Astronomy 102 Lab: Solar Observing

Pre-Lab Assignment: In this lab, you will determine the rotation rate of the Sun, determine the speed of material ejected from the Sun in a coronal mass ejection, and use resources available on the internet to learn more about the visible Sun and the more active regions of the Sun which are not readily visible to the naked eye. When you look at the photosphere of the Sun, you will be able to see some structures, such as limb darkening, sunspots, and granulation. Answer the following questions before coming to lab.

A) What is meant by "limb darkening?"

B) What is a "sunspot?"  

C) The units for velocity are miles per hour, based on distance (miles) divided by time (hours). Let's say you leave your house in your car and head for the interstate highway. At 9 AM you pass mile marker 84. When you pass mile marker 200, you note on your watch that it's 11:30 AM. What is the elapsed time in hours? What is the total distance traveled in this time frame? What is your average velocity during this time frame?

D) You have already done a lab on spectra, including emission and absorption lines. Given the graph below, which of these (emission or absorption line) is represented by the dip? What about the hump?

Objective: This activity will let you use images of the Sun taken from solar observatories to calculate the rotation period of the Sun. You will also measure the expansion rate of a coronal mass ejection and study other solar features and past solar activity.

Background: When the Sun is unusually active, the aurora borealis, or northern lights, can be visible in more of the continental U.S., including here in Champaign. You will use data from that time period, when there were many sunspots and large number of solar flares.

Procedure:
A. The visible Sun: You will find three images taken in white (visible) light on 2003/10/26, 10/27, and 10/28. These images are labeled with the date on which they were taken.

You will now use sunspot positions to track the rotation of the Sun. On the white light images, the sunspots are the dark splotches. Look at the three white light images and find three sunspot groups that appear on each image (each dot will be a group rather than a single spot). You will track the progress of these spots as the Sun rotates.

We will refer to the sunspot groups from left to right as sunspots "1," "2," and "3," respectively. The image we see of the Sun is a circle; if you measure horizontally across a circle, you will get the largest value at the equator and a value that approaches zero as you approach the poles. For each sunspot group, measure the distance across the disk of a horizontal line that passes through the group in centimeters. You only need to measure this on one image. Record the length of this line in the appropriate space below.

If you are having difficulty with this, please consult this example.

1A. Total horizontal distance across the image of the Sun for Sunspot 1: _______________________
1B. Total horizontal distance across the image of the Sun for Sunspot 2: _______________________
1C. Total horizontal distance across the image of the Sun for Sunspot 3: _______________________

Locate each sunspot group on 10/26 and measure the distance from the center of it to the left edge of the image of the Sun. Repeat the procedure for 10/27 and 10/28.

2. Fill in the table below. Round to the nearest tenth of a centimeter.

Distance from sunspot group to the left edge of the Sun (cm)
Date	Sunspot 1	Sunspot 2	Sunspot 3
10/26
10/27
10/28

You will now use your data to determine the rotation rate of the Sun. Calculate the distance traveled by each sunspot group between the 10/26 image and the 10/27 image. Do this by taking the distance on 10/27 and subtracting the distance on 10/26 for each sunspot group.

3A. Distance traveled by Sunspot 1 (10/26–10/27): ____________________
3B. Distance traveled by Sunspot 2 (10/26–10/27): ____________________
3C. Distance traveled by Sunspot 3 (10/26–10/27): ____________________

Calculate the distance traveled by each sunspot group between the 10/27 image and the 10/28 image.

4A. Distance traveled by Sunspot 1 (10/27–10/28): ____________________
4B. Distance traveled by Sunspot 2 (10/27–10/28): ____________________
4C. Distance traveled by Sunspot 3 (10/27–10/28): ____________________

Calculate the average distance traveled by each spot in one day of time. For example:

[Distance traveled by spot 1 (10/26–10/27) + Distance traveled by spot 1 (10/27–10/28)] / 2 = Average distance traveled by spot 1 per day.

5A. Average distance traveled by Sunspot 1 per day. __________________
5B. Average distance traveled by Sunspot 2 per day: __________________
5C. Average distance traveled by Sunspot 3 per day: __________________

To get the time for each sunspot group to cross the front of the Sun, take the total horizontal distance across the Sun for each group (Question 1) and divide it by the average distance traveled by the group in one day. For example: let’s say the total distance across the Sun for a sunspot is 10 cm. (Your values may be different.) The average distance traveled by a sunspot per day is 2.0 cm/day. Thus, the sunspot will take 10 cm / 2.0 cm/day = 5 days to cross the front of the Sun.

6A. Time for Sunspot 1 to cross the front of the Sun: ______________________
6B. Time for Sunspot 2 to cross the front of the Sun: ______________________
6C. Time for Sunspot 3 to cross the front of the Sun: ______________________

You have now calculated the time to cross the front of the Sun. The sunspot would require twice the time you calculated to go completely around the Sun. Calculate the average of your three answers above, then multiply by two to find the rotation period of the Sun.

7. Rotation period of the Sun: ________________________

The rotation period of the Sun near the equator is 25 days. Calculate a percent error for the rotation period you calculated.

