X-ray picture of

Measuring Solar Activity

A printable version of this lesson can be found here

In four parts:

  1. "General Appearance," recognizing common structures in the images
  2. "Changing Look, or Constant Face?," recognizing patterns in five years' worth of images by making graphs
  3. "Long-Term Cycles," recognizing patterns in 250 years' worth of data, comparing to graphs of Part Two, and making predictions about the future
  4. "Connection to the Visible Sun," recognizing spatial relationships by comparing simultaneous images made in different wavelengths, and making testable predictions about the observable Sun
Parts One, Two, and Four can each stand alone. Part Three requires Part Two.

Part One: General Appearance

Let's begin by looking at an image of today's Sun taken by the EIT telescope on the SOHO spacecraft. What do you see? What features stand out? Are they features that you have seen before? If you were to go outside and look at the Sun with your naked eye (NOT a good idea!), would it look like this?

Activity: Make a sketch of what you see in this solar image, being sure to include the most notable or interesting features. Invent names for the different types of features.

WARNING: "Naked eye" means just that -- "naked", as in "unprotected and vulnerable". NEVER look straight at the Sun with your unprotected eyes. You can lose your eyesight, and YOU WILL NEVER GET IT BACK. There are some safe ways to observe the Sun -- find out about these safe methods BEFORE you look at the Sun.

Part Two: A Changing Look, or a Constant Face?

Look again at the image of today's Sun. Do you suppose the Sun always looks like this? Here are some pictures of the Sun in X-rays produced by Yohkoh SXT to show how the Sun appeared on several other dates. Compare the images from several different dates. Is there any difference? Do you suppose there is any pattern to the Sun's appearance through time?

Activity: To look for subtle patterns in data, scientists often plot their data graphically -- pictures show many things that are easily overlooked in tables of numbers. Try making these two graphs to see if any patterns are noticable:

Describe any patterns you see. Would you be able to use these graphs to make predictions of solar activity in the coming months? In the coming years?

Part Three: Long-Term Cycles

Compare your plots from Part Two to these two graphs. The first shows how the number of sunspots has changed, day to day, over the last 250 years. The second shows the solar latitude of sunspots during the course of the last 125 years. (There is a relationship between X-ray active regions and sunspots, so there should be some relationship between your graphs from Part Two and these longer-term records.) How do your graphs compare to the longer record? Can you predict, even roughly, what the Sun will look like next year? When you graduate from college? On your thirtieth birthday?

Activity: Download an image taken on June 7, 2000 . Compare it to the other images you have. Where in the solar activity cycle do you think the sun was when this image was taken? Near minimum/maximum activity? Increasing/Decreasing activity? You can find lots of recent images in the First Light archive at URL:


For each image in the First Light Archive, the date is given in the format YYMMDD_time, where YY is the year, MM is the month, and DD is the day.

Test yourself: Here is another X-ray image. There is no date stamped on the image; it was made sometime between 1991 and 1995. Try to estimate when this picture was taken, based on the amount and location of activity.

The "Mystery Image" in the above Test Yourself activity is randomly selected from a set of twelve solar X-ray pictures. Each time you reload the "Mystery Image" you may get a different picture! A composite poster of all twelve images in the set is viewable here, and you can order your own full-size poster at URL:


Part Four: Connection to the Visible Sun

Except for today's image from SOHO, the images we have examined so far are all X-ray pictures. They show the apperance of the hot outermost layer of the Sun's atmosphere. But since X-rays are invisible to human eyes, you can't see the coronal structures simply by walking outside and looking upwards.

Now here is a visible-light picture of the Sun, and an X-ray picture taken on the same day (1992Jan23). Notice the location of the dark sunspots in the visible-light picture (often called "white light"), and the location of the large active regions in the X-ray picture. Is there any relationship? The visible-light image shows what you would see if you simply walked outside and looked up (although you would have to SHIELD YOUR EYES with a protective filter). Since this is the visible layer of the Sun, it is called the "photosphere," or "sphere of light." The corona (or "crown") lies much higher, many kilometers above the photosphere. See the above warning about viewing the Sun.

Link to an Explanation

Activity: Here are a bunch of white-light and X-ray pictures of the Sun. The white-light images are stamped with the date and time of the observation; the date stamps have been left off of the X-ray images. Use what you have learned about magnetic active regions to match the X-ray pictures with the respective simultaneous white-light images.

If you could walk outside right now and observe the visible Sun, could you make predictions about the appearance of today's Sun in invisible X-rays?

Test yourself: We have provided you with an image of today's Sun made in ultraviolet rays (which are not as high energy as X-rays but are still invisible to the human eye). Make a guess about what the visible-light Sun looks like today. Make a sketch of your prediction. Then download a white-light picture of today's Sun and compare: how accurate was your prediction? One site from which you can get a daily (depending on local weather) solar white-light image is the Big Bear Solar Observatory in California, at URL:


NOTE: All of the X-ray images were taken by the Soft X-ray Telescope (SXT) on the Yohkoh solar observing satellite. Yohkoh/SXT has made more than two million X-ray pictures of the Sun since its launch in 1991, and continued making pictures until December if 2001. For most of 1992, SXT also made pictures in visible light. Towards the end of 1992, however, the harsh radiation from the Sun began to take its toll on the camera and it is no longer used for white-light pictures. You can see the effects of the damaging radiation in some of the 1992 pictures; look for a dark "shadow" near the right-hand edge of the Sun.

Lesson designed by the YPOP Team

For questions about this lesson, please contact David McKenzie

Selected by the sciLINKS program,
a service of National Science Teachers
Association. Copyright 2001.