Scientists are closer to understanding how high-energy particles from the sun — the solar wind — enter Earth’s magnetic field. Thayer School Research Associate Hiroshi Hasegawa and an international team of colleagues have for the first time observed giant space vortices that trap plasma and energy from the solar wind. The finding, published in the August 12 issue of Nature, may help explain how Earth’s magnetic field lets in the solar plasma when it should be acting as a barrier.
The newly discovered vortices, known as products of Kelvin-Helmholtz instabilities, resemble curled ocean waves. “These vortices were really huge structures, about six earth radii across,” says Hasegawa, who has been analyzing the data collected by four satellites dubbed the Cluster. “This is the first time rolled-up Kelvin-Helmholtz vortices have been detected unambiguously. Past observations, which were based on single-spacecraft measurements, could not tell with certainty whether the waves along the magnetopause — the edge of Earth’s magnetic field — were large rolled-up vortices or only small ripples that do not trap the solar wind.”
One reason space physicists and engineers want to understand how the solar wind gets through the magnetopause is because the solar wind causes geomagnetic storms that can disable satellites, disrupt radio and radar systems, and create electrical surges in power transmission lines and telephone wires.
Speeds of Light
Ninety years after the phenomenon was first predicted, Thayer researchers have observed an elusive property of light: in some media, a flash of light breaks into constituent frequencies that travel further and faster than the flash as a whole. Called precursors, these strong, fast constituents have defied detection since 1914. But when Professor Ulf Österberg and Research Associate Seung-Ho Choi beamed 100-femtosecond (10-15) pulses of red laser light into a 70-cm.-long tube of distilled water, they observed a new pulse which attenuated an order of magnitude less than a conventional pulse.
Precursors result from two phenomena: 1) light traveling through any transparent medium splinters into component frequencies; and 2) water, like many transparent materials, transmits certain frequencies of light exceptionally well. Splinters of light traveling at favored frequencies propagate efficiently, while others fade. But as the light travels, the favored frequencies change — and so do the precursors, further complicating detection. “The precursor is like soap in the bathtub,” says Österberg. “It’s like it’s alive, it’s breathing, it’s moving along and changing the whole time.”
Precursors, if controllable, could be useful in medical imaging, underwater communications, radar, and other applications. Österberg and Choi’s discovery was reported in AAAS Science online and in Physical Review Letters.
Red Tide Alert
Professor Daniel Lynch’s ocean circulation models recently helped coastal communities in Casco Bay, Maine, gain their first-ever advance warning of red tide, the annual shellfish contamination caused by toxic algae. The early warning came from a National Oceanic and Atmospheric Administration-funded project that analyzes wind, currents, and other oceanographic data collected by sensors on ships, satellites, and buoys. When data about a patch of toxic algae observed in the Gulf of Maine was fed into Lynch’s computer model, the result was an accurate prediction of where and when the red tide would wash ashore. The forecast allowed Maine officials to close shellfish beds to public harvest before contamination began.
Lynch’s work in Thayer School’s Numerical Methods Lab involves three-dimensional coastal ocean circulation models, including a nonlinear prognostic model that allows the circulation field to evolve over time.
Microwaves for Vision
Professor Stuart Trembly is investigating a less invasive alternative to laser eye surgery: reshaping corneas with microwave thermokeratoplasty (MTK). Microwave energy, applied around the pupil outside the field of vision, causes collagen fibers in the cornea to shrink, flattening the optical surface in the center of the eye. The procedure is fast, requires no cutting, and uses less expensive equipment than laser surgery.
Trembly has advanced the state of MTK therapy with two patented devices. One is an improved applicator with embedded sensors to measure temperature or mechanical strain of the cornea during the procedure; the other is a feedback system that analyzes the signals to determine precisely when the myopia is corrected.
The project, which received support from Thayer School Overseer Ralph Crump ’66 and his wife, Marjorie, created opportunities for student input. Luke Dalton ’99, Th’01 worked on the cooling system for the MTK applicator; Michael Barton Th’04 studied anisotropic thermal conductivity of the cornea; and M.S. candidate April Mohns ’03, Th’04 is currently working on a finite element treatment-planning model.
Trembly is chief scientific officer of ThermalVision, Inc., which was incorporated in the fall of 2002 to bridge the gap from laboratory to market. “Human trials,” says Trembly, “are scheduled to begin in about 18 months.”
