The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL).
In this release:
- Ozone-destroying gas levels spike in Arctic middle atmosphere
- Lunar subsurface features mapped
- Climate models must consider ozone variations
- How L'Aquila earthquake rupture and aftershocks evolved
- Explaining ocean reflectance lines
- Cosmic ray particles flow into solar region
- Groundwater resources declining in northern India
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1. Ozone-destroying gas levels spike in Arctic middle atmosphere
Energetic particle precipitation, in which high-energy electrons and protons from the Sun and magnetosphere hit Earth's upper atmosphere, results in the production of nitrogen oxides (NOx) in the mesosphere and lower thermosphere. Under the right weather conditions, NOx can descend into the stratosphere, where it destroys ozone. Randall et al. describe observations that show that in early 2009, the amount of NOx that descended into the middle atmosphere above the Arctic was about 50 times higher than usual. They argue that the high level of NOx resulted not from increased energetic particle precipitation (which was actually below average at the time) but from unusual meteorological conditions that allowed more NOx to reach the middle atmosphere. The authors note that 2009 is the second time on record in recent years when abnormal weather conditions led to increased descent of NOx in the polar region. They suggest that this may indicate that changes are occurring in the atmosphere and point to a need for better understanding of the interaction between meteorology and space weather.
Title: NOx descent in the Arctic middle atmosphere in early 2009
Authors: C. E. Randall, V. L. Harvey, and J. France: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA; D. E. Siskind: Space Science Division, Naval Research Laboratory, Washington, D. C., USA; P. F. Bernath: Department of Chemistry, University of York, Heslington, UK; C. D. Boone: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada; K. A. Walker: Department of Physics, University of Toronto, Toronto, Ontario, Canada.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039706, 2009; http://dx.doi.org/10.1029/2009GL039706
2. Lunar subsurface features mapped
Recently the Japanese Kaguya spacecraft (also known as SELENE) took radar soundings of the lunar subsurface and detected subsurface echoes in the western nearside maria, the large basaltic plains formed by volcanic eruption. To learn more, Oshigami et al. study the distribution of these subsurface reflectors. They note that these subsurface reflectors are found in a few areas that consist of about 10 percent of the western nearside maria, at apparent depths ranging from hundreds to more than 1000 meters (hundreds to thousands of feet). The authors suggest the reflectors represent thick regolith layers between different basaltic rocks within the maria, not the bottom of the maria. The regolith probably accumulated during periods of volcanic inactivity, the authors report. By comparing the distribution of these subsurface reflectors and the ages of the surface layers, they found that most of the reflectors formed more than 3.4 billion years ago. The results should contribute to scientists' understanding of the geologic history of the Moon.
Title: Distribution of the subsurface reflectors of the western nearside maria observed from Kaguya with Lunar Radar Sounder
Authors: Shoko Oshigami and Yasushi Yamaguchi: Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan; Atsushi Yamaji: Graduate School of Science, Kyoto University, Kyoto, Japan; Takayuki Ono, Atsushi Kumamoto, and Hiromu Nakagawa: Graduate School of Science, Tohoku University, Sendai, Japan; Takao Kobayashi: Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039835, 2009; http://dx.doi.org/10.1029/2009GL039835
3. Climate models must consider ozone variations
Ozone levels vary in time and with height, latitude, and longitude, but most climate models, including those used by the Intergovernmental Panel on Climate Change, do not take the longitudinal variations into account; instead, they use prescribed zonal average ozone levels as input. To study the effect of zonal ozone asymmetries on simulated long-term Southern Hemisphere climate trends, Waugh et al. use a chemistry climate model to compare simulations for 1955 to 2055 that include three-dimensional interactive stratospheric chemistry with simulations in which the zonal mean ozone is prescribed. In agreement with previous studies, the authors find that simulations that prescribe zonal average ozone underestimate the effect of changes in the ozone hole on temperature and circulation trends in the Southern Hemisphere. They conclude that this effect needs to be considered when interpreting completed climate simulations and planning new ones.
Title: Effect of zonal asymmetries in stratospheric ozone on simulated Southern Hemisphere climate trends
Authors: D. W. Waugh and L. Oman: Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland, USA; P. A. Newman and R. S. Stolarski: Atmospheric Chemistry and Dynamics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; S. Pawson and J. E. Nielsen: Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA; J. Perlwitz: Cooperative Institute for Research in Environmental Sciences, University of Colorado, NOAA, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL040419, 2009; http://dx.doi.org/10.1029/2009GL040419
4. How L'Aquila earthquake rupture and aftershocks evolved
In April 2009 the magnitude 6.3 L'Aquila earthquake shook central Italy. By analyzing seismic waves from the main shock and aftershocks, Chiarabba et al. examine the space and time evolution of the seismicity in the region. Using data from seismic stations in the region, they locate the active faults in the region, show where areas of large slip occurred, and describe the geometry of the active faults, which had not been observed through surface observations in past years. They find that seismic activity migrated northward in the first 3 days after the 6 April main shock, and then continued migrating and spreading from the main fault plane to adjacent faults. Most of the aftershocks occurred on the main fault plain of the Paganica fault, a fault that has not been well studied. The authors note that the lack of seismic release in the upper 6 to 7 kilometers (3.7 to 4.3 miles) of a fault located to the north, forming an echelon system, could point to the possibility of a large earthquake in the future. The data can help scientists constrain and validate seismological models and could help in seismic hazard assessment.
