Solar Sleuths Solve 60-Year-Old Mystery of Sun’s Magnetic Waves

Sun's Chromosphere

 

“This new research opens the door to providing a new understanding of the mystery surrounding the Sun’s magnetic waves. This is a crucial step towards explaining the coronal heating problem—where the temperature a few thousand kilometers from the surface—is hotter than the heat source itself,” said Dr. Ben Snow, from the University of Exeter and a co-author of a new study that led to the ground-breaking discovery of why the Sun’s magnetic waves strengthen and grow as they emerge from its surface, which could help to solve the mystery of how the corona of the Sun maintains its multi-million degree temperatures.

For more than 60 years observations of the Sun have shown that as the magnetic waves leave the interior of the Sun they grow in strength but until now there has been no solid observational evidence as to why this was the case.

The corona’s high temperatures have also always been a mystery. Usually the closer we are to a heat source, the warmer we feel. However, this is the opposite of what seems to happen on the Sun—its outer layers are warmer than the heat source at its surface.

Scientists have accepted for a long time that magnetic waves channel energy from the Sun’s vast interior energy reservoir, which is powered by nuclear fusion, up into the outer regions of its atmosphere. Therefore, understanding how the wave motion is generated and spread throughout the Sun is of huge importance to researchers.

The team, which was led by Queen’s University Belfast, included 13 scientists, spanning five countries and 11 research institutes including University of Exeter; Northumbria University; the European Space Agency; Instituto de Astrofísica de Canarias, Spain; University of Oslo, Norway; the Italian Space Agency and California State University Northridge, USA.

The experts formed a consortium called “Waves in the Lower Solar Atmosphere (WaLSA)” to carry out the research and used advanced high-resolution observations from the National Science Foundation’s Dunn Solar Telescope, New Mexico, to study the waves.

“This new understanding of wave motion may help scientists uncover the missing piece in the puzzle of why the outer layers of the Sun are hotter than its surface, despite being further from the heat source,” said Dr. David Jess from the School of Mathematics and Physics at Queen’s.

“By breaking the Sun’s light up into its basic colors, we were able to examine the behavior of certain elements from the periodic table within its atmosphere, including silicon (formed close to the Sun’s surface), calcium and helium (formed in the chromosphere shown at the top of the page where the wave amplification is most apparent).

“The variations in the elements allowed the speeds of the Sun’s plasma to be uncovered. The timescales over which they evolve were benchmarked, which allowed the wave frequencies of the Sun to be recorded. This is similar to how a complex musical ensemble is deconstructed into basic notes and frequencies by visualizing its musical score.”

The team then used super computers to analyse the data through simulations. They found that the wave amplification process can be attributed to the formation of an ‘acoustic resonator,’ where significant changes in temperature between the surface of the Sun and its outer corona create boundaries that are partially reflective and act to trap the waves, allowing them to intensify and dramatically grow in strength.

The experts also found that the thickness of the resonance cavity -the distance between the significant temperature changes—is one of the main factors governing the characteristics of the detected wave motion.

“The effect that we have found through the research is similar to how an acoustic guitar changes the sound it emits through the shape of its hollow body,” said Jess. “If we think of this analogy we can see how the waves captured in the Sun can grow and change as they exit its surface and move towards the outer layers and exterior.”

Source: David B. Jess et al. A chromospheric resonance cavity in a sunspot mapped with seismology, Nature Astronomy (2019).

The Daily Galaxy via Queen’s University Belfast