Soup Posted at 2024-11-11 23:49:59(1Wk ago) Report Permalink URL | ||
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| Mining old data from NASA's Voyager 2 solves several Uranus mysteries by Karen Fox, Molly Wasser and Gretchen McCartney, NASA  The first panel of this artist’s concept depicts how Uranus’s magnetosphere — its protective bubble — was behaving before the flyby of NASA’s Voyager 2. The second panel shows an unusual kind of solar weather was happening during the 1986 flyby, giving scientists a skewed view of the magnetosphere. Credit: NASA/JPL-Caltech When NASA's Voyager 2 spacecraft flew by Uranus in 1986, it provided scientists' first—and, so far, only—close glimpse of this strange, sideways-rotating outer planet. Alongside the discovery of new moons and rings, baffling new mysteries confronted scientists. The energized particles around the planet defied their understanding of how magnetic fields work to trap particle radiation, and Uranus earned a reputation as an outlier in our solar system. Now, new research analyzing the data collected during that flyby 38 years ago has found that the source of that particular mystery is a cosmic coincidence. It turns out that in the days just before Voyager 2's flyby, the planet had been affected by an unusual kind of space weather that squashed the planet's magnetic field, dramatically compressing Uranus's magnetosphere. "If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus," said Jamie Jasinski of NASA's Jet Propulsion Laboratory in Southern California and lead author of the new work published in Nature Astronomy. "The spacecraft saw Uranus in conditions that only occur about 4% of the time." Magnetospheres serve as protective bubbles around planets (including Earth) with magnetic cores and magnetic fields, shielding them from jets of ionized gas—or plasma—that stream out from the sun in the solar wind. Learning more about how magnetospheres work is important for understanding our own planet, as well as those in seldom-visited corners of our solar system and beyond. That's why scientists were eager to study Uranus's magnetosphere, and what they saw in the Voyager 2 data in 1986 flummoxed them. Inside the planet's magnetosphere were electron radiation belts with an intensity second only to Jupiter's notoriously brutal radiation belts. But there was apparently no source of energized particles to feed those active belts; in fact, the rest of Uranus's magnetosphere was almost devoid of plasma. The missing plasma also puzzled scientists because they knew that the five major Uranian moons in the magnetic bubble should have produced water ions, as icy moons around other outer planets do. They concluded that the moons must be inert with no ongoing activity  NASA’s Voyager 2 captured this image of Uranus while flying by the ice giant in 1986. New research using data from the mission shows a solar wind event took place during the flyby, leading to a mystery about the planet’s magnetosphere that now may be solved. Credit: NASA/JPL-Caltech Solving the mystery So why was no plasma observed, and what was happening to beef up the radiation belts? The new data analysis points to the solar wind. When plasma from the sun pounded and compressed the magnetosphere, it likely drove plasma out of the system. The solar wind event also would have briefly intensified the dynamics of the magnetosphere, which would have fed the belts by injecting electrons into them. The findings could be good news for those five major moons of Uranus: Some of them might be geologically active after all. With an explanation for the temporarily missing plasma, researchers say it's plausible that the moons actually may have been spewing ions into the surrounding bubble all along. Planetary scientists are focusing on bolstering their knowledge about the mysterious Uranus system, which the National Academies' 2023 Planetary Science and Astrobiology Decadal Survey prioritized as a target for a future NASA mission. JPL's Linda Spilker was among the Voyager 2 mission scientists glued to the images and other data that flowed in during the Uranus flyby in 1986. She remembers the anticipation and excitement of the event, which changed how scientists thought about the Uranian system. "The flyby was packed with surprises, and we were searching for an explanation of its unusual behavior. The magnetosphere Voyager 2 measured was only a snapshot in time," said Spilker, who has returned to the iconic mission to lead its science team as project scientist. "This new work explains some of the apparent contradictions, and it will change our view of Uranus once again." Voyager 2, now in interstellar space, is almost 13 billion miles (21 billion kilometers) from Earth. | |
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| SoupPosted at 2024-11-18 20:57:50(5Days ago) Report Permalink URL | ||
