The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.
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1. Tracing the world oceans' conveyor belt
A radioactive iodine isotope can likely measure the deepwater circulation in the world's oceans, as evidenced by a study that followed the tracer in the Nordic Seas. Alfimov et al. found that the long-lived isotope can be used to label water masses in the Arctic and North Atlantic oceans and follow circulation patterns through the world's "conveyor belt" that begins with the formation of deepwater in the Nordic Seas. The authors note that iodine-129 has a well-defined manmade input from nuclear reprocessing facilities and is unevenly distributed throughout the global oceans. The researchers used the iodine tracer to report the average deepwater concentrations in the North Atlantic and suggest that while previous research was unable to estimate the contribution from natural and human inputs into the cold, dense water's formation, their study can help determine the mixing and circulation of the Arctic and North Atlantic oceans and may be useful for calculating the global thermohaline [vertical, temperature-driven] circulation.
Title: Tracing water masses with 129-I in the western Nordic Seas in early spring 2002
Authors: Vassili Alfimov, A. Aldahan, G. Possnert, Uppsala University, Uppsala, Sweden.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020863, 2004
2. Measuring large-scale pollution from space
An ozone-sensing satellite instrument currently used to measure emissions from volcanic eruptions was able to calculate the amount of sulfur dioxide released during the month-long 2003 fire at an Iraqi sulfur plant. Carn et al. report that the Earth Probe Total Ozone Mapping Spectrometer (EP TOMS) determined that approximately 600 kilotons [600,000 tons] of the compound was released during the blaze. Their estimate closely corresponds with the sulfur dioxide yield expected from the plant's sulfur inventory before the fire. Despite the satellite's sensitivity limitations in the lower atmosphere, the instrument detected dense plumes of sulfur dioxide in the troposphere from the event, readings that were only possible because of the magnitude of the pollution. The authors suggest that a new generation of ultraviolet imaging will provide better observations for measuring smaller and more frequent pollution events.
Title: Fire at Iraqi sulfur plant emits SO2 clouds detected by Earth Probe TOMS
Simon A. Carn, A. J.Krueger, Joint Center for Earth Systems Technology,
University of Maryland Baltimore County (NASA/UMBC), Baltimore, Maryland,
N. A. Drotkov, Goddard Earth Sciences and Technology Center, University of Maryland Baltimore County, Baltimore, Maryland, USA;
M. A. Gray, L-3 Communications Government Services, Inc., Largo, Maryland, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020719, 2004
3. Tracking strong thunderstorms worldwide
A researcher has provided the first identification of the energy and speed of a lightning-like discharge seemingly related to the strength of its parent thunderstorm. Kenneth B. Eack analyzed the flashes from antennae readings in New Mexico and Florida that measured the radiation and electrostatic field changes from the events. The findings indicate that the average narrow bipolar event transfers approximately 0.3 Coulombs of charge over a distance of 3.2 kilometers [two miles] and that the discharge moves at nearly one-half the speed of light. Eack notes that because the bipolar events' properties differ from typical cloud-to-ground or intracloud lightning, packing exceptionally strong peak power levels, the events can be closely tracked. Also, since the events tend to take place near the center of unusually strong thunderstorms, he suggests that satellite monitors could use the flashes to track such strong thunderstorms worldwide.
Title: Electrical characteristics of narrow bipolar events
Author: Kenneth B. Eack, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021117, 2004
4. New data improves space weather predictions
Data from a recently launched satellite may provide a significantly better measure of the daily changes in the upper atmosphere, allowing for improved estimates and predictions for space weather conditions. Eastes et al. report that solar measurements from the Student Nitric Oxide Explorer satellite, launched in 1998, provide soft X-ray measurements that can be used to estimate solar flux in the Earth-space boundary. The authors found a high correlation between soft X-ray flux and the nitrogen density in the thermosphere. Such density variations affect the drag on orbiting satellites and can interfere with high-frequency communications signals transmitted in the upper atmospheric region. They suggest that the recent broadband photometer observations can replace less reliable radio-frequency data from the 1970s with measurements that are easier to translate into predictions of the thermospheric conditions.
