Pergamon

Chin. Astron. Astrophys. Vol. 20, No.1, pp. 85-91. 1996 A translation of Acta Astron. Sin. Vol. 36. No.3, pp. 288-293, 1995

Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved

SO2751062(96)00012-4 0275-1062196 S24.00+0.00

Two-dimensional He I 10830 hi spectroscopy

of a subflare t

LI Hui FAN Zhong-yu YOU Jian-qi Purple Movnlain Observadory, Chinese Academy of Sciences, Nanjing 210008

Abstract We present a two-dimensional He I 10830 8, spectroscopic observation

of the SF/C 1.9 subflare of 1993-12-29 and a preliminary data analysis. The

main results are: 1) Bright spots above the continuum as proposed by Rust were

not found in the direct monochromatic map, but four small areas of relative

brightening were found in the residual intensity map, whose intensities varied

appreciably in the course of the flare. 2) All the He 110830A bright areas fall

inside the Ha bright regions, but the converse is not true. 3) Of the four bright

areas, two are located in the penumbra, one extends into the umbra, and one

possibly is inside a small pore. 4) All the four bright areas show redshifts or red-

ward asymmetry, giving downward motion of under 16 km/s, which got smaller

as the flare subsided.

These subflare “bright areas” are very similar to the emission spots above the

continuum we previously observed and so can be regarded as having the same

physical origin. Their lesser intensity is probably due to their exciting source

having weaker X-ray emissions.

Key words: solar flare-infrared-two-dimensional spectrum

1. INTRODUCTION

The helium lines, having far higher excitation potentials than the hydrogen and ionized

calcium lines, may provide information not contained in the latter. But in the optical

wavelengths, most helium lines have very small optical thicknesses, and do not respond

to usual surface features or even small flares. He 110830A is the only helium line that

shows absorption on the solar surface, it has an adequate optical thickness and being a

triplet, it is convenient for extracting the physical parameters. Moreover, it is located in

the near infrared range where atmospheric scattering is small, thus, it is quite an ideal

line and has already been used in exploring quiet solar features like coronal holes and the

chromosphere networkl’l. It can also provide diagnostic for transients like active regions and

t Supported by National Natural Science Foundation Received 19940902; revised version 1994-11-22

85

86 LI Hui et al.

flares121. But because of its location in the near-infrared, its detection and measurement is

difficult, consequently, observations on fast phenomena like flares are rare, and the available

results are mutually conflicting. For example, Tandberg-Hanssen in his review articleI

holds that only the cores of flares of classes of 2+ or greater can show emission on the solar

disk, in all other places, He 110830A is in absorption. Harvey and Recely141 on comparing

various data of a 3N/M4.0 large flare, holds that the bright double ribbon in Ha flares

corresponds to a dark double ribbon in He I 10830 A. Rust and Bridge@] from a study of 12

subflares in monochromatic lights found that the subflares all have one ore more small bright

spots with emission above the continuum, outside these bright spots, even within the bright

Ha flare region, He 110830A remains in absorption, apparently unaffected by the flare.

Apart from extremely rare cases, short time scale variation was never seen in He1 10830A

absorption in the course of a flare. Recently, You et all61 summarized the He1 10830A

spectra of 11 small flares of classes <_ 1N and found that 7 had small (5 lo”), brighter-

than-continuum regions and moreover, most of the He I 10830A lines in these bright regions

showed either a bodily redshift or a redward asymmetry. They also found that, outside the

He1 10830A bright spots, but still within the Ho flare region, although there is no change

in the intensity of He 11083OA the average optical thickness is increased. The question

therefore arises, are there two kinds of small flare, in one, bright spots are shown in the

He I 10830 A image, in the other, no such are seen ? Then, Is there any difference in physical

conditions inside and outside such spots . ? In order to go on with further quantitative

calculation we need more observational data as basis. As You et al. used at the time a one-

dimensional Reticon, their failure to detect bright spots in He I 10830 A could have been due

to insufficient spatial coverage. Rust and Bridge, on the other hand, arranged single diodes

into an array and did not obtain any spectral profile, so we cannot exclude the possibility

of false brightening caused by velocity in their case. Hence observation and analysis of a

two-dimensional He I 10830 A flare spectrum in a time series should be useful for tackling this

problem. What we observed this time was a subflare with a weak X-ray emission (SF/C 1.9,

NlOW41), happening to be at the critical state between possible presence and absence of

brightening. Analysis of this actual instance may hopefully lead to understanding.

