheric molecules that cause auroras. Campbell et al. suggest that nitric oxide molecules are promoted into potentially glowing, vibrational excited levels by a short-lived negative ion that is formed as a result of electron impact. Previous studies had reported that chemiluminescence, where nitrogen and oxygen molecules interact to form excited nitric oxide, was the main source of auroral lights produced by nitric oxide molecules. The authors analyzed the electron interactions with nitric oxide molecules in the upper atmosphere and found that the momentary (10-100 femtoseconds or quadrillionths of a second) creation of a negative ion increases the probability of producing the excitation levels that are responsible for auroral lights by up to two orders of magnitude. They suggest that the electron-driven effect results from a chance convergence of nitric oxide's molecular structure and the natural distribution of electrons in the upper atmosphere. Title: Infrared auroral emissions driven by resonant electron impact excitation of NO molecules

Authors:
Laurence Campbell, M. J. Brunger, Flinders University, Adelaide, South Australia; Z. Lj. Petrovic, Institute of Physics, Belgrade, Serbia and Montenegro; M. Jelisavcic, R. Panajotovic, S. J. Buckman, Australian National University, Canberra, Australia.

Source: Geophysical Research Letters (GL) paper 10.1029/2003GL019151, 2004


2. Elephants communicate without a sound?

Elephants may be able to communicate over long distances but it's hard to tell, because they're not talking. Instead, they may be transmitting infrasonic signals through the ground with seismic waves. Gunther et al. studied the range of low-frequency vocal sounds made by African elephants and suggest that the signal's two kilometer [one mile] range can propagate further than the animals' airborne vocalizations. Their study suggests that it is possible for elephants to communicate over longer distances than can be heard, although they propose that it is unlikely that the seismic waves are the primary mode of elephant communication. The researchers found that normal elephant rumblings can be heard at distances between four and five kilometers [two and three miles] under normal atmospheric conditions, but that the ground waves could be useful for communicating a warning or the herd's location during a storm or adverse conditions that would block the airborne sound waves.

Title: Seismic waves from elephant vocalizations: A possible communications mode?

Authors:
Roland H. Gunther, Caitlin E. O'Connell-Rodwell, Simon L. Klemperer, Stanford University, Stanford, California.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019671, 2004


3. Modified equipment can provide high-resolution ionospheric data

Researchers have modified the European Incoherent Scatter Svalbard radar to potentially provide detailed information of even a single event in the polar ionosphere. Oksavik et al. demonstrate a new observation mode for the EISCAT instrument with the potential for monitoring small-scale flow variations in the Earth-space boundary. The authors used the technique to provide high-resolution data for the flow patterns surrounding a single poleward-moving auroral form (PMAF) event, a phenomenon associated with magnetic storms and substorms seen from the polar cap. PMAF events have been studied for more than 20 years as precursors to larger ionospheric disturbances. The researchers suggest that a more specific understanding of a single ionospheric event can help them better understand the larger phenomena that disrupt the planetary magnetic field and affect Earth systems, including communication equipment to satellite transmissions.

Title: High-resolution observations of the small-scale flow pattern associated with a poleward moving auroral form in the cusp

Authors:
Kjellman Oksavik, University of Oslo, Oslo, Norway, and Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland; J. Moen, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, and Arctic Geophysics, University Centre in Svalbard, Longyearbyen, Norway; H. C. Carlson, Air Force Office of Scientific Research, Arlington, Virginia.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019838, 2004


4. Volcanic eruptions' effect on global climate change

Including the atmospheric effects from volcanic eruptions in general circulation models may be the most important factor in improving the accuracy of long-term climate simulations. Vyushin et al. analyzed general circulation model estimates for the 20th century climate and found that volcanic eruptions, rather than the manmade effects from increasing greenhouse gas and aerosol levels, are the most significant factor in determining long-range correlations of surface air temperature. The authors tested more than 30 land and oceanic sites indicative of global climate change, using a full range of manmade and natural climate change factors, including the effects from enhanced greenhouse gases, aerosols, ozone, solar radiation, and volcanic eruptions, and found that the simulations containing volcanic effects most closely matched the observed long-term behavior of surface temperatures, which is a vital scaling factor used in climate model predictions.

