Solve the mystery of overheating the sun with the Parker Solar Probe

It's one of the oldest and oldest mysteries that surround, literally, our sun – why is its outside atmosphere warmer than its surface of fire?

Researchers at the University of Michigan think they have the answer and hope to prove it with the help of NASA's Solar Probe Parker. In about two years, the probe will be the first boat made by humans to enter the area surrounding the sun, where heating is fundamentally different from what was previously observed in the space. This will allow them to test their theory that the heating is due to small magnetic waves propagating in the area.

The resolution of the puzzle would allow scientists to better understand and predict solar time, which can pose a serious threat to the Earth's power grid. And the first step is to determine where warming begins and ends in the outside atmosphere of the sun – a puzzle where theories are not lacking.

Once in this area, the Parker Solar Probe will help determine the cause of the heating by directly measuring the magnetic fields and particles present.

"Whatever the physics behind this overheating, it's a puzzle that has set us back for 500 years," said Justin Kasper, UM professor of climate and space science and principal investigator for the Parker mission. "In just two years, Parker Solar Probe will finally reveal the answer."

The U-M theory is exposed in an article, Strong preferential ionic heating is limited to the surface of the solar surface, published June 4 in The Letters from the Astrophysical Journal.

In this "preferential heating zone" above the sun's surface, the temperatures increase overall. Even stranger, the individual elements are heated to different temperatures, or preferably. Some heavier ions are overheated until they are ten times warmer than the hydrogen present everywhere in this region, warmer than the sun's core.

These high temperatures inflate the solar atmosphere by several times the diameter of the sun, which is why we see the crown extended during solar eclipses. In this sense, Kasper explains, the mystery of coronal warming has been visible to astronomers for more than half a millennium, although high temperatures have only been appreciated in the last century.

This same area is characterized by hydromagnetic "Alfvén waves" that come and go between its outermost edge and the sun's surface. At the farthest end, called Alfvén Point, the solar wind is moving faster than the Alfvén speed and the waves can no longer return to the sun.

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

In trying to estimate how far from the sun's surface this preferential heating stops, the U-M team examined decades of observation of the solar wind by the NASA Wind spacecraft. They examined the magnitude of the increase in helium temperature near the sun that had been washed away by collisions between ions of the solar wind during their journey to Earth. Watching the decay of the helium temperature allowed them to measure the distance to the outer edge of the area.

"We are taking all the data and treating it as a stopwatch to determine how much time has elapsed since the wind overheated," Kasper said. "Since I know how fast this wind is moving, I can convert information at a distance."

These calculations place the outer edge of the overheating zone at about 10 to 50 solar rays from the surface. It was impossible to be more definitive because some values ​​could only be guessed.

Initially, Kasper did not think to compare his estimate of the location of the area to the point of Alfvén, but he wanted to know if there was a physically significant location in the area. space that would produce the outer limit. After reading that the Alfvén point and other surfaces had been observed expanding and contracting with solar activity, he and his co-author, Kristopher Klein, a former post-doctorate at the UM and new professor at the University of Arizona, have reworked their analysis over the years. changes rather than considering the entire Wind mission.

"To my great shock, the outer limit of the preferential heating zone and the Alfvén point moved in a completely predictable parallel manner, despite completely independent calculations," Kasper said. "You superimpose them and they do exactly the same thing over time."

So does the Alfvén point mark the outer edge of the heating zone? And what exactly changes under the Alfvén point that overheats heavy ions? We should know in the next two years. The Parker solar probe was launched in August 2018 and had its first rendezvous with the sun in November 2018 – already approaching the sun like any other human-made object.

In the years to come, Parker will get closer to each pass until the probe falls below Alfvén's point. In their article, Kasper and Klein predict that it should enter the preferential heating zone in 2021 as the boundary expands with increasing solar activity. . Next, NASA will have information directly from the source to answer all kinds of long-standing questions.

"Thanks to Parker Solar Probe, we will be able to definitively determine, through local measurements, which processes lead to solar wind acceleration and preferential heating of certain elements," Klein said. "The predictions in this article suggest that these processes operate under the surface of Alfvén, a region close to the Sun that no spacecraft has visited, which means that these preferential heating processes n & # 39; 39, have never been measured directly before. "

Kasper is the principal investigator of the SWEAP (Solar Wind Electron Electrons Alphas and Protons) survey on the Parker Solar Probe. SWEAP sensors collect solar wind and coronal particles at each encounter to measure speed, temperature, and density, and to unravel the mystery of heating.

The document titled "The strong preferential heating of ions is limited to the surface of the solar surface".