2. OBSERVATION AND DATA TREATMENT

For the observation, we used the Purple Mountain Observatory solar He I 10830 A monochro- matic scanning equipment171, fitted with a TM-860 CCD (590 x 800 pixels). In the direction

of the slit each pixel corresponds to 0.34”, along the dispersion, to 0.0693A. Scanning is

realized by a step motor actuating the secondary mirror of the coelostat, each step corre-

sponding to w 1.8”. To raise the time resolution and save computer disk space, we used

the “opening window” work mode, that is, at each scan, each frame records only part of

the spectral data-the 30 pixels around the centre of the He 11083OA line (- 2%1), and 10

pixels (- 0.7A) in the neighbouring continuum. For dark or flat fields, data from the whole

map are recorded.

On each of the two days, December 28 and 29, 1993, we observed a subflare in the active

region NOAA 7640 and obtained their two-dimensional He I 10830 A spectrum. Because we

did not secure data before the flare maximum on 28, here we shall present mainly the results

He I 10830 AImage 87

observed on 29. The weather at the time of observation was fine, with occasional thin clouds

passing, but the seeing was not good, about 3”t-4tt. The observation started one minute after

the onset of the flare, and ended ten minutes after the maximum. Four sets (labelled G,

H, I, J) of two-dimensional spectral data were obtained, each comprising 50 steps of scan,

scanning an area of about 102”x90tt on the solar surface and just covering the whole Ha

flare bright region. Simultaneous series of front-of-slit Ha photographs were taken. The

relevant data on the flare and the observation are given in Tables 1 and 2, respectively.

Table 1 Data of the Active Region and the Flare

ACTIVE REGION Serial Position sunspot class Magnetic Class 7640 NlOW40 BG FKI

FLARE start Maximum Finish Class X-ray

0218 UT 0220 UT 0237 UT SF c 1.9

Table 2 Summary of Observations

Scan Series Time of Scan (UT) Number Scans Notes G 0219-0220 50 Ha photos H 0222-0223 50 I 0224-0226 50 Ha photos J 0228-0230 50

In the data treatment, each frame of original spectrum is corrected for the appropriate

dark field and applied the flat field correction. When combining the monochromatic images,

four columns of pixels are taken from each frame, corresponding to a monochromatic image of

half-width 0.28 A. By taking different matching pixels we obtain the images at the line centre

and various off-bands, as well as the spectral profile near He I 10830 A at any point of the

image, and we can use the continuum to calibrate the intensity. In the observation, automatic

return to the starting point was effected after completing a scan series. But considering

that there may be mechanical gaps, when comparing the pictures from the four series,

we first examined the combined photosphere images from the continuum and, assuming the

photospheric sunspots would not have changed much in the 11 minutes of observation, made

the necessary offset adjustment, should there be any displacements between the images.

3. RESULTS AND DISCUSSION

In the direct He1 1083OA core intensity map pieced together from the scans (Fig. la) the

whole flare region is in absorption and we cannot see any bright arcsecond-sized areas with

intensities above the continuum, as pointed out by Rust and BridgeL51. But on the residual

intensity map obtained by dividing the observed intensity Ic by the continuum intensity at

the corresponding point, I=,, (Fig. lc), we can clearly see four small areas (labelled by 1, 2,

3, 4) that are brighter than their surroundings. These “bright areas” coincide in position

with bright cores of the Ha flare, their size is 3” - 5”, far smaller than the Ha flare.

