Visible sunspots lie in the

THE SUN

LAB 4

Objectives 1. To determine the size of sunspots 2. To determine the rotation period of the Sun. 3. To study some images of Coronal Mass Ejections from the sun.

Equipment 1. Metric ruler with centimeter markings 2. Calculator 3. Only print pages 1 through 7 of this lab!

Introduction

Ever since Galileo saw sunspots on the surface of the Sun with his telescope, we have been intrigued by these “freckles” and their study has helped us to determine many properties of the Sun. In the last few decades, astronomers have developed some fantastic tools, one of which is SOHO, the Solar and Heliospheric Observatory. This is a 2-ton spacecraft located between Earth and Sun, about 1.5 million kilometers from earth. It continuously “looks” at the Sun and its many instruments have beamed back a tremendous amount of information since its launch in 1995. The cameras on SOHO capture many different types of images, which you will use in this experiment.

The photosphere is the yellow surface of the Sun we see. While Galileo and others like Thomas Herriot of England recorded sunspots after 1611, historical records from Chinese astronomers indicate they were also aware of this phenomenon. However, sunspots are very small, and because it is dangerous to look at the Sun for long periods of time, we have to use special precautions for viewing it. We can use specially filtered solar telescopes or make a special device with a pinhole, lens and mirrors to “project” the sun’s image onto a piece of paper. The latter is a relatively simple way to view the Sun safely, and we have a special device to do this conveniently on campus. You must NEVER focus a telescope or binoculars on the Sun! These devices work by concentrating light to produce a sharp image. In the case of the Sun, the concentrated light can melt the glue holding the lenses in place and completely destroy the telescope or binoculars. More dangerously, the concentrated light can permanently damage the retina in your eyes. For these reasons, it is very important to view the Sun safely. Simply using dark glasses especially during eclipses is not enough.

A total solar eclipse is a particularly fantastic sight and a great opportunity to study some features of the Sun. During a total solar eclipse, the Sun, Moon and Earth all lie in exactly the same plane, with the Moon between the Sun and Earth. By coincidence, the Sun is 400 times bigger than the Moon, but it is also 400 times further away. This means they have the same angular size in the sky, about 0.5 degrees, and the size of the Moon’s disk is the same as the Sun. When they are lined up exactly, and the Moon is at the proper distance from Earth, the Moon’s disk can completely cover up the Sun. However this condition lasts for less than 7.5 minutes and is visible from only a few places on Earth. This makes total solar eclipses rare events that can be frightening if you are not aware of what’s happening! Daylight gradually gives way to darkness and you can actually see the Sun with a “bite” taken out of it. As the bite increases, the sky gets darker, stars become visible, and at “totality” you see a black circle surrounded by a ghostly white light. The black circle is the Sun covered up by the Moon, and the white light is the Sun’s corona, which is always there, but since its light is too dim, it is

2

obscured by the brighter light from the photosphere. Since the Moon is moving, within a few minutes it will uncover the Sun, the corona will once again become invisible, the Sun’s bite will appear in the opposite direction, the sky will gradually brighten and in less than an hour, everything will go back to ‘normal.’

Partial solar eclipses are not as dramatic, but still impressive to see. We encourage you to go the extra mile to view an eclipse if you get the opportunity. But you have to take special precautions to view all solar eclipses. Since the Sun’s light is diminished, it causes the pupils in our eyes to dilate and there is a greater risk of retinal damage. Hence it is very important NOT to look directly at the Sun as the Moon creeps in. Special filters can be used to view the Sun safely and they are now available cheaply at eclipse sites. Keep August 21, 2017, on your calendar when a total solar eclipse will be visible from some parts of U.S.A. Check out the eclipse path in your textbook and make plans to go view it – safely!

Modern solar telescopes use the same principle of a total solar eclipse to study the corona. The front end of the telescope is fitted with a filter that effectively covers up the disk of the Sun, so you see it as a black disk surrounded by the corona or the solar atmosphere. Some of the SOHO images that show a black disk in the middle, are using this principle to study areas outside the photosphere.

While the rotation period of the Earth’s equator and poles is the same (24 hours) the Sun’s equatorial region rotates faster taking less time than the poles. When astronomers first saw sunspots, they speculated whether the black dots were other planets casting a shadow on the Sun. But Galileo soon decided these dots were on the Sun’s surface because they appeared to “flatten out” and look oval shaped when seen closer to the edge of the Sun. It is like sticking a quarter to a balloon. If you see it head-on, the quarter looks like a circle, but if you rotate the balloon and keep the quarter near the edge, it will look like an oval.

