What's in a coronal notch?

Science Nugget: Apr 23, 1999

Introduction

The images below (data on the left, sketch on the right) show what excites our attention:
 
 
 
 

Sorry for the hasty sketch! This shows a "lazy loop," defined as one lying so far away from the vertical as to be almost horizontal. The arrow on the left points roughly at disk center, and the arrow in the lazy loop is meant to bisect it in its (assumed) plane. A line vertical to the solar surface must project to disk center, so to the extent that this "lazy loop" is at all well-behaved geometrically (ideally it's planar and semicircular), it is tilted far away from the local vertical. Above the lazy loop, but below the mass of the active region, one can see the notch. It is a gap in the illuminated coronal field, ie one for which loops visible to SXT (ie, at temperatures above about 2 MK) do not occur.

By way of orientation, the "notch" morphology probably is described elsewhere in other contexts. Frequently an active region will have the appearance of a Conestoga wagon, with large tilted ribs at either end; eruptions and CME precursors often have the Ron Moore's Elbows syndrome, in which the dramatic motions take place bilaterally and avoid what Ron thinks of as the "tie-down" field at the core of the active region.

What does EIT see? What is the plasma beta?

Identifying the notch with the strong magnetic core of the active region makes sense. We confirm that this is reasonable in this case by doing a comparison with a white-light image from MDI (SOHO) - this image was from about two hours prior to the SXT image, and the registration is by appearance only, but:
 
 

One can see (faintly) the small sunspots of NOAA active region 8308 (the region area was only 60 millionths of the hemisphere on April 13), located roughly coincident with the notch. So, the conjecture that the notch corresponds to strong "core" fields seems consistent with the appearance. The next question: why is the notch so invisible to SXT? It is of course because there is no high-temperature material there with enough pressure to radiate soft X-rays; on the other hand, is there heating anyway, but to a lower temperature? In other words, could the plasma beta (ratio of gas pressure to magnetic pressure) be comparable as one goes along horizontally on the axis of the Conestoga wagon (ie, flying above the probable neutral line)? This would have some implications for coronal heating. Frequently, for example, theorists (who often feel as though they have to assume something!) might guess that coronal heating would scale as, say, B2. So let us look at the EIT data, from lower temperatures, and see if we can see the notch.
 
 

       
 

From left to right, in the usual EIT colors, and in ascending order of best temperature: HeII, FeIX, FeXII, FeXIV

Sure enough, the notch remains empty in each of the EIT filters. The 304 (He II) filter (left) - seen in its original full resolution - shows dark counterparts of the "lazy loops". The 171 and 195 filters (center two) show a bright fan originating to the west (right) of the notch, no doubt relevant in some way. But by and large the EIT images establish that little cooler material is adding much gas pressure; if there were still hotter material Yohkoh would see it, so we conclude that the notch really does represent a drastic dropout in the coronal plasma beta, just above the core of the active region. We hesitate to compare Flagstaff, Arizona to the core of an active region, but anyway here's a picture of the Grand Canyon:
 

 
 

The implication might be that our "science nugget" conclusions are as solid as a rock!

The value of beta, or: what good is gas, anyway?

We can crudely analyze the SXT image in terms of emission measure, and hence density. This is always extremely risky because of the obvious heterogeneity of the corona seen at these energies - without knowledge of the geometry, one cannot make a proper model for interpreting the flux. However, pushing ahead, we find the AlMg flux in the heart of the notch to be about 10 DN/sec - DN's are the instrument scale for brightness. This leads to a rough guess at the density of 109 cm-3. This is of course an upper limit, since we don't really see anything in the notch - the density could be quite low indeed. This translates into a plasma beta of less than 0.001, whereas the bright AR loops to the north would have beta = 0.05 or so. The speculations from all this are
  • Coronal heating can vary widely in adjacent regions at comparable values of the magnetic intensity;
  • The gas trapped in a loop has little to do with the structure of an active region;
  • The strong-field core of an active region squeezes the peripheral fields into highly inclined orientations - perhaps this stressful attitude contributes to the stronger heating that the coronal gas sees in such structures.
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    April 24, 1999: Hugh Hudson
    (hudson@isass0.solar.isas.ac.jp)