NASA’s Parker Solar Probe found that S-shaped bends in the magnetic field are not the cause of the intense heating of the solar corona. These switches are visible in the solar wind but not in the corona. This finding casts doubt on the idea that magnetic field collisions at the Sun’s surface create reversals and suggests other mechanisms. Additional data is needed to test the new hypotheses.

Parker Solar Probe Reveals Mysteries

After plunging into the Sun’s corona, NASA’s Parker Solar Probe ruled out S-shaped kinks in the Sun’s magnetic field as the cause of the corona’s scorching heat, according to a study published July 29 in The Astrophysical Journal Letters by the University of Michigan.

An atmosphere similar to the Sun’s corona can be 200 times hotter than the Sun’s surface, despite being farther away from the main heat source at the Sun’s core. The corona’s seemingly physics-defying temperatures have puzzled scientists for decades, but it allows the Sun’s hot soup of charged particles, or plasma, to move fast enough to escape the Sun’s gravitational pull and engulf our solar system as the solar wind.

To solve the mystery, NASA created the Parker Solar Probe to dive into the corona and find the source of the heat. The spacecraft is equipped with a suite of instruments designed by Justin Kasper, professor of climate and space science and engineering, to directly measure the density, temperature and flow of coronal plasma.

Opening Returns

As the probe first approached the Sun, it detected hundreds of S-shaped bends in the Sun’s magnetic field, called switches because they briefly change the direction of the magnetic field, as well as thousands of smaller bends. To some scientists, the reversals seemed like promising sources of heat for the corona and solar wind. The violent S-bends store a lot of magnetic energy, and that energy is likely released into the surrounding plasma as the switches move through space and eventually flatten out.

“This energy has to go somewhere, and it could contribute to heating the corona and accelerating the solar wind,” said Moytaba Ahavan-Tafti, assistant research professor in the university’s Department of Climate and Space Sciences and Engineering and corresponding author of the study.

But to heat the corona, it must pass through reversal switches, so learning where reversal switches occur is critical to understanding their effects on corona temperature. After examining data from Parker Solar Probe’s first 14 orbits, the research team found that S-shaped bends are common in the solar wind near the Sun, but are absent in the corona.

Competing theories

Scientists still don’t agree on what causes this change. Some believe the magnetic field is disrupted by turbulence in the solar wind outside the corona. Others believe the feedback loops begin their journey at the Sun’s surface, when magnetic field lines and loops explosively collide and merge into twisted shapes.

The study’s results rule out the latter hypothesis. If reversals were caused by collisions between magnetic fields on the Sun’s surface, they should be even more common in the corona. But Ahavan-Tafty believes magnetic collisions could still play an indirect role in causing the transition and heating the corona.

A new theory

“Our theory could bridge the gap between two schools of thought regarding S-shaped reversal mechanisms,” Ahavan-Tafty said. “Although they must occur outside the corona, there may be a trigger mechanism inside the corona that causes reversals to occur in the solar wind.”

When magnetic fields hit the Sun’s surface, they vibrate like plucked guitar strings, sending ripples along the magnetic fields into space. At the same time, the energy from the collisions creates very fast plasma flows in the solar wind.

Ahavan-Tafti believes that the fast plasma disrupts magnetic waves in the solar wind, which could also play a role in heating the corona if some of them dissipate in the sun’s atmosphere before reversing.

“The mechanisms that cause the keys to form, and the keys themselves, can heat both the corona and the solar wind,” he said.

Future Research

However, there is currently insufficient data to suggest that triggers at the Sun’s surface, rather than turbulence in the solar wind, are the cause of the transitions.

“Parker Solar Probe’s upcoming trips, starting on December 24, 2024, will collect more data even closer to the Sun. We will use the data to further test our hypothesis,” Akhavan-Tafti said.