Solar wind temperature mystery

During a total solar eclipse, the Sun’s corona and prominences are visible to the naked eye [image credit: Luc Viatour / https://Lucnix.be ]

Expecting a variable, researchers found a constant.
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From traversing sand dunes in the Sahara Desert to keeping watch for polar bears in the Arctic, a group of solar scientists known as the “Solar Wind Sherpas” led by Shadia Habbal, have traveled to the ends of the Earth to scientifically observe total solar eclipses—the fleeting moments when the Moon completely blocks the Sun, temporarily turning day into night.

With the images, they’ve uncovered a surprising finding about the Sun’s wind and its wispy outer atmosphere—the corona—which is only visible in its entirety during an eclipse, says NASA (via Phys.org).

From more than a decade’s worth of total eclipse observations taken around the world, the team noticed that the corona maintains a fairly constant temperature, despite dynamical changes to the region that occur on an 11-year rotation known as the solar cycle.

Similarly, the solar wind—the steady stream of particles the Sun releases from the corona out across the solar system—matches that same temperature.

“The temperature at the sources of the solar wind in the corona is almost constant throughout a solar cycle,” said Shadia Habbal, a solar researcher at the University of Hawaii who led the study. “This finding is unexpected because coronal structures are driven by changes in the distribution of magnetized plasmas in the corona, which vary so much throughout the 11-year magnetic solar cycle.”

The new findings, published in the Astrophysical Journal Letters, are helping scientists better understand the solar wind, which is a key component of space weather that can impact electronics hardware and astronaut activities in space. The results could also help scientists understand a longstanding solar mystery: how the corona gets to be over a million degrees hotter than lower atmospheric layers.

More Than Just Pretty Pictures

Scientists have used total solar eclipses for over a century to learn more about our universe, including deciphering the Sun’s structure and explosive events, finding evidence for the theory of general relativity, and even discovering a new element—helium.

While instruments called coronagraphs are able to mimic eclipses, they’re not good enough to access the full extent of the corona that is revealed during a total solar eclipse. Instead, astronomers must travel to far-flung regions of the Earth to observe the corona during eclipses, which occur about every 12 to 18 months and only last a few minutes.

Through travels to Australia, Libya, Mongolia, Oregon, and beyond, the team gathered 14 years of high-resolution total solar eclipse images from around the world. They captured the eclipses using cameras equipped with specialized filters to help them measure the temperatures of the particles from the innermost part of the corona, the sources of the solar wind.

“That means that whatever is heating the majority of the corona and solar wind is not very dependent on the Sun’s activity cycle,” said Benjamin Boe, a solar researcher at the University of Hawaii involved in the new research.

The finding is surprising as it suggests that while the majority of solar wind is originating from sources that have a roughly constant temperature, it may have wildly different speeds.

“So now the question is, what processes keep the temperature of the sources of the solar wind at a constant value?” Habbal said.

Full article here.