Main Menu > 6: Space Weather > Discover the Solar Cycle

The Reuvan Ramaty High Energy Solar Spectroscopic Imager, RHESSI, will study the active Sun and energetic solar flares.

A flare results from the rapid release of magnetic energy which has built up in the solar atmosphere. Flares are seen as intense brightenings in active regions on the Sun. The amount of energy released in a flare can be ten million times greater than the energy released in a volcanic eruption. This energy is released in a very short time—as little as a few tenths of a second.

In this activity we will investigate one of the fastest and most energetic events on the Sun, a solar flare. During a flare, the Sun releases a large amount of energy in a short period of time. Scientists observe solar flares in many different wavelengths, from low-energy radio waves to high-energy gamma rays. The RHESSI satellite will make much sharper pictures of solar flares in high-energy X-rays and gamma rays than any satellite to date. This activity uses X-ray flare data from GOES, a Geostationary Operational Environmental Satellite, since RHESSI has not yet collected data. GOES satellites primarily monitor environmental conditions on Earth, but there is an X-ray monitor on board which records X-ray emissions from the Sun.

Scientists use a series of letters and numbers to classify the energy level of an X-ray flare. The letters used are A, B, C, M and X, with A being the weakest and X being the strongest. This activity singles out M class flares because they occur fairly often and are always strong enough to show up above the normal X-ray emission level of the Sun.

The list below indicates the total number of X-ray flares observed from the Sun in a given year. The total number of M class flares, or high-intensity flares, is also listed. For each year, calculate the percentage of M class flares by dividing the number of M class flares by the total number of flares and multiplying by 100. Fill in column 4 with your results. Use the graph below to plot the percentage of M class flares over time. If you choose, you could also plot the total number of flares, or the number of M class flares.

 Year Total Number of Flares Number of M Flares Percentage of M Flares 1990 2630 273 1991 3324 590 1992 2815 202 1993 2446 74 1994 1610 25 1995 1124 11 1996 515 4 1997 1141 21 1998 2248 94 1999 2425 170 2000 2661 215

Fill in the values in column 4 for Percentage of M Flares and plot a graph.

Use the graph above to plot the percentage of M Flares over time.

What do you notice from your graph? You should see that the percentage of high-energy flares from the Sun is not constant in time. This is also true of the total number of flares, and the number of M class flares. You should see a maximum percentage of M class flares in 1991 and a minimum percentage in 1996. If you were to extend this graph for a much longer period of time, you would discover that the rise and fall occurs in a regular cycle that repeats itself approximately every eleven years. However, the maxima that occur every eleven years are not always the same height.

Scientists also see this eleven-year activity pattern in the number of sunspots on the Sun, and solar flares are known to originate in regions where there are sunspots. In general, the Sun goes through a regular activity cycle with eleven years from one maximum (or minimum) to the next. The last solar maximum occurred in the year 2000, and the last solar minimum was in 1996. This pattern should be visible in your graph. Can you predict when the next solar maximum, or solar minimum, will be?

In the full version of this activity, X-ray Candles: Solar Flares on Your Birthday, students count the number of high-intensity X-ray flares in their birth month and combine their findings with the rest of the class to obtain and graph complete flare counts for each year.

A full activity that investigates solar x-ray flares can be found at the RHESSI Web site.

An interactive web activity about the correlation between sunspots and flares can be found on the SEGway Web site.

Learn more about the mission at the RHESSI Web site.

 Related to chapter 6 in the print guide.
 For an overview of how the Sun effects the Earth see the Sun-Earth Connection. For a more on the history of human observation of sunspots, see the Galileo Debate. See video of a taken from the SOHO spacecraft.
 Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.) auroras View the full, printable version of the glossary.

Main Menu | Resources | 6: Overview