Solar Activity: What's Going On Out There?
The term solar activity refers to all kinds of disturbances that take place on the sun from moment to moment and from one day to the next. All forms of solar activity, including the 11-year sunspot cycle and some even longer cycles, seem to involve magnetism. Deep inside the sun, a natural dynamo generates new magnetic fields all the time. The magnetic fields rise to the surface and on up to higher layers in the solar atmosphere where they twist around and cause all kinds of trouble. Recent observations show that additional magnetic fields are also generated in the higher layers of the atmosphere.
Astronomers measure magnetic fields on the sun by their effects on solar radiation, using instruments called magnetographs. You can see images taken with these devices on many of the professional solar observatory Web sites. These magnetic field observations show that sunspots are areas of concentrated magnetic fields, and that sunspot groups have north and south magnetic poles. Outside of sunspots, the overall magnetic field of the sun is pretty weak.
Many of the rapidly changing features on the sun and probably all explosions and eruptions seem to be related to solar magnetism. Where there are changing magnetic fields, electrical currents occur (as in a generator), and when two magnetic fields bump into each other, a short circuit — called a magnetic reconnection — can suddenly release huge amounts of energy.
Coronal mass ejections: The mother of solar flares
For decades astronomers believed that the main explosions on the sun were solar flares. Now astronomers know that they were just like the blind man who feels an elephant's tail and thinks that he knows all about the beast when he's touching one of the animal's least significant parts. Observations from space reveal that the primary engines of solar outbursts aren't solar flares, but coronal mass ejections — huge eruptions that occur high in the corona. Often, a coronal mass ejection triggers a solar flare beneath it in the low corona and chromosphere. You can see solar flares in many of the images on professional astronomy Web sites. As the number of sunspots increases over an 11-year sunspot cycle (see the following section), so does the number of flares.
Scientists didn't know about coronal mass ejections for many years because they couldn't see them. Astronomers could only get a good view of the corona at rare intervals during the brief duration of a total eclipse of the sun. But solar flares can be seen at any time, so scientists studied them intensely and overestimated their importance.
When satellite images show a coronal mass ejection that isn't going off, say to the east or to the west from the sun, but that forms a huge expanding ring or halo event around the sun, that's bad news. The halo event means that the coronal mass ejection — about a billion tons of hot, electrified, and magnetized gas — is heading right at Earth at about a million miles per hour. When it strikes the Earth's magnetosphere, dramatic effects sometimes result, as described later in the section "Solar wind: Playing with magnets."
If you see a halo event in one of the satellite images, check the National Oceanographic and Atmospheric Administration (NOAA) Space Environment Center Web site, because NOAA may be forecasting some pretty fierce space weather.
Cycles within cycles: The sun and its spots
Sunspots are regions in the photosphere where the magnetic field is strong and that appear as dark spots on the solar disk. The spots are cooler than the surrounding atmosphere and often appear in groups.
The number of sunspots on the sun varies dramatically over a repeating cycle that lasts about 11 years — the famous sunspot cycle. In the past, people blamed everything from bad weather to a decline in the stock market on sunspots. Usually, 11 years pass between successive peaks (when the most spots occur) of the sunspot cycle, but this period can vary. Further, the number of spots at the peak can vary widely from one cycle to the next. No one knows why.
As a sunspot group moves across the solar disk due to the sun's rotation, the biggest spot on the forward side (the part of the group that leads the way across the disk) is called the leading spot. The biggest spot on the opposite end of the group is the following spot.
Magnetograph observations show definite patterns in most sunspot groups. During one 11-year cycle, all the leading spots in the Northern Hemisphere of the sun have north magnetic polarity, and the following spots have south magnetic polarity. At the same time in the Southern Hemisphere, the leading spots have south polarity, and the following spots have north polarity.
Here's how these polarities are defined: The compass needle that points north on Earth is called a north-seeking compass. A north magnetic polarity on the sun is one that a north-seeking compass would point to. A south magnetic polarity on the sun is one that a north-seeking compass would point away from.
Just when you think that you have it straight, guess what? A new 11-year cycle begins, and the polarities all reverse. In the Northern Hemisphere, the leading spots have south polarity, and the following spots have north polarity. In the Southern Hemisphere, the magnetic polarities reverse, too. If you were a compass, you wouldn't know if you were coming or going.
To encompass all this information, astronomers have defined the sun's magnetic cycle. The cycle is about 22 years long and contains two sunspot cycles. Every 22 years, the whole pattern of changing magnetic fi elds on the sun repeats itself.
The solar "constant": Time to face the changes
The total amount of energy produced by the sun is called the solar luminosity. Of greater interest to astronomers is the amount of solar energy that Earth receives, or the solar constant. Measurements made by solar and weather satellites sent up by NASA in the 1980s revealed very small changes in the solar constant as the sun turns. You may think that Earth receives less energy when dark sunspots are present on the solar disk, but that isn't the case; in fact, the opposite is true: more sunspots, more energy received from the sun. Chalk up another mystery for astronomers to solve.
According to astrophysical theory, the sun was slightly brighter when it was very young than it has been for the last several billion years, and it will cast more energy on Earth ages from now when it becomes a red giant star.
So "solar constant" sounds like wishful thinking, although from day to day and with amateur equipment, constant sounds pretty darn accurate.
Solar wind: Playing with magnets
Coronal mass ejections are usually invisible with amateur equipment but marvelously revealed by satellite telescopes. They spray billion-ton blobs of electrified gas, called solar plasma, permeated with magnetic fields, out into the solar system, where they sometimes collide with Earth's magnetosphere. (The magnetosphere is a huge region around Earth in which electrons, protons, and other electrically charged particles bounce back and forth from high northern latitudes to high southern latitudes, trapped in Earth's magnetic field. It acts as a protective umbrella against coronal mass ejections and the solar wind.)
A type of solar plasma called the solar wind is constantly streaming out from the solar corona. It moves through the solar system at about a million miles per hour (470 kilometers per second) as it passes Earth's orbit.
The solar wind comes in streams, fits, and puffs and constantly disturbs and replenishes Earth's magnetosphere, which becomes compressed in size and swells out again. The disturbances to the magnetosphere, especially those from traveling solar storms such as the coronal mass ejections, can cause displays of the Northern Lights (aurora borealis) and Southern Lights (aurora australis), as well as geomagnetic storms. The geomagnetic storms can shut down power company utility grids (causing blackouts), blow out electronic circuits on oil and gas pipelines, interfere with radio communications, and damage expensive satellites. Some people even claim they can hear aurorae.
Solar disturbances and their effects on the magnetosphere are called space weather. You can see the latest official U.S. government space weather report and forecast at the Web site of the NOAA Space Environment Center.
Four billion and counting: The life expectancy of the sun
Some day, the sun will run out of fuel, so it will die. All good stars must come to an end. What will actually happen is that the sun will swell up and take the form of a red giant star. It will look enormous, and it will evaporate the oceans. Humans will die of the heat (if people still exist). Talk about global warming!
The red giant sun will puff off its outer layers, forming a beautiful, expanding nebula, the kind of shining gas cloud that astronomers call a planetary nebula. But no humans will be here to admire it. The nebula will gradually fade away and all that will remain at its center is a tiny cinder of the sun, a hot little object called a white dwarf star. It won't be much larger than Earth, and although it will be very hot at first, it will be too small to cast much energy on Earth. Whatever's left on the surface of the earth will freeze. And the white dwarf will shine like an ember in a dying campfire, gradually fading away.
Fortunately, we should have about 5 billion years to go before that prospect looms near.