Asian Journal of Physics

Vol 13, Nos 3 & 4 (2004) 245 - 250

Detection of cooler loop-tops in a coronal structure

Jagdev Singh 1, Takashi Sakurai 2 , and Kiyoshi Ichimoto 2
1 Indian Institute of Astrophysics, Bangalore - 560 034, India
2 National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka,Tokyo, 181-8588,JapanWe have obtained off the limb' spectroscopic observations covering a coronal region of 200" x 500" in [Fe xl
6374 A and [Fe xi) 7892 A emission lines simultaneously. Yohkoh soft X-ray images of the sun showed two
independent coronal structures in the observed coronal region on the North-West limb. The structure at 20°
N, overlying a sunspot group showed an increase in FWHM of both the lines and increase in the intensity
ratio of 7892· A to 6374 A line with height above the limb, implying an increase in temperature with heightin the coronal loops. Whereas the coronal structure at 32° N showed decrease in the FWHM of both thelines and also decrease in the intensity ratio with height. These variations with height imply that coronalloop-tops were cooler with respect to the foot points in the coronal structure at 32° during the observing
period of about 11 hours. The reason for such a behaviour of this structure may be absence of an as::;ociatedsunspot group, or the magnetic field configuration of the underlying region or coronal structure itself, or thebeginning of formation of the structure or the decay phase of the structure.

1. Introd uction

The study of hotter loop-tops or cooler loop-tops in a coronal structure are important for the dynamics and heating of solar corona. It is generally believed that. coronal loops are hotter at the top as
compared to their foot points. Kano & Tsuneta 1 showed that the temperature and emission measurepeaks around the loop-top and decrease towards the foot point of the steady coronal loops, using thesoft X-ray images of the sun obtained from Yohkoh. Moses et a1. 2 have found that the intensity ratioof [Fe xii] 195 A to [Fe ix,x] 171 A increases to a maximum at the coronal loop-top, suggesting .thatcoronal loops are the hottest at their top (T = 1.7 MK). Others have measured the line-width ofseveral coronal emission Jines at different times with height above the limb ( e.g. Tsubaki 3; Mariska,Feldman & Doschek 4; Koutchmy, Zhugzda & Locans 5; Singh 6; Hassler et al. 7 ; Wilhelm et al. B;
Doyle, Banerjee & Perez 9; Doschek & Feldman 10; Doschek et al. l1 ) and interpreted the variationof line-width in terms of variation in the non thermal velocitY. Tsubaki3) from the measurement of[Fe xiv] 5303 A line, representing plasma at 1.8 MK found that line-width decreases with height.
Whereas others have used mostly EUV emission lines observed from space and emission lines in thevisible part of the spectrum representing plasma below and around a temperature of 1.0 MK, andreported increase in line-width with height. Generally it has been interpreted in terms of increasein the non thermal velocity due to waves assuming the constant temperature in coronal loops. We
have obtained the emission line profiles of a coronal region in the [Fe xi] 7892 A and [Fe xl 6374
A lines simultaneously. These lines represent plasma at temperatures of 1.3 MK and 1.0 MK andthus are best suited to study the variation of temperature and non thermal velocity in coronal loops
with height.We determined the variation of FWHM of both these lines with height and variationof intensity ratio of 7892 A to 6374 A line simultaneously in the coronal loops. In this paper we
discuss the implications of these variations and report that one of the coronal structure shows cooler
loop-tops as compared to the foot points of the coronal loops.
© Anita Publications. All rights reserved.

