AGU journal highlights -- Dec. 4, 2013

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American Geophysical Union
AGU journal highlights -- Dec. 4, 2013 The following highlights summarize research papers that have been recently published in Geochemistry, Geophysics, Geosystems (G3), Geophysical Research Letters (GRL), and Journal of Geophysical Research-Atmospheres (JGR-D).

In this release:

Interpreting the strongest deep earthquake ever observed Ultrahigh-speed camera captures unexpected lightning attachment Early geodynamo could have been driven by magma ocean in lower mantle Using new satellite data would improve hurricane forecasts New ice core record shows climate variability in West Antarctica Plasma loss mechanisms from Saturn's magnetosphere

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. doi: 10.1002/grl.50977. The doi is found at the end of each Highlight below.

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1. Interpreting the strongest deep earthquake ever observed

Massive earthquakes that strike deep within the Earth may be more efficient at dissipating pent up energy than similar quakes near the surface, according to new research by Wei et al. The authors analyzed the rupture of the most powerful deep earthquake ever recorded.

On 24 May 2013, a magnitude 8.3 earthquake hit deep beneath the Sea of Okhotsk, between Russia's Kamchatka Peninsula and Japan. The main shock of the earthquake was located at 610 kilometers (379 miles) depth, a rupture in the mantle far below the Earth's crust. By inverting seismic waves that were observed during the earthquake, the authors find that this initial shock triggered four subsequent shocks. These four shocks were magnitudes 7.8, 8.0, 7.9, and 7.9. A pressure front from the initial earthquake propagated at a speed of approximately 4.0 kilometers (2.5 miles) per second, setting off three subsequent earthquakes in a line south of the main shock. The rupture of the second follow-up earthquake sent a secondary rupture front back up north, triggering a third aftershock.

In total, the entire earthquake sequence took just 30 seconds, and the bulk of the stress was released by the four major shocks. In similar earthquake swarms that occur near the surface, such a release could take hours to days and would likely include a large number of small aftershocks. Based on this, the authors conclude that deep earthquakes are likely more efficient in dissipating stress than shallow earthquakes.

Source: Geophysical Research Letters, doi:10.1002/grl.50977, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50977/abstract

Title: Rupture Complexity of the Mw 8.3 Sea of Okhotsk Earthquake: Rapid Triggering of Complementary Earthquakes?

Authors: Shengji Wei, Don Helmberger, and Zhongwen Zhan: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA;

Robert Graves: United State Geological Survey, Pasadena, California, USA.

2. Ultrahigh-speed camera captures unexpected lightning attachment

A bolt of lightning takes, from start to finish, just a few fractions of a second, making lightning difficult to image, but ultrahigh-speed cameras are now enabling observations that show new details of the lightning process. Using multiple high-speed cameras, including one that can record at 20 microsecond intervals, capturing 50,000 frames per second, Lu et al. describe the unexpected processes behind a lightning bolt that struck the 440 meter (1,444 foot) tall International Finance Center in Guangzhou, China. From their high-speed records the authors report for the first time the observation of a lightning bolt where the downward leader attached below the tip of the upward connecting leader.

In a standard cloud-to-ground lightning stroke, a channel of ionized air, known as a stepped leader, reaches down, branching out from the cloud. Similarly, upward leaders reach up from the ground or grounded objects. When two leaders touch, a path is formed that facilitates the most intense lightning process, known as the return stroke. This attachment process was thought to occur tip-to-tip, with the tips of the upward and downward leaders being the points of contact. In their observations, however, the researchers find that the downward leader connected to the side of the over 403 meter (1,322 foot) long upward leader, touching at a point over 67 meters (220 feet) below its tip.

Source: Geophysical Research Letters, doi:10.1002/2013GL058060, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GL058060/abstract

Title: Lightning attachment process involving connection of downward negative leader to the lateral surface of upward connecting leader

Authors: Weitao Lu, Ying Ma, Yan Gao, Yang Zhang, and Yijun Zhang: Laboratory of Lightning Physics and Protection Engineering, Chinese Academy of Meteorological Sciences, Beijing, China, and State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China;

Luwen Chen and Qiyuan Yin: Lightning Protection Center of Guangdong Province, Guangzhou, China;

V. A. Rakov: Department of Electrical and Computer Engineering, University of Florida, Gainesville, USA.

3. Early geodynamo could have been driven by magma ocean in lower mantle

A new study suggesting that early in Earth's history the magnetic field may have been generated not in the core but by a magma ocean at the base of the lower mantle could change scientists' understanding of the magnetic history of Earth.

Earth's magnetic field is currently generated by convecting, electrically conductive fluid in the core. However, recent estimates of the properties of Earth's core suggest that the core may not have been able to sustain a geodynamo early in Earth's history.

Ziegler and Stegman propose that Earth's early geomagnetic field could instead have been generated by a magma ocean at the base of the solid lower mantle. Such an ocean has been hypothesized to have existed for billions of years, from very early in Earth's history about 4.5 billion years ago through at least about 2.5 billion years ago.

At the high temperatures and pressures of the lower mantle, the properties of the silicate melts that could have existed in such an ocean are uncertain. Therefore, the authors modeled a range of possible electrical and magnetic properties of a magma ocean and considered paleomagnetic evidence. They conclude that electrical conductivity could have been high enough for a magma ocean at the base of the lower mantle to have been a main driver of a geodynamo early in Earth's history.

Source: Geochemistry, Geophysics, Geosystems, doi: 10.1002/2013GC005001, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GC005001/abstract

Title: Implications of a long-lived basal magma ocean in generating Earth's ancient magnetic field

Authors: L. B. Ziegler: College of Earth, Ocean, and Atmospheric Science, Oregon State University, Corvallis, Oregon, USA;

D. R. Stegman: Ida and Cecil Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, UC San Diego, La Jolla, California, USA.

