Coronagraph observations of two post flare loop systems, recorded photographically in the emissions of Fe 14 (5303 A) and Fe 10 (6374 A), show occasional enhancements at the intersections of some loops. The brightness of such enhancements in the green line gradually increases to a maximum value several times greater than that of the legs of the loops and then declines with a typical lifetime approx. 30 to 60 min. In red line emission the loop systems are usually very faint, but show the same overall type of enhancement, with a lag in maximum brightness relative to that of the green line approx. 10 min. The electron density, derived from the cooling time, is approx. 10 to the 12th power/cu cm

Smartt, Raymond N.

Zhang, Zhenda

English

NASA Technical Reports Server

N 8  8- I162 2 
LOOP INTERACTION IN THE VISIBLE EMISSION CORONA -- 
MORPHOLOGICAL DETAILS 
! Raymond N. Smartt and Zhenda Zhanp' 
National Solar Observatory 
National Optical Astronomy Observatories 
Sunspot, New Mexico 88349 
U.S.A. 
2 
ABS TR A CT 
Coronagraph observations of two post-flare loop systems, recorded photographi- 
cally in the emissions of Fe XIV (5303a) and Fe X (6374a), show occasional enhance- 
ments at the intersections of some loops. The brightness of such enhancements in the 
green-line gradually increases to a maximum value several times greater than that of 
the legs of the loops and then declines with a typical lifetime N 30-60 minutes. In red- 
line emission the loop systems are usually very faint, but show the same overall type of 
enhancement, with a lag in maximum brightness relative to that of the green line N 10 
minutes. The electron density, derived from the cooling time, is - 10'2em-s. 
I. INTRODUCTION 
We discuss here observations of two flare-associated coronal loop systems, as observed in visible 
coronal emission lines, with emphasis on characteristics of loop interaction processes during the post-flare 
phase. 
Interacting coronal loops have been investigated in relation to flare studies. Machado (1982) has 
found that most, if not all, X-ray flares originate in two or more intersecting loop structures. Rust and 
Somov (1984) showed that 3.5 - 11.5 Kev X-ray brightening started near the intersection of two flare 
loops. They assumed a thermal energy source at the point of intersection of the two loops, with equal 
conduction of heat flux into each loop. Transient X-ray brightenings of interconnecting loops have been 
investigated by Spicer and Svestka (1983). They argue that the fast tearing mode or anomalous Joule 
heating (following inductive changes in the field) could cause the brightenings, the mechanism depending 
on the loop history. VLA 6-cm observations have provided evidence of interacting loops that trigger 
flares and the first direct evidence of the coalescence of coronal loops (Kundu et al., 1984; Kundu, 1983, 
1985). These radio observations indicate that in the 20 seconds preceding the impulsive peak 
( T6 N 1.1 x 10gK), the arcade of loops (burst source) changed and ultimately developed into two strong 
bipolar regions or a quadrupole structure whose orientations were such that the field lines were opposed 
to each other near the loop tops. 
The above studies are concerned with the large energies associated with flare production, while the 
events discussed here represent much smaller energies, since they occur in loop systems in the post-flare 
phase, often several hours after the flare occurrence. However, the basic processes associated with 
interacting loops that lead to the release of energy appear to  be the same in both the high- and the low- 
energy cases. 
I 
~ 'Permanent address: Department of Astronomy, Nanjing University, Nanjing, People'a Republic of China 
Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National 
Science Foundation 
129 PR%CEDING PAGE BLANK NOT FILMED 
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11. OBSERVATIONS 
Observations were obtained with the National Solar Observatory/Sacramento Peak 20-cm aperture 
patrol coronagraph. Coronal images in Fe XIV (5303A), Fe X (6374.5A) and images in H a  of the disk 
and of prominences are recorded sequentially on high-resolution (SO-115), 70-mm film (Smartt et al., 
1981). In the normal mode of operation, a set of these four images is recorded in an interval5 3% and 
repeated at 1 minute intervals. Here we describe the loop interaction features observed in the post-flare 
loop systems of April 28, 1980 and November 14, 1980. 
a) Flare-associated loops, April 28, 1980 
These data cover the period 1343 to 1652 UT and 2145 to 2345 UT. The loops are located at a lati- 
tude - 22 O . At 1531 UT, an obvious bright point in the green-line loops occurs at the projected inter- 
section of two loops. Microdensitometer measurements of this feature reveal that it is equivalent to a 
coronal brightness - 5 times larger than that of the well-defined loop legs. Approximately 15 minutes 
before, and 15 minutes later, this feature was no brighter than the other parts of the loop tops. At 1531 
UT, the same feature can be identified in the red-line image, although far less bright and less extensive 
than in the green-line image. It reaches maximum brightness at - 1541 UT, although still exceedingly 
faint. Fine, hair-like lines define the point of intersection of the loops. These combined green- and red- 
line observations suggest that the enhancement is due in part to an increase in temperature, the lag - 10 
minutes in the red-line maximum relative to that of the green-line being a measure of green- to red-line 
cooling time. However, since the feature is visible in the red-line image, but with little other associated 
