In 2002 solar physicist Daniel Seaton stumbled on the phenomenon that has driven his career ever since.
Seaton was working on the TRACE solar observation satellite, a NASA instrument designed to inform our understanding of the Sun’s activity. The Sun, while the reason we exist, remains in many ways, a mystery.
“We saw this one solar flare where there were just all of these weird features that came in from above the flare,” Seaton tells Inverse. Usually, solar flares — large explosions near the surface of the Sun — push material out and away from the star. That wasn’t happening this time.
“In this case, we saw something — we didn’t know what it was at the time — emerge from the top of our images flowing down toward the Sun,” Seaton says. “I was absolutely dumbstruck.”
Today, Seaton is a senior scientist for solar physics in NOAA’s National Centers for Environmental Information — and finally has an answer to the mystery he stumbled on in 2002.
As detailed in a study published Monday in the journal Nature Astronomy, all he had to do to find the answer was point an instrument at a spot in the sky where no one thought you could see anything: the middle corona.
This research represents the best observations yet of this region. It informs a new understanding of the Sun’s mechanics and may lead to improved methods for predicting potentially volatile space weather.
What’s new? — As a star, the Sun doesn’t have a true surface, but its photosphere is about as close as it gets. This roiling and boiling layer of the Sun is dominated by hot, ionized gasses flowing along powerful magnetic field lines. It’s where solar eruptions and flares originate, Seaton says.
Above the photosphere is the thin chromosphere, so named because it glows bright red under certain viewing conditions, followed by the corona, the hot, wispy aura visible during a total solar eclipse. The corona slowly fades into the larger heliosphere — the very outer layer of the Sun’s atmosphere consisting of the solar wind that flows outward through the entire Solar System.
For technical reasons, the middle corona — the region between 350,000 and 3.5 million kilometers above the earth’s surface — has not been well explored. This has left a persistent gap in solar observations.
“This study is the first time we have really high-quality observations of the dynamics in this region,” Seaton says.
In the lower corona, magnetic field lines control the flow of the hot, ionized gasses. But much further out, these magnetic field lines become less powerful, and gasses behave more normally while the solar wind is accelerated outward.
What the study found was in the middle corona, where these two dynamics converge: material ejected from the sun can behave in more complex ways than expected, and even head back toward the sun.
This is what Seaton saw in 2002.
“I think that’s one of the big surprises,” he says. “Sometimes events that are happening very high in the sun’s atmosphere are propagating back to the surface and causing changes near the surface of the sun that ultimately can lead to eruptions.”
How the discovery was made — NOAA doesn’t just focus on Earth weather. The administration makes space weather forecasts as well, which is why the Solar Ultraviolet Imager, or SUVI, was installed on NOAA’s Geostationary Operational Environmental Satellite 17.
In 2018, Seaton and his colleagues decided to do something different with SUVI — they pointed it to the side of the Sun, at the middle corona, just to “see if there’s anything to be seen.”
It’s important to note, Seaton says, that most scientists assumed there was little to see because the density of the corona falls off rapidly as you move farther from the surface of the Sun. Ultraviolet emissions from the corona are thought to be driven by ions bumping into each other, a process called collisional excitement.
The idea was that in the thinner middle corona, there was little point in pointing a telescope where you wouldn’t see anything.
But this study reveals that in the middle corona, ultraviolet light from lower in the corona is scattered by the ions at higher altitudes, “so that we can see things where we ought not to see visible emissions,” Seaton says.
This “resonant scattering” was what made the study possible, Seaton says, and in some ways is a bigger discovery than the fact that the middle corona can influence events closer to the Sun’s surface.
“A big important takeaway is that these ultraviolet imaging techniques work in a part of the corona that nobody thought they would,” he says. “There is a pathway for the future here that to me, as a solar physicist, is super exciting.”
Why this matters — Beyond advancing scientists' understanding of the Sun’s atmosphere and opening a new window for further observations, Seaton says the study’s findings are important for NOAA’s mission to forecast space weather.
Solar flares on the sun’s photosphere can release X-rays that can disrupt radio communications on Earth, drive high-energy particles away from the sun that can menace satellites and astronauts, and touch off coronal mass ejections, or CMEs. When these “big outbursts of plasma, an ionized soup of electrons and protons” interact with Earth’s magnetic field, they can induce strong currents in power lines, potentially shutting down infrastructure without proper action, Seaton explains
NOAA does its best to predict when such solar eruptions are likely to take place so that NASA, power companies, and airlines, for instance, can take precautions.
But “the models we have of space weather right now are really rudimentary compared to Earth weather,” Seaton says. This study, he thinks, “is the first step toward filling in the gaps and bringing out [space weather] models up to the same level as what the terrestrial community has right now.”
What’s next? — The immediate next steps for Seaton are already underway: His team is conducting further analysis of the data they collected using SUVI in 2018.
He also expects there will be more imaging campaigns by other teams using the seam technique going forward. Meanwhile, newer instruments, such as the joint NASA/European Space Agency mission Solar Orbiter, may shed additional light on the middle corona now that scientists know where, and how, to look.
But moreover, Seaton says he sees an overall shift in how solar scientists are approaching research, moving from studying small regions near the sun to exploring the deeper connections between the sun, the corona, and the heliosphere.
“There is a renaissance of observing the Sun,” he says. “It’s happening, and this is a little piece of it.”
Abstract: The ‘middle corona’ is a critical transition between the highly disparate physical regimes of the lower and outer solar coronae. Nonetheless, it remains poorly understood due to the difficulty of observing this faint region (1.5–3 R☉). New observations from the Solar Ultraviolet Imager of a Geostationary Operational Environmental Satellite in August and September 2018 provide the first comprehensive look at this region’s characteristics and long-term evolution in extreme ultraviolet. Our analysis shows that the dominant emission mechanism here is resonant scattering rather than collisional excitation, consistent with recent model predictions. Our observations highlight that solar wind structures in the heliosphere originate from complex dynamics manifesting in the middle corona that do not occur at lower heights. These data emphasize that low-coronal phenomena can be strongly influenced by inflows from above, not only by photospheric motion, a factor largely overlooked in current models of coronal evolution. This study reveals the full kinematic profile of the initiation of several coronal mass ejections, filling a crucial observational gap that has hindered understanding of the origins of solar eruptions. These new data uniquely demonstrate how extreme ultraviolet observations of the middle corona provide strong new constraints on models seeking to unify the corona and heliosphere.