For laymen, the project sounds very curious: For months Nasa researcher Emily Mason has searched with satellite images on the sun for rain. And she found it. Although she discovered no water - but it rains on the star but waterfall plasma. Mason's discovery could help solve a 70-year-old puzzle about solar surface temperatures.

Astronomers have known for some time that it rains on the sun. However, they had previously suspected the rain in the huge magnetic loops, whipping over the sun's surface a million kilometers into space. They are so huge that you can even see them from the earth during a solar eclipse.

Also Mason first evaluated images from this region. For five months, she analyzed data collected by the Solar Dynamics Observatory in the United States over several years. She found nothing, not a single drop of plasma. Finally, she discovered the rain by chance in much smaller, just under 50,000 kilometers high plasma bows, as she reports together with colleagues in the journal "Astrophysical Journal Letters".

NASA's Solar Dynamics Observatory / Emily Mason

"It's raining for hours in the smaller magnet loops," says Mason. The system works much like rain on earth. In memory of:

  • On our home planet, the water is stored in the ocean, lakes and rivers.
  • The sun warms it up so it evaporates.
  • In the atmosphere it cools down again and condenses.
  • Clouds emerge from which it finally causes the earth's gravity to rain.

Thousands of degrees hot plasma raining on the sun

On the sun the whole thing goes like this:

  • The plasma follows the magnetic loops like a looping roller coaster tracks.
  • At the lowest point where the plasma strikes the solar surface, it heats up from a few thousand degrees Celsius to about a million degrees and travels up the magnetic loop.
  • On the way to the outer point of the loop, a few kilometers away from the heat source, it cools and condenses.
  • The gravitation of the sun finally pulls the material back to itself - it is raining.

"Instead of cool water like on Earth, you still have to deal with several thousand degrees of hot material," says Mason. The animation above shows the previously neglected, relatively small but rainy plasma rings.

Miloslav Druckmüller / Vojtech Ru¿in / Úpice observatory

Solar Eclipse: On the left side a big plasma stream can be seen

The discovery also provides clues as to why the temperature at the solar surface is lower than in the outer layer of its atmosphere, also called a corona. Intuitively, one would assume that it gets colder the further one moves away from the sun's surface. However, in the corona temperatures of up to 5.6 million degrees Celsius prevail, whereas on the sun's surface the temperature is only around 5000 degrees.

To date, researchers can not explain the phenomenon exactly. The new findings now at least bordered on the area in which the corona could be heated, says Mason. Because it can rain in a magnetic loop only if the temperature difference between the lower and upper part of the loop is very large.

Plasma jets could contribute to the solar wind

"When it rains above the sun, it means that the plasma in the lower ten percent of the magnetic loop is extremely heated," explains Mason. "We still do not know what exactly heats up the corona, but it has to happen in this area."

Tom Bridgman / NASA Solar Dynamics Observatory / Scientific Visualization Studio

Magnetic loop holds the plasma on its path

The scientist also suspects that the sun's rain contributes to slow solar winds, that is, to the stream of charged particles that the sun constantly emits into space. The reason: Mason discovered the rain system not only in closed magnetic loops, but also on magnetic lines that protrude into space and have only one end contact with the sun's surface.

It assumes that the plasma first heats up in a closed magnetic loop and then transitions to an open magnetic line. This is possible when the magnetic field around the sun changes abruptly, which is more common. Part of the plasma at the magnetic line then cools off and rains down to the sun. The rest part escapes into the universe and could be a part of the solar wind, so the theory.

The researchers hope they will be able to validate the thesis in the next few years using the "Parker Solar Probe" probe. This flies as close to the sun as no other probe before and will also investigate slow solar winds.