Outside of the He I 10830 A bright areas and inside the Ha bright area, He I 10830 A is still

in absorption. In the course of the flare, the residual intensities of the four He1 10830A

88 LI Hui et al.

“bright areas” showed clear variations (Table 3), with amplitudes of about 13%-15% of the

continuum. Bright area 2 had a net redshift, causing abnormal brightening in the residual

intensity map, hence in Table 3 we list also its real residual intensity at the line core.

Fig. 1 (a) He1 10830 monochromatic image of I .he flare region; (b) Monochromatic image of

continuum of the flare region; (c) Residual inten sity image of the flare region at center of He1

10830; (d) H, image of the flare region

He 110830 AImage 89

After comparing with SGD and Huairou magnetograms we found that bright areas 3 and

4 are located on opposite sides of the magnetic inversion line, and they are linked by a bright

arc in the Ha flare map and they are also linked by a faintly bright arc in the He I 10830 A

residual intensity map: very probably they correspond to the two foot-points of the flare

loop. For comparison, Table 3 also lists the data for a location outside the He1 1083019

bright areas but within the Ha bright region (Point 5) and a flare-free location in the active

region (Point 6). We note the central intensities of these two remained constant in the course

of the flare. We also found that at a point next to the IIe I 10830 8, bright arc between bright

areas 3 and 4 inside the Ha bright region, the absorption increased by lo%-20% during the

flare.

Of the four bright areas, two are located inside the penumbra of the sunspot, one extends

even into the umbra, the other one is outside the penumbra, but its continuum intensity is

still lower than in other parts of the photosphere, hence very probably inside a small spot

or pore. Although the signal-to-noise ratio of the spectral profile is low because of the small

number of fields integrated over, the smallest variation detectable after integrating over 10

fields is 6% of the continuum (the 3g value), adequate for resolving the variations in spectra

of the four bright areas (13%-25% relative to the continuum) and we could see that the

profiles of the bright areas were either bodily redshifted, or showing a higher red wing. This

implies that in the bright areas, there is downward motion of matter, at velocities under

16 km/s. The redshift clearly decreased as the flare decayed (Fig. 2).

011 the previous day, December 28, 1993, we also observed a subflare (C8.4) in the

same active region NOAA 7640, but we only recorded the descending phase;-at the time of

our observation the SXR level had already fallen to about C 2.0. For this subflare, all the

He I 10830 A features including the positions of the areas of relative brightening with respect

to the sunspot, the redshift motion etc. are quite similar to the December 29 subflare. This

is presumably because there was little change in the magnetic field configuration over the

two days.

Although in these two flares of NOAA 7640, we did not detect any He1 10830A emission

above the continuum level, the characteristic features of the relative “bright areas” including

their size, motion, correspondence with Ha bright cores, and variation in intensity are

indistinguishable from the He I 10830 A bright spots above the continuum brightness, so the

two should be regarded as having the same properties and physical cause. Whether the

brightening should exceed the continuum depends on the strength of the exciting source of

the line. Phenomenologically, this appears to be related to the soft X-ray intensity, as You

et all61 found that if the SXR class is above C4 (inside a sunspot) or above C6 (outside a

sunspot), then higher-than-continuum emission will be detected in He I 10830 A. The present

result says that when SXR is around C 1.9, such emission is no longer present, but “bright

areas” can be detected in the residual intensity map. But when SXR falls below C 1, then

even the “bright areas” will not be present. This was the case in our J series of December

29. Again, on May 25, 1993, we had observed a SF/B8.6 flare under very good seeing,

yet we never detected any “bright area” around the maximum, very probably because the

brightening was very small and was drowned in our observing error. It seems that SXR

intensity is a necessary condition for exciting bright areas of He1 1083OA, but whether it is

a sufficient condition must await accumulation of further data. Although the observation

90 LI Hui et al.

Table 3 Residual Intensity of He 110830 A Centre and Average Relative Intensity of Continuum

at Selected Points

Selected R esidual Intensity Average Point G H I J Continuum

bright area 1 0.760 0.656 0.708 0.612 655 bright area 2 0.788/0.732* 0.701/0.694 0.65710.647 0.532 466 bright area 3 0.708 0.687 0.714 0.623 595 bright area4 0.774 0.742 0.657 0.656 733

point 5 0.649 0.640 0.643 0.632 728 point 6 0.695 0.706 0.696 0.709 784

*slash separates values before and after correction for velocity field

i <

c I

0. (i t

0

0

.-

1 ;

(I.

