Curtain-like plasma structures explain solar jets' apparent spinning in simulations

An international team of scientists from the University of Sheffield and the Indian Institute of Astrophysics, led by a postdoctoral research fellow from the University of Newcastle, Australia, has discovered that solar jets called spicules, which are 6–8 Mm long, take the form of warped, denser plasma sheets like pleated drapery.

The findings are published in The Astrophysical Journal.

These spicules have usually been interpreted as spike-like or cylindrical structures from two-dimensional plane-of-the-sky observations. In contrast, this work reveals that plasma jets exhibit a pleated, three-dimensional, curtain-like morphology composed of dense plasma structures, and that the observed rotational motion is the result of the two-dimensional projection of more complicated dynamics in the plane of the sky.

At any given moment, hundreds of thousands or even millions of spicules populate the region between the visible solar surface, called the photosphere, and the million-degree solar corona, the sun's outermost atmospheric layer. These jets shoot upward quasi-periodically against the sun's gravity, and most of the spicular material falls back toward the surface.

Solar swirls behind apparent spinning

The major finding is that hundreds of spiraling plasma columns, with diameters reaching hundreds of km, form along with the spicules in the solar atmosphere, exhibiting dynamics comparable to those of cyclones on Earth. The authors coined the term coronal swirling conduits (CoSCos) for these rotating structures, which often extend into the solar corona and connect multiple regions of the solar atmosphere. They are made of plasma, a very hot, magnetized, gas-like form of matter composed of electrons and ions.

Scientists have discovered that spicules are more dynamic and complex than previously thought. In addition to their upward and downward motion, they frequently exhibit apparent rapid spinning and transverse oscillations. While the jetting behavior and the oscillations are relatively well understood, the origin of this rapid spinning has remained a long-standing mystery.

The study found that these short-lived rotations are generated by prevalent plasma swirls wider than Great Britain. The swirls are thought to number many thousands or even millions in the solar atmosphere at any moment.

In this study, the authors use numerical simulations, run, for example, on the U.K. high-performance computing facility Dial/CSD3, to explain the spinning clusters of spicules, providing new insights into their structure and dynamics.

Dr. Sahel Dey (a postdoctoral researcher from the University of Newcastle, Australia) said, "The actual formation of the spicules and the associated rotating plasma draperies are currently very hard to observe directly, because they require ultrahigh-resolution telescopes for satellite observations from multiple vantage points."

Carrying energy into the corona

The mass and energy fluxes carried by the nonlinear waves and the associated spicules are important for heating the upper solar chromosphere and the low corona, and for supplying the plasma that forms the solar wind. The chromosphere is a relatively thin layer between the solar surface and the extremely hot corona. The solar chromosphere appears as a red ring around the sun during total solar eclipses.

Dr. Piyali Chatterjee (Indian Institute of Astrophysics, IIA) added, "These jet-induced CoSCos in our model draw energy from the thermal and magnetic energy reservoirs of the jets and transport it into the solar corona. However, their observational confirmation is yet to be achieved and will require high-resolution measurement of the coronal magnetic field and velocity field."

Professor Robertus Erdélyi (a.k.a. von Fáy-Siebenbürgen; Solar Physics and Space Plasma Research Centre (SP2RC), School of Mathematical and Physical Sciences (MPS), The University of Sheffield) said, "These rotating motions are everywhere in the universe, from sinking water in domestic faucets with a size of centimeters, to tornadoes on Earth and on the sun, solar jets and spiral galaxies with a size of up to 520,000 light-years.

"Our work has shown, for the first time, theoretical evidence of how these ubiquitous swirls in the solar atmosphere could generate a bunch of magnetic plasma waves that drive energy from the lower solar atmosphere into the low corona."

Erdélyi added, "It has been a fascinating question for the scientific community for a long while—how the sun and many other stars supply energy and mass to their upper atmospheres. Our results, as part of an exciting U.K.–India–Australia–Hungary collaboration, involving our very best early-career scientists like Dey, are an important step forward in addressing the supply of the needed nonthermal energy for solar and astrophysical plasma heating.

"We believe these Britain-sized coronal swirling magnetic plasma conduits are also very promising candidates not just for energy but also for mass transportation between the lower and upper layers of the solar atmosphere. Our future research with my colleagues at SP2RC will now focus on this new puzzle."