THE CONVECTION ZONE



Imagine that you are the captain of a new vessel specially built to explore the interior of the Sun. We call interior the part of the Sun which is below its surface and therefore is not directly visible to us. There you are, at the commands of your vessel, on the visible surface of the Sun, which is also called the photosphere. You switch the motors on, point the nose of your vessel toward the center of the Sun and begin your trip. Immediately, you feel that the whole vessel is being shacked continuously. You are in the convection zone. The vibrations you feel are due to the fact that the fluid (we call it plasma) which is around your vessel is in a very turbulent state. It feels as if your vessel had dived into a gigantic pot of boiling water. The temperature at the surface was about 5,700° C but it is increasing as you go deeper and you realize that your vessel will soon begin to melt. Your on board computers calculate that by the time you will reach the bottom of the convection zone, at a depth of about 200,000 km, the temperature will be 2,000,000° C. Way too much for the vessel. You decide that this is definitely too dangerous for your crew and order to go back to the surface.

It would be nice to be able to see the Sun in situ. Unfortunately, this is impossible. Instead, scientists are using telescopes, satellites and computers to study the interior of the Sun. In the following we explain what we have discovered in this way about the convection zone.


    Imaginary trip
Convection zone: where, what, how, why, ...
    1. Where is the Convection Zone Located
    2. What do we see from it
    3. What is it
        It is made out of plasma
        It is convecting (boiling)
        It is turbulent
        It is rotating
        It generates magnetic fields
        It transports magnetic fields
Why using computers to study the convection zone
    1. Challenge
    2. Virtual worlds
    3. Logical procedure
    4. One example: granulation
Learn more about some particular topics
Who studies the convection zone today




CONVECTION ZONE: WHERE, WHAT, HOW, WHY, ...

1. Where is the Convection Zone Located?

The interior of the Sun can be divided into three regions, depending on the kind of transport of energy which is in action: the core, the radiative zone and the convection zone:

Image Credit: Nasa Marshall Space Flight Center



2. What do we see from it

Click on the picture to see a bigger version of it
Image Credit: High Altitude Observatory
We can only see the surface of the convection zone (why?).If we point our telescopes towards the Sun, what we see is a white ball with some dark patches like in this picture. For more information about this picture click here. We call the dark patches sunspots. The amount of sunspots, their shape and their position change all the time. We will see below that the sunspots appear when bundles of magnetic field which where inside the convection zone break through the surface. To see more details of the surface we can zoom with the telescope.



click on some part of the image to see a movie The result looks like this. This picture has been taken by T. Berger using the Swedish solar telescope located on the top of an island called La Palma (one of the Canary Islands off of northwestern Africa).

In this picture, white regions are hot and dark regions are cold.



Several interesting features are visible:

click on some part of the image to see a movie The whole surface is covered with cells. They look very much like the cells you can see at the surface of a pot of boiling water. We call them convection cells because they are due to convection, the physical mechanisms responsible for the boiling water. The bright regions correspond to hot rising material, whereas the dark lanes are the location where the colder material falls down into the Sun. Click on the image on the right to see an animation of these convection cells as seen with the Dutch Open Telescope located on the Spanish island of La Palma.



click on some part of the image to see a movie In addition to the convection cells there are also these dark patches which are visible on the surface. The big ones like in the picture above are called sunspots and their diameter can be as big as the diameter of the earth. The small ones, like in the side picture, are called pores. Both indicate locations where concentrated magnetic fields (imagine bundles of rubber bands) are intersecting the surface of the Sun (How do we know that there is a lot of magnetic field in the sunspots?). Sunspots and pores are very dynamical features. Some times they are no sunspots on the surface, some times there are a lot of them. They are the manifestation of processes which take place in the interior where we cannot look directly.



