The Sun (Continued)

Last time I talked about the spectrum of the Sun, saying that it shines via blackbody radiation, or the fact that hot objects emit electromagnetic radiation. Check pages 138 and 139 of the textbook.

For a demonstration of blackbody radiation, be sure to check the on-line video clips, or better yet, the course CD.

Now let’s look at a graph of the spectrum of the Sun.

Remember again that the human eye is sensitive to light with wavelengths between 400 and 700 nanometers. You can see that this is just where sunlight reaches its maximum brightness.

Note also that the Sun produces a fair amount of radiation in the ultraviolet part of the spectrum below 400 nanometers. This radiation can be very damaging for leaving organisms (the shorter the wavelength, the worse it is). The fact that we don’t have much of this radiation at the surface of the Earth is due to the absorbing effects of ozone in the Earth’s atmosphere.

Last time I mentioned that the three-part structure of the Sun’s atmosphere is crucial to understanding the nature of solar-terrestrial relations. The lowest part is the photosphere which comprises the opaque gases we recognize as the disk of the Sun. As mentioned last time, the temperature of the photosphere is 5800 K. Let’s again look at a recent picture of the Sun. What you see is the photosphere. An amazing fact about the Sun is that its edge seems so sharp, like a solid object, even though it is really composed of very hot gases.

Above the photosphere lie the chromosphere and corona. The corona (Latin for “crown”) is the gleaming halo seen around the Sun at times of total solar eclipse. The solar corona, shown below, is one of the most beautiful objects in astronomy. This picture shows the solar eclipse of 1991, when the Moon blocked out the photosphere so we could see the corona.

The beautiful structuring of the corona is due to solar magnetic fields, as I will describe below.

The corona has a very interesting and puzzling property. The first spectrum of the corona was made in 1869 by two American astronomers, Harkness and Young. They found that the corona has an emission line spectrum, with two very strong emission lines at 530 nanometers (the Green Line) and 637 nanometers (the red line). The coronal spectrum is shown below, which shows a little image of the Sun at a wavelength where the corona emits a lot, you can see the emission lines mentioned above, as well as some others.

While some of the emission lines were identified, others did not correspond to emission lines of known elements that had been measured in the laboratory. For a long time, there was speculation that these emission lines might result from coronium, a chemical element that had not been isolated on Earth, but which was present in the Sun.

The solution finally came in 1939, and was extremely interesting. By way of introduction, note that the element iron has 26 protons in its nucleus, and an intact (electrically neutral) iron atoms has 26 electrons surrounding the nucleus (what physicists and chemists call the atomic number). The element calcium has 20 protons in the nucleus and 20 electrons in the neutral state, so an atomic number of 20.

You can knock electrons out of an atom, if you put enough energy into it. The amount of energy required is called the ionization potential. You can knock a second electron out of the atom as well. That takes more energy because the atom hangs on to its remaining electrons more ferociously than before. Knocking a third electron out takes still more energy, etc.

In 1939, it was discovered, on the basis of laboratory measurements which became possible then that the “red line” was an emission line of an iron atom that had 9 of its 26 electrons stripped away (so-called FeX). The “green line” was an emission from iron with 13 of the 26 electrons removed (FeXIV). The reason this was not figured out in 1869 was that at that time there was not technical ability to produce atoms in this state.

A tough but interesting question for you to think about: What does the presence of FeX and FeXIV tell you about physical conditions in the solar corona?

The corona expands out into space to form a medium which is everywhere flowing away from the Sun. This is called the solar wind. A cartoon representation of it is shown below.

There are a number of spacecraft out in the solar wind, monitoring its properties. The conditions in the solar wind (interplanetary weather) for Sunday, April 18, 2004 are shown below.

Here we can see the flow speed out from the Sun, ranging from 450 kilometers per sec to 350 kilometers per second, and the density, which fluctuates about an average of a few particles per cubic centimeter. This is much lower density than typical vacuum experiments in physics laboratories.

The importance of the solar wind is that it establishes a “medium” or “bridge of matter” between the Sun and the Earth. That has the important consequence that events that happen on the Sun can produce disturbances that propagate through the solar wind and impact the Earth. This is the basis of the field of solar-terrestrial relationships, that I will discuss.

Sunspots

Sunspots are a prominent feature of the Sun. You can see them on the picture above, which shows the state of the Sun as it is now (actually a few days ago). They are regions of slightly cooler temperatures (although only about 1000 degrees Kelvin cooler), and they have very strong magnetic fields. The picture below shows the Sun as it was on September 21, 2000, when a number of large sunspots were on the surface of the Sun.

A close-up view of a sunspot shows structure like below.

Magnetic fields in sunspots have been measured to be thousands of the units called Gauss. By comparison, the Earth’s magnetic field is a bit under one Gauss, and a refrigerator magnetic will have a magnetic field of a couple of hundred Gauss. The fields in sunspots can be a substantial fraction (15-20%) of the field strength in a MRI medical imager. Pretty impressive for a region that can be larger than the Earth in extent.

You can check out the status of sunspots right now by looking at the SOHO web page with today’s picture of the Sun

http://sohowww.nascom.nasa.gov

The magnetic fields in sunspots arch way up into the solar atmosphere, producing loops.

These loops sometimes destabilize, and big blobs of matter are fired off into space. These blobs are called coronal mass ejections. They are actually of practical interest, because if they hit the Earth, they can cause intense auroras, problems with radio communication, etc.

Often associated with coronal mass ejections are solar flares, which are huge explosions on the Sun, always associated with the complex magnetic fields around sunspots.

Last Fall, even though we were a couple of years past solar maximum, or the time of peak solar activity, there was a furious outbreak of huge solar flares and coronal mass ejections. A flare which occurred on November 3, 2004 is the largest that occurred since the beginning of the space age in the late 1950s. A description of these “Halloween Storms” is given on the URL below.

http://www.spacetoday.org/SolSys/Sun/SunStormsRecordYr.html

The 11 Year Solar Cycle

Sunspots, CMEs, and solar explosions called solar flares are all linked together.

Interestingly enough, sunspots don’t just occur at random. They wax and wane in numbers according to an eleven year period. One can think of the Sun having a “heartbeat” with a period of 11 years. The last “solar maximum” was around the beginning of 2001. The latest plot of sunspot counts over the last few years is shown below.

We can see this 11 year cycle going all the way back to the beginning of the age of telescopic astronomy at the beginning of the 1600s.

Notice how the numbers of sunspots wax and wane through the years, but also how definite the 11 year period is. It is telling us something about the inner workings of the Sun, but an good theory of this phenomenon is missing.

The time between about 1650 and 1720 when the Sun seems to have forgotten how to make sunspots is believed to be a real phenomenon, and is referred to as the Maunder Minimum.