American Geophysical Union AGU Journal Highlights - 30 December 2011
Highlights, including authors and their institutions
The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Space Physics (JGR-A), Journal of Geophysical Research-Oceans (JGR-C), Journal of Geophysical Research- Biogeosciences (JGR-G), and Geophysical Research Letters (GRL).
In this release:
1. Cassini data shows Saturn moon may affect planet's magnetosphere
2. Using Loch Ness to track the tilt of the world
3. Alaskan lake bed cores show expanding Arctic shrubs may slow erosion
4. Evaluating the energy balance of Saturn's moon Titan
5. A new way to measure Earth's magnetosphere
6. Waves triggered by lightning leak out of Earth's atmosphere
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1. Cassini data shows Saturn moon may affect planet's magnetosphere
Scientists have been puzzled by periodic bursts of radiation, known as the Saturn kilometric radiation (SKR), that occur in the planet's magnetosphere. These emissions occur at a rate that is close to, but not quite the same as, the rate at which the planet rotates.
New observations from the Cassini spacecraft's flybys of Saturn's moon Enceladus in 2008 are revealing new details about the plasma environment around Enceladus and how it may affect Saturn's magnetosphere. These observations could also shed some light on the SKR rotation rate.
Enceladus sprays out a plume of water vapor and ice from its south pole. This plume produces ionized gas that is a significant source of plasma for Saturn's magnetosphere and E ring. Observations described by Morooka et al. show that the plume also produces negatively charged dust that affects the motion of the plasma in this region. This dust-plasma interaction impacts the dynamics of Saturn's magnetosphere, possibly influencing the rate of SKR emissions.
Source: Journal of Geophysical Research-Space Physics, doi:10.1029/2011JA017038, 2011 http://dx.doi.org/10.1029/2011JA017038
Title: Dusty plasma in the vicinity of Enceladus
Authors: M. W. Morooka, J.-E. Wahlund, and A. I. Eriksson: Swedish Institute of Space Physics, Uppsala, Sweden;
W. M. Farrell: Planetary Magnetospheres Laboratory, Goddard Space Flight Center, Greenbelt, Maryland, USA;
D. A. Gurnett, W. S. Kurth, and A. M. Persoon: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA;
M. Shafiq, M. Andre, and M. K. G. Holmberg: Swedish Institute of Space Physics, Uppsala, Sweden.
2. Using Loch Ness to track the tilt of the world
That the rise and fall of the tide is primarily driven by the gravitational pull of the moon and the Sun is common knowledge, but not all tides are controlled by such a standard mechanism. Researchers working on Loch Ness in Scotland find that rather than the loch's tide being driven directly by this so-called astronomical tide, it is also controlled by a process known as ocean tidal loading. Loch Ness lies just 13 kilometers (8 miles) inshore from the North Sea. The astronomical tide redistributes the ocean to such an extent that the changing mass of water along the coast deforms the seafloor. As the ocean tide ebbs and flows, the surface of the Earth rises and falls.
Through a series of pressure sensors distributed throughout Loch Ness that measured the height of the water, and by ruling out other potential sources, Pugh et al. find that this local shift in the shape of the Earth-like a bowl of water on an unstable table-controls the loch's tide. They find that the tide has a magnitude of 1.5 millimeters (0.06 inches), a measurement made to an accuracy of just 0.1 mm (0.004 in) over the loch's 35 km (22 mi) length. The authors suggest that this sensitivity in measuring the effects of tidal loading surpasses even that possible using Global Positioning Satellite receivers. The authors hope that similar experiments conducted at suitable lakes worldwide could be used to better understand oceanic tidal loading.
Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007411, 2011 http://dx.doi.org/10.1029/2011JC007411
Title: Lunar Tides in Loch Ness, Scotland
Authors: David T. Pugh and Philip L. Woodworth: National Oceanography Centre, Liverpool, United Kingdom;
Machiel S. Bos: CIMAR/CIIMAR, University of Porto, Porto, Portugal.
3. Alaskan lake bed cores show expanding Arctic shrubs may slow erosion
The relationship between permafrost, Arctic vegetation, soil erosion, and changing air temperatures is complicated at best. For instance, rising temperatures melt surface permafrost layers and increase shrub growth. These shrubs can catch drifting snow, insulating the soil during the winter, and accelerate permafrost degradation-facilitating their own proliferation. Alternatively, increased vegetation can shift energy transfer dynamics, cooling the surface and protecting permafrost. Hence, expanding Arctic shrub populations may either reinforce or counteract permafrost erosion. The complexity of the interactions makes firsthand accounts of these dynamics particularly important.
To figure out how the permafrost ecosystem has evolved under modern warming for the northernmost reaches of Alaska, Tape et al. pulled observations from a diverse set of sources. The authors took sediment cores from lake beds in the study area to determine changes in sedimentation rates, and hence watershed erosion, for the past 60-100 years. Tree ring analyses indicate the changing growth rates of tall shrubs, and satellite observations show changes in shrub extent. The authors find that erosion rates were increasing or fluctuating prior to 1980, after which they declined for three of the four lakes under investigation. The authors suggest that this reduction in erosion rate was driven by the observed 18 percent increase in the coverage of tall shrub, whose roots could have helped stabilize the soil. The authors suggest that their technique, of using lake bed soil cores to detect permafrost degradation at the watershed scale, will be particularly important for furthering the understanding of the changing Arctic.
