Interpreting images of Coronal Mass Ejection?

[rite2zz]

Hi, I'm doing the project on using the SOHO satellitto measure motion of coronal mass ejections
here is the link to that page (please visit this page so you know what i'm talking about)
https://www.sciencebuddies.org/mentorin ... ?from=Home

i dont understand one of the steps in that procedure. It is the first step that says
Below is a series of five images taken from one of the coronagraphs on LASCO. In each of the images, the white circle shows the size and location of the Sun. The black disk is the occulting disk blocking out the disk of the Sun and the inner corona. The tick marks along the bottom of the image mark off units of the Sun's diameter. To the right of the disk we can see a CME erupting from the Sun.

My question is...if the white disk is the size of the sun, then what is the huge black disk surrounding it and why does the CME only appear to show at the edge of the big black disk if the whtie one (in the center) is the sun, shouldn't the CME be sort-of attached to the other edge of the white disk (he sun)?

In addition, hwo can you 'measure' the position of one of the feature, i dotn understand how a position is 'measurable'?
thanks in advance for all helpers

[Craig_Bridge]

what is the huge black disk surrounding it and why does the CME only appear to show at the edge of the big black disk if the whtie one (in the center) is the sun,

Your question implies you don't understand what an "occulting disk" is. You might try reading http://www.daviddarling.info/encycloped ... _disk.html

You can also close one eye and use a quarter (or other coin) as an occulting disk while looking at your computer screen. Hold the coin between your eye and the center of the screen and move it closer to your eye then closer to the screen. Anything between the occulting disk and the screen is blocked from view. Keep the coin at a fixed point between your eye and the screen and move a finger or other object between the coin and the screen to see how the diameter of this occlusion cone gets bigger the closer to the screen the occulted object is.

The sun emits a huge amount of energy and will destroy your eyes and most instruments that look directly at it. The image you are refering to appears to have been taken by the C3 instrument. The C3 occulting disk blocks out a central cone that is approximately 3.5 times sun radius at the mean sun distance. In the diagrams the black occulting diameter is shown as approximately 4 sun diameters so it was proably taken when the earth was closer to the sun than the mean.

The C1 occulting disk was only 1.5 sun diameters and after a year of operation, the positional accuracy of SOHO had caused enough incursions (occulting disk not fully blocking the sun) to degrade the C1 instrument. If you back calculate the angular deviation required for an incursion, you will find it isn't much!