Scientists have long wondered why the hot charged particles in our sun’s atmosphere get hotter as they move away from the sun’s surface. A new study may provide an answer, finding that the super-hot nature of the sun’s outer atmosphere, or “corona,” may be linked to the intriguing behavior of small-scale waves in this hazy plasma. These waves, known to scientists as “kinetic Alfvén waves,” or “KAWs,” are wave-like oscillations of magnetic fields that manifest themselves in motions in the sun’s photosphere.
The findings could provide an important clue to the seemingly physics-defying “coronal heating mystery” of why the corona is hundreds of times hotter than the visible solar “surface,” or photosphere, which emits all the Sun’s light we see, The Astrophysical Journal reports.
The team behind the study, led by Syed Ayaz, a researcher at the University of Alabama in Huntsville, suggests that as KAWs spread, they dissipate and heat the solar corona. They thus serve as an important, albeit small-scale, mechanism by which energy is transferred in the sun’s plasma.
The solar corona during an eclipse. john finney photography
Ayaz said the phenomenon could explain why the sun’s visible surface has a temperature of about 10,000 degrees Fahrenheit (5,500 degrees Celsius), while the corona, which marks the uppermost part of the sun’s atmosphere, has a temperature of more than 2 million degrees Fahrenheit (1.1 million degrees Celsius).
“Alfvén waves have been proven for decades to be the best candidates for transporting energy from one place to another,” Ayaz said in a recent statement. “Until now, no solar spacecraft mission has made predictions about these phenomena near the Sun.”
Most of the Sun’s energy comes from its core, where nuclear fusion occurs. This means that the Sun should get hotter as you go deeper into it. Most of our star’s layers follow this principle. However, the corona, despite being millions of miles farther from the Sun’s core than the Sun’s surface, is still much hotter than the photosphere.
Ayaz and his colleagues studied the effects of KAWs in plasmas rising to heights equal to 10 solar radii. At such distances, when the waves interact with the sun’s charged plasma, which is filled with “ions,” atoms stripped of their electrons, they “quickly dissipate, transferring their energy entirely to the plasma particles as heat,” Ayaz said.
The team’s findings suggest that wave energy can reach the corona and heat it, although it is not yet clear to what extent it affects the corona’s temperature.
The new research “provides important insights into the critical problem of converting magnetic field energy to heat plasmas containing charged particles such as protons and electrons,” said Gary Zanck, director of the Center for Space Plasma and Aeronomy Research at the University of Alabama, who was not involved in the work.
A diagram showing the layers of the Sun, including the corona and the underlying photosphere. NASA
The latest findings are backed by data from the European Space Agency’s Solar Orbiter and NASA’s Solar Dynamics Observatory (SDO). SDO previously found that another type of high-frequency, bow-shaped magnetic wave propagating through the corona can also dump large amounts of energy into the Sun’s outer atmosphere over time, helping to heat the million-degree layer.
Similar processes that provide heat to the sun’s corona were the focus of a recent NASA sounding rocket mission. The mission, called MaGIXS-2 — short for the second flight of the Marshall Grazing Incidence X-ray Spectrometer — was launched into space for a few minutes in mid-July to collect X-rays from the sun.
These rays are particularly revealing of how often our star releases bursts of energy, which could help scientists learn more about how the corona heats up.
While scientists continue to piece together the puzzle of how the sun’s corona gets so hot, other heating mechanisms involving the sun’s magnetic field have been ruled out. For example, scientists suspected that certain S-shaped kinks in the sun’s magnetic field contain a lot of magnetic energy that is released into the surrounding plasma, heating it up and accelerating the solar winds that cause storms.
However, an analysis of the first 14 orbits of the Parker Solar Probe around the Sun, presented in a separate paper published in The Astrophysical Journal Letters, found no evidence of the sought-after feature within the corona.
Mojtaba Akhavan-Tafti, a research scientist at the University of Michigan who led the study, said in a statement that the Parker Solar Probe’s upcoming flights to the sun, likely as early as December, could shed light on the long-standing mystery.