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The Sun's 'shimmering' corona

Does our star’s outer atmosphere display variations?
OMearaStephen
It’s amazing how often the Sun’s delicate outer atmosphere, as seen during a total solar eclipse, is described as the shimmering corona. Taken literally, this means the corona shines with a soft, wavering light. But does it? While the use of “shimmering” appears to be largely a case of poetic license — to make an otherwise tranquil phenomenon seem more animated — is there a glimmer of truth to it?

Near or far?

During the March 9, 2016, total solar eclipse, I was in a fishing boat off the coast of Ternate, Indonesia, watching totality through a substantially large hole in the clouds. Several times I saw the corona waver ever so slightly like a mirage, and I wondered if my eyes were deceiving me.

It is true that the Sun’s corona is in a dynamic state. But the Sun is so huge that it would take a light flicker a couple of seconds to cross the corona’s brightest parts — not fractions of a second, as occurs with a shimmer.

Therefore, naked-eye coronal shimmering must be a localized phenomenon, with the source being perhaps air turbulence close to the observer, thin atmospheric vapors passing in front of the corona, involuntary motions in the observer’s eyes, or a combination of any or all of these. The flickering I saw reminded me of the passing of shadow bands, except that I saw them cross the “white sheet” of the corona. Before delving into this curious phenomenon further, let’s look at shadow bands as we traditionally see them.

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