Comet Studies, Redux
Wow — the images of Comet Lulin are simply amazing. I’ve been following its progression in amateur images for the past week or two and it’s like being back in school back when I used to study comet disconnection events in exchange for grad student tuition.
Why do I say that? Well, when I went back to school in 1988 with the intent of amassing enough cred to try for a Ph.D in astrophysics (which I didn’t get, but got something else instead, it’s a long story), one of my first jobs was to study images of Comet Halley. LOTS of images!
My advisor was John C. Brandt, and he was one of the discipline scientists for the International Halley Watch. His specific group was interesed in the Large-scale Phenomena — meaning that we watched the plasma tail of Comet Halley as it changed over the course of the comet’s perihelion approach and departure. We received thousands of images of the comet from August 1985 through July 1986, taken by amateurs and professionals from around the world. Those images allowed us to track fine structure changes in the plasma tail as it encountered various regimes (areas) of the solar wind. In particular, we tracked disconnection events in the tail.
It was my job (along with Marty Snow, who was a grad student at the time) to measure each image and provide astrometric measurements of the movement of structure in the disconnecting tail. We did this by identifying positions of several background stars and essentially triangulating the positions of the coma and tail structures from those. We then input our data into a program that allowed us to calculate the exact positions and, over time, we could figure out how fast the tail structure was moving.
We compared that information to where the comet was in the solar wind and ultimately our team (including Jack Brandt, Marty Snow, Yu Yi, Marlon Caputo, Cora Randall and I) published several papers about our work, tying down the cause of plasma tail disconnection to times when the comet crossed what is called the heliospheric current sheet. As the comet encounters and crosses this region, its tail “breaks off” and reforms due to changes in magnetic polarity. (You can read more about this process here and here (second link will download a pdf document.))
The other outcome of our work was a tome called the International Halley Watch Atlas of Large-Scale Phenomena. After we measured those images, I then prepped them and laid them out for publication — and the book ultimately came out in 1992.
Looking at the images (like the one above) that astrophotographers are providing of Lulin reminded me of those heady days back in the late 80s and early 90s when we pored over Comet Halley images. Today’s imagers have much better equipment and when I look at their work, I mentally start looking for stars to measure and structure to chart. Some things never change!
If you get a chance over the next couple of weeks, go out and look at Comet Lulin. Here’s a handy viewing chart to help you plan your own comet-gazing adventure. Here’s another. And, don’t forget to dress warmly, no matter where you are!