Motions Above A Spikey Arcade

Science Nugget: January 21, 2000

First, an apology

This is a Nugget that is long overdue. Almost exactly one year ago we wrote in a Nugget about motions in an M flare on the Sun and then promised to submit a subsequent Nugget about some of the other motions that were observed in this same event. But then we found other things to write about, other material for Nuggets, and so the followup to that earlier report never got written. In the meantime, the motions in the 20-Jan-99 flare were so exciting that we wrote a paper about it (McKenzie and Hudson 1999). So when we say that Nuggets sometimes lead to publications, we mean it, even if the present Nugget is late.

Let's get on with it

Enough rambling. The 20-Jan-99 flare was exciting because of the beautiful arcade structure and the way we saw it brightening so slowly, as well as the motions that were evident. We invite you to go back and look at the earlier report and watch the movies. What's unusual about this event is that above the arcade there is a fascinating amount of motion, in a direction that's predicted by theoretical models but heretofore unobserved. There are also structures associated with the arcade which we don't fully understand. To show you what I'm talking about, below is a still image showing the arcade. It is much like thousands of other arcades Yohkoh has observed, but in the late phase of the flare vertical rays, or spikes, can be seen to stick out above the top of the arcade (upper left). These we don't understand, though they've been observed in many (easily more than 30) other flares.

When we run a movie of this event (below), we see that not only is there structure above the arcade, there's motion. The movie is made about one-and-a-half hours after the movies from the previous Nugget. This is during the decay phase of the flare. The flare decay phase is often thought of as uninteresting, precisely because there are no motions or changes in morphology.

Here is an MPEG movie of this remarkable occurrence. Note added in proof - this movie does not do justice to the subject. Please refer to the .gif version on the original Nugget.

The flow is detectable because of the movement of the dark "blob-like" features in the images. Note that these features are moving downwards, into the top of the arcade. The speeds are between 100-500 km/sec (200 thousand to 1 million mph), and the dark (in X-rays) features are about 10 thousand kilometers in diameter.

So what is it?

At the moment there's two possible interpretations that we're seriously considering.

First, this flare (and many others like it) was associated with a coronal mass ejection (CME), in which great quantities of material is cast off (or escapes) from the Sun. If some of this material failed to achieve escape velocity (620 km/sec), then it would have to fall back to the surface of the Sun. The downward speed would also be less than 620 km/sec, which is consistent with the observations here. Note that the material would almost certainly not be dense enough to "block out" X-rays; to create the dark appearance we see here, the cool blobs would have to push the hotter, X-ray emitting material out of the way.

Second, the motion could represent the shrinkage of magnetic field lines that have been reconnected as a result of the CME. In the standard picture of the formation of post-CME arcades, the magnetic field which is stretched out by the eruption reconnects underneath the ejected material. Having reconnected, the field lines then shrink back down from their higher, stretched-out state to a more relaxed configuration -- forming the arcade of loops in the process. The motion seen above the arcade might be interpreted as this "shrinkage". If it helps, a cartoon depicting this shrinkage scenario is sketched below.

What's the Big Deal?

I'm glad you asked. First of all, the arcade structures are quite beautiful, and the flow fields are something new we haven't observed before. Something new is always interesting. Moreover, lately we've looked at movies of many other flares like this one, and the same kind of downward flow fields have been detected. This means that we aren't talking about a curiosity involving a single event here, but rather about a more general occurrence: 16 flares in the last two years, to be exact. Another example would be the limb flare of 23-Apr-98 (MPEG, 260 kbyte). In all these events, the speeds of the downward-directed flow field are between 45-500 km/sec. However, at this stage we can't say whether the upper limit is imposed by the physics of the phenomenon, or by a selection effect in the way the images were acquired.

And if the second interpretation above can be demonstrated to be accurate, then we have gained a few implications for reconnection models (and maybe some challenges for the modelers!). First, the speeds of 45-500 km/sec are somewhat slower than typically expected for reconnection outflows, though Forbes and Acton (1996) point out that the speed can be affected by the plasma conditions. Second, the flows are discrete along the length of the arcade; this points to a patchy and intermittent kind of truly 3-D reconnection (a la Klimchuk 1997). Finally, what are the rays that poke up from the top of the arcade? Svestka et al. (1998) thought they might be mini-jets, but we haven't yet found any indication of outward flow in the area above the arcade. We know they're hot, at least 5 million kelvins and maybe as hot as 15 MK.

Forbes, T., and Acton, L. 1996, ApJ, 459, 330

Klimchuk, J. 1997, in ASP Conf. Ser. 111: Magnetic Reconnection in the Solar Atmosphere, ed. R. Bentley and J. Mariska, 319

McKenzie, D., and Hudson, H. 1999, ApJ, 519, L93

Svestka, Z., Farnik, F., Hudson, H., and Hick, P. 1998, Sol. Phys., 182, 179

22-January-2000 D. McKenzie (