We went out on Christmas night and found Comet Machholz just about where the star chart says it’s supposed to be. With a pair of binoculars we could make out a nice fuzzy glob of light. If you could take a picture of it over several minutes’ time, it would look about like this image from two of Europe’s better-known comet photographers. Notice in their view that Machholz seems to have a couple of tails. The one sweeping out from the comet to the upper left is the dust tail and the one sweeping out to the lower left is the plasma tail.
For several years while I was in graduate school I studied comets, including one named Machholz! Our team’s interest was in tracking the evolution of a comet’s plasma tail, which is formed in an interaction between cometary gases and the solar wind. The dust tail, by contrast, is made up of microscopic dust particles streaming off the comet. When I joined the team I began working on analyzing images of Comet Halley and its nicely active plasma tail. We wanted to track it throughout its tour around the Sun, and later on we did the same with others. The biggest result was a book published as part of the International Halley Watch project called the International Halley Watch Atlas of Large-Scale Phenomena. It was published through the University of Colorado in a limited edition run, and I was astonished to find it the other day on Amazon.com (although I did know it was selling at various times on Ebay). In fact, I have eight copies of it here at my office.
For the project I and other team members (Jack Brandt, Yu Yi, Marty Snow, Marlon Caputo) went through about 2,500 images of Comet Halley submitted by astronomers from around the world. We measured the size and angular orientation of the plasma tail and correlated it with the comet’s location in the solar wind. Then we arranged the images in a time sequence starting from late 1985 to about June 1986. If you flip through the book, you can see the tail change drastically as Halley neared the Sun and visited various latitudes of the solar wind.
My job was mostly to analyze the images, and then lay them out in the book for publication. So, while I’m not listed as one of the book’s authors, I am acknowledged for my contributions, and it was a most satisfying and interesting project with which to be associated. And, I learned a lot about comets and the solar wind in the process.
We went on to study other comets: Machholz, Schwassman-Wachman, DeVico, Mueller, Hyakutake, and Hale-Bopp. We gathered up images for each comet, and correlated their positions in the solar wind with their appearances, and also with data from the Ulysses spacecraft, which was in the same “regime” of the solar wind as each of the comets for various periods during their respective orbits. For Hale-Bopp, we took observations at the University of Hawai’i 88-inch telescope on Mauna Kea during November 1996 to see if we could catch the “turn on” of the plasma tail. That was a personal thrill for me, even though our data showed that at the time the plasma tail hadn’t yet started to grow.
Nowadays I’m out of the plasma-tail research business, although some of my colleagues, most notably Jack Brandt (team leader, close friend and co-author, currently at the University of New Mexico), continue to pursue the study of correlations between cometary plasma tails and the solar wind. He continues to publish papers based on data we took during my tenure at CU, as well as on data he’s gathered since that time on other comets. We made some good contributions to the comet research literature. Plasma tails are a fascinating way to see the Sun’s influence on things even as small and evanescent as a comet!