Big comet, big CME... big coincidence?
October 4th, 2011
SOHO-2143 shortly after discovery in the SOHO LASCO C3 field of view. (Click for larger) This is something that comes up time and time again, something I receive many emails about, and something I always have to type out the same answer to. So it's time I put this down in pixels so that I can point to it in the future. I'll start with the bottom line: there still remains zero evidence for a link between sungrazing comets and coronal mass ejections (CMEs) that can't be better explained than by simple coincidence. Now to elaborate...
Discovery
I'll begin with the story of what is one of SOHO's brightest ever Kreutz-group comet discoveries, which was first reported on September 30th, 2011 by four different amateur astronomers within nine seconds of each other! (SOHO comet-hunting is competitive...) It was discovered as it first entered the LASCO C3 field of view, or some 22-million kilometers from the Sun, and was already a bright object that was only ever going to increase in brightness as it raced towards its fiery demise.
Sure enough, by midday on October 1st, SOHO-2143 was saturating the C3 camera to a similar extent as Mercury, allowing us to confidently place it at magnitude 0 or -1 brightness. It reached the LASCO C2 coronagraph later that day, and we were able to follow it as it plunged at around 600km/s into the solar corona. And then just as we saw the last of its tail disappearing behind one side of the C2 occulting disk, we saw a huge CME blast out of the opposite side. A movie of this whole event can be downloaded right here (0.25Mb .mov file). "What are the chances,", I hear people say, "that a huge CME will go off right when the comet hits the Sun? They must be related!". That statement (which often haunts me in my sleep...) in itself contains the first two misconceptions that are a good place to start this discussion.
This large CME in SOHO/LASCO C2 appeared shortly after SOHO-2143 had disappeared behind the C2 occulting disk. (Click for larger)
Misconceptions
Misconception #1: "...right when...".
The solid disk we put on the LASCO C2 images is approximately 2.5 solar radii ("2.5 Rs") in diameter. That put the outer edge at approx 1.4 million km from the solar surface; a distance that takes the comet up to one hour or more* to travel. It is while in this huge "blind spot" that we lose sight of the comet and at some point gain sight of a CME. That really doesn't tell us much about an event that is to have occurred between a tiny comet and a relatively small source region on the solar surface. (More on comet size and source regions later.) Also...
Misconception #2: "...the comet hits..."
This definitely needs clarifying: Kreutz-group comets are called "sungrazers" for a reason; they graze the solar surface, not hit it! This might seem a minor point, and indeed perhaps it is, but it needs to be noted that the perihelion distance (i.e. their closest point to the Sun) can vary anywhere from 1-2Rs, with extreme outliers being close to 1-million kilometers from the solar surface. Yes, a small number of them seem to have a perihelion distance just inside the solar surface, but in reality they rarely get anywhere near that close to the Sun before being vaporized.
Size matters
Now we have those addressed, we'll look at size... because it does matter! The comets discovered by SOHO have nuclei of maybe up to 100m. As comets go, they're tiny, and when faced with the searing several-million degree solar corona produced by the ~1.4-million Km wide Sun... well that's rather daunting for a small, loosely held-together blob of ice and rock! But, not easily swayed, it seems these plucky little ice-balls can -- in rare circumstances -- actually survive long enough to reach their perihelion. (That's another story... stay tuned!) But... even if they do reach somewhere near the solar surface, they need to somehow initiate a magnetically driven eruption that blasts out at up to 2500km/s. I'm lucky enough to be surrounded by some of the best solar physicists in the world, and none of them can think of a reasonable mechanism** in which physics would allow this event to be initiated by any comet, let alone such a tiny one.
Probabilities
When the Sun is highly active, we can see up to ten or twelve CMEs per day, sometimes more; this equates to one CME every two-or-so hours. When we see a big comet disappear behind the LASCO occulting disk, it is hidden behind there for up to one hour before reaching perihelion. So, by my estimation, for every large comet we see in LASCO, there's as much as a 50% chance that a CME will occur while it is behind that disk! (As I stated earlier, statistically we see much lower percentages than this, which itself is unexpected.)
One more nail...
This is the source region that produced the CME, taken from the EUVI (195A) camera on the STEREO-B spacecraft. The <100m comet was hundreds of thousands of kilometers away from here when this happened... So far I have given arguments for why we don't think a comet can cause a CME, so I should also back this up with evidence or, more to the point, a lack of evidence. I mentioned briefly a thing called a "source region"; this is the area in the solar corona from which solar flares and CMEs originate. These source regions are almost always closely associated with sunspots and "active regions" on the solar surface (the bright areas in EUV images). Now that we have the STEREO spacecraft giving us full 360-degree coverage of the Sun, we can see every single active region on the Sun at any given time. This gives us the ability to locate the source of every significant flare/CME that occurs. In the case of the event that various folks are trying to link to SOHO-2143, the eruption clearly comes from an active region that was pointing directly at the STEREO-B spacecraft. While we don't yet have the orbital parameters for this comet (I'll try and make those measurements this week), based on the typical trajectory of Kreutz-group comets it does not seem likely that this comet would have been anywhere near this active region at the time the eruption initiated. Could it have interacted with the comet over a long distance? I guess so. But it is far more likely the CME was initiated by the same mechanisms by which several eruptions happen daily on the solar surface regardless of the presence, or lack thereof, of tiny comets. We can speculate but without observational evidence and physics to support it, we can not link this comet and this CME, or any other that we have seen thus far.
Reality
I have personally seen over 1,600 sungrazing comets; I would honestly LOVE for there to be a link between them and coronal mass ejections. But I can not find one, nor can the scientists I work with think of a reasonable mechanism by which this link can occur. On several occasions I have made a determined effort to prove that comet can cause CMEs, but repeatedly I fail and, if anything, my results hint that a large comet seems to actually preclude CMEs, not initiate them! In one of my studies, I compiled the list of SOHO's 20 brightest (and therefore presumably largest) Kreutz-group comets, and looked for associated CMEs. Out of those twenty, there was not one single CME occur close to perihelion time of the comet, and it was almost uncanny how even during the highest rates of solar activity, there would be a lull as a comet passed by. (This is not a valid result; it's another example of a coincidence between two unrelated events.)
I think that what is happening is that all eyes turn to LASCO when there is a big, beautiful comet such as SOHO-2143, and people will remember it as "the comet that caused the CME". But who remembers, SOHO-6, SOHO-1476, or one of dozens others? They were all very big, bright Kreutz comets... and they had no CME "associated" with them. In fact in July of this year we saw a Kreutz comet that was so big and bright, it was visible in SDO's EUV (extreme ultraviolet) images! There was no CME there, either. When two events happen at roughly the same time, and all eyes are on them, it's easy to say they are related. In this case, and every other we have studied, we are as sure as we can be that they are not. But we are scientists, and we keep looking, and keep studying, and if we DO find this link then be assured that we will announce, publish and publicize it... and this web site is one of the first places we'll do that!
Last minute update: Dr. Phil Plait has just posted a similar article to this, with an excellent video. He highlights one excellent point about this particular event too: in the COR2 movies (~12MB .mov), there are three CMEs that occur while the comet is in the field of view. Why would the third one be related when the first two certainly are not? So his conclusion is much the same as mine: a link is highly unlikley, but it doesn't mean we'll rule it out or stop looking.
* We're assuming a flat plane here but of course in reality the comet is coming in at some angle to what we call the plane of the sky.
** I'm going to get emails from "electric universe" proponents for this comment. Sigh. I did say "reasonable mechanism"...
