The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Space Physics (JGR-A), Journal of Geophysical Research-Solid Earth (JGR-B), Journal of Geophysical Research-Oceans (JGR-C), and Geochemistry, Geophysics, Geosystems.
In this release:
- Mercury's magnetic field measured by MESSENGER orbiter
- Oxygen isotopes improve weather predictability in Niger
- Annual Arctic sea ice less reflective than old ice
- Properties of solitary waves in Lake Constance
- How earthquake properties vary with depth
- Tracking a Jurassic reversal of the Earth's magnetic field
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1. Mercury's magnetic field measured by MESSENGER orbiter
Researchers working with NASA's Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft report the frequent detections of Kelvin-Helmholtz (KH) waves at the edge of the innermost planet's magnetosphere. In six different sets of magnetic field measurements made by the orbiter as it passed through Mercury's magnetopause, the boundary that separates the planet's magnetosphere from the solar wind plasma in the magnetosheath, Sundberg et al. detect the magnetic field oscillations characteristic of fully developed KH waves. Kelvin-Helmholtz waves form when fluids of different speeds travel alongside each other-in this case, the magnetosphere and magnetosheath plasmas-and promote mixing of the plasmas on larger spatial scales, and shorter time scales, than diffusive transport. The observations, which span the first 88 days of MESSENGER's time in orbit, bring Mercury alongside Earth, Saturn, and Venus as planets for which such Kelvin-Helmholtz waves are of importance.
The waves seen at Mercury's magnetopause, however, differ markedly from those at Earth's. The authors' KH wave observations were all made in the postnoon and duskside region of Mercury's magnetosphere, whereas at Earth, KH waves are seen farther toward the nightside on both flanks. Moreover, the measured waves had periods averaging 10-20 seconds, whereas the periods of their terrestrial counterparts are several minutes. Also, the amplitudes of the measured magnetic field oscillations were 2-3 times larger than those seen at Earth. Wave growth at the magnetopause is known to be an important mechanism for transporting material across the largely impermeable boundary, and the authors propose that these newly identified Kelvin-Helmholtz waves could be the source of plasma for Mercury's dayside boundary layer, discovered previously by the MESSENGER mission.
Source:
Journal of Geophysical Research-Space Physics, doi:10.1029/2011JA017268,
2012
http://dx.doi.org/10.1029/2011JA017268
Title: MESSENGER orbital observations of large-amplitude Kelvin-Helmholtz waves at Mercury's magnetopause
Authors: Torbjorn Sundberg: Heliophysics Science Division, NASA Goddard Space Flight Center, Maryland, USA;
Scott A. Boardsen: Heliophysics Science Division, NASA Goddard Space Flight Center, Maryland, USA and Goddard Earth Sciences and Technology Center, University of Maryland, Maryland, USA;
James A. Slavin, Thomas H. Zurbuchen, and Jim M. Raines: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Michigan, USA;
Brian J. Anderson and Haje Korth: The Johns Hopkins University Applied Physics Laboratory, Maryland, USA;
Sean C. Solomon: Department of Terrestrial Magnetism, Carnegie Institution of Washington, District of Columbia, USA.
2. Oxygen isotopes improve weather predictability in Niger
For the African nation of Niger, the effect of seasonal atmospheric variability on the weather is poorly understood. Because most residents rely on local agriculture, improving the predictability of seasonal weather and precipitation availability is crucial. In the summer of 2006, researchers measured the oxygen isotope ratio of rainwater collected in the nation's capital, Niamey, to determine the connection between intraseasonal atmospheric variability and precipitation. Water containing the heavier oxygen-18 isotope rains preferentially over the lighter oxygen-16 version, lighter water evaporates preferentially over the heavier molecule, and the oxygen isotope ratio decreases from the equator to the poles. Thus, the oxygen isotope ratio found in a water sample can indicate the water's history. Previous research found that precipitation oxygen isotope ratios could be used to understand convective processes, but to develop a more nuanced and continuous interpretation researchers need to understand the seasonal shifts in the background atmospheric water vapor ratio.