structure of the whole loop system, it is likely that the integrated path is greater than simply the sum of 
two loops along the line-of-sight. 
At 1931 UT, the green-line loops (Figure 1) have now reached a height H IO6 km. At this time, 
there are fewer loops in the system, and due to the inclination of the plane of the loops relative to the 
line-of-sight , enhancements that occur at  loop intersections are now clearly evident. The large X-point 
enhancement corresponds to a brightness - 3 times larger than unenhanced parts of the loops. This 
enhancement is not confined to the region of intersection, but clearly extends for considerable distances 
along the loops. For example, the lower right-hand part of the rrX'r appears to be enhanced for at least 
2.5 x 10' km from the point of intersection. This type of observation, together with the measured bright- 
ness, confirms that such features arise from localized loop interactions. 
Fig. 1 - Flare associated coronal loops, recorded in Fe XIV (5303A), on April 28, 1980, a t  
1932 UT, showing enhancements at loop interactions. 
ORLGINAL PAGE IS 
OF POOR QUALITY 
130 
b) Flare-associated loops, November 14, 1980 
These loops occurred on the west limb at a latitude of 22 a N, the data spanning the interval 1633 - 
1808 UT. However, due to a fault in the microprocessor control, data were recorded only at intervals of 
a few minutes. The most obvious general characteristic of the loops as they appear in the green-line 
image is the suggestion of two systems of loops, as indicated by the obvious discontinuity in the outline 
of the loop tops. A bright point occurs at this location, maximum at - 1732 UT, as shown in Figure 2. 
Fig. 2. Flare-associated coronal loops, recorded in Fe XIV (5303A) on November 14, 1980, at 
1732 UT, showing a bright enhancement at the intersections of two loop systems. 
The rise time is estimated to  be - 25 minutes, while the decay time is - 30 minutes. As in the cases dis- 
cussed above, a similar bright point also becomes evident in the red-line record, reaching a maximum 
value - 10 minutes after the green-line maximum. This lag time estimate is less precise than in the pre- 
vious cases because of the data gaps. The faint red-line rrXrr enhancement is notable for its narrow 
dimensions. The green-line enhancement has an area - 4.6 x 107km2. 
111. ANALYSIS 
It was noted in Section I1 that the maximum brightness in a red-line enhancement at the X-point 
appears to lag the maximum in the green-line enhancements by about 10 minutes, apparently a measure 
of the cooling time. Moore et al. (1980) find that radiative cooling dominates conductive cooling when 
T < i07K. The radiative cooling time is given by, 
(1) 
3KT 
re = -
where K is the Boltzman constant, T, the plasma temperature before cooling, Ax, the radiative cooling 
coefficient and Ne, the electron density. If we assume that cooling from maximum green-line emission to 
maximum red-line emission is due predominantly to radiative cooling, we can estimate a maximum value 
for the electron density in the region of the enhancement from equation (1). The radiative cooling 
coefficient has a value of about 10-”erg 12m~u-l (Raymond et  al., 1976). 
A p e  
Then, 
N e  ,< 4.3 x 10‘ T/rc, 
where the unit of r, is in minutes. 
The appropriate value for T is uncertain, but since red-line emission can be observed faintly at the 
X-point at the time of maximum green-line emission, we argue that an average value of T in the 
enhancement cannot be far in excess of the characteristic temperature of the green-line. We therefore 
take T = 2 x 106K; then N e  ,< 1012~m-s. 
131 
IV. DISCUSSION 
The question arises concerning the morphology of the loop system that leads to the condition 
required for an X-point brightening. An arcade of loops spanning a field line contains parallel loops with 
currents having the same sense. Our observations typically show much more complex structure in which 
parallel loops appear to be the exception. It therefore seems likely that an encounter between two loops 
that have intersecting planes occurs when the lower loop develops more rapidly than the higher loop, or 
simply if photospheric motion moves the loop footpoints such that contact occurs. The reason for the 
relatively short life-times of the enhancements is not obvious. 
In summary, we have presented here the first evidence of coronal loop interaction in the post-flare 
phase as observed in visible coronal lines. Derived electrons densities are found to be roughly an order of 
magnitude larger than those that characterize the loop system. A definitive analysis of the phenomenon 
will probably require spectral data of the visible emisison as well as coordinated observations with X-ray, 
EUV and radio measurements. 
REFERENCES 
Kundu, M.R.: 1983, Solar Phys.  86,205. 
Kundu, M.R., Machado, M.E., Erskine, F.T., Rovira, M.G., and Schmahl, E. J.: 1984, Astron. Atro- 
Kundu, M.R.: 1985, Proc of the IAU Symp. (No. 170) on Unstable Current Systems and Plasma Insta- 
Machado, M.E.: 1982, Adv. Space Res. 2, 115. 
Moore, R., et al.: 1980, Skylab Solar Workshop 11: Solar Flares, P.A. Sturrock (ed.), p. 341. 
Nakajima, H., Tajima, T., Brunel, F., and Sakai, J.: 1985, Bulletin of Faculty ofEngineering, Toyama 
Raymond, J.C., Cox, D.P., and Smith, B.W.: 1976, Ap.J .  204, 290. 
Rust, D.M., and Somov, B.V.: 1984, Solar Phys.  93, 95. 
Smartt, R. N., Dunn, R. B. and Fisher, R. R.: 1981, S.P.I.E. 288, 395. 
Spicer, D.S., and Svestka, Z.: 1983, Solar Phys.  87,  271. 
phys. 132,241. 
bilities in Astrophys., p. 185. 
University, 36, 39. 
132 


Loop interaction in the visible emission corona: Morphological details

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