246.Jagdev Singh, Takashi Sakurai and Kiyoshi 1chimoto

2. Observations and Data Analysis
Simultaneous spectroscopic observations of a coronal region of 200/1 x 500" above the North-Westlimb were obtained in [Fe xi] 7892 A and [Fe x] 6374 A lines on September 19-20, 1998 with the 25cm
coronagraph, the details of the observations may be seen in a paper by Singh et a1 12 . Here we list
important features of these observations. The raster scans were obtained with a step of 4/1, equal to
the width of the slit used, the length of the slit being 500". The exposure time for the 6374 A linewas 208 and 40s for t.he 7892 A line. Thus the raster scan of 50 steps could be completed in about
35 minutes. The observations of the same coronal region were repeated 9 times at irregular interval
and lasted for 11 hours from morning to evening. The spectra were obtained with a resolution of47 mA per pixel at the 6374 A line and 48 mA per pixel at 7892 A line but the slit width of 160
microns reduced the spectral resolution by a factor of 2.3.
The observed line profiles were corrected for the dark current, pixel response and the scatteredlight component. We m~de Gaussian fit to the corrected emission line profile to compute the peakintensity, line-width and Doppler shift at each location of the observed coronal region. Top rightand left hand side panels of Figure 1 show the intensi.ty distribution of 7892 A and 6374 A lines
respectively in the observed coronal region. Due to different pixel size "of the two CCD cameras
used, image scales per pixel were different. The image of the 7892 A line was rescaled to match thedimensions of the image in 6374 A line. More details of the data analysis are given in the paperby Singh et a1 12 . From the appearance of Yohkoh soft X-ray images of the sun for the period ofSeptember 16-20, 1998 (Solar Geophysical Data I3 ), we found that coronal region we have observedon September 19-20, 1998 has two independent coronal structures, one at a mean latitude of 20°
N and other at 32° N. We, therefore, split the observed _coronal region in two parts according tothe two coronal structures observed on the North-West limb of Yohkoh soft X-ray images, for thepurpose to study the variation of line-widths of both these lines as a function of height and theintensity ratio of 7892 A line to 6374 A line ( here after called as 'intensity ratio') with height. We
selected 250 locations covering most of the coronal structure at 20° N latitude as shown by. + marksin thfl middle two panels of Figure 1 and 100 locations on the other structure as shown by +. marksin the lower most two panels of Figure 1, the number of locations being ac«ording to the extent ofthe ,;oronal structure, to study the variations with height above the limb.
3. :itesults
Figure 2 for the coronal structure at 20° N was generated from the locations selected by the + marks
shown in the middle panels of Figure 1. The top panel of Figure 2 shows the FWHM of 7892 A 'and
6374 A emission lines as a function of height above the limb along with linear fit to the respective
data sets and the bottom panel shows the intensity ratio as a function of height along with a linear
fit. This figure shows that FWHM of both these lines and intensity ratio increases with height. Theslope of the linear fit to the FWHM of 6374 A line data is larger than that of the 7892 A line data.
Top panel of Figure 3 for the coronal structure at 32° N shows the FWHM of 7892 A and 6374 Aemission lines as a function of height above the limb and the bottom panel shows the i"ntensity ratio.
To our surprise Figure 3 shows opposite behaviour to that of Figure 2. Here the FWHM of boththese lines decreased with height and also the intensity ratio decreased with height above the limb.
Also in'.this case slope of the linear fit to FWHM of 6374 A line data is less than that of 7892 A linedata. ' The intensity ratio increased in the structure at 20° by a factor of about 1.6 over a distanceof about 140" whereas it decreased by a small amount in the structure at 32° N. Figures 1-3 are
the typical examples of the observations made at different epochs. The mentioned trend in the data
was seen through out the period of observations of about 11 hours. Of course there were some small

Detection o/cooler loop-tops........................

247

S~pt$ll'lhar 20, 1998

(FaXI 7892 A)(FaX 6374 A)

Figure 1: Top right and left hand panels of the figure show intensity distribution of [Fe xi] 7892 A and [Fe xl6374 A emission lines of the observed coronal region on September 19-20, 1998. In the middle two panels, +marks show locations selected on the coronal structure at 20° N to generate Figure 2 and the + marks in thebottom most panels indicate the coronal structure at 32° N and the locations chosen to generate Figure 3.
variations in the values of the slopes with time. It is difficult to say if these variations were periodic
in nature as the interval between successive observations was large. It may be noted that our results
are valid upto a height of about 150" above the limb and for the steady coronal structures and may
not be possible to extend far in the corona as the decrease in line-width has to stop at some heightin the corona.
4. Discussion and Conclusion

