W E A K E N I N G OF T H E S O L A R E UV L I N E E M I S S I O N

N E A R THE S U N ' S LIM B

M I T S U O K A N N O *

The Astronomical Institute at Utrecht, The Netherlands

(Received 17 May; in revised form 23 July, 1983)

Abstract. The weakening of the EUV line emission near the Sun's limb is studied to acquire information about the absorbers causing the weakening. The equivalent optical thickness of the absorbers for the Lyman continuum is determined as a function of the distance from the center of the solar disk by use of Skylab spectroheliograms in OIV 2554 and OvI 21032. It is found that (1) the weakening cannot be explained by shielding of EUV emitting sources in terms of completely opaque spicules and (2) the distribution of the equivalent optical thickness on the solar disk is extremely fiat with a maximum at a position of ~ 5" above the white-light limb. The results imply that the absorbers are a number of mass blobs consisting of cool chromospheric material which overlies the EUV emitting sources. It is suggested that both the EUV emitting sources and the absorbers are the remnants of He-emitting spicules which are diffused into the corona.

I. Introduction

Withbroe and Mariska (1976) and Withbroe (1977) first suggested that the emission in extreme-ultraviolet (EUV) transition-region lines with wavelengths less than 912 A is weakened by absorption in the hydrogen Lyman continuum due to cool inhomogeneities in the line of sight. Kanno (1979) and Schmahl and Orrall (1979) reported the surprising result that the weakening of the EUV line emission is significant even in spectra recorded at the center of the solar disk. It is difficult to expect such a result from prevailing models of the chromosphere-corona transition region.

Nishikawa (1983) analyzed EUV spectroheliograms of the center of the quiet Sun obtained with the Harvard experiment on Skylab, in order to identify the cool inhomoge- neities causing the weakening. He concluded that EUV-emitting sources in the cell interiors are possily overlaid by cool chromospheric clouds, while the weakening in the network boundaries can be explained also by overlapping of spicules with EUV-emitting sheaths. Kanno and Suematsu (1982) studied the wavelength dependence of the weakening in different types of regions of the solar atmosphere. With the exception of the average active region, they found that the observations are consistent with the 'cloud model' of Schmahl and Orrall (1979), in which the EUV-emitting regions are overlaid by cool structures possibly containing neutral hydrogen. The weakening in the average active region can be explained by overlapping of EUV-emitting spicules as well as by the cloud model.

It is interesting to know the behavior of the weakening near the Sun's limb, which must contain useful information on the absorbers causing the weakening and on the structure of the chromosphere-corona transition region. In this paper we determine the

* Present address: Hida Observatory, University of Kyoto, Kamitakara, Gifu 506-13, Japan.

Solar Physics 89 (1983) 253-259. 0038-0938/83/0892-0253501.05. �9 1983 by D. Reidel Publishing Company.

254 MITSUO KANNO

equivalent optical thickness of the absorbers in the Lyman continuum at positions within and above the white-light limb using Skylab spectroheliograms of the quiet Sun. The distribution of the equivalent optical thickness near the solar limb will be compared with theoretical curves calculated for the two-component model of EUV-emitting sources introduced by Withbroe and Mariska (1976). A suggestion will be given of the origin of the absorbers and the EUV-emitting sources.

2. Equivalent Optical Thickness

The degree of weakening of an EUV line can be expressed by the equivalent optical thickness of the absorbers in the Lyman continuum at 912 A, zn, defined by

I(obs)/I(pred) = exp [ - ZH(2/2H) 3 ] , (1)

where I(obs) is the observed intensity of the fine, I(pred) is the intensity which would be expected in the absence of weakening, 2 is the wavelength of the line, and 2 R = 912 A. That is, Zn corresponds to the optical thickness of the absorbers on the assumption that they overlie the EUV-emitting region. It is assumed that Zn = 0 for EUV lines with wavelengths longer than 912 A.

For an optically thin resonance line I(pred) is given by (Dupree, 1972)

1.74 x 10- 16AgID f G(T) Q(T) dT, (2) I(pred)

where A is the chemical abundance of the relevant element relative to hydrogen; g is the effective Gaunt factor; f the oscillator strength; D a correction factor for the depletion of the ground level when low lying metastable levels are appreciably populated;

G(T) = R(T) T - 1/2 exp( - AE/kT) , (3)

and Q(T) is the differential emission measure defined by Withbroe (1975):

Q(T) = N2(dT/dh) -1 . (4)

In the above expression R(T) is the fractional ion concentration of the ion forming the line, AE is the energy of the line, and dh is the height differential. The intensity is in units of erg cm - z s - 1 sterad- 1

In order to find zn we use the observed intensities of O IV 2 554 and O vl 21032. The temperature rang of line formation is 4.9 < log T ~< 5.5 (log Tma ~ = 5.2) for O IV 2 554 and 5.3 < log T < 5.9 (log Tma x -- 5.5) for O vI 21032, where Tma, is the temperature at the maximum value of G(T) for each ion (Jordan, 1969). We adopt the empirical differential emission measure curves derived by Withbroe (1977) for an average quiet Sun, a quiet cell center, a quiet network, and an average coronal hole, and an average active region. The curves are derived by an iterative method (Withbroe, 1975) using lines with wavelengths above 912 A and lines in the lithium isoeleetronic sequence with log Tm~ • > 5.8, so that they may be free from the effect of weakening (Kanno, 1979). The curves show an interesting feature; for each of the atmospheric regions the

