Seismology of sunspots reveals hidden gaps in their magnetic structure
An international team of scientists from the US, Spain, Colombia, and Australia has used a powerful diagnostic tool, similar to SONAR, to study the magnetic regions on the Sun's surface. Their research confirms a long-standing theory about the complex structure of sunspots, which are not solid as they appear but are full of gaps beneath the surface.
Dr Alina Donea from Monash University's School of Mathematics said: "Through regular telescopes, sunspots look like solid, dark patches on the Sun’s surface. But just beneath the surface, they are not solid at all - they're broken up, full of gaps, and far more complex."
Sunspots are the visible signs of powerful magnetic fields. Where a concentrated bundle of magnetic energy pushes through the surface, it forms a magnetic pole. These strong fields block the flow of hot gas, making the area cooler - and darker - than its surroundings.
"What happens to the heat that sunspots block is still a big mystery," Dr Donea said.
In 1979, leading solar physicist E. N. Parker suggested a bold idea: the missing heat is converted into waves that dive into the Sun, carrying their energy with them deep into the solar interior, to return later - after the sunspot fades. He believed that just beneath the surface, what looks like a solid magnetic field is actually full of gaping holes - broken into many narrow strands only a few hundred kilometers deep.
Now, solar scientists E. G. Broock (Spain), A. M. Cifuentes (Colombia, PhD student at Monash), A.-C. Donea (Australia), and C. Lindsey (USA) have applied a powerful diagnostic algorithm, computational helioseismic holography, to seismic observations from the Helioseismic and Magnetic Imager (HMI) aboard NASA’s spaceborne Solar Dynamics Observatory (SDO).
This technique creates sound-based maps of the layers under sunspots - reaching about a thousand kilometers deeper than what regular telescopes can see from above.
"The inverted landscape shown by these acoustic maps is radically different from the seamless magnetic monoliths our conventional telescopes show us of the sunspot photosphere,” said Dr Lindsey.
It shows sunspot sub-photospheres strewn with multiple compact 'acoustic anomalies' some 4000 kilometers across, packed into their umbrae and inner penumbrae in a layer extending between 500 and 1000 kilometers beneath their overlying photospheres.
These anomalies are called 'strong acoustic scatterers'. The researchers’ finding offers convincing support for Parker’s hypothesis that the magnetic monoliths we see covering sunspot photospheres are fragmented into multiple strands.
The stakes in this discovery are considerable. A huge amount of heat is constantly flowing into the sunspot cross-section. If that heat were not handled properly, it could strongly affect how sunspots evolve.
"If the wave-converting structures that Parker suggested really exist, they could play a key role in keeping sunspots together—and maybe even how they come together in the first place,” Mr Martinez-Cifuentes said.
The helioseismic maps reveal that strong acoustic scatterers are more common than just in large sunspots. This suggests they may play a role in the overall evolution of magnetic regions, which could affect the potential of their associated magnetic configurations to flare or erupt into coronal mass ejections, ones that might hit Earth.
"The data the SDO is giving us is reigniting an important international discussion about the behaviour of sunspots, their role in heat management of magnetic regions, and how this influences the dynamics and evolution of solar activity Dr Donea said : “Listening to the Sun is amazing - it reveals seismic clues about hidden weaknesses in sunspots.”