Solar prominence visible during eclipse


During my conversations with people about the recent total solar eclipse, one question seemed to come up repeatedly. What was that reddish spot at about 6 o’clock during the total phase?

Short answer: The red spot was a solar prominence, a violent uplifting of plasma from the sun’s chromosphere.

A more detailed answer requires an explanation of the sun’s outer structure.

The sun’s “surface” is called its photosphere, the “sphere of light.” Granted, the term “surface” is a bit misleading. The sun is a boiling, seething mass of thermonuclear energy, hot gases exploding like a gigantic hydrogen bomb.

Thus, it’s difficult to define where the surface is. Some astronomy types would say that the photosphere is the lowest level of the sun’s atmosphere. In a sense, the sun is hot gas all the way down. It consists almost entirely of atmosphere. Thus, we’ll arbitrarily define the photosphere as the sun’s “surface.”

Ahem. Where was I? Oh, yes. The photosphere.

The photosphere shines brightly in white light. It’s so bright that looking at it will harm your vision.

Careful eclipse observers probably noticed that just as the eclipse achieved totality, the left edge of the sun was fringed with a thin, reddish arc of light.

If you saw it, you witnessed the sun’s inner atmosphere, called its chromosphere, the “sphere of color.” It resides right above the photosphere.

Above the chromosphere is the sun’s outer atmosphere, the ghostly and breathtaking corona.

The chromosphere is exceedingly thin at only 1,200 miles wide. It is thus just a paper-thin layer compared to the 935,000-mile-wide diameter of the sun.

Also, the chromosphere is mostly transparent. The bright photosphere visually overwhelms it most of the time. We can see it with our eyes only during that magic instant at the beginning of totality when the photosphere is blocked, and an arc of its thin chromosphere is still visible.

The chromosphere is a distinct shade of red. Brad Hoehne, Director of the John Glenn Astronomy Park, calls it “Barbie pink.”

Why pink? The chromosphere consists primarily of hydrogen in the form of plasma, a fourth state of matter distinct from solids, liquids, and gasses. Plasma is the universe’s most abundant state of matter because it exists in such high quantities in stars.

The chromosphere’s plasma consists of hydrogen in a superheated, highly excited state.

A hydrogen atom generally consists of one proton and one electron. The chromosphere’s plasma consists of charged particles — ionized hydrogen with electrons at such high energy levels that they have been torn away from their protons.

The chromosphere’s distinct pinkish color lies in its spectrum.

When you see a rainbow, you experience the spectrum of the sun’s photosphere. The rainbow tells you that the visible portion of the sun, its photosphere, glows with virtually all the colors of the spectrum. Combine all those colors, and you get white.

According to Ohio Wesleyan astronomer Robert Harmon, the chromosphere’s plasma glows only in four distinct colors: red, teal (dark blue-green), blue, and violet. Combine them, and you get pink, the same distinct color that we see in emission nebulae, the clouds of hydrogen in which stars are born.

As a heaving, spinning mass, the sun is bound to generate a complex and powerful magnetic field, twisted, distorted, and ever-changing.

The sun’s enormous gravity is not powerful enough to counter all of its explosive force. Its corona is thus driven into space.

And that’s where prominences (finally!) fit in. At small spots where the gravitational field is powerful and distorted enough, a bit of the pinkish chromosphere bursts violently outward along a line of magnetism and into the corona.

The six o’clock prominence looked tiny. It wasn’t. A typical prominence can be up to ten times the diameter of planet Earth.

Sometimes, those eruptions follow a curved magnetic line back to the sun. The 6 o’clock prominence had a distinct “U” shape in my binoculars.

The force of the sun’s gravity will eventually triumph over the sun’s constantly changing magnetic field and its explosive power. Prominences fall back into the chromosphere or dissipate into the corona. The six o’clock prominence will disappear within a few days or months. It may be gone already, but you’ll need another total eclipse to check for yourself.

Tom Burns is the former director of the Perkins Observatory in Delaware.

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