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New Insight into Magnetic Field of Sun
Keele University astronomers have used data from NASA’s Chandra X-ray Observatory to make a discovery that may have profound implications for understanding how the magnetic field in the Sun and stars like it are generated.
The Sun and stars like it are giant spheres of superheated gas and the Sun’s magnetic field is responsible for producing sunspots, its 11-year cycle, and powerful eruptions of particles from the solar surface. These solar storms can produce spectacular auroras on Earth, damage electrical power systems, knock out communications satellites, and affect astronauts in space.
Researchers have discovered that four old red dwarf stars with masses less than half that of the Sun are emitting X-rays at a much lower rate than expected, a key indicator of a star’s magnetic field strength, suggesting these stars have weaker magnetic field than previously thought.
Since young stars have high levels of X-ray emission and magnetic field strength, this suggests that the magnetic fields of these stars weakened over time and whilst this is a commonly observed property of stars like our Sun, it was not expected to occur in low-mass stars, as their internal structure is very different.
“We have known for decades that the magnetic field on the Sun and other stars plays a huge role in how they behave, but many details remain mysterious” said lead author Nicholas Wright, Ernest Rutherford Fellow at Keele University. “Our result is one step in the quest to fully understand the Sun and other stars.”
The rotation of a star, flow of gas in its interior and convection all play a role in producing its magnetic field. Convection, similar to the circulation of warm air inside an oven, distributes heat from the interior of the star to its surface in a circulating pattern of rising cells of hot gas and descending cooler gas. It occurs in the outer third (by radius) of the Sun, while the hot gas closer to the core remains relatively still. There is a difference in the speed of rotation between these two regions and many astronomers think this difference is responsible for generating most of the magnetic field of the Sun. Since stars rotate more slowly as they age, this also plays a role in how the magnetic field weakens with time.
“In some ways you can think of the inside of a star as an incredibly complicated dance with many, many dancers,” said co-author Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Some dancers move with each other while others move independently. This motion generates magnetic field, but how it works in detail is extremely challenging to determine.”
For stars with much less mass, convection occurs all the way into the core of the star, meaning the boundary between regions with and without convection, thought to be crucial for generating magnetic field in the Sun, does not exist. One school of thought has been that magnetic field is generated mostly by convection in such stars, and as convection doesn’t change as a star ages, their magnetic fields would not weaken much over time.
By studying these low-mass red dwarf stars in X-rays, Wright and Drake were able to test this hypothesis, using NASA’s Chandra X-ray Observatory to look at two and data from the ROSAT satellite for the other two.
“We found that these smaller stars have magnetic fields that decrease as they age, exactly as it does in stars like our Sun,” said Wright. “This really goes against what we would have expected.”
These results imply that the interaction along the convection zone-core boundary does not dominate the generation of magnetic field in stars like our Sun, since the low mass stars studied by Wright and Drake lack such a region and yet their magnetic properties are very similar.
A paper describing these results appeared in the July 28th issue of the journal Nature.
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