When we talk about "coronal loops" in the context of Yohkoh SXT observations, we mean long, slender structures that are seen in the corona. We see them in the SXT images because the plasma contained in these objects is very hot, and dense enough to emit appreciable amounts of X-rays. The loops are formed by magnetic fields that come up into the corona from the lower regions and loop back down into the Sun. The charged particles in the plasma are constrained to move along the path created by the magnetic fields -- for all practical purposes, one can assume that the plasma does not move perpendicular to the magnetic field lines; and so the X-ray emitting plasma tells us both the location and orientation of coronal magnetic fields.
Any image of the hot corona will reveal lots of loops (except during the doldrums of solar minimum, perhaps), as one can easily demonstrate simply by checking out the latest SXT image (at LMSAL, at ISAS, at MSSL). In fact, due to their ubiquitous presence and fundamental physical structure, coronal loops have often been referred to as the "building blocks of the corona". Thus, the theme of the corona seems to be, "All Loops, All the Time."
Lots of our Weekly Nuggets have dealt with loops. Here are a few: [1], [2], [3]; try the Nugget search tool to find more.
In a famous montage of SXT images, examples of the varying appearances of coronal loops were highlighted. This week, we'll focus not on the shape of loops, but on the interesting behavior that these "building blocks" occasionally exhibit. With two examples, we will explore some activity that indicates the flow of plasma, or at least of energy, along the length of the loops.
For Example Number One, we have a loop from 9-Jun-2001. The image
at left shows the location on the Sun. In the movie linked below, we can
see that the loop appears to brighten first at the north end, and the
brightness appears to propagate down to the southern end of the loop.
After that, it seems that the "wave" of brightness travels back up the
loop, to the northern end again. Please see the movie:
To emphasize this behavior, we can station ourselves at various positions
along the length of the loop, and monitor the changing brightness at each
of those locations. (We did this in an earlier
Nugget too, for reference.)
The images below show the locations where we measured the changing
brightness (left), and the light curves from those 6 locations (right).
In the light curve plot, time increases from left to right, and we've
indicated the position along the loop by the letters A-F. (Click to
enlarge.)
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At each of the six locations, the traveling brightness shows up as a peak in the corresponding light curve. As the "wave" moves from A to F and back to A, the surface plot above displays a V shape. (If it helps, an alternative version of the surface plot, with time stamps to show when the peak occurs at each location, is linked here. The slope of the ridges in the surface plot indicate the speed of propagation of the brightness. In this case, we find speeds around 230-310 km/sec.
For Example Number Two, we offer a loop from one day earlier,
9-Jun-2001. The location is shown at left (click to enlarge), and a
movie is linked below. What you'll see is a few recurrent brightenings at
the southern footpoint, and then the loop brightens from the south to the
north. Not once, but twice -- and the northern end of the moving
brightening does not appear to be at the same place both times.
The same light curve treatment shows the difference in the propagation of
X-ray brightness. Again, the images below show the locations where we
measured the changing brightness (left), and the light curves from those 6
locations (right). In the light curve plot, time increases from left to
right, and we've indicated the position along the loop by the letters A-F.
(Click to enlarge.)
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Rather than a V shape, the two traveling brightnesses appear in this surface plot as distinct unconnected ridges. Since the brightness peaks at different times in the different locations, the ridges are slightly slanted. As before, the slope of the ridges in the plot indicate the speed of propagation; the speed here is about 230 km/sec, similar to that seen in the other loop example. (A version with time tickmarks is shown here.)
The images from Yohkoh SXT show the locations of hot plasma. So when we see something like a propagating brightness, we have to ask ourselves, do we see plasma moving from one end of the loop to the other, or are we seeing heat move from one end to the other? Either is possible: If the plasma actually moves along the loop, then one ought to see the loop brightening from end to end, as the hot plasma emits radiation. On the other hand, if the plasma doesn't move, but gets heated at one end of the loop, then the hotter stuff will radiate X-rays, and also conduct the heat further along the loop: one would see different parts of the loop light up as they get heated.
In the present case, there are circumstantial clues to indicate that we're seeing actual motion of plasma. In Example One, the "round trip" propagation of the brightness enhancement is more likely to be due to the motion of material -- probably some plasma is heated at the southern end of the loop, and then moves northward along the loop to form the "return leg" of the trip. In Example Two, we can clearly see that the "wave" of brightness starts from the southern end of the loop, but in the two episodes goes to two different places in the northern end. In both these examples, plasma motion seems a more likely explanation for the observations.
As always, this is only a first impression, and closer analysis (including possibly comparison with data from SOHO and/or TRACE) may prove the conclusions of this Nugget to be so much hogwash.
Please don't forget that the coming week will see a total eclipse of the Sun. On June 21, 2001, the moon will temporarily move directly between the Earth and the Sun, blocking our view. For a discussion of the appearance of the Sun during that time, please see last week's Nugget. Yohkoh will watch the solar eclipse, and probably attempt some observations similar to those during the February 1998 and August 1999 eclipses.
What SXT will see: SXT will see the moon sweep in over the Sun, stop, and reverse itself. An hour and a half later, this will happen again. Simulations of these two events, based on the orbital parameters of Yohkoh, are linked below. In each case, we've only shown the intervals when Yohkoh will be in daylight; the movie frames are 2 minutes apart. The SXT image in the middle is from May 25, one rotation before the eclipse.
Event 2
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