Steps Toward Finding Twist
Previous work by Jessica Windschitl had been done on a program to compute both the twist and the kink in vector magnetograms. My first task was to determine ways to better analyze the data. For instance, the center of the tube was found through simple flux weighting after removing all values below the 3-sigma level (300 G). After looking at a wide range of magnetograms and looking at every threshold between 50G and about 2/3 of the maximum flux, we determined that a cutoff of 50% of the maximum flux was the best way to determine the flux weighted centroid. Another important consideration we found was which method we were using to fit the data with. After reviewing the fitting programs we had access to we found which were most appropriate to our equations and data.
We next tried looking at every pixel surrounding our flux weighted centroid. Perhaps these were better choices for the actual center of the flux tube? Looking at each of these pixels, we were faced with two methods of determining which is the "best" pixel. The first was a simple statistical consideration. Maybe we should choose the pixel with the lowest residual? The other was a theoretical consideration. In our fitting of twist, we used the formula Btheta / Bz = q * r + c, where q represents twist, r represents distance from the center, and c is simply a fitting constant. Clearly the smallest c fits with theory best (since, in theory, c should equal 0). Upon examining the two options we came to the conclusion to use the smallest c value. This did indeed lead to better estimates of the center of the flux tube.