If there is an increase of temperature from certain value, the abundance of [FeX] ions responsiblefor 7892 A line emission will increase as compared to that of [FeIX] ions causing the 6374 A lineemission. Thus the intensity ratio of 7892 A to 6374 A will increase with increase in temperature.
The increase in temperature will also cause the increase in FWHM of the coronal emission lines
due to increase in the thermal velocity of ions. Similarly the decrease in temperature will cause
decrease in the above mentioned parameters of emission lines. Therefore, the observed increase in
FWHM of both the emission lines and simultaneously observed increase in the intensity ratio withheight above the limb in the coronal structure at 20° N latitude imply that temperature in thecoronal loops increased with height. There might be some variations in the non thermal velocities

Jagdev Singh. Takashi Sakurai and Kiyoshi Jchimoro

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Figure 2: Top panel of the figure shows the FWHM of 7892 Aand 6374 Aemission lines as a function of heighta.bove the limb along with the linear fit to the respective data. sets and the bottom panel shows the intensity
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Figure 3: Same as that of Figure 2 but for the coronal structure at 32° N latitude.
also. More similar observations especially in three emission· lines are needed to separate out thetwo components. The increase in temperature in the coronal loops agree with the findings of Kano& Tsuneta1 and Moses et al 2 • It may be noted that there was a sunspot group No. 8331 (SolarGeophysical Data 1998) associated with this structure.
~ereas the obsewed decrease in the FWHM of both these lines and decrease in the intensity
ratio with height above the limb in the coronal structure at 32° N certainly imply a decrease in·temperature with height. The loop-tops in this coronal structure show minimum values of FWHMof these lines and of the intensity ratio, implying that loop-tops are at minimum of temperature.
Hence, we may say that we have discovered 'a coronal structure where loop-tops are cooler than thefoot points of the loops. To find out the reason for this we have scanned the broad band images(Solar Geophysical Data 1998) and soft X-ray images of the sun obtained from Yohkoh for theperiod of September 1998. There was no sunspot group on the visible part of the solar disk at 32° N
latitude close to the west limb where this coronal structure was observed on September 19-20, 1998

250Jagdev Singh, Takashi Sakurai and Kiyoshi Ichimotoat the limb. Also the soft X-ray images did not show any activity in the corresponding region ofthe sun prior to September 19, 1998. Soft X-ray images obtained on September 19-20 show activestructure at 32° N corresponding to coronal structure observed by us in 7892 A and 6374 A lines
simultaneously above the limb. From these observations we may say that the coronal structure at32° was formed at the limb on September 19, at the most one day earlier than our observations andif any sunspot activity was associated with this structure, it must be on the limb or behind the limb,
not visible on the solar disk to us. It is also possible that this structure was short lived and was in thedecay phase when we made observations. With these circumstances and observations it is difficult tofind out the reason for the cooler loop-tops in this coronal structure. May be some peculiar magnetic
field configuration at the photospheric level or in the coronal structure itself caused it.
To conclude, we may say, we have probably for the first time found a coronal structure with looptops cooler as compared to the temperature of the foot points. The cause for this can be beginningof of formation of the coronal structure, decay phase of the coronal structure or peculiar magnetic
field configuration of the J.:egion. More observation of such type, better in three lines are needed todelineate the cause of cooler loop-tops.

Acknowledgments
J. Singh thanks National Astronomical Observatory, Japan for providing him the opportunity towork at NAOJ and Prof. J. H. Sastri, Acting Director to grant him leave of absence to visit NAOJ.
J. Singh is thankful to Dr. S. K. Saha for his help in preparing the final version of the manuscript.

References
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2 Moses D et al., Solar Phys., 175 (1997),571.
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4 Mariska. J T, Feldman U, Doschek G A, Astrophys. J., 226 (1978), 698.
5 Koutchmy S, Zhugzda. Y D, Locans V, Astron. Astrophys. 120 (1983), 185.
6 Singh J, Solar Phys., 95 (1985), 253.
7 Hassler D M, Rottman G J, Shoub E C, Holzer T E, Astrophs. J. Lett. 348 (1990), L77.
8 Wilhelm K et al., Astrophys. J., 500 (1998), 1023.
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10 Doschek G A, Feldman U, Astrophys. J. 529 (2000), 599.
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12 Singh J, Sakurai T, Ichimoto K, Muneer S, Solar Phys. 212 (2003), 343.
13 Solar Geophysical Data, (1998), Prompt Report, No. 65l.

Received: September 10, 2004

Revised: October 23,2004

Accepted: November 12, 2004