8. Percent error: ________________________

9. Describe what aspects of the procedure may contribute to the error in your rotation period determination. ("Human error" is not an acceptable answer. Explain how a person would cause such errors.)

Now, turn your attention to the magnetogram, which was measured by GONG. Compare the white light image on 10/27 with the magnetogram, including the locations of the sunspot groups.

10. Is there a correlation between sunspots in the white light image and structures on the magnetic field image? What does this tell you about the relationship between sunspots and the magnetic field?

B. Coronal Mass Ejections Watch the video of a coronal mass ejection (CME). Look up the definition of a CME in your textbook or consult this page on Space Weather Phenomena so you know what you are looking at. You will see a central black circle that represents the Sun and a second white circle surrounding the first one; this is the disk the SOHO satellite uses to block out the Sun's brilliance.

11. Describe how a CME appears in the video in a few sentences.

We want to measure the speed of one of these ejections from the Sun. Please look at the images of a CME. We need to find the scale of these pictures first, which is similar to what you would find on a road map, where 1 cm would correspond to 10 miles. The white circle at the center of each image represents the disk of the Sun. The tick marks along the bottom of the image mark off units of the Sun's diameter.

Measure the diameter of the outer edge of the white circle in millimeters. The Sun's diameter is actually 1,400,000 kilometers. Plug your measured value into the following equation to calculate the scale of the photographs. Round all answers in this section to three significant digits, such as 18,645 km rounded to 18,600 km.

12. Scale of the photos = 1,400,000 km / _____________ mm = _______________km/mm

We'll follow the progression of the outside of expanding bubble of material on the right side of the Sun. Select a feature that you can see in all five images, such as the extent of the bright structure or the inner edge of the dark loop shape. The spikes disappear over time so they shouldn't be used. Measure the position of the feature in each image and record the information in the table below.

Since the Sun is much larger than a few millimeters across, you convert the position to km by multiplying by the scale factor you calculated in Question 12. For example, if your scale is 100 km/mm, a measurement of 5 mm corresponds to 5 mm :× 100 km/mm = 500 km. Here is an example of the measurements and the conversions.

Calculate the change in position by subtracting between the current position and the previous position. The first line is blank, but second line should have the change between position 1 and position 2. Calculate the speed by dividing the change in position by the change in time.

13. Fill in the table below. Round your answers in this section to three significant digits.

Time	Position (mm)	Position (km)	Change in Position (km)	Change in Time	Average Velocity (km/s)
08:05			—————	———————	———————
08:36				1,860 seconds
09:27				3,060 seconds
10:25				3,480 seconds
11:23				3,480 seconds

14A. There are two primary forces that act on the CME. What force would pull the CME back towards the Sun?

14B. What force would push the CME away from the Sun?

15A. Do you expect the CME to be accelerating, decelerating or moving at a constant speed as it leaves the Sun?

15B. How well does that match the changes in velocity you calculated?

C. The Sun on the Web: Use the websites linked below to answer the following questions.

First, go to spaceweather.com. Look at the data in the left margin to see the most recent sunspot number.

16. Record the date and the sunspot number: ______________________________

The sunspot number tells astronomers if we are around a solar maximum or a solar minimum. The sunspot cycle shows how the sunspot number varies over time. The length of the cycle is the average time between successive solar maxima. Use the plot of monthly averaged sunspot numbers since 1750 to estimate the length of the sunspot cycle. Since no cycle is exactly the same, you will get a more accurate result by calculate the average time over several cycles. Pick two sunspot maxima that are far apart and note the cycle number listed below the peak. For example, the current cycle is "24."

17A. First maximum year: ______________________
17B. Second maximum year: ______________________
17C. First cycle number: ______________________
17D. Second cycle number: ______________________

Determine the time difference between the two sunspot maxima and the difference in cycle numbers. Find the average sunspot cycle length by dividing these two differences.

18A. Year difference: ______________________
18B. Cycle difference: ______________________
18C. Average cycle length: ______________________

Use the SILSO website to find the monthly average sunspot number over the last six cycles to answer the next two questions.

19. What is trend of the sunspot number in the current cycle? Is it closer to a maximum or a minimum?

20. When is the solar maximum of cycle #25 going to occur? Show your work.

21. What was the maximum sunspot number ever recorded? What year did it occur?

D. The spectrum of the solar wind: In the Spectra lab we saw how you can use the spectrum of a glowing object (like a star) to determine its composition. The SOHO satellite has done that with the Sun. One of the instruments is the MTOF or "Mass, Time-of-Flight" spectrometer, which measures the elements in the solar wind.

Look at the CELIAS MTOF Mass Time-of-Flight Spectrometer page.

Look at the spectrum in the graph. The atomic mass of each element is listed across the bottom and the abundance of each element in the solar wind is measured on the vertical axis. The sharpest peaks are the most abundant and the chemical symbols are listed at the top.

22. Which element is the most abundant in the solar wind?

23. Which four elements are the next four most abundant?

24. Are you looking at an absorption line spectrum or an emission line spectrum?