Science

Some erratic “coronal loops” on the Sun may be an optical illusion

A new study suggests that some coronal loops that appear to be coming from the Sun may actually be optical illusions created by folds or wrinkles in large layers of stellar material called coronal veils.

If the discovery turns out to be correct, solar scientists may be forced to rethink this solar phenomenon.

The new model could solve some long-standing mysteries about the coronal loops that make the sun look like a dirty ball of yarn. In addition, the study may provide a 3D model of the phenomenon that researchers have been looking for for six decades.

Related: Watch ‘coronal streamers’ move away from the sun in a large video from Parker Solar Probe

Traditional ideas about coronal loops suggest that they are composed of plasma – a dense gas in which atoms, devoid of electrons, flow along magnetic “tubes” generated by the powerful magnetic field of the Sun. These magnetic tubes are invisible, but the plasma that flows through them like water through a garden hose is invisible. As a result, this plasma can be seen as loop-like “threads” sticking out of our star.

Although this “garden hose” model of coronal loops is in good agreement with physics, there are some problems with it. First, the Sun’s plasma becomes thinner and dimmer as it moves away from its stellar core. The researchers explained that coronal loops, if made of plasma, should show the same dimming with height, but remain consistently bright.

Also, as the Sun’s magnetic field moves away from the star, that field expands to fill space. As a result, coronal loops should swell further away from the Sun if created by this field, but they don’t seem to do so.

“They are not getting as wide as we think,” Anna Malanushenko, a solar physicist at the National Center for Atmospheric Research in Boulder, Colorado, and lead author of the paper describing the results, said in a NASA statement. (will open in a new tab). “Most of them stay too thin and we don’t understand why.”

This ultraviolet image of the sun shows that coronal loops do not swell with altitude.

This ultraviolet image of the sun shows that coronal loops do not swell with height. (Image credit: NASA/SDO)

These inconsistencies led Malanushenko to question the observations of coronal loops. After all, the solar corona and its atmosphere are no strangers to optical tricks.

For reasons researchers don’t yet understand, the solar corona is much hotter than the Sun’s surface, called the photosphere, despite being less dense. This makes the crown optically thin or translucent, like fog. As a result, it is completely covered by the light of the photosphere. To investigate coronal loops, Malanushenko repurposed a 3D solar simulation originally created to study solar flares and then wrote a computer program to observe the model.

This program took 2D snapshots of a simulated sun, showing that this artificial star has coronal loops just like a real one. But the simulation provided Malanushenko with an opportunity that solar scientists don’t get when visualizing a real sun with telescopes: She was able to pause this simulated sun and peer into the 3D structures behind artificial coronal loops, revealing a striking and unusual structure that was very different from hose-like tubes.

“I have no words to describe it because it is unlike anything we see on Earth,” Malanushchenko said. “I mean, this formation is like puffs of smoke, or maybe a veil or puckered curtains.”

Malanushenko developed a simple physical spherical model with a looped curtain of thin material emerging from it to show how the curtain could create the illusion of a coronal loop.

This simplified model compares a garden hose model of coronal loops (left) with a coronary veil model (right).

This simplified model compares a garden hose model of coronal loops (left) with a coronary veil model (right). (Image credit: Anna Malanushenko)

The shadow cast by the model on the wall behind it was the two-dimensional images of the sun that we get with telescopes. The folds and wrinkles of the bedspread created a pattern of dark and light threads, in many ways reminiscent of real coronal loops.

But the shadow loops weren’t really real, Malanushchenko said. Instead, they were caused by an optical illusion – the projection effect. In the same way, coronal veil folds cast the same two-dimensional filamentary shadows on our telescopes as model veil folds cast on a wall.

But even if this hypothesis is correct, the idea is not true for all coronal loops, Malanushenko said. Some are definitely not illusions created by the coronal veil. Even in the simulated star, there have been cases where hose-like formations have formed.

“It would be great if we could say, ‘All of our thoughts were wrong; we have a whole new paradigm,” said Jim Klimchuk, a solar physicist at NASA’s Goddard Space Flight Center in Maryland and co-author of the study. statement. – This is not at all the case – but these veils, I am sure that they are, and now the question is in proportions: veils more often or loops more often?

In addition to determining this, solar physicists will have to figure out what creates coronal veils.

The researchers described the results in a paper published in The Astrophysical Journal. (will open in a new tab).

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