Comet Lulin (C/2007 N3) as imaged by Rolando Ligustri at http://www.castfvg.it/ The lower image shows the comet's plasma tail disconnecting. (Click to embiggenate.)
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!
No, this isn’t about politics, although it’s interesting to note that when I DID write about politics, I had a HUGE spike in readership… but alas, now that I’m back to talking about astronomy, the excitement has died down…
… heavy sigh …
A "visible light" image of the Sun on October 14, 2008
…where was I…
oh yes… gasbags.
Specifically, the Sun.
Yes, it’s a “bag” of gas and it blows a stream of gas and charged particles out in a constatn stream called the solar wind. Traditionally, the Sun goes through some cycles where it’s windier and gasbaggier than usual. These normally tend to correspond to periods of higher sunspot activity and the like.
Well, we had a meeting of our little vodcasting production group today — that’s the small collection of us who are creating vodcasts for MIT Haystack Observatory on the subject of space weather — the stuff that happens in the near-planet environment when our local gasbag star decides to belch some material our way.
Among other things, we got to talking about the current, relatively quiescent solar wind. Everybody who monitors the solar wind (basically solar physicists and atmospheric scientists and space weather experts from around the world) has been remarking on how low the wind levels are, almost like the Sun is holding out on us for some unknown reason. It so happens that they’re the lowest in 50 years. What this means for the long term in solar behavior is probably not a big deal right now, but suffice to say, right now, everybody who was expecting some ramping up of activity as the current solar cycle wears on is ready for some outburst action. Or even just a sunspot, please, anything… do we have to get a famous blonde actress out here to beg the Sun to just “think of the children” now?
Well, perhaps things are not that desperate. The Sun has its little behavioral crotchets, and the more data we take, the more we learn about this star (and presumably others like it). The folks who monitor the Sun continue to do so, adding in this quiescent data to the ever-growing store of solar behavior knowledge. And, they’re hoping for some solar action sooner rather than later.
The Sun on October 14, 2008. The sunspot is the bright spot in the upper center of the Sun.
Now, we did have the promise of a sunspot a couple of days ago, and there was a coronal hole spewing a stream of material that Earth went through. Everybody got ready just in case this portended some new break-out activity. But, as you can see by the picture on the left, there’s not a whole lot of sunspottin’ going on here. The coronal hole (which you can’t see in this particular view) is moving out of our line of sight and there’s another small one moving into our view. So, everybody waits, and we’re pretty sure there’ll be some action soon… we just don’t know when.
The solar wind is of more than just passing interest to me, and not just because I’m working on vodcasts about space weather. Back in the dark ages I worked on a whole passel (technical term meaning “hundreds if not thousands, so many that I had dreams about them at night”) images of Comet Halley during its last apparition in 1985 and 1986. Our research interest was really with Halley’s tail, specifically its plasma tail. A comet’s plasma tail interacts with the solar wind. Specifically, a plasma tail forms when gases streaming off the comet get ionized by interactions with the solar wind. In the process, the plasma tail gains an electrical charge and has the same electrical current propreties (polarity, etc.) as the “regime” (area) of the solar wind in which it forms. (And those properties are “encoded” into the solar wind as it leaves the Sun.)
As the comet goes along in its orbit (and it’s important to know that the plasma tail only exists within about 1.5 to 2.0 astronomical units from the Sun for reasons I’m not going to go into here because this post is already longer than it should be), it experiences the solar wind. If a comet stays in the regime of the solar wind that has the same polarity as its plasma tail, everything is fine and dandy. But, eventually the comet will encounter a regime that has the opposite polarity. The old plasma tail can’t exist there, and so it “breaks off” and a new one starts to grow, capturing the polarity of the new regime. The loss of the old tail is called a “disconnection event.”
The technical term for the underlying process that occurs when the magnetic field line entrained in the plasma tail collides with the magnetic field lines in the solar wind is “magnetic reconnection” and it’s quite a complex process that we see in magnetic fields in places like the solar atmosphere and near-Earth space (and if you want to read more about it, be my guest.) Anyway, I spent a lot of time studying those Halley pictures to pinpoint when its plasma tail disconnection events took place and our team of stalwart grad students and principal investigator (i.e. the PhD astronomer) tried to relate what WE were seeing to what part of the solar wind the comet was encountering.
Chasing comet plasma tail got a lot easier when the Ulysses spacecraft was launched to measure the solar wind as it was streaming away from the Sun. UIysses’s instruments would tell us (essentially) “Okay, I saw the solar wind at this location and it had this polarity and temperature and speed and this many particles loaded into it.”
And we’d say, “Okay, we’re going to look at a comet that we know about that will be experiencing that very piece of solar wind you just measured today, and it will pass through that piece of solar wind in two days. We’ll be able to see what happens to the plasma tail. And, based on what you’re telling us Ulysses, we will predict what will happen to the plasma tail as a result.”
We did that for a bunch of comets (where “bunch” is secret scientist jargon that means “more than two or three”) and by golly, we were able to use solar wind readings to predict what would happen to the plasma tails of those comets. Which was a lot of fun and got me through graduate school (along with some work I did on HST’s GHRS instrument team).
Comet Encke encounters a coronal mass ejection collide. (Courtesy NASA/STEREO, as shown on Boston.com)
I thought of all that fun today when I saw this really cool set of images from the STEREO mission. I actually saw them last year but I must have been busy or something because I didn’t blog about them then.
Anyway, they show Comet Encke experiencing a coronal mass ejection (a huge blast of solar wind) in April 2007. Essentially this little spermy-looking comet is moving along in the solar wind, doing its little comety thing and it collides with a fast-moving jet of material (plasma) spewed out by the Sun. The interaction with the comet and both of its tails (the dusty, inert tail and the electrically charged plasma tail) basically tears them off and we watch new ones grow over the space of a few hours. It was a great thing to see a disconnection event (with the associated magnetic reconnection) happen in “real” time after years of looking at static comet images in sequences and just imagining what the action must have looked like when it was occurring.
