Large amounts of matter are needed to create and sustain the massive plasma structures in the solar corona. New calculations reveal how this is possible.
The new computer simulations are based on a magnetic field structure that is often associated with prominences: the magnetic field lines in the corona form a double arc with a small dip in the middle. As the calculations show, the flame-like prominence forms in this dip and remains trapped there. All relevant layers of the Sun were taken into account, from the corona, the Sun's outer atmosphere, to parts of the convection zone below the Sun's surface. For an animated version, see the institutes website.
© MPS
To the point
- Large and cool: Prominences are cool plasma structures extending several thousand kilometers in the Sun's hot corona. Some persist for weeks.
- Fresh material supply: For prominences to form and remain stable, they need a constant replenishment of plasma. A new study provides insights into their "supply logistics." For the first time, the new computer simulations take into account processes in all involved layers of the Sun - including those below the surface.
- Magnetic field: The Sun's magnetic field is crucial. It lifts plasma from deeper layers into the prominences and directs the inflow through the corona.
- Predicting solar flares: To protect critical infrastructure such as power grids and satellites, it is essential to predict impending solar storms. This research lays the groundwork for future predictions.
Solar prominences come in a wide variety of shapes and sizes-sometimes even simultaneously, as this image of the Sun from February 11, 2014, impressively demonstrates. While most prominences resemble billowing flames, others take on a loop-like shape. Prominences at the edge of the Sun appear bright to the observer, while those on the solar disk appear dark.
© Sun: NASA/SDO/GSFC Visualization Studio/Virtual Solar Observatory; white frames: MPS
At more than one million degrees, the Sun's atmosphere, the corona, is incredibly hot. However, not everywhere. Time and again, huge structures of significantly cooler solar plasma - about 10,000 degrees - appear within the corona. These structures are known as prominences. They span up to several thousand kilometers and often resemble flickering flames that can take on a wide variety of shapes. Despite their delicate appearance, they are massive "chunks of matter": their density exceeds that of the surrounding corona by more than a hundred. In a sense, this is as if a giant mountain were suspended in mid-air. Prominences can remain stable for weeks or even months, yet they also possess explosive potential: if they do not fade away quietly, they culminate in a massive eruption during which the Sun hurls charged particles into space. If the particle cloud spreads toward Earth, it can trigger violent solar storms.
In the current publication in the journal Nature Astronomy, researchers at the Max Planck Institute for Solar System Research (MPS) in Germany investigate how prominences form and what the secret of their longevity is. Their findings reveal that multiple processes are at work, creating a constant balance between material loss and supply.
In complex computer simulations, the researchers model the interaction of magnetic fields and plasma within the Sun. In doing so, they consider not only the Sun's atmosphere, where the prominences manifest, but for the first time also the deeper, cooler layers of our star. There, beneath the Sun's visible surface, turbulent plasma flows generate the Sun's complex, constantly changing magnetic field, which extends even into the corona.
"In the Sun's atmosphere, the magnetic field is the driving force. It also plays a decisive role in all processes that contribute to the formation and maintenance of the prominences," says MPS scientist Lisa-Marie Zeßner-Ondratschek, first author of the new publication. Equally decisive is the temperature gradient within these layers. With a maximum temperature of 20,000 degrees, the lower solar atmosphere, the chromosphere, is significantly cooler than the corona; the underlying solar surface reaches just 6,000 degrees.
Trapped in the magnetic field dip
A sequence of images from the new computer simulations. The flame-like prominence extends up to about 20,000 kilometers into the corona and constantly changes shape. Occasionally, some of its material "rains" back down into the Sun's lower-laying layers (image D).
© MPS
For her calculations, the researcher focused on the smaller prominences, which extend "only" up to 20,000 kilometers into the corona. She assumed a magnetic field architecture that often accompanies prominences: the magnetic field lines take the form of a double arch in the corona. They resemble the two humps of a dromedary or two adjacent mountains in a mountain range. The prominence forms in the dip between the "humps."
As the computer simulations show, a kind of injection process sets the prominence in motion. Driven by turbulent, small-scale magnetic field movements, the chromosphere ejects bursts of cool plasma; this billowing blob remains trapped in the magnetic field dip in the corona. Then the sophisticated supply logistics of the prominences kick in: Although some of the cool plasma repeatedly "rains" back down into lower-lying layers, two processes compensate for the losses: Material is regularly ejected from the chromosphere; additionally - albeit to a lesser extent - hot plasma flows from the corona along the magnetic field lines into the dip, cools down, and "condenses" there.
"Our calculations show, more realistically than ever before, how both processes interact to supply the prominences with material and thus keep them alive," says Lisa-Marie Zeßner-Ondratschek. Earlier simulations, which had only taken the Sun's atmosphere into account, were mainly able to model condensation in the corona. The new publication thus closes a major gap in our knowledge and impressively demonstrates that processes within the Sun's interior are also crucial for understanding - and perhaps one day predicting - the eruptive nature of our star.
