The solar corona—the outermost layer of the Sun, which can be seen only during a total solar eclipse—is made up of superhot plasma. Coronal plasma continually blows away from the Sun, carrying the solar magnetic field with it. This steady stream of charged particles is the solar wind, which perpetually bombards Earth’s magnetic field, driving space weather such as geomagnetic storms (which disrupt electrical grids on Earth) and aurorae.
Despite extensive research into the solar wind and how it interacts with Earth’s magnetosphere, scientists still don’t fully understand how this interplanetary plasma works. One major mystery is its speed: The solar wind doesn’t just gently waft away from the Sun. It travels at speeds of 300 kilometers per second (slow wind streams) and even 700 kilometers per second (fast wind streams). How the solar wind accelerates to those speeds is an unsolved puzzle that has driven plasma astrophysics research for decades.
Any convincing theory of how the fast solar wind attains such high speeds must explain how energy is deposited at large distances from the Sun as well as why the plasma preferentially heats heavier ions over lighter electrons.
Two leading theories on the fast solar wind manage to explain its strange properties. The first, Alfvénic turbulence, points to the collision of oppositely traveling Alfvén waves, low-frequency plasma waves converging like two ripples on a still lake. The second, ion cyclotron waves, calls out high-frequency ripples in the plasma. Each theory has its own issues, and they don’t agree with each other. Alfvénic turbulence can explain the energy transport but fails to explain the preferential heating, whereas ion cyclotron waves do explain this feature but lack an obvious source.
A new study published in Nature Astronomy has sought to reconcile the two theories by applying the recently described “helicity barrier” effect.
Helicity Barrier: Hitting the Brakes in the Solar Corona
The as yet theoretical helicity barrier acts like a dam to the turbulent energy cascade in the solar corona, impeding the forward flow of energy from large to small scales: The energy gets stuck behind the barrier, builds up over time, and eventually overflows the dam to generate ion cyclotron waves that heat ions. Meyrand is a coauthor of the new study.
Jonathan Squire, lead author of the new study and also a physicist at the University of Otago, explained that the team used the “hybrid particle-in-cell” method, which follows billions of ions as they move around randomly and interact with electromagnetic fields. They simulated a tiny patch of the solar wind as it moved outward, stirring up the plasma at large scales to create imbalanced Alfvénic turbulence. This imbalance in turn formed a helicity barrier, which trapped the energy and caused it to increase over time. Energy eventually built up enough to power ion cyclotron waves that led to very efficient heating of protons, thus explaining both the energy transport and ion heating issues.
The results of the study closely match the range of in situ observational data and images captured by NASA’s Parker Solar Probe (PSP), a mission geared in part toward unraveling the mystery of the fast solar wind. “This is really exciting,” Squire said, “because it’s good evidence that the helicity barrier actually occurs in the solar wind, suggesting we’re on the right track.”
Stuart Bale, an astrophysicist at the University of California, Berkeley, and one of the principal investigators of PSP, agreed. “It’s an interesting new model…[and] does have some good concurrence with our data.”
Christopher Chen, an astrophysicist at Queen Mary University of London who wasn’t part of the current study team, also said that the key strength of the proposed model is how it nicely connects several pieces of observational evidence with some recent ideas in plasma turbulence. Chen added that “in coming years, we want to test this rigorously against other heating models using the latest observations from PSP to see if it [their theory] holds up.”
Squire added, “It’s always particularly satisfying when a number of different, seemingly unrelated observations can be explained by one theory.”
Bale, however, cautioned, that “we haven’t solved the problem yet…there’s certainly more to do.”
Squire concurred. “Our study is highly idealized,” he said. “We need more study of how the effect changes with important parameters, such as magnetic field strength, and how it behaves in the complex magnetic geometries of the corona and solar wind.”
As PSP moves closer to the Sun (it flew through the corona last December), the team hopes to do just that.
—Alakananda Dasgupta (@AlakanandaDasg1), Science Writer