WEAKENING OF THE SOLAR EUV LINE EMISSION 255

differential emission measure is nearly constant in the temperature range 4.9 < log T < 5.9. Accordingly, we have from Equations (1) and (2)

( g f D ) o i v ~ 6 ( T ) dT

Ioxv(obs) OIV ' J - exp [ - zi-i (20 iv/2H) 31, (5)

Iovi(obs) P

(g)VD)o v i ] G(T) dT L/

OVI

where the suffixes OIv and OvI refer to OIv 2554 and O w 21032, respectively. Thus we can estimate "c u from the observed intensity ratio Iotv(obs)/Iovi(obs ), if we know the ratio in the right-hand side of Equation (5).

Using recent atomic data, Raymond and Doyle (1981) carried out multilevel calcu- lations over a range of electron pressures to obtain absolute intensities for spectral lines in UV and EUV regions. Their results are expressed as the intensity Ipr(2) integrated over a constant emission measure for four electron pressures Pe = Ne T. Thus the ratio in the right-hand side of Equation (5) can be known from Raymond and Doyle (1981):

(g )oI f G(T) dT 0 IV _ Ipr(554)

(g/Z))ovi i G(r) dr /pr(1032) o v I

(6)

where we adopt their results for Pe = 101s (quiet regions), 1014 (coronal holes), and 1016

(active regions). In order to obtain an impression on the degree of accuracy in estimating z H from

Equation (5), we compare in Table I the values of zIj derived from Equation (5) with those derived so far by other authors using different methods. Table I shows that except for a coronal hole the values of zH derived from Equation (5) are in agreement with the existing results within a factor of 1.3. Although the reason for the discrepancy of the

TABLE I

Equivalent optical thickness of the absorbers causing the weakening

Quiet Sun Quiet cell Quiet network Coronal Active center hole region

Kanno (1979) Schmahl and Orrall (1979) Nishikawa (1983) Kanno and Suematsu (1982)

2,3 . . . . 4.6 4.5 4.8 3.9 6.5 2.1 2.2-2.4 1.7-1.8 - - 2.8 2.9 3.0 2.8 4.3

Average 2.9 3.0 2.8 3.3 5.4 OIV 2554 / OvI 21032 2.3 3.2 2.4 1.3 6.9

256 MITSUO KANNO

coronal hole is unknown, it is probable that a reasonable value of z H can be obtain from Equation (5), so long as there is little difference in the shape of the Q(T) function at different positions on the solar disk. In Section 3 we will make this assumption.

3. Distribution of the Equivalent Optical Thickness near the Sun's Limb

From observations obtained with the Harvard experiment (Reeves etal., 1974) on Skylab a typical quiet limb region of the west limb on 4 September 1973 was selected for study. The region was devoid of coronal holes and active regions, as can be seen from the M g x ;t 625 image of the region. The data consist of spectroheliograms acquired simultaneously over a period of 5 rain in several lines. Each individual spectroheliogram covers a 5' x 5' area crossing the limb with a spatial resolution of 5" x 5". For the present analysis, we utilized the data of O w 2 554 and O v1 21032.

From the data we derived the observed intensity ratio I o iv (obs)/Io v~ (obs) for each pixel of 5" x 5". The ratios foi- the pixels which are located at the same distance from the white-light limb within + 2.5" were averaged to obtain the mean ratios as a function ofp ( = R/Ro), the distance from the center of the solar disk. From the mean ratios we derived v n as a function of p using Equations (5) and (6). For p < 1, the numbers of pixels at the same value of p range from 54 and 66 and the statistical deviation of the

l O -

I ~H -

5 - -

I I i I I

t EQUATION (7) O! O0 ~ O 0

o / _o_--~o ~ ~ o _ _ 2 _ _ _ ~ o ~ O o / __0 0 - - o - - o o / I O OBSERVATIONS // /

/ /

/ / J j.

- "'" "~'L A N E PARALLEL . .---- ~ " " " " LAYER MODEL

0.5 MODEL

_ _ I I I I J I ~

0,88 092 Q96 1.00 p (= R/Ro)

Fig. l. Compar i son o f the observed distribution o f z H near the l imb with the theoretical ones from the spicule sheath model and the plane-parallel layer mode l o f the chromosphere -corona transit ion region (see

the text for details).

