All Quiet on the Solar Wind Front

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 …

…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.

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).

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.