A Beautifully Observed X-ray Jet

Science Nugget: November 10, 2000

Introduction

At the end of last week's Nugget, we teased you with an allusion to X-ray jets. As if to answer this taunt, the Sun produced a particularly pretty example of a jet, just a few hours later. This week, we answer the challenge by taking a good look at the event. For previous Nuggets about jets, we invite you to try the search function on the Refined Nuggets homepage.

The Jet of the Week

On November 3, just north of the center of the solar disk, an eruption of plasma occurred. This plasma was channelled along a "conduit" of magnetic field lines, which appears to be nearly linear in our images. This situation is what we refer to as an X-ray jet. The current example stretched for more than 280 thousand kilometers. We don't know precisely when it began growing, because the jet was already visible when Yohkoh came out of the Earth's shadow; but in the span of just 6 minutes the jet doubled in length. Here's a movie:


Java Script movie -- MPEG movie

The time span is 16:37 UT to 16:52 UT, on 3-Nov-00. These images are extracted from the SXT half-resolution full-Sun frames, so each pixel corresponds to a distance of about 3400 km (in the plane of the sky, at the distance of the Sun). To get a closer look, we have two options: (i) blow up the SXT images, or (ii) grab images from TRACE, which has a sharper eye. The results of both these are below. First, doubling the size of the SXT images yields this movie:


Java Script movie -- MPEG movie

Right away we start to see some interesting details. First this jet is a "repeater", meaning that after the first eruption, there is a second brightening. This is fairly common.

Second, the jet is not perfectly straight. It's slightly curved to the west (right), and even appears to change shape as it progresses. The path and shape are determined by the magnetic field which confines the hot plasma -- an open question is whether the other end of this magnetic field is located on the Sun, some distance away, or far out in space (i.e., is the field closed or open?). In the present case, we believe the jet is open to space.

There is a hint of substructure in the base of jet, meaning that there is physics happening on length scales smaller than 6800 km. It is difficult to tell from these images, however, because there's also a hint of saturation in some of the pixels.

The Benefit of Resolution (Spatial and Temporal!)

For a better look, we have to go to the TRACE images. The TRACE telescope camera has a sharper resolution than that of SXT; each pixel corresponds to a length of about 350 km on the Sun. So for our first look, let's examine the TRACE movie of this same jet, at half-size:


Java Script movie -- MPEG movie

The sharper eye allows us to see that there is a lot of structure in the jet plasma, and in the region at the foot of the jet. The whole region at the base of the jet seems to be erupting, reminiscent of a filament eruption. The higher cadence (i.e., more images per minute) suggests that as the clumpy plasma moves outward along the magnetic field lines, its path is helical and twisting. At the end of the movie, we can see the foot-region brightening again, in preparation for the second "spurt" observed at the end of the SXT movie. We even get the impression that the jet "conduit" itself is glowing faintly, it presence evidenced by plasma bound to its magnetic field. Let's take a closer look.

Maximum Zoom

TRACE aficionados would be unforgiving if we showed the TRACE images at half-size without also providing the full-resolution images. For our last movie, we'll reap the rewards of their hard work. In the interest of bandwidth conservation, we'll only provide an MPEG movie (i.e., no JAVA Script movie this time); but the movie is glorious:


MPEG movie

The path of the jet does seem to have some helical structure, as suggested in the first frame of the movie. But more interestingly, the spatial resolution reveals the connectivity of the magnetic field at the foot of the jet. Please consult the image below, taken from the middle of the TRACE full-resolution movie:

In the foot of the jet, there are coronal loops stretching from one side of the foot region to the other. The dipolar nature of the magnetic field is made apparent, despite the fact that this event is defined by material flowing out into space. Go back and watch the movie again -- you may be able to see the evolution of loops connecting the opposite sides of this foot region. And yet the footpoints of these loops do not leave the boundary of the "conduit". It just so happens that MDI has magnetograms from this time span, but at the time of this writing the analysis is not yet complete. So we will have to wait for a definitive explanation of the connectivity of the magnetic field. In the meantime, we can pile up a load of guesses: clearly, a cartoon is called for here!

"Hey Rocky! Watch me pull a theory out of this hat!"

If we suppose that the "conduit" of the jet represents field lines that are open to space before the jet event, then what would result if some magnetic flux just happened to emerge within the conduit? (bar napkin sketch) It seems possible that a reconnection of the emerged field with the lines open to space could produce the brightening, the energy of the material ejected in the jet, and the apparent connection between footpoints on opposite sides of the jet base. The boundaries of the "conduit" would certainly be perturbed by such an interaction, but not necessarily violated; this might give us a reason why the brightening at the base does not appear to leave the confines of the "conduit" boundary.

This is not so different from previous explanations of jets, like the cartoon of Heyvaerts, Priest, and Rust, or the works of Shimojo and Shibata. In this case, we are simply placing the emergence of flux entirely within the collection of previously-open field lines, and not allowing it to stray outside. Mix in some reconnection, add just a pinch of rank speculation, and voila! -- a cartoon explanation. Quod erat faciendum.

Epilogue

Of course, a theory is useless if it cannot be compared to real data. We'll get to work on the magnetograms, and report back when we have a result. For the time being, please let the following crude overlay image taunt you. It is a composite made by laying an MDI magnetogram (courtesy of SOHO/MDI onto a TRACE image. Both are from 16:37 UT, near the beginning of the flare. There is magnetic field of both polarities (different colored contours) at the base of the jet -- but is it recently emerged? Time, and more analysis, will tell.

November 10, 2000

David McKenzie <mckenzie@isass0.solar.isas.ac.jp>