AGU journal highlights -- Nov. 5 2013

(Press-News.org) Contact information: Mary Catherine Adams
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American Geophysical Union
AGU journal highlights -- Nov. 5 2013 The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Oceans (JGR-C), Geophysical Research Letters, and Space Weather.

In this release:

Anthropogenic aerosols increasing over India Comparing climate impact of different geoengineering methods New physical model calculates airline crews' radiation exposure Magnetic energy determines electron bulk heating Coastal radar observations reveal complex surface circulations Assessing a plan to clear energetic protons from the radiation belt

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: 10.1002/2013JD020507. The doi is found at the end of each Highlight below.

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1. Anthropogenic aerosols increasing over India

Aerosol particles in the Earth's atmosphere scatter and absorb light differently at different wavelengths, thereby affecting the amount of incoming sunlight that reaches the planet's surface and the amount of heat that escapes, potentially altering the planet's climate. Most recent regional studies of aerosol trends have used satellite data to examine aerosol levels over ocean regions; fewer regional studies have measured aerosol over land.

A network of aerosol observatories known as ARFINET was set up in the mid-1980s to measure aerosols over the Indian subcontinent. Babu et al. analyzed the data from ARFINET observations to look at the long-term trends in aerosol optical depth, a measure of the concentration of aerosols in the atmosphere. They find statistically significant increases in aerosol optical depth at most stations over the past 2 decades. In fact, they report that "a phenomenal increase in aerosol loading has taken place."

Aerosol levels and the rate of increase varied seasonally, the authors find. The rate of increase was high during the dry months of December through March, but there was no strong trend during the premonsoon (April to May) and summer monsoon periods (June through September).

The authors also used simulations to determine the contributions of different aerosol chemicals to the total aerosol optical depth. They find that contributions to total aerosol from dust have decreased, but the contributions from anthropogenic emissions have increased over the past 2 decades.

Source: Journal of Geophysical Research-Atmospheres, doi: 10.1002/2013JD020507, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JD020507/abstract

Title: Trends in Aerosol Optical Depth over Indian region: Potential causes and impact indicators

Authors: S. Suresh Babu, M. R. Manoj, K. Krishna Moorthy, Mukunda M Gogoi, Vijayakumar S Nair, Sobhan Kumar Kompalli: Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India;

S. K. Satheesh: Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India;

K. Niranjan: Department of Physics, Andhra University, Visakhapatanam, India;

K. Ramagopal: Department of Physics, Sri Krishnadevaraya University, Anantapur, India;

P. K. Bhuyan: Department of Physics, Dibrugarh University, Dibrugarh, India;

Darshan Singh: Department of Physics, Punjabi University, Patiala, India.

2. Comparing climate impact of different geoengineering methods

If efforts to control greenhouse gas emissions do not succeed in combating global warming, some scientists and policy makers may consider geoengineering the climate, even though this could be a risky strategy with potential drawbacks. One geoengineering options is solar radiation management, which involves adjusting the amount of sunlight reaching the planet through one of several possible methods, including injecting sulfur into the stratosphere to block incoming sunlight, putting mirrors in space to reflect sunlight, or injecting sea salt into the air above the oceans to increase the reflectivity of clouds. All of these methods could potentially have a cooling effect, but the regional climate effects and effects on precipitation patterns could differ.

Using an Earth system model, Niemeier et al. compared the effects of these three methods. In their simulations, the forcing from an increase in greenhouse gases in a scenario known as representative concentration pathway 4.5 was balanced by these solar radiation management techniques over 50 years. They also considered a scenario in which radiative forcing was kept constant by fixing greenhouse gas levels at 2020 values.

The authors find that while all three solar radiation management techniques led to similar global mean temperature increases compared to the mean climate around the year 2020, the precipitation patterns varied considerably among the methods. Similar to previous studies, they find that all three solar radiation management techniques would result in lower total global precipitation than fixing greenhouse gas levels. However, injecting salt or sulfur into the air would reduce precipitation more than mirrors would. Salt injection would also change the land/sea distribution of precipitation, with increasing precipitation over land and decreasing precipitation over oceans.

In addition, with all three methods, the difference in temperature between the poles and the equator would decrease, though salt injection would lead to a much stronger reduction of warming in the tropics than the other methods.

