Observation of Sun's Splashing Coronal Rain

Plasma rain in the sun’s atmosphere makes a splash when it lands. New observations from the European Space Agency’s Solar Orbiter have revealed previously unseen details of how this coronal rain falls, including bright fireball effects and sudden upward surges in plasma.

“These are the highest resolution images we have ever obtained from the solar corona,” says solar physicist Patrick Antolin of Northumbria University in Newcastle upon Tyne, England. He presented the results at the National Astronomy Meeting in Cardiff, Wales, the week of July 3 and in a paper to be published in Astronomy & Astrophysics.

The corona is the sun’s wispy upper atmosphere, the sizzling tangle of plasma and magnetism that is visible during a total eclipse (SN: 6/30/19). When clumps of scorching-hot plasma in the corona suddenly cool, they condense and fall toward the solar surface, just like water droplets in Earth’s atmosphere. This coronal rain has been observed before, but details of its formation and falling were fuzzy (SN: 5/24/18).

The 2020 launch of Solar Orbiter promised to change that (SN: 2/9/20). The probe is making passes over the sun’s unexplored polar regions, carrying high resolution cameras and other instruments to investigate solar mysteries. In late March 2022, Solar Orbiter made its closest approach to the sun to date, swooping within 49 million kilometers of our star — about a third of the distance between the sun and Earth.

In images from the spacecraft taken during that close encounter, Antolin and colleagues discovered a new feature in the coronal rain. As the plasma raindrops fell, the region immediately below them brightened. The researchers think the brightening came from other plasma below the falling rain getting compressed and heated, similar to the way meteors in Earth’s atmosphere can create fireballs just ahead of the falling rocks (SN: 2/15/13).

“This is the first time we’ve seen that kind of compression from the rain falling this clearly,” says Emily Mason, a solar physicist at Predictive Science, a research company based in San Diego, who was not involved in the study. “The resolution just wasn’t there before.”

Antolin and colleagues also saw a rebound and upward flow of material after coronal raindrops smacked onto the chromosphere, the thin layer of plasma that lies between the sun’s visible surface and the corona.

“This rain is as dense as the chromosphere,” Antolin says. “It can fall to the chromosphere and then make a splash.” This sort of splash was predicted in computer simulations but had never been observed on the actual sun before.

On their own, the observations won’t directly solve the biggest solar mysteries, like why the diffuse corona sizzles at millions of degrees Celsius higher than the surface of the sun (SN: 8/20/17). But more observations of the same phenomena can help figure out details of the coronal environment, like how easily its gas compresses, or what its chemical composition is.

“We are not able to send a probe to the inner part of the corona because it’s so hot,” Antolin says. “So we can use these observations as indirect probes of the local environment.”

The observations are “an important litmus test for Solar Orbiter itself,” Mason says. “It’s good to see what Solar Orbiter is capable of.”

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