Title: Volcanic forcing improves atmosphere-ocean coupled general circulation model scaling performance

Authors:
Dmitry Vyushin, I. Zhidkov, S. Havlin, S. Brenner, Bar-Ilan University, Ramat-Gan, Israel; A. Bunde, Institute for Theoretical Physics, Justus-Liebig University Giessen, Giessen, Germany.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019499, 2004


5. First detection of flare from total solar irradiance observations

An analysis of total solar irradiance data has allowed researchers to make the first detections of a solar flare from radiation-sensing satellites. Woods et al. report that total solar irradiance dropped by an unprecedented 0.34 percent during the strong solar storm in October-November 2003, because of large, dark sunspots observed by two NASA satellite missions. They also present data showing marked short-term solar radiation increases during one of the strongest X-class events seen during the event. The two-week solar storm included unusually powerful flare and particle eruptions that produced 44 M (moderate)-class flares and 11 X (extreme)-class flares, including a powerful X28-sized flare. The satellite data provide the first definitive measurement of a flare detected using total solar irradiance instruments, which also recorded unexpected ultraviolet variations over a broad wavelength from the Sun's storm.

Title: Solar irradiance variability during the October 2003 solar storm period

Authors:
Thomas N. Woods, Francis G. Eparvier, Juan Fontenla, Jerald Harder, Greg Kopp, William E. McClintock, Gary Rottman, Byron Smiley, Martin Snow, University of Colorado, Boulder, Colorado.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019571, 2004


6. Connecting the San Andreas Fault through Southern California

A web of faults links the San Andreas Fault over a discontinuous 80-kilometer [50 mile] region in southern California. Carena et al. analyzed a gap in the famous fault line that runs from the Mojave Desert to the Coachella Valley and suggest that a network of seismically active faults likely connects the two strands of the 1,200-kilometer [750-mile] San Andreas Fault. The researchers examined the three-dimensional geometry of the fault system in the complex region, reaching nearly 20 kilometers [10 miles] below the Earth's surface. They report that the San Andreas devolves into a series of faults with varying configurations such that it would require an unlikely sequence of fault ruptures to trigger a massive earthquake involving both strands of the fault. The authors also modeled several possible fault rupture scenarios for earthquakes in the Los Angeles area to determine the likelihood of a complex rupture.

Title: Lack of continuity of the San Andreas Fault in southern California: Three-dimensional fault models and earthquake scenarios

Authors:
Sara Carena, John Suppe, Princeton University, Princeton, New Jersey; Honn Kao, Pacific Geoscience Centre, Geological Survey of Canada, Sidney, British Columbia, Canada.

Source: Journal of Geophysical Research-Solid Earth (JB) paper 10.1029/2003JB002643, 2004


7. Portion of Greenland magnetism comes from extraterrestrial source

Part of the magnetic dust observed in Greenland ice cores may come from extraterrestrial sources, although most of the surface magnetism of ice also comes from iron-rich particles deposited by airborne dust. Lanci et al. used magnetic testing techniques on ice cores from central Greenland to trace the iron oxide content and total dust concentration in polar sources. They suggest that measuring the magnetic strength of dust deposited on the icy surface can provide researchers an improved method to estimate the source of airborne aerosols and reconstruct ancient climate changes. Such magnetic testing methods had previously been applied to environmental analyses of windblown particles deposited on land but had never been used within ice. The authors examined the faint magnetic signal from mineral particles trapped under layers of frozen sediment in Greenland and, although they cannot rule out alternative sources of iron dust contamination, found that the observed magnetization is closely correlated with the dust concentration in the ice.

Title: Magnetization of Greenland ice and its relationship with dust content

Authors:
Luca Lanci, University of Urbino, Urbino, Italy, and Rutgers University, Piscataway, New Jersey; D. V. Kent, Rutgers University, Piscataway, New Jersey, and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York; P. E. Biscaye, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York; J. P. Steffensen, University of Copenhagen, Copenhagen, Denmark.

