Colin Roald

Affiliations

• Sturrock solar theory group
• Solar Oscillations Investigation
• Center for Space Science and Astrophysics
• Stanford University
• Yohkoh Soft X-ray Telescope

Research interests

Coronal heating. The outer atmosphere of the Sun is made of million-degree plasma (mostly ionised hydrogen), streaming outward at all times at hundreds of kilometres a second---it is this corona that becomes visible during eclipses. (It's actually there all the time, about as bright as the full moon; it's made invisible only by the overpowering glare of the body of the Sun itself.)

Nobody has a really satisfactory explanation for how the corona gets to be so hot. The surface of the Sun is "only" around 5500°C (5800 K), while the corona can be as much as a thousand times hotter. There are several competing theories, including heating by dissipation of sound waves or magnetosonic waves, and heating by frequent small flares (so-called nanoflares). Both these ideas suffer from the mundane problem that when we go look at the Sun, there doesn't appear to be enough waves or flares to actually do the heating needed.

Currently I'm working on tests-of-principle on an alternate idea: heating resulting from magnetic reconnection in the photosphere. The photosphere is threaded with tangled magnetic field lines, which are shuffled about all the time by the boiling turbulence of the sun's surface. When two lines collide, the crossing magnetic fields drive sharp currents which release energy, more or less like an electric stove element. But also in the right circumstances, a reconnection may release significant tension in the field lines, in which case the sudden relaxation would fling gas and radiate energy up into the corona.

I am trying to test this idea both theoretically, by running the numbers to see if we can expect enough energy to be available at all, and observationally, by looking for a correlation between magnetic field strengths in the photosphere and thermal x-ray emission from the corona.

Solar neutrinos. Neutrinos are subatomic particles so insubstantial that most zip through the earth without noticing that it's here. They're produced by the zillion in the nuclear burning deep in the solar core, and because of their insubstantiality, they're the only signal that can come to Earth directly from there, without being significantly obscured by the opaque mass of the Sun itself. (Energy transferred by photons takes 170,000 years to work its way through the dense gas to the surface, and thence 8 minutes to leap from there to the Earth!)

Neutrinos are, of course, extremely difficult to observe while they're passing by, but with application of enough statistics we can extract a little information from the few we do snag. For example, it seems neutrino counts vary with the rotation of the Sun, which implies some sort of structure in the deep interior that we "see" as the Sun turns. The counts also show a 155-day periodicity (which is not understood at all), but do not seem to correlate with the 11-year solar activity cycle.

Dynamo theory. In the solar cycle, approximately every 11 years, the Sun's magnetic field rises to a maximum, weakens, and reverses direction entirely. In sync with this oscillation, sunspot numbers rise and fall; flares, coronal mass ejections, and magnetic storms come and go; and the corona itself brightens, tangles up, dims and smooths out. My thesis work was done on models of magnetic field evolution deep in the solar interior---so-called dynamo theory---which is thought to be the real driver of all this solar-cycle variability.

Publications

Roald, C.B., 1999, "A two-layer \alpha\omega-dynamo model, and its implications for 1-D dynamos," to appear in the proceedings of the Workshop on Stellar Dynamos, 1998 September 28-30, Medina del Campo, Valladolid, Spain (preprint astro-ph/9904369).

Roald, C.B., 1998, "Nonlinear solar dynamo models with magnetic back-reactions," Ph.D. Thesis, University of Rochester, Rochester, NY.

Roald, C.B., 1998, "A two-layer \alpha\omega-dynamo model with dynamic feedback on the \omega-effect," MNRAS 300, 397

Roald, C.B., & Thomas, J.H., 1997, "Simple solar dynamo models with variable alpha and omega effects," MNRAS 288, 551

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