discovery: rain on the sun

May 14, 2019

The Earth and the Sun share something pretty weird in common:

Wait for it… It. Is. Rain.

Researchers have detected “plasma rain” pouring down over the solar surface last month, which may also explain the big astronomical mistery:

Why the heck is the sun’s outer atmosphere so much hotter than the sun’s surface?

(Quick astro reminder: The visible surface of the sun has a temperature of over 5.500° C. Backing away from this inferno should cool things down, right? Nope, it doesn’t. Instead, the sun’s upper atmosphere, corona, scorches at millions of degrees — a temperature 200 to 500 times higher than that of the roaring furnace below.)

So, this amazing new plasma rain data, collected using high-resolution telescopes mounted on NASA’s Solar Dynamics Observatory, showed that coronal rain works similarly to rain on Earth — well, with a few exceptions. Compared to rain on our little planet, plasma rain on the sun is millions of °C hotter.

Millions.

The sun, a ball of hydrogen and helium that is constantly fusing those elements together, creating the heat that basically keeps us alive here on Earth. Our solar star is also a hotbed of magnetic activity that periodically vents out rivers of charged particles which, if Earth gets in the way, cause auroras in the high atmosphere and can even short out satellites.

Also, plasma, which is an electrically charged gas, doesn’t form a pound on the surface, like water here on Earth. Instead, the plasma traces the magnetic field lines, or loops, that emerge from the sun’s surface. However, instead of (average) 15 °C plain H2O, you have a million °C plasma which traces magnetic loops that emerge from the sun’s surface. That is a small yet hot hot hot difference between the two, though.

How does solar rain form?

At the above-mentioned loop’s foot points, the plasma is superheated to over million °C, then, it expands up the loop and gathers at its peak away from the original heat source.

As the plasma cools off, it condenses and gravity pulls it down the loop’s legs as coronal rain.

To be exact: The track that plasma follows up and out of the sun is kind of like a car on a roller coaster. At the peak of the loop — the top of the roller coaster — plasma is at its coolest, because it’s farthest away from the sun. At that highest point, some of it cools and falls back down to the sun as precipitation, just like rain on Earth.