4. Using new satellite data would improve hurricane forecasts

To track and forecast the development of dangerous tropical cyclones, the National Weather Service's National Centers for Environmental Prediction uses a model known as the Hurricane Weather Research and Forecasting (HWRF) system. HWRF is an operational model that calculates a hurricane's likely path and intensity. The system currently does not use data pulled from weather satellites due to mixed results when data were assimilated from early weather satellites.

Launched in 2011, the National Oceanographic and Atmospheric Association's Suomi National Polar-Orbiting Partnership (NPP) satellite is the agency's most advanced Earth-observing satellite to date. Suomi NPP's observations are not currently used in the HWRF model. Focusing on four major hurricanes from 2012—Sandy, Isaac, Beryl, and Debbie—Zou et al. added the observations of Suomi NPP's microwave radiometer to the data stream that feeds into HWRF and addressed key methods that could improve the model's accuracy.

The authors find that including the Suomi NPP observations can help correct an eastward bias in hurricane track forecasts. The Suomi NPP data also improved intensity predictions, although they note that this is mostly because hurricane intensity depends strongly on the improved prediction of the storm's path. Even for Hurricane Sandy, which had a highly unusual storm track, the inclusion of the Suomi NPP data provided a path prediction that is closest to what actually occurred. The authors say that if the Suomi NPP data had been included in HWRF at the time, forecasters would have had an accurate assessment of Sandy's path 4 days before the storm made landfall in the United States, a full day of extra warning.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/2013JD020405, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JD020405/abstract

Title: Impacts of Assimilation of ATMS Data in HWRF on Track and Intensity Forecasts of 2012 Four Landfall Hurricanes

Authors: X. Zou: Deptment of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, Florida, USA;

F. Weng: NOAA Center for Satellite Applications and Research, College Park, Maryland, USA;

B. Zhang: Earth Resources Technology, Inc., Laurel, Maryland, USA;

L. Lin: Earth Resources Technology, Inc., Laurel, Maryland, USA, and Joint Center for Satellite Data Assimilation, College Park, Maryland, USA;

Z. Qin: Deptment of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, Florida, USA, and Nanjing University of Information Science and Technology, Nanjing, China;

V. Tallapragada: NOAA NCEP Environmental Modeling Center, College Park, Maryland, USA.

5. New ice core record shows climate variability in West Antarctica

A 308-year ice core record provides new data on climate variability in coastal West Antarctica and shows that a clear warming trend has occurred in recent decades. To study climate over the past 3 centuries, Thomas et al. analyzed stable isotopes in the ice core, which provide a record of past temperatures. They observe that climate variability in coastal West Antarctica is strongly driven by sea surface temperatures and atmospheric pressure in the tropical Pacific.

The authors report that their ice core record shows that the region warmed since the late 1950s at a rate similar to that observed in the Antarctic Peninsula and central West Antarctica. However, the authors note that this recent warming trend is similar in magnitude to warming and cooling trends that occurred in the mid-nineteenth and eighteenth centuries in their record, indicating that in this coastal West Antarctic location the effects of human-induced climate change in recent years have not exceeded natural climate variability over the past 300 years.

Source: Geophysical Research Letters, doi:10.1002/2013GL057782, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GL057782/abstract

Title: A 308 year record of climate variability in West Antarctica

Authors: Elizabeth R. Thomas, Thomas J. Bracegirdle and John Turner: British Antarctic Survey, Cambridge, UK;

Eric W. Wolff: Department of Earth Sciences, University of Cambridge, Cambridge, UK.

6. Plasma loss mechanisms from Saturn's magnetosphere

Since the first up-close observations of Saturn, made by the Pioneer 11 probe in 1979, a great deal has been learned about the dynamics of the gas giant's magnetosphere. In-depth observations made by the Cassini orbiter, which has been circling Saturn since 2004, have revealed fundamental differences between the behavior of Saturn's magnetosphere and that of the Earth's magnetosphere. Earth's magnetospheric plasma is largely populated by ions captured from the solar wind, whereas Saturn's plasma comes predominantly from water vapor that spews from massive geysers on the southern end of its icy moon Enceladus. Ionized water vapor from Enceladus streams out at 12 to 250 kilograms (27 to 551 pounds) per second, yet observations show that the concentration of plasma in Saturn's magnetosphere is at a relatively steady level. This discrepancy has left researchers searching for potential plasma loss mechanisms. In a review, Thomsen highlights the progress made in recent years in understanding this question.

According to the author, the main force driving plasma from Saturn's magnetosphere derives from the planet's fast rotation, which takes just 10.7 hours and produces currents in the magnetosphere that drive the plasma outward. These currents do not produce a uniform outflow—observations have shown interlocking fingers of cold inner magnetospheric plasma flowing outward and hot outer magnetospheric plasma flowing inward to take its place. Once it reaches the outer magnetosphere, the author says, the plasma can be lost to the solar wind, either crossing through the magnetopause or being swept down the magnetotail. Observations made using the Cassini orbiter have shown mass loss through magnetic reconnection in the magnetotail, but current estimates suggest that this mechanism is inadequate to remove all of the plasma emerging from the inner magnetosphere.

Source: Geophysical Research Letters, doi:10.1002/2013GL057967, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GL057967/abstract

Title: Saturn's Magnetospheric Dynamics

Authors: M. F. Thomsen: Planetary Science Institute, Tucson, Arizona, USA.

### Contact: Thomas Sumner
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