NASA’s Parker Solar Probe has ruled out S-shaped bends in the sun’s magnetic field as a cause of the corona’s searing temperatures, according to a study by the University of Michigan published in The Astrophysical Journal Letters.
The sun’s corona, which resembles a crown, can be 200 times hotter than the sun’s surface despite being farther from the heat source at the sun’s core. The corona’s heat’s baffling defiance of physics has puzzled scientists for many years, enabling the rapid movement of the sun’s charged particles or plasma to escape its gravitational pull and envelop our solar system as the solar wind.
To unravel the solar mystery, NASA developed the Parker Solar Probe to venture into the sun’s corona and uncover its heat source. The probe, equipped with cutting-edge instruments designed by Justin Kasper, a U-M professor, is capable of directly measuring the corona’s plasma density, temperature, and flow.
Upon its initial approach to the sun, the probe identified numerous S-shaped bends in the sun’s magnetic field, known as switchbacks, which briefly reverse the direction of the magnetic field, along with numerous gentler bends. Many scientists view these switchbacks as potential heat sources for the corona and solar wind. Their pronounced S-shape stores significant magnetic energy, which is likely released into the surrounding plasma as the switchbacks journey through space and eventually straighten out.
“That energy has to go somewhere, and it could be contributing to heating the corona and accelerating the solar wind,” said Mojtaba Akhavan-Tafti, U-M assistant research scientist of climate and space sciences and engineering and the corresponding author of the study.
In order to understand the influence of switchbacks on the corona’s temperature, it’s crucial to identify where they form. Analyzing data from the Parker Solar Probe’s initial 14 orbits around the sun, the research team found that while S-shaped bends are common in the solar wind near the sun, they are absent inside the corona.
Debate persists among scientists regarding the cause of switchbacks. Some propose that turbulence in the solar wind beyond the corona bends the magnetic field, while others suggest that switchbacks originate at the sun’s surface, where turbulent magnetic field lines explosively collide and form bent shapes.
The study’s findings refute the latter theory. If switchbacks were a result of magnetic collisions at the sun’s surface, they would be even more prevalent within the corona. Nonetheless, Akhavan-Tafti believes that magnetic collisions may still play an indirect role in the formation of switchbacks and the heating of the corona.
“Our theory could fill the gap between the two schools of thought on S-shaped switchback generation mechanisms,” Akhavan-Tafti said. “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.”
When magnetic fields clash on the sun’s surface, they create vibrations that travel through the magnetic fields into space, while also generating rapid streams of plasma in the solar wind. Akhavan-Tafti suggests that the fast plasma can distort the magnetic waves into switchbacks in the solar wind, potentially contributing to the heating of the corona if these waves dissipate within the solar atmosphere before becoming switchbacks.
“The mechanisms that cause the formation of switchbacks, and the switchbacks themselves, could heat both the corona and the solar wind,” he said.
Currently, there is insufficient data to definitively conclude whether triggers at the sun’s surface or turbulence in the solar wind are the primary cause of switchbacks.
“Parker Solar Probe’s upcoming trips into the sun, as early as 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.
Journal reference:
M. Akhavan-Tafti and S. L. Soni. In Situ Mechanisms are Necessary for Switchback Formation. The Astrophysical Journal Letters, 2024; DOI: 10.3847/2041-8213/ad60bc
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