One of the deeply confusing aspects of our Sun (and other stars) is their temperature structure. Starting in the core, the Sun is millions of degrees kelvin and supports nuclear burning. As you leave the nuclear burning core and climb first into the radiative zone and then the convective zone, the temperature systematically drops until it reaches a temperature of several 1000 degrees at a star’s surface. This makes sense. In the core, the gas is being compressed under the pressure of all the upper layers of the star gravitationally pushing down. The pressure allows nuclear reactions to release energy in a form that can heat things up: specifically light. That light then interacts with stellar material, being absorbed and reabsorbed over and over as it loses energy and goes on a random walk through the radiative region (think light bulb heating the air around it), and then (think of the lava lamp material above a light bulb) it also gives off energy as it heats cells of material at the base of the convective zone that rise and convectively give off heat as the cells rise (and then, when cool, sink back down).
So far so good.
The problem is, as you then move away from the surface of the Sun, you enter regions where the temperatures again go up – A lot – like back to millions of degrees hot levels of a lot!
And no one fully knows why. This is a very counter intuitive situation. Imagine that the surface of a lava lamp was 23C and the air half an inch away was 200C! In a press conference Wednesday, astronomers announced that they think they may have found a starting point for understanding what is going on in this bizarre situation.
In a pair of presentations given by Bart De Pontieu (Lockhead Martin Solar and Astrophysics Laboratory) and Scott McIntosh (Southwest Research Institute) it was shown that a combination of sound waves and magnetic fields can channel energy (and heat is a form of energy) into the Sun’s Chromosphere. In their models, they find that sound waves propagate through the convection zone, and the energy within the sound waves can escape in locations where broken magnetic field lines form solar spicules (a time of flame shaped thing). The sound waves trigger shocks that super heat fountains of material that is ejected into the chromosphere. When they compared their models to actual high-speed images they found excellent correspondence between modeled expectations and reality.
During the press conference’s questions session, one of the journalists asked, (to paraphrase), “Why should we think that 10 years from now we’ll be saying that the question of ‘Why is the Chromosphere so hot’ was definitively answered in 2007?” While that may sound like a really obnoxious question for a generally well-behaved room full of science writers, it was actually a really honest question. We still don’t fully understand the Sun’s magnetic field or exactly what causes the field lines to break and reconnect is a bit hairy to try and understand and model. We still don’t fully understand how convection works in the Sun either. We are incrementally building better and better tools for modeling what we observe, but our theoretical models include lots of assumptions. To say we can definitively announce anything that includes both magnetic fields and convection is, um, optimistic.
But this is a start. I honestly think that 10 years from now, as we continue to build a fully refined understanding of what is going on, the papers written on the results shown in this press conference will be cited. Tomorrow’s understanding builds on yesterdays results. Sometimes science goes in leaps of ingenuity. This is not one of those times, but it is still solid science.
Next Up: Tidal Streams…