It’s been a mystery for more than half a century: why, in the short distance from the Sun’s surface to its corona, or outer atmosphere, does the temperature leap from a few thousand to a few million degrees? The answer, researchers say, might lie in hot jets of plasma erupting from the Sun’s surface1.
“It’s truly a breakthrough in the longstanding puzzle of how the corona gets so hot,” says Rob Rutten, a solar physics expert at Utrecht University in the Netherlands who was not involved with the work. “The jets behave like bullets shot upwards, causing hot coronal temperature fronts in front of them.”
Over the years, theorists have offered various explanations for the hot corona. One idea is that the Sun’s violent inner motion shakes its magnetic field lines, sending waves through the atmosphere and into the corona that deposit their energy as heat2. Another posits that the magnetic field lines become so twisted that they snap, accelerating and heating the coronal gas3. However, there has been little observational evidence to support either of these theories.
Plasma jets have also been considered as a possible heating mechanism. These jets are known to travel several hundred kilometres from the ‘chromosphere’ layer just above the Sun’s surface to the corona. Yet in the past, rough observations of plasma jets suggested them to be too cool for coronal heating, with temperatures similar to that of the chromosphere itself — just a few thousand degrees.
In a paper published today in Science1, however, Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and his colleagues show that most of the plasma making up the jets is heated to hundreds of thousands of degrees on its way to the corona, with a small fraction reaching millions of degrees. On the basis of the jets’ frequency and intensity, the researchers estimate that they deliver energy “of the order that is required” for the corona to sustain its high temperature. “We are not saying that this is the only mechanism to heat the corona,” says De Pontieu. “Clearly, however, these events deserve more attention.”
via Nature News.