Why does the Sun’s Corona sizzle at 1 million° F?

The Sun's corona, invisible to the human eye except when it appears briefly as a fiery halo of plasma during a solar eclipse, remains a puzzle even to scientists who study it closely. 

Located about 1,300 miles [2100 km] from the star's surface, it is more than hundred times hotter than lower layers much closer to the fusion reactor at Sun's core.

There are various theories, none yet conclusive that explain the sizzling heat of the corona - magnetic energy lines that reconnect in the upper atmosphere and release explosive energy and energy waves dumped in the corona, where they are converted to thermal energy.

Now a team of physicists, led by New Jersey Institute of Technology's Gregory Fleishman have discovered a phenomenon that may help to determine the physical mechanisms that heat the Sun's upper atmosphere to 1 million degrees Fahrenheit and higher. 

With a series of observations from NASA's Solar Dynamics Observatory [SDO], the team has found regions in the corona with elevated levels of heavy metal ions contained in magnetic flux tubes - concentrations of magnetic fields which carry an electrical current. 

The images, captured in the extreme [short wave] ultraviolet band [EUV], reveal disproportionally large, by a factor of five or more concentrations of multiply charged metals compared to single-electron ions of hydrogen, than exist in the photosphere.

The iron ions reside in what the team calls "ion traps" located at the base of coronal loops, arcs of electrified plasma directed by magnetic field lines. The existence of these traps, implies that there are highly energetic coronal loops, depleted of iron ions, which have thus far eluded detection in the EUV range. Only metal ions, with their fluctuating electrons, produce emissions which make them visible.

"We knew that something really intriguing happens at the interface between the photosphere - the Sun's surface and the corona, given the noticeable disparities in the chemical composition between the two layers and the sharp rise in plasma temperatures at this junction," Fleishman says.

"These observations suggest that the corona may contain even more thermal energy than is directly observed in the EUV range and that we have not yet accounted for," he says. 

"What we know of the corona's temperature comes from measuring EUV emissions produced by heavy ions in various states of ionization, which depends on their concentrations, as well as plasma temperature and density," he adds. "The non-uniform distribution of these ions in space and time appears to affect the temperature of the corona."

The metal ions enter the corona when variously sized solar flares destroy the traps, and they are evaporated into flux loops in the upper atmosphere.

Energy releases in solar flares and associated forms of eruptions occur when magnetic field lines, with their powerful underlying electric currents, are twisted beyond a critical point that can be measured by the number of turns in the twist. 

The largest of these eruptions cause what is known as space weather, the radiation, energetic particles and magnetic field releases from the Sun powerful enough to cause severe effects in Earth's near environment, such as the disruption of communications, power lines and navigation systems.

Image Credit: NASA's Solar Dynamics Observatory
Source: New Jersey Institute of Technology