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When it rains, it pours… on the Sun

Last Updated on Tuesday, 24 June 2014 12:15
Published on Monday, 23 June 2014 15:41

Just like on Earth, the Sun has spells of bad weather, with high winds and showers of rain. But unlike the all-too-frequent storms of the UK and Ireland, rain on the Sun is made of electrically charged gas (plasma) and falls at around 200,000 kilometres an hour from the outer solar atmosphere, the corona, to the Sun's surface. And the thousands of droplets that make up a 'coronal rain' shower are themselves each as big as Ireland.

Scullion figure 1A mosaic of images obtained using the Swedish Solar Telescope (SST), revealing evidence of a large-scale coronal rain shower pouring relentlessly into the dark sunspot on the surface of the Sun. The coronal rain shower appears as the labelled giant arching water-fall-like flow. At the top left the Earth is depicted on the same scale. Credit: E. Scullion / SSTNow a team of solar physicists, led by Dr Eamon Scullion of Trinity College Dublin, have pieced together an explanation for this intriguing phenomenon, with imagery that shows a 'waterfall' in the atmosphere of the Sun. Dr Scullion will present their work at the National Astronomy Meeting (NAM 2014) in Portsmouth on Tuesday 24 June 2014.

Discovered almost 40 years ago, solar physicists are now able to study coronal rain in great detail thanks to state-of-the-art satellites like the NASA Solar Dynamics Observatory (SDO) and ground-based observatories like the Swedish 1-m Solar Telescope (SST). The scientists see regular and massive shifts in the solar 'climate', but despite decades of research have until now been unable to understand the physics of coronal rain.

It turns out that the process through which hot rain forms on the Sun is surprisingly similar to how rain happens on Earth. If the conditions in the solar 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 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 origins of solar coronal heating nonetheless remain one of the longest standing puzzles in solar physics.

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. Dr Scullion and his team have now developed a new insight into how coronal rain forms. Together with collaborators in Trinity College Dublin and the University of Oslo in Norway, he suggests a model of 'catastrophic cooling', where an exceptionally rapid fall in temperature causes material to change from rarefied coronal gas to 'raindrops'.

The team used images from the Swedish Solar Telescope based on La Palma in the Canary Islands, a telescope that produces some of the sharpest images of the Sun available. 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 have been assembled into a movie and show how a solar flare precedes a 'rain shower'.

Dr Scullion comments: "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."

 

Media contacts

NAM 2014 press office landlines: +44 (0) 02392 845176, +44 (0)2392 845177, +44 (0)2392 845178

Dr Robert Massey
Mob: +44 (0)794 124 8035
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Anita Heward
Mob: +44 (0)7756 034 243
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Dr Keith Smith
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An ISDN line is available for radio interviews. To request its use, please contact Sophie Hall via This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Science contact

Dr Eamon Scullion
Trinity College Dublin
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Image, animations and captions

https://www.ras.org.uk/images/stories/NAM/2014/Scullion_figure_1.jpg
A mosaic of images obtained using the Swedish Solar Telescope (SST), revealing evidence of a large-scale coronal rain shower pouring relentlessly into the dark sunspot at the surface of the Sun. The coronal rain shower appears as the labelled giant arching water-fall-like flow. At the top left the Earth is depicted on the same scale. Credit: E. Scullion / SST

https://www.ras.org.uk/images/stories/NAM/2014/Scullion_movie.mov
A movie made from a series of high-resolution observations (44-km per pixel image scale), taken over a period of 1 hour and 10 minutes using the Swedish 1-m Solar Telescope (SST) and showing coronal rain formation. The field-of-view is dominated by small-scale jets (spicules), as observed in the red-wing of the H-alpha 656.3-nm spectral line. A part of a C-class solar flare is seen as a bright flash in the top-left edge of the image (approx. half-way through the movie), which is followed by many dark threaded flows (i.e. coronal rain) in the second half of the movie, which appear to be falling back to the solar surface from bottom-right to top-left. The field of view in the image is 16,000 by 20,000 kilometres. Credit: E. Scullion / SST

 

Further information

Dr. Eamon Scullion is a Government of Ireland Post-doctoral Research Fellow (supported by the Irish Research Council) working with Prof Peter Gallagher also of the Trinity College Dublin, Ireland. He collaborated with Prof Luc Rouppe van der Voort of the University of Oslo in Norway.

This observational campaign was financially supported by SOLARNET.

 

Notes for editors

The RAS National Astronomy Meeting (NAM 2014) will bring together more than 600 astronomers, space scientists and solar physicists for a conference running from 23 to 26 June in Portsmouth. NAM 2014, the largest regular professional astronomy event in the UK, will be held in conjunction with the UK Solar Physics (UKSP), Magnetosphere Ionosphere Solar-Terrestrial physics (MIST) and UK Cosmology (UKCosmo) meetings. The conference is principally sponsored by the Royal Astronomical Society (RAS), the Science and Technology Facilities Council (STFC) and the University of Portsmouth. Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website and via Twitter.

The University of Portsmouth is a top-ranking university in a student-friendly waterfront city. It's in the top 50 universities in the UK, in The Guardian University Guide League Table 2014 and is ranked in the top 400 universities in the world, in the most recent Times Higher Education World University Rankings 2013. Research at the University of Portsmouth is varied and wide ranging, from pure science – such as the evolution of galaxies and the study of stem cells – to the most technologically applied subjects – such as computer games design. Our researchers collaborate with colleagues worldwide, and with the public, to develop new insights and make a difference to people's lives. Follow the University of Portsmouth on Twitter.

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter.

The Science and Technology Facilities Council (STFC) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory. Follow STFC on Twitter.

The Swedish 1-m Solar Telescope is operated on the island of La Palma by the Institute for Solar Physics of the Royal Swedish Academy of Sciences and the Instituto de Astrofisica de Canarias.