Shrinkage of Coronal X-ray loops

Science Nugget: September 29, 2000

Introduction

One of the earliest big discoveries by the Yohkoh mission may have been the notion that the solar corona above active regions is continuously expanding outwards into space. This result was possible, in part, to the high temporal continuity of the SXT movie. It was brand-new stuff. One can confirm this effect very simply -- run the Yohkoh movie backwards, and come away with the impression that the sun is systematically sucking up the solar corona.

Perhaps this notion of an ever-expanding solar corona is an obvious one, based solely on thermodynamic (or perhaps statistical) considerations alone: there is always a tendency towards increased entropy. There's always a tendency to fill a vacuum, water will always find a way into a dry pair of shoes, and college dorm rooms will always be trashed by the end of the semester.

Always on the lookout for something new, this nugget considers the opposing view: shrinking coronal loops.

How so?

The idea of shrinking loops is not a new one. The concept has been explored by numerous authors, including J. Wang, Z. Svestka, and Forbes and Acton. Furthermore, this topic has been explored to some degree by our own Hugh Hudson in the literature and also here in our own database o' Science nuggets.

It's easy to imagine a scenario where active region loops will want to expand: they tend to be light, hot, and the magnetic field has a tendency to push away from all the other magnetic field (in a great effort to fill all available space). There's a ready source of new magnetic pressure from "emerging flux", whose name describes the phenomenon. But how is it possible for a loop structure to shrink? Some ideas include...

Magnetic Implosion during solar flares,
A change in coronal currents, giving rise to increased magnetic tension,
Opposing magnetic footpoints moving towards one another, increasing the magnetic tension in the "tie-down field"),
A change in the aspect of the loop as the sun rotates it over the limb,
An increase in magnetic pressure in overlying loops, or perhaps...
None Of The Above.

Show me the data.

We return to the ApJ paper of J. Wang because he's got some wonderful figures available over the web. One of the many striking plates from his paper:

The loops to the South (left) can be seen to be contracting towards the active region, while the loops to the East (top) are slowly expanding outwards. Wang felt this was a real contracting event as the loop height decreased smoothly while the footpoint separation remained relatively constant.

I'm still not buying it.

In SXT data, we've found Yet Another Event supporting the notion of contracting loops, carefully annotated in the following figure:

The outline of the loop structure is shown, in time, by the curves illustrated on the above plot. This is perhaps better demonstrated in the following movie:

The entire movie takes place over a 5-hour period. One explanation might be that solar rotation changes the aspect of this loop structure, but the footpoints (to the right of the solar limb) move far less quickly than do the looptop structures, which suggests to us that the aspect isn't changing that much. As we mentioned earlier, there are several possibible explanations for this shrinkage. We think that perhaps it has something to do with a change in the magnetic configuration of this active region, somewhere just over the limb (or behind the bright core of this region) where we can't really tell what's going on.

September 29, 2000

Brian Handy <handy@isass0.solar.isas.ac.jp>