WEAKENING OF THE SOLAR EUV LINE EMISSION 257

mean ratios ranges from 18 to 38~o, while for p > 1 they range from 25 to 48 and from

26 to 60%, respectively. Figure 1 shows the 'observed' value of rH against p by open circles. It is apparent that

the distribution of zH inside the limb is extremely flat as compared to the cos 0law, while outside the limb a maximum appears at a position of ~ 5" above the limb. The light solid line in Figure 1 represents the least-square fit of the distribution of ~xa for p < 1,

zH : 2.4 (see 0) 0.27 , (7)

which is consistent with the value of ~n at the center of the quiet Sun in Table I. The value of zH for p > 1 seems to have a minimum, which may be a fictitious one because the O vl 21032 emission has an appriciable contribution from the corona (e.g. Dupree, 1972).

We conclude that the absorbers causing the weakening are not in a layer with plane-parallel geometry, but must be an aggregation of mass blobs which are at an average height of 5" above the limb.

4. Comparison with the Two-Component Model

It is interesting to examine whether the observed distribution of ~H near the limb can be explained by shielding of the EUV emission due to Ha-emitting spicules which are known to be significantly opaque for the Lyman continuum. We study it using the two-component model of the chromosphere-corona transition region introduced by Withbroe and Mariska (1976). They considered the effects of EUV emission by both spicules and thin structures with plane-parallel geometry.

For 0.8 ~< p < 1.0 the total intensity along the line of sight due to emission by spicules can be expressed by (Withbroe and Mariska, 1976)

/~ = I o P~ ~ e x p ( - k ~ ) , (8) n ~ l k = O

where Io is the specific intensity radiated by a spicule perpendicular to its axis, P,, is the probability that there are n spicules along the line of sight, and z, is the optical thickness of a spicule along the line of sight. For the extreme values of % we have

I~(z, = O) = IoN~m, (9)

and /~(% = (~) = Io[1 - e x p ( - N / m ) ] , (10)

where N and m are defined by Equations (3) and (5) of Withbroe and Mariska (1976). For a model in which all the EUV line emission comes from spicules (spicle sheath

model), the equivalent optical thickness of the weakening can be calculated by

/~(% = ~)//~(~s = 0) = exp ( - ~H(~/2H)3], (11)

if spicules are completely opaque for the Lyman continuum. Equation (11) gives the upper limit of zi-i by shielding due to spicules.

2 5 8 MITSUO KANNO

The intensity of an optically thin line due to emission from a plane-parallel layer can be expressed by (Withbroe and Mariska, 1976)

oo

Ip = 1(0)# -1 Z Pnexp(-nzs) , (12) n ~ 0

where 1(0) is the intensity from the layer at the center of the solar disk and # = cos 0. In the above expression absorption by spicules is taken into account and the plane- parallel layer, whose thickness is to be much less than the height of spicules, is assumed to be located at the top of the homogeneous chromosphere from which spicules are originating. For the extreme values of z~, we have from Equation (12)

Ip(Z, = 0) = I(0)#- a, (13)

and

Ip(v s = oo) = I ( O ) # - ' e x p ( - N / m ) . (14)

For a model in which all the EUV line emismsion comes from the plane-parallel layer (plane-parallel layer model), the upper limit of ~n by shielding due to spicules can be estimated from an equation corresponding to Equation (11).

We calculated ZH for O IV 2 554, using the same values of quantities on the properties of spicules as in Withbroe and Mariska (1976). The theoretical distributions of ZH by shielding due to spicules are shown in Figure 1 by the dark solid line (spicule sheath model) and the dashed line (plane-parallel layer model). Except for the extreme limb, the theoretical values of zri representing the upper limits are much less than the observed ones. Furthermore, the theoretical curves of "b- H are similar to those from the cos 0 law, which is inconsistent with the observations.

We conclude that the weakening cannot be explained by shielding of EUV emitting sources in terms of completely opaque spicules. The present author's suggestion (Kanno, 1979) that shielding due to spicules may cause the weakening is not correct; a major part of the weakening comes from unknown absorbers consisting of cool chromospheric material. We now suggest that the absorbers causing the weakening correspond to a number of mass blobs which are diffused into the corona from Ha-emitting spicules, while the EUV emission from transition-region lines also originates from the mass blobs. A study of the spicule-remnant model of the chromosphere-corona transition region mentioned above is in progress.

5. Conclusions

We have studied the weakening of the EUV line emission near the Sun's limb, using the Skylab spectroheliograms in Ow 2554 and OvI 21032: (1)The distribution of ~H on the solar disk, which 5s a measure of the degree of weakening, is extremely flat as compared to the cos 0 law. A maximum of ~ia appears at a position of ,-~ 5" above the limb. (2) The weakening cannot be explained by shielding of EUV emitting sources in terms of completely opaque spicules. We have suggested that the absorbers causing the

WEAKENING OF THE SOLAR EUV LINE EMISSION 259

weakening are an aggregation of mass blobs which are the remnants of Ha-emitting spicules.

Acknowledgements

The author would like to thank Prof. Dr C. de Jager and Dr O. Namba for improving the original manuscript. Thanks are also due to Dr T. Nishikawa for the assistance in processing the observational material. Part of this work was carried out during the author's stay at the Astronomical Institute at Utrecht. His stay was supported by the Japan Society for the Promotion of Science (JSPS) and the Netherlands Organization for Applied Scientific Research (TNO).

References

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