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Astronomical Laboratory 29:137 Fall 2007 Project 3: Solar rotation period and properties of solar prominences Summary Images of the sun obtained using the solar hydrogen alpha (Hα) telescope in the large dome (Van Allen hall roof) are used to find the rotation period of the sun and to find the speed of ejecta from a solar prominence. Background The rotational period of an object is the time it takes to revolve once on its axis. For example, the rotation period of the Earth is one sidereal1 day. In order to observe the rotational period of any rotating sphere, we can measure how many degrees of longitude a surface feature moves in a given time interval. We then solve for the period by taking the ratio of one full rotation (360 ) to the observed angular motion and multiplying by the time interval. For instance, if we observe that a given feature moves 25 of longitude in 2 days, then the rotation period is: 360 P 2 days 29 days 25 This gives us an easy way to measure the rotation of the sun. All we need to do is find some feature, such as a sunspot, and see how far it moves in a given time. Active regions of the sun come in a number of varieties, such as sunspots, flares and prominences. All active regions have large magnetic fields by definition. Sunspots are relatively cool areas of the sun's surface which A sidereal day is the time for one rotation as measured in a fixed frame (e.g. the stars). For the Earth, this is ~23h56m3s, as opposed to 24h, which is the average solar day (noon to noon). This distinction is unimportant for this lab. 1 Astronomical Laboratory 29:137 Fall 2007 clearly stand out from the rest of the surface of the sun because of their relative darkness. The temperature of a sunspot is about 4200 K compared to a temperature of about 5800 K for the rest of the sun. If we could see only the spots without the sun, they would appear bright red. Flares occur in regions of the sun where the magnetic field gets twisted very tightly by the differential rotation of the sun. This magnetic twisting then releases energy. These active regions emit very high energy photons, such as X-rays. Prominences (which sometimes cause coronal mass ejections) are regions of cool gas in the corona of the sun. When seen against the disk, these appear as dark filaments, but when seen above the Sun’s limb, they can be very bright. Coronal mass ejections are caused by the eruption of prominences, and are blasts of gas which move out through the Sun’s corona, and on into the solar system. Procedure Observing 1. Since observations must take place during the day, you need be able to access the roof and dome during the day. A key for the roof and the large dome will be available in the lab room (665). As well as a sign-up sheet. Each team can reserved 1 hour per day in advance. 2. Pick a day that is mostly clear. A few clouds are ok, but too many will make observations difficult. 3. Initialize the telescope and begin controlling the telescope with the Sky software. 4. Make absolutely certain that both night-sky telescopes have their lens caps on. 5. Set the telescope to track the Sun. Make sure the TV cable (see diagram of camera control box below) is plugged into the correct camera (the camera control box is attached to the black arm supporting the telescope mount). 6. Turn on the TV monitor. Use the joystick to slowly center the Sun on the TV monitor (as pictured at right). Astronomical Laboratory 29:137 Fall 2007 7. Adjust the shutter duration and gain settings. Adjust the shutter duration using the knob on the camera control box (see picture below). Different shutter settings are indicated on the white square underneath the knob. Adjust both of these settings while watching the TV monitor to get a feel for how they change the image. When observing sunspots in Part A, you will want to use a low shutter setting, perhaps 0 or 1, and then you will want to adjust the gain until you have the best resolution on the sunspots. When observing Coronal Mass Ejections, move the shutter to one of the highest settings, look at the outer edge of the Sun for anything that is coming off the surface and then readjust the gain to provide the best resolution of those features. Gain Knob Shutter Duration TV Cable 8. To capture an image from the video camera (that is, the image displayed on the TV monitor) you will need to use an additional piece of software. This is called FlashBus Spectrim FBG and is displayed on the desktop as an icon named, FBSpectrimG. Opening this software will bring up a window with a live image of the Sun that should be identical to the one on the TV monitor. Resize the FBSpectrimG window by pulling and dragging on the lower right edge until the top of the window reads “640x480” instead of “320x240”. 9. To capture the image go to File, Save Image and select a directory to save to. See lab instructor for an assigned image directory if you don’t already have one. Be sure to note the time that the images were taken (not necessarily the same as when they were saved). A sensible scheme to keep track of this might be to name the image file like: yyyy-mmdd_hhmmss.tif. Select TIF for the image type. 10. Take data for 10 minutes, saving your images your local image directory. Be sure to note the time that the images were taken where you saved them. Try to get at least 10 good images, but feel Astronomical Laboratory 29:137 Fall 2007 free to take more. (Note: Don’t sacrifice quality for quantity and speed here!) 11. For the expansion speed of a coronal mass ejection (henceforth CME) the process is slightly different. Increasing the shutter duration will allow you to see any CME’s (if there are any). If may also be necessary to adjust the ring on the telescope seen in the illustration below and indicated