Source: Journal of Geophysical Research-Atmospheres (JD) paper 10.1029/2003JD004433, 2004


8. Global warming melting Alps?

Researchers propose that an observed rise in the ice-melting rate during the summer and an extension of the melting periods through October in the Alps may be caused by global warming. Vincent et al. analyzed more than 50 years of data showing the annual mass balance changes for two glaciers in the Alps and report anomalous ice melting that was likely caused by climate change. The authors note an ice reduction of nearly half a centimeter [two-tenths of an inch] per day over the widely spaced glaciers, corresponding to a recorded 20 watts per square meter [two watts per square foot] rise in energy at the glacier surface since the 1950s. The researchers note, however, that historical observations indicate that glaciers formed in the world's mountainous regions during the Little Ice Age have slowly retreated since the end of that period. They suggest that the high ice depletion rates seen over the past 20 years is likely driven by warmer summer temperatures that may be initiated by human influences.

Title: Ice ablation as evidence of climate change in the Alps over the 20th century

Authors:
Christian Vincent, E. Le Meur, National Center for Scientific Research, Saint Martin d'Heres, France; G. Kappenberger, Meteo Swiss, Locarno Monti, Switzerland; F. Valla, National Center for Agricultural Mechanization, Water, and Drilling, Saint Martin d"heres, France; A. Buader, M. Funk, School of Hydraulic Engineering, Hydrology, and Glaciology, Zurich, Switzerland.

Source: Journal of Geophysical Research-Atmospheres (JD) paper 10.1029/2003JD003857, 2004


9. Worst case scenario of methane hydrate release

A worst-case scenario of climate change from the possible future release of submerged methane hydrates predicts catastrophic warming in the atmosphere and rising sea level similar to conditions that preceded the last ice age. Renssen et al. simulated the climate response from a massive release of methane from gas hydrates in the oceans, using a three-dimensional model to estimate the changes to the atmosphere-sea ice-ocean system over 2,500 years. Although the researchers do not speculate on what could initiate the temperature increase, their results indicate that an incremental oceanic warming above a few degrees Celsius [Fahrenheit] could initiate a chain reaction that would raise the water temperatures in the intermediate depths and disturb even more frozen hydrates. The current study provides the most detailed examination of the potential warming caused by a methane hydrate-fueled enhancement of the greenhouse effect.

Title: Modeling the climate response to a massive methane release from gas hydrates

Authors:
Hans Renssen, C. J. Beets, D. Kroon, Vrije University Amsterdam, The Netherlands; T. Fichefet, H. Goosse, George Lemaitre Institue for Astronomy and Geophysics, Catholic University of Louvain, Louvain-la-Neuve, Belgium.

Source: Paleoceanography (PA) paper 10.1029/2003PA000968, 2004


10. Global Positioning System for Mars?

A new study examines the factors that would enable researchers to create a Martian version of the Global Positioning System widely used on Earth. Mendillo et al. investigated the planet's ionospheric characteristics with radio signal data taken from the Mars Global Surveyor spacecraft and analyzed how local time, latitude, and solar cycle patterns would affect Mars' electron content and contribute to errors in estimating exact locations on the planet's surface. They note that, as seen on Earth, a planet's ionosphere imparts a delay on radio transmissions between an orbiting satellite and ground receiving stations that can hinder precise location of ground sites. The magnitude of the delay effect on Mars would depend on the radio frequency selected for its satellite navigation system, or it could be overcome by using a dual-frequency system. The authors suggest that a constellation of GPS-like satellites could be introduced to improve navigation and provide continual monitoring of Martian features and locations with an expected margin of error of around one meter [three feet].

Title: Ionospheric effects upon a satellite navigations system at Mars

Authors:
Michael Mendillo, Steven Smith, Carlos Martinis, Jody Wilson, Center for Space Physcis, Boston University, Boston, Massachusetts; Xiaoqing Pi, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California; David Hinson, Stanford University, Stanford, California.

Source: Radio Science (RS) paper 10.1029/2003RS002933, 2004

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