Zvyagin S.A.,Helmholtz Center Dresden |
Aimar E.,Pj Afarik University |
Ozerov M.,Helmholtz Center Dresden |
Wosnitza J.,Helmholtz Center Dresden |
And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics
Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron-spin-resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field Hsat=48.5 T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the density matrix renormalization group calculations for a spin-1/2 Heisenberg chain with a staggered transverse field. © 2011 American Physical Society. Source
Flaherty K.M.,Wesleyan University |
Hughes A.M.,Wesleyan University |
Rosenfeld K.A.,Harvard - Smithsonian Center for Astrophysics |
Andrews S.M.,Harvard - Smithsonian Center for Astrophysics |
And 5 more authors.
Turbulence can transport angular momentum in protoplanetary disks and influence the growth and evolution of planets. With spatially and spectrally resolved molecular emission line measurements provided by (sub)millimeter interferometric observations, it is possible to directly measure non-thermal motions in the disk gas that can be attributed to this turbulence. We report a new constraint on the turbulence in the disk around HD 163296, a nearby young A star, determined from Atacama Large Millimeter/submillimeter Array Science Verification observations of four CO emission lines (the CO(3-2), CO(2-1), 13CO(2-1), and C18O(2-1) transitions). The different optical depths for these lines permit probes of non-thermal line-widths at a range of physical conditions (temperature and density) and depths into the disk interior. We derive stringent limits on the non-thermal motions in the upper layers of the outer disk such that any contribution to the line-widths from turbulence is <3% of the local sound speed. These limits are approximately an order of magnitude lower than theoretical predictions for full-blown magnetohydrodynamic turbulence driven by the magnetorotational instability, potentially suggesting that this mechanism is less efficient in the outer (R 30 AU) disk than has been previously considered. © 2015. The American Astronomical Society. All rights reserved. Source
Crawled News Article
This spring, Caleb Kinsella was one of the thousands of high school seniors around the country anxiously waiting to find out if he got into the college he wanted. At 6 p.m. one day, his top school, the University of Florida (UF), would update its website with every applicant’s acceptance status all at once. UF is a great school, in the top 50 in the country according to U.S. News and World Report, and Kinsella (who, full disclosure, is my nephew) wasn’t sure he would make the cut with his just-okay grades and test scores. So he was thrilled and a little surprised when he found out that he’d gotten in. Except, the more he looked, the more it seemed like something wasn't quite right. "It kind of fooled me at first," Kinsella told me. "It said, ‘Congratulations, you’ve been admitted to the University of Florida.’ But shortly after, I discovered it was actually the ‘Path to Campus Enrollment.’ Is that what it’s called? The PaCE program." Kinsella had been accepted to a year-old program at UF that lets students who don’t quite make the cut for traditional admission take their first two years of classes online or at a community college for a 25% discount in tuition. They can start taking classes on UF’s campus only after they earn 60 credits, and start as juniors. This combination of online and offline education is new, but gaining in popularity. Many institutions around the country, including the University of Colorado and , offer so-called hybrid degrees for bachelor's or master's students in many fields of study. This is all part of an expansion in online education that’s been progressing fitfully for most of this decade, an experiment involving millions of young people whose results are far from certain. While U.S. News and World Report ranks more than 200 online bachelor's programs, fewer than half of them report their graduation rates. Of the 69 programs that did, only 16 graduate more than half of their students. And those who did manage to complete their courses took a long time: Only 35% of programs had students who graduated within six years. Online education began to capture the educational world’s imagination in a serious way in 2012, which the New York Times called "the year of the MOOC," or massive open online class. Startups like Coursera, Udacity, and EdX promised to permanently change the way young people learned. In a TED talk from that year, Coursera cofounder Daphne Koller spoke about MOOC’s ability to solve problems as diverse as the legacy of apartheid in South Africa and the burden of student debt in America. Coursera’s goal, she said, was "to take the best courses from the best universities, and provide them to everyone around the world, for free." Ben Maddox, New York University's chief instructional technology officer and one of its primary online education pioneers, calls this time "MOOC fever." It was "a time of heightened awareness and expectations," he told me, which "caused us to think about instruction in new and different ways. It was energizing." One of the earliest MOOC providers was Udacity, founded by former Google VP and professor Sebastian Thrun. He started the company in early 2012 after being involved in an online education pilot program at Stanford. "MOOCs didn’t come about because of years of careful planning," he told Fast Company. "They came about because I put my Stanford classes online, and I had no idea what would ensue." But just a year later, it was clear that MOOCs were far from changing the world. A study by the University of Pennsylvania found that student engagement with courses fell drastically after the first week, and that completion rates averaged just 4%. By the end of that year, Thrun was giving interviews to outlets including Fast Company, saying Udacity had "a lousy product." He pivoted his company’s focus from changing the world to helping people in life-change careers. MOOCs are of course