AGU journal highlights - 24 August 2004


Highlights, including authors and their institutions

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


1. Cloud drop charge may affect precipitation

The first reliable aircraft measurement of the average charges on cloud drops has revealed that typical charges may be high enough to influence precipitation. Beard et al. report typical cloud drop charges more than an order of magnitude larger than previous measurements in similar clouds obtained from mountain observation locations. The authors used wing-mounted laser probes to measure cloud particle sizes and a reverse flow device to slow and evaporate cloud drops so that their charge could be measured with a filter electrometer. This novel combination of instruments resulted in a charge sensitivity of one-seventh of an electron per drop. The researchers' initial aircraft results -- previous instruments were not sensitive enough to accurately observe the representative charges -- indicate that cloud drop charges in layer clouds may be high enough to influence microphysical processes that promote precipitation.

Title: Aircraft measurements of high average charges on cloud drops in layer clouds

Kenneth V. Beard, Harry T. Ochs III, University of Illinois, Urbana-Champaign, Illinois, and Illinois State Water Survey, Champaign, Illinois, USA;
Cynthia H. Towhy, Oregon State University, Corvallis, Oregon, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020465, 2004


2. Remotely detecting Arctic cloud properties

Scanning mixed water/ice clouds in the Arctic from satellites or aircraft can provide enhanced physical and radiative data for the climactically significant cloud cover and improve global cloud and circulation models. Chylek and Borel analyzed high-resolution satellite cloud particle data from above Point Barrow, Alaska, near the often-cloudy region where the Arctic Ocean meets the freshwater Bering Strait, and report the composition of a mixed water-ice cloud commonly seen in the area. The microphysical cloud properties are known to affect the surface energy balance and the Arctic ice pack, and an improved understanding of their structure would help researchers estimate the increase of global warming in the polar regions. The authors detail the pockets of pure water, ice, and mixed layers observed inside the cloud with a high degree of accuracy and suggest that additional space- or aircraft-based measurements could be used to further refine scientists' understanding of the clouds.

Title: Mixed phase cloud water/ice structure from high spatial resolution satellite data

Petr Chylek, Los Alamos National Laboratory, Los Alamos, New Mexico, and New Mexico State University, Las Cruces, New Mexico, USA;
Christoph Borel, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020428, 2004


3. What caused the 2002 major Antarctic warming?

Increased planetary wave activity during the Antarctic winter of 2002 was the likely source of weak mid-level atmospheric winds that caused unprecedented warming in the Southern Polar stratosphere. Dowdy et al. analyzed the first "major" warming, as indicated by a high-latitude temperature increase that also causes a shift in the polar vortex, in the Southern Hemisphere. The authors report that mesospheric winds in 2002 were considerably different from other years, because of unusually active planetary wave activity. This led to a breakdown of normal atmospheric circulation patterns that lasted for more than two weeks before the Antarctic returned to its typical winter flow. They note that planetary waves in the Southern Hemisphere generally have a smaller amplitude than those in the Northern Hemisphere, evidenced by a stronger winter vortex in the Antarctic. The researchers suggest that a better understanding of the anomalous circulation can allow them to improve their predictions for atmospheric circulation patterns worldwide.

Title: The large-scale dynamics of the mesosphere-lower thermosphere during the Southern Hemisphere stratospheric warming of 2002

Andrew J. Dowdy, Robert A. Vincent, University of Adelaide, Adelaide, Australia;
Damian J. Murphy, Australian Antarctic Division, Kingston, Tasmania, Australia;
Masaki Tsutsumi, National Institute of Polar Research, Tokyo, Japan;
Dennis M. Riggin, Colorado Research Associates, Boulder, Colorado, USA;
Martin J. Jarvis, British Antarctic Survey, Cambridge, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020282, 2004


4. Alternative water source for water in Martian channels near Tharsis

A dynamic groundwater model applied to Mars suggests that large channels seen emanating from the planet's Tharsis region may have come from ancient snowfall on the volcanic rise. Harrison and Grimm propose that the flood formations known as the Chryse outflow channels were formed by millions of cubic kilometers [miles] of water -- more than can be stored in the region's crust -- supplied by precipitation that fell over Tharsis during periodic cooling at low latitudes. The researchers report that the water entered the Martian crust either through precipitation or melting snow, producing significant groundwater pressures that created strong discharges at the origin of the channels. The authors compared their model results to a previous theory, suggesting that the channels may have been derived from melting at the Martian South Polar ice cap and found that the Tharsis provides a more efficient supply to the channels.

