Could We See the Sun Erupt on Eclipse Day?

The Sun is rapidly approaching solar maximum, a time when activity such as sunspots, flares, and solar eruptions known as coronal mass ejections, become common. So, what are the chances of seeing such an eruption during totality on April 8th?

The chance depends on the year: The Sun goes through an 11-year cycle of activity, from minimum to maximum and back again. The solar eclipse of 2017, witnessed by hundreds of millions in the U.S., occurred just before the minimum of the cycle. Now, the 2024 eclipse will be occurring near maximum.

This change in solar activity has several impacts on how we view the ethereal corona that’s revealed during totality. For one, the well-organized solar magnetic field during minimum restricts the brightest coronal streamers to the equatorial region. But as solar magnetic fields become more tangled, the corona becomes more evenly distributed around the disk. Pink prominences, which are loops or filaments extending above the Sun, are also more likely to be visible.

Coronal mass ejections (CMEs) likewise become more common; according to Joan Burkepile (University Corporation for Atmospheric Research), major eruptions occurred two to three times a day in February and March. In these events, the Sun’s magnetic fields tangle and snap, expelling blobs of gas into interplanetary space. Upon arrival at Earth, such blobs can create aurorae or, less optimistically, threaten satellite systems or power grids.

With these eruptions exploding off the Sun at least a couple times a day, the chances of a CME appear high. “The question is not, ‘how likely is a CME,’ but, ‘how likely is it that there will not be a CME,’” says Dan Seaton (Southwest Research Institute). “Odds are pretty good that there will be an eruption around the time of the eclipse.”

The question is, will we see it?

Could We See a Solar Eruption During Totality?

“In principle, I think a CME could be visible to the naked eye, especially a big one,” Seaton answers. In fact, there are good examples of likely CMEs recorded for posterity. A drawing by Gugliemo Temple from the total solar eclipse of 1860 — when the Sun was also at a maximum in its cycle — shows a circular feature that’s distinct from the streaming corona. It’s probably a solar eruption.

Another example might be found in Chaco Canyon, in which a petroglyph has been interpreted to depict the solar corona at totality. Archaeohistorian Kim Malville (University of Colorado, Boulder) has made the case that the rock drawing in New Mexico shows not only a solar eclipse — specifically that of July 11, 1097 — but also a CME. According to tree-ring data, the Sun was also in the more active phase of its cycle that year.

“Those curvy lines, if they are indeed representations of observations of the corona, must have been stylized recollections of something that looked turbulent and tumbled,” Malville says. “It may have been a cooperative endeavor in which several observers contributed to the recollection of what they had seen: the brief apparition of a deity, never seen before.”

More recently, astronomical observations of the 2010 and 2012 total solar eclipses have captured CMEs during totality. So have amateur astrophotographers, such as Miloslav Druckmüller and Andreas Möller, who captured a clear CME after the 2020 eclipse over Argentina, or Petr Horálek, Josef Kujal, and Milan Hlaváč, who found a beautiful one during the 2023 eclipse in Australia. (See below for those photos.) But those events were photographed rather than seen with the unaided eye.

“With binoculars (used carefully, only during totality!) I think it would be very possible to see a CME close to the Sun, but probably only a big event very close to the Sun would be really noticeable without,” Seaton adds.

A lot of factors determine whether a CME’s visibility. “The visibility time would vary depending on many factors including angle, speed, and how dense the CME was,” says Amir Caspi (also at Southwest Research Institute). A side-directed CME would be more obvious than one directed straight toward Earth. Also, the more a CME has spread out, the harder it will be to see. Other factors, such as the brightness contrast between the CME and background light — coming not only from the corona but also from sunlight scattered off interplanetary dust — also come into play.

Considering those effects, Sarah Gibson (also at University Corporation for Atmospheric Research) estimates a lower chance of actually seeing a CME during totality, around 5-10%. Burkepile agrees the chance is low, putting the likelihood at less than 20%. Once an eruption goes off, it would only be visible for a couple hours at most; the fastest ones might only be visible for 30 minutes, Burkepile adds.

What Would a Solar Eruption During Totality Look Like?  

As long as the angle is right, viewers should be able to distinguish an eruption from the corona itself. “A sideways-directed CME might look like a circle, or a partial circle,” Gibson notes. “CMEs are faster than the local solar wind, at least at first (when they are still near the Sun), and they plow ahead with their circular shape more or less intact.”

Writing on the Solar Eclipse Mailing List, retired professional astronomer Steve Edberg notes that he saw a side-directed CME in 1980, which was later confirmed. He described it as having the “same color (silvery satin) as the rest of the corona, but it had (in this case) a noticeable S-shape as seen with binoculars.”

The loop-like shape makes it easier to differentiate a CME from coronal streamers, which stretch out away from the Sun due to the solar wind, Gibson adds.

Despite CMEs’ explosive nature, they will appear stationary to any individual observer. “Although CMEs are fast — even thousands of kilometers per second — the Sun’s diameter is more than 1 million kilometers,” Gibson says.  “So in the few minutes of [totality], its motion is not likely to be noticeable.” Photography from different sites along the eclipse path have recorded motion in past CMEs, though, and research projects from high-altitude research planes as well as volunteers distributed on the ground could follow CMEs in more detail.

If you think you’ve seen a CME on April 8th, there are now easy ways to check after the fact. The cameras of the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO) take almost continuous imagery of the Sun, made available in real-time (or nearly so, depending on when the satellite can downlink data) on this NASA site.

Besides imaging the Sun itself, SOHO’s wide-field cameras also image the solar corona, using an occulting disk (rather than the Moon) to block out the Sun’s brilliant light — to see these images, look at the LASCO C2 and LASCO C3 cameras. You can create a movie to watch events unfold over time here: SOHO Movie Theater.

In the end, the chances are low — but not zero — that anyone will see a bubble near the Moon’s edge during totality. But it can’t hurt to look. “You never know,” Burkepile says. “I have a friend who likes to say it is better to be lucky than good.  Let's hope we are lucky.”

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