Treating Prostate Cancer
Thayer School and Dartmouth Medical School researchers are investigating a novel approach to using photodynamic therapy (PDT) to treat early-stage prostate cancer. Professor Brian Pogue, research associate Bin Chen, and adjunct Professor Jack Hoopes are testing their hypothesis that PDT is effective when used to consecutively target prostate cancer cells and the tumor’s vascular system. PDT involves fewer side effects than prostate surgery, which can cause impotence and incontinence.
PDT is a two-step process. The subject is injected with a photosensitizer, then the tumor is treated by laser light irradiation. By adjusting the interval between drug administration and light irradiation, PDT can be directed at either tumor cells or blood vessels.
Current PDT protocols typically use either a relatively long drug-light interval to target tumor cells or a short drug-light interval to cause vascular occlusion. The Thayer School team’s protocol applies the same long-interval cellular-targeting PDT but follows it immediately with short-interval vascular-targeting PDT. The researchers believe that targeting the tumor’s vascular system is crucial since a single vessel supplies oxygen and nutrients for thousands of tumor cells.
Ongoing studies are focused on assessment of the anti-tumor effect in prostate cancer and the abilty to spare normal surrounding tissues. The research is jointly funded by the Department of Defense and the National Institutes of Health.
Detecting Breast Cancer
A team of Dartmouth engineers and medical researchers headed by Professor Keith Paulsen has released preliminary findings on how various imaging technologies can be used to detect breast cancer. Reporting in the May issue of Radiology, the journal of the Radiological Society of North America, the interdisciplinary team from Thayer School, Dartmouth Medical School, Norris Cotton Cancer Center, and Dartmouth-Hitchcock Medical Center, described baseline data that identify an array of tissue properties that can differentiate healthy breasts from cancerous ones.
For example, electrical impedance spectral imaging, by measuring the impedance of cell membranes, can distinguish electrical characteristics that vary from healthy to cancerous tissue. Microwave imaging spectroscopy sends microwave energy, which is sensitive to water, through the breast, while near infrared spectral imaging sends infrared light, which is sensitive to blood. Both techniques can distinguish between healthy cells and cancerous cells, which tend to have more water and blood than regular tissue.
Phase 1 of the study concentrated on 23 healthy women. The second phase focuses on women who have had abnormal mammograms. “We’re just now getting into the really exciting part,” says Paulsen. “We’ve started to get some information on what the normal breast is like, and now we have some information on the abnormal tissue.”
The project is at least 10 years away from providing commercial versions of the tests. “There’s a lot of ways we can improve the instrumentation,” Paulsen says, “and we’re still trying to understand what these images mean. These are new types of images that no one has ever looked at before.” Ideally, the imaging techniques will be combined into a single medical procedure.
Paulsen, with Professor Paul Meaney and Larry Gilman Th’95, has edited Alternative Breast Imaging: Four Model-Based Approaches, to be published this November by Springer.
De-icing Planes and Bridges
Professor Victor Petrenko‘s thin-film, pulse electro-thermal de-icing (PETD) method for airplanes had its first in-flight testing this year. The Goodrich Corp., which holds the license for PETD aerospace applications, bonded a thin titanium-alloy skin onto the outboard wings of a twin-engine plane and tested it in an icing wind tunnel, then in flight behind an icing tanker (an aircraft with a tail-mounted icing spray boom), and finally on several cross-country flights under natural icing conditions.
“The test pilots were quite pleased with the system,” says Dave Sweet, director of research and development at Goodrich. “All the ice was removed instantly with very little runback ice generated.” Testing on a business jet is scheduled for this winter.
“The beauty of the method,” says Petrenko, “is that only a very thin layer of ice directly at the ice-material interface is heated.” A single pulse of electricity melts the interfacial ice, and any ice build-up slides right off. Regular pulsing keeps surfaces ice-free while maintaining low overall power consumption.
Another PETD test is taking place on Sweden’s four-year-old Uddevalla Bridge, which has to be closed each winter because ice falls from its towers and cables. In August Petrenko traveled to Sweden to meet with the Swedish Road Administration and several companies involved in the de-icing project. The process involves heating a stainless-steel foil coating on the bridge towers and cables with a second-long electrical pulse. Full-scale implementation of a PETD system for the bridge will begin next year.
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