Source: Journal of Geophysical Research-Biogeosciences, doi: 10.1029/2011JG001795, 2011 http://dx.doi.org/10.1029/2011JG001795
Title: Twentieth century erosion in Arctic Alaska foothills: The influence of shrubs, runoff, and permafrost
Authors: Ken D. Tape: Institute of Arctic Biology, University of Alaska-Fairbanks, Alaska, USA;
David Verbyla: Department of Forest Sciences, School of Natural Resources and Agricultural Sciences, University of Alaska-Fairbanks, Alaska, USA;
Jeffrey Welker: Environment and Natural Resources Institute and Biology Department, University of Alaska-Anchorage, Alaska, USA.
4. Evaluating the energy balance of Saturn's moon Titan
To understand the weather and climate on Earth as well as on other planets and their moons, scientists need to know the global energy balance, the balance between energy coming in from solar radiation and thermal energy radiated back out of the planet. The energy balance can provide interesting information about a planet. For instance, Jupiter, Saturn, and Neptune emit more energy than they absorb, implying these planets have an internal heat source. Earth, on the other hand, is in near equilibrium, with energy coming in approximately equaling energy going out, though a small energy imbalance can lead to global climate change.
Saturn's moon Titan is the only moon in the solar system with a thick atmosphere, and scientists have been interested in exploring ways in which Titan is similar to Earth. To learn more about Titan, Li et al. calculated its energy balance. The absorbed energy has been measured by various telescopes and spacecraft; the emitted energy was recently measured by instruments onboard NASA's Cassini spacecraft. The authors compared total absorbed solar power with total emitted thermal power and find that the global energy budget of Titan is in equilibrium within the measurement error.
Source: Geophysical Research Letters, doi:10.1029/2011GL050053, 2011 http://dx.doi.org/10.1029/2011GL050053
Title: The global energy balance of Titan
Authors: Liming Li, Mark A. Smith, and Xun Jiang: Department of Earth and Atmospheric Sciences, University of Houston, Houston,Texas, USA;
Conor A. Nixon and Richard K. Achterberg: Department of Astronomy, University of Maryland, College Park, Maryland, USA;
Nicolas J. P. Gorius, Amy A. Simon-Miller, and F. Michael Flasar: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
Barney J. Conrath and Peter J. Gierasch: Department of Astronomy, Cornell University, Ithaca, New York, USA;
Kevin H. Baines, Robert A. West, and Ashwin R. Vasavada: Jet Propulsion Laboratory, Caltech, Pasadena, California, USA;
Andrew P. Ingersoll and Shawn P. Ewald: Division of Geological and Planetary Sciences, Caltech, Pasadena, California, USA.
5. A new way to measure Earth's magnetosphere
Researchers have demonstrated the potential use of a new way to measure properties of Earth's magnetosphere, the magnetic bubble that surrounds the planet. Zhai et al. used a property known as Faraday rotation for radio tomographic imaging of the magnetosphere. Faraday rotation occurs when a linearly polarized light wave travels through a magnetized medium such as the magnetosphere. The magnetic field causes the plane of polarization to rotate, and the amount of rotation is directly proportional to the electron density in the medium and to the magnetic field. Therefore, because Earth's magnetic field is known, researchers can use measurements of Faraday rotation to reconstruct electron density in the magnetosphere.
Using receivers on the Wind spacecraft, the researchers measured the polarization of radio signals transmitted by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. They used the polarization data to reconstruct a two-dimensional electron density image of Earth's magnetosphere in the north polar region. The researchers find that the electron density determined by this method agrees well with empirical models of electron density. Such measurements could lead to improved understanding of large-scale processes in the magnetosphere.
Source: Journal of Geophysical Research-Space Physics, doi:10.1029/2011JA016743, 2011 http://dx.doi.org/10.1029/2011JA016743
Title: Magnetospheric radio tomographic imaging with IMAGE and Wind
Authors: Y. Zhai: Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey, USA;
S. A. Cummer: Electrical and Computer Engineering Department, Duke University, Durham, North Carolina, USA;
J. L. Green: Planetary Sciences Division, NASA Headquarters, Washington, DC, USA;
B. W. Reinisch: Center for Atmospheric Research, University of Massachusetts, Lowell, Massachusetts, USA;
M. L. Kaiser: Space Weather Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
M. J. Reiner: Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington, DC, USA; NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
K. Goetz: School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA.
6. Waves triggered by lightning leak out of Earth's atmosphere
Lightning flashes can generate electromagnetic waves in the atmosphere. When these waves have a particular frequency, they can resonate in the cavity formed by the Earth's surface and the bottom of the ionosphere. This phenomenon, known as a Schuman resonance, has been observed from the ground and used to study a variety of atmospheric properties. Now, Simoes et al. report the first observations of Schumann resonances from a satellite. They detected the extremely low frequency waves with the Communications/Navigation Outage Forecasting System (C/NOFS) satellite, which studies ionospheric conditions. The researchers suggest the fact that these waves were detected in the ionosphere, outside of the surface- ionosphere cavity, indicates that some of these waves are unexpectedly leaking out into space. Therefore scientists may need to revise models of extremely low frequency wave propagation in the ionosphere.
Source: Geophysical Research Letters, doi:10.1029/2011GL049668, 2011 http://dx.doi.org/10.1029/2011GL049668
Title: Satellite observations of Schumann resonances in the Earth's ionosphere
Authors: Fernando Simoes, Robert Pfaff, and Henry Freudenreich: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
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Journal
Geophysical Research Letters