Research Highlights

Sitting 150 million kilometers away from the Earth, the Sun produces puzzling phenomena, like solar flares, that physicists are working to understand. One of these puzzles involves the Sun's tachocline, a belt of heat transition. “A tachocline is when the radiative interior of a star rotates like a solid ball, but the convection zone [an unstable outer heat layer in a star] rotates differently,” explained former JILA graduate student Loren Matilsky. “For geometric reference in the Sun, the outer 30% by radius is the convection zone, and the inner 70% by radius is the radiative interior.” Before leaving JILA to become a postdoctoral researcher at the University of California Santa Cruz, Matilsky collaborated with JILA Fellow Juri Toomre and his group at JILA to study the Sun's tachocline using computer simulations. In a new paper published in The Astrophysical Journal Letters, Matilsky and Toomre developed a new type of simulation, one where the tachocline is self-consistent and not artificially enforced, meaning that it arises on its own. According to Matilsky: “As far as we know, it's the first time this type of self-consistent tachocline behavior has been published for a fully nonlinear fluid dynamical global simulation.”

Of the many different objects in the galaxy, M-dwarf stars, also known as red dwarf stars, are of particular interest to astrophysicists. These small objects are the most common type of star in the universe and have unique properties. “If you lay out all of the different types of stars [in a plot of stellar color versus brightness] we can see, based on what color they are and how bright they are, [that] most stars fall on a line we call the ‘main sequence’,” explained graduate student Connor Bice. “That's where they are born, and they stay in that same spot for most of their lives. Down at the tail end of that [line] are red dwarfs. They're the least massive, the coldest, and the smallest type of main-sequence stars.” Bice is a researcher in JILA Fellow and astrophysicist Juri Toomre's group, and both he and Toomre have been looking at some of a red dwarf's unique properties, mainly their magnetic fields and convective flows. In a new paper published in the Astrophysical Journal, Bice and Toomre have found a link between the star’s convective cycles, or the heat cycles in a star’s atmosphere, and its magnetic fields, using fluid dynamics simulations.

Senior research associate Brad Hindman of the Toomre group uses helioseismology to understand what’s happening under the surface of the Sun. Helioseismology is a lot like the ultrasound tests used to evaluate medical conditions. However, there’s a big difference: physicians already have a good idea of the basic structures they are probing with sound waves. Helioseismologists don’t. They study sound that travels below the Sun’s surface to learn about the structure and behavior of the Sun’s convection zone, which comprises the outer third of the Sun. However, if they misinterpret the nature of the sounds they analyze, then they are likely to miss the mark in determining what’s happening inside the Sun.

Our comfortably middle-aged Sun completes a rotation once every 28 days. In contrast, young Sun-like stars spin much faster, sometimes whipping around 10 times as quickly. According to widely accepted theory, these young suns build magnetic fields in their convection zones by dynamo processes. Observations of these stars indicate strong magnetic activity.

Juri Toomre and his group concentrate their stellar research close to home--just 93 million miles away, to be precise. They want to answer the question: What dynamic processes occur deep within the Sun? To find out, they use a powerful combination of computer simulations and helioseismology (which analyzes sound waves produced by the Sun to probe its internal structure.) The researchers believe that working out the details of the Sun's internal structure should lead to explanations for the 22-year sunspot cycle and other regular surface features such as the Sun's consistent, but variable, rotation rate.