Title: Tharsis recharge: A source of groundwater for Martian outflow channels

Authors: Keith P. Harrison, R. E. Grimm, Southwest Research Institute, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020502, 2004


5. Magnetotail on Saturn's Titan

Fast-moving solar plasma piles up around Saturn's moon Titan, producing a Mars- and Venus-like magnetotail on the side of the satellite opposite the direction of the particle flow. Kallio et al. present the results from a three-dimensional kinetic model that included both subsonic and supersonic plasma and its interaction with Titan's dense atmosphere. Their results confirm some of the moon's unusual features, such as the absence of a bow shock perimeter in front of the plasma flow, originally observed during a 1980 flyby from the Voyager 1 spacecraft that relayed information about its magnetic field shape and behavior. The authors found that subsonic plasma flow produces an asymmetric and twisted magnetotail on the wake side of the moon, in agreement with the Voyager results. The researchers suggest that while previous data provided valuable global information about the properties of the plasma and magnetic field near Titan, the current study is the first to present a hybrid model capable of measuring the charged plasma flow on both sides of the moon.

Title: Titan in subsonic and supersonic flow

Authors: Esa Kallio, I. Sillanpaa, P. Janhunen, Finnish Meteorological Institute, Helsinki, Finland.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020344, 2004


6. Magnetic field map for Mars

A new technique to measure the magnetic field strength on Mars has provided a partial map of the planet's low-altitude magnetic field and additional detail that could identify the age of Martian crust. Lillis et al. adapted a method originally used to determine the magnetic field strength of the Moon's surface that uses the angular distribution of electrons reflected from a body's crustal magnetic field to any planet or moon with a significant atmosphere. The authors analyzed data from the electron reflectometer on the Mars Global Surveyor mission and present a map of approximately 20 percent of the planet. The map, taken from nighttime passes to reduce solar wind distortions, reveals two new magnetic sources in the northern lowlands and shows the topology of magnetic sources in the region. Their map reaches closer to the planet's surface than previously published magnetic maps, to altitudes where the magnetic fields are substantially stronger and indicates that the northern lowlands plains overlie a much older surface.

Title: Mapping crustal magnetic fields at Mars using electron reflectometry

Robert J. Lillis, D. L. Mitchell, R. P. Lin, University of California, Berkeley, California, USA;
J. E. P. Connerney, M. H. Acuna, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020189, 2004


7. Midlatitude ionospheric disturbance likely initiated from space

The first simultaneous observations of medium-scale traveling ionospheric disturbances in the Northern and Southern hemispheres suggests that the upper atmospheric phenomenon is likely caused by electrical forces from space. Otsuka et al. present observations of a wave-like airglow in the Earth-space boundary recorded at night over Japan and northern Australia that allow the researchers to map the structure of the event. The authors found that the ionospheric perturbation stretched in a mirror image on either side of geomagnetic field lines, lending credence to a proposal that the disturbances are caused by an electric field imbalance, perhaps initiated by plasma instabilities. The researchers used highly sensitive airglow imagers to identify the medium-scale anomalies, which were previously thought to have been caused by gravity waves propagating upward from the ground.