From July 2010 to May 2011, Tremoy et al. measured the atmospheric water vapor and precipitation oxygen isotope ratios in Niamey. They find that the water vapor ratio varied regularly throughout the year, with minima during both the summer monsoon and the winter dry season and maxima in between. The authors suggest that the summer decline is driven by convection associated with the monsoon and that the dry season decrease is due to both atmospheric subsidence and air arrivals from midlatitudes. The fall maxima are caused by weakening convection, and the spring peak is associated with oxygen-18 enriched air moving in from the south. The authors also detect a number of shorter-period shifts in water vapor isotopic composition, which they suggest are driven by convective processes, like evaporation and subsidence, and daily atmospheric mixing, potentially opening the door for oxygen isotope measurements to be used to study atmospheric variability and dynamics and thus the origin of Niger's moisture.
Source:
Geophysical Research Letters, doi:10.1029/2012GL051298, 2012
http://dx.doi.org/10.1029/2012GL051298
Title: A 1-year long delta-O-18 record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales
Authors: Guillaume Tremoy and Olivier Cattani: Laboratoire des Sciences du Climat et de l'Environnement, UMR 8212, Institut Pierre Simon Laplace, CEA-CNRS-UVSQ, Gif-sur-Yvette, France;
Francoise Vimeux: Laboratoire des Sciences du Climat et de l'Environnement, UMR 8212, Institut Pierre Simon Laplace, CEA-CNRS-UVSQ, Gif-sur-Yvette, France and Laboratoire HydroSciences Montpellier, UMR 5569, Institut de Recherche pour le Developpement, CNRS-IRD-UM1-UM2, Montpellier, France;
Salla Mayaki and Ide Souley: Institut des RadioIsotopes, Universite Abdou Moumouni de Niamey, Niamey, Niger;
Camille Risi: Laboratoire de Meteorologie Dynamique, Institut Pierre Simon Laplace, UPMC-CNRS, Paris, France;
Guillaume Favreau and Monique Oi: Laboratoire HydroSciences Montpellier, UMR 5569, Institut de Recherche pour le Developpement, CNRS-IRD-UM1- UM2, Montpellier, France.
3. Annual Arctic sea ice less reflective than old ice
In the Arctic Ocean, the blanket of permanent sea ice is being progressively replaced by a transient winter cover. In recent years the extent of the northern ocean's ice cover has declined. The summer melt season is starting earlier, the winter freeze is happening later, the areal extent of the ice has decreased, and more ice is failing to last through the summer. A key uncertainty in this ongoing climate transformation is how seasonal sea ice affects and responds to climate dynamics as compared to the traditional multiyear sea ice. Tackling an important branch of this issue, Perovich and Polashenski analyze how the albedo of seasonal sea ice changes throughout the summer melt season. The ice's albedo affects how much sunlight enters the system and hence influences biological productivity, ice extent, and future rates of warming.
For four years, the authors measured the albedo every 2.5 meters (8 feet) along a 200-m (656-ft) stretch of seasonal ice off the northern coast of Alaska. They find that though the albedo of snow-covered winter seasonal ice is the same as that of multiyear ice, the equivalence fades rapidly with the summer thaw. They find that seasonal sea ice albedos experience seven distinct phases: cold snow, melting snow, pond formation, pond drainage, pond evolution, open water, and refreezing. Though the albedos of seasonal and multiyear ice experience similar transitions, the rate and extent for the two types of ice vary drastically with the potential for a large effect on the Arctic Ocean energy budget. The authors find that over the course of one melt season nearly 40 percent more energy would enter an ocean system with seasonal sea ice cover than one with multiyear ice.
Source: Geophysical Research Letters, doi:10.1029/2012GL051432, 2012
http://dx.doi.org/10.1029/2012GL051432
Title: Albedo evolution of seasonal Arctic sea ice
Authors: Donald K. Perovich: CRREL, ERDC, Hanover, New Hampshire, USA and Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA;
Christopher Polashenski: CRREL, ERDC, Hanover, New Hampshire, USA.
4. Properties of solitary waves in Lake Constance
Solitary waves-large individual waves that can travel long distances holding their shape, when normal waves would tend to flatten out-occur in oceans and in lakes, both on the surface and as internal waves below the surface. In lakes, these waves can affect circulation and mixing, and influence aquatic ecosystems, but many studies of the properties and effects of internal solitary waves in lakes are based on limited observations. From observations recorded over six years in Lake Constance, in Germany, Preusse et al. studied seasonal changes in the properties of internal solitary waves. Their study, which included 219 wave trains with a range of numbers of waves per train, amplitude, propagation depth, and other properties, shows that internal solitary waves are a regular occurrence. They find that a substantial number of the solitary waves are strongly nonlinear and that solitary wave properties vary with the stratification of the lake, which changes with season.
