Why is the Sun’s Corona So Hot? One Hypothesis Down, Many to Go

The temperature of the Sun’s corona is a minimum of 100 times hotter than the Sun’s surface, despite the corona being far less dense and extending millions of miles from the Sun’s surface, as well. But why is this? Now, a recent study published in The Astrophysical Journal could eliminate a longstanding hypothesis regarding the processes responsible for the corona’s extreme heat, which could help them better understand the Sun’s internal processes. This study holds the potential to help scientists gain greater insight into the formation and evolution of our Sun, which could lead to better understanding stars throughout the universe, as well.

For the study, the researchers analyzed data from the first 14 laps conducted by NASA’s Parker Solar Probe around the Sun with the goal of ascertaining how the magnetic field causes S-shaped bends, often called magnetic switchbacks due to their behavior in causing sudden reversals in the magnetic field’s direction. The goal of the study was to determine the source of the switchbacks, which are known to store energy from the magnetic field, to better understand how they could potentially heat the corona and solar wind.

“That energy has to go somewhere, and it could be contributing to heating the corona and accelerating the solar wind,” said Dr. Mojtaba Akhavan-Tafti, who is an assistant research scientist of climate and space sciences and engineering at the University of Michigan and lead author of the study.

The debate regarding the origin of the switchbacks has been disputed for some time within the scientific community, with scientists currently favoring two potential hypotheses: switchbacks originate from the magnetic field bending due to the solar wind’s extreme activity that occurs past the corona, and the other origin being from the surface of the Sun.

The results of the study show switchbacks do not originate from the surface of the Sun, which the researchers attribute to the lack of the number of switchbacks inside the corona. In contrast, if the Sun’s surface was the origin of the switchbacks, it is hypothesized the switchback numbers inside the corona would be far greater. Therefore, the study’s results eliminate one of the two competing hypotheses regarding the origin of switchbacks in the Sun.

“Our theory could fill the gap between the two schools of thought on S-shaped switchback generation mechanisms,” said Dr. Akhavan-Tafti. “While they must be formed outside the corona, there could be a trigger mechanism inside the corona that causes switchbacks to form in the solar wind.”

He follows this with, “The mechanisms that cause the formation of switchbacks, and the switchbacks themselves, could heat both the corona and the solar wind.” 

The study of the Sun’s magnetic field reversal dates to the 1970s when the two German-US Helios spacecraft, dubbed Helios-1 and Helios-2, observed this reversal behavior when Helios-2 traveled just over 43.432 million kilometers (26.99 million miles) from the Sun with Helios-1 being 3 million kilometers (1.9 million miles) behind it. This distance record was broken by the Parker Solar Probe in October 2018 and has since achieved a jaw-dropping distance of 7.26 million kilometers (4.51 million miles) from the Sun, which was accomplished in September 2023.

The Helios missions were followed by the first observations of switchbacks conducted by the NASA/ESA Ulysses probe that studied the Sun’s southern and northern polar regions in 1994 and 1995, respectively. More recently, remnants of switchbacks were observed by the ESA/NASA Solar Orbiter in September 2020 when the spacecraft was just over 146 million kilometers (91 million miles) from the Sun.

As noted, discovering the origin of switchbacks could help scientists better understand the internal processes of the Sun, and specifically the behavior of the solar wind, which contributes to space weather that can cause massive damage to orbiting satellites and electronic ground stations on Earth.

What new discoveries will scientists make about the origins of switchbacks on the Sun in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!