According to a team of researchers led by Dr Eamon Scullion of Trinity College Dublin, rains on the Sun are made of plasma and fall at about 200,000 km an hour from the Sun’s corona (the outer atmosphere) to its surface.
This mosaic reveals evidence of a large-scale coronal rain shower pouring relentlessly into the dark sunspot at the surface of the Sun. The coronal rain appears as the labeled giant arching water-fall-like flow. At the top left, the Earth is depicted on the same scale. Image credit: E. Scullion / SST.
Solar physicists discovered coronal rains almost 40 years ago. But despite decades of research they have until now been unable to understand the physics of these rains.
Using NASA’s Solar Dynamics Observatory and ground-based observatories like the Swedish 1-m Solar Telescope, Dr Scullion and his colleagues studied coronal rains in great detail.
In June 2012, they observed a giant‘waterfall of solar material pouring down from the outer atmosphere of the Sun into a dark sunspot on its surface. Another set of images shows how a solar flare precedes a rain.
The results show that the process through which hot rains form on the Sun is surprisingly similar to how rains happen on our planet.
“Showers of rain and waterfalls on the Sun are quite something, though I wouldn’t recommend taking a stroll there anytime soon. But the parallels with weather on Earth are both striking and surprising,” explained Dr Scullion, who will present the findings at the 2014 National Astronomy Meeting in Portsmouth, UK, on 24 June 2014.
The scientists suggest a model of ‘catastrophic cooling,’ where an exceptionally rapid fall in temperature causes material to change from rarefied coronal gas to ‘raindrops.’
If the conditions in the Sun’s atmosphere are just right, then clouds of hot, dense plasma can naturally cool and condense and eventually fall back to the solar surface as droplets of coronal rain.
In another parallel with terrestrial weather, the material that makes up the hot rain clouds reaches the Sun’s corona through a rapid evaporation process. But here the evaporation is caused by solar flares, the most powerful explosions in the Solar System that are thought to help heat the Sun’s outer atmosphere.
The torrential rain storms, driven by solar flares, may play a fundamental role in controlling the mass cycling of the solar atmosphere and act as a kind of solar-scale thermostat in regulating the temperature fluctuations of the solar corona.
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Eamon Scullion et al. Flare-driven Coronal Rain with Ground (SST/CRISP) and Synoptic Space Observatories. National Astronomy Meeting, June 24, 2014
