Coronal Rain

Rain on the Sun? Dr. Patrick Antolin, from University of St Andrews (UK), gives us an explanation for this amazing phenomenon in our latest #TheScienceOfEST post.



Movie of coronal rain on AR 12367. Credit: IRIS (NASA, LMSAL)


What if I told you that it rains on the Sun? “Yet another crazy scientist” — would be your answer!

Well, probably both are true. It does rain on the Sun! But it is a special kind of rain. Certainly not water, but plasma in a catastrophic cooling and condensation state. It is a phenomenon that we call coronal rain, a cousin of the famous prominences, and it is linked to the mechanism of thermal instability, which leads to the recombination of the free electrons in the fully ionised plasma of the solar corona.

The solar corona is the most external layer of the solar atmosphere and it is famous for its million Kelvin temperatures. Mysteriously, like snow flakes in the oven, the hot corona hosts large amounts of this hundred times cooler and denser material called coronal rain. Among the images or movies about the Sun that have taken your breath away chances are your favourite one is about coronal rain. This spectacular phenomenon is seen as cool material seemingly appearing out of nowhere and streaming down along magnetic loops that compose most of the solar corona.

One of the most striking examples of coronal rain occurs during the decay phase of solar flares, when the flaring, overly dense loops cool down and show the cool material raining down along their legs.

Many mysteries still surround this phenomenon. Why is it clumpy and stranded? It seems ubiquitous over active regions, but how much of the solar atmospheric mass cycle does it actually represent? How likely is it that a loop will exhibit coronal rain during its lifetime? What is its link to the spatiotemporal properties of the coronal heating mechanisms? These and many other important questions make coronal rain a prime target for next generation instrumentation such as the European Solar Telescope. 

The movie shows observations of active region 12367 near the solar limb and the associated coronal rain. The data were taken with the IRIS slit-jaw camera in the 1400 A filter on 2015 June 11. This filter samples plasma at upper chromosphere/transition region temperatures.


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