Penn AstroLab 2:
The Sun from Soho

Last Updated 11 Jan 1999

Name:__________________________Date project began: ____________________

Section #:_______ Instructor:___________________

Project Goals

  1. Learn about the SOHO mission and spacecraft.
  2. To perform virtual observations of the Sun using data from SOHO.
  3. Learn to recognize prominences, filaments, plages and sunspots and relate these phenomena to solar activity.
  4. Estimate the rotation period of the Sun.

Background Information

Studies of the Sun provide detailed knowledge of a typical star's temperature, density, chemical composition, and dynamics. Other stars in the Galaxy are still largely inaccessible to the same degree of scrutiny that we are able to give the Sun. So, when we gain insight into the nature of the Sun and the physical processes occurring in it, we learn something about those billions of other suns that populate our galaxy. Today, you will concentrate on the Sun's active regions. Observations of active regions help us to understand the Sun's magnetic field. Changes occur in solar active regions on time scales of days, hours, and sometimes minutes. The Sun also has a long term activity cycle of 22 years. The three main layers of the Sun's atmosphere (photosphere, chromosphere, and corona, from "lowest" to "highest") are affected by this activity in different ways. You will learn to recognize active regions in these levels and try to understand how active features in one level of the solar atmosphere relate to those in other levels.


We recommend that you keep a "laboratory notebook" for these exercises. In addition to recording your work on and answers to the questions, you can make notes related to these activities and questions that you have concerning the material presented. Also, if you have access to a printer, you can include printouts of the images and any maps generated here. If you are conversant with HTML and the web, you can also keep an "on-line" lab notebook!

Start running a web browser on a computer. We will start at the SOHO Home Page:

which has a number of interesting links. To find out about the SOHO Mission and its instruments, follow the links shown in the menu-bar at the left on that page: - mission description (English) - instrument description

Feel free to browse a bit before proceeding.

The main instrument that provides the data we will be using is the Extreme ultraviolet Imaging Telescope (EIT). This camera provides images in narrow bandpasses corresponding to four important atomic spectral lines of ionized Iron (Fe) and Helium (He):

Database Entry EIT band description
SOHO EIT, 171 A, Full Disk Fe IX, X 171 Å images
SOHO EIT, 195 A, Full Disk Fe XII 195 Å images
SOHO EIT, 284 A, Full Disk Fe XV 284 Å images
SOHO EIT, He II line, 304 , Full Disk He II 304 Å images

Note: the latest images from SOHO and ground-based solar observatories can be found at

which also gives a caption telling which instruments the data was taken with.

Step 1: Starting Data

Go to

which is the SOHO summary database. This is the site where images of the Sun from SOHO are received daily and archived. This will present a list of SOHO data available for that day. If you get a message that there is no SOHO data availbable for that day, work your way backward, then proceed to the previous day in the synoptic database and again look for SOHO data for that day. Continue as necessary until you find the latest available SOHO images. Again, you may wish to browse the various choices before continuing.

Write down the date of the latest SOHO data here:_________________________

Note: Click on the button at the top of the screen marked "Synoptic Data". You will be offered a variety of images to view from various ground-based observatories. Browse these images a bit if you wish. You should backtrack to the SOHO Data when you are done looking to proceed with the exercise.

Step 2: Investigations

  1. Choose to view the image called "SOHO EIT He II Line, 304 A, Full Disk" which shows the Sun as seen in the spectral line of singly-ionized Helium at a wavelength of 304 Å. Scan the solar disk. If you look carefully at "blank" areas of the Sun deviod of the loops and really bright areas indicative of prominences and sunspots (see below), you may notice that it appears mottled. This effect is called "supergranulation.". Choose a representative patch of the Sun, and sketch the granulation pattern as best you can:

  2. Scan around the surface for areas that look white against the red-orange background of the Sun. These are "sunspots". Note that when viewed in the normal continuum light, they will appear dark, since they are cooler than surrounding regions. Are there any notable sunspot(s) on the disk? If so, choose a particularly notable one and sketch it.

