Coronal rain is the most dramatic manifestation of cooling in the solar corona. It corresponds to cold, dense clumps of plasma appearing on a time scale of minutes in the corona, which preferentially fall towards the solar surface along coronal rings. Now, solar astronomers using ESA’s Solar Orbiter have observed a never-before-seen fireball phenomenon occurring within the coronal rain.
This image, taken with ESA’s Solar Orbiter on 1 April 2022, shows a partial section of the Sun with gas at 1 million degrees; the red paths correspond to some of the rain tracks analyzed by Antolin et al. Image credit: Patrick Antolin / ESA / Solar Orbiter / EUI / HRI.
Coronal rain is a process of condensation in which some of the fiery material from the Sun aggregates due to sudden localized temperature drops.
The corona, which is the outermost part of the Sun’s atmosphere, is made up of gases at temperatures of a million degrees, and rapid drops in temperature produce super-dense clusters of plasma up to 250 km wide.
These fireballs hurtle towards the Sun as gravity pulls them in at over 100km per second.
In spring 2022, ESA’s Solar Orbiter (SolO) sailed very close to the Sun at a distance of only 49 million km, enabling the best spatial resolution ever obtained of the solar corona.
Along with the first high-resolution images of the coronal rain clumps, SolO observed the heating and compression of the gas immediately beneath them.
The resulting peak in intensity under the lumps indicates that the gas has heated up to a million degrees, lasting for a few minutes as they fall.
On Earth, shooting stars occur when meteoroids enter our atmosphere at high speeds and burn up.
But the sun’s corona is thin and low-density and doesn’t remove much material from the clumps, so scientists think most shooting stars reach the solar surface intact.
Their impacts have never been observed until now, and SolOs observations have revealed that this process can produce a short, strong brightening with an upward surge of material and shock waves that reheat the overlying gas.
Shooting stars and meteors in Earth’s atmosphere feature a trail behind the path of the meteors, formed when heated material underneath removes parts of the object, a process called ablation. The same happens to comets orbiting the Sun.
However, ablation does not occur in the solar corona due to its magnetic field.
Instead, the infalling gas is partially ionized and follows magnetic field lines, which act like giant pipes that convey the gas.
The compression and underlying heat prevents the clumps from producing tails and makes the phenomenon much more difficult to capture on the Sun.
The inner solar corona is so hot that we may never be able to probe it in situ with a spacecraft, said Dr. Patrick Antolin of Northumbria University.
However, SolO orbits close enough to the Sun that we can detect small-scale phenomena occurring within the corona, such as the effect of rain on the corona, allowing us a valuable indirect probe of the coronal environment that is crucial for understanding its composition and thermodynamics.
The detection of coronal rain alone is a huge leap forward for solar physics because it gives us important clues to key solar mysteries, such as how it heats up to millions of degrees.
If humans were alien beings capable of living on the surface of the Sun, we’d be constantly rewarded with incredible views of shooting stars, but we’d have to watch our heads.
The study was published in the journal Astronomy and astrophysics.
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P. Antolin et al. 2023. Extreme ultraviolet fine structure and variability associated with coronal rain revealed by Solar Orbiter/EUI HRIEUV and SPICE. YY, in print; doi: 10.1051/0004-6361/202346016
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