Title: The 2009 L'Aquila (central Italy) Mw6.3 earthquake: Main shock and aftershocks
Authors: C. Chiarabba, A. Amato, M. Anselmi, P. Baccheschi, I. Bianchi, M. Cattaneo, G. Cecere, L. Chiaraluce, M. G. Ciaccio, P. De Gori, G. De Luca, M. Di Bona, R. Di Stefano, L. Faenza, A. Govoni, L. Improta, F. P. Lucente, A. Marchetti, L. Margheriti, F. Mele, A. Michelini, G. Monachesi, M. Moretti, M. Pastori, N. Piana Agostinetti, D. Piccinini, P. Roselli, D. Seccia, and L. Valoroso: Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039627, 2009; http://dx.doi.org/10.1029/2009GL039627
5. Explaining ocean reflectance lines
Ocean reflectance images show thin surface lines with spacing between 100 meters (328 feet) and 10 kilometers (6.2 miles) that have been difficult to explain. McWilliams et al. propose a new dynamical explanation for the formation of these surface lines. Their proposed mechanism, cold filamentary intensification, shows how surface lines sharpen and become concentrated to produce thin filaments. They draw an analogy between cold filamentary intensification and the similar phenomenon of frontogenesis, the formation of an atmospheric or oceanic front. In addition, the authors ran an ocean circulation simulation that demonstrates the occurrence of cold filamentary intensification in the southeastern Pacific Ocean. They conclude that cold filamentary intensification could provide a fundamental interpretation of features seen in visual and radar reflectance images and could explain strong vertical velocities and material flux in the upper ocean.
Title: Cold filamentary intensification and oceanic surface convergence lines
Authors: J. C. McWilliams, F. Colas, M. J. Molemaker: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039402, 2009; http://dx.doi.org/10.1029/2009GL039402
6. Cosmic ray particles flow into solar region
Anomalous cosmic ray (ACR) particles originate as interstellar neutral gas that flows into the heliosphere, becomes ionized, is picked up by the solar wind, and is accelerated to high energies in the outer heliosphere. These ACRs then flow back into the inner heliosphere by the processes of diffusion and drift in the Sun's magnetic field (SMF) that is carried into the heliosphere by the solar wind. The drift motion depends on the SMF polarity, which reverses every 11 years. To improve understanding of the relative importance of the diffusion and drift processes, Cummings et al. used three satellites to measure the radial and latitudinal distribution of ACR oxygen from early 2007 to mid 2008. This was the first such measurement made when the SMF points inward north of the heliospheric current sheet (HCS), the wavy plane separating the inward and outward pointing regions of the SMF. This measurement indicates that the HCS is not an efficient conduit for these particles to the inner heliosphere if the waviness is too large, as during this period. The authors suggest that these results will help scientists correctly model cosmic ray transport.
Title: Radial and latitudinal gradients of anomalous cosmic ray oxygen in the inner heliosphere
Authors: A. C. Cummings, R. A. Mewaldt, and E. C. Stone: Space Radiation Laboratory, California Institute of Technology, Pasadena, California, USA; C. Tranquille and R. G. Marsden: Research and Scientific Support Department, ESTEC, Noordwijk, Netherlands.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039851, 2009; http://dx.doi.org/10.1029/2009GL039851
7. Groundwater resources declining in northern India
Groundwater resources have been declining sharply in northern India, one of the most highly populated and heavily irrigated regions of the world. To measure the rate of groundwater loss in recent years, Tiwari et al. use satellite gravity observations from the Gravity Recovery and Climate Experiment (GRACE) satellite, which measures changes in mass by detecting variations in the gravity field. GRACE makes it possible to directly monitor regional changes in water storage without the need for local sampling. The authors estimate that the total loss rate for the region during the period from April 2002 to June 2008 was 54 cubic kilometers (13 cubic miles) per year, probably the largest rate of groundwater loss from any comparably sized region on Earth. They note that this loss is roughly equivalent to the mass loss from melting Alaskan glaciers during the same period, and could have contributed about 0.16 millimeters (0.006 inches) per year to global sea level rise. The authors predict that if this trend continues, a serious water crisis in northern India could soon result.
Title: Dwindling groundwater resources in northern India, from satellite gravity observations
Authors: V. M. Tiwari: National Geophysical Research Institute, CSIR, Hyderabad, India and Department of Physics, University of Colorado, Boulder, Colorado, USA; J. Wahr: Department of Physics and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; S. Swenson: Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado, USA.
Source: Geophysical Research Letters (GRL) paper 10.1029/2009GL039401, 2009; http://dx.doi.org/10.1029/2009GL039401
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Geophysical Research Letters