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| Theoretical astrophysicist proposes solution to enigma of Crab Nebula's 'zebra' pattern by Brendan M. Lynch, University of Kansas  Crab Nebula. Credit: NASA A theoretical astrophysicist from the University of Kansas may have solved a nearly two-decade-old mystery over the origins of an unusual "zebra" pattern seen in high-frequency radio pulses from the Crab Nebula. His findings have just been published in Physical Review Letters. The Crab Nebula features a neutron star at its center that has formed into a 12-mile-wide pulsar pinwheeling electromagnetic radiation across the cosmos. "The emission, which resembles a lighthouse beam, repeatedly sweeps past Earth as the star rotates," said lead author Mikhail Medvedev, professor of physics & astronomy at KU. "We observe this as a pulsed emission, usually with one or two pulses per rotation. The specific pulsar I'm discussing is known as the Crab Pulsar, located in the center of the Crab Nebula 6,000 light years away from us." The Crab Nebula is the remnant of a supernova that appeared in 1054. "Historical records, including Chinese accounts, describe an unusually bright star appearing in the sky," said the KU researcher. But unlike any other known pulsar, Medvedev said the Crab Pulsar features a zebra pattern—unusual band spacing in the electromagnetic spectrum proportional to band frequencies, and other weird features like high polarization and stability. "It's very bright, across practically all wave bands," he said. "This is the only object we know of that produces the zebra pattern, and it only appears in a single emission component from the Crab Pulsar. "The main pulse is a broadband pulse, typical of most pulsars, with other broadband components common to neutron stars. However, the high-frequency interpulse is unique, ranging between 5 and 30 gigahertz—frequencies similar to those in a microwave oven." Since this pattern was discovered in a 2007 paper, the KU researcher said the pattern had proved "baffling" for investigators.  Medvedev modeled wave diffraction off a circular reflecting region with radially varying index of refraction outside of it to better understand the Crab Nebula's zebra pattern. Credit: Mikhail Medvedev "Researchers proposed various emission mechanisms, but none have convincingly explained the observed patterns," he said. Using data from the Crab Pulsar, Medvedev established a method using wave optics to gauge the density of the pulsar's plasma—the "gas" of charged particles (electrons and positrons)—using a fringe pattern found in the electromagnetic pulses. "If you have a screen and an electromagnetic wave passes by, the wave doesn't propagate straight through," Medvedev said. "In geometrical optics, shadows cast by obstacles would extend indefinitely—if you're in the shadow, there's no light; outside of it, you see light. But wave optics introduces a different behavior—waves bend around obstacles and interfere with each other, creating a sequence of bright and dim fringes due to constructive and destructive interference." This well-known fringe pattern phenomenon is caused by consistent constructive interference but has different characteristics when radio waves propagate around a neutron star. "A typical diffraction pattern would produce evenly spaced fringes if we just had a neutron star as a shield," the KU researcher said. "But here, the neutron star's magnetic field generates charged particles constituting a dense plasma, which varies with distance from the star. | |
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| SoupPosted at 2024-11-19 00:46:17(5Days ago) Report Permalink URL | ||
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| Hubble sees aftermath of galaxy's scrape with Milky Way by NASA  This artist's concept shows the Large Magellanic Cloud, or LMC, in the foreground as it passes through the gaseous halo of the much more massive Milky Way galaxy. The encounter has blown away most of the spherical halo of gas that surrounds the LMC, as illustrated by the trailing gas stream reminiscent of a comet's tail. Still, a compact halo remains, and scientists do not expect this residual halo to be lost. The team surveyed the halo by using the background light of 28 quasars, an exceptionally bright type of active galactic nucleus that shines across the universe like a lighthouse beacon. Their light allows scientists to "see" the intervening halo gas indirectly through the absorption of the background light. The lines represent the Hubble Space Telescope's view from its orbit around Earth to the distant quasars through the LMC's gas. Credit: NASA, ESA, Ralf Crawford (STScI) A story of survival is unfolding at the outer reaches of our galaxy, and NASA's Hubble Space Telescope is witnessing the saga. The Large Magellanic Cloud, also called the LMC, is one of the Milky Way galaxy's nearest neighbors. This dwarf galaxy looms large on the southern nighttime sky at 20 times the apparent diameter of the full moon. Many researchers theorize that the LMC is not in orbit around our galaxy, but is just passing by. These scientists think that the LMC has just completed its closest approach to the much more massive Milky Way. This passage has blown away most of the spherical halo of gas that surrounds the LMC. Now, for the first time, astronomers have been able to measure the size of the LMC's halo—something they could do only with Hubble. In a new study, available on the preprint server arXiv and