Northern Lights: Sun Activity
The story of the Northern Lights, or the Aurora Borealis, begins at the Sun. Looking up at the Sun, we may think there isn't much happening there, but it turns out there is a lot happening on the Sun and it's constantly changing! It is important to understand the Sun since life on earth is so intimately tied to it. And if we want to understand how the beautiful colors of the aurora came to be, we need to understand solar activity.
Click here to see what is happening on the Sun today.
- The Solar Cycle
- Solar Flares
- Coronal Mass Ejections
The Sun goes through an 11 year cycle of activity. "Solar minimum" is when there are very few, if any, noticeable features on the Sun. "Solar maximum" refers to when the Sun is very active and contains many noticeable features such as sunspots or flares. This diagram shows an image of the Sun each year during one of its cycles. Can you guess which images correspond to solar minimum and which ones correspond to solar maximum?
This “Butterfly Diagram” below tracks the north (yellow) and south (blue) poles of magnetic field on the Sun. We can see that they are flipped every 11 years, which is what creates the solar cycle. This flipping stirs up the Sun and creates the abundance of activity that we see during solar maximum.
The next Solar maximum is predicted to peak in 2013. A lot of sunspots will begin to appear as strong magnetic field loops from beneath the surface of the Sun start to poke through. The looping field lines carry extremely energetic, charged particles with them. As the lines bend and twist like rubber bands, they can sometimes snap off, carrying the plasma out into space.
Sunspots appear as darker regions on the Sun. They are regions of concentrated magnetic field and usually appear before solar flares and coronal mass ejections. Sunspots are representative of solar activity, and are therefore more likely to appear during solar maximum. Though just a small spot on the Sun, some can grow to be bigger than Jupiter! While it is never safe to look at the Sun directly, if you have the right filter, you can sometimes see them without any magnification. They are not holes, but cooler regions, making them appear dark against the hot, bright surface of the rest of the Sun.
When the Sun is very active, solar flares are more likely to appear, although the exact time is currently unpredictable. Flares are massive explosions on the Sun. They are always associated with sunspots, where there is a lot of energy from the twisting magnetic fields that pierce the surface. The video at right shows the evolution of a flare over 4 hours. You can see the 10 million degree Celsius plasma strung along the magnetic loops. When the loops of these fields suddenly break or slip, the energy released causes a huge amount of plasma to splash up.
A solar prominence is also a splashing up of plasma and stretching, twisting magnetic field lines; however, prominences usually occur in regions that do not have sunsponts. They extend away from the surface of the Sun in huge loops or with one end disconnected.
When a solar flare or prominence is extremely powerful, the loop can break off and has enough energy to fly away, and the material is lost from the Sun. This is called a Coronal Mass Ejection, or CME. The video at right blocks the bright Sun in the center to allow the steady solar wind to show up. You can see the huge disturbance as a CME bulges up and streams away from the Sun. The white fuzzy spots that then flash across are from the charged materials carried by the CME hitting the detector.
CMEs occur about once a week during solar minimum, and up to two or more a day during solar maximum. About 70 a year fly in the direction of Earth, but less than 10 will be powerful enough to produce large geomagnetic storms. A CME generally takes 3-4 days to reach earth, so we have a couple days warning to look for the effects.