Comets in the Solar Wind

Comet C/2012 S1 (ISON) is a dynamically new sungrazing comet. This makes it a particularly unique object that we hope will be a big help in our understanding of Oort Cloud comets and primitive solar system material. Sungrazing comets are pretty useful in this respect, as they get much closer to the Sun that "ordinary" comets, and thus their surface gets much hotter than your typical comet. The result is that material that typically stays more or less "inert" on a comet's icy surface may begin to vaporize, giving us the chance to spectroscopically look at the comet and distinguish new compositional elements. Fun stuff for sure, but the truly unique thing about sungrazers is that they server a dual purpose: they also teach us about the Sun!

Historically, comets were our first clue that there exists a solar wind - an ever-present outflow of particles from the Sun - that causes their sublimated material to flow behind them in the long tails we see. This realization was a big deal as we had no way of sending probes into space so see what was actually going on out there. Instead, the comets were our probes!

Comets as Solar Probes


The amazing Comet Lovejoy in STEREO/SECCHI HI-1A
Now that we're in the space age we do have plenty of spacecraft out there sampling the solar wind and its embedded magnetic fields, but most of those are either close to Earth or at least in more-or-less the same orbit as Earth. The region very close to the Sun is extremely hostile to spacecraft, and very difficult to get to. We are right now working on two imaging instruments ("WISPR" and "SOLOHi") that will go on NASA's Solar Probe Plus, and ESA's Solar Orbiter satellites, both of which will get very close to the Sun. But they don't launch for at least three more years, and naturally their lifespan is limited as all spacecraft are. That is where sungrazing comets like C/2012 S1 (ISON) help fill in the gaps! We can observe them as they approach the Sun, and look at the way they interact with the solar wind and solar outflows. From this, we can deduce properties of the near-Sun environment in ways that are simply not possible without actually sending probes there. And best of all - comets are zero-cost!

So let's take a look at some examples of comets in the solar wind, and specifically in the Heliospheric Imager 1 ("HI-1") camera on NASA's STEREO satellite. First, I'll start with the short animation of comet C/2011 W3 (Lovejoy) that you see opposite.

I did a lot of processing to enhance that image, but what you're basically seeing is the solar wind and solar outflow coming from the upper-right corner of the animation (the Sun is outside of the field of view by about 5-degrees), and comet Lovejoy racing in towards it. You can see how Lovejoy's tail is waving and rippling as the solar wind blows past it. We can look at data such as this and determine the velocity of the solar wind at the comet, and try to model the density fluctuations we see there. (Ambient solar wind is on the order of 200-300km/s, btw, so it's a tad breezy out there!)

In 2007 we witnessed Comet 2P/Encke being hit by a coronal mass ejection and completely (though only temporarily) losing its entire tail! [Click to animate]
One of the most amazing events we've witnessed so far with the HI-1 cameras happened in April 2007 when comet 2P/Encke flew through our field of view. Click on the image opposite to see an animation of what happened...

Yep - the comet had its tail completely ripped off by a coronal mass ejection (CME) passing over it! (Thankfully Encke's tail quickly grew back!) In addition to that major disconnection event, if you watch the full sequence, you can see Encke's tail experienced lots of interaction with the solar wind during this period.

From this sequence we learned a few things. It was the best view we've ever had of a so-called "disconnection event" in a comet tail, and the first time we'd been able to definitively track the solar feature (CME in this instance) that caused the disconnect. The preliminary conclusion (outlined in this paper written by my colleagues [small PDF]) was that the CME's own magnetic field caused a "magnetic reconnection" in the interplanetary magnetic field (IMF) that was draped around the comet. In essence, the IMF surrounding the comet was pulled away by the stronger local magnetic field in the CME front, and that took the comet's tail with it!

So this helps us understand a little more about magnetic fields in CMEs, and also helped us pin-point exactly the location and direction of the CME, since we knew exactly where in 3-D space the comet was. We can use this kind of info to validate our models of CME direction, velocity, and location, for example. So again, a lot of invaluable solar physics data here!

