The following highlights summarize research papers in Geophysical Research Letters (GRL). 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 (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.
1. Sudden pressure increase in magma reservoir triggered by gigantic lava-dome collapse on Montserrat
The 13 July 2003 collapse of the Soufrière Hills Volcano (SHV) lava dome on Montserrat, British West Indies, loosened more than 210 million cubic meters [270 million cubic yards] of the dome into the Caribbean Sea, making it the largest such event worldwide in the historical record. Voight et al. analyzed pressure changes within the SHV's magma chamber, recorded by dilatometers placed within four 200-meter-[700-foot-]deep boreholes strategically located around the volcano. The near sites underwent expansion while the far site experienced contraction, they found, indicative of a deep-source volume expansion. The strain data was evaluated to determine the depth of the pressure source, and suggested a slightly flattened ellipsoidal-shaped magma chamber source at a centroid depth of roughly six kilometers [four miles]. Source volume was weakly constrained, suggesting an average radius of about one kilometer [0.6 mile]. The observed increase in pressure was consistent with the magma containing pressurized bubbles that expanded when the ambient pressure was reduced; under such conditions the pressure recovery from bubble growth could exceed the initial pressure decrease. Thus the magma within the chamber was likely supersaturated with gas before the dome collapse.
Title: Unprecedented pressure increase in deep magma reservoir triggered by lava-dome collapse
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024870, 2006
2. A simplified model of dissolution of rock fractures
During dissolution in porous or fractured rock, channels become a means for rapid transport of fluid within the rock matrix, playing an important role in fundamental and applied geophysics problems, such as the development of limestone caverns or the sequestration of carbon dioxide. Through analysis of the coupling between fluid flow and the chemical kinetics governing this transport, Szymczak and Ladd revealed that as dissolution proceeds, the channels interact and compete for available flow, causing the shorter ones to cease transport. As a result, the number of channels decreases with time while the distance between them increases, leading to a scale-invariant, power-law distribution of channel lengths. The authors characterized this distribution of evolving channels with a simple mathematical model of resistor networks. The results of this model were then compared with pore-scale simulations of fracture dissolution that employed a complex microscopic, three-dimensional numerical model, discovering that despite their model's simplicity, it retains the essential features of nonlinear interaction between the channels.
Title: A network model of channel competition in fracture dissolution
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025334, 2006
3. Aerosol transport and scavenging in clouds and rainwater
Beryllium-7 and Lead-210 are widely used tracers for studying atmospheric transport, exchange, and aerosol removal processes, and to test global circulation models. Following their production in the atmosphere, Beryllium-7 and Lead-210 are quickly absorbed into aerosol particles and are removed by radioactive decay and by wet and dry deposition, with mean residence times in the lower atmosphere ranging from days to more than one month. Su and Huh studied Beryllium-7 and Lead-210 in the lower atmosphere, focusing on the differences in their respective abundances in cloudwater and rainwater. They collected 47 cloudwater and 39 rainwater samples over a mountain region in northern Taiwan from February 2004 to March 2005 and found that cloudwater samples have higher Beryllium-7 and Lead-210-specific activities and lower pH values than concurrent rainwater samples. Variation in the isotopes' activity ratios were controlled by the different altitudes and transport pathways of air masses from multiple source regions. Their results show that Beryllium-7 and Lead-210 can be removed from aerosol particles by hydrogen ions and that their uptake behaviors and concentrations in cloud and rain waters are strongly dependent on pH.
Title: Measurements of 7Be and 210Pb in cloudwaters: Toward a better understanding of aerosol transport and scavenging
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025042, 2006
4. Corrections of the Indian gauge data used for early assessments of the great 26 December 2004 earthquake
Several studies generated quickly after the great Sumatra-Andaman 26 December 2004 earthquake postulated that slip occurred slowly over the region because of the discrepancy between the time of earthquake rupture and the time of local subsidence seen by a tide gauge at Port Blair, in India's Andaman islands. Singh et al. have, however, discovered that the time kept by the clock of the tide gauge data was incorrect when the earthquake occurred, causing previous investigators to believe that regional slip occurred long after the initial rupture. Previous investigators were unable to visit the site to note this discrepancy. Using the new data, the authors found that a mixed mode of slip occurred in the region, with half the total slip occurring seismically in less than five minutes after the rupture arrival, and the rest developing over the next 30 minutes. By meshing seismic, GPS, and the new tide gauge data, the authors hypothesized that slow slip occurred over the northern half of the rupture area during the first 35 minutes, and over the rest of the rupture area during the first hour.
