By For most folks on Earth, the spectacular sight of the sun’s fiery corona can be witnessed only during a total solar eclipse. But even before the widely watched astronomical event on April 8, researchers at Predictive Science Inc. in San Diego had a clue as to what eclipse enthusiasts would see when they looked skyward (SN: 4/8/24).
Since 1994, the private company has been creating sophisticated computer simulations of the sun’s dynamic and magnetized atmosphere for scientists who want to better understand the atmosphere’s evolution. These simulations incorporate continuously updated information about the corona to provide forecasts of its appearance during eclipses.
Such predictions are extremely important for “our colleagues who are studying the corona,” says Cooper Downs, an astrophysicist at the company. “It can help with planning to say, ‘Okay this streamer is in the prediction, maybe we should point our instruments there.’”
A few minutes before the April 8 event, the company published its final prediction for that eclipse (though the model kept running with new data for two weeks afterward.) That prediction got the corona’s appearance fairly close, though not quite perfect, forecasting several long streams that ended up in slightly different places in reality. Observers were still able to get good data, Downs says, since they relied on other sources such as satellite imagery for planning and focused on large and stable streamers.
Still, he wasn’t disappointed by the model’s imperfections. It just points, he says, to the need for more holistic observations of the sun.
We have limited intel on the sun’s atmosphere
Total solar eclipses provide a rare opportunity to get more information about the sun’s corona, Downs says. “When the moon is in front of the sun, it’s like the perfect occulter.”
Together, observations and simulations bring researchers closer to understanding long-standing mysteries such as why the corona, at a scorching million or so degrees Celsius, is so much hotter than the roughly 5,500° C surface (SN: 8/20/17). They are also crucial for forecasting space weather events where the sun blasts out radiation and charged particles that can scramble satellite communications and affect electronic equipment on our planet.
Currently, space weather predictions are far worse than terrestrial weather forecasts. “They’re not even close,” Downs says.
A lot of that comes down to limited data. Atmospheric researchers have access to weather stations and balloons all over Earth, while those studying our local star have mainly a single two-dimensional view of the sun at any given time. From this limited perspective, solar scientists try to re-create the full three-dimensional structure of the corona and infer things like its temperature, density, outflows and magnetic structure.
To see the hazy corona, researchers need to mute the sun’s brilliance. They can do that with an artificial coronagraph, an opaque disk inside a telescope that blocks out the sun and some of the space around it to make its atmosphere visible. But during an eclipse, instruments can see the entirety of the corona from the sun’s surface outward. And the moon’s extensive shadow dims the entire sky, making it easier to see faint features in the corona.
Creating predictions about how the corona will appear during an eclipse is an important part of validating computer models of how the corona works, says Chip Manchester, a solar physicist at the University of Michigan in Ann Arbor who also creates such simulations. If the model matches observations, it provides extra assurance that it’s getting the underlying physics correct.
Solar maximum made predictions difficult in 2024
In 2017, forecasting for the total solar eclipse that swept the United States that year (SN: 8/11/17) was helped by the fact that the sun was near solar minimum, a low point in its 11-year activity cycle. During solar minimum, the sun is relatively stable, with sudden changes few and far between. Downs was able to put out his company’s prediction for how the corona would appear seven days ahead of the eclipse. The projections fairly closely matched reality.
This year, the sun is approaching the most active part of its cycle: solar maximum. And that drove much of the mismatch between eclipse prediction and reality. During solar maximum, the sun is a roiling tempest, with frequent flares bursting forth with no warning. Information about eruptions on its unseen backside couldn’t be incorporated into the simulations until the sun rotated and brought that hidden activity into view.
“You could immediately see: Oh no, there’s new stuff that the model doesn’t have yet,” Downs says. “There’s nothing we can do. We can’t make up data.”
For the April eclipse, the company had access to extremely up-to-date information from NASA’s Solar Dynamics Observatory, which watches the sun from Earth orbit (SN: 4/21/10). The simulations got an extra boost from the European Space Agency’s Solar Orbiter, a satellite traveling around the sun that got to witness the eclipse while off to one side relative to our planet, taking magnetic readings and other data (SN: 2/9/20).
Ideally, Manchester says, solar scientists would have at least three spacecraft spaced equally apart in orbit around the sun. “Then you would see what’s coming around from the backside.”
That setup won’t be available for the foreseeable future. But ESA has plans to bolster its observations of the corona with the PROBA-3 mission, which will block out the sun’s central regions with a coronagraph to study its outer atmosphere and is expected to launch this year. In 2029, the agency will also send up the Vigil spacecraft, a space weather observatory that can watch potentially hazardous solar activity before it rolls into view from Earth.
Downs is already working with the PROBA-3 team to simulate how their views of the sun will look after launch and is hopeful that the satellite’s data will improve predictions for the next total solar eclipse, which will grace the Arctic and far-Western Europe two years from now. “We are definitely looking forward to 2026,” he says.
