Download Project 3: Solar rotation period and properties of solar prominences

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.
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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.
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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.