The gas and plasma making up the Sun is a soup of moving charged particles which generate a strong magnetic field. The magnetic field lines get twisted as the equatorial regions move faster and at some point they push out of the photosphere and re-enter it, creating a very strong local magnetic field, which appears as a sunspot. The sunspot is dark because it is cooler than the surrounding gas, its temperature is about 4000 K compared to 5800 K for the surrounding photosphere. But why is it cooler? Remember the gas in the photosphere is in constant convective motion. The strong local magnetic field in the sunspot hinders this motion, and hot gases are unable to enter the sunspot region, causing a small area which is cooler – and hence dark.

Since magnetic fields are characterized by two poles, north and south, sunspots usually occur in pairs, with field lines linking the two. If the field is strong enough, gas in the chromosphere can get trapped in an arc above the surface, and it is visible as a giant loop called a prominence. These can be visible in solar telescopes and SOHO has also photographed many impressive prominences.

Two other events photographed on the surface of the Sun are solar flares and coronal mass ejections. Flares occur when the magnetic fields around sunspots get so twisted, they rearrange themselves and shoot out a tremendous burst of energy in the form of X-rays and ultraviolet light. Massive disturbances on the Sun’s surface also can cause giant bubbles of gas filled with charged particles from the corona to shoot outwards, which is called a coronal mass ejection. If it is directed towards Earth, it will reach us in a few days and the high energy particles can cause havoc to our satellites and telecommunication grids.

3

Pre-lab Questions

1. How can astronomers and astronomy students study the surface of the Sun? (a) By focusing the sun in their refracting telescope (b) By projecting the Sun’s image on a white screen (c) By focusing the sun in binoculars (d) All are okay to use

2. Why is total solar eclipse a worthwhile experience?

(a) You can see the corona (b) Stars become visible in the day time sky (c) It does not happen very often (d) All of the above make it a worthwhile experience

3. Where do sunspots lie?

(a) In the corona (b) In the photosphere (c) In the chromosphere (d) In the radiative zone

4. The sun rotates differentially. This means

(a) Its rotation rate is different from that of the planets (b) Its rotation rate depends on the temperature of its layers (c) Its rotation rate is faster at the equator than the poles (d) Its rotation rate depends on the magnetic field

5. Sunspots are cooler regions on the Sun because

(a) The photosphere is cooler than the corona (b) The photosphere is cooler than the chromosphere (c) The strong magnetic field causes prominences (d) The strong magnetic field prevents hot gases from entering this region

6. If there is a coronal mass ejection on the Sun,

(a) It will hit earth within 8 minutes since it travels at the speed of light (b) It can disturb earth’s magnetic field within a few days (c) It can cause auroras and radio blackouts within a few days (d) Both b and c are possible

Lab Exercise You are presented with pictures of 12 sunspots marked A, B, and C taken by SOHO on a daily basis from 6-22 to 7-3. These are at the end of the lab. Superimposed on the Sun is a “longitude grid” which enables you to estimate how much each sunspot has moved each day. You will use these 12 images to first figure out the size of each sunspot and then the rotation rate of the Sun.

While you are advised to print out these instructions to work on them, we do not recommend you print out the images! Not only will you be wasting a lot of printer ink, but printers can distort the images, rendering them inaccurate. Therefore we recommend you only print pages 1 to 7 and keep the images on pages 8-20 on the computer screen while you take the measurements required in the lab.

4

Estimating the Size of Sunspots A. Go to the image at the end of the lab for 6-25. Take a good look at your centimeter

ruler, and locate the 1 cm, 2 cm etc markings. Notice that there are 10 thin lines between each centimeter. The distance between each of the thin lines is 1 millimeter. You will be taking the first set of readings in millimeters.

B. Measure the diameter of the Sun in millimeters and record it in the table below.

Remember 1 centimeters = 10 millimeters

C. Measure the size of sunspot A, B and C in millimeters and record it in the table below.

D. Repeat steps B and C for 6-26 and 6-27.

E. Find the average of the values for each column, that is add the numbers in each column and divide by 3.

7. Size of Sunspots

Sun’s diameter Sunspot A Sunspot B Sunspot C

6-25

6-26

6-27

Average

F. Since we know the size of the Sun, we can set up a ratio to find the size of the sunspots

(Sunspot diameter)real = (Sun diameter)real (Sunspot diameter)mm (Sun diameter)mm

OR (Sunspot diameter)real = (Sun diameter)real x (Sunspot diameter)mm

(Sun diameter)mm

G. Use the bold equation above to calculate the diameter of each sunspot. Take the real diameter of the Sun to be 1.4 x 106 km. This will give you the diameter of each sunspot in km.

8. Diameter of Sunspot A =

Diameter of Sunspot B = Diameter of sunspot C =

5

9. The Earth’s diameter is 1.3 x 104 km. Was each sunspot bigger, smaller or about the same size as the Earth?

Calculating Solar Rotation

H. Look at the images from 6-22 to 7-3 carefully and record in the table below the longitude for each sunspot. You will have to estimate to the best of your ability when the sunspot is in-between 2 longitude lines.

