The Sun Continued

The Corona

Above the photosphere, the layers of the Sun are transparent. They actually are glowing, emitting light, but the photosphere is so bright that its light swamps the fainter glow of the upper layers. The upper layers are faint partly because they are so thin - there's not that much gas to glow.

The upper layer of the Sun is the corona. You can see it in this image from the solar eclipse of July 11, 1991. Here's another excellent photo of that eclipse. It extends more than 500,000 miles above the photosphere and glows a beautiful pearly color. The only time you can actually SEE this color is during total solar eclipse. What you CAN'T see is that the corona also emits a lot of X-ray radiation. We are protected from it by our ionosphere, so study of this X-ray emission must be done from orbit. The X-ray emission results from the extremely high temperature of the corona; that temperature is around 1,000,000K. The Sun seen in X-rays looks somewhat strange. The photosphere is dark because it is too cool to emit X-rays. The corona is hot enough to do it.

The mystery of the coronal temperature seems to be finally solved. For a long time it was not obvious why the corona had such a high temperature when it was sitting atop the photosphere, which has a temperature of 5780K. The answer is likely to be magnetic field activity just above the photosphere; these violently flipping fields could accelerate the atoms in the corona to million-degree speeds. The coronal loops shows clearly in ultraviolet images from the TRACE satellite. Here's an image of the Sun in ultraviolet from SOHO.


Sunspots are magnetic phenomena. When a loop of solar magnetic field pops up from the photosphere and makes a loop above the surface, sunspots occur. Measurements of the spectrum in sunspots reveals the strong magnetic field found there. This magnetic field interacts with the ionized gas of the photosphere and locks it in place; it can't take part in the convection. The natural result is that the gas cools somewhat; the umbra (center) is at about 4500K. That dark center is actually still glowing; it's just enough fainter that a photo exposed correctly for the surrounding photosphere shows the umbra as black. Expose correctly for the umbra and the glow can be seen.

Sunspots have been observed for centuries. They come and go in cycles approximately 11 years long. The polarity reverses each time, so the full cycle is about 22 years. Our book only shows sunspot activity for the 20th century, but records go back farther than that. Decent data exist from about 1650 onward. There was an interesting period from about 1645 to 1715 when almost no sunspots were seen. It is known as the Maunder Minimum, named for English astronomer Edward Maunder (1851-1928). This period corresponded with what was called the Little Ice Age in Europe. Maybe the Sun has some kind of effect on our weather??

Sunspots can, on occasion, be seen with the naked eye. There are occasions when the setting Sun is so obscured by dust in the air that it is so dim you can look right at it. On rare occasions a small speck will be noticed on the Sun. Ancient Chinese observers did that, and they recorded the event. Those are naked-eye sunspots. They can actually be seen (when they exist) anytime by using a number 14 electric welder's glass filter. There have been several of them in the last 2 years. This very large group was photographed on 7 April 1947. This large spot should have been naked-eye visible.

Solar Effects on Earth

The Sun's activity actually has some direct effects on Earth, but you likely have heard of only one of them. The Northern (and Southern) Lights are a direct result of solar activity inducing large currents in Earth's magnetic field. The Lights are frequently visible from the northern states and Canada; only rarely are they seen here at the low latitude of Texas. This is the Aurora over Canada.

Another effect was not observed until the 20th century. Solar activity can disrupt the ionosphere and, as a result, disrupt high-frequency radio communication. When such a disruption occurs (usually as the result of a large solar flare), radio bands that are normally alive with signals are quite dead; the few signals that are heard are quite weak. It is really spooky to hear. Checking solar conditions usually confirms the flare.

The last solar effect involves the relative motion of a wire across the lines of a magnetic field. When this happens, a voltage is induced in the wire. This animation might help. TXU moves wires in a magnetic field to generate our electric power. It doesn't matter whether the wore moves or the field moves - the effect is the same. When solar activity compresses the Earth's magnetic field and causes it to move, long wires can have a large voltage indeed, and, if the circuit is completed through the Earth, large currents also. It takes LOOOONG wires for this to happen. Who has really long wires? Electric utility organizations - their transmission lines are very long.

This induced voltage has been responsible for a number of power outages. One on March 13, 1989 took down a power grid in Canada. In New Jersey, a $36,000,000 power transformer was fried by currents induced by the flare. This phenomenon is more likely in the northern states and Canada; it would be rare here.

Also look at Space Weather. This contains a link to a picture of the fried transformer.

The Source of the Sun's Energy

The Sun's prodigious energy output comes from the fusion reaction that takes in 6 protons (hydrogen nuclei and produces one helium atom plus two protons. This is a three-step process called the proton-proton chain. The alchemists tried for years to transmute one element into another but never succeeded. The Sun does it constantly, making helium from hydrogen.

Look at Figure 9.26 or this graphic image. Six protons go into the reaction and two protons plus a helium nucleus come out. Six particles go in and six particles come out. If you add up the masses going in and coming out, you find that 0.7% or so of the mass disappears (data on p. 261). That is the source of the energy. Einstein found that e=mc2, which means that matter and energy are interchangeable. Matter can change to energy and vice versa. A little bit of matter can produce a LOT of energy, and that's the Sun's secret. The gamma rays pictured at the top and bottom of the figure represent the energy released in the reaction. Note that the SUM of mass and energy is conserved in the process.

Helium was, in fact, discovered in the Sun before it was known on Earth. Astronomer Joseph Lockyer found it in 1868 using the relatively new technique of spectroscopy. Pierre Janssen of France independently discovered it the same year. An unidentified yellow line in the Sun's spectrum was ascribed to something called Helium. It was finally discovered in rocks on Earth in 1895. It is actually a product of radioactive decay in the Earth.

The NCAR High Altitude Observatory has a good outline of advances in solar physics.

Try looking at images and movies from SOHO (Solar and Heliospheric Observatory).