The Magnetic Sun

Up to now, we have described the Sun as a ball of gas, or plasma, but we have not said much about its magnetic fields.  We could sort of get away with that when we talked about the interior and surface of the Sun, because there the gas energy dominates the magnetic field, pushes it around, and concentrates it in small bundles.  This is what makes the sunspots.  Below is a close-up image of the sun in visible light, showing these little bundles of magnetic field -- you can also see the photospheric granulation, which sort of looks like the pile of a rug.  The sunspots are dark, as we said in the previous lecture, because the magnetic fields in the sunspot inhibit the flow of heat into them, so they are cooler (at 4,000 K) than the surrounding photosphere (which is at 6,000 K). Notice that the biggest sunspot in the lower right (it is 3-4 times the size of Earth across) has a dark part, called the umbra, and a lighter part called the penumbra.  Think of the magnetic fields as a bundle of ropes all gathered together, so that when we look down on a sunspot we see a cross-section of them.  In the penumbra the rope strands are lying nearly flat, which is what makes the striation pattern.  In the middle of the sunspot, these magnetic rope strands stick out of the surface.

Above the visible surface (the photosphere) of the Sun, the gas (or plasma) still exists, but is much less dense.  In these regions above the surface (the chromosphere, corona, and solar wind), the magnetic field energy dominates the gas energy, and it is the magnetic field that is "king."  It gets to tell the gas where to go.  That is why, when we look at a picture of the corona, we see loops -- these are the strands of magnetic field that come out of the surface, arch through the corona, and usually come back down to the surface.

TRACE image of coronal loops in the UV

The magnetic fields of the Sun are the cause of all of the activity that make the Sun such a dynamic place.  If there were no magnetic fields, the Sun would be completely static and unchanging over billions of years!

We talked about planetary magnetic fields, especially the magnetic field of the Earth and of Jupiter.  Recall that these magnetic fields require that the planet have an electrically conducting fluid inside (molten iron in the case of Earth, and metallic hydrogen in the case of Jupiter), and have a fairly rapid rotation rate.  The circulating fluid generates the magnetic field.  The Sun also obeys these rules -- the plasma from which it is made is highly conducting, and the Sun rotates once every 28 days or so.  One thing that is different about the Sun is that it has a convection zone.  So magnetic fields that are generated inside the Sun can actually be brought to the surface.  When they break through, they form the sunspots.

We can watch a movie of the magnetic fields.  In this movie, magnetic fields coming out of the Sun (magnetic north poles) are shown as white, and those going into the Sun (magnetic south poles) are black.  Notice that sunspots appear in white and black pairs.

When we watch the Sun over many years, we will notice that the sunspots are much more common at certain times, and less common at other times.  If we count the number of sunspots, we find that the Sun goes through an 11-year activity cycle, where the number of sunspots becomes almost zero, then grows to a large number, then back to near zero over approximately 11 years.  Here is the changing sunspot number over the last 8 years.  In 1995 we were in sunspot minimum, when there were almost no sunspots.  In 2002 we passed sunspot maximum, and we are now well no the way to the minimum of the cycle.  The image of sunspots for today shows that the Sun has a few spots, but rather small ones.

The plot below shows the previous sunspot cycle and the start of the current cycle in a somewhat different way.  The chart on the right is just the sunspot number as before, but the chart on the left shows the latitude dependence of the sunspot locations on the Sun.  Notice that early in the cycle, the sunspots generally start appearing near 30 degrees latitude on the Sun.  At the current time of the solar cycle, they appear at about 20 degrees, and late in the cycle they appear near the equator.  This is called a butterfly diagram.

 

The next plot shows the butterfly diagram for the last 120 years!  Notice that the sunspot cycle is very regular, but seems to have been growing stronger over the last few cycles.

This butterfly diagram and at least two other effects, Joy's Law, and Hale's Law, can be explained in a rough way by the operation of a solar dynamo.  With the addition of Hale's Law, the solar cycle is actually seen to be a 22-year cycle.

Magnetic fields can be measured in the photosphere using the Zeeman Effect.  Below is an "image" of magnetic field strength showing not only the strength of the sunspot regions, but also their magnetic polarity.  White in the image represents fields coming out of the Sun (north magnetic polarity) while black represents fields going into the Sun (south polarity).

 

Magnetic Fields: The sunspots in the image at right are revealed to be areas of strong, opposite polarity magnetic fields, which extend to even larger areas than the spots themselves.

Notice the lighter areas, called faculae, which are found near sunspots and are also areas of strong magnetic field.

Sunspots and their surrounding magnetic field areas are called active regions.  Active regions are the site of solar flares, vast explosions of energy that send high-energy particles (electrons and protons mostly, but some helium atoms and a very few heavier elements) into space.  They are also involved in a phenomenon called coronal mass ejections (CMEs), which are spectacular "bubbles" of magnetic field and plasma that lift off from the Sun and expand quickly into interplanetary space.  Sometimes these CMEs are directed toward Earth, and when they run into the Earth's magnetosphere they can cause magnetic storms.  Look at Spaceweather.com, or the Space Weather Now page to see what is cooking at the moment.