separate from the online classes offered by a university (for one thing, university classes aren’t free). But the volume of attention MOOCs generated made traditional educational institutions interested in offering online classes, as well, whatever the problems might be; many universities continued to design and expand their own online programs, even as enthusiasm for MOOCs waned. According to a 2016 report from the U.S. Department of Education, 5.5 million American students, or 25% of all college students, were taking some online classes. That was in 2013, and the number has continued to grow, if slowly. There are several reasons that online classes are particularly attractive to today’s colleges. For one, they have the potential to drastically increase revenue to institutions facing lower traditional enrollment. College enrollments fell by almost a million students between 2011 and 2013, the largest two-year drop since the U.S. Census Bureau began to collect these statistics in 1966. With an online course, a professor can design a curriculum once, and her university can run it again and again, with minimal updates, into the far future. The number of students isn’t limited to those who can physically move to your campus and attend classes, or dependent on the availability of a professor who'd maybe rather be writing a book. Anyone, anywhere, can take classes at any time. NYU's Maddox disputes that online classes are a revenue generator for schools. "It’s like cloud computing," he said. "It has all sorts of advantages, but the decision to go to cloud isn’t a lot of cost savings in the end." It’s the same with online education, he says. "To promote a high-quality, adaptive educational experience? We’re still seeing if that’s a cost savings in the end." In the best cases, universities have been learning lessons from MOOCs’ failures as they design their courses. For one, online classes tend to do best when students are a bit older and pursuing a specific educational goal. At the new Udacity, Thrun told me, the typical student is now 24 to 50 years old and looking to gain skills to help in their lives and careers. "They rarely come to us and say, ‘I want to learn something interesting,’" he explains. "This attitude increases their chances of success, because there’s something tangible they want from us. It’s not just self-enlightenment. That motivation is important." NYU also focuses its online offerings around graduate programs in areas that lend themselves to concrete truths and remote evaluation of progress, like computer science and engineering. Education providers are also realizing that online classes are especially attractive when they offer cost savings to the student. Not only is crushing student debt one of the primary reasons a student might second-guess college attendance, but a lower price tag matches the "less-than" perception of online classes. Recognizing this, Udacity now offers a complete master's degree in computer science through a partnership with Georgia Tech and AT&T for only $7,000. Many online programs are finding that students succeed best when they’re engaged with the content. "If you go to Stanford, and instead of teaching students and mentoring them, you just gave them all the books for all the classes, I think you’d see the same low completion rates" that many MOOCs have, Thrun told me. "To me, the MOOC is the book. The video book. It’s a component, but not all there is to it." Increasing engagement is also driving another Udacity project. Thrun said he is "actually experimenting with meetups," and looking at "what happens if someone takes charge [in a class]. And it does positively impact the outcomes." Students getting together to learn, in person, with "someone taking charge." Interesting (and familiar!) idea. So how do these trends look on the ground? At UF, officials tried to tick all these boxes with its online offerings, though it has faced several major problems. Elizabeth D. Capaldi Phillips was recruited from Arizona State University to run the new program, now dubbed UF Online (of which PaCE is a major part), only to resign after two months. Her position went unfilled for a year. The new head, Evangeline J. Tsibris Cummings, was appointed in July 2015, just months before PaCE admitted its first class of students. Meanwhile, the university cancelled its contract with Pearson to administer UF Online after it failed to attract out-of-state students who pay higher tuition. That first class ended up being very small, despite its reduced price tag. PaCE accepted 3,000 students its first year, but had only 235 people agree to take part in its online experiment (for comparison, about 50% of students accepted to UF in a more traditional way choose to enroll). Part of this, perhaps, was a focus in the first year on the fact that students hadn’t gotten full campus enrollment, and not that they had gotten into PaCE. "We got less than 10% of the students that we sent letters out to," Cummings told Fast Company, "but frankly I think that is tremendous, considering it was brand new. There was some consternation in the initial launch where folks didn't know what it was. I think there was some initial confusion about how it related to their on-campus admissions decision, and so we've learned from that." Paige Fry, a member of that class, remembers calling her mother in tears when she read her PaCE acceptance. "I didn’t know what it was or what it meant, and I was very upset," she told me over the phone. However, after evaluating UF’s program, she thought their online journalism program was a good match for her. Students of that first class were encouraged to move to Gainesville, where UF is located, she says, but were faced with a complex network of school activities in which they weren’t allowed to participate. They couldn’t use the school gym, but they could rush fraternities and sororities. They couldn’t live in the dorms, but could sit in the student section at football games. For two years, they are literally not allowed to officially participate in university