Understanding what is going on in the convection zone by looking only at the surface is a very complicated task. Scientists have found two ways of doing this:




3. What is the convection zone?

It is made out of plasma. The convective zone, like the rest of the Sun, is made up entirely of plasma. A plasma is a 'gas' that conducts electrical currents, just like a wire does. The 'gas' contained in neon light bulbs is a plasma for example. The plasma in the convective zone is mainly made up of hydrogen (70% by mass), helium (27.7% by mass) plus small quantities of carbon, nitrogen and oxygen.


It is convecting (boiling). As we have seen above, the bottom of the convection zone is heated by the radiations coming out of the radiative zone, a bit like a room is heated by a radiator. The temperature at the bottom of the convection zone is 200,000° C. At the same time the top of the convection zone (surface of the Sun) is being cooled by the creation of light. The temperature at the surface is only about 5700° C. Thus, there exist a large temperature difference between the base and the surface of the convection zone.

This difference in the temperature results in a physical phenomenon called convection, which you are familiar with since you surely have seen a pot of boiling water. Initially, when you put a pot of water on the cooker, both the water at the top and at the bottom of the pot are at the same temperature, namely the temperature in your kitchen. Then you turn on the cooker and the bottom of the pot becomes hotter and hotter until, after a while, you see bubbles appearing at the surface. These bubbles are very similar to the cells observed at the surface of the Sun.

When a blob of water touches the bottom of the pot, it is heated very rapidly and becomes much hotter than the water at the top. Hot water is lighter than cold water (why is that?). So, our blob of water is lighter than the cooler water at the top. Remember that light 'stuff' always floats on top of heavy 'stuff' (as for example the marshmallows in hot chocolate). Thus, the blob of water rises toward the surface. At the surface of the pot, the contrary happens: the water in contact with the surface is cooled instead of being heated and therefore becomes heavier and sinks. The result is that individual blobs of liquid carry heat as they rise and then give up some of it before falling and picking some more. This is how convection transports energy from the top of the radiative zone (the heater) up to the surface of the Sun where light is formed (the cooler).


It is turbulent. The plasma in the convection zone is very much NOT viscous (what is more viscous: water, oil or air?). An immediate consequence of this is that the motions of the plasma in the convection zone (like the motions of water in the pot) become very complicated. This is referred to as turbulence. Turbulence is present everywhere in our life and is a fascinating and very actual subject of research. To learn more about turbulence click here.

The image below gives an idea of how complicated the motions in a turbulent flow can be. The picture has been generated by N. Brummell with a computer and represents the temperature (red is hot and black is cold) in a small slice of the convection zone. The complicated patterns give you an idea of how turbulent the flow is.

Temperature in a convection simulation
Image Credit: N. Brummell

This image illustrates how computers can be used to study the convection zone of the Sun. More about that below.


It is rotating. A big difference between the boiling pot of water and the convection zone is that the Sun is rotating. This affects the motions of the blobs of plasma. Instead of rising and sinking vertically, they go up and down in a swirling way. This effect of the rotation on the motions is called the Coriolis force and is also present on the earth: look at the motion of the water when you empty the tub after taking a bath. Does it swirl always in the same direction?

Another important aspect of the solar rotation is that, unlike the Earth, the Sun does not rotate like a solid body. Observations of its surface have revealed that the equatorial regions rotate faster than the poles. If you were standing on the solar equator it would take you 26 days to go round the Sun, while if you where standing close to one of the solar poles it would take you about 32 days. This is called differential rotation.

A lot of what we know today about the way the Sun rotates and other global motions in its interior have been discovered thanks to the helioseismology.


It generates magnetic fields. The interaction of the convective turbulent motions of the gas in the convection zone and the differential rotation leads to the generation of electric currents and solar magnetic fields. This process is called the solar dynamo mechanism. On the earth we use dynamos to produce electricity and we call them electricity generators. You find them in a car, on a bike and also in a power station. The basic principle is always the same: out of a rotational motion electricity and magnetic field are created. Click here to learn more about that.

The magnetic field generated in the convection zone has important properties:

It tends to agglomerate into bundles of magnetic field or magnetic flux tubes

It is buoyant. This means that it is lighter than its surroundings. The consequence is that once it has been created it tends to rise toward the surface.

