One of the Biggest Mysteries Surrounding the Sun May Soon Be Solved

Scientists have long been unable to explain the mystery of why the Sun's outer atmosphere—or corona—is hotter than its surface. Now, a team of researchers has proposed a new hypothesis for why this might be the case.

However, they will need to wait some two years before they can put their idea to the test. This is when NASA's Parker Solar Probe will become the first man-made spacecraft to venture into the area where this unusual heating takes place and take measurements of the magnetic fields and particles there.

"Whatever the physics is behind this superheating, it's a puzzle that has been staring us in the eye for 500 years," Justin Kasper, a professor of climate and space sciences from the University of Michigan and a principal investigator for the Parker mission, said in a statement. "In just two more years the Parker Solar Probe will finally reveal the answer."

According to a study published in The Astrophysical Journal Letters, the heating in the corona could be caused by small magnetic waves that travel back and forth between its outermost edge and the star's surface.

The scientists propose that these so-called "Alfvén waves" reach a point known as the "Alfvén point" where they can no longer travel back to the Sun.

"When you're below the Alfvén point, you're in this soup of waves," Kasper said. "Charged particles are deflected and accelerated by waves coming from all directions."

In this "zone of preferential heating," temperatures rise overall, but strangely, some individual elements are heated more than others. For example, some heavier ions—charged atoms or molecules—are superheated until they are ten times hotter than the surrounding hydrogen, and indeed the core of the Sun itself.

These high temperatures cause the Sun's atmosphere to grow many times larger than the diameter of the star itself, and they're also responsible for the extended corona that is visible during solar eclipses.

The scientists still don't know whether the Alfvén point marks the end of the heating zone. Also, the processes that take place below this point to superheat the heavy ions remain a mystery.

But the team hopes the answers to these questions—and—more will be revealed when Parker enters the heating zone—which they predict will happen in 2021. Once there, the spacecraft's sensors will be able to collect data that could cast light on the heating mystery.

"With Parker Solar Probe we will be able to definitively determine through local measurements what processes lead to the acceleration of the solar wind and the preferential heating of certain elements," Kristopher Klein, co-author of the study from the University of Arizona, said in a statement.

"The predictions in this paper suggest that these processes are operating below the Alfvén surface, a region close to the Sun that no spacecraft has visited, meaning that these preferential heating processes have never before been directly measured," he said.

Understanding why the Sun's outer atmosphere is hotter than its surface could help scientists to better predict solar weather, which has the potential to disrupt power systems on Earth.

In late October 2018, Parker passed within 26.55 million miles of the Sun's surface, breaking the record for the closest approach to our star by a man-made object, according to NASA.

Over the course of its mission, Parker will repeatedly break this record before making a final close approach of 3.83 million miles from the Sun's surface in 2024. The previous close approach record was set in April 1976 by the German-American spacecraft Helios 2.