classes. This was because the university had decided that in order to make the program as low cost as possible, PaCE students were not required to pay university activity fees. The school has since reconsidered this position, and now allows PaCE students to pay additional fees and use all student services. Fry eventually accepted, and took six classes her first semester, in which she got straight As. She also told me that many PaCE students semi-secretly sit in on the classes in which they’re officially enrolled online. "In general, I’m a self-motivated person," she told me. She works part-time, lives at home, and does her coursework whenever she wants. "I wake up at whatever time, and then sit down to do it," she told me, describing a pretty enviable daily routine. However, it must be said, it's a routine that’s not overly different from one I experienced in college and graduate school, which I attended in person. Fry is obviously the kind of focused, goal-driven person who succeeds in online education. But what about someone with less focus? In other words, a typical college freshman, who may feel a little overwhelmed? And even more specifically, one who didn’t have the drive in high school to excel enough to receive a regular enrollment slot at a state school like UF? "Honestly, if you asked me as a friend what recommendation I’d have about this, I’d have no recommendation at this point," said Thrun. "I can see the general concern: Oh my god, maybe it’s not the right fit, he’s not going to be successful, he’s going to lose interest, and he’s going to give up on college forever. I understand that logic, and, boy, I wish I had a word of wisdom." In general, Thrun says, the education a student gets can vary widely from program to program, and even class to class. UF, for instance, is one of the few that reports graduation rates: 63% of online students graduated within four years, just slightly under its 67% rate among traditional students. This is likely among the reasons its online bachelor's program comes in at number 11 in the nation in the rankings. But this doesn’t guarantee the success of the PaCE program. So, is Kinsella, whose best friend was also accepted to PaCE, going to attend? How does he feel about being part of this experiment? "Excited, obviously," he told me. "It felt great. It’s like, our favorite school." He’s confident he can succeed. "I’m going to be there, and it’s going to be a lot of fun, but it just comes down to me, myself."
Naugolnykh K.,University of Colorado and
39th International Congress on Noise Control Engineering 2010, INTER-NOISE 2010
A sound wave in unstable convective atmospheric layer can stimulate perturbations development. High-intensity atmospheric perturbations such as cyclones and thunderstorms generate infrasound, which is detectable at large distances from the source. The waveconvection instability can produce variations in the level of infrasound radiation by the developing cyclone, and this can serve as a precursor of intense atmospheric events. Source
Crawled News Article
Greenhouse gases released as global warming thaws permafrost in Arctic regions could increase the economic toll of climate change by trillions of dollars, scientists say. The release of the carbon dioxide and methane normally trapped in the frozen ground could increase the economic damage from climate change by as much as $43 trillion by the end of the next century, a study by U.S. and British researchers predicts. The researchers at the University of Colorado and the University of Cambridge have created the first models of the economic impact of melting Arctic permafrost and added that to the $326 trillion impact already predicted by other climate and economic modeling. The researchers combined two models, one estimating total likely emissions of greenhouse gases from melting permafrost, the other calculating the temperature increases that would result in climate-related impacts worldwide, to create their prediction for 2200. "There is almost nothing in the literature on this," says Chris Hope, a policy modeling expert at Cambridge. "We are the first to combine a physical model and an integrated assessment model in this way," says Hope, lead author of the study appearing in the journal Nature Climate Change. The researchers based their estimates on the assumption that anthropogenic greenhouse gas emissions resulting from human activities such as fossil fuel burning would continue until at least 2100. By that year, release of methane and carbon dioxide from melting permafrost could push global temperatures up by between 0.11 and 0.25 degrees Celsius over what would already be present due to human-caused emissions, they predict. By 2100, those human activities by themselves are expected to boost carbon dioxide levels by 75 percent over what they are today, they say. The Arctic — where around 1,700 gigatons of carbon are trapped in permafrost — is warming at twice the rate of the global average, the researchers note, and if that rate continues, hundreds of billions of tons of greenhouse gases will be released as the permafrost thaws. "These results show just how much we need urgent action to slow the melting of the permafrost in order to minimize the scale of the release of greenhouse gases," Hope says. The researchers acknowledge that their models have a large uncertainty factor due to something known as the "transient climate response," a difficulty in gauging just how sensitive global temperatures are to atmospheric carbon dioxide concentrations. "If you assume a bigger transient climate response, the model climate is warmer for the same amount of carbon dioxide input," says study senior author Kevin Schaefer at the National Snow and Ice Data Center at the University of Colorado. "If it's a smaller value, then the warming is less." Still, says Hope, the study findings point to a clear need to reduce human-caused emission levels as much as possible and as soon as possible. "There's only one way to stop the thawing of permafrost, and that's to stop climate change," he says.