Magnetic field have internal
tension like rubber bands It is elastic like a rubber band. Thus, if you try to push sideway on it (red arrows), its internal tension pushes back in the opposite direction (blue arrow).

Magnetic field lines are
always closed Magnetic field lines have no ends: the magnetic field always close on itself.

Magnetic field lines can
reconnect If a magnetic field line is deformed so much that in some place it makes an X, it can reconnect. That is how magnetic field lines can split or merge.

Because the flows in the convection zone are turbulent, the magnetic field generated by the dynamo action in the Sun has a very complicated structure. This is an example of it, again taken from a numerical simulation (click on the image to see an animation of it):

Click on the image to see an animation of it
Image Credit: MHD group of the University of Chicago

It transports magnetic fields. Once it has been created, the magnetic field is being moved and deformed by the convective flows. Eventually, a substantial amount of it is stored near the bottom of the convection zone in what is called the overshooting region. In this storage place, the magnetic field is aligned with the toroidal direction, a bit like a thin donuts.

How the magnetic field ends up stored in this manner is still a very mysterious problem. What we know is that the differential rotation plays an important role in this. Imagine you have a magnetic field line which is like a meridian (it is oriented north to South). Because the equator of the Sun rotates faster than the poles, it winds the field line around the Sun like it is illustrated in this picture:

Deformation of a meridional line by the differential rotation
Image Credit: Nasa Marshall Space Flight Center

In this manner two thin donuts of magnetic fields are created around the Sun: one slightly above the equator and another one slightly below the equator. Question: apart from the effect of the differential rotation, is there any other reason to believe that the magnetic field is stored more or less in this form? Answer = another question: do the active regions (new group of sunspots) appear in some special locations on the solar surface? Answer.

If a substantial part of the magnetic field generated by the dynamo tends to be stored near the bottom of the convection zone, then we have to explain how the magnetic field escape from this storage place and rise up to the surface. The explanation found by the scientists is summarized in this computer generated picture:

Magnetic tube rising through the convection zone
Image Credit: Peter Calligari, Fernando Moreno-Insertis and Manfred Schuessler

The yellow hemisphere represents the bottom of the convection zone and the transparent one represents its surface. The green line is a bundle of magnetic field lines which has risen from the bottom of the convection zone (where it was stored) up to the surface. The little superimposed picture with the two sunspots shows how the bundle of magnetic field lines appears to us when it emerges at the surface. This picture was generated with computers in an attempt to understand the mechanisms that govern the emergence of new magnetic regions on the surface of the Sun. It illustrates well how computers can be useful to study the interior of the Sun.




WHY USING COMPUTERS TO STUDY THE CONVECTION ZONE

1. The challenge

We cannot look inside the convection zone. As we have explained above, we only see the surface of the Sun. The surface is not quiet at all. If you look at it for a while you discover motions everywhere: granulation, new sunspots appearing, oscillations, ... This surface activity is difficult to understand because most of it is the manifestation of something which happens inside but that we cannot see. Thus, the challenge is to understand what happens inside the convection zone by looking only at the surface.

2. Virtual worlds

One way of going around this problem is to use computers. Who says computers, says virtual worlds. Indeed, a nice thing about the computers is that they are very good at representing virtual worlds. Thus, if we were able to construct a virtual Sun in the computer, then we could study this computerized Sun. The advantage is that because this Sun would be in the computer we would be able to see what is going on anywhere we want in its interior. Neat, isn't it?

This plan seems to be very straightforward but there is a little problem. How do we make sure that the Sun in the computer is reasonably similar to the real one? In effect, if the virtual Sun has nothing to do with the reality, then studying this computer generated Sun does not tell us a lot about the real one. Speaking in terms of computer games, we want our virtual world to be like the one you find in a flight simulator, not like the one you find in a fantasy game: in a flight simulator if you make a bad maneuver your virtual plane may fall and crash; in a fantasy game, the hero may be able to do impossible things, like jumping higher than a 20 stores building. The main difference between the two games is that the flight simulator is based on the laws of physics, whereas the fantasy game is based on the imagination of its authors. So, we have to make sure that our virtual Sun is based only on the laws of physics and that it has nothing to do with fantasy. How do we do that?