0.

I I I I I I0830 10830.3 10x31

Cf'avelength (l\)

I

r --. (d)

-(

-7i3dmk . . , Wavelength (A)

Fig. 2 Time variation of the profile of He 110830 A at bright area 2. The four panels (a)-(d) . . . . . --

He I 10830 AImage 91

used by Harvey and Recely141 was a large 3N/M4/0 flare, the He I 1083OA data were obtained

some 15 hours after the start of the flare, when the SXR had fallen to about C 2.4, and from

the small diagram given we cannot see whether there were any small “bright areas” in the

residual intensity map. If there indeed were not any, then such “bright areas” depend also

on the nonthermal processes of the flare. Our observed size of the bright areas, lo-25

square arcsecond or (0.3 - 1.3)x lOi cm2, is very close to the size of the invariant region of

electron sedimentation given by applying the thick target model to the region of Ha redshift,

(2.2&0.7)x 1017cm2 (large flares) or (1.4&1.0)x 1017cm2 (small flares)l’l, while our measured

infall velocity is somewhat smaller than given by the Ha measurement,-this may be related

to the lag between the time of observation and the impulsive phase, or to the lines coming

from different heights. The He1 10830A bright areas occur most often in the penumbra of

sunspots, less often in the umbra and in small pores, and they coincide in position with

the Ha bright cores, -this is close to the law of occurrence of white-light flareslg~‘ol. It

seems that the small “bright areas” clearly differ from the surrounding flare region in their

properties. Very probably, the diffuse region of absorption surrounding a He I 10830 8, bright

area is related to the SXR field, an increase in SXR increases the density of the He1 triplet

energy levels, while the He I 10830 A bright areas correspond to the footpoints of hard X-ray

emission loops and so are directly related to nonthermal electrons.

Of course, some of the conclusions can only be confirmed after confrontation with space

observed SXR and HXR images, but some of the features of the He I 10830 A “bright areas”

found here, including their size, redshift velocity, relative position on the solar surface, and

their correlations with Hex bright cores and X-ray intensity provide certain observational con-

straints on future discussions of flare models and, especially, “chromospheric condensation

zones”121, as well as give a handle for researching into the mechanism of He1 10830A.

ACKNOWLEDGMENT We thank Colleague Xie Xiao-wei for help with the photographs

of this paper.

[II 121 [31

[*I

[51

161

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References

Giovanelli Ft. G., Hall D., Solar Phys., 1977, 52, 211

Fisher G. H., Canfield R. C., McClymount A. N., ApJ, 1985, 289,425

Tandberg-Hanssen E., In Solar Activity, Blaisdell, Walthum, Mass., 1967, 250

Harvey K. L., Recely F., Solar Phys., 1984,91,127

Rust D. M., Bridges III, C. A., Solar Phys., 1975,43, 129

You Jianqi, Wang Chuanjin, Fan Zhongyu, In: G. X. Ai, E. Hiei, Y. J. Ding et al., eds., Proceedings of the first China-Japan Seminar on Solar Physics, Kunming TonDar Institute, 1993,148

You Jian-qi, Wang Chuan-jin, Lu Jin & Fan Zhong-yu, CAA 1994, 18, 238 = AAnS 1993, 34, 374

Zarro D. M., Wulser J. P., Canfield R. C., In: Ft. M. Winglee, A. L. Kiplinger eds., Max’91 Workshop, Univ. of Colorado, Boulder: Colorado, 1991, 77

McIntosh P., Donnelly R. F., Solar Phys., 1972, 23

Sakurai et al., Publ. Astron. Sot. Japan, 1992,44, L7