Title: The correspondence between thermospheric neutral densities and broadband measurements of the total solar soft X-ray flux
R. Eastes, Florida Space Institute, Kennedy Space Center, Florida, USA;
S. Bailey, University of Alaska-Fairbanks, Fairbanks, Alaska, USA;
B. Bowman, Air Force Space Command, Peterson Air Force Base, Colorado, USA;
F. Marcos, J. Wise, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA;
T. Woods, University of Colorado, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020801, 2004
5. Analyzing meteors over Antarctica
Results from a meteor radar system recently installed at the South Pole indicate that most meteoric activity occurs during the Antarctic summer around a very concentrated region of the sky. Janches et al. present observations from a very high frequency radar system designed to study upper atmospheric winds and analyze the day-night and seasonal variability of meteoric activity over the South Pole.
Although such meteor flux has been monitored and studied at low- and mid-latitudes, the difficulty in placing and operating equipment in extreme geographical locations has inhibited study over the Antarctic. Their findings agree with observations from the Arctic region of sporadic meteor activity, which deposits extraterrestrial material in both poles. The authors suggest that the radar information can provide data on the frequency, mass and size of falling meteors, which will help them understand the origin of the space particles and other atmospheric phenomena in the polar regions.
Title: Diurnal and seasonal variability of the meteoric flux at the South Pole measured with radars
Diego Janches, E. M. Lau, S. K. Avery, J. P. Avery, S. de la Pena, Cooperative
Institute of Research in Environmental Sciences, University of Colorado, Boulder,
S. E. Palo, University of Colorado, Boulder, Colorado, USA;
N. A. Makarov, Institute for Experimental Meteorology, Scientific Production Association, Obninsk, Russia.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021104, 2004
6. Finding the source moons for Saturn's outer ring
An analysis of dust particles in Saturn's E-ring may allow researchers to learn more about some of the planet's moons that created its famous bands. Juhasz and Horanyi created a three-dimensional model that followed the trajectories of a large number of particles in Saturn's outermost E-ring and captured seasonal variations in the particles' density and distribution. Their theoretical estimates matched previous remote observations and will be used to analyze the Cassini spacecraft measurements as the NASA/ESA mission repeatedly passes through Saturn's E-ring. The authors point out that data from Cassini's Cosmic Dust Analyzer instrument will identify the moon of origin of the particles by measuring their size and speed. The instrument will also be able to measure the chemical composition of the particles, which combined with other remote observations, will allow scientists to identify the surface properties of the moons where the grains originated.
Title: Seasonal variations in Saturn's E-ring
Antal Juhasz, KFKI Research Institute for Particles and Nuclear Physics, Budapest,
Mihaly Horanyi, University of Colorado, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020999, 2004
7. Measuring energetic ions upstream of Earth's bow shock
Simultaneous measurements of energetic ions by two Cluster spacecraft show that the density of the ions decreases exponentially with distance from the shock upstream of the interplanetary magnetic field. Kis et al. determined the gradient of energetic particles at distances between 0.5-2.8 the radius of Earth during a period of high solar wind speed and report that the particle density decreases rapidly the further it moves from the planetary bow shock. The authors show that these energetic ions are essentially confined to a region of several Earth's radii in front of the bow shock. They note that the interplanetary charged particles are scattered parallel to Earth's magnetic field in the region upstream of the shock and suggest that the ions may be produced by the bow shock itself due to acceleration out of the solar wind.