Title: Slow slip below Port Blair, Andaman, during the great Sumatra-Andaman Earthquake of 26 December 2004
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025025, 2006
5. Monitoring urban carbon dioxide emissions on small spatial and time scales
Monitoring the amounts and patterns of fossil fuel emissions will be critical to estimating future carbon dioxide concentrations in the atmosphere. Seeking to improve fossil fuel emissions monitoring, Pataki et al. used a tunable diode laser absorption spectrometer to measure carbon dioxide mixing ratios and associated carbon isotope compositions in the atmosphere over Salt Lake City, Utah, between 15 December 2004 and 20 January 2005. They found a pronounced diurnal pattern that reflected the contribution of gasoline versus natural gas combustion to atmospheric carbon dioxide , where natural gas combustion varied from 30-40 percent of total manmade carbon dioxide released during evening rush hour and 60-70 percent during pre dawn hours. They also observed that during a brief warming period, the proportional contribution of natural gas combustion decreased as air temperatures rose, indicating changes in residential heating patterns. These data show that for the first time, atmospheric measurements can be used to infer patterns of energy and fuel usage on hourly to daily time scales.
Title: High resolution atmospheric monitoring of urban carbon dioxide sources
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024822, 2006
6. Currents in the equatorial Atlantic Ocean
In the equatorial Atlantic Ocean, Antarctic Intermediate Water (AAIW), characterized by its low salinity, is found at depths between 500 and 1000 meters [2,000 and 3,000 feet]. Because the dynamics of this layer is poorly known, Ollitrault et al. studied the positions of acoustic float data collected over 10 years within the equatorial AAIW. They documented five current jets along lines of latitude between 6 degrees North and 6 degrees South. Three of these jets, the Equatorial Intermediate Current (0 degrees), Southern Intermediate Counter Current (2 degrees South), and Northern Intermediate Counter Current (2 degrees North), had been discovered by previous investigators; two jets, the South Equatorial Intermediate Current (4 degrees South) and the North Equatorial Intermediate Current (4 degrees North), were discovered by the authors. The jets are robust features in the Atlantic equatorial circulation, each spanning 3000 kilometers [2,000 miles], and except for the Equatorial Intermediate Current, coherent vertically between 800 and 1000 meters 2,600 and 3,300 feet]. Large seasonal fluctuations also exist within the jets: within 1 degree of the equator, AAIW at 800 meters [2,600 feet] flows westward in northern hemisphere summer and fall and eastward during the winter, whereas the flow at 1000 meters [3,300 feet] is eastward in late fall and winter.
Title: Zonal intermediate currents in the equatorial Atlantic Ocean
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025368, 2006
7. Particle acceleration at the heliospheric Termination Shock
The heliospheric Termination Shock, the boundary where speed of the solar wind drops abruptly as it begins to feel the effects of interstellar wind, has been believed for several decades to be the accelerator of anomalous cosmic rays. Anomalous cosmic rays are produced from neutrals in the interstellar medium that drift into the heliosphere, where they are ionized and energized. Last winter, NASA's Voyager 1 spacecraft crossed the Termination Shock near its nose, but did not observe the acceleration in particle speed necessary for the Termination Shock to be responsible for the generation of anomalous cosmic rays at that location. McComas and Schwadron hypothesized that the lack of observed higher energy particles is a natural consequence of the magnetic geometry in the region ahead of the flattened shock. Instead, particle acceleration should occur back along the flanks of the shock, where the magnetic field has progressively longer connection times to accelerate particles. The authors predict that this will be observed when Voyager 2 reaches the Termination Shock significantly further back from its nose.
Title: An explanation of the Voyager paradox: particle acceleration at a blunt termination shock
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025437, 2006
8. Weather in mesospheric ice layers
Ice clouds exist during summer just below the mesopause (80-90 kilometers [50-60 miles] in altitude). Usually occurring above the poles, these clouds are also detected at mid-latitudes. The mechanisms that govern their formation are not fully known. Berger and Lübken studied ice layers in the mesosphere through a new ice model that applied background conditions from the Leibniz Institute Middle Atmosphere (LIMA) model. The LIMA/ice model coupled the mesosphere to the troposphere and stratosphere, and showed that daily fluctuations of background conditions, such as temperatures and winds, significantly determine the shape of ice layers. Furthermore, these layers temporarily redistribute water vapor within the upper mesosphere through freeze-drying. This action dries polar regions at these altitudes and causes the accumulation of water vapor at mid-latitudes where ice particles preferably sublimate, or turn directly into vapor. This sublimation locally increases water vapor concentrations by an order of magnitude above background levels, which in turn supports the formation of ice particles in combination with local temperature minima. The authors' model is the first to successfully reproduce the appearance of mesospheric clouds at mid-latitudes and to introduce short term variability (weather) in mesospheric ice layers.
