Sunday, April 7, 2013

The Sun Part 5: How The Inner Layers Work


The energy from fusion in the Sun's core travels outward and eventually leaves the Sun altogether in the form of electromagnetic (EM) radiation.  Protons, electrons and neutrinos also leave the Sun's surface, with a great deal of kinetic energy. These particles plus the EM radiation is what solar wind is made of and it eventually reaches far past the orbit of Pluto .

Gamma Ray's Long Journey

How does electromagnetic energy, originating in the core, make its way outward through the thick dense hot plasma of the Sun? In the Sun's core, energy from fusion dissipates outward into the surrounding radiative zone, the thick orangey yellow layer in the diagram below.

The plasma surrounding the core is so incredibly dense that gamma rays emitted during the fusion reaction can hardly travel anywhere before they are absorbed by electrons. These electrons then re-emit the photons in all directions, with each re-emission taking a having ever so slightly lower energy because each collision absorbs a tiny bit of it. Because the radiative zone cools in an outward gradient, net energy eventually moves outward, but these countless absorption-reemission events and the indirect path taken should slow the net movement of energy greatly. Exactly how much these processes slow the movement of photons outward is still up for debate and estimates vary widely.

Some online courses mention another factor that may slow down outward photon movement.  In the core environment, the energy is so intense that matter and light exist close to thermodynamic equilibrium. This could slow down the outward dissipation of energy even more. Nuclear fusion creates positrons as well as gamma rays. The positrons react with electrons in the plasma, annihilating each other and creating even more gamma rays, and possibly interfering with the slow outward absorption-emission progress outward. The reaction, an electron (e-) annihilating with a positron (e+), creating two gamma photons (γ), and vice versa, is shown by a double arrow, below.

γ + γ ↔ e+ + e-

However, even in the center of the Sun, temperatures are not likely to be high enough to make the reaction above truly reversible. A temperature of 1010°C is required, and the core is about 16 x 106°C, so the reaction is favoured to the right. It seems likely, then, that no new electrons are created in the Sun's core, affecting net outward photon movement. However, because new positrons are created, additional gamma rays flood the core plasma and these photons may impact net photon movement.

The above reaction is quite interesting. At everyday energies, you will never see an electron annihilating with a positron and emitting gamma radiation because at these low energies, the reaction is very strongly favoured to the right, but at intense energies, such as inside a collider, many positrons are often emitted and these quickly annihilate with electrons and release gamma radiation. What is interesting is that even though at everyday energies the reaction favours both electrons and positrons (everything to the right of the arrow), you will not see any positrons. This is part of a big unanswered question in physics: Why isn't there antimatter in our current universe? I invite you to explore this question in the Scientific Explorer article, The Hadron Epoch. To conclude this reaction discussion, inside the intense heat and pressure in the solar core, the reaction only weakly favours the production of photons, if it does at all, so the reason it takes a very long time for energy to actually move outward through the core is mostly due to countless absorption-emission events and very convoluted zigzag paths photons would have to take because of absorption and re-emission in random directions.

All these factors slow the movement of radiation outward so much that most physicists estimate it takes about 20 million years for a photon created in the core to make it the surface (although there is debate as I mentioned). From there it takes a mere 8 minutes or so to reach Earth, as sunlight, because almost no collisions interfere with the photon trajectories in the near-empty space in between. 20 million years is also how long physicists estimate it would take the Sun to cool off to a stable state if nuclear fusion suddenly stopped.

Each gamma photon is ultimately converted into several million much lower energy photons that eventually escape the surface of the Sun, and strike Earth.  Almost all sunlight photons range in wavelength between 100 nm (nanometers) and 1 mm. Sunlight is mostly made of visible photons, but significant ultraviolet and infrared photons contribute as well, resulting in a fairly short band (between the red lines) in the EM spectrum shown below.

The graph below shows the radiation spectrum of sunlight at the top of the atmosphere as well as at sea level. Although the graph cuts off at both ends, small amounts of higher energy photons such as X-rays and very low energy photons such as radio waves also come from the Sun. We will look at the origin of X-rays in a moment.
Robert A. Rohde;wikipedia
The Sun Glows White-Hot

This distribution is similar to the distribution of photons created by a black body at about 5500°C, the Sun's surface temperature, and that spectrum is basically what you see above. Both radiation and heat come from nuclear fusion. This means that all the gamma rays produced in the Sun's core and transformed into lower energy photons at the surface are mixed up in the Sun's "heat glow." This means the photons in the graph above come from two sources - some come from the core fusion reaction and others come directly from the kinetic motion of white hot plasma and gases at the surface, like the glow from white-hot steel. The majority of photons from both sources fall into the same energy range.

If you look at the radiation spectrum above you'll notice several dips. These are absorption bands of common gases - oxygen, water vapour and carbon dioxide - in the atmosphere. Electrons in gas molecules absorb radiation just like free electrons in plasma do, preventing some of it from striking Earth's surface.