Source: Journal of Geophysical Research-Atmospheres, doi: 10.1002/2013JD020445, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JD020445/abstract

Title: Solar irradiance reduction via climate engineering: Impact of different techniques on the energy balance and the hydrological cycle

Authors: U. Niemeier and H. Schmidt: Max Planck Institute for Meteorology, Hamburg, Germany;

K. Alterskjær and J.E. Kristjánsson: Department of Geosciences, Meteorology and Oceanography Section, University of Oslo, Oslo, Norway.

3. New physical model calculates airline crews' radiation exposure

Airline pilots and crews, who spend hundreds of hours each year flying at high altitude, are exposed to increased doses of radiation from galactic cosmic rays and solar energy particles, enough so that airline crew members are actually considered radiation workers by the International Commission on Radiological Protection.

The radiation to which crew members are exposed changes with conditions in space and in the atmosphere, and with the behavior of the Earth's magnetic field. Radiation exposure changes with latitude and altitude. It also ebbs and flows with the 11-year solar cycle. Steps can be taken to protect crew members against the radiation hazard, such as diverting to lower altitude or latitude paths when conditions change and the radiation risk increases. Doing so, however, requires having an accurate early warning.

The nowcast of atmospheric ionizing radiation for aviation safety (NAIRAS) model, described by Mertens et al., is a new physics-based model that can calculate the radiation hazard. In general the model's predictions are accurate to within 50 percent of measured doses. This accuracy increases to 25 percent near the poles where, because of the properties of the Earth's magnetic field, the risk is the highest.

Source: Space Weather, doi:10.1002/swe.20100, 2013 http://onlinelibrary.wiley.com/doi/10.1002/swe.20100/abstract

Title: NAIRAS aircraft radiation model development, dose climatology, and initial validation

Authors: Christopher J. Mertens and Ryan B. Norman: NASA Langley Research Center, Hampton, Virginia, USA;

Matthias M. Meier: DLR - German Aerospace Center, Institute of Aerospace Medicine, Radiation Biology, Cologne, Germany;

Steven Brown: George Mason University, School of Physics, Astronomy, and Computational Sciences, Fairfax, Virginia, USA;

Xiaojing Xu: Science Systems & Applications, Inc., Hampton, Virginia, USA.

4. Magnetic energy determines electron bulk heating

When magnetic field lines interact at the magnetopause, the boundary between the solar magnetic field and the Earth's magnetic field, a process known as magnetic reconnection, causes magnetic energy to be converted into kinetic energy and heat. Magnetic reconnection is a collisionless process, and its dynamics are still not fully understood. Past studies of reconnection have produced conflicting findings as to whether, and how much, reconnection heats electrons. Although reconnection has been found to heat electron to 10 million kelvins in the Earth's magnetotail, it does not appear to heat electrons in the solar wind.

Drawing on observations of 79 instances of magnetic reconnection as recorded by NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites, Phan et al. studied the extent to which various physical properties affect the magnitude and occurrence of electron bulk heating. The authors find that bulk electron heating depends primarily on the amount of available magnetic energy in the solar wind.

From their observations the authors empirically determined that around 2 percent of the magnetic energy in the solar wind is converted to bulk electron heating. This finding, they suggest, may be universal for plasmas both in space and in the laboratory. It could help explain why there is strong electron heating in the Earth's magnetotail but essentially no healing in the solar wind during reconnection. They suggest that it could also be used to investigate the role of magnetic reconnection in heating the solar corona.

Source: Geophysical Research Letters, doi:10.1002/grl.50917, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50917/abstract

Title: Electron Bulk Heating in Magnetic Reconnection at Earth's Magnetopause: Dependence on the Inflow Alfvén Speed and Magnetic Shear

Authors: T. D. Phan and M. Oieroset: University of California, Berkeley, California, USA;

M. A. Shay: University of Delaware, Newark, Delaware, USA;

J. T. Gosling: University of Colorado, Boulder, Colorado, USA;

M. Fujimoto: ISAS, Japan;

J. F. Drake: University of Maryland, College Park, Maryland, USA;

G. Paschmann: MPE, Garching, Germany;

J. P. Eastwood: Imperial College, London, United Kingdom;

V. Angelopoulos: University of California at Los Angeles, California, USA.

5. Coastal radar observations reveal complex surface circulations

The behavior of nearshore ocean surface currents has important effects on the coastal ecosystem, with the alongshore propagating waves affecting how nutrients, salt, and heat are distributed and also helping transport marine organisms. Using a network of 61 high-frequency radar stations off the U.S. West Coast, Kim et al. got a detailed look at the motion of the coastal ocean. They found that there are essentially two distinct sets of poleward propagating waves driving the nearshore flow.

