Exercise #1: Getting familiar with the Size and Appearance of the Sun

Station 1: In this first station we simply present some images of the Sun to familiarize yourself with what you will be seeing during the remainder of this lab. Note that this station has no questions that you have to answer, but you still should take time to familiarize yourself with the various features visible on/near the Sun, and get comfortable with the specialized, filtered image shown here.

• The first image in this station is a simple ``white light'' picture of the Sun as it would appear to you if you were to look at it in a telescope that was designed for viewing the Sun. Note the dark spots on the surface of the Sun. These are ``sunspots'', and are dark because they are cooler than the rest of the photosphere.

• When we take a very close-up view of the Sun's photosphere we see that it is broken up into much smaller ``cells''. This is the ``solar graunulation'', and is shown in picture #2. Note the size of these granules. These convection cells are about the size of New Mexico!

• To explore what is happening on the Sun more fully requires special tools. If you have had the spectroscopy lab, you will have seen the spectral lines of elements. By choosing the right element, we can actually probe different regions in the Sun's atmosphere. In our first example, we look at the Sun in the light of the hydrogen atom (``H-alpha''). This is the red line in the spectrum of hydrogen. If you have a daytime lab, and the weather is good, you will get to see the Sun just like it appears in picture #3. The dark regions in this image is where cool gas is present (the dark spot at the center is a sunspot). The dark linear, and curved features are ``prominences'', and are due to gas caught in the magnetic field lines of the underlying sunspots. They are above the surface of the Sun, so they are a little bit cooler than the photosphere, and therefore darker.

• Picture #4 shows a ``loop' prominence located at the edge (or ``limb'') of the Sun (the disk of the Sun has been blocked out using a special telescope called a ``coronograph'' to allow us to see activity near its limb). If the Sun cooperates, you may be able to see several of these prominences with the solar telescope. You will be returning to this image in Exercise #2.

Station 2: Here are two images of the Sun taken by the SOHO satellite several days apart (the exact times are at the top of the image). (8 points)

• Look at the sunspot group just below center of the Sun in image 1, and then note that it has rotated to the western (right-hand) limb of the Sun in image 2. Since the sunspot group has moved from center to limb, you then know that the Sun has rotated by one quarter of a turn ($90^\circ$).

• Determine the precise time difference between the images. Use this information plus the fact that the Sun has turned by 90 degrees in that time to determine the rotation rate of the Sun. If the Sun turns by 90 degrees in time $t$, it would complete one revolution of 360 degrees in how much time?

  
  
  
  
  
  
  
  
  
  

• Does this match the rotation rate given in your textbook or in lecture? Show your work.

  
  
  
  
  
  
  
  
  
  

In the second photograph of this station are two different images of the Sun: the one on the left is a photo of the Sun taken in the near-infrared at Kitt Peak National Observatory, and the one on the right is a ``magnetogram'' (a picture of the magnetic field distribution on the surface of the Sun) taken at about the same time. (Note that black and white areas represent regions with different polarities, like the north and south poles of the bar magnet used in the second part of this lab.) (7 points)

• What do you notice about the location of sunspots in the photo and the location of the strongest magnetic fields, shown by the brightest or darkest colors in the magnetogram?

  
  
  
  
  
  
  
  

• Based on this answer, what do you think causes sunspots to form? Why are they dark?

Station 3: Here is a picture of the corona of the Sun, taken by the SOHO satellite in the extreme ultraviolet. (An image of the Sun has been superimposed at the center of the picture. The black ring surrounding it is a result of image processing and is not real.) (10 points)

• Determine the diameter of the Sun, then measure the minimum extent of the corona (diagonally from upper left to lower right).

  
  
  
  
  
  
  
  

• If the photospheric diameter of the Sun is 1.4 million kilometers (1.4 x $10^6$ km), how big is the corona? (HINT: use unit conversion!)

  
  
  
  
  
  
  
  

• How many times larger than the Earth is the corona? (Earth diameter=12,500 km)

  
  
  
  
  
  
  

Station 4: This image shows a time-series of exposures by the SOHO satellite showing an eruptive prominence. (15 points)

• As in station 3, measure the diameter of the Sun and then measure the distance of the top of the prominence from the edge of the Sun in the first (earliest) image. Then measure the distance of the top of the prominence from the edge of the Sun in the last image.

  
  
  
  
  
  
  
  
  
  

• Convert these values into real distances based on the linear scale of the images. Remember the diameter of the Sun is 1.4 x $10^6$ kilometers.

  
  
  
  
  
  
  
  
  
  

• The velocity of an object is the distance it travels in a certain amount of time (vel=dist/time). Find the velocity of the prominence by subtracting the two distances and dividing the answer by the amount of time between the two images.

  
  
  
  
  
  
  
  
  
  

• In the most severe of solar storms, those that cause flares, and ``coronal mass ejections'' (and can disrupt communications on Earth), the material ejected in the prominence (or flare) can reach velocities of 2,000 kilometers per second. If the Earth is 150 x $10^6$ kilometers from the Sun, how long (hours or days) would it take for this ejected material to reach the Earth?

  
  
  
  
  
  
  
  
  

Station 5: This is a plot of where sunspots tend to occur on the Sun as a function of latitude (top plot) and time (bottom plot). What do you notice about the distribution sunspots? How long does it take the pattern to repeat? What does this length of time correspond to? (3 points)