Title: Multi-spacecraft observations of diffuse ions upstream of Earth's bow shock
A. Kis, M. Scholer, B. Klecker, Max-Planck-Institute for Extraterrestrial Physics,
E. Mobius, L. M. Kistler, H. Kucharek, University of New Hampshire, Durham, New Hampshire, USA;
E. A. Lucek, Imperial College, London, United Kingdom;
H. Reme, J. M. Bosqued, Laboratory of Space Astrophysics, Toulouse, France.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020759, 2004
8. Details of North African tectonics
Measurements of the coastal shoreline movement after a strong 2003 earthquake in Algeria have constrained the quake's slip location and may help refine estimates for the little-studied North African plate boundary. Meghraoui et al. analyzed the coastal tectonic motion from the 6.8-magnitude May 2003 Zemmouri earthquake, which was the strongest temblor in the region in nearly three centuries. The authors used Global Positioning System and ground-based measurements to record the ground deformation and report a maximum uplift of approximately 0.75 meters [2.5 feet] and an average of nearly half a meter [two feet] along the North African coast. The shifts are caused by the convergence of the African and Eurasian tectonic plates, although little is known about the plate movement in the area. Based on their study of the seismic activity of the recent earthquake, the researchers estimate that the fault likely initiated along the sea bottom between 5-10 kilometers [3-6 miles] offshore.
Title: Coastal uplift and thrust faulting associated with the Mw=6.8 Zemmouri (Algeria) earthquake of 21 May, 2003
M. Meghraoui, Z. Cakir, P.-J. Alasset, Institute of Global Physics, Strasbourg,
S. Maouche, Center for Geology and Space Physics, Algiers, Algeria;
B. Chemaa, F. Benamouda, National Institute for Cartography and Remote Detection, Algiers, Algeria;
A. Aoudia, Abdus Salem International Center for Theoretical Physics, Trieste, Italy;
A. Harbi, A. Ayadi, Center for Geology and Space Physics, Algiers, Algeria, and Abdus Salem International Center for Theoretiocal Physics, Trieste, Italy;
Y. Bouhadad, Center for Seismic Phenomena, Algiers, Algeria.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020466, 2004
9. Observations of metallic ions in boundary layer
The first direct nighttime observation of charged metallic particles in a boundary layer between Earth's mesosphere [altitude 30-80 kilometers, 20-50 miles] and ionosphere [altitude 50-500 kilometers, 30-300 miles] may help researchers better model interactions within the region. Roddy et al. provide measurements of the atmospheric content between 80-220 kilometers [50-140 miles] from a series of rocket observations in 2003 that observed the height of the thin boundary layer at night. The layer regularly shifts in altitude, often descending during nighttime, and can affect satellites traveling in low-Earth orbit and communications signals. Currently, however, space environment models are unable to predict the timing and size of the disturbances. The new research shows an abundance of iron and magnesium in a boundary layer below 140 kilometers [90 miles], unlike previous studies that found that the layers were primarily made up of nitrogen and oxygen gases with no metallic ions. The authors suggest that the metallic ions are likely carried from lower atmospheric levels into the ionosphere.
Title: Relative concentrations of molecular and metallic ions in midlatitude intermediate and sporadic-E layers
Patrick A. Roddy, G. D. Earle, University of Texas at Dallas, Richardson, Texas,
C. M. Swenson, C. G. Carlson, Utah State University, Logan, Utah, USA;
T. W. Bullett, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020604, 2004
10. Is Earth's interior really layered?
An analysis of seismic techniques used to estimate the Earth's interior has found that a commonly used surface waves analysis method may provide an erroneous interpretation of subsurface features. Van Wijk and Levshin show that seismic signals could bounce off topographical variations or underground unevenness and skew the results from the current testing method that assumes that dispersion of surface waves is caused by layers in Earth's structure. The authors report that wave scattering disperses seismic signals that are now used to provide assessments of deep underground layers, possibly producing inaccurate maps of the planet's interior. They provide evidence from laboratory testing and numerical models of such seismic dispersion from non-layered subsurface features, which could easily be misinterpreted as evidence of layering. The researchers propose that surface wave data should be combined with other types of geophysical data to create more accurate estimates of the subsurface structure.
Title: Surface wave dispersion from small vertical scatterers
Kasper van Wijk, Colorado School of Mines, Golden, Colorado, USA;
A. L. Levshin, University of Colorado, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021007, 2004