- Paper 1:
- Title: Helioseismology challenges models of solar convection
- Authors: Laurent Gizon and Aaron C. Birch
- First Author’s Institution: Max-Planck-Institut fur Sonnensystemforschung, Germany
- Paper 2:
- Title: Anomalously Weak Solar Convection
- Authors: Shravan Hanasoge, Thomas L. Duvall Jr and Katepalli R. Sreenivasan
- First Author’s Institution: Dept. of Geosciences, Princeton University
While, in comparison to other stars, we know a lot about our sun, the closest star can still surprise us from time to time. Hanasoge et al. measure convection currents in the sun, finding that subsurface flows are 20-100 times slower than what is predicted in widely used theoretical models.
Why should we care about convection in the sun?
This discovery has wide-reaching ramifications. For instance, convection in the sun plays important roles in solar activity. It carries hot plasma up to the surface of the sun, where the plasma can interact with the sun’s magnetic field to generate stunning filaments, prominences, and coronal mass ejections (those links are movies). Convection might even help drive the solar magnetic cycle, an 11-year (or 22-year, if you consider magnetic reversals) cycle in which solar activity, as measured through the number of sunspots, decreases and then increases. But the farthest-reaching ramifications result from using knowledge of convection in the sun to make guesses about the convection patterns in other stars.
What is convection, and how does it work in the sun?
Convection is a type of fluid instability that occurs when a blob of fluid slightly less dense than its surroundings, if displaced upward or downward, will continue traveling in that direction. In the sun, convection can be characterized by the large scale pattern of hot gas rising from about 0.7 solar radii up to the sun’s surface, where is cools and then sinks again. Although on large scales this process is somewhat orderly, forming convective patterns called granules and super-granules that are visible on the surface of the sun, convection is turbulent on small scales, meaning that it becomes increasingly difficult to predict the motions of individual fluid blobs of progressively smaller sizes. Although convection is primarily an up-and-down motion, this turbulence can produce horizontal flows, including longitudinal flows, which move in the direction of changing longitude as opposed to latitude or radius.
How did Hanasoge et al. measure the sun’s slow convection?
Hanasoge et al. use time-distance helioseismology to map the longitudinal flow of convection currents just below the sun’s surface at 0.96 solar radii. Typical helioseismology measures global modes of pressure waves about the sun’s surface. On the other hand, time-distance helioseismology examines how sound waves propagate around a particular point on the surface, much like the seismology of earthquakes, or the spreading of ripples around a stone you throw into a pond (check out this video Dopplergram of the Sun for a visualization of this effect). The basic method of time-distance helioseismology is:
- Measure the Doppler velocities at many points on the surface of the sun as a function of time. At each point, the Doppler velocity versus time is the “wavefield.” This wavefield is illustrated in Figure 1.
- Pick a pair of points on the surface. Cross-correlate their wavefields over time.
- There should be a special time, the delay time, that maximizes the cross-correlation. This corresponds to the time it took the signal from one point to travel to the other point. A sound wave goes into the sun, reflects, and comes back up to the surface, so the delay time tells you about the flow in the layers the sound wave traveled through.
- Compare the delay times for different points on the sun’s surface. If you know the speed of sound at the various layers the sound wave traveled through, you can calculate the flow at those layers.
The authors find that the longitudinal flow at 0.96 solar radii is 20-100 times slower than models predict. For a comparison between the model and the theory, see Figure 2.
What happens next?
The Kepler Mission has resulted in a treasure trove of precise asteroseismology measurements of many stars other than our sun. It will be interesting to see what role, if any, the discovery of slow convection in our sun will have on our interpretation of the Kepler stars, and other stars that have been characterized by asteroseismology. Whenever the sun surprises us, we have to rethink the physics of stars!