Rain on the Sun
Plasma on the Sun cools and falls to produce a phenomenon called coronal rain in 2012.
NASA/GSFC/SDO
The outermost layer of the Sun’s atmosphere—the corona—is millions of degrees as hot as the layer closer to the core. That coronal-heating problem is one of the primary mysteries in solar and stellar physics (see the article by Nour E. Raouafi, Physics Today, November 2022, page 28 ). One way to investigate it is by studying coronal loops and coronal rain.
Coronal loops are arcs of plasma protruding from the photosphere—the Sun’s visible surface—that project up into the corona and follow the shape of the Sun’s magnetic field. Because they are visibly distinct from the background, the loops are easier to observe than many other solar features. The particulars of how they form, how long they persist, and what they look like on smaller scales are still mysterious but essential for solving the coronal-heating problem. Coronal loops are best observed in the extreme UV (EUV), but high-resolution EUV observations have been especially rare.
Using the EUV imager at high resolution and the Spectral Imaging of the Coronal Environment (SPICE), 2 of the 10 instruments on Solar Orbiter, Patrick Antolin (Northumbria University) and colleagues around the world looked at a special subset of coronal loops that exhibit coronal-rain events. Those events often, but not always, accompany coronal loops during their cooling stage. But unlike water rain on Earth, coronal rain falls along coronal loops as clumps of plasma, at temperatures of 103–105 K, hundreds of times as cool as the coronal environment but hundreds of times as dense, as seen in the top figure. With the two instruments, the astronomers were able to follow the cooling evolution of loops and see that bundles of rain clumps often fall together, making the cooling of the corona incredibly effective. Moreover, they found that loops hosting coronal rain are widespread. Since only a special heating distribution leads to coronal rain, that may help astronomers to solve the coronal-heating problem.
Using the unprecedented resolution of SPICE, the team could also measure the size of a rain clump for the first time in EUV. Barely two pixels on the camera, clumps were measured as small as 260 km across. The researchers also noticed something else—an associated bright feature below a falling clump. While the clump of plasma is dark in EUV, the compression of the surrounding plasma and heat created forms an emission signal in the images, as seen in the bottom figure.
But it was the brightening, not the emission signal, that was the most important part of coronal rain. Much like meteors falling to Earth help astronomers understand our planet’s atmosphere, the brightening produced by the rain can help scientists understand the composition of the corona. Furthermore, when the rain hits the chromosphere, there is an upward splash. And when clumps of plasma fall at roughly 100 km/s—about four times as fast as rain on Earth—the rebound is significant.
The dark feature indicated by the black arrows corresponds to the extreme-UV absorption produced by a rain clump. The bright feature indicated by the red arrow is compression and heating produced by the rain clump as it falls.
P. Antolin et al., https://arxiv.org/abs/2305.11691
For the first time, astronomers were able to identify the rebound and shock flow as the clumps of falling plasma struck the chromosphere. They appear as bright, upward-propagating features. The shock partly reheats the coronal loop and may be part of the reason the corona is hotter than expected. Astronomers have long predicted that phenomenon, and the rebound observations give additional weight to the theory.
The coronal-heating problem isn’t solved with those new observations alone, but a better understanding of the morphology of coronal loops, how the rain forms and falls, and how much rain there is can be used to uncover how energy is transported between layers of the Sun and converted into heat. (P. Antolin et al., Astron. Astrophys., 2023, doi:10.1051/0004-6361/202346016 .)