If you think our weather is bad, wait until you have seen the weather on the Sun, where there are 'rainstorms' of plasma, each droplet the size of Ireland and produced by a cycle of heating, evaporation and cooling.

It rains giant raindrops of plasma on the Sun. Image: NASA/SDO.
 
"The parallels with weather on Earth are both striking and surprising," says Dr Eamon Scullion of Trinity College Dublin, who is presenting the research at the Royal Astronomical Society's National Astronomy Meeting at the University of Portsmouth this week.

The droplets fall towards the Sun's surface, known as the photosphere, at a rate of 55 kilometres per second and were discovered 40 years ago. However, an explanation had always eluded solar physicists. Using imagery from the one-metre Swedish Solar Telescope in La Palma in the Canary Islands, Scullion's team were able to solve the mystery by observing the conditions necessary for 'rainclouds' of plasma to form and then precipitate out.

Plasma close to the photosphere becomes super-heated by the enormous amounts of energy that are unleashed by solar flares, instigating rapid evaporation into the Sun's upper atmosphere, the corona. When conditions are right the plasma cools and condenses, falling back towards the photosphere as 'rain'. The process of heating and cooling releases energy into the corona, helping to keep it hotter than a million degrees Celsius. In that sense, the plasma rain acts like a solar thermostat for the corona.

An image of an active region on the Sun with sunspots. A 'waterfall' of coronal rain is visible pouring onto the sunspots. The scale of Earth is shown at the too left. Image: E Scullion/Swedish Solar Telescope.
 
The corona is also heated by phenomena called spicules, more commonly known as solar jets that shoot thousands of kilometres above the photosphere. Now, Dr Eon Lee of the University of St Andrews has developed a computer simulation that shows what happens in another kind of solar jet, known as a blowout. Presenting his model at the National Astronomy Meeting, he describes blowouts as being like miniature coronal mass ejections, which are vast eruptions of solar plasma into space that can cross Earth's path and interact with our planet's magnetic field. The blowouts have also been observed to interact with Earth's magnetic field, but on a much smaller scale.

The new model shows how the blowouts are triggered by outbursts of the Sun's magnetic field that occur at the base of the jets, causing them to carry twisted filaments of plasma between 10,000 and 100,000 degrees Celsius hot. Compared to the corona, however, these filaments are relatively cool. However, the blowouts also spark a process called magnetic reconnection, whereby magnetic field lines join. This drives hot plasma in the corona into the blowout, so that both hot and cooler plasma is ejected into space. Lee's computer model appears to fit very well with observations of the blowouts made with Japan's solar observing spacecraft, Hinode, finally solving the mystery of the miniature mass ejections.