Title: Weather in mesospheric ice layers
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024841, 2006
9. The history of the Martian dynamo
Based on Mars' strongly magnetized crust, a planetary dynamo (a process through which molten core material moving in the presence of a magnetic field acts to regenerate that field) must have existed in the planet's early history, although none exists today. Using the sensitive electron reflection magnetometry technique, Lillis et al. discovered a magnetic anomaly under the Hadriaca Patera volcano, located near Hellas Basin in the southern highlands, and used the volcano's geomorphology to develop two possible hypotheses about early Mars. In their first, the volcano formed earlier than the oldest datable surfaces on Mars, acquiring its magnetic signature from a dynamo-driven global magnetic field. The Hellas impact, occurring after the cessation of the dynamo, erased its surface expression but did not demagnetize the underlying crust, leaving Hadriaca with a positive magnetic anomaly. In their second, the volcano formed after the Hellas impact and acquired its magnetic signature from a global magnetic field, implying that the Martian dynamo was active several hundred million years later than previously suggested. The authors say that one of these scenarios must be true; either would provide new insight into early Mars.
Title: Unusual magnetic signature of the Hadriaca Patera Volcano: Implications for early Mars
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024905, 2006
10. Martian lowlands are not young
The crust of Mars can be described as ancient southern highlands and lower, thinner, and apparently younger northern lowlands. The origin of this difference, or dichotomy, has been controversial: some propose that internal processes (like plate tectonics) were responsible, while others suggest that large meteor impacts caused the northern portion's low elevations. NASA's Mars Orbiter Laser Altimeter has revealed a larger-than-expected population of visible and buried impact basins, or Quasi-Circular Depressions, on Mars, which provides new constraints on the age of the Martian lowlands. Herbert Frey mapped and counted these depressions to determine the total population of craters (both visible and buried) and thereby obtain a more complete record of crater retention for a given area. His investigations show that the Martian lowlands have similar crater density to areas of the southern highlands known to be of Early Noachian age--more than four billion years old--with large lowland basins possibly older. This suggests that the lowland crust formed and became low no later than 500 million years after Mars formed, which better limits the likely process of origin.
Title: Impact constraints on the age and origin on the lowlands of Mars
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024484, 2006
11. The Artic influences sub-polar wind patterns and European climate
In the Arctic, fluctuations in sea ice extent will impact the Earth's albedo, or the fraction of sunlight that is reflected back by Earth. This feedback mechanism is likely the most important factor for the polar amplification of global warming. To look for such feedbacks, Dethloff et al. used an atmosphere-ocean general circulation model run over 500 years that, compared to past models, includes improved representation of snow cover over land, sea-ice cover, and melt ponds. Detailed analysis of the first nine years of simulations showed that although warming may occur in the mid-latitudes, Arctic cooling will result from a polar vortex churning cold air from the Rockies northward. This indicates that the Arctic exerts a strong influence on the mid- and high-latitude climate, because of its ability to modulate the strength of sub-polar westerlies and storm tracks. Moreover, the improved representation revealed Arctic Oscillation-like fluctuations in the middle troposphere and in middle latitudes that can strongly impact European climate.
Title: A dynamical link between the Arctic and the global climate system
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025245, 2006
12. Monitoring magma movements within Stromboli volcano, Italy
Rapid analysis of Very-Long-Period seismic events, which commonly accompany magmatic and/or hydrothermal transport, can help determine the source location and source mechanism flow and shed light on the geometries of volcanic conduits before and after eruptions. Following a landslide and tsunami in 2002 and an eruption in 2003 associated with Italy's Stromboli volcano, scientists have maintained a 13-station network of broadband seismometers at the volcano, which have collected a large archive of Very-Long-Period events. Using this archive, Auger et al. analyzed and classified 2,001 Very-Long-Period events that occurred during 10 days in September 2004. From this, they show the feasibility of providing continuous remote analyses of Very-Long-Period events on Stromboli in real time. The authors then developed an automated procedure to accomplish this task, results of which are shown in real time on the Internet. They note that such analysis to detect and quantify radical changes in magma feeding systems as they occur will be useful in the evaluation of volcanic hazards.
Title: Real-time monitoring and massive inversion of source parameters of Very-Long-Period (VLP) seismic signals: An application to Stromboli Volcano, Italy
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL024073, 2006
13. High-density patches in the polar ionosphere
Patches are islands of high density ionospheric plasma, typical of daytime mid-latitudes, found in the nighttime low-density plasma of the polar regions. The mechanisms by which patches are produced have not yet been illuminated, despite several decades of attention to this topic. To better describe this phenomenon, Carlson et al. designed and implemented an experiment that mapped ionospheric densities, temperatures, and velocities in order to detect patches within the polar ionosphere. They then traced the patches back in time to their initial plasma source region at mid-latitudes. The authors also looked for signatures of patch generation processes in and between these two regions, and found that at times when the boundary between low and high density plasma relaxed poleward, patches were pinched off and injected into the polar ionosphere. Moreover, the authors observed that during reconnection of poleward-moving interplanetary magnetic field lines with the Earth's magnetosphere, which form bright ultraviolet cusp aurorae, ionospheric patches are transported to the poles. This suggests that current research on cusp aurorae can shed further light on the formation of ionospheric patches.
Title: Direct observations of injection events of subauroral plasma into the Polar Cap.
Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025230, 2006
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