Using the coastal radar observations, along with a simplified ocean circulation model and surface wind measurements, the authors determine that one set of waves moves northward at 100 to 300 kilometers (about 60 to 190 miles) ¬per day, while the second set travels northward at around 10 kilometers (about 6 miles) per day. In agreement with previous studies, the authors conclude that the higher speed signals are likely coastally trapped waves. The cause of the slower speed signals, however, is less certain. The authors suggest that the slower signals may be the scattered or reflected remnants of the faster waves, or they may be caused by nearshore advection or pressure gradients.

Source: Journal of Geophysical Research-Oceans, doi:10.1002/jgrc.20400, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrc.20400/abstract

Title: Poleward propagating subinertial alongshore surface currents off the U.S. West Coast

Authors: Sung Yong Kim: Division of Ocean Systems Engineering, School of Mechanical, Aerospace & Systems Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea;

Bruce D. Cornuelle and Eric J. Terrill: Scripps Institution of Oceanography, La Jolla, California, USA;

Burt Jones: Red Sea Research Center, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia;

Libe Washburn: Department of Geography/ICESS, University of California, Santa Barbara, Santa Barbara, California, USA;

Mark A. Moline: School of Marine Science and Policy, College of Earth, Ocean, and Environment, University of Delaware, Newark, Delaware, USA;

Jeffrey D. Paduan: Department of Oceanography, Graduate School of Engineering and Applied Sciences, Naval Postgraduate School, Monterey, California, USA;

Newell Garfield: Geosciences Department and Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, California, USA;

John L. Largier: Bodega Marine Laboratory, University of California, Davis, California, USA;

Greg Crawford: Faculty of Science and Technology, Vancouver Island University, Nanaimo, British Columbia, Canada;

P. Michael Kosro: College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.

6. Assessing a plan to clear energetic protons from the radiation belt

The Earth's radiation belts have been a known hazard to satellites since at least 1962, when an American high-altitude nuclear weapons test named Starfish Prime produced an artificial belt that disabled the first commercial communications satellite, TelStar 1. In the years since the Cold War, thousands of satellites have been put into orbit, and surface charging, high-energy protons, high-energy electrons known as "killer electrons," and other hazards of the inner magnetosphere have continued to take their toll. Satellites can be hardened against these radiation hazards, but some researchers have recently floated a more radical idea: if specially designed transmitters are put into space and set to emit tightly tuned waves, known as electromagnetic ion cyclotron (EMIC) waves, they could potentially push the highly energetic protons out of the Earth's inner radiation belt, a high-tech cattle catcher clearing the satellite's path.

The plan is theoretically possible and not without precedent. Researchers have previously used a different type of wave to scatter high-energy electrons out of the outer radiation belt. However, the specific details, such as which frequencies of waves would work best, are more elusive.

Researchers trying to calculate the wave-particle interaction are faced with a choice: they can model the interaction either nonlinearly or quasi-linearly. Quasi-linear solutions typically simplify the dynamics, whereas nonlinear computations are more difficult and take longer to perform. Soria-Santacruz et al. modeled the interaction between EMIC waves spread over a large area and high-energy protons using both techniques. They find that for all energies and interaction angles studied, the quasi-linear and nonlinear solutions generally agree. They suggest that researchers should be free to use the less computationally expensive quasi-linear approach in future work.

Source: Geophysical Research Letters, doi:10.1002/grl.50925, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50925/abstract

Title: Scattering rates of inner belt protons by EMIC waves: A comparison between test particle and diffusion simulations

Authors: M. de Soria-Santacruz and M. Martinez-Sanchez: Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;

K. G. Orlova: Department of Earth and Space Sciences, UCLA, Los Angeles, California, USA and Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia;

Y. Y. Shprits: Department of Earth and Space Sciences, UCLA, Los Angeles, California, USA and Skolkovo Institute of Science and Technology, Moscow, Russia and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

### Contact: Mary Catherine Adams
Phone (direct): +1 202 777 7530
E-mail: mcadams@agu.org

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