to be published in The Astrophysical Journal Letters, researchers were surprised to find that it is so extremely small, about 50,000 light-years across. That's around 10 times smaller than halos of other galaxies that are the LMC's mass. Its compactness tells the story of its encounter with the Milky Way. "The LMC is a survivor," said Andrew Fox of AURA/STScI for the European Space Agency in Baltimore, who was principal investigator on the observations. "Even though it's lost a lot of its gas, it's got enough left to keep forming new stars. So new star-forming regions can still be created. A smaller galaxy wouldn't have lasted—there would be no gas left, just a collection of aging red stars." Though quite a bit worse for wear, the LMC still retains a compact, stubby halo of gas—something that it wouldn't have been able to hold onto gravitationally had it been less massive. The LMC is 10 percent the mass of the Milky Way, making it heftier than most dwarf galaxies. "Because of the Milky Way's own giant halo, the LMC's gas is getting truncated, or quenched," explained STScI's Sapna Mishra, the lead author on the paper chronicling this discovery. "But even with this catastrophic interaction with the Milky Way, the LMC is able to retain 10 percent of its halo because of its high mass." A gigantic hair dryer Most of the LMC's halo was blown away due to a phenomenon called ram-pressure stripping. The dense environment of the Milky Way pushes back against the incoming LMC and creates a wake of gas trailing the dwarf galaxy—like the tail of a comet. "I like to think of the Milky Way as this giant hairdryer, and it's blowing gas off the LMC as it comes into us," said Fox. "The Milky Way is pushing back so forcefully that the ram pressure has stripped off most of the original mass of the LMC's halo. There's only a little bit left, and it's this small, compact leftover that we're seeing now."  This artist's concept illustrates the Large Magellanic Cloud's (LMC's) encounter with the Milky Way galaxy's gaseous halo. In the top panel, at the middle of the right side, the LMC begins crashing through our galaxy's much more massive halo. The bright purple bow shock represents the leading edge of the LMC's halo, which is being compressed as the Milky Way's halo pushes back against the incoming LMC. In the middle panel, part of the halo is being stripped and blown back into a streaming tail of gas that eventually will rain into the Milky Way. The bottom panel shows the progression of this interaction, as the LMC's comet-like tail becomes more defined. A compact LMC halo remains. Because the LMC is just past its closest approach to the Milky Way and is moving outward into deep space again, scientists do not expect the residual halo will be lost. Credit: NASA, ESA, Ralf Crawford (STScI) As the ram pressure pushes away much of the LMC's halo, the gas slows down and eventually will rain into the Milky Way. But because the LMC has just gotten past its closest approach to the Milky Way and is moving outward into deep space again, scientists do not expect the whole halo will be lost. Only with Hubble To conduct this study, the research team analyzed ultraviolet observations from the Mikulski Archive for Space Telescopes at STScI. Most ultraviolet light is blocked by the Earth's atmosphere, so it cannot be observed with ground-based telescopes. Hubble is the only current space telescope tuned to detect these wavelengths of light, so this study was only possible with Hubble. The team surveyed the halo by using the background light of 28 bright quasars. The brightest type of active galactic nucleus, quasars are believed to be powered by supermassive black holes. Shining like lighthouse beacons, they allow scientists to "see" the intervening halo gas indirectly through the absorption of the background light. Quasars reside throughout the universe at extreme distances from our galaxy. The scientists used data from Hubble's Cosmic Origins Spectrograph (COS) to detect the presence of the halo's gas by the way it absorbs certain colors of light from background quasars. A spectrograph breaks light into its component wavelengths to reveal clues to the object's state, temperature, speed, quantity, distance, and composition. With COS, they measured the velocity of the gas around the LMC, which allowed them to determine the size of the halo. Because of its mass and proximity to the Milky Way, the LMC is a unique astrophysics laboratory. Seeing the LMC's interplay with our galaxy helps scientists understand what happened in the early universe, when galaxies were closer together. It also shows just how messy and complicated the process of galaxy interaction is. Looking to the future The team will next study the front side of the LMC's halo, an area that has not yet been explored. "In this new program, we are going to probe five sightlines in the region where the LMC's halo and the Milky Way's halo are colliding," said co-author Scott Lucchini of the Center for Astrophysics | Harvard & Smithsonian. "This is the location where the halos are compressed, like two balloons pushing against each other." | |
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