November 15, 2013
Powering The Sun’s Incredibly Hot Atmosphere
John P. Millis, PhD for redOrbit.com - Your Universe Online
The core of the Sun is a nuclear furnace, burning at 15 million degrees Kelvin. This heat travels to the surface, creating surface plasma that is still a balmy 6,000 Kelvin. However, a strange thing happens above the surface of the Sun; in the solar atmosphere, known as the corona, temperatures can exceed a million degrees Kelvin.
Researchers have long suspected that the cause for this dramatic heating is due to interactions of the magnetic field above the solar surface, but there were competing theories as to how the energy would actually be released into the atmosphere.
But now the research team of Michael Hahn and Daniel Wolf Savin has found that the source of the heating can be traced to magnetic oscillations known as Alfven waves. "This is the big, unanswered question in solar physics, and nearly everyone in the field is somehow working on trying to solve it," according to Savin. "We really had no idea where the research would lead us, but we were hoping to at least be able to add another piece to the puzzle. We did not expect it to be such a big piece."
Using the Extreme Ultraviolet Imaging Spectrometer on the Japanese satellite Hinode, the astronomers were able to measure vibrations in the solar magnetic field above a polar coronal hole. These oscillations transfer energy from the magnetic field into the atmosphere.
"This is a fundamentally important finding," said Ilia Roussev, NSF program director for solar terrestrial research. "This issue is the holy grail of solar physics. If this allows us to better understand the mechanics, then it has tremendous consequences."
While theorists have posited that Alfven waves could be the driving source behind the heating, the technology was not available to probe the magnetic field vibrations. It wasn’t until the launch of the Hinode observatory, and its ultraviolet spectrometer that such observations became possible. "Until that time, we could only see the sun in white light; we didn’t have UV observations. But, now we do,” Roussev noted.
Some astronomers are still cautious about the way forward. Since systems like the Sun are incredibly complex, they often have multiple mechanisms operating in parallel to create the observed phenomenon.
"Some in the community have responded enthusiastically to our findings; others more cautiously, but that is to be expected," Savin said. "Others, including us, have pointed out that there may not be just one solution to the problem as there are different structures on the Sun. Our work is relevant for coronal holes, which are the source of the fast solar wind. A different mechanism or mechanisms may be operating in the quiet sun."
All of this is part of a greater effort to understand our central star and the effects that it can have on our rocky world. "Ultimately, this kind of research does provide new perspective on space weather, which is known to affect the Earth" said Hahn. "Understanding these fundamental processes improves our understanding, of not just the solar corona, but also of space weather."