AGU journal highlights - 29 July 2004

I. Highlights, including authors and their institutions

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.

1. Evidence for unique particle distribution in noctilucent clouds

The first observations of the ultraviolet spectrum within polar mesospheric clouds may provide a clue to explain a mysterious "hump" previously seen within the cloud's spectrum. (The mesosphere ranges from 30 to 80 kilometers [20 to 50 miles] in altitude.) Carbary et al. analyzed satellite spectrographic images and were able to separate the particle scattering and distribution within the high-latitude clouds. The authors identified an unexpected hump in the clouds' middle spectrum that suggests that particles are separated into two distinct groups within the clouds. They report that the separation corresponds with previous observations indicating that smaller particles within the clouds fall in a sedimentation-like process to the lower portions of the cloud while larger particles remain at its top. They also propose that atmospheric gravity and tidal waves likely contribute to the two distinct sides of the noctilucent clouds [i.e., clouds that glow at night]. The new observations lend credence to the unique bi-modal distribution of particles within the clouds.

Title: Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds

Authors:
James Carbary, D. Morrison, G. J. Romick, Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020101, 2004

2. Special section on San Andreas Fault Observatory project

The 28 June issue of Geophysical Research Letters features a collection of papers about the San Andreas Fault Observatory at Depth (SAFOD), a project designed to drill into the hypocentral zone of repeating low-magnitude earthquakes on the San Andreas Fault. Hickman et al. introduce the first of two special sections, which includes nine research articles discussing surface-based investigations of earthquake source parameters and the crustal structure near the SAFOD site. A second section will run in a separate issue and will include studies of the mineralogy, mechanical and physical properties of rock adjacent to the 2.2-kilometer [1.4 mile] -deep borehole. The researchers propose that the overall research effort will provide new insights into the composition and properties of fault zone materials at depth and answer unresolved questions about processes controlling faulting and earthquake generation within the San Andreas Fault Zone.

Title: Introduction to special section: Preparing for the San Andreas Fault Observatory at Depth

Authors:
Stephen Hickman, William Ellsworth, U.S. Geological Survey, Menlo Park, California;
Mark Zoback, Stanford University, Stanford, California.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020688, 2004

3. Plumbing the depths of the San Andreas Fault

A set of deep seismic monitors that stretches for more than 1,200 meters [3,900 feet] within the San Andreas Fault can likely provide unique information about the volatile fault system and allow unprecedented monitoring for earthquakes in Southern California. Chavarria et al. present vertical seismic profile results from the seismic monitors of the San Andreas Fault Observatory at Depth project. The authors state that the array of 32 seismometers, reaching more than two kilometers [one mile] below ground, can allow them to detect microearthquake waves and gain valuable insights into the fault line's structure. The researchers also note that the ratio of primary to secondary waves recently observed within the fault indicates the likelihood of two faults intersecting at the exact location of the experiment. In addition, the ratios may be caused by increasing fluid saturation, which could account for the study's findings and be correlated to creep distribution associated with the area.

Title: The SAFOD Pilot Hole seismic array: Wave propagation effects as a function of sensor depth and source location

Authors:
J. Andres Chavarria, Peter E. Malin, Eylon Shalev, Duke University, Durham, North Carolina, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2003GL019382, 2004

4. Solar variation may drive geomagnetic cycles

A 75-year analysis of interplanetary magnetic field (IMF) data indicates that the 22-year geomagnetic activity cycle is more affected by internal solar variation than was previously thought. Echer and Svalgaard report that IMF polarity is unsteady, and its variations are not symmetric around times of solar minimum, when the Sun's polar fields are strong. The polarity changes are important to estimating solar wind behavior and space weather conditions affected by geomagnetic fields. The authors suggest that polarity changes correspond with the Sun's solar cycle, as the solar system's current sheet is more stable during the initiation of solar cycle and is more disturbed by high-speed streams as the solar cycle concludes. The researchers note that the field's polarity changes are not as stable as was previously estimated, a finding that could have significance to understanding the presumed 22-year cycle.

Title: Asymmetry in the Rosenberg-Coleman effect around solar minimum revealed by wavelet analysis of the interplanetary magnetic field polarity data (1927-2002)

Authors:
Ezequiel Echer, National Institute of Space Research, Sao Jose dos Campos, Sao Paulo, Brazil;
L. Svalgaard, Easy Toolkit, Inc., Houston, Texas, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020228, 2004

5. Creating artificial high-altitude auroras

Researchers have created the first annular (ring-like) artificial aurora at high-altitudes, using a high-frequency energy pulse to stimulate charged particles and create optical emissions in the ionosphere. Kosch et al. used the European Incoherent Scatter facility to beam radio waves into the high atmosphere approximately 235 kilometers [145 miles] above its location in Norway and produce visible emissions over northern Scandinavia. The researchers scanned various locations and altitudes in the sky for years before discovering the optimal height and angle for creating the optical emissions, which they suggest are generated by electrons colliding with neutral gas in the thermosphere. Such high-frequency radio wave induced emissions are identical to those created in natural auroras. The authors were able to repeatedly create the emissions, which survive for nearly one minute before collapsing as they descend in altitude. Their experiment provides evidence for the high-altitude plasma turbulence thought to be similar to processes found in natural auroras.

