Sunspots and the Solar Cycle

Sunspots are dark regions on the photosphere of the Sun.  These features are observed daily by astronomers around the world and by satellites in space.  One of the most important pieces of data collected by scientists is the sunspot number.  The sunspot number is basically the number of sunspots on the Sun on a given day but it is determined in a very precise manner.  The daily sunspot numbers are averaged by month and by year.  The location (solar latitude and longitude) and the magnetic polarity (north or south) are also measured.  Over time a huge amount of data has been collected.  Below you are asked to analyze some of this data.

1. Over the past 300 years determine the maximum yearly sunspot number and the year(s) during which it occurred. 

2. Over the past 300 years determine the minimum yearly sunspot number and the year(s) during which it occurred.

3. For the year 2000 only, determine the maximum and minimum monthly sunspot numbers and the months during which each occurred.

4. Looking at the yearly values, there is an obvious pattern of up and down that occurs over the years.  The peaks of the pattern are called solar maxima and the valleys are called solar minima.  How many solar maxima have occurred since 1700?

5. Consider the solar minimum to be a “starting point” in this “sunspot cycle”.  A complete cycle then consists of a rising and then a falling of sunspot numbers.  Are we nearest the beginning, the middle, or the end of the current cycle?

6. Determine the average number of years for a complete sunspot cycle by dividing the number of years by the number of cycles.  Count at least 20 cycles for this purpose.

7. Now use the graphs to search for the greatest variation from the average length of the sunspot cycle that you found in the previous question.  In other words, what has been the shortest sunspot cycle?  the longest?

8. Using your results predict the dates of the next solar maximum and the next solar minimum. 

9. Looking at the last 300 years are there any long-term changes, trends, or cycles apparent in the yearly sunspot numbers?  Explain.

10. Now consider the butterfly diagram.  It reveals a cyclical pattern in the location of the sunspots.  In the year 1988 were most sunspots located near the equator or near latitudes of 30°S and 30°N?  How about in the year 1995?

11. Note the vertical lines separating the cycles in the butterfly diagram.  With what does each line correlate – a solar maximum or a solar minimum or with something else?

12. During a single sunspot cycle what can be said about the general location of sunspots – begin where, move in what direction, and end where?

13. Finally consider the magnetic butterfly diagram.  During the year 1986 what was the magnetic polarity of the Sun’s geographic north pole (+90°)?  the geographic south pole (-90°)?  Repeat for year 1996.

14. Note that the sunspots also have magnetic polarity that show up in this diagram.  Are there apparent changes in the sunspots’ polarity from one cycle to the next?  Explain.

15. Based on the magnetic data what can be said about the changes in the Sun’s overall magnetic field that occur from one solar minimum to the next?

16. Determine the number of years for the Sun’s magnetic field to undergo a complete cycle.  This is called the solar cycle.

This shows the yearly average number of sunspots plotted for each year.

This shows how sunspot numbers vary from month to month.  The yearly may be thought of as the average of the twelve monthly values (which are averaged from daily values).

The next six graphs are close-ups of the yearly averages with individual values circled:

This is called a “butterfly diagram”.  It shows the solar latitudes of sunspots observed during that time period.  Latitude is the angular distance north of the Sun’s equator.

This is a magnetic butterfly diagram that maps the Sun’s magnetic field at its surface.  White areas equal north magnetic fields; black areas equal south magnetic fields.