Neutrinos, X-rays and Red Light

Along with gamma rays, neutrinos are also produced in the fusion reaction. Neutrinos aren't a kind of photon. They are fundamental fermion particles with no charge and possibly no mass as well. They are almost entirely invisible to matter. They experience almost no interactions in the plasma so they escape immediately out through the layers of the Sun and then go right through Earth as well, with almost no interaction with matter here either.

The surface and atmosphere of the Sun are sources of yet more photons. Just above the surface, hydrogen is cool enough to exist as excited atomic hydrogen. These excited atoms emit a series of photons depending on the energy level of the excited electron. The majority of emissions are of a specific wavelength in the visible red spectrum. This is why this layer of the Sun's atmosphere glows red, and it is called the chromosphere (it is overwhelmed by other photons so the Sun doesn't look red; you need a special filter to see it). We will explore this layer in more detail in The Sun Part 6. There is a layer of atmosphere above this one, within the corona, that is mysteriously extremely hot. Electrons in the hot plasma, accelerated by powerful magnetic fields, emit X-rays and extreme ultraviolet photons. An X-ray imager can pick out bright loops of this plasma arcing within the corona. We will look at these loops as well as why this upper layer is so unexpectedly hot in The Sun Part 7.

The immensely hot compressed plasma of the Sun is a very complex fluid system. In the previous Sun articles, we've seen how fusion works and how it ties into the Sun's life cycle. This plasma is also loaded with powerful electrical and magnetic properties. We'll explore how these properties create amazing solar surface structures such as sunspots, coronal loops and coronal mass ejections - structures that are visible from Earth and make up solar weather.


What is magnetism?

Magnetism comes from two sources - electrical currents (induction) and the intrinsic magnetic moment of every electron, proton and neutron. Both electric currents and magnetic moments create fields of force called magnetic fields. You feel this force as a push or pull when you bring two magnets together. They will attract each other or push against each other, because magnetism is a dipole - it exhibits two opposite poles, which attract each other while like poles repel. Explore the origin of magnetism in my series of articles called Magnetism Explained.

Magnetism, like electricity, comes from atoms. Every electron is a tiny magnet. It is a magnetic dipole because it has two qualities - a special kind of intrinsic spin and an electric charge. Any rotating charged body creates a changing electric field and a changing electric field generates a magnetic field. Electricity and magnetism are intimately intertwined. A changing magnetic field also generates an electric field. In fact, these two fields are different aspects of one single force, called electromagnetism in physics.

An atom has a magnetic moment that is the sum of all the magnetic moments of its particles. Each atomic nucleus has a magnetic moment that comes from its protons and neutrons. However, if the number of protons and neutrons is equal (as in an alpha particle in solar plasma) then the total magnetic moment is zero because their spins cancel out. Electrons also contribute magnetism. Each magnetic moment has a direction, just as a magnet does. In most materials, the magnetic moments are directed in all different directions. They cancel each other out, and these materials display no noticeable magnetism. But in some materials, the atoms line up in such a way that the dipoles tend to be pointed in the same direction. These materials display magnetism, a macroscopic phenomenon composed of countless microscopic phenomena all added together. Metal atoms are arranged in neat crystal lattices in a sea of mobile conducting electrons. Certain iron compounds have atomic metal lattices that are especially well suited to lining up electron spins so these materials display powerful magnetism.

The magnetism in a star is a bit different. Here, magnetism is not so much the sum of all the magnetic moments of the free electrons in the plasma (although they contribute magnetism), but rather electromagnetic induction. The Sun's core and radiative zone plasma rotates and the plasma in the convective zone surrounding them rotates and experiences convection as well. To get an idea of what convection looks like, see the image below.
This is a snapshot model of thermal convection in Earth's mantle. It is made of solid crystalline pressurized rock that undergoes slow creeping convective movement. It's not plasma so it lacks the turbulence caused by powerful magnetic forces and electrical currents, making this a much simpler system that is far easier to model than the convective zone plasma in the Sun.

In order to have convection you need a sufficient temperature gradient to drive the motion. The temperature gradient is not sufficient to drive convection in the core or in the surrounding radiative zone, so despite enormous temperatures and intense subatomic kinetic energy, these regions are very calm. Plasma in the core and in radiative zone rotates smoothly, following the rotation of the Sun itself. It's the convective zone where matters get very complicated.

Electrons flow inside the convective zone of the Sun as they follow the plasma's convective currents and rotational currents. These electron flows are changing electric currents that induce corresponding magnetic fields. Likewise, changing magnetic fields inside the Sun induce electric currents, so there is always feedback going on, which greatly complicates the system. The complex interplay of electric and magnetic fields generates tremendous forces that can compress the plasma fluid and accelerate plasma particles. The gigantic plasma system of the Sun is highly turbulent and chaotic, generating tremendous forces that make its motion very difficult to predict.