Title: Novel artificial optical annular structures in the high latitude ionosphere over EISCAT

Authors:
Mike Kosch, A. Senior, F. Honary, Lancaster University, Lancaster, United Kingdom;
M. T. Rietveld, EISCAT Scientific Association, Ramfjordbotn, Norway;
I. W. McCrea, Rutherford-Appleton Laboratory, Didcot, United Kingdom;
A. J. Kavanagh, High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA;
B. Isham, EISCAT Scientific Association, Longyearbyen, Norway.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019713, 2004

6. Strong El Nino makes a good test for circulation models

The unusual cloud behavior seen during the 1997-1998 El Nino makes the unusually strong tropical oscillation an effective measure to judge the accuracy of general circulation models. Lu et al. tested Version 3 of the Hadley Centre Atmospheric Model against the recent El Nino event, whose strength caused unexpected sea surface temperature variations, affected cloud distribution, and initiated the collapse of the tropical Pacific's normal Walker circulation. The authors report that the Hadley model was able to reproduce the effects of the El Nino, including the observed lower clouds in the western portion of the Pacific and the higher clouds seen in its eastern section. They also compared the model to satellite measurements at the top of the atmosphere and show that the Hadley model was in reasonable agreement with those measurements for the unusual climactic event. The researchers suggest that enough data exists for the abnormally strong 1997-1998 El Nino to make it a useful barometer to test the reliability of future models.

Title: The 1997/98 El Nino: A test for climate models

Authors:
Riyu Lu, Institute of Atmospheric Phyics, Chinese Academy of Sciences, Beijing, China;
Buwen Dong, Hadley Centre for Climate Prediction and Research, Met Office, Exeter, United Kingdom;
Robert D. Cess, Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York, USA;
Gerald L. Potter, Lawrence Livermore National Laboratory, Livermore, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019956, 2004

7. Blackout reveals power plant pollution

The summer 2003 electrical power blackout in the northeastern United States and eastern Canada brought to light the amount of pollution produced by power plants in the region. Marufu et al. show that the unexpected power plant shutdown in August 2003 allowed researchers an opportunity to examine the changes in airborne pollution in the atmosphere compared to normal circumstances. The authors compared the levels of atmospheric pollution immediately following the blackout to historical observations and to areas that were not affected by the power loss. They found reductions of nearly 90 percent of sulfur dioxide in the air, roughly half the normal airborne ozone, and approximately 70 percent of light scattered by pollution in the 24-hour period following the blackout. The researchers also note that smog and haze reductions during the period enhanced the visibility range in the U.S. and Canada by more than 40 kilometers [20 miles].

Title: The 2003 North American electrical blackout: An accidental experiment in atmospheric chemistry

Authors:
Lackson T. Marufu, Brett F. Taubman, Bryan Bloomer, Charles A. Piety, Bruce G. Doddridge, Jeffrey W. Stehr, Russell R. Dickerson, University of Maryland, College Park, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL019771, 2004

8. Spring increase in Arctic cloudiness

A comparison of ground and satellite cloud data over the Arctic seas indicates a strong increase in cloudiness during spring over the past two decades. Axel J. Schweiger reports that the two data sets show a nearly five percent increase per decade in cloudiness associated with a drop in surface pressure and an increase in atmospheric circulation patterns. A conflicting earlier report had indicated a seasonal drop in cloudiness or only a slight increase. Schweiger's analysis used observations from independent instruments over the Arctic seas, leading him to suggest that the increased cloudiness indicates a significant climate variation that began near 1980. He notes that a decrease in cloudiness occurs from December to February, which may result in a cooling at the Arctic surface and contribute to greater sea ice growth. The spring increase may also accelerate ice melting, suggestive of how changes in ice reflectivity and cloud cover affect the global climate and weather patterns.

Title: Changes in seasonal cloud cover over the Arctic seas from satellite and surface observations

Author:
Alex J. Schweiger, Applied Physics Laboratory and Polar Science Center, University of Washington, Seattle, Washington.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020067, 2004

9. Revised estimate for water volume through ocean straits

Imposing a maximum limit on volume discharge for oceanic straits can allow researchers an improved measure to estimate water movement through the channels. Melvin E. Stern presents a modified formula for estimating an upper boundary on volume discharge in the Denmark Strait, one of the most documented major straits in the world. Such straits have important climactic significance because they control the movement and transfer of dense, cold bottom waters that move to the ocean floor and are subsequently recirculated in the subtropical ocean basins. Stern's result optimizes the unknown parameters in the cross-stream velocity profile. His data proposes a new extreme value for volume discharge that is approximately half of the previous maximum. His projection of 2.7 Sverdrup (a measure of water movement equal to one million cubic meters [264.2 million gallons] per second) as the average water flow also agrees with previous measurements for the Denmark Strait.

Title: Transport extremum through Denmark Strait

Author:
Melvin E. Stern, Florida State University, Tallahassee, Florida, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020184, 2004

10. Revising noise limits for ocean studies

The sound limitations used to protect fish and marine wildlife from underwater seismic testing may need to be revised after a new study found that the noise is louder in shallow waters than originally thought. Tolstoy et al. report that the sound pressure from compressed-air pulses used in seismic experiments can reverberate from the seafloor in shallow waters, magnifying and enhancing the exposure of low-frequency acoustic waves through the ocean ecosystem. The authors also found that sounds in deep regions within the Gulf of Mexico were quieter than anticipated by model predictions. Currently, permits are required for U.S.-flagged ships that mandate a safety radius for potentially damaging sounds before such testing can take place, as the energy can affect animal behavior. While the researchers note that their study found reductions in high-frequency sounds that most affect marine mammals, they suggest that the existing guidelines that do not include standards for the frequency of the sound source should be re-examined.

Title: Broadband calibration of R/V Ewing seismic sources

Authors:
Maya Tolstoy, J. B. Diebold, S. C. Webb, D. R. Bohnenstiehl, E. Chapp, R. C. Holmes, M. Rawson, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL020234, 2004