Coronal rain is the most dramatic display of cooling in the Sun’s corona. It corresponds to cool and dense clumps of plasma appearing over a timescale of minutes in the corona, that preferentially fall towards the solar surface along coronal loops. 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 April 1, 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 condensation process in which some of the Sun’s fiery material clumps together due to sudden, localised temperature drops.
The corona, which is the outermost part of the Sun’s atmosphere, is formed of gas at million-degree temperatures, and quick drops in temperature produce super-dense clumps of plasma that reach 250 km wide.
These fiery balls plummet back down towards the Sun as gravity pulls them in at over 100 km per second.
In spring 2022, ESA’s Solar Orbiter (SolO) cruised super close to the Sun at a distance of only 49 million km, allowing 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 gas immediately underneath them.
The resulting spike in intensity below the clumps indicates that the gas is heated up to a million degrees, which lasts for a few minutes as they fall.
On Earth, ‘shooting stars’ happen when meteoroids enter our atmosphere at high speeds and burn up.
But the Sun’s corona is thin and low in density and does not strip much material off the clumps, so scientists think most of the ‘shooting stars’ make it to the solar surface intact.
Their impacts have never been observed until now and SolO’s observations have revealed that this process can produce a brief, strong brightening with an upward surge of material and shock waves that reheat the gas above.
‘Shooting stars’ and meteors in Earth’s atmosphere are characterised by a trace behind the meteor’s path, formed when heated material underneath strips off 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 because of its magnetic field.
Instead, falling gas is partially ionized and follows the magnetic field lines, which act as giant tubes that funnel the gas.
The compression and heat underneath prevents the clumps from producing tails and makes the phenomenon much harder to capture on the Sun.
“The inner solar corona is so hot we may never be able to probe it in situ with a spacecraft,” said Northumbria University’s Dr. Patrick Antolin.
“However, SolO orbits close enough to the Sun that it can detect small-scale phenomena occurring within the corona, such as the effect of the rain on the corona, allowing us a precious indirect probe of the coronal environment that is crucial to understanding its composition and thermodynamics.”
“Just detecting coronal rain is a huge step forward for solar physics because it gives us important clues about the major solar mysteries, such as how it is heated to millions of degrees.”
“If humans were alien beings capable of living on the Sun’s surface, we would constantly be rewarded with amazing views of shooting stars, but we would need to watch out for our heads.”
The study was published in the journal Astronomy & Astrophysics.
_____
P. Antolin et al. 2023. Extreme-ultraviolet fine structure and variability associated with coronal rain revealed by Solar Orbiter/EUI HRIEUV and SPICE. A&A, in press; doi: 10.1051/0004-6361/202346016