  3. Now look for huge loops of gas sticking off the Sun; these are called "prominences." The loop-like structure of these reflects the looping of magnetic field line pushed out from the surface of the Sun. Are there any notable prominences(s) visible on the Sun's limb? If so, choose a prominent prominence and sketch it.

  4. Now that you have made an initial tour of the Sun, try and get a feel for where things are on the full disk. Sketch the full disk of the Sun. Be sure to label any prominences and sunspots, especially those you sketched in detail above.

  5. The physical equatorial radius of the Sun is 696000 kilometers. Compare the extent of the features found above, especially sunspots and prominences, to the apparent diameter of the Sun's disk in the images and thus estimate their physical sizes (in km). Compare these to the radius of the Earth 6378 km.

  6. Go back one page, and select the "SOHO EIT, 171 A, Full Disk" image. This is a view of the Sun in the Fe IX/X line of highly ionized Iron. Note the different appearance of features in general (not just the different "color"). Sunspots may be easier to see in this image than in the He II line. Backtrack and view the images in the other Iron lines under the "SOHO EIT, 195 A, Full Disk" and "SOHO EIT, 284 A, Full Disk" links. Do you recognize any of the sunspots/groups of spots you saw in the other image, in particular, the one you drew in detail? Sketch it in one of the Iron lines (please note which one) and compare it to the He II sketch you made previously.

  7. Back up to the SOHO Data page for your starting date, and choose the image labeled "SOHO MDI, Magnetogram, longi. comp., Full Disk". This image is different than the EIT pictures, as it was taken using the Michelson Doppler Imager (MDI) onboard SOHO which is tuned to a set of spectral lines sensitive to the magnetic field direction (eg. N and S polarity, or out from and into the Suns surface). The bright and dark spots on the MDI image show regions of strong magnetic field of differing polarity. You should note that the dark regions are found near white regions, and that these correspond to active regions such as the sunspots. Sketch the detal from the MDI image for your sunspot region, and speculate upon the geometry of the magnetic field lines near the sunspot.

  8. Choose one group of sunspots to investigate. Below, draw a circle to represent the Sun, and mark the area where this sunspot group appears. You now want to view previous images of the Sun to track them across the disk. If you can manage it, the best thing is to open a new browser window then click ahead as before to select previous dates, or you can click backward two pages on your original web browser and select the day previous to this one. In either case, look at the "SOHO EIT, 171 A, Full Disk" Fe IX/X image. Note that because of missing days of data, you may have to play a bit and choose a different ending date for this part of the exercise (please note this dates used on your drawing below).

    Can you still see that group of sunspots? If not, choose another group to work with. Draw another circle, and mark the spot where you now see this group of spots. Continue to examine and draw images from previous days, until you reach the day when the spots are no longer visible.

  9. On what date did the sunspots first appear? On which limb of the Sun? Why did the sunspots appear to move across the surface of the Sun in this particular manner?

  10. Based on your observations, what is the rotation period of the Sun? Estimate your uncertainties also.

    ROTATION PERIOD OF SUN: _____________________ +/- _____________________ days

  11. From your observations, do you see any relationship between the sunspots, prominences or other structures on the Sun?

  12. Go back to the synoptic database for a particular day, and look at the ground-based images. The H-Alpha image is a good choice. In the full disk image, compare the brightness at the limb to the brightness in the center. Describe what you see. Is this consistent with the idea of "limb darkening"? Explain.

Further Explorations

These are some optional related activities you can pursue on-line or off:


The University of Pennsylvania Online AstroLab Project was created by Deborah Goldader and Steven T. Myers. Use of this AstroLab for educational purposes is granted by the authors.

The use of SOHO images for educational purposes has been strongly encouraged by NASA, and we are grateful for their use in this exercise. All SOHO images are courtesy of the various SOHO instrument consortia. SOHO is a project of international cooperation between ESA and NASA.

© Deborah Goldader, Steven T. Myers 1997,1998,1999,