Fluid jets on bass speaker solve mystery of Sun’s ‘burning prairie’

The origin and nature of solar spicules – jets of plasma ejected from the Sun’s surface which early observers likened to prairie fires – have remained a matter of contention for astrophysicists since their discovery in 1877.

Researchers in India and the UK have now tried to demystify spicules using numerical computer simulation and table-top lab experiments with an audio speaker. They poured a polymeric fluid on the cone of a bass speaker and noticed that after a specific frequency, fluid jets flew off the speaker. Solar plasma is woven together by magnetic field lines, very similar to the long chains that weave these polymeric fluids, the scientists contend. The underlying physics of these seemingly dissimilar phenomena is the same, the Indian team, which conducted the speaker and numerical experiments says.

The UK scientists analysed high-resolution image sequences captured by the Interface Region Imaging Spectograph (IRIS), a NASA solar observation satellite with special filters, to create refined images of spicules. The Indian team then matched these images to their numerical simulation and with analogous fluid experiments in the lab. The plots generated by the fluid experiments in the lab and the numerical simulation matched closely, quantitatively and qualitatively, says lead author Piyali Chatterjee at the Indian Institute of Astrophysics in Bengaluru.

Her co-author Robertus Erdelyi, a solar physicist studying spicules at the University of Sheffield, told Nature India that the study unifies two different areas of physics — polymer and plasma — to show through mathematical modelling that similar forces are responsible for jet formation.

“The analogy is fascinating and demonstrates how very different physical environments can generate very similar phenomena,” Mihalis Mathioudakis, a Professor of Astrophysics at Queen’s University in Belfast, not involved in the study, told Nature India. Physicist Rajaram Nityananda at the Azim Premji University (APU) Bengaluru agrees that this is “definitely something new – a first announcement of a new approach to an old problem.”

The findings¹ challenge the current thinking about these enigmatic structures, says study co-author Murthy OVSN, an experimental physicist at APU.

Spicules: Spikes on the solar surface

Solar spicules are powerful jets of hot plasma propelled from the lower to the higher parts of the solar atmosphere, explains Mathioudakis. Plasma is the fourth state of matter, the other three being solid, liquid and gas. More than 99% of the known universe is composed of plasma, says Erdelyi.

Axel Brandenburg, Professor at the Nordic Institute for Theoretical Physics (NORDITA), Stockholm, says, the word spicules refers to the ‘spikes’ at the solar surface, best seen through a special type of optical filter, the H-alpha filter. “They are an old phenomenon, well known also to amateur astronomers.”

Mathioudakis says it is fascinating that solar spicules are about 10,000 times less dense than air at sea level on Earth. “Yet they produce some fascinating imaging.”

Scientists have suggested several theories for spicule formation including magnetic reconnection, the breaking and reconnecting of oppositely directed magnetic field lines in plasma; p-modes or pressure-driven oscillations at the Sun’s surface resulting from sound waves travelling through the solar interior; and Alfvén waves, a type of magnetohydrodynamic wave responsible for transfer of energy and information in plasma.

However, none of these models are sufficiently convincing.

Researchers have used intricate microphysics to explain this curious phenomenon, says Murthy.

“Even without using specialised microphysics, we were able to get not only spicules double the height of those obtained previously, but a forest of them,” notes Chatterjee.

From their experiments, the team found that four main elements are enough to form a forest of jets of solar spicules – a fluid medium, gravity, vibrations or oscillations and anisotropy or directionality.

Brandenburg says identifying these four minimum ingredients of spicule formation is a major achievement.

Mathioudakis and Brandenburg add a caveat: most of the study focuses on two dimensions. The team is now extending its model in a 3D environment.

Why spicules are important

The Sun’s outermost layer, corona, which lies beyond the photosphere and the chromosphere, is made up of very hot plasma. The coronal plasma continually blows away from the Sun, carrying the solar magnetic field with it. This is the solar wind, which continually bombards Earth's magnetic field, driving space weather events.

The origin of solar wind is one of the unsolved puzzles in modern plasma astrophysics, says Erdelyi. Spicules are believed to power the solar wind and are also thought to cause heating of the corona. How do the outer layers of the solar atmosphere manage to reach temperatures higher than the inner layers? "Spicules may hold the answer to this most puzzling problem in astrophysics", Mathioudakis adds.