Finally, I've been working with images from the SOHO satellite since 2003, and STEREO images since launch in 2006, and in all those years, this is my absolute favorite movie. It's short, but beautiful!!


Comet McNaught in STEREO HI-1A in 2007. My favorite comet movie of all time (so far...)
This is comet C/2006 P1 (McNaught) in 2007, known to some as the Great Comet of 2007, which was one of the most spectacular comets in the past few decades, particularly for those in the Southern Hemisphere. We were extremely fortunate with our SECCHI imaging instruments on STEREO, as we had only just opened the doors on the HI-1 telescope for the first time when McNaught made its appearance.

As far as I'm concerned - and I've never yet had anyone disagree - this is one of the most beautiful and spectacular sequences of images of a comet ever recorded. The fine structure in the tail, and the feather-like appearance, is just truly gorgeous. (You can download some still frames and a full-res .mov file of this sequence over on the Sungrazer Website - I highly recommend you do that!)

So what are we seeing here? First, you'll notice Venus as the bright planet in the lower-mid-left and Mercury enters from the right towards the end. But the sequence is of course dominated by McNaught's jaw-dropping feathery tai!

The basic process for causing those structures - also known as "striae" - is the fragmentation of larger dust grains that leave the comet's nucleus and then interact in various ways with the solar wind. This is a pretty new area of research and the small-scale details and processes are not actually well understood, so we have to use some slightly hand-wavy arguments to get our models to work. But one of my colleagues, Dr. Geraint Jones at UCL in the UK, has done some excellent work modeling features like this in McNaught and a couple of other recent near-Sun comets, and his efforts definitely give us some very valuable information about both the near-Sun conditions.

Prospects for ISON in the solar wind

Now I am going to get a little speculative and wrap up by saying a few words about what comet ISON in particular might teach us about the solar wind and near-Sun conditions.

First, Comet ISON doesn't have the size, the dust production rates, or the necessary proximity to the spacecraft, to reproduce what we saw with McNaught. Instead, I suspect our view of it in the HI-1 cameras will be very reminiscent of Encke and Lovejoy: a thin tail that is continuously buffeted by the solar wind. We are fortunate that we have two STEREO spacecraft, and thus two HI-1 cameras, that are in very different locations in space. We are going to make some special observations to ensure that we get simultaneous images of ISON in those cameras so that we can compare the features from the two viewpoints. This is a huge bonus that should enable stereoscopic tracking of features in the tail.

Another plus point: we are at, or near... or vaguely hovering around... what passes for solar maximum. The Sun has been all kinds of weird this last cycle, but regardless, we can never rule out the possibility of an ISON-directed coronal mass ejection. If that happens, that will be exciting! And if we catch an event like that in both spacecraft imagers at once, it would truly be a jackpot for us! This is something we have absolutely no way to predict, but if it does happen, the consequences for ISON would be the same as for Encke: the tail would probably be ripped off, but would grow back pretty quickly.

Once ISON gets really close to the Sun, that puts it in a whole new domain - the solar corona. Comet Lovejoy was an almost unprecedented source of information about the solar magnetic fields, electron density, and a couple more things. Once ISON is in that region - assuming it survives that far, of course - a whole different set of processes will begin that are alien to all but the largest of sungrazing comets. That's an entirely different discussion, and indeed a whole new blog post, that I will save for another day!

The bottom line here is that sungrazing comets are truly unique in the way they teach us about both comets, and about near-Sun conditions. When we add in the factor that ISON is a relatively big, pristine chunk of the solar system's building blocks, I hope you can begin to appreciate why we have formulated such a high-profile observing campaign. Comet ISON holds an unprecedented potential for new science data, and we have at our hands an unprecedented armada of ground and space-based observatories that constitute the most technologically advanced array of astronomical instrumentation in history. Regardless of whether ISON fragments, survives, dazzles, or fizzles, we will unquestionably gain a huge volume of new data and new scientific insights - and that's what the CIOC is here to help facilitate!

Keep up-to-date on the latest ISON and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions stated on there, and in my blog posts, are my own.