3. Logical procedure

First it is important to realize that to describe a given process, some physical ingredients are more important than others. For example, if you are writing a football computer game, you need to include correctly the effect of gravity on the ball so that its trajectory looks realistic. If you want to be more refined, you can also include the effect of the resistance of the air. The difference, though, with the motions calculated using only gravity is not so important. In this sense, gravity is more fundamental than the resistance of the air: if you forget about the effect of the air, your motions are approximately correct, if you forget about gravity, they are very different from reality.

When we construct our virtual Sun in the computer we do not want to include all the physical ingredients at once. First of all, because no computers is big enough and fast enough to tackle all the complication of the Sun at once. Secondly, because we want to discover which are the important physical processes which govern the dynamics in the interior of the convection zone, and which ingredients are secondary.

Imagine that we have observed the surface of the Sun and that we have noticed something intriguing that we would like to understand. To do this we proceed by try and error in an organized way:

Scheme of the logical process
  1. Taking into account our knowledge of the universal laws of physics, we make a prediction about which physical mechanism could cause what we see at the surface of the Sun.
  2. Then, we use the computer to create a virtual Sun (or part of the Sun -- like the convection zone) which obeys this mechanism that we just predicted.
  3. Once we have the new virtual Sun, we compare its surface with what is observed on the surface of the real Sun.
  4. Then there are two possibilities:
    • The two surfaces are similar. This indicates that the physical ingredients that we are using may not be to far from reality. Thus, we might have discovered something new about the Sun.
    • The surfaces are different. This indicates that either the mechanism we thought about has nothing to do with the Sun, or the mechanism is correct but something else happens: we did not put the right amount of this effect in our model; maybe there are some other more important physical ingredients in play that we did not think about; ... we need to begin again in 1.
We may have to go through the logical circle a lot of times in order to try a new idea or adjust the parameters of a mechanism that we think could be a good explanation of something which really happens in the convection zone. But this is not a problem because computers are perfect for this kind of repetitive job:

4. One example: granulation

One problem for the study of which computers have been used extensively is the solar granulation. When we look at the surface with a telescope we see something like in the picture A in the image below.

Observation of granulation
Image Credit: Nic Brummell

Zooming in, the surface of the Sun appears like in the picture C. This looks very much like the surface of a convecting liquid and. So, it suggests that convection could be the mechanism responsible for the motions in the this part of the Sun. In order to test this idea we can use the computer in the following manner.

First we built in the computer a convecting layer. We make a box in which we put a liquid which has properties similar to those of the solar plasma. We heat the box from below (we tell the computer that the bottom has a very high temperature) and we cool it at the top (we tell the computer that the surface temperature is much smaller than the bottom one). Then, we give to the computer the laws of physics that it will use. We tell the computer: given a blob of liquid, this how you calculate its speed, this is how you calculate its temperature, ... etc.

Once this is done, we tell the computer: go ahead and begin calculating how the liquid in the box is evolving. If we have made no error, after a little while the liquid in the virtual box begins to boil, or to convect.

The next step is to compare the surface of our virtual box with the image of the surface granulation on the Sun. What we see in our computer is this (click on the image to see an animation of the simulation).

Simulation of granulation
Image Credit: T. Emonet & F. Cattaneo

They look kind of similar. This is nice because it indicates that we are on the right tracks. Now we can begin to ask some details about convection and we can answer them by looking at our numerical simulation. For example: what does determine the horizontal size of the cells? How does the temperature look inside?, ....


LEARN MORE ABOUT SOME PARTICULAR TOPICS




WHO STUDIES THE CONVECTION ZONE TODAY