The Sun is a complicated collection of magnetic fields of varying strengths pointing in different directions, some of which line up to create ultra-powerful localized magnetic fields such as sunspots and other, even more powerful surface phenomena that we will look at in following articles.

The Sun Is A Magnetic Dynamo

There is a relatively thin layer of plasma between the convection zone and the calm radiative zone underneath where a tremendously powerful magnetic generator is set up, generating a field so powerful that it pervades the entire solar system. This is the transition layer. There is a sharp transition between the uniform rotation of plasma in the radiative zone and the much more complex movement of plasma in the convective zone. The graph below shows how plasma rotates uniformly (single red line left) in the radiative zone and then changes into several different rotation rates in the convective zone. The transition layer is where the red lines branch off.

Puzl bustre;en.wikipedia
The transition region experiences tremendous shear, as several horizontal layers slide past one another, and it is here where physicists believe a magnetic dynamo generates an extraordinarily intense magnetic field in addition to, and far more powerful than, the induced internal magnetic fields that complicate the motion of plasma in the convective zone.

The shear produced in the transition layer means that electric currents within the plasma are dragged through the pre-existing magnetic fields set up by convection, distorting them in the process. The drawing of an armature in a motor, shown below, might help you visualize a dynamo. An armature is made up of copper wire wound around an iron core (the circular structure) that is made to rotate inside a stationary magnetic field created by two magnets (see the three N-S poles).

The rotation drags and distorts the magnetic field lines created by the two stationary magnets. In the same way, magnetic field lines are dragged along with the plasma fluid in the Sun, amplifying the pre-existing magnetic field (see how close the field lines become; that means it's more powerful there).

Every 11 Years, The Sun's Magnetic Field Reverses Polarity

The dragging of magnetic lines is also behind the mysterious 11-year cycle in solar activity. Magnetic fields are drawn as magnetic field lines so we can visualize them. They show us the direction of the field and how relatively strong it is. Around a magnetic dipole (the Sun), lines of force, called magnetic field lines, run from the North pole to the South pole. These lines have elasticity and tension like a rubber band does. As the Sun rotates, the equator rotates faster than the poles do and magnetic field lines stretch out and wind around it, like stretchy thread wound around a spool. The effect is shown below (imagine the Sun is the yellow sphere).

The magnetic field lines experience increasing tension. Their potential energy increases, an effect called the Omega effect. The lines themselves also twist, like twisting a length of twine, because of the motion of convection - columns of hot plasma rise and sink. This too increases the potential energy of the magnetic field, and this is called is the Alpha effect.

Like a rubber band that is increasingly stretched and twisted, eventually something has to give. Around the time of solar maximum, once every 11 years or so, the magnetic field reverses itself, so North becomes South and vice versa, and the stored up potential energy is released, a bit like a tightly wound spool unwinding itself all at once. As reversal approaches, sunspots (areas where intense magnetic loops poke up through the photosphere) appear, peppering the Sun's surface. The magnetic fields of sunspots can be extremely powerful, as much as a thousand times more powerful than the background dipole magnetic field of the Sun. Solar flares, sudden areas of brightening on the Sun's surface, are more common and severe as well.

The differential rotation of the Sun builds up magnetic potential energy, but what finally triggers a reversal? The answer appears to be yet another kind of plasma flow in the Sun. The Sun also experiences a kind of plasma flow called meridional flow, and it is this flow that might be the actual trigger for magnetic reversal. Plasma flows from the equator toward the poles at the surface and in the opposite direction just beneath the surface. The surface flow is slow but the subsurface flow is even slower, taking about 11 years to travel from the poles back to the equator. The strength and the structure of the meridional flow varies greatly over each 11-year cycle. Both the Omega and Alpha effects, on the other hand, are constant, they vary very little. This suggests that the meridional flow has something to do with the 11-year solar cycle. The surface meridional flow carries the powerful sunspot magnetic fields toward the poles. This means that south-directed magnetic flux goes to the north magnetic pole and vice versa, weakening the dipole magnetic field, perhaps until it eventually breaks down and changes polarity altogether. The 11-year solar cycle isn't completely understood by physicists. The SOHO spacecraft, a joint project between NASA and ESA, is currently investigating the solar cycle. It continuously takes colourful and fascinating images of the Sun using a variety of imagers. To view the very latest collection, click here.

For a more technical discussion, try this review of current dynamo models of the solar cycle, available in PDF and HTML.

Next we will explore the outer layers of the Sun - the photosphere and corona, where violent storms take place, in The Sun Part 6.

1 comment:

  1. I understand that photons collide with particles in the sun, but why would the particles absorb the photons, then later emit more photons? Are there any conditions that would cause the particle to emit a photon, or is it just random?