Title: Geomagnetic conjugate observations of medium-scale traveling ionospheric disturbances at midlatitude using all-sky airglow imagers

Yuichi Otsuka, K. Shiokawa, T. Ogawa, Nagoya University, Toyokawa, Japan;
P. Wilkinson, IPS Radio and Space Services, Haymarket, New South Wales, Australia.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020262, 2004


8. First principles of new deep-Earth mineral

Researchers have determined the first principles for a recently discovered mineral that may be the most common material in the Earth's core-mantle boundary. Tsuchiya et al. obtained the first elasticity principles from a single crystal and of a larger amount of the rock-like magnesium-silicate mineral identified in high-pressure, high-temperature simulations of the Earth's core-mantle boundary region. The authors compared the thermodynamic properties and seismic velocities of the newfound post-perovskite mineral to perovskite, a known material found in abundance on the Earth's mantle and thought to make up almost the entirety of the planet's interior. They note that the new material is a heated and pressurized version of the perovskite and show that the new mineral possesses remarkably variable elastic properties, more so than simple perovskite, including an unusual layered structure but does not display the expected elasticity. The researchers suggest that the unusual elasticity could explain some of the abnormal seismic observations in the thick layer above the core-mantle boundary region.

Title: Elasticity of post-perovskite MgSiO3


Taku Tsuchiya, Jun Tsuchiya, Koichiro Umemoto, Renata M. Wentcovitch, Minnesota Supercomputing Institute for Digital Technology and Advanced Computation, University of Minnesota, Minneapolis, Minnesota, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020278, 2004


9. Pumping iron in the Southern Ocean

Iron addition to the Southern Ocean encourages phytoplankton growth as in other oceanic regions, lending credence to the Iron Hypothesis: that iron availability had a significant effect on paleoclimate temperatures. Turner et al. report the findings of two iron-addition experiments in the Southern Hemisphere, where naturally low iron levels and low light from deep mixed layers were thought to suppress plankton growth. The authors report that iron enrichment benefitted all the major groups of algae, stimulating growth that produces the trace gas dimethylsulfide. They found up to eight-fold increases in the gas, which is known to affect climate cooling by enhancing cloudiness, and found similar enhancement of the algal precursor of dimethylsulfide during the study. Because the Southern Ocean plays a significant role in modulating the global climate, large-scale iron fertilization has been proposed as a possible strategy for mitigating the increase of man-made carbon in the atmosphere.

Title: Iron-induced changes in oceanic sulfur biogeochemistry

Sue M. Turner, P. S. Liss, University of East Anglia, Norwich, United Kingdom;
M. J. Harvey, C. S. Law, National Institute of Water and Atmosphere, Wellington, New Zealand;
P. D. Nightingale, Plymouth Marine Laboratory, Plymouth, Devon, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020296, 2004


10. Atmospheric distribution of bromine monoxide

A new method to estimate tropospheric bromine monoxide levels has allowed researchers to separate the atmospheric layers of the ozone-destroying compound. Schofeld et al. combined two measurement sets to provide the first direct measure of tropospheric bromine monoxide at a midlatitude site. Bromine-containing compounds are formed naturally and artificially and are known to have a significant effect on the global ozone balance. The authors analyzed two years of spectroscopic measurements of bromine monoxide columns in Lauder, New Zealand, and found that although existing models were able to roughly predict the amounts of atmospheric bromine monoxide, the distribution of the compound in the stratosphere-troposphere layers was misrepresented. Their direct measures suggests that stratospheric bromine levels are larger than the values used in most ozone assessment models. The researchers also report, contrary to current assumptions, that the background tropospheric levels are substantially smaller and are not necessarily constantly or uniformly distributed worldwide.

Title: Retrieved tropospheric and stratospheric BrO columns over Lauder, New Zealand

Authors: Robyn Schofield, National Institute of Water and Atmospheric Research, Omakau, Central Otago, New Zealand, and University of Auckland, Auckland, New Zealand;
K. Dreher, B. J. Connor, P. V. Johnston, A. Thomas, National Institute of Water and Atmospheric Research, Omakau, Central Otago, New Zealand;
D. Shooter, University of Auckland, Auckland, New Zealand;
M. P. Chipperfield, University of Leeds, Leeds, United Kingdom;
C. D. Rodgers, University of Oxford, Oxford, United Kingdom;
G. H. Mount, Washington State University, Pullman, Washington, USA.

Source: Journal of Geophysical Research-Atmospheres (JD) paper 10.1029/2003JD004463, 2004

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