What do plasma jets in the Sun have in common with paint dancing on top of an audio speaker? Quite a lot, it turns out!
Led by scientists in Bengaluru, a team of interdisciplinary researchers from India and UK, have explained the origin of 'spicules' on the Sun, using laboratory experiments as an analogy.
Jets of plasma - the fourth state of matter consisting of electrically charged particles - occur just about everywhere in the Sun's chromosphere, which is the atmospheric layer just above the Sun's visible surface. These jets, or spicules, appear as thin grass-like plasma structures that constantly shoot up from the surface and are then brought down by gravity. Modelers have struggled to match the observed characteristics of a forest of such spicules despite invoking exotic microphysics. The authors have proposed a simple mechanism and a set of four conditions under which such a forest of spicules can occur- an almost periodic but strong buffeting against gravity in a fluid whose behavior depends on direction in space. The amount of energy and momentum that these spicules can carry is of fundamental interest in solar and plasma astrophysics, where the processes by which plasma is supplied to the solar wind, and the solar atmosphere is heated to a million degrees celsius, still remains a puzzle. In trying to explore the underlying physics of spicule dynamics, they turned to an improbable device: an audio speaker. A bass speaker responds to excitation at low frequencies like the rumbling sounds you hear in movies. When you place a liquid above such a speaker and turn on the music, so to speak, the free surface of the liquid becomes unstable beyond a particular frequency and starts vibrating. A beautiful example of Faraday excitation observed in nature is when droplets of water splashes on the back of a partially submerged male alligator during mating display. However, a fluid like paint or shampoo will result in unbroken jets when excited on a speaker since its long polymer chains give it a directionality. The authors of the article realized that the physics underlying these paint jets must be analogous to the solar plasma jets. They then asked what it would take to generate such jets in plasma? The solar plasma can be imagined as threaded by magnetic field lines, much like the long chains in polymer solutions like paint or shampoo. This makes both the systems anisotropic, with properties varying with the direction in space. Mathematically too, there exists an analogy in the treatment of stresses involved, though there are obvious differences as well.
Spurred by the visual similarity between the solar spicules and the jets of paint on the speaker, they investigated the roles of magnetic field on the Sun using state-of-the-art numerical simulations of the solar plasma. In parallel, the role of polymer chains by using slow motion videography on Faraday waves in polymeric solutions was captured. They found that the jets are kept intact against instabilities by the magnetic field in the Sun, and by the polymer chains in the polymeric solution respectively.
The findings appeared in the journal Nature physics
The team from Bengaluru, India includes: Mr. Sahel Dey, a Ph.D student at IIA and IISc, Dr. Piyali Chatterjee from IIA and Dr. Murthy O. V. S. N. from the Azim Premji University. The team from the UK comprises Dr Marianna Korsos from Aberystwyth University and Drs Jiajia Liu and Chris Nelson from the Queen's University Belfast, and Prof. Robertus Erdelyi from the University of Sheffield, UK.