by an arrow. This ring adjusts the filter and you will see that rotating this ring will allow you to optimize the contrast of the image. Analysis Part A: Rotation Period of the Sun 1. Run MaxIm. Click on File, then Open. Click on the “look in” box and find the directory where you saved your images. Open all the images you took by clicking on the first name, holding down the shift key on your keyboard, and clicking the final name. 2. Find the pixel scale of the image. To do this, place your cursor at the top of the image of the sun, at the center. Record the coordinates of this point. Now, move to the bottom of the image of the sun, keeping the x-coordinate constant. Record the coordinates of the bottom of the sun. Subtract the y-coordinates. This is the diameter of the sun in pixels. Divide the diameter of the sun in km (see Appendix H) by the diameter of the sun in pixels to get the number of km in a pixel. 3. You will need to make a blink movie in order to see any motion of active regions on the surface of the sun. Select View, then Blink from the menu to open the blink window. Click on your first image, then the button labeled >>. Click on the next image and the button again, Astronomical Laboratory 29:137 Fall 2007 then click OK. Click Play to start the movie. Adjust the skip rate to a comfortable level (1 sec is good). Note if any motion of the active regions occurs. (Don’t be surprised to see no motion at first, as your second image should be no more than a minute after the first!) 4. Use Helios to determine the solar latitude ( ) and longitude ( ) of an active sunspot region at least four times over a period of a week or more. 5. The period is determined by plotting the solar longitude of a given feature versus time and fitting a linear fit. Use Graphical Analysis to determine the uncertainty in the slope and hence the derived period. 6. Extra credit. If there are several sunspots visible in both sets of images, calculate the period as a function of latitude. (Note: unlike the Earth, which rotates like a solid body, the sun is a gaseous body and rotates differentially. This means the period depends on the solar latitude. Try to determine the dependence of period on latitude. Part B: Expansion Speed of a CME 1. Load all of your images that showed prominences into MaxIm. 2. First make a blink movie in order to see any motion of active regions on the surface of the sun. Select View, and then Blink from the menu to open the blink window. Click on your first image, then the button labeled >>. Click on the next image and the button again, then click OK. Be sure to keep the images in the order that they were taken Click Play to start the movie. Adjust the skip rate to a comfortable level (1 sec is good). 3. To measure the expansion speed of the CME, we need to know the times the images were taken, as well as a pixel measurement of the expansion distance. Pick two good images taken far enough apart that there is a measurable change. Write down the times these images were taken, and take the difference for the total expansion time. 4. On the two images, measure the change in position of the “end” of the prominence. Zooming in may be helpful in making accurate measurements. Record the (x,y) coordinates on the first image, then do the same for the second image. Use the Pythagorean Theorem to get the total distance for the expansion. 5. You will want to convert the pixel distance into more familiar units, since a speed given in pixels per second does not give much information. Astronomical Laboratory 29:137 Fall 2007 Use the image scale you calculated in part A to convert distance into km. 6. Divide the distance you calculated by the time in seconds. This is the expansion speed of the CME. Appendix: Helios, A Sun Program Measure the position of a sunspot Helios will calculate the position of a sunspot in solar coordinates (latitude, longitude) if you enter the date and time of the observation and the x and y coordinates of the sunspot (see diagram), and the diameter of the Sun in pixels. Unfortunately, there is not a particularly elegant way to find the x and y coordinates of a sunspot. A method has been developed in MaxIm that has proven effective. 1. Find the center position of the Sun in pixels. X0,Y0 This can be done using the line profile tool in MaxIm to overlay a straight line on the image and center it by eye horizontally on the Sun’s outline. Write down the y-pixel position (as shown above, circled) when it the line look centered. Repeat for a vertical line, using the x-pixel value. You now have the center coordinates of the Sun’s image in pixels (x0 , y0). You can use this same method of overlaying a line from the line profile tool to measure the diameter of the Sun in pixels. Astronomical Laboratory 29:137 Fall 2007 2. Measure the x and y pixel positions of the sunspots In MaxIm, place the cursor over a sunspot and record the (x,y) coordinates. Subtract the Sun’s center pixel positions from the sunspot pixel positions to find the x and y coordinates of the sunspot. These can be entered in Helios. If the center pixel coordinates of the Sun were found to be (x0 , y0) and the sunspot pixel coordinates were found to be (xspot , yspot) the x and y coordinates of the sunspot would be (xspot – x0 , yspot – y0). 3. Input x,y coordinates into Helios to get the solar latitude and longitude of each feature Now open Helios and input the date and time when the image was taken. Helios needs to know the location of the observatory being used also, use 41.5 N 91 W as in the example below. The diameter of the Sun in pixels is entered under Disk Diam and the x and y coordinates of the sunspot are entered under Sunspot Disk Location & Size. The heliographic latitude and longitude will then appear at the bottom of the window in the area circled below.