Sunspot A Sunspot B Sunspot C

6-22 Not visible

6-23

6-24

6-25

6-26

6-27

6-28

6-29

6-30

7-1

7-2

7-3 Not visible Not visible

10. Record the values of each sunspot’s longitude for 6-24, 6-27, and 6-30.

I. Next, let’s calculate how many degrees each sunspot travelled in one day. For example if sunspot A was at -60 degrees on 6-22 and at -45 degrees on 6-23, it has travelled 15 degrees (60-45) from 6-22 to 6-23. Fill in the table below. Take special care between 6-26 and 6-28 when the longitudes go from negative to positive! Think before you record.

Sunspot A Sunspot B Sunspot C

6-22 to 6-23 N/A

6-23 to 6-24

6-24 to 6-25

6-25 to 6-26

6-26-to 6-27

6-27-to 6-28

6-28-to 6-29

6-29-to 6-30

6-30-to 7-1

7-1 to 7-2

7-2 to 7-3 N/A N/A

6

11. Record the number of degrees each sunspot travelled from 6-24 to 6-25, 6-27 to 6- 28, and 6-30 to 7-1.

12. Find the average of all the numbers in the sunspot A column, that is add up all the

numbers and divide by the number of entries (probably 10). Do the same for sunspot B and sunspot C. This gives the average number of degrees each sunspot travels in one day. Average degrees travelled by sunspot A = X =

Average degrees travelled by sunspot B = Y =

Average degrees travelled by sunspot C = Z=

13. Find the average of the degrees travelled by A, B, and C. Use X, Y, Z from step 12. Average degrees travelled by A, B and C = X + Y + Z /3 =

14. The Earth also travels 1 degree per day, so add one degree to your answer in 13. This gives the number of degrees the Sun travels in one day. Number of degrees travelled by the Sun in 1 day =

15. How many days will it take the Sun to travel 360 degrees? Use your answer in 14 to figure this out. This is what we were looking for – the time for one rotation of the Sun.

Calculating the speed of a coronal mass ejection

J. Take a look at the 4-image collage that was taken by SOHO on November 25 and 26 2000. The images were taken by different instruments, hence they look different. The upper left image was taken by Instrument MDI on Nov 25 at 16:00 UT. The upper right was taken by EIT at 195 A at 18:48 UT, the lower left by Lasco 14 hours later on Nov 26 at 08:06 UT and the lower right 3 hours later at 11:42 UT.

K. The upper left image is a sunspot group which is visible as a flare, in the upper right

image. It has become a full-blown coronal mass ejection (CME) in the lower left image and expanded out from the Sun in the lower right image.

L. The black circle in the center of the two lower images is the Lasco cover or filter. The white circle is the actual Sun.

M. In the lower left image, measure the size of the Sun (white circle) in millimeters, and measure the height of the CME above the surface of the Sun. The CME’s are the bright white streaks AND the lighter gas cloud all the way to the edge of the image to the green arrow. In the lower right image measure the Sun (white circle) in millimeters, and measure height of the CME above the surface of the Sun. The CME is the white cloud that goes almost to the edge of the image to the yellow arrow.

16. For lower left image Size of Sun in mm = (Sun’s Diameter)mm

Height of CME in mm = (CME Height)mm

7

17. Use the same type of equation used in step F to figure out how far from the Sun’s surface the CME lies. (CME Height)real = (Sun’s Diameter)real x (CME Height)mm

(Sun’s diameter)mm

18. For the lower right image Size of Sun in mm = (Sun’s Diameter)mm

Height of CME in mm = (CME Height)mm

19. For lower right image

(CME Height)real = (Sun’s Diameter)real x (CME Height)mm

(Sun’s diameter)mm

20. How far did the CME travel in the 3.6 hours between the two images? To do this, subtract your answer to # 17 from your answer to # 19.

21. What was the speed of the CME? Since speed is distance/time, CME’s speed = your answer to # 20/3.6 hours. Your answer will be in km/h.

22. Earth is 150 x 106 km from the Sun. How long will it take this CME to reach Earth? Use the equation Time = distance /speed found in # 21

23. Write your conclusions for this lab. What did you learn? What surprised you? Were the equations and numbers difficult to understand? Give us some feedback for improvements.

This lab has been adapted from SOHO resources for educators. Images are courtesy of SOHO Consortium. SOHO is a project of international cooperation between ESA and NASA. Grading rubric: 1-6: 0.5 points each 7-9: 1.0 point each 10: 3 points 11, 12: 2 points each 13-22: 1 point each 23; 2 points

8

C

A

9

C

C B

A

10

B

C

A

11

C

B

A

12

C

B

A

13

C

B

A

14

C

B

A

15

C

B

A

16

C

B

A

17

C

B

A

18

C

B

A

19

B

20

C M

E H eigh

t to h

ere!

C M

E H eigh

t to h

ere !