Source:
Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007403, 2012
http://dx.doi.org/10.1029/2011JC007403
Title: Seasonal variation of solitary wave properties in Lake Constance
Authors: M. Preusse: Limnological Institute, University of Konstanz, Konstanz, Germany, Department of Mathematics and Statistics, University of Konstanz, Konstanz, Germany;
H. Freistuhler: Department of Mathematics and Statistics, University of Konstanz, Konstanz, Germany;
F. Peeters: Limnological Institute, University of Konstanz, Konstanz, Germany.
5. How earthquake properties vary with depth
A new study shows systematically how seismic properties vary with depth. Lay et al. analyzed recent large and great earthquakes, including the 2004 Sumatra- Andaman (magnitude 9.2), 2010 Chile (magnitude 8.8), and 2011 Tohoku (magnitude 9.0) earthquakes. They define four domains of seismogenic behavior along megathrust faults according to depth. In domain A, the shallowest, reaching to about 15 kilometers (about 9 miles) below sea level, large tsunami-generating earthquakes can occur. In domain B, extending from about 15- to 35-km (9- to 22- mi) depth, great earthquake events with large slip but diffuse short-period energy occur. In domain C, from 35- to 55-km (22- to 34-mi) depth, smaller isolated megathrust patches rupture, producing bursts of coherent short-period energy in both great ruptures and in moderate-sized events. In domain D, which extends from about 30- to 45-km (19- to 28-mi) depth in subduction zones where relatively young ocean lithosphere is being underthrust with shallow plate dip, low-frequency earthquakes, seismic tremor, and slow slip events occur. Below this zone, stabile sliding or ductile flow takes place.
Source:
Journal of Geophysical Research-Solid Earth, doi:10.1029/2011JB009133, 2012
http://dx.doi.org/10.1029/2011JB009133
Title: Depth-varying rupture properties of subduction zone megathrust faults
Authors: Thorne Lay: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA;
Hiroo Kanamori: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA;
Charles J. Ammon: Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA;
Keith D. Koper: Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA;
Alexander R. Hutko: Incorporated Research Institutions for Seismology Data Management Center, Seattle, Washington, USA;
Lingling Ye, Han Yue, and Teresa M. Rushing: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA.
6. Tracking a Jurassic reversal of the Earth's magnetic field
Roughly 180 million years ago, during the height of the Jurassic period, the Earth's magnetic field flipped, bringing the magnetic north pole once again into the Northern Hemisphere. This so-called van Zijl reversal, named for the researcher who first described it, is the second-oldest well-documented geomagnetic reversal. Such perturbations of the Earth's magnetic field, which tend to take place over about 10,000 years, and possibly much less, have been identified as occurring up to several billion, and as recently as 780,000, years ago. An open question exists about the effect of such reversals on the properties of the Earth's magnetic field, including the structure it takes, and the consequent effects on its shape, size, and strength. Drawing on newly identified records of the van Zijl reversal, Moulin et al. describe the serpentine travels of the transitional magnetic pole and the variable strength of the paleomagnetic field.
Analyzing the orientations of magnetic minerals found encased within rock samples drawn from an ancient lava field in Lesotho, a small country encompassed within South Africa, and from another field in South Africa itself, the authors tracked the shifting geographic location of the ancient magnetic pole. They find that over a short period, possibly only a few centuries, the pole leapt from a location oriented around 45 degrees south to one near 45 degrees north. The paleomagnetic pole then drifted through around 20 degrees latitude as it moved to the southeast. Finally, the pole moved to a stable location centered near the geographic north pole. The authors find that leading up to the magnetic reversal, the strength of the magnetic field weakened to roughly 10 - 20 percent of its normal value, a depression that only decayed once the pole's location stabilized.
Source:
Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003910, 2012
http://dx.doi.org/10.1029/2011GC003910
Title: The "van Zijl" Jurassic geomagnetic reversal revisited
Authors: Maud Moulin: Departement de Geologie, Universite Jean Monnet, Saint Etienne, France;
Vincent Courtillot, Frederic Fluteau, and Jean-Pierre Valet: Equipe de Paleomagnetisme, Institut de Physique du Globe, Paris, France and Sciences de la Terre, de l'Environnement et des Planetes, Universite Paris Diderot, Paris, France.